Molecular basis of neurogenesis, retinal and spinal cord development
The research interests of my laboratory center on understanding the molecular events that lead to the determination, differentiation and survival of highly specialized sensory neurons and cells. The mammalian sensory system carries external and internal sensory information to the central nervous system, where it is processed to coordinate motor responses. The establishment of these sensory circuits in the adult depends critically on the generation of distinct neuronal types and sensory receptors at proper times and positions during embryogenesis as well as on their maintenance throughout life. Despite the importance of sensory neurons and cells, however, the molecular principle underlying their formation and survival still remains poorly understood. My laboratory employs a variety of molecular, genetic and bioinformatic approaches to identify and study transcription factors and signaling molecules that are required for programming development of the retina, inner ear, spinal cord, and other CNS areas. A major focus of our work is to develop knockout and transgenic animal models to study roles of these regulatory genes during normal sensorineural development, as well as to elucidate how mutations in these genes may cause sensorineural disorders such as blindness and deafness.
Our laboratory utilizes two general molecular genetic approaches to understand the biological roles that a regulatory gene plays during vertebrate neurogenesis. One is a loss-of-function approach involving targeted gene disruption in mouse embryonic stem (ES) cells to produce mice deficient for a particular gene. The other is a gain-of-function approach involving retrovirus- or plasmid-mediated overexpression of a particular gene in the chick and mouse embryonic tissues. These complementary approaches coupled with bioinformatic analysis have allowed us to identify a number of regulatory factors that are required for fate commitment, differentiation and/or survival of various sensory neurons and cells. For instance, targeted deletion of the winged helix/forkhead transcription factor gene Foxn4 largely eliminates amacrine neurons and completely abolishes horizontal cells in the retina, while misexpressed Foxn4 strongly promotes an amacrine cell fate, indicating that Foxn4 is both necessary and sufficient for commitment to the amacrine cell fate, and is non-redundantly required for competence acquisition for the genesis of horizontal cells. In the spinal cord, loss of Foxn4 function eliminates V2b interneurons and causes a fate-switch to V2a interneurons while overexpression of Foxn4 promotes the V2b fate. We further demonstrate that Foxn4 specifies the V2b fate by regulating the Dll4/Notch1 signaling pathway. Nr4a2 is an orphan nuclear receptor expressed in a subset of postmitotic GABAergic amacrine cells and their precursors during retinogenesis. Its targeted inactivation results in the loss of a subpopulation of GABAergic amacrine neurons whereas misexpressed Nr4a2 promotes GABAergic amacrine cell differentiation, suggesting that Nr4a2 is both necessary and sufficient to confer amacrine precursors with a GABAergic phenotype. These and other similar studies should provide a framework for understanding the regulatory gene network that governs the specification and differentiation of diverse neuronal types and subtypes in the mammalian nervous system.
Figure 1. Effect of targeted Foxn4 deletion on the formation of different retinal cell types. (A-F) Retinal sections from Foxn4+/+ and Foxn4lacZ/lacZ mice were immunostained with the indicated antibodies. Foxn4 inactivation results in a dramatic decrease of amacrine cells immunoreactive for syntaxin, a complete absence of horizontal cells immunoreactive for Lim1 (arrows), and a significant increase in photoreceptors immunoreactive for recoverin. (G) Schematic illustration of the retinal phenotype in Foxn4 null mice.
Misra K, Luo H, Li S, Matise M, Xiang M. (2014) Asymmetric activation of Dll4-Notch signaling by Foxn4 and proneural factors activates BMP/TGFβ signaling to specify V2b interneurons in the spinal cord. Development 141:187-98
Xiang M. (2013) Intrinsic control of mammalian retinogenesis. Cell. Mol. Life Sci. 70:2519-32
Luo H, Jin K, Xie Z, Qiu F, Li S, Zou M, Cai L, Hozumi K, Shima DT, Xiang M. (2012) Forkhead box N4 (Foxn4) activates Dll4-Notch signaling to suppress photoreceptor cell fates of early retinal progenitors. Proc. Natl. Acad. Sci. USA 109:E553-62
Jin K, Jiang H, Mo Z, Xiang M. (2010) Early B-cell factors are required for specifying multiple retinal cell types and subtypes from postmitotic precursors. J. Neurosci. 30:11902-16
Jiang H, Xiang M. (2009) Subtype specification of GABAergic amacrine cells by the orphan nuclear receptor Nr4a2/Nurr1. J Neurosci 29:10449-10459