Peroxisomes Are Critical for the Development and Maintenance of B1 and Marginal Zone B Cells but Dispensable for Follicular B Cells and T Cells

Lipid metabolism in B cell biology

In recent years, the field of immunometabolism has solidified its position as a prominent area of investigation within the realm of immunological research. An expanding body of scientific literature has unveiled the intricate interplay between energy homeostasis, signalling molecules, and metabolites in relation to fundamental aspects of our immune cells. It is now widely accepted that disruptions in metabolic equilibrium can give rise to a myriad of pathological conditions, ranging from autoimmune disorders to cancer. Emerging evidence, although sometimes fragmented and anecdotal, has highlighted the indispensable role of lipids in modulating the behaviour of immune cells, including B cells. In light of these findings, this review aims to provide a comprehensive overview of the current state of knowledge regarding lipid metabolism in the context of B cell biology.

Peroxisome deficiency underlies failures in hepatic immune cell development and antigen presentation in a severe Zellweger disease model

Peroxisome biogenesis disorders (PBDs) represent a group of metabolic conditions that cause severe developmental defects. Peroxisomes are essential metabolic organelles, present in virtually every eukaryotic cell and mediating key processes in immunometabolism. To date, the full spectrum of PBDs remains to be identified, and the impact PBDs have on immune function is unexplored. This study presents a characterization of the hepatic immune compartment of a neonatal PBD mouse model at single-cell resolution to establish the importance and function of peroxisomes in developmental hematopoiesis. We report that hematopoietic defects are a feature in a severe PBD murine model. Finally, we identify a role for peroxisomes in the regulation of the major histocompatibility class II expression and antigen presentation to CD4+ T cells in dendritic cells. This study adds to our understanding of the mechanisms of PBDs and expands our knowledge of the role of peroxisomes in immunometabolism.

Tissue-specific roles of peroxisomes revealed by expression meta-analysis

Peroxisomes are primarily studied in the brain, kidney, and liver due to the conspicuous tissue-specific pathology of peroxisomal biogenesis disorders. In contrast, little is known about the role of peroxisomes in other tissues such as the heart. In this meta-analysis, we explore mitochondrial and peroxisomal gene expression on RNA and protein levels in the brain, heart, kidney, and liver, focusing on lipid metabolism. Further, we evaluate a potential developmental and heart region-dependent specificity of our gene set. We find marginal expression of the enzymes for peroxisomal fatty acid oxidation in cardiac tissue in comparison to the liver or cardiac mitochondrial β-oxidation. However, the expression of peroxisome biogenesis proteins in the heart is similar to other tissues despite low levels of peroxisomal fatty acid oxidation. Strikingly, peroxisomal targeting signal type 2-containing factors and plasmalogen biosynthesis appear to play a fundamental role in explaining the essential protective and supporting functions of cardiac peroxisomes.

Peroxisome Deficiency in Cochlear Hair Cells Causes Hearing Loss by Deregulating BK Channels

The peroxisome is a ubiquitous organelle in rodent cells and plays important roles in a variety of cell types and tissues. It is previously indicated that peroxisomes are associated with auditory function, and patients with peroxisome biogenesis disorders (PBDs) are found to have hearing dysfunction, but the specific role of peroxisomes in hearing remains unclear. In this study, two peroxisome-deficient mouse models (Atoh1-Pex5^-/- and Pax2-Pex5^-/- ) are established and it is found that peroxisomes mainly function in the hair cells of cochleae. Furthermore, peroxisome deficiency-mediated negative effects on hearing do not involve mitochondrial dysfunction and oxidative damage. Although the mammalian target of rapamycin complex 1 (mTORC1) signaling is shown to function through peroxisomes, no changes are observed in the mTORC1 signaling in Atoh1-Pex5^-/- mice when compared to wild-type (WT) mice. However, the expression of large-conductance, voltage-, and Ca2⁺ -activated K⁺ (BK) channels is less in Atoh1-Pex5^-/- mice as compared to the WT mice, and the administration of activators of BK channels (NS-1619 and NS-11021) restores the auditory function in knockout mice. These results suggest that peroxisomes play an essential role in cochlear hair cells by regulating BK channels. Hence, BK channels appear as the probable target for treating peroxisome-related hearing diseases such as PBDs.

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.

The thioredoxin system: Balancing redox responses in immune cells and tumors

The thioredoxin (TRX) system is an important contributor to cellular redox balance and regulates cell growth, apoptosis, gene expression, and antioxidant defense in nearly all living cells. Oxidative stress, the imbalance between reactive oxygen species (ROS) and antioxidants, can lead to cell death and tissue damage, thereby contributing to aging and to the development of several diseases, including cardiovascular and allergic diseases, diabetes, and neurological disorders. Targeting its activity is also considered as a promising strategy in the treatment of cancer. Over the past years, immunologists have established an essential function of TRX for activation, proliferation, and responses in T cells, B cells, and macrophages. Upon activation, immune cells rearrange their redox system and activate the TRX pathway to promote proliferation through sustainment of nucleotide biosynthesis, and to support inflammatory responses in myeloid cells by allowing NF-κB and NLRP3 inflammasome responses. Consequently, targeting the TRX system may therapeutically be exploited to inhibit immune responses in inflammatory conditions. In this review, we summarize recent insights revealing key roles of the TRX pathway in immune cells in health and disease, and lessons learnt for cancer therapy.

The peroxisome: an up-and-coming organelle in immunometabolism

Peroxisomes are essential metabolic organelles, well known for their roles in the metabolism of complex lipids and reactive ionic species. In the past 10 years, peroxisomes have also been cast as central regulators of immunity. Lipid metabolites of peroxisomes, such as polyunsaturated fatty acids (PUFAs), are precursors for important immune mediators, including leukotrienes (LTs) and resolvins. Peroxisomal redox metabolism modulates cellular immune signaling such as nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation. Additionally, peroxisomal β-oxidation and ether lipid synthesis control the development and aspects of the activation of both innate and adaptive immune cells. Finally, peroxisome number and metabolic activity have been linked to inflammatory diseases. These discoveries have opened avenues of investigation aimed at targeting peroxisomes for therapeutic intervention in immune disorders, inflammation, and cancer.

The physiological functions of human peroxisomes

Peroxisomes are subcellular organelles that play a central role in human physiology by catalyzing a range of unique metabolic functions. The importance of peroxisomes for human health is exemplified by the existence of a group of usually severe diseases caused by an impairment in one or more peroxisomal functions. Among others these include the Zellweger spectrum disorders, X-linked adrenoleukodystrophy, and Refsum disease. To fulfill their role in metabolism, peroxisomes require continued interaction with other subcellular organelles including lipid droplets, lysosomes, the endoplasmic reticulum, and mitochondria. In recent years it has become clear that the metabolic alliance between peroxisomes and other organelles requires the active participation of tethering proteins to bring the organelles physically closer together, thereby achieving efficient transfer of metabolites. This review intends to describe the current state of knowledge about the metabolic role of peroxisomes in humans, with particular emphasis on the metabolic partnership between peroxisomes and other organelles and the consequences of genetic defects in these processes. We also describe the biogenesis of peroxisomes and the consequences of the multiple genetic defects therein. In addition, we discuss the functional role of peroxisomes in different organs and tissues and include relevant information derived from model systems, notably peroxisomal mouse models. Finally, we pay particular attention to a hitherto underrated role of peroxisomes in viral infections.