William A. Mohler, Ph.D.
University of Connecticut Health Center

New Approaches to Characterizing Cellular Changes in Sarcopenia.

Sarcopenia, progressive loss of muscle mass and quality, is the principle component of the profound changes in body composition that occur with aging, and it is a large factor in loss of strength, deficits in gait, and reduced metabolic rate in the elderly. We will apply two novel methods to characterize the cellular changes underlying the problem of sarcopenia:

1. Live tissue imaging by Second Harmonic Imaging Microscopy (SHIM). Within the past year we have discovered that endogenous proteins within the skeletal muscle myofilament lattice generate remarkably high-contrast, optical sectioning images when viewed by SHIM. This new mode of non-linear laser-scanning microscopy requires no fixation or staining of tissue, acquires 3-dimensional sub-micron detail throughout the full depth of thick muscle tissue, is impervious to photobleaching, should not be obscured by age-accumulated autofluorescence in the tissue. This allows high-resolution histology of native muscle tissue samples, with essentially no sample preparation. Furthermore, the signal in SHIM is sensitive to the local density and alignment of contractile proteins within muscle sarcomeres, and therefore should allow quantitative assessment of not only the size and numbers of muscle fibers, but also the “health” of the contractile cytoskeleton within them. SHIM can also be combined with two-photon epi-fluorescence microscopy to allow simultaneous imaging of both the second harmonic and fluorescence profiles of the same sample. Several aspects of aging will be studied at the levels of both histology and molecular structure in muscle tissue of humans, mice, and nematodes using these novel imaging techniques.

2. In vitro assay of muscle regenerative capacity through studies of cultured satellite myoblasts. Like bone, muscle is subject to a continual rate of turnover during aging. In the case of muscle, fibers damaged by use or injury are repaired by proliferation and fusion of cells descended from tissue-resident satellite myoblasts. With the progression of sarcopenia, weakened muscle is more subject to damage, yet less efficient at repair. This begs the questions of how many proliferation-competent/fusion-competent satellite cells can actually be activated for regeneration in aged muscle tissue, and whether they are able to respond to exogenous stimulatory treatments. To approach these questions, we will use a virus-transduced reporter-gene assay system for high-throughput quantitation of the differentiation and fusion of myoblasts. Satellite cells dissociated from aged and young mice will be compared with respect to the key parameters of proliferative capacity, and ability to fuse. Once aged myoblast cultures are established, they will be tested for response to a panel of known pharmacological stimulators and inhibitors of proliferation, differentiation, and fusion. As these agents have generally been characterized on only cultures of fetal or neonatal myoblasts, we will ask whether the same responses can be elicited from aged myoblasts.

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