Neural regulation of bone mineral homeostasisnew insights from fish

Resumo

This PhD thesis focuses on the neuro-regulatory role of the brain on bone homeostasis and development, a vibrant area of research at the intersection of bone biology and neurobiology. In this project, we have uncovered a new member of the parathyroid-hormone family, Pth4, and characterized its function as a novel brain-derived key regulatory signal of bone mineral metabolism during embryonic development and adult homeostasis in fish. This project is cross-disciplinary and combines cutting-edge approaches from comparative genomics, cell and developmental neurobiology, reverse genetics, bone physiology and evolutionary biology. Comparative analyses of vertebrate genomes reveal that the Pth4 is an ancient parathyroid hormone in vertebrates, but secondarily lost in eutherian mammals. Transgenic fish lines and loss-of-function experiments by targeted laser ablation enabled mapping a subset of hypothalamic Pth4-expressing neurons with axonal projections to the brainstem and spinal cord. These play an essential role in larval bone mineralization. We demonstrate that pth4 expression is directly regulated by Runx2a, and that Pth4 can activate downstream cAMP signaling mediated by Pth receptors. Gain-of-function experiments by pth4 ectopic expression in transgenic zebrafish demonstrate that Pth4 can alter bone mineral density and calcium/phosphorus levels as well as affect expression of genes involved in phosphate homeostasis. Our findings culminate in a new model for the evolution of the PTH family roles in bone mineral homeostasis in the context of the vertebrate transition from aquatic to terrestrial environments. We feel this PhD project will have broad interest not only for researchers of Bone Biology and Neurobiology, but also Developmental and Cell Biology, Comparative Genomics and Evolutionary Biology. Furthermore, we believe that the discovery and functional characterization of Pth4 will have the potential to translate novel strategies for intensive finfish culture (design of new phosphate improved diets) and novel therapeutic agents to treat human bone diseases.