Mice Lacking Skeletal Muscle Actin Show Reduced Muscle Strength and Growth Deficits and Die during the Neonatal Period

Abstract

All four of the muscle actins (skeletal, cardiac, vascular, and enteric) in higher vertebrates show distinct expression patterns and display highly conserved amino acid sequences. While it is hypothesized that each of the muscle isoactins is specifically adapted to its respective tissue and that the minor variations among them have developmental and/or physiological relevance, the exact functional and developmental significance of these proteins remains largely unknown. In order to begin to assess these issues, we disrupted the skeletal actin gene by homologous recombination. All mice lacking skeletal actin die in the early neonatal period (day 1 to 9). These null animals appear normal at birth and can breathe, walk, and suckle, but within 4 days, they show a markedly lower body weight than normal littermates and many develop scoliosis. Null mice show a loss of glycogen and reduced brown fat that is consistent with malnutrition leading to death. Newborn skeletal muscles from null mice are similar to those of wild-type mice in size, fiber type, and ultrastructural organization. At birth, both hemizygous and homozygous null animals show an increase in cardiac and vascular actin mRNA in skeletal muscle, with no skeletal actin mRNA present in null mice. Adult hemizygous animals show an increased level of skeletal actin mRNA in hind limb muscle but no overt phenotype. Extensor digitorum longus (EDL) muscle isolated from skeletal-actin-deficient mice at day 2 to 3 showed a marked reduction in force production compared to that of control littermates, and EDL muscle from hemizygous animals displayed an intermediate force generation. Thus, while increases in cardiac and vascular smooth-muscle actin can partially compensate for the lack of skeletal actin in null mice, this is not sufficient to support adequate skeletal muscle growth and/or function.Actin forms the core of the thin filaments that are found in essentially all eukaryotic cells. It is required for cellular functions ranging from the generation and translation of mechanical force via a sliding-filament mechanism involving myosin filaments to the formation of rigid structures such as those found in intestinal microvilli and stereocilia. The actin gene family in vertebrates is comprised of six closely related proteins that are expressed in complex developmental and tissue-specific patterns (17,33). All six of the functional actin genes reside on different chromosomes. This multigene family appears to have arisen by duplication after the separation of the vertebrates and urochordates (11). Two nonmuscle actins, cytoplasmic ␤-and ␥-actin, are found in nonmuscle cells, and four actins which are very similar to one another (skeletal, cardiac, vascular, and enteric actin) comprise the major isoforms found in the adult muscle types for which they are named.The primary sequences of the six isoactins are very similar. The cytoplasmic actins differ from the muscle actins at about 25 of the 374 amino acid residues that make up their primary structure. These replacements a...

Requirement for serum response factor for skeletal muscle growth and maturation revealed by tissue-specific gene deletion in mice

“…Alternatively, the reduction in expression of one or more SRF target genes, ␣-actin, for example, could perturb myofiber growth through secondary mechanisms. In this regard, ␣-skeletal actin knockout mice die during the perinatal period from abnormalities in skeletal muscle growth and force generation (46). Consistent with the notion that SRF plays a role in hypertrophic growth of skeletal muscle, SRF expression is up-regulated during load-induced hypertrophy of skeletal muscle (47), and the CArG box in the ␣-skeletal actin promoter is a target for hypertrophic signaling (48).…”

CREB-binding protein (CBP) regulates β-adrenoceptor (β-AR)−mediated apoptosis

“…48 However, cardiac skeletal a-actin (Aska) was significantly increased in the KO tissue. On the basis of gene knock-out studies, it is believed that Aska contributes to muscular strength and contractility, 49 and thus our results suggest that blocking Bim expression in the heart will maintain the bAR-mediated inotropic function and only prevent the apoptotic arm of the signal transduction pathway.…”

Cell biology of embryonic migration

“…Mouse knockouts of cell migration components or other proteins with direct effects on cell migration Protein Function Type Phenotype Reference α skeletal muscle actin Actin Cytoskeleton Total Postnatal death at P1-9, marked loss of body weight; upregulation of other actin isoforms (Crawford et al, 2002) α smooth muscle actin Actin Cytoskeleton Total Viable; impaired vascular contractility and blood pressure homeostasis; upregulation of other actin isoforms (Schildmeyer et al, 2000) α cardiac actin Actin Cytoskeleton Total Perinathal lethality; cardiac hypertrophy and heart muscle abnormalities; upregulation of other actin isoforms (Kumar et al, 1997) β non-muscle actin Actin Cytoskeleton Total Death after E9.5 (Shawlot et al, 1998) γ non-muscle actin Actin Cytoskeleton Skeletal muscle-specific Muscle weakness, necrosis and degeneration (Sonnemann et al, 2006) Tropomyosin Actin Cytoskeleton Total Death before morula stage (Hook et al, 2004) Mena Actin Cytoskeleton Total Viable, with misrouted axons and defects in the nervous system (Lanier et al, 1999) Mena, VASP, Evl triple null Actin Cytoskeleton Total Defects in brain development, neuritogenesis, and neural tube closure Filamin-B Actin Cytoskeleton Total Skeletal malformations and impaired microvascular development (Zhou et al, 2007) Gelsolin (or ADF) Actin Cytoskeleton Total Defects in fibroblast and platelet motility and lamellar responses (Witke et al, 1995) Nonmuscle myosin II-B Actin Cytoskeleton Total Embryonic and perinatal lethality with severe heart defects (Tullio et al, 1997) Myosin heavy chain II-A Actin Cytoskeleton Total Failure in embryonic patterning, embryonic lethality by E7.5 (Conti et al, 2004) Cardiac alpha myosin, heavy chain Actin Cytoskeleton Total Embryonic lethality between E11 and 12 with gross heart defects (Jones, 1996) (Imamoto and Soriano, 1993;Nada et al, 1993) Ephrin B1 Transmembrane signaling Total Neural crest cell misguidance (cranial and cardiac, but not trunk) (Davy et al, 2004) Angiomotin Transmembrane signaling Total Death between E11-E11.5, severe vascular insufficiency in intersomitic regions, dilated vessels in the brain (Aase et al, 2007) Birth Defects Res C Embryo Today. Author manuscript; available in PMC 2009 June 1.…”