Bone Morphogenetic Protein (BMP) signaling directs the development of multiple organs and tissues in embryogenesis, and is the causative factor in multiple congenital and adult diseases, including cardiovascular and limb defects, kidney disease, pulmonary hypertension, and is important in medical applications such as orthopedics, endodontics, and tissue engineering. BMP heterodimers have been shown to exhibit consistently higher signaling activity to BMP homodimers and thus are beginning to be used in therapeutics. Understanding how BMP heterodimers signal more effectively or exclusively in some contexts, will lead to their more successful use in applications like tissue engineering, bone repair and regeneration. The zebrafish offers a paradigm of exclusive BMP heterodimer signaling in patterning cells across the dorsoventral embryonic axis as a morphogen, with different levels specifying distinct cell types. This context will be exploited to test hypotheses for the more effective signaling by BMP heterodimers. Numerous extracellular modulators of BMP regulate its levels spatiotemporally in the embryo. Using phosphorylation of Smad5 (P-Smad5) as a direct readout of BMP signaling, a quantitative method will used to determine how these extracellular factors shape the signaling gradient in space and time. How P-Smad5 translates the gradient into distinct gene expression domains that specify cell fate will also be addressed. In most vertebrates, polarity of the egg determines anterior-posterior polarity of the embryo and is essential to establishing the embryonic dorsal-ventral axis. Egg polarity originates during early stages of oogenesis when distinct animal-vegetal domains are established in the oocyte. The first polarized structure in vertebrate oocytes is the Balbiani body (Bb), a large, membrane-less structure conserved from insects to mammals that in frogs and fish is composed of ribonucleoproteins, ER, golgi, an enrichment of mitochondria, and RNAs destined to the vegetal pole of the oocyte and egg. The Bb is a transient structure that dissociates at the oocyte cortex delivering its contents and determining the vegetal pole. Although fundamental to forming the major axes of most vertebrate embryos, vertebrate oocyte polarity has been little studied due to the inaccessibility of these early oocyte stages within the ovary and the difficulty of genetic analysis in adult females. Through a mutant screen in the zebrafish, two genes were discovered that establish oocyte polarity, bucky ball (buc) and macf1 (microtubule actin crosslinking factor 1). Buc is required to form the Bb, whereas Macf1, an unusually large cytoskeletal linker protein, is required for its dissociation. Buc is a highly disordered protein that can undergo a phase transition to form amyloid-like fibers in vitro. The role of these proteins in aggregating the Bb and dissociating it will be studied. Two newly identified Bb resident proteins will be investigated for their functions in Bb formation and its dissociation. Importantly, inappropriate amyloid protein aggregates in cells are hallmarks of neurodegenerative disease. Thus, these studies are relevant to understanding how these aggregates form and can be dissociated in therapeutics.
Studies of the Bone Morphogenetic Protein (BMP) signaling mechanism and how signaling is regulated to generate diverse cellular responses will impact the myriad of biological contexts, where BMP signaling acts in congenital and adult disease, and for its use in orthopedics, endodontics, tissue engineering, and bone repair. Cell polarity is essential to the normal function of all epithelial tissues and organ systems, and loss of cell polarity underlies cancer metastasis, thus understanding mechanisms of cell polarity establishment, as studied here, is of great significance to human health. Studies of how proteins form aggregates and how the aggregates can be regulated to dissociate, as studied here in oocytes, has important medical applications for diseases associated with inappropriate protein aggregation, including neurodegenerative diseases of the synucleinopathies.
