The tight interrelationship between peroxisomes and mitochondria is illustrated by their cooperation in lipid metabolism, antiviral innate immunity and shared use of proteins executing organellar fission. In addition, we previously reported that disruption of peroxisome biogenesis in hepatocytes severely impacts on mitochondrial integrity, primarily damaging the inner membrane. Here we investigated the molecular impairments of the dysfunctional mitochondria in hepatocyte selective Pex5 knockout mice. First, by using blue native electrophoresis and in-gel activity stainings we showed that the respiratory complexes were differentially affected with reduction of complexes I and III and incomplete assembly of complex V, whereas complexes II and IV were normally active. This resulted in impaired oxygen consumption in cultured Pex5(-/-) hepatocytes. Second, mitochondrial DNA was depleted causing an imbalance in the expression of mitochondrial- and nuclear-encoded subunits of the respiratory chain complexes. Third, mitochondrial membranes showed increased permeability and fluidity despite reduced content of the polyunsaturated fatty acid docosahexaenoic acid. Fourth, the affected mitochondria in peroxisome deficient hepatocytes displayed increased oxidative stress. Acute deletion of PEX5 in vivo using adeno-Cre virus phenocopied these effects, indicating that mitochondrial perturbations closely follow the loss of functional peroxisomes in time. Likely to compensate for the functional impairments, the volume of the mitochondrial compartment was increased several folds. This was not driven by PGC-1α but mediated by activation of PPARα, possibly through c-myc overexpression. In conclusion, loss of peroxisomal metabolism in hepatocytes perturbs the mitochondrial inner membrane, depletes mitochondrial DNA and causes mitochondrial biogenesis independent of PGC-1α.

Peroxisome biogenesis disorders (PBD) are a group of multi-system human diseases due to mutations in the PEX genes that are responsible for peroxisome assembly and function. These disorders lead to global defects in peroxisomal function and result in severe brain, liver, bone and kidney disease. In order to study their pathogenesis we undertook a systematic genetic and biochemical study of Drosophila pex16 and pex2 mutants. These mutants are short-lived with defects in locomotion and activity. Moreover these mutants exhibit severe morphologic and functional peroxisomal defects. Using metabolomics we uncovered defects in multiple biochemical pathways including defects outside the canonical specialized lipid pathways performed by peroxisomal enzymes. These included unanticipated changes in metabolites in glycolysis, glycogen metabolism, and the pentose phosphate pathway, carbohydrate metabolic pathways that do not utilize known peroxisomal enzymes. In addition, mutant flies are starvation sensitive and are very sensitive to glucose deprivation exhibiting dramatic shortening of lifespan and hyperactivity on low-sugar food. We use bioinformatic transcriptional profiling to examine gene co-regulation between peroxisomal genes and other metabolic pathways and we observe that the expression of peroxisomal and carbohydrate pathway genes in flies and mouse are tightly correlated. Indeed key steps in carbohydrate metabolism were found to be strongly co-regulated with peroxisomal genes in flies and mice. Moreover mice lacking peroxisomes exhibit defective carbohydrate metabolism at the same key steps in carbohydrate breakdown. Our data indicate an unexpected link between these two metabolic processes and suggest metabolism of carbohydrates could be a new therapeutic target for patients with PBD.

Brown adipocyte in brown adipose tissue (BAT) specializes in expending energy through non-shivering thermogenesis, a process that produces heat either by uncoupling protein 1 (UCP1) dependent uncoupling of mitochondrial respiration or by UCP1 independent mechanisms. Apart from this, there is ample evidence suggesting that BAT has an endocrine function. Studies in rodents point toward its vital roles in glucose and lipid homeostasis, making it an important therapeutic target for treating metabolic disorders related to morbidities such as obesity and type 2 diabetes. The rediscovery of thermogenically active BAT depots in humans by several independent research groups in the last decade has revitalized interest in BAT as an even more promising therapeutic intervention. Over the last few years, there has been overwhelming interest in understanding brown adipocyte’s developmental lineages and how brown adipocyte uniquely utilizes energy beyond UCP1 mediated uncoupling respiration. These new discoveries would be leveraged for designing novel therapeutic interventions for metabolic disorders.

Objectives Peroxisomes play a crucial role in lipid and reactive oxygen species metabolism, but their importance for pancreatic β-cell functioning is presently unknown. To examine the contribution of peroxisomal metabolism to β-cell homeostasis in mice, we inactivated PEX5, the import receptor for peroxisomal matrix proteins, in an inducible and β-cell restricted manner ( Rip-Pex5 −/− mice). Methods After tamoxifen-induced recombination of the Pex5 gene at the age of 6 weeks, mice were fed either normal chow or a high-fat diet for 12 weeks and were subsequently phenotyped. Results Increased levels of very long chain fatty acids and reduced levels of plasmalogens in islets confirmed impairment of peroxisomal fatty acid oxidation and ether lipid synthesis, respectively. The Rip-Pex5 −/− mice fed on either diet exhibited glucose intolerance associated with impaired insulin secretion. Ultrastructural and biochemical analysis revealed a decrease in the density of mature insulin granules and total pancreatic insulin content, which was further accompanied by mitochondrial disruptions, reduced complex I activity and massive vacuole overload in β-cells. RNAseq analysis suggested that cell death pathways were affected in islets from HFD-fed Rip-Pex5 −/− mice. Consistent with this change we observed increased β-cell apoptosis in islets and a decrease in β-cell mass. Conclusions Our data indicate that normal peroxisome metabolism in β-cells is crucial to preserve their structure and function.