By releasing secreted factors to the blood upon receiving neuronal input neuroendocrine cells represent the main integrators connecting nervous and endocrine systems. The pineal gland is a small neuroendocrine structure in the dorsal diencephalon crucial for the regulation of circadian rhythm, partially via secretion of melatonin. Several nuclei at the base of the diencephalon form the hypothalamus, a master gland from which various factors controlling body temperature, hunger, thirst, fatigue and reproduction are released.
The highly conserved homeodomain factor brain specific homeobox (bsx) is expressed in both the developing pineal complex and hypothalamus. Using TALEN-mediated targeted mutagenesis in zebrafish, we generated the bsxm1376 mutant allele, in which a truncated Bsx protein lacking a functional DNA-binding domain is translated. In the rodent hypothalamus the orexigenic neuropeptide AGRP has previously been described as being significantly downregulated upon loss of functional Bsx. In bsxm1376/m1376 larvae we observed drastically reduced agrp expression in the hypothalamus, suggesting a similar Bsx function in zebrafish.
Very little is known about the role of Bsx in the pineal complex. In situ hybridization revealed pineal gland expression of tph2, a key enzyme in melatonin synthesis, to be missing in bsxm1376/m1376. Loss of Tph2 suggests absence of melatonin in bsxm1376/m1376 and thus disturbed activity regulation during dark cycles. Reduction of irbp and gngT1 expression further indicates disturbed photoreception via the pineal complex in bsxm1376/m1376.
The teleost pineal complex more recently has also been implicated in background adaptation as well as the establishment of both anatomical and functional asymmetries in the epithalamus. We analyzed expression of agrp2, a neuropeptide crucial for melanocyte contraction, which we found to be significantly reduced in bsxm1376/m1376. Furthermore, bsxm1376/m1376 larvae display abnormalities in epithalamus asymmetry as revealed by absence of parapineal otx5 expression as well as symmetrical expression of lov in the habenula. Through epistasis analysis including floating head and one-eyed pinhead mutants as well as injections of otx5 and crx morpholinos, we seek to integrate Bsx into a comprehensive model of pineal complex development. Detailed knowledge about the mechanisms by which Bsx regulates neuroendocrine development or differentiation in not only one but two neuroendocrine brain areas could contribute to a better understanding of numerous diseases associated with the neuroendocrine system.