Low blood phosphate (Pi) reduces muscle function in hypophosphatemic disorders. We previously found that mice with conditional ablation of the house-keeping phosphate transporters Pit1 and Pit2 in skeletal muscle developed a generalized skeletal muscle myopathy and died in the second week of life. This phenotype can be rescued by adenoviral or germline expression of HA-tagged human PIT1 under the control of the cytomegalovirus/chicken beta-actin/rabbit beta-globin gene (CAG) promoter in a cross with our previously described transgenic HA-hPIT1tg/+ mouse line.
Pit1 and Pit2 expression in skeletal muscle peaks at 120 days of life and subsequently declines by 50% in 18 mo. old wildtype (WT) mice. To address the question, whether HA-hPIT1 could compensate for the decline of Pit1 expression with age in WT mice, we tested forelimb grip strength (GS), voluntary wheel running (VWR) and assessed the energy metabolism of mice with germline (HA-hPIT1tg/+) and skeletal muscle (sm) targeted transgenic CAG-mediated constitutive expression of HA-hPIT1 (smHA-hPITtg/tg). Both mouse lines are fertile and healthy without apparent reduction in lifespan when compared to WT mice. Furthermore, HA-hPIT1tg/+ mice have improved muscle function when compared to wildtype (WT) littermates, whose GS and VWR declines progressively between 4 and 12 mo. of age. Small animal MRI showed reduced body weight and fat mass at 9 mo., while lean mass was preserved. Metabolic cage studies revealed that HA-hPIT1tg/+ mice consume more feed and water, have higher energy expenditure, and an increased respiratory exchange ratio (females: 0.92+/-0.006, n=8, p<0.001, vs. WT 0.85+/-0.01, n=12; males: 0.91+/-0.007, n=10, p=0.04, vs. WT 0.87+/-0.01, n=9). Observed differences were generally more pronounced in males. Evaluation of the smHA-hPIT1tg/tg line has thus far progressed to 2 mo. of age, at which male VWR activity is significantly increased (36810+/-2146 wheel turns/day, n=7, p=0.008 vs. WT 28245+/-1618, n=11), while body weight and GS are still unchanged at this age compared to WT. In the aggregate, our findings are consistent with a shift of substrate utilization in muscle mitochondrial metabolism, whereby transgenic expression of HA-hPIT1 reverses the aging-related decline in skeletal muscle function of mice. These data further support an important, but thus far little explored role of metabolic phosphate sensing by PIT1 in skeletal muscle.