The physiological function of neural stem cells

Stem cells play an important role in tissue formation, maintenance, and disease. However, much of what we know about many stem cells is based upon their behavior in culture. To better understand the role of stem cells in tissue development/maintenance or in disease, it will be important to characterize progenitor function both in vivo and in vitro. To this end, I have examined the physiological significance of central and peripheral nervous system (CNS and PNS) stem cells in various contexts.;Neural crest stem cells (NCSCs) are self-renewing, multipotent progenitors that give rise to all lineages of the PNS. Although NCSCs persist in peripheral nerves throughout late gestation, their physiological function was unknown. Cre-recombinase fate mapping revealed that within peripheral nerves, endoneurial fibroblasts, in addition to myelinating and non-myelinating Schwann cells, are neural crest-derived, whereas perineurial cells, pericytes, and endothelial cells are not. The multilineage differentiation of nerve NCSCs into glial and non-glial derivatives in developing nerves appeared to be regulated by Neuregulin, Notch ligands, and Bone Morphogenic Proteins. These factors are expressed in developing nerves, and in culture cause nerve NCSCs to generate Schwann cells and fibroblasts, but not neurons. Since neurofibromas (tumors of the PNS) contain mainly glia and fibroblasts, these findings raised the question of whether NCSCs can be transformed by neurofibromin deficiency.;Neurofibromatosis Type I is one of the most prevalent disorders of the nervous system, and is caused by mutations in neurofibromin 1 (NF1), a gene encoding a RasGap protein. This disease is characterized by the development of neurofibromas, which contain most PNS cell types including glia, neurons, and fibroblasts. Since these cell types arise from multipotent NCSCs, we analyzed the function of NF1 in NCSCs isolated from all regions of the developing PNS. At E13, NF1 deficiency increased the frequency, colony size, and self-renewal potential of NCSCs in regions of the PNS where neurofibromas commonly occur. In contrast, NF1 deficiency had little effect on NCSCs from regions where neurofibromas rarely occur, despite equal levels of NF1 expression and Ras pathway activation in all regions. Consistent with the role of NF1 as a negative Ras regulator, Ras pathway inhibition also regionally affected NCSC function. The mechanism by which NF1 and the Ras pathway regionally regulate NCSC function remains to be determined.;At later time points in development, NF1 deficiency did not increase the persistence or frequency of NCSCs, arguing against a NCSC origin for neurofibromas that arise postnatally. However, it remains possible that NF1-/- cells may de-differentiate postnatally to acquire stem cell characteristics in the presence of additional mutations. To test whether NF1-related tumors contained stem cells in the presence of additional mutations, we generated NF1+/- mice on a p16Ink4ap19Arf-deficient background, since p16Ink4a and p19 Arf are commonly mutated in human malignant peripheral nerve sheath tumors (MPNSTs). These mice developed MPNSTs that contained self-renewing, multipotent progenitors. These findings may improve our understanding of NF1-related tumor development, perhaps leading to new therapies.;CNS stem cells persist throughout the lifespan and neurogenesis continues in the adult mammalian brain, though neurogenesis and neural progenitor function in the olfactory bulb and hippocampus are known to decline during mammalian aging. It has been proposed that this is partially caused by the senescence of progenitors with age, but whether known senescence-associated genes are responsible remains uncertain. We studied the age-related decline in forebrain neurogenesis and progenitor function in aging wild-type and p16 Ink4a-deficient mice. Aging p16Ink4a-deficient mice showed a significantly smaller decline in subventricular zone (SVZ) progenitor function and olfactory bulb neurogenesis than did their wildtype counterparts. Declining SVZ progenitor function and olfactory bulb neurogenesis during aging are thus caused partly by increasing p16Ink4a expression.;NCSCs also persist throughout adult life, at least in the enteric nervous system, raising the question of whether neurogenesis continues throughout life in PNS. We were unable to detect neurogenesis in the adult PNS under most normal, diseased, and injured physiological conditions. Thus, the physiological role of adult gut NCSCs remains uncertain.