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Contractility and myosin heavy chain content of skeletal muscle engineered from adult and aged rats

Functional three-dimensional skeletal muscle constructs (myooids) were engineered from myogenic cells harvested from the muscles of adult and aged rats.  Myooids formed by self-organization of myogenic cells in the absence of an artificial scaffold and were maintained in culture.  Myooid excitability and contractility were measured at days 1, 3, 7, and 24 after myooid formation.  Skeletal muscle myosin heavy chain (MHC) content was measured and MHC isoforms were separated on SDS-PAGE gels and quantified.  Adult and aged rat myooids had only ~35-60% of the skeletal muscle MHC content of control skeletal muscle from rats, the remaining MHC content consisting of isoforms found in cultured fibroblasts but not in control skeletal muscle.  In addition, myooids expressed only developmental isoforms of skeletal muscle MHC, known to generate less specific force than adult isoforms.  Myooids from aged rats generated greater normalized force than myooids from adult rats, but only 1-3% of that for adult skeletal muscle controls.  Normalizing for skeletal muscle MHC content and isoform expression predicts myooid forces of ~6-18% of control adult skeletal muscles.  We hypothesize that the remaining force deficit is due to cellular and sub-cellular disorganization—Myooids lack the density and organization of sarcomeric arrays seen in skeletal muscle.