Evolution of the TRH Neuropeptide Pathway and its Growth Regulation Function in Echinoderms

Cocurullo, Maria (2022). Evolution of the TRH Neuropeptide Pathway and its Growth Regulation Function in Echinoderms. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.00014231

Abstract

The Thyrotropin-releasing hormone (TRH) is a hormone produced by the hypothalamus having a central role in the regulation of the Hypothalamus-Pituitary-Thyroid axis in vertebrates. Therefore, it has a central role in regulating metabolism, growth and reproduction. However, its wide distribution throughout the brain and in other regions of the body suggests that it works also as a neurotransmitter and neuromodulator and that is implicated in a variety of other functions. However, very little is known about its role in invertebrates and how the TRH signalling evolved.

The aim of this work was to characterize and assess the role of the TRH pathway during the development of the sea urchin Strongylocentrotus purpuratus. Combining classic Whole Mount In Situ Hybridization and immunostaining with information from scRNA-seq data generated in the Arnone lab, it was possible to characterize the cells that are producing the TRH (TRHergic) and the ones that are responding thorough the TRH receptor (TRHR). Moreover, it was investigated how the TRHergic neurons are integrated in the larva sensory system, focusing on three components: the larval apical organ (considered to be the larva central nervous system), the dopaminergic system (which is known to regulate arm growth in response to food), and the photoreceptor system mediated by the Go-Opsin3.2 (known to be coexpressed in the TRHergic cells of S. purpuratus). A first characterization of the TRHergic cells was performed also in other sea urchin species, mainly the Mediterranean Paracentrotus lividus, highlighting similarities and differences at morphological level that might be the result of a differential regulation and/or function in these species, therefore, providing initial information on how this signalling system evolved in closely related species.

Furthermore, knock-down experiments using morpholino oligonucleotides to disrupt the TRH signalling showed that this neuropeptide is required for arm growth in S. purpuratus. Similarly, the Opsin 3.2 knock-down showed to affect arm elongation.

Based on these evidences, it was set up a protocol to evaluate the larval phenotypic response to food availability under the classic 12h light:12h dark cycle and in constant dark, showing for the first time the role of the light in such response.

In conclusion, this work described the complex interaction between the TRH pathway and other components of the larva sensory/neurosecretory system, laying the foundations for further studies aiming to elucidate how the sensory/neurosecretory system develops and evolved, and how it is controlling phenotypic and developmental adaptation to the environment.

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