Klotho deficiency promotes skeletal muscle weakness and is associated with impaired motor unit connectivity

Muscle wasting and weakness are important clinical problems that impact quality of life and health span by restricting mobility and independence, and by increasing the risk for physical disability. The molecular basis for this has not been fully determined. Klotho expression is downregulated in conditions associated with muscle wasting, including aging, chronic kidney disease, and myopathy. The objective of this study was to investigate a mechanistic role for Klotho in regulating muscle wasting and weakness. Body weight, lean mass, muscle mass, and myofiber caliber were reduced in Klotho-deficient mice. In the tibialis anterior muscle of Klotho null mice, type IIa myofibers were resistant to changes in size, and muscle composition differed with a higher concentration of type IIb fibers to the detriment of type IIx fibers. Glycolytic enzymatic activity also increased. The composition of the soleus muscle was unaffected and myofiber caliber was reduced comparably in type I, IIa, and IIx fibers. Muscle contractile function declined in Klotho-deficient mice, as evidenced by reduced absolute twitch and torque, and decreased rates of contraction and relaxation. RNA-sequencing analysis identified increased transcriptional expression of synaptic and fetal sarcomeric genes, which prompted us to test effects on muscle innervation. Klotho-deficiency induced morphological remodeling of the neuromuscular junction, myofiber denervation, and a functional loss of motor units. Loss of motor units correlated with absolute torque. Collectively, our findings have uncovered a novel mechanism through which Klotho-deficiency leads to alterations to the muscle synapse affecting motor unit connectivity that likely influences muscle wasting and weakness. Key points summaryO_LIMaintaining skeletal muscle mass and function is critical to preserve physical capacity and independence. Clinical observations implicate longevity factor Klotho as a key regulator of muscle mass and weakness. Low Klotho levels are reported to correlate with muscle weakness and frailty. C_LIO_LIUsing Klotho null mice, our study shows that Klotho-deficiency promotes skeletal muscle weakness and impaired motor unit connectivity. C_LIO_LIRNA-sequencing analysis identified altered expression of sarcomeric and synaptic genes suggesting changes to the muscle synapse in Klotho-deficient mice. C_LIO_LIHistopathological analyses revealed Klotho-deficiency is associated with reduced myofiber caliber, altered muscle composition, and increased prevalence of NCAM+ denervated fibers. Imaging of the NMJ further showed morphological changes and reduced area of synaptic contact. C_LIO_LIOverall, our findings show that Klotho regulates the structure and function of the NMJ affecting motor unit connectivity which may have an important role in the pathogenesis of muscle wasting and weakness. C_LI