Knock-in of integrin β1D affects primary but not secondary myogenesis in mice

Abstract

Integrins are extracellular matrix receptors composed of α and β subunits involved in cell adhesion, migration and signal transduction. The β1 subunit has two isoforms, β1A ubiquitously expressed and β1D restricted to striated muscle. They are not functionally equivalent. Replacement of β1A by β1D (β1D knock-in) in the mouse leads to midgestation lethality on a 50% Ola/50% FVB background [Baudoin, C., Goumans, M. J., Mummery, C. and Sonnenberg, A. (1998). Genes Dev. 12, 1202-1216]. We crossed the β1D knock-in line into a less penetrant genetic background. This led to an attenuation of the midgestation lethality and revealed a second period of lethality around birth. Midgestation death was apparently not caused by failure in cell migration, but rather by abnormal placentation. The β1D knock-in embryos that survived midgestation developed until birth, but exhibited severely reduced skeletal muscle mass. Quantification of myotube numbers showed that substitution of β1A with β1D impairs primary myogenesis with no direct effect on secondary myogenesis. Furthermore, long-term primary myotube survival was affected in β1D knock-in embryos. Finally, overexpression of β1D in C2C12 cells impaired myotube formation while overexpression of β1A primarily affected myotube maturation. Together these results demonstrate for the first time distinct roles for β1 integrins in primary versus secondary myogenesis and that the β1A and β1D variants are not functionally equivalent in this process.

Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse

“…Although a5b1, a4b1 and av integrins are not essential for myogenic cell fusion itself in vitro (Yang et al, 1996;Blaschuk et al, 1997;Taverna et al, 1998), cell engagement with the fibronectin matrix present within the myotome in vivo may serve to promote the approximation of myogenic cells, increasing the opportunities for fusion. The fact that b1D integrin knock-in and conditional b1 integrin knock-out mouse embryos show defects in myotube formation (Cachaço et al, 2003;Schwander et al, 2003) supports this hypothesis.…”

The impact of alternative splicing in vivo: Mouse models show the way

“…Mice homozygous for this mutation showed multiple defects in development and embryonic lethality around day 11.5 . Later experiments suggested compromised nutrition of the embryo caused by malformation of the placenta and decreased muscle formation (both tissues showing exclusive expression of the b1A isoform in wild type) to be the main reasons for the embryonic lethality (Cachaco et al 2003). Interestingly, some features of the defects observed in development of the neuronal tube resembled those found in Fibronectin deficient embryos (George et al 1993), which points to an essential interaction between Integrin b1A and Fibronectin in this process.…”

Integrins in regulation of tissue development and function

“…Indeed, it was shown that the expression of CD9, the integrin-associated tetraspanin that regulates in vitro myoblast and sperm-egg fusion [104], was strongly reduced on the surface of β1-deficient myoblasts [101]. Furthermore, analysis of knockin embryos in which β1D had been replaced by β1A [105] revealed a reduction in skeletal muscle mass, when the mice were bred on a predominantly FVB background [106]. This reduction appeared to be due to impaired primary myogenesis, whereas there was no direct effect on secondary myogenesis.…”

“…The use of cultured β1-null progenitor cells and the analysis of β1 chimeric mice have shown that β1 integrins are required for colonization of the fetal haematopoietic organs but not for the development of haematopoietic stem cells within those organs [105]. Studies using mice chimeric for either the α4, α5, or αv subunit, which are the β1-associated α subunits on leucocyte progenitors, indicate that none of these integrins is required for colonization of the fetal haematopoietic organs but they do implicate α4 in the multilineage development of haematopoietic cells and in lymphocyte homing to lymph nodes and Peyer's patches [106,107].…”