As life expectancy increases in the United States, age-related muscle weakness becomes a growing public health concern. Declines in strength limit mobility and increase susceptibility to injury in older individuals. Increased frailty not only impacts the quality of life of the individual but also places additional strain on the health care system. For these reasons and others, a more profound understanding of the aging process in skeletal muscle is critical for enabling Americans to remain healthy later in life. Depression of skeletal muscle excitability has been identified as a contributing factor to reduced strength in older individuals. In mammalian skeletal muscle, the L-type Ca channel serves as the voltage sensor for excitation-contraction (EC) coupling by triggering Ca release from the sarcoplasmic reticulum (SR) in response to depolarization of the plasma membrane. It has been established that impaired EC coupling contributes to age-related muscle weakness. This depression of excitability (termed """"""""EC uncoupling"""""""") in aging muscle is characterized by a reduction in SR Ca release which is a direct consequence of a fewer number of L-type channels present in the plasma membrane. Members of the RGK (Rad, Rem, Rem2, Gem/Kir) family of monomeric G proteins have been demonstrated to reduce L- type channel membrane expression, raising the possibility that RGK proteins mediate plastic changes in skeletal muscle that ultimately result in EC uncoupling. Thus, the purpose of this proposal is to investigate a potential role for RGK proteins in age-related muscle weakness.
Specific Aim #1 will seek to establish an innovative model system for the study of RGK protein function in adult skeletal muscle. To do so, cDNA plasmids encoding RGK proteins will be delivered to mouse muscle via in vivo electroporation. The validity of this new model system will be assessed with patch-clamp electrophysiology and Ca imaging. Using the methodology developed in Specific Aim #1, Specific Aim #2 will probe a role for RGK proteins in EC uncoupling in dennervated muscle.
Specific Aim #3 will employ real-time qRT-PCR and western blotting to determine whether RGK protein expression is increased in older mice. The findings of this study may lead to therapeutic applications in which pharmacological (or even genetic) targeting RGK proteins would slow the progression of age-related muscle weakness. In addition, information gathered in the course of this study may also contribute to a better understanding of muscle weakness associated with cachexia, diabetes and degenerative neuromuscular disease.
Age-related muscle weakness impacts the health and daily activities of a growing segment of the American population. Although depression of skeletal muscle excitability has been identified as a contributing factor to strength deficits in aging muscle, little is known about how this loss of muscle excitability develops over time. Thus, the overall goal of this research proposal is to provide information about the molecular mechanisms that underlie muscle weakness in older individuals.
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