Expansion of CNS Precursor Pools
1999; Cell Press; Volume: 22; Issue: 1 Linguagem: Inglês
10.1016/s0896-6273(00)80668-6
ISSN1097-4199
Autores Tópico(s)Developmental Biology and Gene Regulation
ResumoThe circuitry of the cerebellar cortex is remarkably simple, with just two principal neurons, the Purkinje cell and the granule cell. The high ratio of granule cells to Purkinje cells, ∼200:1 in the mouse, provides a finely controlled neural system for motor coordination and balance. In this issue of Neuron, Wechsler-Reya and Scott (1999) present evidence that Sonic Hedgehog (Shh), produced by Purkinje cells, is the mitogen that controls granule cell number. The granule neuron arises through an unusual developmental scheme. Born along the dorsal aspect of the epithelium lining the fourth ventricle in a zone called the rhombic lip, granule cell precursors move away from the classical germinal zone onto the surface of the developing cerebellar anlage and form a displaced germinal zone (8Miale I. Sidman R.L. Exp. Neurol. 1961; 4: 277-296Crossref PubMed Scopus (777) Google Scholar). By birth, this external germinal zone (EGL) covers the roof of the cerebellar anlage. In the early postnatal period, a massive expansion of the pool of EGL precursor cells occurs, generating some 108 mature granule neurons in the cerebellar cortex. Several classes of mechanisms, each involving cell–cell interactions, have previously been proposed to regulate the number of granule cells. 3H-thymidine incorporation studies on purified EGL cells revealed robust proliferation in the absence of mitogens, suggesting an autocrine signal among granule precursor cells (3Gao W. Heintz N. Hatten M.E. Neuron. 1991; 6: 705-715Abstract Full Text PDF PubMed Scopus (229) Google Scholar). A role for Purkinje cell–granule cell interactions was suggested by cell ablation studies (9Smeyne R. Chu T. Lewin A. Bian F. Crisman S. Kunsch C. Lira S. Oberdick J. Mol. Cell. Neurosci. 1995; 6: 230-251Crossref PubMed Scopus (172) Google Scholar) and by analyses of mutant mice, including a number of Purkinje cell degeneration mutations (1Caviness V.S.J. Rakic P. Annu. Rev. Neurosci. 1978; 1: 297-326Crossref PubMed Scopus (501) Google Scholar). Previously, a role for Shh in regulating granule cell growth and development was suggested by studies of patched (ptc). Ptc is a component of the receptor complex that transduces Shh signaling (7Ingham P. Taylor A. Nakano Y. Nature. 1991; 353: 184-187Crossref PubMed Scopus (342) Google Scholar). In the absence of Shh, Ptc binds to and inhibits the signaling component of the complex, smoothened. When Shh binds to Ptc, it causes a dissociation and disinhibition of smoothened and induction of a signaling cascade for proliferation. Thus, loss of Ptc function leads to a sustained activity of smoothened and deregulated proliferation. Consistent with a role for Shh in cerebellar development, mutations in ptc have been related to the development of medulloblastomas (a devastating tumor thought to arise in granule cell precursors) in humans and mice (4Goodrich L. Milenkovic L. Higgins K. Scott M. Science. 1997; 277: 1109-1113Crossref PubMed Scopus (1366) Google Scholar). In the paper in this issue by Wechsler-Reya and Scott, the authors demonstrate that Shh is produced in the Purkinje cell layer, with components of the Shh pathway, ptc and ptc2, as well as the transcription factors gli1 and gli2 (5Hammerschmidt M. Brook A. McMahon A. Trends Genet. 1997; 13: 14-21Abstract Full Text PDF PubMed Scopus (481) Google Scholar), abundant in EGL cells (see figure), and they propose that Shh signaling acts to control the number of granule cells as the cerebellar circuitry is forming. 3H-thymidine incorporation studies in cultures of purified granule neurons demonstrate that Shh provides a mitogenic signal to EGL cells in this period, stimulating the rapid proliferation of the cells. With this assay, Wechsler-Reya and Scoti show that Shh is 100-fold more potent than previously reported granule cell mitogens and indicate that the action of Shh is dose dependent, with maximal growth stimulation at 3 μg/ml. Thus, the authors propose that Shh provides a signal that bulks out the EGL precursor pool from a thin layer of cells into a robust proliferative zone some 8–10 cells in depth. 3H-thymidine incorporation studies also revealed the surprising fact that basic fibroblast growth factor (bFGF) mitigates the effect of Shh on granule cell growth. In the presence of both bFGF and Shh, far less growth stimulation was seen. This is consistent with prior studies showing a role for bFGF in the differentiation of granule cells (6Hatten M. Lynch M. Rydel R. Sanchex J. Joseph-Silverstein J. Moscatelli D. Rifkin D. Dev. Biol. 1988; 125: 280-289Crossref PubMed Scopus (319) Google Scholar, 10Tao Y. Black I. DiCicco-Bloom E. J. Comp. Neurol. 1996; 376: 653-663Crossref PubMed Scopus (93) Google Scholar) and suggests that bFGF may regulate the action of Shh. To show that Shh acts in situ, the authors treated tissue slices with Shh and monitored the number of BrdU-labeled cells. Whereas labeled cells in control tissue slices exited the zone by migration along the radial glial fibers, labeled cells in treated slices were localized to the EGL. This suggested that the granule cell precursors respond to Shh by maintaining their proliferative status. The final proof of a role for Shh was provided by studies with blocking antibodies against Shh (2Ericson J. Morton S. Kawakami A. Roelink H. Jessell T. Cell. 1996; 87: 661-673Abstract Full Text Full Text PDF PubMed Scopus (738) Google Scholar). When hybridoma cells secreting blocking antibodies were injected into animals in the early postnatal period, the mitotic zone of the EGL thinned from 8–10 cells to 2–3 cells deep. These studies provided evidence that Shh functions in the expansion of the pool of neuronal precursor cells in the EGL. In this paper, we see the first evidence for the molecular basis of a signaling loop between Purkinje cells and granule cells that may act to control the number of granule neurons. This is a remarkable finding, because it suggests that the control of the formation of neural circuits commences much earlier than previously imagined. Whereas previous models held that neuronal number is controlled by programmed cell death after synaptic connections form, Wechsela-Reya and Scott show that Purkinje cells control the number of granule cells generated during neurogenesis, the first step of development. The timing of Purkinje cell and granule cell differentiation, with the Purkinje cell maturing just prior to the granule cell, is therefore key to cerebellar development. At present, the mechanisms that control Shh expression in Purkinje cells, as well as expression of the Shh signaling pathway in the granule cell precursors, are unknown. Unraveling this mechanism promises important insights on the development of the cerebellar cortex.
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