Met and Hgf signaling controls hypaxial muscle and lateral line development in the zebrafish

Abstract

Somites give rise to a number of different embryonic cell types, including the precursors of skeletal muscle populations. The lateral aspect of amniote and fish somites have been shown to give rise specifically to hypaxial muscle,including the appendicular muscle that populates fins and limbs. We have investigated the morphogenetic basis for formation of specific hypaxial muscles within the zebrafish embryo and larvae. Transplantation experiments have revealed a developmentally precocious commitment of cells derived from pectoral fin level somites to forming hypaxial and specifically appendicular muscle. The fate of transplanted somites cannot be over-ridden by local inductive signals, suggesting that somitic tissue may be fixed at an early point in their developmental history to produce appendicular muscle. We further show that this restriction in competence is mirrored at the molecular level, with the exclusive expression of the receptor tyrosine kinase met within somitic regions fated to give rise to appendicular muscle. Loss-of-function experiments reveal that Met and its ligand, hepatocyte growth factor, are required for the correct morphogenesis of the hypaxial muscles in which met is expressed. Furthermore, we demonstrate a requirement for Met signaling in the process of proneuromast deposition from the posterior lateral line primordia.

hgf/c‐met expression and functional analysis during zebrafish embryogenesis

“…Loss of HGF or Met function in the mouse results in defective delamination and migration of these muscle progenitors from the dermomyotome and, therefore, they are absent from their target regions Maina et al, 1996;Dietrich et al, 1999). Previous work using zebrafish has also demonstrated that Met signaling is required for muscle progenitor cell migration from the somites to form fin hypaxial muscle (Haines et al, 2004). Furthermore, this work showed that Met is necessary for deposition of pro-neuromasts by the migrating posterior lateral line primordia.…”

“…To corroborate these results, we designed two additional MOs, one targeting the c-met 5Ј UTR and the other a splice blocking MO that prevents correct splicing of c-met transcripts as confirmed by RT-PCR and DNA sequencing (data not shown). It was previously shown that c-met morphants exhibit reduced myod staining of fin buds (Haines et al, 2004;Carney et al, 2007). Therefore, we verified the specificity of all three MOs by examining fin bud myod expression using in situ hybridization ( Fig.…”

“…Lastly, maternally deposited Met protein may be sufficient to mask any early defects caused by MO gene knockdown. Indeed, two previous zebrafish studies also did not identify strong Met loss of function phenotypes during early development (Haines et al, 2004;Tallafuss and Eisen, 2008).…”

“…Previous work has documented zebrafish c-met expression in tissues necessary for formation and development of muscle and the posterior lateral line (Haines et al, 2004), spinal cord motor neurons (Segawa et al, 2001;Tallafuss and Eisen, 2008), and inferior and superior extraocular muscles (Lin et al, 2006). c-met expression (using a 496-bp probe, GenBank CK690356) has also been described as part of a large-scale in situ hybridization screen directly submitted to the Zebrafish Information Network ; http://zfin.org).…”

“…We utilized a previously validated c-met MO whose target sequence overlaps the AUG translation initiation codon (Haines et al, 2004). Other than slightly shortened embryonic axes, the injected embryos did not display any gross morphological phenotypes through 24 hr (data not shown).…”