VEGF and Skeletal Muscle Adaptation during Chronic Overload

Huey, Kimberly

Abstract

Skeletal muscle normally responds to chronic elevations in load and activity by increasing mass, strength, and/or fatigue resistance. However, inadequate responses can lead to the muscle dysfunction and weakness that is often observed with aging or diseases such as cancer and diabetes. Hypertrophy induced by functional overload (FO) provides an in vivo model to study muscle growth. Myogenesis and angiogenesis are both necessary to increase muscle mass. An essential factor for regulating capillary maintenance, endothelial cell and myocyte survival, and exercise-induced angiogenesis is Vascular Endothelial Growth Factor (VEGF). Therefore, our central hypothesis is that VEGF expressed by mature myofibers is critical for FO-induced hypertrophy, angiogenesis and myocyte survival and that Hsp25 phosphorylation (pHsp25) is a key step in these processes. Utilizing FO in mice with a unique inducible muscle specific VEGF deletion (mVEGF-/-) or mice overexpressing wild type Hsp27 (human form of Hsp25) (hspTG) or a non-phosphorylatable Hsp27 mutant (mut-hspTG), we will test the hypothesis that (p)Hsp25 is a critical mediator of the angiogenic, myogenic and/or anti-apoptotic VEGF functions that stimulate hypertrophy and improved contractile function. This project will determine if a VEGF-(p)Hsp25 pathway is a key mechanism facilitating a balance between hypertrophy and capillarity during FO.
Aim 1 experiments will test if myofiber- expressed VEGF is essential for increasing mass, strength, and fatigue resistance through a (p)Hsp25 dependent mechanism. FO-induced changes in plantaris mass, maximal isometric force, fatigability, and growth (fiber cross sectional area and contractile protein accretion) will be measured in mVEGF+/+, mVEGF-/-, wild type (WT), hspTG, or mut-hspTG mice 7, 14, or 30 days after sham (S) or FO surgery.
Aim 2 experiments will test if myofiber-expressed VEGF is necessary for increasing muscle capillarity and reducing apoptosis through a (p)Hsp25 mechanism in response to FO. Experiments will compare mVEGF+/+, mVEGF-/, WT, hspTG, mut-hspTG mice at 7, 14 and 30 d for changes in capillary:fiber ratio, capillary density, and apoptosis. Western blot and immunohistochemistry will quantify/localize VEGF, (p)Hsp25, caspase-3, (p)Akt, and (p)p38. (p)Hsp25 interactions with caspase-3 and/or Akt will be determined by immunoprecipitation. Caspase-3 activity will be quantified by a luciferase substrate assay. The inability of skeletal muscle to respond to a growth stimulus ultimately impairs muscle function and negatively impacts quality of life and mobility especially in the aging population. These studies, utilizing inducible muscle specific VEGF deletion or transgenic Hsp27 models will provide evidence of the physiological impact of the loss of VEGF and/or the VEGF-(p)Hsp25 pathway on muscle function and adaptation. Discovering VEGF/(p)Hsp25-dependent mechanisms in muscle will lead to the development of methods to ameliorate the well-known decline in muscle function that occur with injury, disease and/or aging.

Public Health Relevance

With chronic disease and/or aging there is a significant decrease in muscle strength and integrity that ultimately limits mobility and increases susceptibility to injury. Unlike healthy individuals the ability to increase muscle mass and strength through mechanical loading may be impaired due to reduced VEGF and/or heat shock protein 25 (Hsp25). Thermal, cell-based, or pharmacological strategies to augment VEGF and/or Hsp25 expression can have a positive impact on maintaining and improving muscle function in individuals with muscle weakness, thereby improving quality of life and reducing long-term medical costs.