Mengqing Xiang
	Professor Department of Pediatrics UMDNJ-Robert Wood Johnson Medical School Member Cancer Institute of New Jersey Ph.D., 1991, University of Texas M.D. Anderson Cancer Center Tel:  [732] 235-4491 Fax: [732] 235-4850 xiang@cabm.rutgers.edu

POU domain transcription factor, Brn3, axon pathfinding, apoptosis, retinogenesis, retinal ganglion cell, inner ear hair cell, sensory ganglia.

Our research interests center on understanding the molecular mechanisms that govern the determination and differentiation of the highly specialized sensory neurons. We employ molecular genetic approaches in animal models to identify and study transcription factors that are required for programming development of the retina, inner ear, and somatosensory ganglia. In addition, we explore the possible linkage of these factors to some hereditary sensorineural diseases including blindness and deafness and develop animal models for these disorders.

Our laboratory utilizes two general approaches to understand the biological roles that a transcription factor 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 the gene. The other is a gain-of-function approach involving retrovirus-mediated overexpression of the gene in the chick and mouse embryonic tissues. These complementary approaches have allowed us to achieve a comprehensive understanding of the crucial functions of the Brn3 subfamily of POU domain transcription factors in senserineural development.

The Brn3 (Brn3a, Brn3b and Brn3c) genes encode three closely related transcription factors characterized by the presence of a DNA-binding POU domain. They represent vertebrate homologs of the C. elegans Unc-86, a gene essential for proper development of multiple neural lineages, including mechanosensory neurons. Similar to mutations in Unc-86, targeted deletion of Brn3b in mice causes the loss of a large set of retinal ganglion cells and optic nerve hypoplasia. It appears that Brn3b is required for the survival of retinal ganglion cells and involved in multiple processes of their early and terminal differentiation. Brn3b null mutations result in a wide range of differentiation defects in retinal ganglion cells that include downregulation of early ganglion cell markers, defasciculation of axon fibers, axon misrouting, and disruption of correct retinotectal projections.

Deletion of Brn3a results in developmental defects in the trigeminal, dorsal root, and inner ear sensory ganglia, and in the brainstem. As a result, Brn3a^-/- neonates exhibit uncoordinated limb and trunk movements, and impaired suckling response. In the trigeminal and inner ear sensory ganglia, Brn3a is required for regulating the expression of the Trk neurotrophin receptors and involved in proper axon guidance of sensory neurons. Targeted null mutation of Brn3c leads to loss of all the cochlear and vestibular hair cells by apoptosis, resulting in complete deafness and profound deficit in the vestibular system. We have recently shown that the generation of inner ear hair cells can initially occur in Brn3c^-/- embryos, indicating an essential role for Brn3c in terminal differentiation and survival but not in fate specification of hair cells. In the human, a small deletion in the Brn3c coding region has been linked to autosomal dominant hearing loss.

We have conducted overexpression analyses in the chick embryo to understand the mechanism that causes the functional differences of Brn3 genes during retinogenesis. We find that Brn3b, Brn3a and Brn3c all have similar DNA-binding and transactivating activities, and that the POU domain is minimally required for these activities. Consequently, we show that all these Brn3 proteins have a similar ability to promote development of ganglion cells when ectopically expressed in retinal progenitors. During chick retinogenesis, cBrn3c instead of cBrn3b exhibits a spatial and temporal expression pattern characteristic of ganglion cell genesis and its misexpression can also increase ganglion cell production. Based on these data, we propose that all Brn3 factors are capable of promoting retinal ganglion cell development, and that this potential may be limited by the order of their expression in vivo.

We are also applying the gain-of-function approach in the chick embryo to investigate the genetic regulatory network(s) that controls retinal ganglion cell development. We find that during retinogenesis, the chicken Ath5 (Cath5) bHLH transcription factor gene is expressed in retinal progenitors as well as differentiating ganglion cells but absent in terminally differentiated ganglion cells. Forced expression of both Cath5 and the mouse Ath5 gene (Math5) in retinal progenitors activates the expression of cBrn3c following central-to-peripheral and temporal-to-nasal gradients. As a result, both Cath5 and Math5 proteins have the ability to promote the development of ganglion cells. Moreover, we find that forced expression of all three Brn3 genes can stimulate the expression of cBrn3c as well. We further show that Ath5 and Brn3 proteins are all capable of transactivating a Brn3b promoter. Thus, the Ath5 and Brn3 factors participate in a transcriptional cascade to regulate the determination and differentiation of retinal ganglion cells.

Schematic illustrating regulatory relationships between Ath5 and Brn3 factors during retinal ganglion cell development. In the chicken, Cath5 directly activates cBrn3c expression which in turn activates the expression of cBrn3a and cBrn3b. The maintenance of cBrn3c expression may be achieved via positive autoregulation and feedback activation by cBrn3a and cBrn3b. An analogous transcriptional cascade controls retinal ganglion cell development in the mouse.

Selected Publications1