Critical to the possibility of using stem cells to replace neurons in neurodegenerative disease is understanding the mechanisms that control the proliferation of stem cells and their differentiation into neurons (see related news item). Among the more important regulators of neural development uncovered to date are the hedgehog family of signaling molecules, including the one named after the video game character Sonic the hedgehog (Shh). However, Shh is also found in many areas of the adult nervous system, including the hippocampus. In the present study, David Schaffer and Karen Lai of the University of California at Berkeley, along with Brian Kaspar and Fred Gage of the Salk Institute in San Diego, California, sought to determine whether Shh plays a role in regulating the ongoing neurogenesis that replenishes the granule cells of the dentate gyrus in the adult hippocampus.

Although the researchers were not able to detect Shh itself in the hippocampus (a limitation of the available antibodies, the authors write), antibodies to the Shh receptor Patched revealed high hippocampal expression of the receptor in brain sections, as well as in neural progenitor cells isolated from the hippocampus. In culture, the addition of Shh to these stem cells stimulated cell proliferation. The cells generated by this means appear to retain their multipotency.

The researchers then moved their experiments into live animals. With the introduction of Shh cDNA to the rat hippocampus via a viral vector, cell proliferation increased 3.3-fold. Conversely, the direct injection of cyclopamine-an inhibitor of the hedgehog signaling pathway -into the hippocampus reduced hippocampal cell proliferation to half that of vehicle-injected animals.

The authors remind us of one caveat in the quest to harness adult stem cells to the task of replacing lost neurons, namely, the possibility that proliferation-promoting molecules will also promote tumor formation. Just as the proposed use of cylcopamine (or other hedgehog pathway inhibitors) to treat medulloblastoma might have a negative effective on normal hippocampal cell proliferation (with potential attendant cognitive side effects), the use of molecules that increase activity in the hedgehog pathway might favor the growth of such tumors.-Hakon Heimer.

Reference:
Lai K, Kaspar BK, Gage FH, Schaffer DV. Sonic hedgehog regulates adult neural progenitor proliferation in vitro and in vivo. Nat Neurosci. 2 Dec 2002. DOI:10:1038/nn983. Abstract