Hyperphosphatemia Contributes to Skeletal Muscle Atrophy in the Absence and Presence of Chronic Kidney Disease

PROJECT SUMMARY Chronic kidney disease (CKD) is a health epidemic that increases risk of death due to various comorbidities. Alterations in mineral metabolism are a hallmark of advanced CKD, where elevations in serum phosphate levels (hyperphosphatemia) have been suspected to directly contribute to tissue damage and mortality.

Hyperphosphatemia is clinically tackled in CKD patients by administration of phosphate binders and introduction of low phosphate diets. While the contributions of hyperphosphatemia to vascular calcification have been established and the underlying signaling mechanism in vascular smooth muscle cells has been

studied, the potential direct effects of elevated phosphate on other tissues and cell types are not understood. In our study, we focus on skeletal muscle wasting and atrophy which affects many patients with CKD. In our preliminary studies, we have characterized the skeletal muscle phenotype in two mouse models of CKD, one

genetic and one diet-induced, and our molecular, histological and functional analyses indicate reduced muscle strength and mass as well as reduced size of individual myofibers and activation of atrophy gene programs. Since these mice develop hyperphosphatemia, and our preliminary in vitro studies indicate that phosphate

elevations can induce atrophy in differentiated skeletal muscle cells (myotubes), we hypothesize that in CKD, elevated serum phosphate levels can directly affect skeletal muscle tissue and contribute to skeletal muscle wasting. Since phosphate uptake via specific phosphate transporters is essential for all cells and tissues for

house-keeping functions and survival, we cannot use loss-of-function approaches to study the direct effects of phosphate on skeletal muscle in mice. Instead, we will determine whether administration of a low phosphate diet, when initiated early on, protects CKD mice from developing skeletal muscle atrophy. Our phenotypic

analyses will include structural and functional studies by MRI and histology, and also focus on fibrosis, inflammation and increased fat content which are observed in skeletal muscle tissue of CKD patients. Our in vivo study is accompanied by cell culture experiments which analyze direct effects of phosphate on myotubes.

We will determine the involvement of phosphate transporters and the signaling mechanisms that mediate phosphate-induced atrophy in cultured myotubes. Our preliminary studies also indicate that healthy mice receiving a high phosphate diet for three months develop signs of skeletal muscle atrophy. While the injury

seems to be milder than in CKD mice, this finding indicates that hyperphosphatemia by itself, in the absence of kidney injury, can damage skeletal muscle. We will determine if prolonged administration of a high phosphate diet further increases skeletal muscle damage, and whether after three months of high phosphate diet the

transition to a diet with normal phosphate content reverses skeletal muscle injury. If successful, our study would indicate that lowering phosphate in CKD has protective effects on skeletal muscle tissue, and that diets rich in phosphate salts, such as all processed foods, can cause skeletal muscle injury in healthy individuals.