Abstract
Skeletal muscle aging is accompanied by mass reduction and functional decline, as a result of multiple factors, such as protein expression, morphology of organelles, metabolic equilibria, and neural communication. Skeletal muscles are formed by multiple fibers that express different Myosin Heavy Chains (MyHCs) and have different metabolic properties and different blood supply, with the purpose to adapt their contraction to the functional need. The fine interplay between the different fibers composing a muscle and its architectural organization determine its functional properties. Immunohistochemical and histochemical analyses of the skeletal muscle tissue, besides evidencing morphological characteristics, allow for the precise determination of protein expression and metabolic properties, providing essential information at the single-fiber level. Aiming to gain further knowledge on the influence of aging on skeletal muscles, we investigated the expression of the MyHCs, the Succinate Dehydrogenase (SDH) activity, and the presence of capillaries and Tubular Aggregates (TAs) in the tibialis anterior muscles of physiologically aging C57BL/6J mice aged 8 (adult), 18 (middle aged), and 24 months (old). We observed an increase of type-IIB fast-contracting fibers, an increase of the oxidative capacity of type-IIX and -IIA fibers, a general decrease of the capillarization, and the onset of TAs in type-IIB fibers. These data suggest that aging entails a selective modification of the muscle fiber profiles.
Highlights
The contractile properties of a muscle are the result of finely coordinated functional properties of the different cells of which it is composed
The Myosin Heavy Chain (MyHC) isoforms expressed in a muscle fiber are the main markers of its contractile and metabolic properties [2,3]
Skeletal muscle aging includes macroscopic effects, such as the change of the muscle architecture [7], and modifications at the cellular level, which involve the alteration of the morphology, fiber-type shift [8], change in the excitation–contraction coupling proteins [9], capillaries network [10], and the onset of tubular aggregates (TAs), as a sign of skeletal muscle damage [11,12,13], among the others
Summary
The contractile properties of a muscle are the result of finely coordinated functional properties of the different cells of which it is composed. The idea that skeletal muscle function declines due to an overall skeletal muscle architecture modification, or a loss of the fine interplay that characterizes muscle contraction, rather than a change in single fiber parameters [7,14], guided our interest toward further elucidation of the properties of skeletal muscle during the aging progression In this context, immunohistochemical and histochemical analyses of the skeletal muscle tissue allow for both the precise determination of protein expression and metabolic properties at the single-fiber level and provide important information on the overall structure of the tissue. In order to elucidate whether aging entails modification of structural, immunohistochemical, and metabolic properties, we performed a study on physiologically aging C57BL/6J mice, which entailed the analyses of the expression of the MyHC isoforms, the presence of capillaries, Succinate Dehydrogenase (SDH) activity, and presence of TAs to define the fiber profile of the fast-contracting muscle tibialis anterior of adult, middle-aged, and old mice [15,16]. Our data show that aging tibialis anterior muscle undergoes specific modifications, such as the increase of type-IIB fibers, the increase of SDH activity in a fiber-type specific manner, and the onset of TAs in type-IIB fibers, leading to suggest that the decline in skeletal muscle performance upon age progression is a multifactorial process
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