Split top: A maternal cathepsin B that regulates dorsoventral patterning and morphogenesis

Abstract

The vertebrate embryonic dorsoventral axis is established and patterned by Wnt and bone morphogenetic protein (BMP) signaling pathways, respectively. Whereas Wnt signaling establishes the dorsal side of the embryo and induces the dorsal organizer, a BMP signaling gradient patterns tissues along the dorsoventral axis. Early Wnt signaling is provided maternally, whereas BMP ligand expression in the zebrafish is zygotic, but regulated by maternal factors. Concomitant with BMP activity patterning dorsoventral axial tissues, the embryo also undergoes dramatic morphogenetic processes, including the cell movements of gastrulation, epiboly and dorsal convergence. Although the zygotic regulation of these cell migration processes is increasingly understood, far less is known of the maternal regulators of these processes. Similarly, the maternal regulation of dorsoventral patterning, and in particular the maternal control of ventral tissue specification, is poorly understood. We identified split top, a recessive maternal-effect zebrafish mutant that disrupts embryonic patterning upstream of endogenous BMP signaling. Embryos from split top mutant females exhibit a dorsalized embryonic axis, which can be rescued by BMP misexpression or by derepressing endogenous BMP signaling. In addition to dorsoventral patterning defects, split top mutants display morphogenesis defects that are both BMP dependent and independent. These morphogenesis defects include incomplete dorsal convergence, delayed epiboly progression and an early lysis phenotype during gastrula stages. The latter two morphogenesis defects are associated with disruption of the actin and microtubule cytoskeleton within the yolk cell and defects in the outer enveloping cell layer, which are both known mediators of epiboly movements. Through chromosomal mapping and RNA sequencing analysis, we identified the lysosomal endopeptidase cathepsin Ba (ctsba) as the gene deficient in split top embryos. Our results identify a novel role for Ctsba in morphogenesis and expand our understanding of the maternal regulation of dorsoventral patterning.

Tris(1,3-dichloro-2-propyl) phosphate disrupts dorsoventral patterning in zebrafish embryos

“…Similar to DMP, TDCIPP exposure resulted in a concentration-dependent increase in the severity of dorsalization, suggesting that TDCIPP exposure results in inhibition of normal BMP signaling and downstream effects on dorsoventral patterning. Moreover, the presence of split yolk phenotypes (while rare) also supports the conclusion that TDCIPP interferes with BMP-related pathways, as these phenotypes are characteristic of zebrafish split top mutants deficient in BMP signaling ( Langdon et al, 2016 ).…”

“…Within the current study, we discovered that, while the majority of epiboly-delayed embryos appear normal at 24 hpf, 76% of epiboly-arrested embryos resulted in dorsalized phenotypes, suggesting that epiboly arrest—but not epiboly delay—at 6 hpf leads to dorsalization by 24 hpf. Previous studies have shown that mutations in early zygotic genes lead to epiboly defects, disruptions in dorsoventral patterning, and severe deformities in surviving embryos ( Haffter et al, 1996 ; Kane et al, 1996 ; Kane, McFarland & Warga, 2005 ; Langdon et al, 2016 ). Therefore, the link between epiboly arrest and dorsalization observed following TDCIPP exposure suggests that TDCIPP may interfere with the normal function of early zygotic gene products.…”

Kinesin-1 promotes chondrocyte maintenance during skeletal morphogenesis

“…Cathepsin D mediates early stages of perichondral ossification [ 68 ], and although myopathy was reported in zebrafish morphants no cartilage phenotypes were noted [ 69 ]. Cathepsin B is elevated in in vitro models of osteoarthritis [ 70 ] and promotes early embryonic patterning in zebrafish via BMP signaling, but no jaw phenotypes were noted in mutants [ 71 ]. Collectively, the absence of kif5Blof like phenotypes in these mutants suggests that Kif5B modulates cell maintenance during chondrogenesis by a mechanism that is distinct from these cathepsins.…”

Cathepsin B & D and the Survival of Early Embryos in Red Spotted Grouper, Ephinephelus akaara

“…General lysosomal enzymes, phosphatases and proteinases have been suggested to be involved in yolk protein degradation of eggs in various animals including avians, amphibians, fish, crustaceans and insects ( Perona & Vallejo, 1982 ; Lemanski & Aldoroty, 1977 ; Vogel & Gerster, 1997 ; Carnevali et al, 1999b ). Products of maternal cathepsin transcripts including cathepsin B & D are associated with oocyte development in the ovary, and their RNA is depleted during embryo development ( Carnevali et al, 2008 ; Follo et al, 2013 ; Fernandez et al, 2013 ; Langdon et al, 2016 ). It has been suggested that transcript level of cathepsin B & D can determine the quality of eggs and embryos in a pelagic fish species ( Palomino et al, 2017 ).…”