Low blood phosphate (Pi) reduces muscle function in hypophosphatemic disorders. However, it is unknown, which Pi transporters are required and whether hormonal changes due to hypophosphatemia contribute to muscle function in these disorders.
To address these questions we evaluated a series of tissue-specific knockout mice lacking one or two copies of the house-keeping Pi transporters Pit1 and Pit2 in skeletal muscle (sm). These mice were generated using a cross between floxed transporter alleles and human skeletal actin (HAS)-Cre.
Simultaneous tissue-specific deletion of both transporters in smPit1-/-;smPit2-/- mice caused marked skeletal muscle atrophy at birth, which severely impaired mobility, resulting in death by postnatal day P13. smPit1-/-, smPit2-/- and three allele mutant mice are fertile and have normal body weights, suggesting that the two transporters provide a high degree of redundance in skeletal muscle, however, these mice show a gene-dose dependent reduction in running activity resembling the impaired endurance seen in Hyp mice. In contrast to Hyp mice, which develop hypophosphatemia due to high circulating fibroblast growth factor 23 (FGF23), interestingly grip strength is preserved in our mice, which have low FGF23. Further evaluation of the mechanism shows reduced ERK1/2 activation and stimulation of AMP kinase in skeletal muscle from P10 smPit1-/-;smPit2-/- mice consistent with energy-stress. This can be modeled in C2C12 myoblasts, in which we can show that oxygen consumption rate (OCR) is regulated by Pi transport-dependent and ERK1/2-dependent metabolic Pi sensing pathways downstream of Pit1 and Pit2.
We here show for the first time that Pit1 and Pit2 are essential for normal myofiber function and survival. Insights gained from smPit1-/-;smPit2-/- and three allele knockout mice improve our understanding of metabolic Pi sensing in skeletal muscle, and of the relative contribution of hormonal changes to hypophosphatemic myopathy when compared to Hyp mice. Building on these data we plan to develop alternative strategies for the treatment of hypophosphatemic and possibly of other myopathies.