What is the best immunocytochemical marker for endogenous stem cells?

Endogenous populations of stem cells are proving to be highly relevant to the treatment of degenerative disease and to the initiation of cancer. It is crucial, therefore, to know how many of these cells exist in tissues and how their numbers are manipulated by disease and treatments. Markers for stem cells are constantly being discovered, and many are proving of great use. However, different markers do not always recognize the exact same populations of cells. Therefore, the confusion of the incidence of endogenous stem cells persists. The discrepancy between the recognition potential of different markers can be allocated, in part, to the bias in the experimental strategy to discover them. For example, the assumption that endogenous stem cells are dividing will lead to the discovery of markers of division, and may be adequate for the identification of stem cells in an actively proliferating state, and therefore in certain conditions (e.g. injury) or in certain locations (e.g. subventricular zone in the case of the adult brain). In contrast, the study of specific survival pathways operating in stem cells may lead to the discovery of markers that recognize both the proliferating and the quiescent stem cell sub-populations. I will discuss the advantages and disadvantages of these different strategies as well as the potential to use them in tandem to extract information from the tissue. I will also expand upon one strategy that was based on elucidating distinct signaling pathways in stem cells and subsequent establishment of a novel marker of neural stem cells: We have elucidated a novel signal transduction pathway that regulates stem cell numbers in vitro and in vivo. At the heart of this pathway lies a specific phosphorylation on the serine residue of STAT3. This modification is downstream of several cell surface receptors including Notch (through a non-canonical branch), Tie2, insulin, and FGF. Downstream of STAT3-serine phosphorylation is the transcription factor Hes3 and sonic hedgehog. Of particular value to in vivo interventions is that, unlike neural stem cells, many cell types do not rely on STAT3-serine phosporylation. This pathway, therefore, is of particular importance to immature cells, providing specificity when performing manipulations in vivo. Using Hes3 immuno-reactivity as a novel marker of neural stem cells, we have shown that the adult central nervous system contains a widespread multipotent precursor, suggesting relevance of this approach to diseases that affect many and multiple areas in the brain and spinal cord. Pharmacological activation induces many fold increases in the numbers of Hes3+ cells as well as rescue of neurons and motor skill recovery in models of neurodegenerative disease. The combined use of pro- and anti-angiogenic factors that have the same activating effect on neural stem cells allows for maximal stem cell stimulation with minimal vascular side-effects.