Sonic hedgehog-dependent synthesis of laminin α1 controls basement membrane assembly in the myotome

Abstract

Basement membranes have essential structural and signalling roles in tissue morphogenesis during embryonic development, but the mechanisms that control their formation are still poorly understood. Laminins are key components of basement membranes and are thought to be essential for initiation of basement membrane assembly. Here, we report that muscle progenitor cells populating the myotome migrate aberrantly in the ventral somite in the absence of sonic hedgehog (Shh) signalling, and we show that this defect is due to the failure to form a myotomal basement membrane. We reveal that expression of Lama1, which encodes laminin α1, a subunit of laminin-111, is not activated in Shh-/- embryos. Recovery of Lama1 expression or addition of exogenous laminin-111 to Shh-/-;Gli3-/- embryos restores the myotomal basement membrane, demonstrating that laminin-111 is necessary and sufficient to initiate assembly of the myotomal basement membrane. This study uncovers an essential role for Shh signalling in the control of laminin-111 synthesis and in the initiation of basement membrane assembly in the myotome. Furthermore, our data indicate that laminin-111 function cannot be compensated by laminin-511.

Hedgehog signalling acts upstream of Laminin alpha1 transcription in the zebrafish paraxial mesoderm

“…In mouse skeletal muscles, a Laminin 1-and Laminin 5-containing basal lamina associated with embryonic muscles is gradually replaced by a Laminin 2-containing basal lamina in adult muscles [14,[17][18][19][20][21]. As in mouse embryos, Laminin 1 is the earliest subunit detected in the tail bud and presomitic mesoderm of zebrafish embryos, closely followed by Laminin 2 in adaxial cells and Laminin 4 and 5 in slow and fast muscle cells [8,[22][23][24].…”

“…In the mouse, loss of somitic Lama1 expression in Sonic Hedgehog (Shh)-deficient embryos impairs the assembly of the myotomal basement membrane and results in aberrant migration and differentiation of muscle progenitor cells [17]. Likewise, lamc1 (sleepy (sly)) mutation disrupts the pre-patterning of adaxial cells and Hedgehogmediated induction of muscle pioneer and medial fast muscle cell differentiation in zebrafish embryos [22].…”

“…Given our previous findings that Shh signalling is required for Lama1 expression in the mouse paraxial mesoderm and neural tube [17], we asked whether Hedgehog signalling was necessary for lama1 expression in the zebrafish embryo.…”

“…In contrast, no Lama1 expression has been observed in the chick and mouse notochord, although Laminin-111 is deposited in the peri-notochord basement membrane [14,17,52]. Likewise, the spatial distribution of lama1 transcripts in the paraxial mesoderm differs between zebrafish and mouse embryos.…”

“…Likewise, the spatial distribution of lama1 transcripts in the paraxial mesoderm differs between zebrafish and mouse embryos. Although deposited in the myotomal basement membrane assembled at the surface of muscle progenitor cells, Laminin 1 is transcribed by sclerotomal, and not myogenic cells in the mouse embryo [17]. In contrast, lama1 is transcribed by adaxial, slow and fast muscle cells in the zebrafish embryo and contributes to the somitic basement membrane and myotendinous junction [23,24,53].…”