Islet transcription factors and the evolution of the vertebrate pineal gland

TABARES F; CHASELON D; CIRIO MC; DE SOUZA FS

Chascomus

Taller; V Taller de Biología Celular y de Desarrollo; 2022

The pineal gland develops from the dorsal roof of the diencephalon of vertebrates. Its main function is to secrete melatonin during night time, a hormone that participates in the adaptation of the body to the day/night cycle. In mammals, melatonin synthesis by pinealocites is regulated by a circuit that depends on photoreceptors located in the retina, while the pineal of non-mammalian vertebrates expresses various opsins and exhibits intrinsic light-sensing capability. In amphibians, in particular, the pineal complex includes a deep part as well as a superficial frontal organ (Stirnorgan). In order to explore the similarities and differences of pineal development at the molecular level, we are characterising the expression of transcription factors of the Islet family, namely Isl1 and Isl2. In the mouse, Islet expression coincides with the maturation of pinealocytes, showing an early coexpression with Pax6, which is a molecular marker for pineal progenitors and early maturing pinealocytes. Islet+ cells do not express mitotic markers like phosphohistone H3 (pH3) and coexpress tryptophan hydroxylase (Tph), an enzyme necessary for melatonin synthesis, indicating that Islet marks mature pinealocytes. In frog embryos (Xenopus laevis), both paralogues Isl1 and Isl2 are expressed in Tph+ maturing pinealocytes. In addition, in late Xenopus embryos (st 48/50), Islet expression is observed in the deep pineal as well as in the frontal organ, the main light-sensing part of the amphibian pineal complex. Since it is known that Islet genes are crucial for the development of the retina, it would seem that these proteins play conserved roles in the development of light-sensing structures in vertebrates.