Human Prototypic Disease Models for Geriatric Sarcopenia
Diabetes and diabetic sarcopenia
Reduced insulin actions due to insulin resistance and/or depletion seem to be involved
in the aggravation of muscle dysfunction. Recent-onset type 2 diabetes (T2D) is associated
with sarcopenia in elderly (≥75 years), but not in middle-aged or younger subjects
[9]. The older participants in this study possessed a higher insulin resistance, whereas
the younger participants predominantly exhibited a reduced insulin secretion capacity,
which does not only indicate a different pathological pathway of T2D manifesting earlier
or later in life, but also a stronger association of sarcopenia with insulin resistance
than with insulin depletion. This assumption is affirmed by further studies showing
that insulin resistance is an independent risk factor of decreased skeletal muscle
mass [10].
Insulin is an anabolic and metabolic hormone increasing muscle protein synthesis and
limiting protein degradation [11]. It induces a pathway involving phosphoinositide3-kinase (PI3K), phosphoinositol
triphosphate, protein kinase B/AKT, TBC1 domain family member (TBC1D) 4 and TBC1D1,
resulting in the translocation of glucose transporter type 4 (GLUT4) containing vesicles
to the plasma membrane [12] and the dis-inhibition of glycogen synthesis by phosphorylation of glycogen synthase
kinase 3. AKT also induces the activation of mTOR (mechanistic target of rapamycin),
which promotes protein synthesis, and inhibits forkhead box protein O (FOXO) activity,
thus reducing the expression of E3 ubiquitin ligases that mediate atrophy [13] ([Fig. 3]). AKT overexpression in rodent muscle causes hypertrophy and increases glucose disposal,
whereas defects in the AKT pathway are detectable in leptin receptor-null mice and
obese Zucker rats [14]. A decreased mTOR pathway activity is present in T2D patients [15]. In the insulin resistant state, mTOR signaling is diminished and thus protein synthesis
is decreased whereas protein degradation is disinhibited [16]. However, despite the obvious significance of insulin for muscle metabolism and
anabolism, its therapeutic potential for the prevention and alleviation of sarcopenia
remains unclear. In a retrospective observational study, insulin treatment was protective
against the decline of skeletal muscle index (SMI) in the lower, but not the upper
extremities [17]. Insulin therapy stimulates protein anabolism in younger, but not in older diabetic
subjects and failed to prevent atrophy in elderly [18]. Particularly elderly people may be affected by a differential insulin resistance,
in which they might be still sensitive to insulin-mediated glucose uptake, but not
anymore to insulin-mediated protein synthesis [19]. In the elderly, supra-physiological insulin doses seem to be necessary to overcome
insulin resistance and to stimulate protein synthesis and muscle anabolism [20]. However, permanent hyper-insulinemia has acknowledged negative effects and is therefore
an inappropriate approach. Altogether, beneficial effects of insulin on muscle function
and mass in elderly are questionable and are certainly no justification for the initiation
of an insulin therapy.
It seems encouraging to target insulin resistance itself. The insulin-sensitizing
glitazones are promising agents against atrophy [18]. They improve mitochondrial activity and limit protein degradation. In mice, rosiglitazone
reduces the activity of caspase-3 and proteasome in muscle, thus reducing proteolysis
and atrophy [21], and treatment with rosiglitazone led to an improvement of muscle mass [14]. The positive effect might be mediated by a disinhibition of the PI3K/AKT pathway
via improved insulin sensitivity. Another possible explanation might be the activation
of the AKT-mTOR pathway following stimulation of peroxisome proliferator-activated
receptor (PPAR) γ by rosiglitazone ([Fig. 3]). However, in elderly obese non-diabetic individuals, pioglitazone did not prevent
muscle loss [22] and the unfavorable benefit/risk profile currently limits glitazone use even as
anti-diabetic agents.
The biguanide metformin is another drug potentiating insulin actions. Metformin activates
AMP-activated protein kinase (AMPK) [23], which is a cellular energy sensor stimulated by a raising AMP/ATP ratio and increasing
glucose and fatty acid uptake and oxidation in skeletal muscle [24]. Thus, AMPK plays an essential role in muscle energy balance during ongoing exercise.
Furthermore, AMPK mediates long-term effects of exercise, such as mitochondrial biogenesis
[25]. AMPK can furthermore protect against age-related functional and mitochondrial impairment
via promotion of myocyte macroautophagy, which is essential for myocyte maintenance
[26]. The beneficial effects of AMPK apply mainly on muscle metabolism. The influence
on muscle mass seems to be less favorable. In aging rodents, an inverse relationship
of AMPK activation and load-induced muscle hypertrophy was demonstrated [27]. AMPK stimulates myofibrillar protein degradation via increased FOXO expression,
down-regulates the mTOR pathway, thus restricting protein synthesis [28], may negatively influence the differentiation of satellite cells [29] and stimulate myostatin expression [30], hereby triggering muscle protein degradation while downregulating muscle protein
synthesis ([Fig. 3]). An in vitro study showed that metformin increased myostatin expression in cultured myotubes at
a concentration of 0.5 mM [31]. Thus, AMPK agonists may have the potential to induce muscle atrophy. The net effect
of metformin on muscle function and its clinical impact in alleviating or even aggravating
sarcopenia remain to be determined. A case control study indicated a protective effect
of metformin against frailty in T2D [32] and a randomized study demonstrated a significantly higher gait speed after a 16-week
double blind intervention with metformin (3×500 mg/d) in pre-frail elderly participants
without diabetes. However, handgrip strength and muscle mass remained unchanged after
metformin therapy [33]. These latter results are in line with the above-mentioned AMPK effects. Handgrip
strength tests isometric hand muscle contraction, which predominantly involves type
II muscle fibers, whose main energy source is anaerobic ATP metabolism. In contrast,
the gait speed test requires dynamic rhythmic and repeated muscle contractions, which
primarily relay on aerobic glucose metabolism. Altogether, metformin and AMPK activation
may alleviate muscle metabolism in endurance exercise, but not muscle mass and/or
power.
Blockers of the ATP-sensitive potassium (KATP) channels, such as the sulfonylureas
and glinides may induce atrophy. Glibenclamide has been demonstrated to induce muscle
atrophy in rats and in humans and repaglinide has been identified as an atrophic agent
in preclinical studies [18].
Glucagon-like peptide-1 (GLP-1) exerts anti-apoptotic effects on pancreatic β-cells
and increases glucose sensitivity, proliferation and transcription of proinsulin [34]. The intracellular GLP-1 signaling involves PI3K and other proliferative pathways
[18]. Further, GLP-1 has various beneficial extra-pancreatic effects, such as improvement
of endothelial and cardiac function and decrease of ischemic myocardial damage [35]. These effects are thought to be at least partially mediated by an improved tissue
glucose uptake. In cultured human and rat skeletal muscle cells, GLP-1 increases GLUT4
gene expression and glucose uptake. In rat skeletal muscle, GLP-1 improves insulin
sensitivity [36]. But effects of GLP-1 analogues on age-dependent muscle atrophy in animals and humans
are currently unknown und warrant further examination. A pilot study involving nine
participants revealed a modest increase in skeletal muscle index (SMI) after 24 weeks
of liraglutide treatment, whereas body mass index (BMI) and android fat decreased
[37].
Dipeptidyl peptidase 4 inhibitors (DPP4i) inhibit endogenous GLP-1 degradation and
display a protective effect in preliminary studies. In rats, sitagliptin (40 mg/kg
twice daily) restored GLUT4 expression in the heart, soleus und gastrocnemius muscles
[38]. In 37 T2D patients treated with DPP4i, the SMI was higher than in T2D patients
without DPP4i treatment [39].
Sodium glucose co-transporter inhibitors are also not well studied regarding muscle
integrity, but two weeks of dapagliflozin treatment increased insulin-mediated tissue
glucose uptake by 18% [40] and another study found an overall weight loss without reduction of muscle mass
in 50 participants with T2D treated with dapagliflozin, indicating a neutral effect
of this agent [41].
Taken together, the interwoven pathogenic processes of T2D and sarcopenia may offer
chances for shared therapeutic approaches. However, the so far known antidiabetic
drug effects on muscle integrity are mainly unfavorable [18] ([Table 1]). Up to date, there is no exercise-mimicking drug and the only established measures
to improve both muscle function and insulin sensitivity are exercise-based interventions
[42]. Apart from compliance problems, elderly individuals are often too frail to undertake
the required degree of exercise. Therefore, currently available data primarily stress
the importance of an early detection of muscle deterioration in diabetic patients
in order to prevent accelerated progression of both sarcopenia and T2D.
Table 1 Currently available anti-diabetic drugs and their effects on muscle integrity.
drug class (tested agent)
|
net effect
|
mechanisms/present data
|
Insulin
|
neutral/beneficial
|
stimulates protein anabolism (only in younger, not in older subjects)
|
Glitazones (rosiglitazone)
|
beneficial, but the risk profile limits their use
|
improve mitochondrial activity and decrease protein degradation; improve muscle mass
|
Biguanides (metformin)
|
unclear
|
improvement of muscle metabolism, but potential to induce atrophy
|
Sulfonylurea (glibenclamide)
|
unfavourable
|
induce atrophy in rat and human
|
Glinides (repaglinide)
|
unfavourable
|
induce atrophy in vitro and in rodents
|
GLP-1 analogues (liraglutide)
|
possibly beneficial
|
increases GLUT4 gene expression and glucose uptake; increase in SMI
|
DPP4-Inhibitors (sitagliptin)
|
possibly beneficial
|
higher SMI
|
Sodium-glucose-co-transporter inhibitors (dapagliflozin)
|
unclear, possibly neutral
|
SMI unchanged
|
Legend: No data are available for Amylin (Pramlintide/Symlin) and glucagon secretion
inhibitors.
Aside from therapeutic efforts targeting sarcopenia in T2D, it has to be kept in mind
that sarcopenia may be one of several major diabetes-related complications that do
not necessarily improve with intensification of antidiabetic treatment, but may even
be influenced negatively by “overtreatment”. Especially in older people, preserving
mobility might be more important than a tight blood sugar control. Thus, the current
knowledge about the influence of antidiabetic therapy on sarcopenia is – at least
to date - one more argument for a less strict and individually adapted antidiabetic
therapy in older individuals. Further, whereas reduction of dietary energy intake
and weight loss are classically recommended for T2D treatment and improve insulin
sensitivity, such interventions may worsen muscle loss in elderly, at least unless
concurrent exercise and nutritional supplementation are ensured [43]. Possibly, but unproven, weight loss supported by GLP1-analogues may produce more
favorable outcomes.
Cushing’s syndrome and sarcopenic obesity
Cushing’s syndrome (CS) characterized by an excessive secretion of cortisol is known
to be associated with obesity, muscle atrophy ([Fig. 1]) and a decline in physical activity. Surgical treatment of clinical hypercortisolism
due to pituitary adenoma or adrenal gland tumors is effective in the control of cortisol
excess, but the functional impairments due to the glucocorticoid-induced myopathy
remain despite biochemical remission. Thus, Berr and colleagues could show that hand
grip strength and proximal muscle performance were both significantly decreased in
patients suffering from active CS compared to obese controls [44]. Six months (short-term) and 2 years (long-term) after surgery, grip strength was
still decreased and the chair rising test (CRT) performance also remained impaired.
Another study investigating the relationship between CS, muscle atrophy and obesity
indicates that CS could even be seen as a prototypic disease model for sarcopenic
obesity [45]. CS patients did show a severe decrease in muscle function measured by hand grip
strength and CRT, but did not lose muscle mass. Fat infiltration into the muscle might
be the reason for decreased muscle quality as CS patients with impaired glucose metabolism
showed strongest deterioration of muscle function.
To gain insights into the pathophysiological mechanism leading to muscle atrophy in
patients suffering from CS, rodent models were introduced, based on adrenocorticotropic
hormone (ACTH) induced hypercortisolism and consecutive muscle atrophy. Thus, rats
were infused with ACTH to produce excessive cortisol in the adrenal glands. Gene expression
analysis of skeletal muscle of these rats resulted in an increased expression of FOXO3a
which itself induces the expression of the E3 ubiquitin ligases MuRF1 and atrogin-1
([Fig. 3]). Both of these genes were highly expressed in the infused rats. Furthermore, the
FOXO3a promotor was targeted by the activated glucocorticoid receptor (GR) which usually
resides in the cytosol, but translocates into the nucleus upon glucocorticoid binding
and controls protein degradation in skeletal muscle [46]. A reduction in AKT activity upon GR activation results not only in FOXO3a activation,
but also in FOXO1 activation [47]. Both are involved in muscle atrophy through two main pathways: activation of the
ATP-UPS and autophagy. The ubiquitination and subsequent degradation of muscle proteins
is driven by two E3 ligases MuRF-1 and atrogin-1. MuRF-1 usually targets muscle structural
proteins like myosin heavy chain, actin, myosin-binding protein C and troponin I,
whereas the targets of atrogin-1 are involved in protein synthesis like eukaryotic
translation initiation factor 3 (eIF3) and protein translation initiation factor [48]. Besides the UPS activation, FOXO3a also mediates autophagy via the regulation of
autophagy-related genes such as microtubule-associated protein 1 A/1B-light chain
3 (LC3) and BCL2/adenovirus E1B 19- kDa-interacting protein 3 (BNIP3) [49].
As CS is not only associated with muscle atrophy, but in parallel with obesity, a
closer look at the molecular pathology of sarcopenia in the context of accompanying
obesity seems to be important as obesity appears to be a promoting factor which exacerbates
the development of sarcopenia in both, young patients suffering from CS and the older
people developing sarcopenia. Obesity is defined as abnormal or extensive fat accumulation
that negatively affects health. Sarcopenia and obesity can co-occur, and synergistically
are associated with worse functional decline and outcomes than either condition alone
[10]. Therefore, Baumgartner and colleagues first described the term sarcopenic obesity
as a skeletal mass index that is less than two standard deviations below the sex-specific
reference for a young, healthy population, with a percentage of body fat greater than
27% in men and 38% in women (roughly a BMI of 27 kg/m2) [50]. Several reasons have been hypothesized how obesity contributes to a decline in
skeletal muscle. Under both conditions, in the sarcopenic state as well as in obesity,
the muscle is infiltrated with fat, which makes paracrine signaling via (adipo-) cytokines
between fat and muscle cells possible. One major cause of muscle loss is the reduced
capacity of muscle cell renewal as muscle cell progenitors differentiate due to increased
levels of adipokines in an adipocyte-like phenotype. Pro-inflammatory adipokines and
cytokines such as interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α) and C-reaktive
protein (CRP) are described as confounders of pathological processes in sarcopenic
obesity and lead to a low-grade inflammatory state which will further be described
in CKD and uremic sarcopenia. Thus, besides the role of TNF-α in programmed cell death
it also upregulates the expression of the muscle atrophying ubiquitin ligase MuRF1
via the nuclear factor ‘kappa-light-chain-enhancer’ of activated B-cells (NFκ-B) pathway.
Other causes are the abnormal protein biosynthesis rate and anabolic resistance to
exercise. There seems to be a cross-talk between the hypothalamo-pituitary axis and
the nutritional status which leads to a higher catabolic state of metabolism in obese
patients and a higher susceptibility of muscle wasting under energy restriction [51]. Further mechanisms are the obesity derived intracellular lipotoxicity which leads
to an increase of lipids and fatty acids in muscle cells causing oxidative stress.
Reactive oxygen species (ROS), chronic inflammation, insulin resistance, increased
levels of leptin and a decrease of adiponectin lead to mitochondrial damage and apoptosis
in skeletal muscle. Further, pituitary induced high circulating levels of glucocorticoids
in CS may directly modulate bone remodeling and metabolism causing deterioration of
the structural integrity of bones which is associated with a high risk of fractures
[52]. Thus, CS is often accompanied by a secondary osteoporosis which leads in combination
with muscle atrophy and obesity even to an osteosarcopenic obesity, a new phenotype
in geriatrics [53]. Hypercortisolism affects bone metabolism with various mechanisms. It is known that
glucocorticoid excess inhibits bone formation, reduces the replication of bone forming
cells, prevents the differentiation of osteoblasts, induces apoptosis of osteoblasts
by activating caspase 3 and inhibits collagen type I synthesis [54]. Treatment of pituitary hormone excess such as surgery of pituitary adenomas improve
skeletal health in some patients, whereas others seem to remain a high risk of fractures.
As muscle, fat and bone tissue exert an immense cross-talk, it would be interesting
to study patients suffering from CS and associated secondary osteosarcopenic obesity
in more detail to find out more about pathologies leading to geriatric sarcopenia
often accompanied by osteoporosis.
Since about three out of four European adults are overweight, and sarcopenia and obesity
can co-occur or even both occur in patients suffering from CS, a closer look at the
molecular pathology of the CS driven sarcopenia/sarcopenic obesity is important [55]. The profound decrease in life quality of patients suffering from CS with muscle
function impairments emphasize the need for further studies clarifying the exact mechanisms
by which muscle function is affected. However, this knowledge about potential pathomechanisms
in steroid-driven myopathy could also be beneficial for elder sarcopenic obese patients
as research in these patients is hampered by confounding comorbidities and polypharmacy.
As CS patients are frequently free of comorbidities and as CS is potentially curable,
CS could be a prototypic disease model for further research in sarcopenic obesity.
Pituitary induced high circulating levels of glucocorticoids in CS may directly modulate
bone remodeling and metabolism causing deterioration of the structural integrity of
bones which is associated with a high risk of fractures [52]. Thus, CS is often accompanied by a secondary osteoporosis which leads in combination
with muscle atrophy and obesity even to an osteosarcopenic obesity. Hypercortisolism
affects bone metabolism with various mechanisms. It is known that glucocorticoid excess
inhibits bone formation, reduces the replication of bone forming cells, prevents the
differentiation of osteoblasts, induces apoptosis of osteoblasts by activating caspase
3 and inhibits collagen type I synthesis [54]. Treatment of pituitary hormone excess such as surgery of pituitary adenomas improve
skeletal health in some patients, whereas others seem to remain a high risk of fractures.
As muscle, fat and bone tissue exert an immense cross-talk, it would be interesting
to study patients suffering from CS and associated secondary osteosarcopenic obesity
in more detail to find out more about pathologies leading to geriatric sarcopenia
often accompanied by osteoporosis.
Chronic kidney disease and uremic sarcopenia
Uremic sarcopenia describes prevalently occurring muscle abnormalities developing
in the uremic milieu in patients suffering from chronic kidney disease (CKD). As the
loss of muscle mass in uremic patients is much more intensive and the first signs
of sarcopenia are observed in younger patients than usually expected, the pathogenic
mechanisms involved might give important insights when prototypically modelling sarcopenia
in human patients. However, muscle abnormalities in renal patients receiving dialysis
are usually not defined by a change in muscle physiology, but instead by reduced muscle
force, significant muscle wasting and selective structural changes including type
II fiber atrophy (with a greater fiber atrophy of type IIB compared to type IIA),
small cross-sectional area (CSA), fiber grouping and mitochondrial aberrations with
a decrease in mitochondrial enzyme concentration and activity [56]. The prominent and progressive reduction of muscle mass in uremic sarcopenia appears
to be the answer of a disrupted skeletal muscle homeostasis with a multifactorial
etiology involving hormonal, immunologic and myocellular causes, inflammation, reduction
of protein intake, metabolic acidosis, increased angiotensin II, abnormalities in
insulin/insulin-like growth factor 1 (IGF-1), myostatin expression, satellite cell
inactivation, mechanical changes like physical inactivity and comorbidities [56].
Many of these numerous causes leading to uremic sarcopenia have an increase in muscle
proteolysis and a decrease in muscle protein synthesis in common. The major cause
in CKD muscle wasting is thought to be the activation of the UPS which leads to excessive
protein degradation and especially an increase in the cleavage of the 14 kDa actin
fragment [57]. Both, inflammation and metabolic acidosis strongly trigger the activation of the
ATP-UPS. Low-grade inflammation which is common in CKD is marked by increased circulating
levels of the inflammatory markers CRP, IL-6 and TNF-α. Thus, it was shown, that muscle
mass in dialysis patients inversely correlates with an increase in CRP and IL-6 levels.
However, TNF-α as the major inflammation factor that triggers protein degradation,
directly activates the ubiquitin proteasome and induces muscle wasting through the
activation of the NFκB pathway by increasing the expression of antrogenes (MuRF-1,
atrogin-1 and MAFbx) or by attenuation of the insulin-stimulated protein synthesis
via FOXO [58].
Metabolic acidosis as another major causative pathological mechanism of sarcopenia
development in CKD, occurs mainly through a strong accumulation of the uremic toxin
indoxyl sulfate (IS), which activates on the one hand also the UPS, but also leads
on the other hand to oxidative stress and metabolic changes in muscle cells. IS moreover
induced mitochondrial network disintegration as consequence to metabolic changes such
as upregulation of antioxidative responses (pentose phosphate pathway and glutathione
metabolism) which were related to nuclear factor-2 (Nrf2) activation, and impaired
mitochondrial ATP production due to a downregulation of the tricyclic acid (TCA) cycle,
the glutamine metabolism and the mitochondrial oxidative phosphorylation. The IS induced
mitotoxicity and immense ATP shortage is especially detrimental in skeletal muscle
which are highly metabolic and require vast quantities of mitochondria for ATP production.
However, the association between plasma IS levels and muscle mass was also clinically
examined in CKD patients undergoing peritoneal dialysis and indicates a decrease in
muscle mass when IS levels are elevated [59]. Besides the above hypothesized pathomechanistic pathway induced by increased IS
levels, other studies describe a close relationship between IS accumulation, oxidative
stress, inflammation and muscle atrophy. Thus, accelerated IS causes muscle breakdown
not only through an increase of ROS, but also through an increase of inflammatory
cytokines such as TNF-α, IL-6 and transforming growth factor-β1 (TGF-β1) resulting
in an enhanced expression of the muscle atrophy related genes myostatin and atrogin-1.
The renin-angiotensin-aldosterone–system (RAAS) is highly activated under conditions
of CKD and also seems to provide a strong input to the development of uremic sarcopenia.
Thus, angiotensin II is highly elevated, which leads to a decrease of phospho-actin,
resulting in an increase of caspase-3 which enhances actin cleavage. Not only muscle
proteolysis is elevated, but also apoptosis of muscle cells, both promoting muscle
wasting. Angiotensin II is also responsible for low circulating and skeletal IGF-1
levels which enhances muscle wasting as well [60]. Other chronic conditions which are associated with a dysregulation of RAAS and
muscle dystrophy are congestive heart failure and liver cirrhosis [61]
[62].
Another risk factor for the development of sarcopenia in CKD represents the disturbances
in insulin levels and resistance, which is described to be low in CKD. One pathway
through which insulin is involved in uremic myopathy is the activation of the UPS
through PI3K activation. But PI3K can also activate an intracellular signaling cascade
involving AKT-P, which is involved in protein degradation through FOXO and in decreased
protein synthesis through GSK-1 and mTOR ([Fig. 3]). However, low insulin resistance, which correlates linearly with the decline of
renal function, leads also to a dramatic decrease in the use of glucose as energy
source and an impaired glucose metabolism [63].
Other mechanisms that might influence protein homeostasis are disturbances in appetite
with changes in ghrelin and leptin levels and also the inactivation of satellite cells
which then reduce their MyoD and myogenin expression resulting in a reduced replacement
of injured muscle cells [64].
Uremic muscle wasting is also highly complex and progressive when comparing with sarcopenia.
The pathogenesis of both, geriatric and uremic sarcopenia is similar, which makes
it important to investigate the commonalities in an interdisciplinary approach.
Klinefelter’s syndrome and hypogonadal sarcopenia
Klinefelter’s syndrome (KS) (47XXY) as the most abundant sex chromosome disorder is
characterized by hypogonadism, gynecomastia and azoospermia. It is strongly associated
with a changed body composition, leading to an increase in fat mass and a decrease
in both, muscle mass and bone mineral mass [65]. Affected men show also lower aerobic capacity and a diminished muscle strength
and performance. Furthermore, KS often goes along or even results in T2D, metabolic
syndrome and cardiovascular events [66]. The treatment of choice is the supplementation with testosterone which is the most
widely prescribed medication in the USA so far. As with advancing age hypogonadism
appears in elderly people leading to a decrease of muscle mass and performance, KS
seems to be a good prototypic disease model to study hypogonadism and decreased levels
of testosterone in a young population leading to the induction of muscle wasting and
thus, a “secondary sarcopenia” that we name hypogonadal sarcopenia [67].
Testosterone as the most important anabolic steroid hormone is known to be involved
in a variety of developmental and biological processes including the maintenance of
muscle and bone and the inhibition of adipogenesis. With advancing age, the muscle
mass, which is in the age of 40 years about 60% of total body mass, decreases up to
about 40% when reaching the age of 70 years [67]
[68]. At the same time, bone mineral density decreases and adipose tissue increases leading
to sarcopenia, frailty and obesity. Testosterone levels decrease dramatically with
age and age-induced hypogonadism is not rare in today’s aging population. Concerning
the decline of testosterone levels, Feldman et al. preformed some cross-sectional
and longitudinal studies [69]. They could determine in a first study that total testosterone levels decline 0.8%
per year. Both, free and albumin-bound testosterone decrease 2% per year after the
4th decade of life. In a second study, which was longitudinal, the total testosterone
decline was 1.6% per year and the decline of bioavailable testosterone was 2–3% per
year. In a third study they could show that the testosterone level in general and
the free testosterone index declined progressively from the 3rd to 9th decade of life with a decline for total testosterone levels being 0.11 nmol/L per
year and being 0.0049 nmol testosterone per nmol sexual hormone binding globuline
(SHBG) per year for the free testosterone index. Several studies followed from other
groups leading to the same results that testosterone levels decrease with age [69].
The reason for the negative effects induced by low testosterone levels while aging,
is not only a low androgen production in general, but also an accelerated testosterone
metabolism or malfunctioning androgen receptors. Testosterone seems to build an extrinsic
factor which prevents sarcopenia or will enhance when levels are low. The exact metabolic
function of testosterone in promoting skeletal muscle function remains poorly understood.
It is known or seems to be important, that testosterone increases muscle protein synthesis,
which is important for muscle formation, growth and maintenance. Furthermore, testosterone
plays also a crucial role in muscle regeneration via the activation of satellite cells
[67].
Many studies have been performed to investigate the effects of testosterone, but also
to find out more about the exact role of testosterone in muscle, bone and fat metabolism.
The testosterone replacement therapy (TRT) is widely used [67]. However, the sample sizes of the studies are often small. Furthermore, most of
the studies were performed in the USA and Europe, which does not allow any prediction
about i. e. the Asian population. Results are very contradictory. Several studies
showed an association of frailty and testosterone concentrations, clearly indicating
that testosterone plays an important role in the development of sarcopenia [70]. Muscle mass and muscle strength could be shown to be related with testosterone
concentrations in many studies. Interestingly, testosterone level and physical performance
showed rather inconsistent relationships. Most likely this can be explained by a diversity
of used tests. Physical performance is difficult to measure in the elderly and depends
on many factors in addition to muscle strength. Therefore, testosterone supplementation
was given to young and middle-aged men suffering from hypogonadism and indeed, beneficial
effects could be determined like the increase in weight and muscle mass, as well as
a reduction in fat mass, whereas the side effects were very low [69]. However, the clinical advantages of a TRT in elderly men with age-associated decline
of testosterone remains unclear. Thus, since the risk/benefit ratio of androgen replacement
therapy is unclear, it is suggested to avoid this therapy in elder people until better
outcome data are available. Although testosterone is used since 1940 as therapeutic
drug, the negative side effects especially cardiovascular events are redoubtable.
Therefore, SARMs (selective androgen receptor molecules) were developed which will
be discussed later in the part of managing and treating sarcopenia [71].
Therefore, patients suffering from KS seem to represent a good prototypic disease
model to investigate in both, young (40 years) and very young (20 year) men the tremendous
effects of hypogonadism and to study the outcome of testosterone supplementation on
intracellular signaling and muscle mass as well as muscle performance.
Amyotrophic lateral sclerosis and neurogenic sarcopenia
Amyotrophic lateral sclerosis (ALS) is a rapid progressive neurodegenerative disease
caused by degeneration of brachial and lumbar somatic motor neurons resulting in spinal,
bulbar, respiratory and axial muscle weakness. Furthermore, the neuromuscular junction
degenerates and the skeletal muscle becomes denervated thus causing initial muscles
weakness and following atrophy. Reported age of onset for ALS varies between 40 and
65 years [72]. Mean survival is typically between 3 and 5 years after disease onset. Respiratory
failure occurs during disease progression, which is the main cause of death in ALS
[73]. Today, most cases are considered as a sporadic type (sALS), however there is growing
evidence for ALS associated gene mutations [74]. There is clinical and animal experimental debate that pathological changes might
occur at the neuromuscular junction (NMJ) prior to motor neuron degeneration and onset
of clinical symptoms [75]. This hypothesis proposes that ALS is a distal axonopathy in the early phase of
the disease onset [76]. The “dying-back” or “transsynaptic degeneration” theory has obtained much attention
in recent years in the context of ALS pathophysiology. According to this hypothesis,
motor neurons and nerve terminals show pathological changes prior to motor neuron
degeneration and the onset of clinical symptoms. Interestingly, evidence of distal
axonopathy has been found in other neurodegenerative diseases like Alzheimer’s disease
or Parkinson’s disease, where axonal defects occur prior to cell death and the loss
of axonal function correlates strongly with the onset of functional decline [77]
[78]. These two neurodegenerative mechanisms, loss of motor neurons and degeneration
of NMJs, are also found as causes for sarcopenia. While disease progression in ALS
is rapid and leads through muscular respiratory failure to death, neurogenic sarcopenia
is a chronic condition leading to dysmobility, falls, fractures and loss of independency
at advanced age in the ninth decade.
Irrespectively of the pathomechanisms, both diseases are characterized by a loss of
motor units and muscle fiber atrophy. A motor unit comprises a motor neuron and the
group of muscle fibers it innervates. The number of muscle fibers innervated by a
motor neuron (size of motor unit) varies widely from very small innervation ratios
in the hand and eye muscles (<10 fibers per motor neuron) to very large innervation
ratios in the trunk and proximal limb muscles (>500 fibers per motor neuron). Several
electrophysiological Motor Unit Number Estimation (MUNE) methods for estimating the
number of motor units have been used in ALS as an objective measurement to monitor
disease progression [79]. The MUNE technique for human muscle was described for the first time in 1971 [80]. This technique has been developed further and modified in different ways over the
years. Many MUNE techniques are time-consuming and technically difficult to perform
or invasive with discomfort for the patients. For this reason, Nandedkar et al. introduced
a technique called the Motor Unit Number Index (MUNIX) in 2004 to obtain a parameter
related to the number and size (MUSIX) of motor units (MU) [81]. This technique is non-invasive, and easy and quick to perform. The Motor Unit Number
Index is amethod for assessing the number (MUNIX) and size (Motor Unit Size Index
- MUSIX) of motor units. Investigations have demonstrated the good reliability and
validity of MUNIX values [82]
[83]. Investigations to study the relevance of MUNIX in sarcopenic patients have shown
that MUNIX values of sarcopenic patients lie between healthy subjects and ALS patients
[84]. This finding indicates that loss of motor neurons most probably plays a prominent
pathogenic role in the development of sarcopenia. In another study it was shown that
patients with pathological MUNIX and MUSIX have a significant increased odds ratio
of 3.09 of being sarcopenic [85].
Compared to this neurocentric view of muscle fiber atrophy due to a transsynaptic
degeneration of the motor neuron, the primary degeneration of the NMJ and its dying-back
mechanism constitute a myocentric view of muscle fiber loss. In this context the motor
neuron synthetized proteoglycan agrin, which is secreted in the synaptic cleft and
induces acetylcholine receptor (AChR) clustering in the postsynaptic membrane, is
important for the formation and stability of the NMJ [86]. The postsynaptic induced effect of agrin is mediated through activation of the
muscle specific kinase (MuSK) and low-density lipoprotein receptor-related protein
(Lrp4) in the plasma membrane. Agrin is inactivated by cleavage from neurotrypsin,
a synaptic protease, which frees a soluble 22 kDa C-terminal agrin fragment (CAF)
that can be detected in human serum [87]. Experiments with transgenic mice overexpressing neurotrypsin in spinal motor neurons
have shown the full sarcopenia phenotype, including a reduced number of muscle fibers,
increased heterogeneity of fiber thickness, more centralized nuclei, fiber-type grouping
and an increased proportion of type I fibers. Several studies have shown a correlation
between CAF concentrations and muscle mass and or muscle performance, qualifying CAF
as a marker for sarcopenia [88]. Interestingly, the injection of a soluble neural agrin fragment has considerably
improved muscle pathology caused by the disassembly of the NMJ in mice [89].
The described overlap in the pathogenesis of both diseases makes it obvious to study
ALS, especially in early stages of the disease, in parallel with neurogenic sarcopenia.
This interdisciplinary approach between myologists, neurologists and geriatricians
may lead to new translational diagnostic and therapeutic steps in the treatment of
both diseases.
Potential Biomarkers for Sarcopenia
Techniques
[Table 2] represents an overview of the current knowledge about diagnostic tools and biomarkers
which play a role in the detection of sarcopenia risk, development and process. Some
of these biomarkers are also involved to a greater extent in some of the above described
prototypic disease models for sarcopenia. However, up to now, the clinical gold standard
to diagnose the decline in muscle mass is dual-energy X-ray absorptiometry (DEXA).
For research, computed tomography (CT) and magnetic resonance tomography (MRT) are
also used [90]. Limitations of these techniques are not only, that they are quite expensive and
only available in large clinics and institutions, but also that they are incapable
to detect the risk of developing a muscle atrophy when the muscle is not wasted yet
[91].
Table 2 Overview about diagnostic tools and biomarkers for sarcopenia.
diagnostic tools/biomarkers for sarcopenia
|
additional information
|
diseases
|
techniques:
|
Computed Tomography (CT), Magnetic Resonance Tomography (MRT), Dual-Energy X-ray Absorptiometry
(DEXA
|
measurement of muscle mass
|
all
|
Motor Unit Number Estimation (MUNE), Motor Unit Number Index (MUNIX), Motor Unit Size
Index (MUSIX)
|
elecrophyiological method for motor unit number and size estimations
|
ALS
|
histological aspects:
|
muscle fiber number, fiber grouping, fiber size heterogeneity
|
decrease in atrophying muscle resulting in reduced muscle mass, strength and power
|
ALS; CKD
|
motor neuron degeneration/aonopathy/loss of neuromuscular junction (NMJ)
|
motor neurons and nerve terminals show pathological changes prior to motor neuron
loss
|
ALS
|
type II muscle fiber atrophy
|
cross-sectional area (CSA) of fibers
|
CSA smaller in atrophying muscle
|
ALS; CKD
|
giant and non-functional or old mitochondria often with no outer membrane
|
occur in atrophying muscle
|
CKD
|
blood and/or urinary markers:
|
myostatin
|
inhibitor of muscle growth
|
Diabetes
|
follistatin (FST)
|
myostatin inhibitor
|
Diabetes; CS
|
creatinine
|
urinary creatinine levels are often elevated when muscle mass cheanges
|
CKD
|
C-terminal agrin fragments (CAF)
|
responds to resistance exercise training; involved in motor neuron loss.
|
ALS
|
ciliary neuotrophic factor (CNTF)
|
decreased levels result in lower hand grip strength
|
ALS
|
indoxyl sulfate
|
causes metabolic acidosis and often leads to muscle atrophy
|
CKD
|
angiotensin II
|
increased levels lead to enhanced muscle proteolysis and an increased muscle cell
apoptosis
|
CKD
|
signaling molecules:
|
muscle.specific RING-finger 1 (MURF1=TRIM63), ubiquitin tripartite motif containing
protein 32 (TRIM32) and atrogin 1
|
atrogene
|
CS, Diabetes
|
insulin growth factor-1 (IGF-1)
|
increase food intake and growth hormone
|
CS, CKD
|
inflammatory markers:
|
interleukine-1 (IL-1)
|
increased inflammatory signalsare involved in muscle atrophy
|
CKD
|
interleukine-6 (IL-6), tumor necrosis factor α (TNF-α), C-reactive protein (CRP)
|
increased inflammatory signalsare involved in muscle atrophy
|
Diabetes, CKD
|
tartrate- resistant acid phosphatase 5a (TRACP5a)
|
|
|
hormones:
|
testosterone
|
decline in muscle mass and strength
|
KS
|
insulin
|
insulin resistance often leads to muscle degeneration
|
Diabetes, CKD, CS
|
microRNAs:
|
miR-1, miR-208, miR-486
|
results in decrease of sattelite cell renewal and decreased IGF1-PI3K-Akt signaling
|
CS
|
Legend: There are improving techniques and different biomarkers to determine sarcopenia
risk, development and process. Most of them are under investigation, but are not used
in a standardized diagnosis today. Some of these biomarkers seem to be involved to
a greater extent in some of the previous described model diseases which often lead
to secondary sarcopenia. (Abbreviations: ALS: amyotrophic lateral sclerosis; CKD:
chronic kidney diseases; CS: Cushing’s syndrome.)
For muscle function and strength measurements a variety of procedures exist. For geriatric
patients hand grip strength and gait speed are the gold standards. However, they are
always dependent on motivation and cooperation of patients in all age groups and time
consuming in clinical practice. Diagnostic tools discovered in the research of ALS
pathology which are also used in (neurogenic) sarcopenia diagnosis are the estimation
of the motor unit number and size by MUNIX and MUSIX [81].
Thus, new biomarkers and diagnostic tools, which are cheap and easily available are
tremendously needed. Regarding the knowledge about the sarcopenic pathophysiology
and the pathophysiology of the prototypic disease models of sarcopenia in non-geriatric
patients with diseases associated with “secondary sarcopenia” or muscle wasting, promises
many potential biomarkers. These markers need to be investigated in more detail ([Table 2]).
Histological aspects
In general, all muscle wasting conditions show enormous histological abnormalities
including one or more of the following described effects: reduced muscle fiber number,
fiber type grouping, heterogeneity in fiber size, giant and non-functional mitochondria
as well as old mitochondria without outer membrane. Many of these are seen especially
in the pathology of CKD resulting in uremic sarcopenia. Other histological changes
are motor neuron degeneration, axonopathy and loss of NMJs which is of course one
main aspect in ALS pathology leading to neurogenic sarcopenia ([Table 2]).
Blood and/or urinary markers
In sarcopenic patients numerous proteins and molecules which are dysregulated can
be detected in increased or decreased levels often even in the blood serum or plasma
and in urine samples. Therefore, many are suggested as potential biomarkers and can
be used as diagnostic tools for sarcopenia ([Table 2]). In degenerating muscle, myostatin as the main autocrine inhibitor of muscle growth
is most probably highly upregulated and represents a putative marker for muscle atrophy,
which is highly relevant in diabetic sarcopenia, but also in all other types of “secondary
sarcopenia” [92]. In line, follistatin (FST) as naturally occurring strong inhibitor of myostatin
signaling and thus, being a positive regulator of muscle growth, is also suggested
as a potential biomarker. Urinary creatinine levels are often elevated, when muscle
mass is changed in CKD. Another circulating biomarker is CAF which is increased in
1/3 of sarcopenic patients and represents an evidence for motor neuron loss. It is
decreased in sarcopenia, but responds positively to resistance exercise training [93]. On the other side, factors like ciliary neurotrophic factor (CNTF), which is also
known to play a crucial role in ALS pathology, are upregulated in sarcopenia shown
by a lowered hand grip strength [85]. Moreover, mitochondrial enzymes are suggested as indicators for sarcopenia. Thus,
levels of peroxisome proliferator-activated receptor c coactivator 1α (PGC-1α) decreases
dramatically in sarcopenia [94]. It is important for mitochondrial function and integrity, decreases FOXO and MURF-1
signaling and its decrease results in an increase of TNF-α [95]. The cross-talk of mitochondrial enzymes with cytoplasmic or nuclear molecules is
disturbed and there are also a lot of signaling molecules which can serve as biomarkers
in atrophying muscle conditions. Changed levels are often detected in CKD and uremic
sarcopenia ([Table 2]).
Signaling molecules
Atrogenes like the muscle-specific RING finger 1 (MURF1=TRIM63) and ubiquitin tripartite
motif containing protein 32 (TRIM32) are upregulated as well as atrogin-1 ([Table 2]).
Inflammatory markers
Muscle degradation goes also along with an increase of inflammatory markers and cytokines
such as IL-1, IL-6, TNF-α and CRP. Chronic inflammation and thus a change in inflammatory
factors is often seen in pathologies of endocrine diseases such as CS, diabetes and
CKD ([Table 2]).
Hormones
Also hormonal factors have been postulated to be good biomarkers as they are substantially
involved in sarcopenia development. Testosterone levels decrease about 1% per year
from the age of 35 years. This decline is followed by a reduction in muscle mass and
strength and plays of course the most significant role in hypogonadism [96]. Insulin resistance is also often associated with atrophying muscle and is involved
not only in diabetes, but also in CS and CKD development ([Table 2]).
microRNAs
Besides all these proteins as biomarkers, also molecules like miRNAs demonstrate promising
markers for sarcopenia and muscle degeneration. The decreased levels of miR-1 (often
associated with cortisol excess and thus relevant for CS), miR-208 and miR-486 result
in a decrease of satellite cell renewal and decreased IGF1-PI3K-AKT signaling leading
to muscle degeneration [6] ([Table 2]).
Potential Therapeutics and Future Classification
General
Up to now, no pharmacological treatment for sarcopenia is available, although a lot
of substances are currently in clinical studies ([Table 3]). Today’s recommendation in general treatment against sarcopenia is: (I) resistance
training, which decreases especially the functional decline in lower limb muscles,
(II) together with a high protein diet, in particular leucine-rich protein (whey)
to enhance muscle mass and function, (III) often in combination with vitamin D supplementation
to increase muscle strength [6]
[7] ([Table 3]).
Table 3 Overview about current treatment strategies for sarcopenia.
Therapy/drug
|
Effects
|
Side effects
|
resistance exercise
|
increase muscle mass and power; reversing frailty
|
potential for falls; muscle injuries
|
whey protein
|
increase of muscle mass; synergy with exercise to increase muscle strength and power
|
minimum increased creatine levels
|
vitamin D
|
enhances muscle strength
|
-
|
testosterone
|
increase muscle mass, strength, power and function
|
potential for falls; muscle injuries; minimal increased creatinine levels; fluid retention;
increased hematocrit; short term worsening of sleep apnea; effects on prostate cancer;
possible increase in cardiovascular events
|
SARMs
Nadrolone (steroidal SARM); MK0773 (TFM-4AS-1) (4-aza steroidal SARM); LGD-4033, BMS-564929
(non-steroidal SARM); Enobosarm
|
increases muscle mass, fiber area, small increase in muscle power
|
increased cardiovascular failure
|
ghrelin
Anamorelin, BIM-28125, BIM-28131, macimorelin (agonist); Capromorelin, MK-0677 (receptor
agonist)
|
increases muscle mass, strength and appetite, reduces TNF-α and myostatin levels
|
Fatigue; atrial fibrillation; dyspnea increased heart failure
|
growth hormone
|
increases nitrogen retention; increases muscle mass, but not strength
|
Arthralgia; muscle pain; edema; carpal tunnel syndrome; hyperglycemia
|
myostatin and activin II receptor inhibitors
MYO-029, AMG 745, LY2495655, REGN1033 (myostatin antibody); ACE-011, ACE-083 (ActivinII
receptor ligand trap); Bimagrumab (Activin receptor inhibitor)
|
enhances muscle mass; increases muscle fiber diameter; decreases fat mass; increases
bone mass
|
urticaria; aseptic meningitis; telangiectasia, epistaxis, and changes in gonadotrophin
levels
|
mixed agonist/antagonist B1, B2, B3 activity
Espindolol
|
increases muscle mass, decreases fat mass, increased hand grip strength
|
|
angiotensin converting enzyme inhibitor
Perindopril
|
increases distance walked, improves 6-min walking distance, decreases hip fractures
|
Hypotension; hyperkalemia; muscle cramps, numbness
|
fast skeletal muscle troponin activators
Terasemtiv
|
improves muscle function
|
|
Legend: There are many pharmaceutical targets which were and are still tested in the
treatment of sarcopenia or other muscle wasting diseases.
Testosterone and selective androgen receptor modulators
The first therapeutic agent, that was used against muscle wasting was testosterone
in the early 1940ies [97]. Testosterone levels physiologically decrease about 1–2% per year in both, men and
woman, starting in the mid-thirties [68]. A lot of focus was put on testosterone as it seems to be an all-round hormone in
regard to muscle mass, strength and power. It seems to be important in satellite cell
activation and increased muscle protein synthesis ([Fig. 3]) [6]
[98]. It is even possible to decrease the amount of fat mass by a considerable reduction
of adipose-derived stem cells and to increase bone strength and bone mineral density
[70]. However, a lot of studies using testosterone as treatment of muscle wasting were
carried out and besides all the positive effects, unfortunately also a lot of negative
side effects were occurring such as cardiovascular events which led to a new way of
treatment by using selective androgen receptor modulators (SARMs). One of the first
SARMs used is nadrolone, a steroidal SARM, which was proved to increase muscle fiber
area and thus, muscle mass, but there was no increase in muscle strength [6]. Another steroidal SARM is known as MK0773 which also increases muscle mass, but
the study was earlier terminated because of increased signals of heart failure [6]. Two non-steroidal SARMs are LGD-033 and BMS-564929. Although a further SARM called
enobosarm could maintain body mass and increase stair climb power in patients with
cancer, SARMs have shown no advantage over testosterone why the way in developing
a treatment against muscle wasting went back to testosterone and its new trials [99] ([Table 3]).
Ghrelin and growth hormone
Ghrelin, ghrelin agonists (anamorelin, BIM-28125, BIM-28131 and macimorelin) or ghrelin
receptor agonists (capromorelin and MK-0677) were shown to be beneficial. Ghrelin
is important for an increase in food intake, increased levels of growth hormone (GH)
and an increase in muscle mass. However, an increase in muscle strength was missing.
Using directly GH was another attempt to treat muscle wasting conditions. Although
GH treatment showed in old men a significant increase in lean body mass, there were
many associated side effects such as arthralgia, muscle pain, edema, carpal tunnel
syndrome and hyperglycemia [6] ([Table 3]).
Myostatin and activin II receptor inhibitors
Myostatin – the main inhibitor of muscle growth and satellite cell production. As
myostatin activates the activin II receptor causing a signaling cascade through SMAD
molecules, this group of intervention is known as “myostatin and activing II receptor
inhibitors”. Antibodies against myostatin were tested in the treatment of muscle wasting
conditions and these are MYO-029, AMG 745, LY295655 and REGN1033, whereas the activing
II receptor inhibitors are ACE-011, ACE-031, ACE-083 and bimagrumab. Positive effects
were obtained like enhancement of muscle mass, increased muscle fiber diameter, decreased
fat mass, positive effects on muscle strength, but unfortunately also a lot of negative
side effects were occurring which finally lead to a determination of using these compounds
[6] ([Table 3]).
Mixed agonist/antagonist B1, B2, B3 activity, ACE inhibitors and fast skeletal muscle
troponin activators
Espindolol, is a non-specific mix of B1/B2 adrenergic receptor blocker that reduces
catabolism and at the same time increases anabolism. It increases muscle mass and
reduces fat mass. Perindopril is an angiotensin converting enzyme (ACE) inhibitor
and seems to improve especially osteosarcopenic characteristics, whereas Terasemtiv
is a fast skeletal muscle troponin activator which seems to improve muscle power [6].