Klotho: a paracrine mediator of skeletal muscle regeneration

Thanks to advances in medical technology, people are living longer than ever before. While this increase in lifespan is exciting, it is not necessarily accompanied with a longer health-span. Increasing age is typically accompanied by a progression of tissue dysfunction. Therefore, strategies to identify key factors that promote healthy aging are needed. Age-related declines in skeletal muscle result in impaired regeneration and functional immobility. Such declines are associated with an increased morbidity in an elderly population, thereby making the need for interventions to counteract the effect of age on skeletal muscle vitality an important public health concern. Fortunately, elegant studies using heterochronic parabiosis have suggested systemic factors, such as proteins and extracellular vesicles (EVs), present in a youthful circulation may “rejuvenate” healing capacity of aging tissues. Studies in this thesis revealed the role of one such circulating longevity factor, Klotho, in mediating the regenerative response of skeletal muscle. Specifically, findings demonstrated that Klotho is upregulated within the muscle and circulation of young mice after an injury, but that the response attenuated with aging. This decreased Klotho expression resulted in disrupted muscle progenitor mitochondrial ultrastructure and myogenic lineage progression, ultimately contributing to a blunted skeletal muscle regenerative capacity. However, supplementation with Klotho restored mitochondria of aged cells and enhanced functional regeneration of aged muscle in vivo. These findings suggest a paracrine role of Klotho in the skeletal muscle regenerative cascade. Yet, while Klotho plays a major role in cellular maintenance at the organismal level, the protein itself is highly unstable and prone to degradation. Therefore, we sought to identify whether there may be mechanisms for protecting Klotho expression within the circulation, for example through storage within circulating EVs. Indeed, population-level profiling of EVs revealed Klotho mRNA expression in an age-dependent manner. This loss of Klotho mRNA over time is important, as the transfer of Klotho mRNA from young EVs to target aged cells enhanced target cell myogenic capacity and mitochondrial function. In a gain-of-function paradigm, we showed that aged muscle transplanted with young EVs displayed an increased functional regeneration, though the benefit was blunted when EVs were isolated from mice heterozygously deficient of Klotho. Collectively, these studies suggest a novel therapeutic role for Klotho, which we propose may ultimately be used to mitigate the impaired muscle healing capacity of the growing geriatric population.