ABSTRACT
Spinal muscular atrophy (SMA) is characterized by degeneration of motor neurons of the spinal cord associated with muscle paralysis and caused by mutations of the survival motor neuron gene (SMN). To determine whether SMN gene defect in skeletal muscle might have a role in SMA pathogenesis, deletion of murine SMN exon 7, the most frequent mutation found in SMA, has been restricted to skeletal muscle by using the Cre-loxP system. Mutant mice display ongoing muscle necrosis with a dystrophic phenotype leading to muscle paralysis and death. The dystrophic phenotype is associated with elevated levels of creatine kinase activity, Evans blue dye uptake into muscle fibers, reduced amount of dystrophin and upregulation of utrophin expression suggesting a destabilization of the sarcolemma components. The mutant mice will be a valuable model for elucidating the underlying mechanism. Moreover, our results suggest a primary involvement of skeletal muscle in human SMA, which may contribute to motor defect in addition to muscle denervation caused by the motor neuron degeneration. These data may have important implications for the development of therapeutic strategies in SMA.
Subject(s)
Exons/genetics , Muscle, Skeletal/metabolism , Muscle, Skeletal/pathology , Muscular Atrophy, Spinal/genetics , Muscular Dystrophies/pathology , Nerve Tissue Proteins/genetics , Sequence Deletion/genetics , Animals , Biomarkers , Cell Size , Creatine Kinase/metabolism , Cyclic AMP Response Element-Binding Protein , Cytoskeletal Proteins/metabolism , Dystrophin/metabolism , Evans Blue/metabolism , Fluorescent Antibody Technique , Membrane Proteins/metabolism , Mice , Motor Neurons/metabolism , Motor Neurons/pathology , Muscle Fibers, Skeletal/metabolism , Muscle Fibers, Skeletal/pathology , Muscular Dystrophies/enzymology , Muscular Dystrophies/metabolism , Neuromuscular Junction/metabolism , RNA-Binding Proteins , SMN Complex Proteins , Sarcolemma/metabolism , Sarcolemma/pathology , UtrophinABSTRACT
Deletion of the murine survival of motor neuron gene (SMN) exon 7, the most frequent mutation found in spinal muscular atrophy (SMA) patients, directed to neurons but not to skeletal muscle, enabled generation of a mouse model of SMA providing evidence that motor neurons are the primary target of the gene defect. Moreover, the mutated SMN protein (SMNDeltaC15) is dramatically reduced in the motor neuron nuclei and causes a lack of gems associated with large aggregates of coilin, a coiled-body-specific protein. These results identify the lack of the nuclear targeting of SMN as the biochemical defect in SMA.
Subject(s)
Cell Nucleus/metabolism , Muscular Atrophy, Spinal/genetics , Nerve Tissue Proteins/genetics , Animals , Base Sequence , Cyclic AMP Response Element-Binding Protein , DNA Primers , Disease Models, Animal , Exons , Gene Deletion , Genes, Lethal , Homozygote , Mice , Motor Neurons/metabolism , Muscular Atrophy, Spinal/metabolism , Nerve Tissue Proteins/chemistry , Phenotype , RNA-Binding Proteins , SMN Complex ProteinsABSTRACT
Spatially and temporally regulated somatic mutations can be achieved by using the Cre/LoxP recombination system of bacteriophage P1. In order to develop gene knockouts restricted to striated muscle, we generated a transgenic mouse line expressing Cre recombinase under the control of the human alpha-skeletal actin promoter. Specific excision of a loxP-flanked gene was demonstrated in striated muscle, heart and skeletal muscle, in a pattern very similar to the expression of the endogenous alpha-skeletal actin gene. Therefore, the reported transgenic line can be used to target inactivation or activation of a given gene to the skeletal muscle lineage.
