Alicia M. Ebert

Ca2+ channel-independent requirement for MAGUK family CACNB4 genes in initiation of zebrafish epiboly

CACNB genes encode membrane-associated guanylate kinase (MAGUK) proteins once thought to function exclusively as auxiliary β subunits in assembly and gating of voltage-gated Ca2+ channels. Here, we report that zygotic deficiency of zebrafish β4 protein blocks initiation of epiboly, the first morphogenetic movement of teleost embryos. Reduced β4 function in the yolk syncytial layer (YSL) leads to abnormal division and dispersal of yolk syncytial nuclei, blastoderm retraction, and death, effects highly similar to microtubule disruption by nocodazole. Epiboly is restored by coinjection of human β4 cRNA or, surprisingly, by mutant cRNA encoding β4 subunits incapable of binding to Ca2+ channel α1 subunits. This study defines a YSL-driven zygotic mechanism essential for epiboly initiation and reveals a Ca2+ channel-independent β4 protein function potentially involving the cytoskeleton.

Voltage‐gated calcium channel CACNB2 (β2.1) protein is required in the heart for control of cell proliferation and heart tube integrity

Background: L‐type calcium channels (LTCC) regulate calcium entry into cardiomyocytes. CACNB2 (β2) LTCC auxiliary subunits traffic the pore‐forming CACNA subunit to the membrane and modulate channel kinetics. β2 is a membrane associated guanylate kinase (MAGUK) protein. A major role of MAGUK proteins is to scaffold cellular junctions and multiprotein complexes. Results: To investigate developmental functions for β2.1, we depleted it in zebrafish using morpholinos. β2.1‐depleted embryos developed compromised cardiac function by 48 hr postfertilization, which was ultimately lethal. β2.1 contractility defects were mimicked by pharmacological depression of LTCC, and rescued by LTCC stimulation, suggesting β2.1 phenotypes are at least in part LTCC‐dependent. Morphological studies indicated that β2.1 contributes to heart size by regulating the rate of ventricle cell proliferation, and by modulating the transition of outer curvature cells to an elongated cell shape during chamber ballooning. In addition, β2.1‐depleted cardiomyocytes failed to accumulate N‐cadherin at the membrane, and dissociated easily from neighboring myocytes under stress. Conclusions: Hence, we propose that β2.1 may also function in the heart as a MAGUK scaffolding unit to maintain N‐cadherin‐based adherens junctions and heart tube integrity. Developmental Dynamics 241:648–662, 2012.