Failure of microtubule-mediated peroxisome division and trafficking in disorders with reduced peroxisome abundance

Hypomorphic mutation of PEX3 with peroxisomal mosaicism reveals the oscillating nature of peroxisome biogenesis coupled with differential metabolic activities

Impaired peroxisome assembly caused by mutations in PEX genes results in a human congenital metabolic disease called Zellweger spectrum disorder (ZSD), which impacts the development and physiological function of multiple organs. In this study, we revealed a long-standing problem of heterogeneous peroxisome distribution among cell population, so called "peroxisomal mosaicism", which appears in patients with mild form of ZSD. We mutated PEX3 gene in HEK293 cells and obtained a mutant clone with peroxisomal mosaicism. We found that peroxisomal mosaicism can be reproducibly arise from a single cell, even if the cell has many or no peroxisomes. Using time-lapse imaging and a long-term culture experiment, we revealed that peroxisome biogenesis oscillates over a span of days; this was also confirmed in the patient's fibroblasts. During the oscillation, the metabolic activity of peroxisomes was maintained in the cells with many peroxisomes while depleted in the cells without peroxisomes. Our results indicate that ZSD patients with peroxisomal mosaicism have a cell population whose number and metabolic activities of peroxisomes can be recovered. This finding opens the way to develop novel treatment strategy for ZSD patients with peroxisomal mosaicism, who currently have very limited treatment options.

Fine-tuning cell organelle dynamics during mitosis by small GTPases

During mitosis, the allocation of genetic material concurs with organelle transformation and distribution. The coordination of genetic material inheritance with organelle dynamics directs accurate mitotic progression, cell fate determination, and organismal homeostasis. Small GTPases belonging to the Ras superfamily regulate various cell organelles during division. Being the key regulators of membrane dynamics, the dysregulation of small GTPases is widely associated with cell organelle disruption in neoplastic and non-neoplastic diseases, such as cancer and Alzheimer's disease. Recent discoveries shed light on the molecular properties of small GTPases as sophisticated modulators of a remarkably complex and perfect adaptors for rapid structure reformation. This review collects current knowledge on small GTPases in the regulation of cell organelles during mitosis and highlights the mediator role of small GTPase in transducing cell cycle signaling to organelle dynamics during mitosis.

OXPHOS deficiencies affect peroxisome proliferation by downregulating genes controlled by the SNF1 signaling pathway

How environmental cues influence peroxisome proliferation, particularly through organelles, remains largely unknown. Yeast peroxisomes metabolize fatty acids (FA), and methylotrophic yeasts also metabolize methanol. NADH and acetyl-CoA, produced by these pathways enter mitochondria for ATP production and for anabolic reactions. During the metabolism of FA and/or methanol, the mitochondrial oxidative phosphorylation (OXPHOS) pathway accepts NADH for ATP production and maintains cellular redox balance. Remarkably, peroxisome proliferation in Pichia pastoris was abolished in NADH-shuttling- and OXPHOS mutants affecting complex I or III, or by the mitochondrial uncoupler, 2,4-dinitrophenol (DNP), indicating ATP depletion causes the phenotype. We show that mitochondrial OXPHOS deficiency inhibits expression of several peroxisomal proteins implicated in FA and methanol metabolism, as well as in peroxisome division and proliferation. These genes are regulated by the Snf1 complex (SNF1), a pathway generally activated by a high AMP/ATP ratio. In OXPHOS mutants, Snf1 is activated by phosphorylation, but Gal83, its interacting subunit, fails to translocate to the nucleus. Phenotypic defects in peroxisome proliferation observed in the OXPHOS mutants, and phenocopied by the Δgal83 mutant, were rescued by deletion of three transcriptional repressor genes (MIG1, MIG2, and NRG1) controlled by SNF1 signaling. Our results are interpreted in terms of a mechanism by which peroxisomal and mitochondrial proteins and/or metabolites influence redox and energy metabolism, while also influencing peroxisome biogenesis and proliferation, thereby exemplifying interorganellar communication and interplay involving peroxisomes, mitochondria, cytosol, and the nucleus. We discuss the physiological relevance of this work in the context of human OXPHOS deficiencies.

Competitive Microtubule Binding of PEX14 Coordinates Peroxisomal Protein Import and Motility

Human PEX14 plays a dual role as docking protein in peroxisomal protein import and as peroxisomal anchor for microtubules (MT), which relates to peroxisome motility. For docking, the conserved N-terminal domain of PEX14 (PEX14-NTD) binds amphipathic alpha-helical ligands, typically comprising one or two aromatic residues, of which human PEX5 possesses eight. Here, we show that the PEX14-NTD also binds to microtubular filaments in vitro with a dissociation constant in nanomolar range. PEX14 interacts with two motifs in the C-terminal region of human ß-tubulin. At least one of the binding motifs is in spatial proximity to the binding site of microtubules (MT) for kinesin. Both PEX14 and kinesin can bind to MT simultaneously. Notably, binding of PEX14 to tubulin can be prevented by its association with PEX5. The data suggest that PEX5 competes peroxisome anchoring to MT by occupying the ß-tubulin-binding site of PEX14. The competitive correlation of matrix protein import and motility may facilitate the homogeneous dispersion of peroxisomes in mammalian cells.

A Functional SMAD2/3 Binding Site in the PEX11β Promoter Identifies a Role for TGFβ in Peroxisome Proliferation in Humans

In mammals, peroxisomes perform crucial functions in cellular metabolism, signaling and viral defense which are essential to the viability of the organism. Molecular cues triggered by changes in the cellular environment induce a dynamic response in peroxisomes, which manifests itself as a change in peroxisome number, altered enzyme levels and adaptations to the peroxisomal morphology. How the regulation of this process is integrated into the cell's response to different stimuli, including the signaling pathways and factors involved, remains unclear. Here, a cell-based peroxisome proliferation assay has been applied to investigate the ability of different stimuli to induce peroxisome proliferation. We determined that serum stimulation, long-chain fatty acid supplementation and TGFβ application all increase peroxisome elongation, a prerequisite for proliferation. Time-resolved mRNA expression during the peroxisome proliferation cycle revealed a number of peroxins whose expression correlated with peroxisome elongation, including the β isoform of PEX11, but not the α or γ isoforms. An initial map of putative regulatory motif sites in the respective promoters showed a difference between binding sites in PEX11α and PEX11β, suggesting that these genes may be regulated by distinct pathways. A functional SMAD2/3 binding site in PEX11β points to the involvement of the TGFβ signaling pathway in expression of this gene and thus peroxisome proliferation/dynamics in humans.

Regulation of peroxisomal trafficking and distribution

Peroxisomes are organelles that perform a wide range of essential metabolic processes. To ensure that peroxisomes are optimally positioned in the cell, they must be transported by both long- and short-range trafficking events in response to cellular needs. Here, we review our current understanding of the mechanisms by which the cytoskeleton and organelle contact sites alter peroxisomal distribution. Though the focus of the review is peroxisomal transport in mammalian cells, findings from flies and fungi are used for comparison and to inform the gaps in our understanding. Attention is given to the apparent overlap in regulatory mechanisms for mitochondrial and peroxisomal trafficking, along with the recently discovered role of the mitochondrial Rho-GTPases, Miro, in peroxisomal dynamics. Moreover, we outline and discuss the known pathological and pharmacological conditions that perturb peroxisomal positioning. We conclude by highlighting several gaps in our current knowledge and suggest future directions that require attention.

The HIV-1 Accessory Protein Vpu Downregulates Peroxisome Biogenesis

People living with HIV can experience accelerated aging and the development of neurological disorders. Recently, we reported that HIV-1 infection results in a dramatic loss of peroxisomes in macrophages and brain tissue. This is significant because (i) peroxisomes are important for the innate immune response and (ii) loss of peroxisome function is associated with cellular aging and neurodegeneration. Accordingly, understanding how HIV-1 infection causes peroxisome depletion may provide clues regarding how the virus establishes persistent infections and, potentially, the development of neurological disorders. Here, we show that the accessory protein Vpu is necessary and sufficient for the induction of microRNAs that target peroxisome biogenesis factors. The ability of Vpu to downregulate peroxisome formation depends on the Wnt/β-catenin pathway. Thus, in addition to revealing a novel mechanism by which HIV-1 uses intracellular signaling pathways to target antiviral signaling platforms (peroxisomes), we have uncovered a previously unknown link between the Wnt/β-catenin pathway and peroxisome homeostasis.

Human immunodeficiency virus type 1 (HIV-1) establishes lifelong infections in humans, a process that relies on its ability to thwart innate and adaptive immune defenses of the host. Recently, we reported that HIV-1 infection results in a dramatic reduction of the cellular peroxisome pool. Peroxisomes are metabolic organelles that also function as signaling platforms in the innate immune response. Here, we show that the HIV-1 accessory protein Vpu is necessary and sufficient for the depletion of cellular peroxisomes during infection. Vpu induces the expression of four microRNAs that target mRNAs encoding proteins required for peroxisome formation and metabolic function. The ability of Vpu to downregulate peroxisomes was found to be dependent upon the Wnt/β-catenin signaling pathway. Given the importance of peroxisomes in innate immune signaling and central nervous system function, the roles of Vpu in dampening antiviral signaling appear to be more diverse than previously realized. Finally, our findings highlight a potential role for Wnt/β-catenin signaling in peroxisome homeostasis through modulating the production of biogenesis factors.

Expanding the spectrum of PEX16 mutations and novel insights into disease mechanisms

Zellweger syndrome spectrum disorders are caused by mutations in any of at least 12 different PEX genes. This includes PEX16, an important regulator of peroxisome biogenesis. Using whole genome sequencing, we detected previously unreported, biallelic variants in PEX16 [NM_004813.2:c.658G>A, p.(Ala220Thr) and NM_004813.2:c.830G>A, p.(Arg277Gln)] in an individual with leukodystrophy, spastic paraplegia, cerebellar ataxia, and craniocervical dystonia with normal plasma very long chain fatty acids. Using olfactory-neurosphere derived cells, a population of neural stem cells, we showed patient cells had reduced peroxisome density and increased peroxisome size, replicating previously reported findings in PEX16 cell lines. Along with alterations in peroxisome morphology, patient cells also had impaired peroxisome function with reduced catalase activity. Furthermore, patient cells had reduced oxidative stress levels after exposure to hydrogen-peroxide (H2O2), which may be a result of compensation by H2O2 metabolising enzymes other than catalase to preserve peroxisome-related cell functions. Our findings of impaired catalase activity and altered oxidative stress response are novel. Our study expands the phenotype of PEX16 mutations by including dystonia and provides further insights into the pathological mechanisms underlying PEX16-associated disorders. Additional studies of the full spectrum of peroxisomal dysfunction could improve our understanding of the mechanism underlying PEX16-associated disorders.

Super-resolution imaging reveals the sub-diffraction phenotype of Zellweger Syndrome ghosts and wild-type peroxisomes

Peroxisomes are ubiquitous cell organelles involved in many metabolic and signaling functions. Their assembly requires peroxins, encoded by PEX genes. Mutations in PEX genes are the cause of Zellweger Syndrome spectrum (ZSS), a heterogeneous group of peroxisomal biogenesis disorders (PBD). The size and morphological features of peroxisomes are below the diffraction limit of light, which makes them attractive for super-resolution imaging. We applied Stimulated Emission Depletion (STED) microscopy to study the morphology of human peroxisomes and peroxisomal protein localization in human controls and ZSS patients. We defined the peroxisome morphology in healthy skin fibroblasts and the sub-diffraction phenotype of residual peroxisomal structures (‘ghosts’) in ZSS patients that revealed a relation between mutation severity and clinical phenotype. Further, we investigated the 70 kDa peroxisomal membrane protein (PMP70) abundance in relationship to the ZSS sub-diffraction phenotype. This work improves the morphological definition of peroxisomes. It expands current knowledge about peroxisome biogenesis and ZSS pathoethiology to the sub-diffraction phenotype including key peroxins and the characteristics of ghost peroxisomes.

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.

New splicing variants of mitochondrial Rho GTPase-1 (Miro1) transport peroxisomes

The mechanisms underlying microtubule-dependent long-distance movement of peroxisomes in mammalian cells are unclear. Okumoto et al. identify splicing variants of human mitochondrial Rho GTPase-1 (Miro1) that localize to peroxisomes and that link these organelles to microtubule-dependent transport complexes including TRAK2.

Microtubule-dependent long-distance movement of peroxisomes occurs in mammalian cells. However, its molecular mechanisms remain undefined. In this study, we identified three distinct splicing variants of human mitochondrial Rho GTPase-1 (Miro1), each containing amino acid sequence insertions 1 (named Miro1-var2), 2 (Miro1-var3), and both 1 and 2 (Miro1-var4), respectively, at upstream of the transmembrane domain. Miro1-var4 and Miro1-var2 are localized to peroxisomes in a manner dependent on the insertion 1 that is recognized by the cytosolic receptor Pex19p. Exogenous expression of Miro1-var4 induces accumulation of peroxisomes at the cell periphery and augments long-range movement of peroxisomes along microtubules. Depletion of all Miro1 variants by knocking down MIRO1 suppresses the long-distance movement of peroxisomes. Such abrogated movement is restored by reexpression of peroxisomal Miro1 variants. Collectively, our findings identify for the first time peroxisome-localized Miro1 variants as adapter proteins that link peroxisomes to the microtubule-dependent transport complexes including TRAK2 in the intracellular translocation of peroxisomes in mammalian cells.

Coupling organelle inheritance with mitosis to balance growth and differentiation

Balancing growth and differentiation is essential to tissue morphogenesis and homeostasis. How imbalances arise in disease states is poorly understood. To address this issue, we identified transcripts differentially expressed in mouse basal epidermal progenitors versus their differentiating progeny and those altered in cancers. We used an in vivo RNA interference screen to unveil candidates that altered the equilibrium between the basal proliferative layer and suprabasal differentiating layers forming the skin barrier. We found that epidermal progenitors deficient in the peroxisome-associated protein Pex11b failed to segregate peroxisomes properly and entered a mitotic delay that perturbed polarized divisions and skewed daughter fates. Together, our findings unveil a role for organelle inheritance in mitosis, spindle alignment, and the choice of daughter progenitors to differentiate or remain stem-like.

The respiratory chain inhibitor rotenone affects peroxisomal dynamics via its microtubule-destabilising activity

Peroxisomes and mitochondria in mammalian cells are closely linked subcellular organelles, which maintain a redox-sensitive relationship. Their interplay and role in ROS signalling are supposed to impact on age-related and degenerative disorders. Whereas the generation of peroxisome-derived oxidative stress can affect mitochondrial morphology and function, little is known about the impact of mitochondria-derived oxidative stress on peroxisomes. Here, we investigated the effect of the mitochondrial complex I inhibitor rotenone on peroxisomal and mitochondrial membrane dynamics. We show that rotenone treatment of COS-7 cells alters peroxisome morphology and distribution. However, this effect is related to its microtubule-destabilising activity rather than to the generation of oxidative stress. Rotenone also induced alterations in mitochondrial morphology, which-in contrast to its effect on peroxisomes-were dependent on the generation of ROS but independent of its microtubule-active properties. The importance of our findings for the peroxisome-mitochondria redox relationship and the interpretation of in cellulo and in vivo studies with rotenone, which is widely used to study Parkinson's disease, are discussed.

MicroRNAs upregulated during HIV infection target peroxisome biogenesis factors: Implications for virus biology, disease mechanisms and neuropathology

HIV-associated neurocognitive disorders (HAND) represent a spectrum neurological syndrome that affects up to 25% of patients with HIV/AIDS. Multiple pathogenic mechanisms contribute to the development of HAND symptoms including chronic neuroinflammation and neurodegeneration. Among the factors linked to development of HAND is altered expression of host cell microRNAs (miRNAs) in brain. Here, we examined brain miRNA profiles among HIV/AIDS patients with and without HAND. Our analyses revealed differential expression of 17 miRNAs in brain tissue from HAND patients. A subset of the upregulated miRNAs (miR-500a-5p, miR-34c-3p, miR-93-3p and miR-381-3p), are predicted to target peroxisome biogenesis factors (PEX2, PEX7, PEX11B and PEX13). Expression of these miRNAs in transfected cells significantly decreased levels of peroxisomal proteins and concomitantly decreased peroxisome numbers or affected their morphology. The levels of miR-500a-5p, miR-34c-3p, miR-93-3p and miR-381-3p were not only elevated in the brains of HAND patients, but were also upregulated during HIV infection of primary macrophages. Moreover, concomitant loss of peroxisomal proteins was observed in HIV-infected macrophages as well as in brain tissue from HIV-infected patients. HIV-induced loss of peroxisomes was abrogated by blocking the functions of the upregulated miRNAs. Overall, these findings point to previously unrecognized miRNA expression patterns in the brains of HIV patients. Targeting peroxisomes by up-regulating miRNAs that repress peroxisome biogenesis factors may represent a novel mechanism by which HIV-1 subverts innate immune responses and/or causes neurocognitive dysfunction.

Host cells employ a myriad of antiviral defense systems but most viruses have developed effective countermeasures. Viruses such as HIV that cause lifelong infections are particularly successful in subverting the host antiviral response. While mitochondria have long been known to be critical hubs for antiviral signaling, it has only recently become apparent that peroxisomes are also important for this process. Peroxisomes are small and numerous structures that are best known for their roles in lipid metabolism. New evidence suggests that pathogenic viruses such as West Nile and Dengue viruses block the production of peroxisomes by sequestering and degradation a critical biogenesis factor. In the present study, we report that HIV significantly reduces the number of peroxisomes in infected cells via a completely novel mechanism. Specifically, HIV-infected cells express high levels of microRNAs that inhibit production of proteins required for peroxisome formation. Interestingly, levels of these microRNAs were elevated in the brains of patients with HIV-associated neurocognitive disorders. Thus, as well as affecting antiviral signaling, loss of peroxisomes during HIV infection may contribute to development of neurological disorders. Understanding how pathogenic viruses affect peroxisome biogenesis and cognate antiviral signaling may ultimately lead to novel therapeutic avenues and prevention of long-term sequelae.

Mitochondrial changes and oxidative stress in a mouse model of Zellweger syndrome neuropathogenesis

Zellweger syndrome (ZS) is a peroxisome biogenesis disorder that involves significant neuropathology, the molecular basis of which is still poorly understood. Using a mouse model of ZS with brain-restricted deficiency of the peroxisome biogenesis protein PEX13, we demonstrated an expanded and morphologically modified brain mitochondrial population. Cultured fibroblasts from PEX13-deficient mouse embryo displayed similar changes, as well as increased levels of mitochondrial superoxide and membrane depolarization; this phenotype was rescued by antioxidant treatment. Significant oxidative damage to neurons in brain was indicated by products of lipid and DNA oxidation. Similar overall changes were observed for glial cells. In toto, these findings suggest that mitochondrial oxidative stress and aberrant mitochondrial dynamics are associated with the neuropathology arising from PEX13 deficiency.

Mechanism of impaired microtubule-dependent peroxisome trafficking and oxidative stress in SPAST-mutated cells from patients with Hereditary Spastic Paraplegia

Hereditary spastic paraplegia (HSP) is an inherited neurological condition that leads to progressive spasticity and gait abnormalities. Adult-onset HSP is most commonly caused by mutations in SPAST, which encodes spastin a microtubule severing protein. In olfactory stem cell lines derived from patients carrying different SPAST mutations, we investigated microtubule-dependent peroxisome movement with time-lapse imaging and automated image analysis. The average speed of peroxisomes in patient-cells was slower, with fewer fast moving peroxisomes than in cells from healthy controls. This was not because of impairment of peroxisome-microtubule interactions because the time-dependent saltatory dynamics of movement of individual peroxisomes was unaffected in patient-cells. Our observations indicate that average peroxisome speeds are less in patient-cells because of the lower probability of individual peroxisome interactions with the reduced numbers of stable microtubules: peroxisome speeds in patient cells are restored by epothilone D, a tubulin-binding drug that increases the number of stable microtubules to control levels. Patient-cells were under increased oxidative stress and were more sensitive than control-cells to hydrogen peroxide, which is primarily metabolised by peroxisomal catalase. Epothilone D also ameliorated patient-cell sensitivity to hydrogen-peroxide. Our findings suggest a mechanism for neurodegeneration whereby SPAST mutations indirectly lead to impaired peroxisome transport and oxidative stress.

Microporation is an efficient method for siRNA-induced knockdown of PEX5 in HepG2 cells: evaluation of the transfection efficiency, the PEX5 mRNA and protein levels and induction of peroxisomal deficiency

The pathomechanism of peroxisomal biogenesis disorders (PBDs), a group of inherited autosomal recessive diseases with mutations of peroxin (PEX) genes, is not yet fully understood. Therefore, several knockout models, e.g., the PEX5 knockout mouse, have been generated exhibiting a complete loss of peroxisomal function. In this study, we wanted to knockdown PEX5 using the siRNA technology (1) to mimic milder forms of PBDs in which the mutated peroxin has some residual function and (2) to analyze the cellular consequences of a reduction of the PEX5 protein without adaption during the development as it is the case in a knockout animal. First, we tried to optimize the transfection of the hepatoma cell line HepG2 with PEX5 siRNA using different commercially available liposomal and non-liposomal transfection reagents (Lipofectamine(®) 2000, FuGENE 6, HiPerFect(®), INTERFERin™, RiboJuice™) as well as microporation using the Neon™ Transfection system. Microporation was found to be superior to the transfection reagents with respect to the transfection efficiency (100 vs. 0-70%), to the reduction of PEX5 mRNA (by 90 vs. 0-50%) and PEX5 protein levels (by 70 vs. 0-50%). Interestingly, we detected that a part of the cleaved PEX5 mRNA still existed as 3' fragment (15%) 24 h after microporation. Using microporation, we further analyzed whether the reduced PEX5 protein level impaired peroxisomal function. We indeed detected a reduced targeting of SKL-tagged proteins into peroxisomes as well as an increased oxidative stress as found in PBD patients and respective knockout mouse models. Knockdown of the PEX5 protein and functional consequences were at a maximum 48 h after microporation. Thereafter, the PEX5 protein was resynthesized, which may allow the temporal analysis of the loss as well as the reconstitution of peroxisomes in the future. In conclusion, we propose microporation as an efficient and reproducible method to transfect HepG2 cells with PEX5 siRNA. We succeeded to transiently knockdown PEX5 mRNA and its protein level leading to functional consequences similar as observed in peroxisome deficiencies.

Central serotonergic neuron deficiency in a mouse model of Zellweger syndrome

Zellweger syndrome (ZS) is a severe peroxisomal disorder caused by mutations in peroxisome biogenesis, or PEX, genes. A central hallmark of ZS is abnormal neuronal migration and neurodegeneration, which manifests as widespread neurological dysfunction. The molecular basis of ZS neuropathology is not well understood. Here we present findings using a mouse model of ZS neuropathology with conditional brain inactivation of the PEX13 gene. We demonstrate that PEX13 brain mutants display changes that reflect an abnormal serotonergic system - decreased levels of tryptophan hydroxylase-2, the rate-limiting enzyme of serotonin (5-hydroxytryptamine, 5-HT) synthesis, dysmorphic 5-HT-positive neurons, abnormal distribution of 5-HT neurons, and dystrophic serotonergic axons. The raphe nuclei region of PEX13 brain mutants also display increased levels of apoptotic cells and reactive, inflammatory gliosis. Given the role of the serotonergic system in brain development and motor control, dysfunction of this system would account in part for the observed neurological changes of PEX13 brain mutants.

Low dose tubulin-binding drugs rescue peroxisome trafficking deficit in patient-derived stem cells in Hereditary Spastic Paraplegia

Hereditary Spastic Paraplegia (HSP) is a genetically heterogeneous group of disorders, diagnosed by progressive gait disturbances with muscle weakness and spasticity, for which there are no treatments targeted at the underlying pathophysiology. Mutations in spastin are a common cause of HSP. Spastin is a microtubule-severing protein whose mutation in mouse causes defective axonal transport. In human patient-derived olfactory neurosphere-derived (ONS) cells, spastin mutations lead to lower levels of acetylated α-tubulin, a marker of stabilised microtubules, and to slower speed of peroxisome trafficking. Here we screened multiple concentrations of four tubulin-binding drugs for their ability to rescue levels of acetylated α-tubulin in patient-derived ONS cells. Drug doses that restored acetylated α-tubulin to levels in control-derived ONS cells were then selected for their ability to rescue peroxisome trafficking deficits. Automated microscopic screening identified very low doses of the four drugs (0.5 nM taxol, 0.5 nM vinblastine, 2 nM epothilone D, 10 µM noscapine) that rescued acetylated α-tubulin in patient-derived ONS cells. These same doses rescued peroxisome trafficking deficits, restoring peroxisome speeds to untreated control cell levels. These results demonstrate a novel approach for drug screening based on high throughput automated microscopy for acetylated α-tubulin followed by functional validation of microtubule-based peroxisome transport. From a clinical perspective, all the drugs tested are used clinically, but at much higher doses. Importantly, epothilone D and noscapine can enter the central nervous system, making them potential candidates for future clinical trials.

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.

Localization of Rab proteins to peroxisomes: a proteomics and immunofluorescence study

A proteomics screen was initiated to identify Rab proteins regulating transport to and away from peroxisomes. Mass spectrometry-based protein correlation profiling of rat liver organelles and immunofluorescence analysis of the peroxisome candidate Rab proteins revealed Rab6, Rab10, Rab14 and Rab18 to associate with the peroxisomal membrane. While Rab14 localized to peroxisomes predominantly in its dominant-active form, other Rab proteins associated with peroxisomes in both their GTP- and GDP-bound state. In summary, our data suggest that Rab6, Rab10, Rab14 and Rab18 associate with the peroxisomal compartment and similar as previously shown for Rab8, Rab18 in its GDP-bound state favors peroxisome proliferation.

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.

A patient-derived stem cell model of hereditary spastic paraplegia with SPAST mutations

Hereditary spastic paraplegia (HSP) leads to progressive gait disturbances with lower limb muscle weakness and spasticity. Mutations in SPAST are a major cause of adult-onset, autosomal-dominant HSP. Spastin, the protein encoded by SPAST, is a microtubule-severing protein that is enriched in the distal axon of corticospinal motor neurons, which degenerate in HSP patients. Animal and cell models have identified functions of spastin and mutated spastin but these models lack the gene dosage, mutation variability and genetic background that characterize patients with the disease. In this study, this genetic variability is encompassed by comparing neural progenitor cells derived from biopsies of the olfactory mucosa from healthy controls with similar cells from HSP patients with SPAST mutations, in order to identify cell functions altered in HSP. Patient-derived cells were similar to control-derived cells in proliferation and multiple metabolic functions but had major dysregulation of gene expression, with 57% of all mRNA transcripts affected, including many associated with microtubule dynamics. Compared to control cells, patient-derived cells had 50% spastin, 50% acetylated α-tubulin and 150% stathmin, a microtubule-destabilizing enzyme. Patient-derived cells were smaller than control cells. They had altered intracellular distributions of peroxisomes and mitochondria and they had slower moving peroxisomes. These results suggest that patient-derived cells might compensate for reduced spastin, but their increased stathmin expression reduced stabilized microtubules and altered organelle trafficking. Sub-nanomolar concentrations of the microtubule-binding drugs, paclitaxel and vinblastine, increased acetylated α-tubulin levels in patient cells to control levels, indicating the utility of this cell model for screening other candidate compounds for drug therapies.

A designed point mutant in Fis1 disrupts dimerization and mitochondrial fission

Mitochondrial and peroxisomal fission are essential processes with defects resulting in cardiomyopathy and neonatal lethality. Central to organelle fission is Fis1, a monomeric tetratricopeptide repeat (TPR)-like protein whose role in assembly of the fission machinery remains obscure. Two nonfunctional, Saccharomyces cerevisiae Fis1 mutants (L80P or E78D/I85T/Y88H) were previously identified in genetic screens. Here, we find that these two variants in the cytosolic domain of Fis1 (Fis1ΔTM) are unexpectedly dimeric. A truncation variant of Fis1ΔTM that lacks an N-terminal regulatory domain is also found to be dimeric. The ability to dimerize is a property innate to the native Fis1ΔTM amino acid sequence as we find this domain is dimeric after transient exposure to elevated temperature or chemical denaturants and is kinetically trapped at room temperature. This is the first demonstration of a specific self-association in solution for the Fis1 cytoplasmic domain. We propose a three-dimensional domain-swapped model for dimerization that is validated by a designed mutation, A72P, which potently disrupts dimerization of wild-type Fis1. A72P also disrupts dimerization of nonfunctional variants, indicating a common structural basis for dimerization. The obligate monomer variant A72P, like the dimer-promoting variants, is nonfunctional in fission, consistent with a model in which Fis1 activity depends on its ability to interconvert between monomer and dimer species. These studies suggest a new functionally important manner in which TPR-containing proteins may reversibly self-associate.

Membrane elongation factors in organelle maintenance: the case of peroxisome proliferation

Separation of metabolic pathways in organelles is critical for eukaryotic life. Accordingly, the number, morphology and function of organelles have to be maintained through processes linked with membrane remodeling events. Despite their acknowledged significance and intense study many questions remain about the molecular mechanisms by which organellar membranes proliferate. Here, using the example of peroxisome proliferation, we give an overview of how proteins elongate membranes. Subsequent membrane fission is achieved by dynamin-related proteins shared with mitochondria. We discuss basic criteria that membranes have to fulfill for these fission factors to complete the scission. Because peroxisome elongation is always associated with unequal distribution of matrix and membrane proteins, we propose peroxisomal division to be non-stochastic and asymmetric. We further show that these organelles need not be functional to carry on membrane elongation and present the most recent findings concerning members of the Pex11 protein family as membrane elongation factors. These factors, beside known proteins such as BAR-domain proteins, represent another family of proteins containing an amphipathic α-helix with membrane bending activity.

PEX14 is required for microtubule-based peroxisome motility in human cells

We have established a procedure for isolating native peroxisomal membrane protein complexes from cultured human cells. Protein-A-tagged peroxin 14 (PEX14), a central component of the peroxisomal protein translocation machinery was genomically expressed in Flp-In-293 cells and purified from digitonin-solubilized membranes. Size-exclusion chromatography revealed the existence of distinct multimeric PEX14 assemblies at the peroxisomal membrane. Using mass spectrometric analysis, almost all known human peroxins involved in protein import were identified as constituents of the PEX14 complexes. Unexpectedly, tubulin was discovered to be the major PEX14-associated protein, and direct binding of the proteins was demonstrated. Accordingly, peroxisomal remnants in PEX14-deficient cells have lost their ability to move along microtubules. In vivo and in vitro analyses indicate that the physical binding to tubulin is mediated by the conserved N-terminal domain of PEX14. Thus, human PEX14 is a multi-tasking protein that not only facilitates peroxisomal protein import but is also required for peroxisome motility by serving as membrane anchor for microtubules.

PEX13 deficiency in mouse brain as a model of Zellweger syndrome: abnormal cerebellum formation, reactive gliosis and oxidative stress

Delayed cerebellar development is a hallmark of Zellweger syndrome (ZS), a severe neonatal neurodegenerative disorder. ZS is caused by mutations in PEX genes, such as PEX13, which encodes a protein required for import of proteins into the peroxisome. The molecular basis of ZS pathogenesis is not known. We have created a conditional mouse mutant with brain-restricted deficiency of PEX13 that exhibits cerebellar morphological defects. PEX13 brain mutants survive into the postnatal period, with the majority dying by 35 days, and with survival inversely related to litter size and weaning body weight. The impact on peroxisomal metabolism in the mutant brain is mixed: plasmalogen content is reduced, but very-long-chain fatty acids are normal. PEX13 brain mutants exhibit defects in reflex and motor development that correlate with impaired cerebellar fissure and cortical layer formation, granule cell migration and Purkinje cell layer development. Astrogliosis and microgliosis are prominent features of the mutant cerebellum. At the molecular level, cultured cerebellar neurons from E19 PEX13-null mice exhibit elevated levels of reactive oxygen species and mitochondrial superoxide dismutase-2 (MnSOD), and show enhanced apoptosis together with mitochondrial dysfunction. PEX13 brain mutants show increased levels of MnSOD in cerebellum. Our findings suggest that PEX13 deficiency leads to mitochondria-mediated oxidative stress, neuronal cell death and impairment of cerebellar development. Thus, PEX13-deficient mice provide a valuable animal model for investigating the molecular basis and treatment of ZS cerebellar pathology.

PEX11 family members are membrane elongation factors that coordinate peroxisome proliferation and maintenance

Dynamic changes of membrane structure are intrinsic to organelle morphogenesis and homeostasis. Ectopic expression of proteins of the PEX11 family from yeast, plant or human lead to the formation of juxtaposed elongated peroxisomes (JEPs),which is evocative of an evolutionary conserved function of these proteins in membrane tubulation. Microscopic examinations reveal that JEPs are composed of independent elongated peroxisomes with heterogeneous distribution of matrix proteins. We established the homo- and heterodimerization properties of the human PEX11 proteins and their interaction with the fission factor hFis1, which is known to recruit the GTPase DRP1 to the peroxisomal membrane. We show that excess of hFis1 but not of DRP1 is sufficient to fragment JEPs into normal round-shaped organelles, and illustrate the requirement of microtubules for JEP formation. Our results demonstrate that PEX11-induced JEPs represent intermediates in the process of peroxisome membrane proliferation and that hFis1 is the limiting factor for progression. Hence, we propose a model for a conserved role of PEX11 proteins in peroxisome maintenance through peroxisome polarization, membrane elongation and segregation.

Differential expression of peroxisomal matrix and membrane proteins during postnatal development of mouse brain

In peroxisomal biogenesis disorders, serious neurological abnormalities can be observed in the patients and the respective knockout mouse models. As a prerequisite for a better understanding of the relationship between the absence of peroxisomes and the observed neuropathology, knowledge of the regional and cell-type specific distribution of peroxisomal proteins in mouse brain is necessary. Therefore, we investigated the expression of distinct peroxins, peroxisomal membrane and matrix proteins (e.g. Pex5p, Pex14p, Pex13p, PMP70, catalase, peroxisomal thiolase, Acox1, "SKL"-PTS1 proteins) by indirect immunofluorescence 1) in primary cultures of the medial neocortex, hippocampus, and cerebellum of newborn mice and 2) in paraffin sections of mouse brain of different ages (newborn-adult). Quantitative analysis revealed a comparable abundance (number/microm(2)) of peroxisomes in cultured neurons and astrocytes of all three brain regions. In contrast, catalase immunoreactivity was higher in cultured astrocytes than in neurons. In mouse brain tissue, the abundance of peroxisomes decreased by half during postnatal development, also exhibiting prominent differences between distinct brain regions and cell types. Catalase protein levels in neuronal peroxisomes, however, decreased much more strongly in the neocortex, CA1-3 areas of the hippocampus, dentate gyrus, cerebellar nuclei, and cerebellar cortex but remained high in Bergmann glia and other astrocytes, epithelial cells of the choroid plexus, and ependyma. Similar age-dependent changes were found for thiolase and Acox1 protein levels. Developmental changes were confirmed by Western blot analysis using enriched peroxisomal and cytosolic fractions of the brain tissue as well as by measurement of catalase activity.

Segmentation of cultured neurons using logical analysis of grey and distance difference

The molecular and cellular bases of neuronal cell death that underpin a wide range of neurodegenerative disorders are still not well understood. One approach to investigating neuronal death is through systematic studies of the changing morphology of cultured brain neurons in response to cellular challenges. Image segmentation methods developed to date to analyze such changes have been limited by the low contrast of cells in unstained neuronal cultures and the unimodal histograms generated by these analyses. In this paper we present new algorithms based on logical analysis of grey and distance difference of images that successfully circumvent these problems. Two key parameters of this analysis, window width and logical threshold, are automatically extracted for use in logical level technique, and spurious regions are detected and removed through use of a hierarchical window filter. The efficacy of the developed algorithms is demonstrated here through an analysis of cultured brain neurons from newborn mice.

Generalised and conditional inactivation of Pex genes in mice

During the past 10 years, several Pex genes have been knocked out in the mouse with the purpose to generate models to study the pathogenesis of peroxisome biogenesis disorders and/or to investigate the physiological importance of the Pex proteins. More recently, mice with selective inactivation of a Pex gene in particular cell types were created. The metabolic abnormalities in peroxisome deficient mice paralleled to a large extent those of Zellweger patients. Several but not all of the clinical and histological features reported in patients also occurred in peroxisome deficient mice as for example hypotonia, cortical and cerebellar malformations, endochondral ossification defects, hepatomegaly, liver fibrosis and ultrastructural abnormalities of mitochondria in hepatocytes. Although the molecular origins of the observed pathologies have not yet been resolved, several new insights on the importance of peroxisomes in different tissues have emerged.

Growth and Division of Peroxisomes

Peroxisomes are ubiquitous subcellular organelles, which are highly dynamic and display large plasticity in response to cellular and environmental conditions. Novel proteins and pathways that mediate and control peroxisome formation, growth, and division continue to be discovered, and the cellular machineries that act together to regulate peroxisome number and size are under active investigation. Here, advances in the field of peroxisomal dynamics and proliferation in mammals and yeast are reviewed. The authors address the signals, conditions, and proteins that affect, regulate, and control the number and size of this essential organelle, especially the components involved in the division of peroxisomes. Special emphasis is on the function of dynamin-related proteins (DRPs), on Fis1, a putative adaptor for DRPs, on the role of the Pex11 family of peroxisomal membrane proteins, and the cytoskeleton.