Metabolic adaptation allows Amacr-deficient mice to remain symptom-free despite low levels of mature bile acids

Mouse Models to Study Peroxisomal Functions and Disorders: Overview, Caveats, and Recommendations

During the last three decades many mouse lines were created or identified that are deficient in one or more peroxisomal functions. Different methodologies were applied to obtain global, hypomorph, cell type selective, inducible, and knockin mice. Whereas some models closely mimic pathologies in patients, others strongly deviate or no human counterpart has been reported. Often, mice, apparently endowed with a stronger transcriptional adaptation, have to be challenged with dietary additions or restrictions in order to trigger phenotypic changes. Depending on the inactivated peroxisomal protein, several approaches can be taken to validate the loss-of-function. Here, an overview is given of the available mouse models and their most important characteristics.

Mice with a deficiency in Peroxisomal Membrane Protein 4 (PXMP4) display mild changes in hepatic lipid metabolism

Peroxisomes play an important role in the metabolism of a variety of biomolecules, including lipids and bile acids. Peroxisomal Membrane Protein 4 (PXMP4) is a ubiquitously expressed peroxisomal membrane protein that is transcriptionally regulated by peroxisome proliferator-activated receptor α (PPARα), but its function is still unknown. To investigate the physiological function of PXMP4, we generated a Pxmp4 knockout (Pxmp4^-/-) mouse model using CRISPR/Cas9-mediated gene editing. Peroxisome function was studied under standard chow-fed conditions and after stimulation of peroxisomal activity using the PPARα ligand fenofibrate or by using phytol, a metabolite of chlorophyll that undergoes peroxisomal oxidation. Pxmp4^-/- mice were viable, fertile, and displayed no changes in peroxisome numbers or morphology under standard conditions. Also, no differences were observed in the plasma levels of products from major peroxisomal pathways, including very long-chain fatty acids (VLCFAs), bile acids (BAs), and BA intermediates di- and trihydroxycholestanoic acid. Although elevated levels of the phytol metabolites phytanic and pristanic acid in Pxmp4^-/- mice pointed towards an impairment in peroxisomal α-oxidation capacity, treatment of Pxmp4^-/- mice with a phytol-enriched diet did not further increase phytanic/pristanic acid levels. Finally, lipidomic analysis revealed that loss of Pxmp4 decreased hepatic levels of the alkyldiacylglycerol class of neutral ether lipids, particularly those containing polyunsaturated fatty acids. Together, our data show that while PXMP4 is not critical for overall peroxisome function under the conditions tested, it may have a role in the metabolism of (ether)lipids.

Lipids and cancer: Emerging roles in pathogenesis, diagnosis and therapeutic intervention

With the advent of effective tools to study lipids, including mass spectrometry-based lipidomics, lipids are emerging as central players in cancer biology. Lipids function as essential building blocks for membranes, serve as fuel to drive energy-demanding processes and play a key role as signaling molecules and as regulators of numerous cellular functions. Not unexpectedly, cancer cells, as well as other cell types in the tumor microenvironment, exploit various ways to acquire lipids and extensively rewire their metabolism as part of a plastic and context-dependent metabolic reprogramming that is driven by both oncogenic and environmental cues. The resulting changes in the fate and composition of lipids help cancer cells to thrive in a changing microenvironment by supporting key oncogenic functions and cancer hallmarks, including cellular energetics, promoting feedforward oncogenic signaling, resisting oxidative and other stresses, regulating intercellular communication and immune responses. Supported by the close connection between altered lipid metabolism and the pathogenic process, specific lipid profiles are emerging as unique disease biomarkers, with diagnostic, prognostic and predictive potential. Multiple preclinical studies illustrate the translational promise of exploiting lipid metabolism in cancer, and critically, have shown context dependent actionable vulnerabilities that can be rationally targeted, particularly in combinatorial approaches. Moreover, lipids themselves can be used as membrane disrupting agents or as key components of nanocarriers of various therapeutics. With a number of preclinical compounds and strategies that are approaching clinical trials, we are at the doorstep of exploiting a hitherto underappreciated hallmark of cancer and promising target in the oncologist's strategy to combat cancer.

Peroxisomes in brain development and function

Peroxisomes contain numerous enzymatic activities that are important for mammalian physiology. Patients lacking either all peroxisomal functions or a single enzyme or transporter function typically develop severe neurological deficits, which originate from aberrant development of the brain, demyelination and loss of axonal integrity, neuroinflammation or other neurodegenerative processes. Whilst correlating peroxisomal properties with a compilation of pathologies observed in human patients and mouse models lacking all or individual peroxisomal functions, we discuss the importance of peroxisomal metabolites and tissue- and cell type-specific contributions to the observed brain pathologies. This enables us to deconstruct the local and systemic contribution of individual metabolic pathways to specific brain functions. We also review the recently discovered variability of pathological symptoms in cases with unexpectedly mild presentation of peroxisome biogenesis disorders. Finally, we explore the emerging evidence linking peroxisomes to more common neurological disorders such as Alzheimer’s disease, autism and amyotrophic lateral sclerosis. This article is part of a Special Issue entitled: Peroxisomes edited by Ralf Erdmann.

Hepatic dysfunction in peroxisomal disorders

The peroxisomal compartment in hepatocytes hosts several essential metabolic conversions. These are defective in peroxisomal disorders that are either caused by failure to import the enzymes in the organelle or by mutations in the enzymes or in transporters needed to transfer the substrates across the peroxisomal membrane. Hepatic pathology is one of the cardinal features in disorders of peroxisome biogenesis and peroxisomal β-oxidation although it only rarely determines the clinical fate. In mouse models of these diseases liver pathologies also occur, although these are not always concordant with the human phenotype which might be due to differences in diet, expression of enzymes and backup mechanisms. Besides the morphological changes, we overview the impact of peroxisome malfunction on other cellular compartments including mitochondria and the ER. We further focus on the metabolic pathways that are affected such as bile acid formation, and dicarboxylic acid and branched chain fatty acid degradation. It appears that the association between deregulated metabolites and pathological events remains unclear.

AMACR amplification and overexpression in primary imatinib-naïve gastrointestinal stromal tumors: a driver of cell proliferation indicating adverse prognosis

Non-random gains of chromosome 5p have been observed in clinically aggressive gastrointestinal stromal tumors, whereas the driving oncogenes on 5p remain to be characterized. We used an integrative genomic and functional approach to identify amplified oncogenes on 5p and to evaluate the relevance of AMACR amplification at 5p13.3 and its overexpression in gastrointestinal stromal tumors. Thirty-seven tumor samples, imatinib-sensitive GIST882 cell line, and imatinib-resistant GIST48 cell line were analyzed for DNA imbalances using array-based genomic profiling. Forty-one fresh tumor samples of various risk categories were enriched for pure tumor cells by laser capture microdissection and quantified for AMACR mRNA expression. AMACR-specific fluorescence in situ hybridization and immunohistochemistry were both informative in tissue microarray sections of 350 independent primary gastrointestinal stromal tumors, including 213 cases with confirmed KIT /PDGFRA genotypes. To assess the oncogenic functions of AMACR, GIST882 and GIST48 cell lines were stably silenced against their endogenous AMACR expression. In 59% of cases featuring 5p gains, two major amplicons encompassed discontinuous chromosomal regions that were differentially overrepresented in high-risk cases, including the one harboring the mRNA-upregulated AMACR gene. Gene amplification was detected in 19.7% of cases (69/350) and strongly related to protein overexpression (p<0.001), although 52% of AMACR-overexpressing cases exhibited no amplification. Both gene amplification and protein overexpression were significantly associated with epithelioid histology, larger size, increased mitoses, higher risk levels, and unfavorable genotypes (all p≤0.03). They were also independently predictive of decreased disease-free survival (overexpression, p<0.001; amplification, p=0.020) in the multivariate analysis. Concomitant with downregulated cyclin D1, cyclin E, and CDK4, AMACR knockdown suppressed cell proliferation and induced G₁-phase arrest, but did not affect apoptosis in both GIST882 and GIST48 cells. In conclusion, AMACR amplification is a mechanism driving increased mRNA and protein expression and conferring aggressiveness through heightened cell proliferation in gastrointestinal stromal tumors.

Phytol is lethal for Amacr-deficient mice

α-Methylacyl-CoA racemase (Amacr) catalyzes the racemization of the 25-methyl group in C27-intermediates in bile acid synthesis and in methyl-branched fatty acids such as pristanic acid, a metabolite derived from phytol. Consequently, patients with Amacr deficiency accumulate C27-bile acid intermediates, pristanic and phytanic acid and display sensorimotor neuropathy, seizures and relapsing encephalopathy. In contrast to humans, Amacr-deficient mice are clinically symptomless on a standard laboratory diet, but failed to thrive when fed phytol-enriched chow. In this study, the effect and the mechanisms behind the development of the phytol-feeding associated disease state in Amacr-deficient mice were investigated. All Amacr-/- mice died within 36weeks on a phytol diet, while wild-type mice survived. Liver failure was the main cause of death accompanied by kidney and brain abnormalities. Histological analysis of liver showed inflammation, fibrotic and necrotic changes, Kupffer cell proliferation and fatty changes in hepatocytes, and serum analysis confirmed the hepatic disease. Pristanic and phytanic acids accumulated in livers of Amacr-/- mice after a phytol diet. Microarray analysis also revealed changes in the expression levels of numerous genes in wild-type mouse livers after two weeks of the phytol diet compared to a control diet. This indicates that detoxification of phytol metabolites in liver is accompanied by activation of multiple pathways at the molecular level and Amacr-/- mice are not able to respond adequately. Phytol causes primary failure in liver leading to death of Amacr-/- mice thus emphasizing the indispensable role of Amacr in detoxification of α-methyl-branched fatty acids.

Role of AMACR (α-methylacyl-CoA racemase) and MFE-1 (peroxisomal multifunctional enzyme-1) in bile acid synthesis in mice

Bile acid analysis of wild-type, Mfe-1−/−, Amacr−/− and Amacr−/−Mfe-1−/− mouse models shows that peroxisomal multifunctional enzyme 1 can participate in bile acid synthesis in both AMACR-dependent and AMACR-independent pathways.