Here is the surprising twist: deleting cyclophilin D made male mice more vulnerable to diet-induced obesity, yet it also increased ADP-stimulated mitochondrial respiration specifically in skeletal muscle. Mitochondrial dysfunction is a hallmark of obesity-associated metabolic disease, and the mitochondrial permeability transition pore is a key regulator of cellular energy homeostasis.

Cyclophilin D, encoded by Ppif, is the principal regulatory component of this pore and has been implicated in insulin secretion, adipocyte survival, and whole-body energy balance. The unresolved question is whether Ppif loss produces a consistent metabolic phenotype during high-fat diet exposure and whether systemic changes reflect tissue-specific mitochondrial oxidative capacity.

Cyclophilin D is a matrix peptidyl-prolyl cis-trans isomerase that facilitates mitochondrial permeability transition pore opening by directly binding pore constituents, including the adenine nucleotide translocator. Beyond cell death regulation, cyclophilin D has emerged as a broader regulator of mitochondrial metabolism, with implications for whole-body energy balance and glucose homeostasis.

Reports on its metabolic role are highly divergent, describing protection from, susceptibility to, or altered progression of diet-induced obesity and glucose intolerance in deficient mice. Whether whole-body metabolic alterations are mechanistically linked to tissue-specific mitochondrial oxidative capacity has not been fully elucidated.

The study characterized male whole-body Ppif knockout mice during thirty-four weeks of standard or high-fat diet exposure, using body composition, indirect calorimetry, glucose and insulin tolerance testing, and mitochondrial respiratory measurements across tissues. The stated finding is that cyclophilin D deficiency exacerbates diet-induced obesity in a dietary context-dependent manner and selectively affects adiposity, energy expenditure, and skeletal muscle mitochondrial respiration.

The mice were maintained on a mixed C57BL/6:129 genetic background, and the study used males housed on a twelve-hour light and twelve-hour dark cycle at twenty-two degrees Celsius. They received either standard chow or a high-fat diet containing sixty percent fat, with food and water available without restriction.

High-fat diet feeding began at twenty point nine weeks of age and continued for thirty-four weeks. Fat and lean mass were quantified using NMR technology, while energy expenditure, respiratory quotient, food intake, and locomotor activity were assessed with combined indirect calorimetry.

After at least twenty-four hours of acclimation to metabolic cages, measurements were collected simultaneously over seventy-two hours. Glucose and insulin tolerance testing followed a six-hour fast and used intraperitoneal glucose or insulin injections. At twenty point nine weeks on standard diet, knockout mice had significantly higher body weight and fat mass, while lean mass was comparable between genotypes.

Food intake remained unchanged, but dark-phase locomotor activity was significantly increased in knockout mice relative to wild-type mice. Energy expenditure adjusted for lean mass was comparable between genotypes during both the light and dark phases. Glucose tolerance showed no difference, while blood glucose at one hundred twenty minutes during the insulin tolerance test was higher in knockout mice, without a statistically significant overall area-under-the-curve difference.

The mice were divided into weight-matched standard-diet and high-fat-diet cohorts, and body weight and body composition were monitored for thirty-four weeks. Under standard diet, body weight trajectories and percentage weight gain were comparable between wild-type and knockout mice throughout monitoring.

Under high-fat diet, knockout mice showed significantly greater body weight from weeks twenty-eight to thirty-four. After thirty-four weeks, high-fat-diet knockout mice had greater fat mass gain and lean mass gain, while neither measure differed between genotypes under standard diet.

In the standard-diet baseline panels, the body-weight bars in panel b are separated by roughly two grams, and the fat-mass bars in panel c by about two grams, while panel d shows nearly overlapping lean-mass bars. In the high-fat-diet trajectory in panel n, the knockout curve pulls away from the wild-type curve late in the study, with the gap reaching roughly fifteen grams by the final time point.

After sixteen weeks of high-fat feeding, knockout mice showed significantly increased body weight and fat mass, while lean mass was comparable across genotypes. Continuous seventy-two-hour monitoring showed comparable average food intake and locomotor activity between groups.

The dark-phase respiratory quotient was significantly lower in knockout mice, indicating higher utilization of lipids as fuel substrate. After adjustment for lean mass, ANCOVA confirmed a significant genotype effect on energy expenditure, with lean mass a strong predictor and a significant genotype-by-lean-mass interaction.

At week fifteen of standard-diet exposure, glucose excursions and area-under-the-curve values were comparable in wild-type and knockout mice. High-fat diet produced more pronounced and higher glucose excursions in both groups, but the genotype difference was not statistically significant.

During insulin tolerance testing, standard-diet groups had comparable glucose decreases, while high-fat-diet groups had a blunted glucose-lowering response relative to standard-diet animals. Overall, Ppif deficiency did not significantly alter glucose tolerance or insulin sensitivity under either diet at these timepoints.

In the lower insulin-tolerance panels, both diet groups show glucose falling sharply by thirty minutes and approaching their lowest levels around sixty to ninety minutes, with a modest rebound by one hundred twenty minutes. The adjacent area-under-curve plots show substantial point-to-point spread within each genotype, especially for the high-fat diet, while the bar heights remain closely aligned.

Mitochondrial respiration assays and citrate synthase activity measurements were performed in isolated quadriceps, brown adipose tissue, and liver mitochondria after thirty-four weeks of standard or high-fat diet. In quadriceps mitochondria, ADP-stimulated State III respiration was significantly increased in knockout mice under both diets, with no additional significant differences at other respiratory states.

Respiratory control ratio values in quadriceps were not different between genotypes under either dietary condition. In brown adipose tissue, oxygen consumption profiles and respiratory control ratio values were comparable between wild-type and knockout mice under both diets.

Whole-body Ppif deficiency exacerbated age-onset obesity when twenty-week-old mice were exposed to high-fat diet for up to thirty-four weeks. Under standard diet, deficient mice had modestly elevated fat mass despite unchanged food intake, lean mass, and weight trajectories.

Energy expenditure adjusted for lean mass was higher in deficient mice under both diets, with a significant genotype-by-lean-mass interaction in high-fat-fed knockout mice. After thirty-four weeks of high-fat diet, obese Ppif-deficient mice showed selectively increased ADP-stimulated oxidative phosphorylation in quadriceps, but not in brown adipose tissue or liver.

The increase occurred without changes in mitochondrial abundance or coupling efficiency. These data indicate that cyclophilin D loss enhances skeletal muscle oxidative responsiveness while other metabolic tissues remain comparatively unaffected.

Together, the findings establish diet-dependent and tissue-specific effects of cyclophilin D on whole-body metabolism, particularly within skeletal muscle. A major discrepancy with earlier reports of reduced obesity susceptibility in Ppif-deficient mice arises from differences in energy expenditure analysis.

Using ANCOVA with lean mass as the covariate, the study identifies lean mass as the primary determinant of energy expenditure and shows that genotype modifies the energy-expenditure and lean-mass relationship. A significant lean-mass-by-genotype interaction does not necessarily mean higher energy expenditure in knockout versus wild-type mice when lean mass is comparable.

The calorimetry data, together with increased lean mass in high-fat-fed knockout mice, identifies skeletal muscle as a key tissue for future investigation. A limitation is the lack of causality between enhanced mitochondrial respiratory capacity, altered energy expenditure, and higher adiposity.

Future studies may assess substrate utilization, metabolic flux, and indirect calorimetry at earlier stages of high-fat diet exposure. Cyclophilin D deficiency worsened obesity during high-fat feeding while selectively enhancing quadriceps mitochondrial respiration. The result points to a diet- and tissue-specific metabolic role, but the study does not establish causality between respiration, energy expenditure, and adiposity.