The zebrafish maternal factor pollywog is required for yolk syncytial layer morphogenesis

Abstract

In teleosts, the Yolk Syncytial Layer (YSL) is functionally similar to the anterior visceral endoderm found in mice and is required for morphogenesis of the overlying blastoderm. The YSL undergoes dramatic reorganization during early development through processes that mirror the morphogenetic movements of the blastoderm. The YSL and YSL nuclei (YSN) undergo epiboly, and during convergence and extension movements of the blastoderm, the YSN underneath the animal cap also converge and extend underneath the axial hypoblast. Our work with pollywog ( pwg ) maternal-effect mutants highlights the delicate control of the YSL during yolk morphogenesis, and provides novel insight into understanding which tissues of the embryo are affected by loss of a cohesive YSL. I found that pollywog encodes the zebrafish mitogen activated protein kinase kinase kinase 4 ( map3k4 ) gene and that it acts upstream of p38a MAPK in the YSL. I show that this pathway acts in the YSL along with a mixer gene family member, mix-type homeobox gene 1 ( mxtx1 ), to non-autonomously coordinate extracellular matrix deposition and morphogenetic movements in the overlying blastoderm. Our data describes an early and novel role for Map3k4, p38a and Mxtxl activity that is required for proper morphogenesis of the YSL and the blastoderm. In embryos lacking maternal Map3k4, the YSL undergoes a rapid and catastrophic retraction and the YSN lose their normal distribution around the yolk. The prechordal plate of pwg mutant embryos deflect laterally or plunge into the yolk, and the overall animalward extension of the prechordal plate is diminished. I also show that the anterior neural plate of pwg mutant embryos fail to converge dorsally to the same extent as in wild embryos. These data show that the p38 MAPK pathway is essential for maintaining normal yolk cell equilibrium during early development and that without proper cues from the YSL, the blastoderm cannot complete its morphogenetic movements. Incuded in this thesis is work highlighting the alpha-actinin gene family in zebrafish. alpha-actinins are actin microfilament crosslinking proteins. Vertebrate actinins fall into two classes: the broadly-expressed actinins 1 and 4 ( actn1 and actn4 ) and muscle-specific actinins, actn2 and actn3 . Members of this family have numerous roles, including regulation of cell adhesion, cell differentiation, directed cell motility, intracellular signaling and stabilization of f-actin at the sarcomeric Z-line in muscle. Here I identify five zebrafish actinin genes including two paralogs of ACTN3 . I describe the temporal and spatial expression patterns of these genes through embryonic development. All zebrafish actinin genes have unique expression profiles, indicating specialization of each gene. In particular the muscle actinins display preferential expression in different domains of axial, pharyngeal and cranial musculature. There is no identified avian actn3 and approximately 16% of humans are null for ACTN3 . Duplication of actn3 in the zebrafish indicates that variation in actn3 expression may promote physiological diversity in muscle function among vertebrates.