Keywords
ABC transporters - Alzheimer's diseases - amyloid-β - P-glycoprotein - multidrug resistance
protein 1
Introduction
Central nervous system (CNS) barriers, consisting of the blood–brain barrier (BBB)
and blood–cerebrospinal fluid barrier, are responsible for the protection of the microenvironment,
which is vital in the regulation of neuronal functions.[1] There is increasing evidence that abnormal expression of efflux transporters (ATP-binding
cassette [ABC]) in the BBB, a semipermeable barrier composed of endothelial cells,
causes CNS-related diseases such as Parkinson's disease and Alzheimer's disease (AD).
In addition, the dysfunctions that occur in these transporters cause the disruption
of the integrity of the BBB, the deterioration of the CNS balance, and consequently,
the exacerbation of CNS diseases, including AD.[1]
The treatment of millions of patients suffering from AD associated with microvascular
dysfunction and/or degeneration is still a mystery despite years of research efforts.
Therefore, there is a great need to identify new therapies that target the underlying
causes of AD, prevent or eliminate existing symptoms.[2] A dense accumulation of amyloid-β (Aβ), that is, a peptide resulting from amyloid
precursor protein (APP) processing, and deposition of hyperphosphorylated tau protein
appear in the CNS as AD pathology.[3]
[4] AD is also closely associated with microvascular pathologies, such as degeneration
and/or dysfunction in the microvascular structure, which are key places in the exchange
of nutrients in the brain between the brain and circulating blood. These pathologies
cause Aβ accumulation by disrupting the integrity of the BBB and preventing the clearance
of neurotoxic molecules (such as Aβ) in the CNS.[1] Microtubule-associated tau proteins and the Aβ40, Aβ42, the most common form of Aβ produced by cleavage of APP, represent the molecular-level
character of AD. Although studies conducted in the brains of AD patients have determined
100 times more Aβ plaque formation compared with controls, the physiopathological
effect of AD is not fully elucidated. Hyperphosphorylation of tau proteins, another
molecular marker determined in patients with AD, causes microtubular collapse.[5] The accumulation of these molecules in the brain increases as the neurotoxic molecules,
which accumulate due to the deterioration of the BBB balance, are not cleaned in the
CNS. One of the families of transporter that can mediate Aβ homeostasis and play key
roles in AD pathophysiology is ABC transporters.
Enabling the movement of their substrates through intracellular organelles and cell
membranes, ABC transporters, which provide the movement of its substrates from intracellular
organelles and cell membranes, ensure the homeostasis of the body.[6] Recently, the emphasis on the role of ABC carriers in CNS disorders associated with
high levels of Aβ, such as AD, has increased research into the causes of changes or
dysfunction in the processes and pathways in which the carrier family is involved.
The contribution of ABC carriers to AD physiopathology is not fully known, and studies
on this subject will contribute to the resolution of the relationship between AD process
and ABC carriers and accordingly to the development of new treatment strategies.[6]
[7]
AD is characterized by three main changes occurring in the CNS: (1) Aβ plaque accumulation
that prevents synaptic communication, (2) the accumulation of hyperphosphorylated
tau proteins that inhibit the transport of molecules inside neurons, and (3) neuronal
cell loss of the limbic system.[5] The basis of AD is explained by two main theories: (1) the “Aβ cascade hypothesis”
emphasizing that it develops with the accumulation of oligomeric Aβ species in the
CNS and (2) the “neurovascular hypothesis” that tries to determine the accumulation
of Aβ species. According to the neurovascular hypothesis, among the causes of Aβ accumulation
in the CNS, the absence or reduction of Aβ degradation, reduction of Aβ clearance,
or increase of Aβ influx along the BBB is indicated.[8] Mechanisms leading to Aβ accumulation in AD are excessive Aβ production as a result
of mutations in APP or genes, and impairment of clearance of Aβ due to changes in
Aβ aggregation properties and/or Aβ removal processes. ABC transporters are responsible
for AD physiopathology by preventing direct or indirect Aβ accumulation in the BBB
by active transport.[5]
In our review, we aimed to evaluate the roles and physiopathological features of human
ABC carrier members expressed in the CNS and associated with AD pathogenesis.
ABC Transporters
Representing the largest family of integral proteins, ABC genes use ATP energy to transport various molecules across all cell membranes.[9] ABC proteins are classified according to the sequence and organization of ATP-binding
domains. Usually, unidirectional ATP pumps move hydrophobic compounds from the cytoplasm
to extracellular or intracellular organelles such as the endoplasmic reticulum and
mitochondria.[10] ABC transporters transfer metabolic wastes to the cytoplasm and from the cytoplasm
to the blood. ABC transporters also prevent xenobiotics from entering the brain.[1]
Genes are divided into subfamilies based on the order of domains similarity in their
structure in the nucleotide-binding folds and transmembrane domains. ABC transporters
are divided into seven subfamilies and it is known that ∼50 ABC transporters (ABCA1–13,
ABCB1–11, ABCC1–6, CFTR, ABCC8–12, ABCD1–4, ABCE1, ABCF1–3, ABCG1,2,4,5,8) are expressed
in the human genome.[10]
[11] Human ABC transporter proteins are localized in many tissues such as brain, liver,
intestine, etc.[12]
[13] Their presence in many tissues and their involvement in many physiological and biochemical
processes are an indication of the importance of ABC transporters. Abnormalities or
dysfunctions that will occur in these transporters emphasize their relationship with
a wide range of diseases, from cystic fibrosis to cancer, especially neurodegenerative
diseases.[14]
ABC transporters similarly function as a second selective permeability barrier in
neurons and glial cells. Because of all of these properties of ABC transporters, they
have essential detoxification and neuroprotective roles in the brain.[15]
[16] Changes in ABC carrier expressions or functions lead to β clearance disturbances,
causing AD-associated cerebral amyloid angiopathy (CAA).[17]
[18]
[19]
[20]
ABCA Subfamily
The largest ABC gene subfamily is ABCA which consists of a lot of amino acids.[21] ABCA full transporter is divided into two groups: ABCA1 like (ABCA1–4,7,12,13) and
ABCA5 like (ABCA5,6,8–10).[11]
[22] ABCA1 and ABCA2 proteins, which play a role in apolipoprotein-dependent cholesterol
efflux, sterol homeostasis, lipid transport, and metabolism, share ∼50% sequence homology.[23]
[24]
ABCA1: This transporter is highly expressed in brain cells ([Table 1]). The fact that ABCA1 mediates the flow of cholesterol from astrocytes and microglia
in the CNS facilitates lipidation of ApoE.[25] Cholesterol level constitutes an important component of AD pathogenesis because
it regulates the properties of the membrane where enzymes necessary for Aβ production
are localized. ApoE, which is an important factor for brain cholesterol homeostasis,
provides development, repair, and nerve growth to brain cells. The association of
altered cholesterol metabolism in the brain with increased AD is indicative of the
critical role that ApoE has in AD. Individuals carrying the ApoE-ε4 alleles, one of
the ApoE isoforms, have a greater risk of developing AD because these alleles directly
interact with Aβ and promote Aβ aggregation and plaque formation.[26]
[27] In the AD mouse model overexpressing J20-human APP (hAPP), ABCA1 deficiency was
shown to cause reduced lipid flux and a dramatic decrease in brain ApoE levels. With
studies in mice overexpressing hAPP, it is shown that ABCA1 deficiency increases Aβ40:Aβ42 ratio.[28] A decrease in ABCA1 causes greater accumulation of amyloid, increasing ABCA1 protein
or function decreases amyloid accumulation.[4] In the study by Kim et al (2013), it was found that neuronal ABCA1 mRNA and protein
levels in the hippocampus of the brains of AD patients were not affected and were
significantly upregulated compared with the cerebellum.[4] The authors thought that this upregulation was related to the pathological process
of AD. Similar to these findings, it was determined that hippocampal ABCA1 expression
was upregulated in an age-dependent manner in the experimental results performed in
APP/PS1 mice. It has been reported that cholesterol efflux decreases in astrocytes
and microglia in mice with ABCA1 deficiency, resulting in cellular accumulation of
cholesterol and decreased ApoE lipidation. Moreover, it has been reported that a decrease
in lipidated ApoE level decreases amyloid-proteolytic degradation that increases the
risk of AD.[25]
Table 1
ABC transporters expression in human brain cell and their roles in AD
|
ABC subtype
|
Expression
|
Role
|
|
ABCA1
|
BEC, CP, CPEC, VEC, astrocyte, microglia, oligodendrocyte, neuron
|
ABCA1 loss, Aβ accumulation in endothelial cells
Increased ABCA1 function in astrocytes reduces Aβ accumulation
Decreases the influx of Aβ across the BBB
|
|
ABCA2
|
BEC, astrocyte, microglia, oligodendrocyte, neuron
|
ABCA2 increase APP and Aβ levels
|
|
ABCA5
|
BEC, astrocyte, microglia, oligodendrocyte, neuron
|
Reduce the formation of Aβ40 and Aβ42
|
|
ABCA7
|
BEC, astrocyte, microglia, oligodendrocyte, neuron
|
Inhibits Aβ production
plays a role in APP processing
leads to enhanced Aβ secretion
|
|
ABCB1/P-gp
|
BEC, CP, CPEC, VEC, astrocyte, microglia, pericyte, oligodendrocyte, neuron
|
ABCB1 is downregulated in brain endothelial cells in AD
Transport the Aβ and also plays a role in clearance the Aβ from the brain
|
|
ABCC1
|
BEC, CP, CPEC, VEC, astrocyte, microglia, pericyte, neuron
|
Decrease Aβ levels
Lack of ABCC1 increase Aβ40 and Aβ42 levels
|
|
ABCG1
|
BEC, CP, CPEC, astrocyte, microglia, oligodendrocyte, neuron,
|
Regulate cellular homeostasis and not appear to have direct role AD
|
|
ABCG2/BCRP
|
BEC, astrocyte, microglia, neuron, pericyte
|
Prevents amyloid accumulation and resting the passage of circulating Aβ
|
|
ABCG4
|
BEC, CP, CPEC, VEC, astrocyte, microglia, neuron
|
Response to clearance Aβ
Suppresses ABCG4 increases Aβ secretion
|
Abbreviations: Aβ, amyloid-β; ABC, ATP-binding cassette; AD, Alzheimer's disease;
APP, amyloid precursor protein; BBB, blood–brain barrier; BEC, brain capillary endothelial
cell; CP, choroid plexus; CPEC, choroid plexus epithelial cell; VEC, ventricular ependymal
cell.
Note: Pericytes, an essential component of neurovascular unit, also known as BBB gatekeepers,
carry nutrients and waste molecules between blood and brain interstitial fluid.
ABCA2: ABCA2 is mostly expressed in the hippocampus and especially white matter ([Table 1]).[29]
[30]
[31] The functional studies of ABCA2 show that ABCA2 plays a role in neural transmembrane
lipid transport and metabolism.[10] Decreasing the expression of ABCA2 leads to an increase in levels of low-density
lipoprotein. In a study comparing ABCA2 expression levels in different brain regions
of patients with AD, the highest level of ABCA2 was found in the temporal and frontal
regions.[32] These study results emphasize the importance of precisely defining the positive
correlation between Aβ and ABCA2 in the pathogenesis of AD. In another study consistent
with these findings, it was reported that ABCA2 expression increased in the temporal
region in AD patients, and it was determined that the excessive increase in this protein
expression was affected by Aβ clearances. Further, unlike ABCA1, it is reported that
ABCA2 downregulation by siRNA decreases Aβ production.[33] The literature findings suggest downregulating ABCA2 to reduce Aβ production as
a therapeutic approach to AD therapy. Therefore, it is important to conduct studies
testing potential therapeutics in the treatment of AD of downregulating ABCA2.
ABCA5: ABCA5 is expressed in many brain regions affected by AD, such as the hippocampus
([Table1]). In the study conducted by Fu et al (2015), they determined that ABCA5 expression
was significantly higher in the hippocampus of individuals with AD compared with the
control groups.[34] In the same study, they determined that the decrease in Aβ level in cells transfected
with ABCA5 was due to the change in APP processing (not APP expression).[34]
ABCA7: ABCA7 has mainly expressed in brain microglial cells ([Table 1]). There is evidence that ABCA7, showing 54% homology with ABCA1, may be involved
in AD pathology through Aβ accumulation and lipid metabolism.[35] Studies have reported that ABCA7 genes are strongly linked to the immune response, cholesterol metabolism, and amyloid
hypothesis affected in AD.[36]
[37] Many investigators suggest that the relationship between ABCA7 and AD pathology
is not direct but through lipid metabolism or immune response. There are different
opinions regarding the role of ABCA7 in Aβ homeostasis, such as that ABCA7 stimulates
the efflux of cholesterol and inhibits Aβ production, and ABCA7 deletion increases
the levels of Aβ by changing the clearance of amyloid plaques. ABCA7 has a role in
phagocytosis of cytotoxic or antigenic molecules released from cells during apoptosis
to maintain tissue homeostasis.[4] Although the underlying mechanisms have not been fully elucidated, it can be predicted
that failure to clear phagocytes in apoptotic cells may cause inflammation and thus
AD.[38] Studies on the functional role of ABCA7 in AD development show that ABCA7 modulates
APP processing. The protective effect of ABCA7 in AD is explained by the fact that
ABCA7 expression inhibits Aβ production in cells coexpressing APP, while its suppression
causes the opposite effect[39]
[40] ([Table 1]). The further cognitive decline that occurs in the cognitive functions of AD patients
is explained by some researchers, by the high levels of ABCA7 and by the regulatory
function of ABCA7 in phagocyte function. This demonstrates the importance of protein
functionality as well as protein aggregation to prevent the progression of AD.[40]
[41]
ABCB Subfamily
The ABCB subfamily contains both full transporter (four) and semitransporter (seven).
ABCB full and half transporters are divided into two groups: ABCB1,4,5,11 and ABCB2,3,6–10,
respectively.[10]
ABCB1 (P-glycoprotein, P-gp): According to other ABCB family members, ABCB1 is the most studied carrier in CNS.
The ABCB1 gene, which is also expressed in cells in the BBB and identified in neuronal cells,
is thought to play a role in the transport of compounds that cannot be transmitted
by diffusion to the brain ([Table 1]). It is thought to serve as the general defense mechanism protecting from the poisoning
of potentially harmful lipophilic compounds that could penetrate the carrier BBB.
In studies, it is argued that depending on the expression, localization, potency,
and multispecificity of this transporter subtype act as an important barrier to the
xenobiotics' entrance to the brain.[42] Studies report that ABCB1 is required for normal clearance of Aβ.[43] It is reported that the accumulation of CAA and Aβ increases the risk of developing
AD associated with impaired ABCB1 activity. ABCB1, the efflux transporter that prevents
various compounds from entering the brain and pumps metabolic waste products from
the brain, plays a crucial role in neuroprotection and general CNS homeostasis.[6] Experimental results in Mdrla/b−/− double knockout mice determined that the brain clearance of Aβ40 and Aβ42 was significantly lower 30 minutes after intracerebral microinjection of Aβ compared
with control animals. Studies report an inverse correlation between Aβ accumulation
in the human BBB and vascular P-gp expression in AD. Postmortem study results also
report that several cortical brain regions of AD patients have decreased ABCB1 function
in the BBB.[44] Consistent with human studies, study findings in mice also show that P-gp expression
and function in the BBB is significantly reduced in mice with high Aβ brain levels.
In 2005, impaired Aβ clearance was determined in control mice after treatment with
ABCB1 inhibitors in ABCB1 knockout mice. Thus, the pathological link between ABCB1
and AD in humans enabled the age-related decline of ABCB1 expression to be identified
as ABCB1 as an important Aβ exporter.[7]
[25] The literature findings highlight the importance of new therapeutic strategies aimed
at restoring the BBB ABCB1 expression and function enhancing Aβ clearance and lower
Aβ brain levels in AD.[7]
[45] It is thought that Aβ also plays a role in the intracerebral/intracellular distribution.[46]
[47] Studies of AD mouse models overexpressing APP lacking ABCB1 show that breast cancer
resistance protein (BCRP) function partially compensates for ABCB1-mediated Aβ clearance.
It is possible to say that a small decrease in ABCB1 expression and/or activity in
the brain plays an important role in causing Aβ deposits due to a decrease in Aβ clearance.
Given the correlation between ABCB1 and Aβ, increased clearance of Aβ through ABCB1
upregulation is important for therapeutic approaches developed to slow or stop the
progression of AD. In a study of tissue samples taken from the medial temporal lobe,
it was reported that ABCB1 expression was inversely proportional to the accumulation
of Aβ40 and Aβ42.[48] In the study in which wild type and mdr1a−/b− knockout mice were injected with 125I-Aβ40 and 125I-Aβ42, it was shown that P-gp null animals had significantly higher Aβ40 and Aβ42 levels in their brains compared with control animals.[44] The study using (R)-11C verapamil and positron emission tomography in 2012 is the first evidence to demonstrate
the direct relationship between AD pathogenesis and decreased ABCB1 function. In the
same year, the correlation between decreased ABCB1 due to aging was revealed by this
research group.[49] In the study using C57BL/6 mice, it was shown that mice exposed to rifampicin and
caffeine increased ABCB1 regulation in the BBB (2.7-fold and 1.5-fold, respectively)
compared with the control group and the upregulation was correlated with BBB increasing
Aβ40 clearance.[15] Another evidence that a deficiency in the ABCB1 transporter can lead to an extra
accumulation of Aβ in the brain of mice is provided by the study of Wang et al (2006).[50]
Loperamide, an opioid with analgesic effect and not crossing the BBB and an ABCB1
substrate, shows a significant antinociception effect when administered to mice with
the ABCB1 inhibitor cyclosporine A. ABCB1 has a fairly broad spectrum of compound
substrates, from small molecules such as morphine to peptides such as Aβ.[12] Studies showing the physiological importance of ABCB1 in protecting the brain and
the difficulties in delivering therapeutic drugs to the CNS, it was demonstrated that
ABCB1 substrates such as ivermectin and drugs that do not normally penetrate the brain
cause a 5- to 50-fold increase in the brain to plasma levels of drugs in the mdr1
knockout mice without ABCB1. The study using APP/PS1 mice showed that it was determined
that the ABCB1 expression of AD mice was downregulated compared with the control group.
In the results of the study using huperzine A, which is extracted from Huperzia serrata and which is a powerful acetylcholinesterase inhibitor, it has been reported that
ABCB1 is a substrate and exhibits neuroprotective properties when used in the treatment
of AD by targeting nicotinic and muscarinic receptors. Huperzine increases the brain-to-plasma
concentration ratio in ABCB1-deficient mice and that the increase in this distribution
may be mediated by ABCB1.[51]
ABCC Subfamily
This subfamily includes 12 full transporters with export activity and one pseudogene
is divided into two groups: ABCA1–6,10–12 and ABCA13, respectively.[10]
[48]
ABCC1 (multidrug resistance protein 1, MRP1): There is some evidence in the literature regarding the protective physiological
role of ABCC1 from AD. ABCC1 efflux is important in the removal of cerebral Aβ through
the BBB. It has been shown that Abcc1 deficiency elevates Aβ40 and Aβ42 levels in the brains of APP/PS1 mice. In one study, increase in ABCC1 export activity
in BBB was associated with a decrease in Aβ accumulation in AD-induced mice. In another
study supporting the role of ABCC1 in Aβ clearance, it was reported that treatment
with the thiethylperazine, an ABCC1 agonist, significantly reduced cerebral Aβ accumulation.[52] ABCC1 protein expressed in brain cells ([Table 1]).[53]
[54] In the study performed in APP/PS1 × ABCB1−/− mice, finding a decrease in Aβ brain levels, it was suggested that Mrp1 activation
was the cause of decreased Aβ brain levels.[55] The most effective ABC transporter that affects Aβ brain load is ABCC1. Up to 14-fold
increase in Aβ42 was detected in ABCC1 knockout mice. There are also studies reporting that ABCC1
can reduce amyloid load in APPPS1 mice by up to 80% through functional activation.[52]
ABCC4: In the literature, there are no studies reporting ABCC4 transport function in brain
capillary endothelium or capillary endothelium cells, but it is reported that ABCC4
mRNA is expressed in the choroid plexus.[55] ABCC4 has been reported to mediate the efflux of organic anions, glutathione-, sulfate-,
or glucuronate-conjugated drugs. Although the importance of ABCC4 in drug transport
in the BBB is not exactly clear, it is thought to contribute to the BBB function by
protecting the brain from xenobiotics.[56]
ABCG Subfamily
The ABCG subfamily consists of six half-carriers with a transmembrane domain. The
subfamily consists of five half transporters members (ABCG1,2,4,5,8).[11]
[22]
ABCG1: ABCG1 that responsible for regulating cellular lipid homeostasis is expressed in
brain cells ([Table 1]). There are a limited number of studies reporting different views on the role of
ABCG1 in AD. Coexpression of ABCG1 and APP has been shown to increase secreted levels
of Aβ, indicating that ABCG1-mediated regulation of APP traffic supports the potential
development of AD.[57] Other researchers have reported that ABCG1 significantly reduced Aβ production in
cells expressing APP, suggesting that ABCG1 inhibits AD pathogenesis.
ABCG2 (BCRP): ABCG2 transporters are mostly expressed in the lumen membrane of BBB cells and pericytes,
microglia, astrocytes, and neural progenitor ([Table 1]).[58] Studies examining the function of ABCG2 in AD and Aβ transport report different
results. Samples from AD patients showed no changes in ABCG2 protein expression in
the hippocampus and another study reported that ABCG2 was not involved in Aβ42 transport of ABCG2 in hAPP mice,[59] in contrast to these studies, the expression of ABCG2 mRNA and protein levels compared
with control, it has been reported to increase significantly in AD/CAA patients.[60] In a study designed to determine the action of ABCG2 in AD, greater Aβ accumulation
was found of ABCG2 knockout animals injected with Aβ than in control mice. This situation
was explained by the researchers as ABCG2 may play a role in obstructing Aβ. Given
that ABCB1 expression is downregulated while ABCG2 expression is increased in AD brains,
studies investigating both transporters in the same brain regions emphasize that carriers
are expressed simultaneously. ABCG2 interacts directly with Aβ and has been found
to promote the cellular flow of Aβ40 and Aβ42 in the BBB.[60]
[61] Although studies in mouse and human AD brains have reported that upregulation of
BCRP is effective in reducing Aβ accumulation,[62]
[63] there are also studies reporting that there is no difference between BCRP levels
in healthy and AD individuals.[59]
ABCG4: ABCG4 protein is expressed in brain cell such as astrocyte and microglia ([Table 1]). Studies have reported high expression level of ABCG4 in microglial cells located
close to senile plaques in the AD brain. A study was determined that ABCG4 was able
to export Aβ in ABCB1/ABCG2-deficient mice and its expression in the brain increased.[63] In another study, it has also been shown that ABCG4 expression in microglial cells
is significantly upregulated in AD patients. In the study of AD mouse models, it has
been reported that ABCG4 activity to eliminate amyloid deposits contributes to Aβ
degradation through microglia-mediated phagocytosis and even completes the proposed
role for ABCA7.[63]
Conclusion
Although the interaction of many mechanisms in neurodegenerative diseases creates
many risk factors, our knowledge about all of these interaction mechanisms or the
results of these interactions is still not sufficient. It is almost impossible to
investigate possible combinations of mechanisms in humans by trial and error. It is
still unclear whether the change in expression and function of ABC members is a lead
or an outcome of AD, but it is clear that they play a significant role in AD physiopathology.
There is a need for studies that better describe the changes in expression, function
and transport functions and mechanisms of ABC transporters. Results from the literature
support the regulation of ABC members as curative to ameliorate or prevent the progression
of AD. It has a major pharmacological importance to develop drugs targeting carriers,
especially because of the strong evidence that ABCB1 and ABCA1 carriers have in AD
pathology.
More studies are needed to elucidate the effect of dysfunction or genetic variation
that occurs in ABC carriers that are directly or indirectly in AD pathology, on the
production of neurotoxic molecules, especially Aβ. Few studies in the literature have
focused on changes in AD and Aβ carrier expression. The elaboration of ABC transporter
functions with molecular studies is important in the development of treatment strategies
targeting ABC carriers in neurodegenerative diseases such as AD.
