Our long-term goal is to understand the molecular and cellular mechanisms by which growth factors, neuropeptides and neurotransmitters control neural development and function, including neuro-glial interaction, axonal guidance and synapse formation, anxiety and learning and memory. Our currently focus is on neuregulin-, neurotrophin-, corticotropin-releasing factor-, and acetylcholine-dependent signaling pathways and their dynamic interplay via mouse genetic, neuroanatomical and molecular and cellular approaches.
Neuregulins and their receptors (erbB2, erbB3, and erbB4, all of which are members of the epidermal growth factor receptor gene family) have been implicated in diverse bioactivities. We have previously generated erbB2-deficient embryos, which die of cardiac phenotype before embryonic day 11(E11). Mutant embryos displayed an absence of neural crest (NC) derived cranial sensory ganglia, exhibited no exits of somatic cranial motor axons, and showed abnormal central neuroglial development. To study the roles of erbB2 in neural development after E11, we have employed a genetic strategy to rescue the cardiac phenotype by generating transgenic mice expressing the rat erbB2 cDNA under the control of a cardiac specific promoter. When this transgene was crossed into an erbB2 null background, mutant embryos survived to birth. Spinal sensory and motor axons exited and projected normally and were fasciculated in their intermediate target fields. However, within their final target fields, they became de-fasciculated and axonal trajectories were severely altered. The failure to make correct target connections led to elevated neuronal cell death. Glial (Schwann) cells were completely absent in the peripheral nerves, yet Schwann cell precursors were present and proliferated normally. These Schwann cell precursors exhibited a markedly decreased ability to migrate into the projection nerve pathway. These results demonstrate that erbB2 is required for Schwann cell development. The elimination of Schwann cells suggests that Schwann cells are essential to normal axonal fasciculation and guidance. Mutant mice also displayed abnormal synapse formation at neuromuscular junction. A series of experiments are aimed to understand the role of erbB2 in neural development after birth and to delineate cellular and molecular mechanisms governing glial development, axonal guidance and synapse formation. The experiments include the generation of erbB2 conditional mutant mice via the Cre-loxP system that permits stage- and tissue-specific inactivation of the erbB2 gene in neurons, glial, or muscle cells.
Neurotrophins promote the survival of distinct populations of central and peripheral neurons by interacting with two classes of receptors: p75 and tyrosine receptor kinases (trks: trkA, trkB and trkC). In contrast to the trks, p75 is expressed in both neuronal and nonneuronal cells, binds all neurotrophins with low affinity, and lacks a kinase domain. Its role in neural development is incompletely understood. p75 mutant mice display a significant loss of sensory, motor and basal forebrain cholinergic neurons and exhibit impairments in sensorimotor tasks and cognitive tests. These results suggest that the function of p75 is to promote neuronal survival during development. Protein homology analysis, however, reveals that p75 belongs to the tumor necrosis factor receptor gene family, members of which mediate cell death. Our results suggest that p75 plays a role in mediating axotomy-induced cell death of neonatal sensory and motor neurons. Our goal is to elucidate the cellular and molecular mechanisms underlying how p75 exerts these two seemingly opposite cellular activities via a series of experiments, including yeast two-hybrid screens and conditional mutation.
Several lines of evidence suggest that neuregulin and neurotrophin signaling pathways interact with each other, e.g. in Schwann cell migration. Further studies are to understand how these two pathways via erbB2 and p75 receptors work in concert during neural development.
Smith, G. W., Aubrey, J.-M., Dellu, F., Contarino, A., Bilezikjian, L.M., Gold, L., Chen, R., Marchuk, Y., Hauser, C., Bentley, C.A., Sawchenko, P.E., Koob, G., Vale, W. and Lee, K.-F. (1998). Cortico
Peterson, D. A., Leppert, J. T., Lee, K.-F. and Gage, F. H. (1997). Basal forebrain neuronal loss in mice lacking neurotrophin receptor p75. Science 277:837-839.
Lee K.-F., Simon H., Chen H., Bates B., Hung M.-C. and Hauser C. (1995). Requirement for the neuregulin receptor erbB2 in neural and cardiac development. Nature 378:394-398.
Lee, K.-F., Buchman, K., Landis, S.C. and Jaenisch, R. (1994). Dependence on p75 for innervation of some sympathetic targets. Science 263:1447-1449.
Lee, K.-F., Li, E., Huber, L.J., Landis, S.C., Sharpe, A.H., Chao, M.V. and Jaenisch, R. (1992). Targeted mutation of the gene encoding the low affinity NGF receptor p75 leads to deficits in the perip