Transient complex interactions of mammalian peroxisomes without exchange of matrix or membrane marker proteins

Mitochondrial and peroxisomal fission in cortical neurogenesis

The human brain is unique in its cellular diversity, intricate cytoarchitecture, function, and complex metabolic and bioenergetic demands, for which mitochondria and peroxisomes are essential. Mitochondria are multifunctional organelles that coordinate various signaling pathways central to neurogenesis. The dynamic morphological changes of the mitochondrial network have been linked to the regulation of bioenergetic and metabolic states. Specific protein machinery is dedicated to mitochondrial fission and fusion, allowing organelle distribution during cell division, organelle repair, and adaptation to environmental stimuli (excellent reviews have been published on these topics [Kondadi and Reichert, 2024; Giacomello et al., 2020; Tilokani et al., 2018; Kraus et al., 2021; Navaratnarajah et al., 2021]). In parallel, peroxisomes contain over 50 different enzymes which regulate metabolic functions that are critical for neurogenesis (Berger et al., 2016; Hulshagen et al., 2008). Peroxisomes share many of the components of their fission machinery with the mitochondria and undergo fission to help meet metabolic demands in response to environmental stimuli (Schrader et al., 2016). This review focuses primarily on the machinery involved in mitochondrial and peroxisomal fission. Mitochondrial fission has been identified as a critical determinant of cell fate decisions (Iwata et al., 2023, 2020; Khacho et al., 2016; King et al., 2021; Prigione and Adjaye, 2010; Vantaggiato et al., 2019; Kraus et al., 2021). The connection between alterations in peroxisomal fission and metabolic changes associated with cellular differentiation remains less clear. Here, we provide an overview of the functional and regulatory aspects of the mitochondrial and peroxisomal fission machinery and provide insight into the current mechanistic understanding by which mitochondrial and peroxisomal fission influence neurogenesis.

Quantitative analysis of peroxisome tracks using a Hidden Markov Model

Diffusion and mobility are essential for cellular functions, as molecules are usually distributed throughout the cell and have to meet to interact and perform their function. This also involves the cytosolic migration of cellular organelles. However, observing such diffusion and interaction dynamics is challenging due to the high spatial and temporal resolution required and the accurate analysis of the diffusional tracks. The latter is especially important when identifying anomalous diffusion events, such as directed motions, which are often rare. Here, we investigate the migration modes of peroxisome organelles in the cytosol of living cells. Peroxisomes predominantly migrate randomly, but occasionally they bind to the cell's microtubular network and perform directed migration, which is difficult to quantify, and so far, accurate analysis of switching between these migration modes is missing. We set out to solve this limitation by experiments and analysis with high statistical accuracy. Specifically, we collect temporal diffusion tracks of thousands of individual peroxisomes in the HEK 293 cell line using two-dimensional spinning disc fluorescence microscopy at a high acquisition rate of 10 frames/s. We use a Hidden Markov Model with two hidden states to (1) automatically identify directed migration segments of the tracks and (2) quantify the migration properties for comparison between states and between different experimental conditions. Comparing different cellular conditions, we show that the knockout of the peroxisomal membrane protein PEX14 leads to a decrease in the directed movement due to a lowered binding probability to the microtubule. However, it does not eradicate binding, highlighting further microtubule-binding mechanisms of peroxisomes than via PEX14. In contrast, structural changes of the microtubular network explain perceived eradication of directed movement by disassembly of microtubules by Nocodazole-treatment.

Organelle Membrane Extensions in Mammalian Cells

Organelles within eukaryotic cells are not isolated static compartments, instead being morphologically diverse and highly dynamic in order to respond to cellular needs and carry out their diverse and cooperative functions. One phenomenon exemplifying this plasticity, and increasingly gaining attention, is the extension and retraction of thin tubules from organelle membranes. While these protrusions have been observed in morphological studies for decades, their formation, properties and functions are only beginning to be understood. In this review, we provide an overview of what is known and still to be discovered about organelle membrane protrusions in mammalian cells, focusing on the best-characterised examples of these membrane extensions arising from peroxisomes (ubiquitous organelles involved in lipid metabolism and reactive oxygen species homeostasis) and mitochondria. We summarise the current knowledge on the diversity of peroxisomal/mitochondrial membrane extensions, as well as the molecular mechanisms by which they extend and retract, necessitating dynamic membrane remodelling, pulling forces and lipid flow. We also propose broad cellular functions for these membrane extensions in inter-organelle communication, organelle biogenesis, metabolism and protection, and finally present a mathematical model that suggests that extending protrusions is the most efficient way for an organelle to explore its surroundings.

PEX11β and FIS1 cooperate in peroxisome division independently of mitochondrial fission factor

Peroxisome membrane dynamics and division are essential to adapt the peroxisomal compartment to cellular needs. The peroxisomal membrane protein PEX11β (also known as PEX11B) and the tail-anchored adaptor proteins FIS1 (mitochondrial fission protein 1) and MFF (mitochondrial fission factor), which recruit the fission GTPase DRP1 (dynamin-related protein 1, also known as DNML1) to both peroxisomes and mitochondria, are key factors of peroxisomal division. The current model suggests that MFF is essential for peroxisome division, whereas the role of FIS1 is unclear. Here, we reveal that PEX11β can promote peroxisome division in the absence of MFF in a DRP1- and FIS1-dependent manner. We also demonstrate that MFF permits peroxisome division independently of PEX11β and restores peroxisome morphology in PEX11β-deficient patient cells. Moreover, targeting of PEX11β to mitochondria induces mitochondrial division, indicating the potential for PEX11β to modulate mitochondrial dynamics. Our findings suggest the existence of an alternative, MFF-independent pathway in peroxisome division and report a function for FIS1 in the division of peroxisomes. This article has an associated First Person interview with the first authors of the paper.

Fission Impossible (?)-New Insights into Disorders of Peroxisome Dynamics

Peroxisomes are highly dynamic and responsive organelles, which can adjust their morphology, number, intracellular position, and metabolic functions according to cellular needs. Peroxisome multiplication in mammalian cells involves the concerted action of the membrane-shaping protein PEX11β and division proteins, such as the membrane adaptors FIS1 and MFF, which recruit the fission GTPase DRP1 to the peroxisomal membrane. The latter proteins are also involved in mitochondrial division. Patients with loss of DRP1, MFF or PEX11β function have been identified, showing abnormalities in peroxisomal (and, for the shared proteins, mitochondrial) dynamics as well as developmental and neurological defects, whereas the metabolic functions of the organelles are often unaffected. Here, we provide a timely update on peroxisomal membrane dynamics with a particular focus on peroxisome formation by membrane growth and division. We address the function of PEX11β in these processes, as well as the role of peroxisome–ER contacts in lipid transfer for peroxisomal membrane expansion. Furthermore, we summarize the clinical phenotypes and pathophysiology of patients with defects in the key division proteins DRP1, MFF, and PEX11β as well as in the peroxisome–ER tether ACBD5. Potential therapeutic strategies for these rare disorders with limited treatment options are discussed.

Edgetic Perturbations Contribute to Phenotypic Variability in PEX26 Deficiency

Peroxisomes share metabolic pathways with other organelles and peroxisomes are embedded into key cellular processes. However, the specific function of many peroxisomal proteins remains unclear and restricted knowledge of the peroxisomal protein interaction network limits a precise mapping of this network into the cellular metabolism. Inborn peroxisomal disorders are autosomal or X-linked recessive diseases that affect peroxisomal biogenesis (PBD) and/or peroxisomal metabolism. Pathogenic variants in the PEX26 gene lead to peroxisomal disorders of the full Zellweger spectrum continuum. To investigate the phenotypic complexity of PEX26 deficiency, we performed a combined organelle protein interaction screen and network medicine approach and 1) analyzed whether PEX26 establishes interactions with other peroxisomal proteins, 2) deciphered the PEX26 interaction network, 3) determined how PEX26 is involved in further processes of peroxisomal biogenesis and metabolism, and 4) showed how variant-specific disruption of protein-protein interactions (edgetic perturbations) may contribute to phenotypic variability in PEX26 deficient patients. The discovery of 14 novel protein-protein interactions for PEX26 revealed a hub position of PEX26 inside the peroxisomal interactome. Analysis of edgetic perturbations of PEX26 variants revealed a strong correlation between the number of affected protein-protein interactions and the molecular phenotype of matrix protein import. The role of PEX26 in peroxisomal biogenesis was expanded encompassing matrix protein import, division and proliferation, and membrane assembly. Moreover, the PEX26 interaction network intersects with cellular lipid metabolism at different steps. The results of this study expand the knowledge about the function of PEX26 and refine genotype-phenotype correlations, which may contribute to our understanding of the underlying disease mechanism of PEX26 deficiency.

Peroxisomal Membrane Contact Sites in Mammalian Cells

Peroxisomes participate in essential cellular metabolic processes, such as oxidation of fatty acids (FAs) and maintenance of reactive oxygen species (ROS) homeostasis. Peroxisomes must communicate with surrounding organelles to exchange information and metabolites. The formation of membrane contact sites (MCSs), where protein-protein or protein-lipid complexes tether the opposing membranes of two organelles, represents an essential means of organelle crosstalk. Peroxisomal MCS (PO-MCS) studies are emerging but are still in the early stages. In this review, we summarize the identified PO-MCSs with the ER, mitochondria, lipid droplets, and lysosomes in mammalian cells and discuss their tethering mechanisms and physiological roles. We also highlight several features of PO-MCSs that may help future studies.

Organelle interplay-peroxisome interactions in health and disease

Peroxisomes are multifunctional, dynamic, membrane-bound organelles with important functions in cellular lipid metabolism, rendering them essential for human health and development. Important roles for peroxisomes in signaling and the fine-tuning of cellular processes are emerging, which integrate them in a complex network of interacting cellular compartments. Like many other organelles, peroxisomes communicate through membrane contact sites. For example, peroxisomal growth, positioning, and lipid metabolism involves contacts with the endoplasmic reticulum (ER). Here, we discuss the most recent findings on peroxisome-organelle interactions including peroxisome-ER interplay at membrane contacts sites, and functional interplay with mitochondria, lysosomes, and lipid droplets in mammalian cells. We address tether proteins, metabolic cooperation, and the impact of peroxisome interactions on human health and disease.

The peroxisome: an update on mysteries 2.0

Peroxisomes are key metabolic organelles, which contribute to cellular lipid metabolism, e.g. the β-oxidation of fatty acids and the synthesis of myelin sheath lipids, as well as cellular redox balance. Peroxisomal dysfunction has been linked to severe metabolic disorders in man, but peroxisomes are now also recognized as protective organelles with a wider significance in human health and potential impact on a large number of globally important human diseases such as neurodegeneration, obesity, cancer, and age-related disorders. Therefore, the interest in peroxisomes and their physiological functions has significantly increased in recent years. In this review, we intend to highlight recent discoveries, advancements and trends in peroxisome research, and present an update as well as a continuation of two former review articles addressing the unsolved mysteries of this astonishing organelle. We summarize novel findings on the biological functions of peroxisomes, their biogenesis, formation, membrane dynamics and division, as well as on peroxisome-organelle contacts and cooperation. Furthermore, novel peroxisomal proteins and machineries at the peroxisomal membrane are discussed. Finally, we address recent findings on the role of peroxisomes in the brain, in neurological disorders, and in the development of cancer.

A role for Mitochondrial Rho GTPase 1 (MIRO1) in motility and membrane dynamics of peroxisomes

Peroxisomes are dynamic organelles which fulfil essential roles in lipid and ROS metabolism. Peroxisome movement and positioning allows interaction with other organelles and is crucial for their cellular function. In mammalian cells, such movement is microtubule-dependent and mediated by kinesin and dynein motors. The mechanisms of motor recruitment to peroxisomes are largely unknown, as well as the role this plays in peroxisome membrane dynamics and proliferation. Here, using a combination of microscopy, live-cell imaging analysis and mathematical modelling, we identify a role for Mitochondrial Rho GTPase 1 (MIRO1) as an adaptor for microtubule-dependent peroxisome motility in mammalian cells. We show that MIRO1 is targeted to peroxisomes and alters their distribution and motility. Using a peroxisome-targeted MIRO1 fusion protein, we demonstrate that MIRO1-mediated pulling forces contribute to peroxisome membrane elongation and proliferation in cellular models of peroxisome disease. Our findings reveal a molecular mechanism for establishing peroxisome-motor protein associations in mammalian cells and provide new insights into peroxisome membrane dynamics in health and disease.

The making of a mammalian peroxisome, version 2.0: mitochondria get into the mix

A recent report from the Laboratory of Heidi McBride (McGill University) presents a role for mitochondria in the de novo biogenesis of peroxisomes in mammalian cells. Peroxisomes are essential organelles responsible for a wide variety of biochemical functions, from the generation of bile to plasmalogen synthesis, reduction of peroxides, and the oxidation of very-long-chain fatty acids. Like mitochondria, peroxisomes proliferate primarily through growth and division of pre-existing peroxisomes. However, unlike mitochondria, peroxisomes do not fuse; further, and perhaps most importantly, they can also be born de novo, a process thought to occur through the generation of pre-peroxisomal vesicles that originate from the endoplasmic reticulum. De novo peroxisome biogenesis has been extensively studied in yeast, with a major focus on the role of the ER in this process; however, in the mammalian system this field is much less explored. By exploiting patient cells lacking mature peroxisomes, the McBride laboratory now assigns a role to ER and mitochondria in de novo mammalian peroxisome biogenesis by showing that the formation of immature pre-peroxisomes occurs through the fusion of Pex3-/Pex14-containing mitochondria-derived vesicles with Pex16-containing ER-derived vesicles.

The Peroxisome-Mitochondria Connection: How and Why?

Over the past decades, peroxisomes have emerged as key regulators in overall cellular lipid and reactive oxygen species metabolism. In mammals, these organelles have also been recognized as important hubs in redox-, lipid-, inflammatory-, and innate immune-signaling networks. To exert these activities, peroxisomes must interact both functionally and physically with other cell organelles. This review provides a comprehensive look of what is currently known about the interconnectivity between peroxisomes and mitochondria within mammalian cells. We first outline how peroxisomal and mitochondrial abundance are controlled by common sets of cis- and trans-acting factors. Next, we discuss how peroxisomes and mitochondria may communicate with each other at the molecular level. In addition, we reflect on how these organelles cooperate in various metabolic and signaling pathways. Finally, we address why peroxisomes and mitochondria have to maintain a healthy relationship and why defects in one organelle may cause dysfunction in the other. Gaining a better insight into these issues is pivotal to understanding how these organelles function in their environment, both in health and disease.

Active diffusion and microtubule-based transport oppose myosin forces to position organelles in cells

Even distribution of peroxisomes (POs) and lipid droplets (LDs) is critical to their role in lipid and reactive oxygen species homeostasis. How even distribution is achieved remains elusive, but diffusive motion and directed motility may play a role. Here we show that in the fungus Ustilago maydis ∼95% of POs and LDs undergo diffusive motions. These movements require ATP and involve bidirectional early endosome motility, indicating that microtubule-associated membrane trafficking enhances diffusion of organelles. When early endosome transport is abolished, POs and LDs drift slowly towards the growing cell end. This pole-ward drift is facilitated by anterograde delivery of secretory cargo to the cell tip by myosin-5. Modelling reveals that microtubule-based directed transport and active diffusion support distribution, mobility and mixing of POs. In mammalian COS-7 cells, microtubules and F-actin also counteract each other to distribute POs. This highlights the importance of opposing cytoskeletal forces in organelle positioning in eukaryotes.

The mechanisms underlying the positioning of eukaryotic organelles remain elusive. Here Lin et al. use imaging and a mathematical model to show that microtubule-based transport and active diffusion and actin-based polar drift act together to facilitate even distribution of peroxisomes in filamentous fungi.

Peroxisomes are platforms for cytomegalovirus' evasion from the cellular immune response

The human cytomegalovirus developed distinct evasion mechanisms from the cellular antiviral response involving vMIA, a virally-encoded protein that is not only able to prevent cellular apoptosis but also to inhibit signalling downstream from mitochondrial MAVS. vMIA has been shown to localize at mitochondria and to trigger their fragmentation, a phenomenon proven to be essential for the signalling inhibition. Here, we demonstrate that vMIA is also localized at peroxisomes, induces their fragmentation and inhibits the peroxisomal-dependent antiviral signalling pathway. Importantly, we demonstrate that peroxisomal fragmentation is not essential for vMIA to specifically inhibit signalling downstream the peroxisomal MAVS. We also show that vMIA interacts with the cytoplasmic chaperone Pex19, suggesting that the virus has developed a strategy to highjack the peroxisomal membrane proteins' transport machinery. Furthermore, we show that vMIA is able to specifically interact with the peroxisomal MAVS. Our results demonstrate that peroxisomes constitute a platform for evasion of the cellular antiviral response and that the human cytomegalovirus has developed a mechanism by which it is able to specifically evade the peroxisomal MAVS-dependent antiviral signalling.

HyPer Family Probes: State of the Art

SIGNIFICANCE

Hydrogen peroxide (H2O2) is not only a key mediator of oxidative stress but also one of the most important cellular second messengers. This small short-lived molecule is involved in the regulation of a wide range of different biological processes, including regulation of cellular signaling pathways. Studying the role of H2O2 in living systems would be challenging without modern approaches. A genetically encoded fluorescent biosensor, HyPer, is one of the most effective tools for this purpose.

RECENT ADVANCES

HyPer has been used by many investigators of redox signaling in various models of different scales: from cytoplasmic subcompartments and single cells to tissues of whole organisms. In many studies, the results obtained using HyPer have enabled a better understanding of the roles of H2O2 in these biological processes. However, much remains to be learned.

CRITICAL ISSUES

In this review, we focus on the uses of HyPer. We provide a general description of HyPer and its improved versions. Separate chapters are devoted to the results obtained by various groups who have used this biosensor for their experiments in living cells and organisms.

FUTURE DIRECTIONS

HyPer is an effective tool for H2O2 imaging in living systems as indicated by the increasing numbers of publications each year since its development. However, this biosensor requires further improvements. In particular, much brighter and more pH-stable versions of HyPer are necessary for imaging in mammalian tissues. Antioxid. Redox Signal. 24, 731-751.

Reevaluation of the role of Pex1 and dynamin-related proteins in peroxisome membrane biogenesis

Analysis of Pex1 and dynamin-related protein function indicates peroxisomes multiply mainly by growth and division in Saccharomyces cerevisiae, whereas no evidence was found for the previously proposed role for Pex1 in peroxisome formation by fusion of ER-derived preperoxisomal vesicles.

A recent model for peroxisome biogenesis postulates that peroxisomes form de novo continuously in wild-type cells by heterotypic fusion of endoplasmic reticulum–derived vesicles containing distinct sets of peroxisomal membrane proteins. This model proposes a role in vesicle fusion for the Pex1/Pex6 complex, which has an established role in matrix protein import. The growth and division model proposes that peroxisomes derive from existing peroxisomes. We tested these models by reexamining the role of Pex1/Pex6 and dynamin-related proteins in peroxisome biogenesis. We found that induced depletion of Pex1 blocks the import of matrix proteins but does not affect membrane protein delivery to peroxisomes; markers for the previously reported distinct vesicles colocalize in pex1 and pex6 cells; peroxisomes undergo continued growth if fission is blocked. Our data are compatible with the established primary role of the Pex1/Pex6 complex in matrix protein import and show that peroxisomes in Saccharomyces cerevisiae multiply mainly by growth and division.

Disturbed mitochondrial and peroxisomal dynamics due to loss of MFF causes Leigh-like encephalopathy, optic atrophy and peripheral neuropathy

BACKGROUND

Mitochondria are dynamic organelles which undergo continuous fission and fusion to maintain their diverse cellular functions. Components of the fission machinery are partly shared between mitochondria and peroxisomes, and inherited defects in two such components (dynamin-related protein (DRP1) and ganglioside-induced differentiation-associated protein 1 (GDAP1)) have been associated with human disease. Deficiency of a third component (mitochondrial fission factor, MFF) was recently reported in one index patient, rendering MFF another candidate disease gene within the expanding field of mitochondrial and peroxisomal dynamics. Here we investigated three new patients from two families with pathogenic mutations in MFF.

METHODS

The patients underwent clinical examination, brain MRI, and biochemical, cytological and molecular analyses, including exome sequencing.

RESULTS

The patients became symptomatic within the first year of life, exhibiting seizures, developmental delay and acquired microcephaly. Dysphagia, spasticity and optic and peripheral neuropathy developed subsequently. Brain MRI showed Leigh-like patterns with bilateral changes of the basal ganglia and subthalamic nucleus, suggestive of impaired mitochondrial energy metabolism. However, activities of mitochondrial respiratory chain complexes were found to be normal in skeletal muscle. Exome sequencing revealed three different biallelic loss-of-function variants in MFF in both index cases. Western blot studies of patient-derived fibroblasts indicated normal content of mitochondria and peroxisomes, whereas immunofluorescence staining revealed elongated mitochondria and peroxisomes. Furthermore, increased mitochondrial branching and an abnormal distribution of fission-mediating DRP1 were observed.

CONCLUSIONS

Our findings establish MFF loss of function as a cause of disturbed mitochondrial and peroxisomal dynamics associated with early-onset Leigh-like basal ganglia disease. We suggest that, even if laboratory findings are not indicative of mitochondrial or peroxisomal dysfunction, the co-occurrence of optic and/or peripheral neuropathy with seizures warrants genetic testing for MFF mutations.

Peroxisomes, lipid droplets, and endoplasmic reticulum "hitchhike" on motile early endosomes

Intracellular transport is mediated by molecular motors that bind cargo to be transported along the cytoskeleton. Here, we report, for the first time, that peroxisomes (POs), lipid droplets (LDs), and the endoplasmic reticulum (ER) rely on early endosomes (EEs) for intracellular movement in a fungal model system. We show that POs undergo kinesin-3- and dynein-dependent transport along microtubules. Surprisingly, kinesin-3 does not colocalize with POs. Instead, the motor moves EEs that drag the POs through the cell. PO motility is abolished when EE motility is blocked in various mutants. Most LD and ER motility also depends on EE motility, whereas mitochondria move independently of EEs. Covisualization studies show that EE-mediated ER motility is not required for PO or LD movement, suggesting that the organelles interact with EEs independently. In the absence of EE motility, POs and LDs cluster at the growing tip, whereas ER is partially retracted to subapical regions. Collectively, our results show that moving EEs interact transiently with other organelles, thereby mediating their directed transport and distribution in the cell.

Proliferation and fission of peroxisomes - An update

In mammals, peroxisomes perform crucial functions in cellular metabolism, signalling and viral defense which are essential to the health and viability of the organism. In order to achieve this functional versatility peroxisomes dynamically respond to molecular cues triggered by changes in the cellular environment. Such changes elicit a corresponding response in peroxisomes, which manifests itself as a change in peroxisome number, altered enzyme levels and adaptations to the peroxisomal structure. In mammals the generation of new peroxisomes is a complex process which has clear analogies to mitochondria, with both sharing the same division machinery and undergoing a similar division process. How the regulation of this division process is integrated into the cell's response to different stimuli, the signalling pathways and factors involved, remains somewhat unclear. Here, we discuss the mechanism of peroxisomal fission, the contributions of the various division factors and examine the potential impact of post-translational modifications, such as phosphorylation, on the proliferation process. We also summarize the signalling process and highlight the most recent data linking signalling pathways with peroxisome proliferation.

The different facets of organelle interplay-an overview of organelle interactions

Membrane-bound organelles such as mitochondria, peroxisomes, or the endoplasmic reticulum (ER) create distinct environments to promote specific cellular tasks such as ATP production, lipid breakdown, or protein export. During recent years, it has become evident that organelles are integrated into cellular networks regulating metabolism, intracellular signaling, cellular maintenance, cell fate decision, and pathogen defence. In order to facilitate such signaling events, specialized membrane regions between apposing organelles bear distinct sets of proteins to enable tethering and exchange of metabolites and signaling molecules. Such membrane associations between the mitochondria and a specialized site of the ER, the mitochondria associated-membrane (MAM), as well as between the ER and the plasma membrane (PAM) have been partially characterized at the molecular level. However, historical and recent observations imply that other organelles like peroxisomes, lysosomes, and lipid droplets might also be involved in the formation of such apposing membrane contact sites. Alternatively, reports on so-called mitochondria derived-vesicles (MDV) suggest alternative mechanisms of organelle interaction. Moreover, maintenance of cellular homeostasis requires the precise removal of aged organelles by autophagy—a process which involves the detection of ubiquitinated organelle proteins by the autophagosome membrane, representing another site of membrane associated-signaling. This review will summarize the available data on the existence and composition of organelle contact sites and the molecular specializations each site uses in order to provide a timely overview on the potential functions of organelle interaction.

No peroxisome is an island - Peroxisome contact sites

In order to optimize their multiple cellular functions, peroxisomes must collaborate and communicate with the surrounding organelles. A common way of communication between organelles is through physical membrane contact sites where membranes of two organelles are tethered, facilitating exchange of small molecules and intracellular signaling. In addition contact sites are important for controlling processes such as metabolism, organelle trafficking, inheritance and division. How peroxisomes rely on contact sites for their various cellular activities is only recently starting to be appreciated and explored and the extent of peroxisomal communication, their contact sites and their functions are less characterized. In this review we summarize the identified peroxisomal contact sites, their tethering complexes and their potential physiological roles. Additionally, we highlight some of the preliminary evidence that exists in the field for unexplored peroxisomal contact sites.

Peroxisome-mitochondria interplay and disease

Peroxisomes and mitochondria are ubiquitous, highly dynamic organelles with an oxidative type of metabolism in eukaryotic cells. Over the years, substantial evidence has been provided that peroxisomes and mitochondria exhibit a close functional interplay which impacts on human health and development. The so-called "peroxisome-mitochondria connection" includes metabolic cooperation in the degradation of fatty acids, a redox-sensitive relationship, an overlap in key components of the membrane fission machineries and cooperation in anti-viral signalling and defence. Furthermore, combined peroxisome-mitochondria disorders with defects in organelle division have been revealed. In this review, we present the latest progress in the emerging field of peroxisomal and mitochondrial interplay in mammals with a particular emphasis on cooperative fatty acid β-oxidation, redox interplay, organelle dynamics, cooperation in anti-viral signalling and the resulting implications for disease.

Emerging roles of mitochondria in the evolution, biogenesis, and function of peroxisomes

In the last century peroxisomes were thought to have an endosymbiotic origin. Along with mitochondria and chloroplasts, peroxisomes primarily regulate their numbers through the growth and division of pre-existing organelles, and they house specific machinery for protein import. These features were considered unique to endosymbiotic organelles, prompting the idea that peroxisomes were key cellular elements that helped facilitate the evolution of multicellular organisms. The functional similarities to mitochondria within mammalian systems expanded these ideas, as both organelles scavenge peroxide and reactive oxygen species, both organelles oxidize fatty acids, and at least in higher eukaryotes, the biogenesis of both organelles is controlled by common nuclear transcription factors of the PPAR family. Over the last decade it has been demonstrated that the fission machinery of both organelles is also shared, and that both organelles act as critical signaling platforms for innate immunity and other pathways. Taken together it is clear that the mitochondria and peroxisomes are functionally coupled, regulating cellular metabolism and signaling through a number of common mechanisms. However, recent work has focused primarily on the role of the ER in the biogenesis of peroxisomes, potentially overshadowing the critical importance of the mitochondria as a functional partner. In this review, we explore the mechanisms of functional coupling of the peroxisomes to the mitochondria/ER networks, providing some new perspectives on the potential contribution of the mitochondria to peroxisomal biogenesis.

Identification of the kinesin KifC3 as a new player for positioning of peroxisomes and other organelles in mammalian cells

The attachment of organelles to the cytoskeleton and directed organelle transport is essential for cellular morphology and function. In contrast to other cell organelles like the endoplasmic reticulum or the Golgi apparatus, peroxisomes are evenly distributed in the cytoplasm, which is achieved by binding of peroxisomes to microtubules and their bidirectional transport by the microtubule motor proteins kinesin-1 (Kif5) and cytoplasmic dynein. KifC3, belonging to the group of C-terminal kinesins, has been identified to interact with the human peroxin PEX1 in a yeast two-hybrid screen. We investigated the potential involvement of KifC3 in peroxisomal transport. Interaction of KifC3 and the AAA-protein (ATPase associated with various cellular activities) PEX1 was confirmed by in vivo colocalization and by coimmunoprecipitation from cell lysates. Furthermore, knockdown of KifC3 using RNAi resulted in an increase of cells with perinuclear-clustered peroxisomes, indicating enhanced minus-end directed motility of peroxisomes. The occurrence of this peroxisomal phenotype was cell cycle phase independent, while microtubules were essential for phenotype formation. We conclude that KifC3 may play a regulatory role in minus-end directed peroxisomal transport for example by blocking the motor function of dynein at peroxisomes. Knockdown of KifC3 would then lead to increased minus-end directed peroxisomal transport and cause the observed peroxisomal clustering at the microtubule-organizing center.

Peroxisome degradation in mammals: mechanisms of action, recent advances, and perspectives

Peroxisomes are remarkably dynamic organelles that participate in a diverse array of cellular processes, including the metabolism of lipids and reactive oxygen species. In order to regulate peroxisome function in response to changing nutritional and environmental stimuli, new organelles need to be formed and superfluous and dysfunctional organelles have to be selectively removed. Disturbances in any of these processes have been associated with the etiology and progression of various congenital neurodegenerative and age-related human disorders. The aim of this review is to critically explore our current knowledge of how peroxisomes are degraded in mammalian cells and how defects in this process may contribute to human disease. Some of the key issues highlighted include the current concepts of peroxisome removal, the peroxisome quality control mechanisms, the initial triggers for peroxisome degradation, the factors for dysfunctional peroxisome recognition, and the regulation of peroxisome homeostasis. We also dissect the functional and mechanistic relationship between different forms of selective organelle degradation and consider how lysosomal dysfunction may lead to defects in peroxisome turnover. In addition, we draw lessons from studies on other organisms and extrapolate this knowledge to mammals. Finally, we discuss the potential pathological implications of dysfunctional peroxisome degradation for human health.

Lumenal peroxisomal protein aggregates are removed by concerted fission and autophagy events

We demonstrated that in the yeast Hansenula polymorpha peroxisome fission and degradation are coupled processes that are important to remove intra-organellar protein aggregates. Protein aggregates were formed in peroxisomes upon synthesis of a mutant catalase variant. We showed that the introduction of these aggregates in the peroxisomal lumen had physiological disadvantages as it affected growth and caused enhanced levels of reactive oxygen species. Formation of the protein aggregates was followed by asymmetric peroxisome fission to separate the aggregate from the mother organelle. Subsequently, these small, protein aggregate-containing organelles were degraded by autophagy. In line with this observation we showed that the degradation of the protein aggregates was strongly reduced in dnm1 and pex11 cells in which peroxisome fission is reduced. Moreover, this process was dependent on Atg1 and Atg11.

Peroxisome interactions and cross-talk with other subcellular compartments in animal cells

Peroxisomes are remarkably plastic and dynamic organelles, which fulfil important functions in hydrogen peroxide and lipid metabolism rendering them essential for human health and development. Despite great advances in the identification and characterization of essential components and molecular mechanisms associated with the biogenesis and function of peroxisomes, our understanding of how peroxisomes are incorporated into metabolic pathways and cellular communication networks is just beginning to emerge. Here we address the interaction of peroxisomes with other subcellular compartments including the relationship with the endoplasmic reticulum, the peroxisome-mitochondria connection and the association with lipid droplets. We highlight metabolic cooperations and potential cross-talk and summarize recent findings on peroxisome-peroxisome interactions and the interaction of peroxisomes with microtubules in mammalian cells.