Ontogenetic expression and neural regulation of the homeobox genes (Crx, Otx2, Pax6, and Pax4) in the retina and pineal gland of the Sprague Dawley rat

M. RATH; S. COON; J. KIM; E. MUÑOZ; M. BAILEY; D. KLEIN; M. MØLLER (EXPOSITOR).

Adelaide, Australia.

Simposio; From Molecular Clocks to Human Health, Australasian Chronobiology Society.; 2007

DNA microarray methodology has made it possible to perform large scale screenings of gene expression in the pineal gland relative to other tissues. By use of this technique, several hundred genes have been detected with a high expression in the pineal gland of the Sprague Dawley rat, many of which exhibit a day/night rhythm, (Klein et al., in preparation). Some of these genes are homeobox genes, which encode transcription factors structurally characterized by a DNA-binding homeodomain and known to switch on a cascade of genes important for organogenesis including the development of the pineal gland and the retina. We have investigated members of the orthodenticle homeobox gene family (Crx and Otx2), which play a fundamental role in development of photoreceptors and rostral brain regions; and, the Pax gene family (Pax6 and Pax4). Pax6 is essential for pineal and retinal development; both Pax genes are involved in development of the endocrine pancreas. All four homeobox genes are expressed in the pineal gland and the retina as indicated by several methods ( in situ hybridization,  Northern blot analysis and qRTPCR). Expression of each gene follows a characteristic developmental pattern in both tissues. In the pineal gland Crx, Otx2, and Pax6 are expressed during both day and night; Otx2 expression is not affected by removal of the sympathetic input to the gland. In contrast, Pax4 expression in the pineal and the retina exhibits a prominent circadian rhythm with a nadir during night time. This night time decrease in Pax4 expression is abolished after superior cervical ganglionectomy in the pineal gland, but not in the retina. Adrenergic activation of the pineal gland lowers Pax4 expression in the pineal. Our results show a strong expression of four homeobox genes in the retina and pineal gland supporting the hypothesis of a common ancestral cell for the pinealocyte and retinal photoreceptor. Our data not only indicate that these homeobox genes are required for normal pineal gland and retinal development, but also that they may play a role in the adult organs, perhaps by controlling expression of the phototransduction/melatonin synthesis genes. The role of Pax4 protein in pineal physiology remains to be elucidated.