RhoA/ROCK pathway activity is essential for the correct localization of the germ plasm mRNAs in zebrafish embryos

Abstract

Zebrafish germ plasm is composed of mRNAs such as vasa and nanos and of proteins such as Bucky ball, all of which localize symmetrically in four aggregates at the distal region of the first two cleavage furrows. The coordination of actin microfilaments, microtubules and kinesin is essential for the correct localization of the germ plasm. Rho-GTPases, through their effectors, coordinate cytoskeletal dynamics. We address the participation of RhoA and its effector ROCK in germ plasm localization during the transition from two- to eight-cell embryos. We found that active RhoA is enriched along the cleavage furrow during the first two division cycles, whereas ROCK localizes at the distal region of the cleavage furrows in a similar pattern as the germ plasm mRNAs. Specific inhibition of RhoA and ROCK affected microtubules organization at the cleavage furrow; these caused the incorrect localization of the germ plasm mRNAs. The incorrect localization of the germ plasm led to a dramatic change in the number of germ cells during the blastula and 24hpf embryo stages without affecting any other developmental processes. We demonstrate that the Rho/ROCK pathway is intimately related to the determination of germ cells in zebrafish embryos.

Maternal miR-202-5p is required for zebrafish primordial germ cell migration by protecting small GTPase Cdc42

“…In migratory PGCs, Rac1 was responsible for formation of actin-rich structures and RhoA promoted retrograde actin flow, which were required for bleb formation ( Ridley, 2015 ). Chemokine-guided germ cells within zebrafish embryos required the function of the small Rho GTPases Rac1 and RhoA ( Kardash et al, 2010 ; Xu et al, 2012 ; Miranda-Rodriguez et al, 2017 ). Cdc42 was involved in cell cytoskeleton organization and cell migration ( Heasman and Ridley, 2008 ) and controlled membrane invagination via the clathrin-independent pinocytic pathway ( Sabharanjak et al, 2002 ).…”

Aggregation, segregation, and dispersal of homotypic germ plasm RNPs in the early zebrafish embryo

“…Once formed along the furrow, germ plasm RNP aggregates appear as electron‐dense material in contact with the cytoskeleton at the base of the cleavage furrow (Knaut et al, ). Animal germ plasm RNPs originally become recruited to the furrow as an elongated rod spanning the length of the furrow, which then during furrow maturation transitions into a compact aggregate at each distal end (Pelegri et al, ; Miranda‐Rodríguez et al; ; Eno et al, ). The latter process is closely followed by the arrival and incorporation of vegetal RNPs into the compacting germ cell aggregate (Hashimoto et al, ; Theusch et al, ).…”

“…Two mechanisms which animals use to differentiate primordial germ cells from somatic cells are preformation and epigenesis (Wylie, 1999;Extavour and Akam, 2003;Magnúsdóttir and Surani, 2014;Bertocchini and Chuva de Sousa Lopes, 2016). Zebrafish, like several other genetic models (C. elegans, Xenopus, and Drosophila) rely on the preformation mechanism, using maternally derived germ plasm as a specialized structure required (Hashimoto et al, 2014;Miranda-Rodríguez et al, 2017) and sufficient (Bontems et al, 2009) for the specification of the germline. Previous studies show that zebrafish germ plasm consists of ribonucleoparticles (RNPs) with RNAs originating from the two poles (animal and vegetal) of the egg (Yoon et al, 1997;Howley and Ho, 2000;Hashimoto et al, 2004;Theusch et al, 2006;Kosaka et al, 2007).…”

Heat Shock Factor 5 Is Essential for Spermatogenesis in Zebrafish

“…Severe developmental abnormality in testicular cell types in hsf5 À/À mutant testis could be a reason for their misregulation. Interestingly, rho kinase rock2, misregulated in hsf5 À/À testis, was shown to be required for proper localization of germplasm RNAs (Miranda-Rodríguez et al, 2017).…”