Inorganic phosphate (Pi) is critical for cellular functions, including ATP synthesis. The mitochondrial Pi transporter PIC is implicated in cardiomyopathy and myopathy in humans. Mice lacking the cytomembrane Pi transporters Pit1 and Pit2 die in the first weeks of life due to a progressive myopathy, whereas three allele mutants have reduced exercise capacity, highlighting the importance of Pi transport and sensing in muscular and cardiac health.
We next generated mice with germline (HA-hPIT1tg/+) and skeletal muscle-specific (smHA-hPIT1tg/tg) over-expression of HA-tagged-hPIT1. Homozygous germline HA-hPIT1 is lethal within a few days of birth and heterozygous mice display a 18-28% reduction in lifespan. MRI analysis at 9 months showed a 25% reduction in body weight in HA-hPIT1tg/+ mice (F(emales): 22±0.4g, p=0.01; M(males): 29±0.8g, p<0.0001) (p values subsequently always compared to wildtype mice). These mice consumed more food (F: 6.0±0.2g, p=0.07; M: 5.6±0.3g, p=0.003) and water (F: 9.2±1.2ml, p<0.0001; M: 7.0±0.8ml, p=0.002), had higher energy expenditure (F: 18±0.5kcal, p=ns; M: 16±0.5kcal, p=0.02), and an increased respiratory exchange ratio (F: 0.92±0.006, p<0.0001; M: 0.91±0.007, p=0.008).
Male smHA-hPIT1tg/tg mice demonstrated a 10% increase in lean body weight (35±1.0g, p=0.04) with no significant changes in other metabolic parameters. Forelimb grip strength and voluntary wheel running endurance tests conducted at 2, 4, 8, and 12 months of age showed that the decline in endurance with age was slower in transgenic male mice (HA-hPIT1tg/+: p=0.03; smHA-hPIT1tg/tg: p=0.02) but unchanged in females. The decline in absolute grip strength with age was similar across all groups.
The hypermetabolic state and shift to carbohydrate oxidation in HA-hPIT1tg/+ mice suggest an important role for Pit1 in metabolic Pi sensing. The gender-dependent effects on endurance and weight in mice are indicative of Pit1’s significant role, particularly in skeletal muscle. Understanding the function of these cytomembrane Pi transporters and their interaction with the mitochondrial transporter PIC is crucial, given its implications in human diseases like cardiomyopathy and myopathy. Our findings offer valuable insights into the mechanisms of Pi sensing and its impact on muscle and cardiac function, highlighting potential therapeutic targets for mitochondrial muscular dystrophies and related conditions.