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.

The peroxisome: an update on mysteries 2.0.

Peroxisomes are key metabolic organelles, which contribute to cellular lipid metabolism, e.g. the ß-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.

Cytochemical Detection of Peroxisomes in Light and Electron Microscopy with 3,3'-diaminobenzidine.

Peroxisomes are ubiquitous dynamic and multifunctional organelles that contribute to numerous anabolic and catabolic pathways, being essential for human health and development. Their best known functions include the oxidation of fatty acids and metabolism of hydrogen peroxide with catalase as a marker enzyme. Indeed, historically, it was the cytochemical staining of catalase in many different cells and tissues that revealed the ubiquitous presence of peroxisomes in almost all animal and plant cells. In this chapter, the method for cytochemical staining of catalase with the alkaline 3, 3'-diaminobenzidine (DAB) is described. Since aldehyde fixation is a prerequisite for staining of catalase with DAB, a method for perfusion fixation of rat liver with glutaraldehyde is presented prior to the cytochemical staining method and the subsequent tissue processing for light and electron microscopy.

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.

The peroxisome: an update on mysteries.

Peroxisomes contribute to several crucial metabolic processes such as ß-oxidation of fatty acids, biosynthesis of ether phospholipids and metabolism of reactive oxygen species, which render them indispensable to human health and development. Peroxisomes are highly dynamic organelles that rapidly assemble, multiply and degrade in response to metabolic needs. In recent years, the interest in peroxisomes and their physiological functions has significantly increased. This review intends to highlight recent discoveries and trends in peroxisome research, and represents an update as well as a continuation of a former review article. Novel exciting findings on the biological functions, biogenesis, formation and degradation of peroxisomes, on peroxisomal dynamics and division, as well as on the interaction and cross-talk of peroxisomes with other subcellular compartments are addressed. Furthermore, recent findings on the role of peroxisomes in the brain are discussed.

Reactive oxygen species and peroxisomes: struggling for balance.

Reactive oxygen species (ROS) can surely be considered as multifunctional biofactors within the cell. They are known to participate in regular cell functions, for example, as signal mediators, but overproduction under oxidative stress conditions leads to deleterious cellular effects, cell death and diverse pathological conditions. Peroxisomal function has long been linked to oxygen metabolism due to the high concentration of H(2)O(2)-generating oxidases in peroxisomes and their set of antioxidant enzymes, especially catalase. Still, mitochondria have been very much placed in the centre of ROS metabolism and oxidative stress. This review discusses novel findings concerning the relationship between ROS and peroxisomes, as they revealed to be a key player in the dynamic spin of ROS metabolism and oxidative injury. An overview of ROS generating enzymes as well as their antioxidant counterparts will be given, exemplifying the precise fine-tuning between the opposing systems. Various conditions in which the balance between generation and scavenging of ROS in peroxisomes is perturbed, for example, exogenous manipulation, ageing and peroxisomal disorders, are addressed. Furthermore, peroxisome-derived oxidative stress and its effect on mitochondria (and vice versa) are discussed, highlighting the close interrelationship of both organelles.

The peroxisome: still a mysterious organelle.

More than half a century of research on peroxisomes has revealed unique features of this ubiquitous subcellular organelle, which have often been in disagreement with existing dogmas in cell biology. About 50 peroxisomal enzymes have so far been identified, which contribute to several crucial metabolic processes such as beta-oxidation of fatty acids, biosynthesis of ether phospholipids and metabolism of reactive oxygen species, and render peroxisomes indispensable for human health and development. It became obvious that peroxisomes are highly dynamic organelles that rapidly assemble, multiply and degrade in response to metabolic needs. However, many aspects of peroxisome biology are still mysterious. This review addresses recent exciting discoveries on the biogenesis, formation and degradation of peroxisomes, on peroxisomal dynamics and division, as well as on the interaction and cross talk of peroxisomes with other subcellular compartments. Furthermore, recent advances on the role of peroxisomes in medicine and in the identification of novel peroxisomal proteins are discussed.

Compartment-dependent management of H(2)O(2) by peroxisomes.

Peroxisomes (PO) are essential and ubiquitous single-membrane-bound organelles whose ultrastructure is characterized by a matrix and often a crystalloid core. A unique feature is their capacity to generate and degrade H(2)O(2) via several oxidases and catalase, respectively. Handling of H(2)O(2) within PO is poorly understood and, in contrast to mitochondria, they are not regarded as a default H(2)O(2) source. Using an ultrasensitive luminometric H(2)O(2) assay, we show in real time that H(2)O(2) handling by matrix-localized catalase depends on the localization of H(2)O(2) generation in- and outside the PO. Thus, intact PO are inefficient at degrading external but also internal H(2)O(2) that is generated by the core-localized urate oxidase (UOX). Our findings suggest that, in addition to the PO membrane, the matrix forms a significant diffusion barrier for H(2)O(2). In contrast, matrix-generated H(2)O(2) is efficiently degraded. We further show that the tubular structures in crystalloid cores of UOX are associated with and perpendicularly oriented toward the PO membrane. Studies on metabolically active liver slices demonstrate that UOX directly releases H(2)O(2) into the cytoplasm, with the 5-nm primary tubules in crystalloid cores serving as exhaust conduits. Apparently, PO are inefficient detoxifiers of external H(2)O(2) but rather can become an obligatory source of H(2)O(2)--an important signaling molecule and a potential toxin.

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.

Peroxisomes and oxidative stress.

The discovery of the colocalization of catalase with H2O2-generating oxidases in peroxisomes was the first indication of their involvement in the metabolism of oxygen metabolites. In past decades it has been revealed that peroxisomes participate not only in the generation of reactive oxygen species (ROS) with grave consequences for cell fate such as malignant degeneration but also in cell rescue from the damaging effects of such radicals. In this review the role of peroxisomes in a variety of physiological and pathological processes involving ROS mainly in animal cells is presented. At the outset the enzymes generating and scavenging H2O2 and other oxygen metabolites are reviewed. The exposure of cultured cells to UV light and different oxidizing agents induces peroxisome proliferation with formation of tubular peroxisomes and apparent upregulation of PEX genes. Significant reduction of peroxisomal volume density and several of their enzymes is observed in inflammatory processes such as infections, ischemia-reperfusion injury and hepatic allograft rejection. The latter response is related to the suppressive effects of TNFalpha on peroxisomal function and on PPARalpha. Their massive proliferation induced by a variety of xenobiotics and the subsequent tumor formation in rodents is evidently due to an imbalance in the formation and scavenging of ROS, and is mediated by PPARalpha. In PEX5-/- mice with the absence of functional peroxisomes severe abnormalities of mitochondria in different organs are observed which resemble closely those in respiratory chain disorders associated with oxidative stress. Interestingly, no evidence of oxidative damage to proteins or lipids, nor of increased peroxide production has been found in that mouse model. In this respect the role of PPARalpha, which is highly activated in those mice, in prevention of oxidative stress deserves further investigation.

Mammalian peroxisomes and reactive oxygen species.

The central role of peroxisomes in the generation and scavenging of hydrogen peroxide has been well known ever since their discovery almost four decades ago. Recent studies have revealed their involvement in metabolism of oxygen free radicals and nitric oxide that have important functions in intra- and intercellular signaling. The analysis of the role of mammalian peroxisomes in a variety of physiological and pathological processes involving reactive oxygen species (ROS) is the subject of this review. The general characteristics of peroxisomes and their enzymes involved in the metabolism of ROS are briefly reviewed. An expansion of the peroxisomal compartment with proliferation of tubular peroxisomes is observed in cells exposed to UV irradiation and various oxidants and is apparently accompanied by upregulation of PEX genes. Significant reduction of peroxisomes and their enzymes is observed in inflammatory processes including infections, ischemia-reperfusion injury, and allograft rejection and seems to be related to the suppressive effect of tumor necrosis factor-alpha on peroxisome function and peroxisome proliferator activated receptor-alpha. Xenobiotic-induced proliferation of peroxisomes in rodents is accompanied by the formation of hepatic tumors, and evidently the imbalance in generation and decomposition of ROS plays an important role in this process. In PEX5-/- knockout mice lacking functional peroxisomes severe alterations of mitochondria in various organs are observed which seem to be due to a generalized increase in oxidative stress confirming the important role of peroxisomes in homeostasis of ROS and the implications of its disturbances for cell pathology.

Biochemical and morphological effects of K-111, a peroxisome proliferator-activated receptor (PPAR)alpha activator, in non-human primates.

K-111 has been characterized as a potent peroxisome proliferator-activated receptor (PPAR)alpha activator. Antidiabetic potency and amelioration of disturbed lipid metabolism were demonstrated in rodents, which were accompanied by elevations of peroxisomal enzymes and liver weight. To examine the possible therapeutic application of K-111 we have now assessed its efficacy in non-human primates with high transferability to humans. For this purpose obese, hypertriglyceridaemic, hyperinsulinaemic prediabetic rhesus monkeys were dosed sequentially with 0, 1, 3 and 10mg/kg per day orally over a period of 4 weeks each. In addition, the effect of K-111 on the peroxisome compartment was analyzed in cynomolgus monkeys using liver samples obtained following a 13-week oral toxicity study. In prediabetic monkeys, the reduction of hyperinsulinaemia and improvement of insulin-stimulated glucose uptake rate indicated amelioration of insulin resistance. These effects were nearly maximal at a dose of 3mg/kg per day, while triglycerides and body weight were lowered significantly in a dose-dependent manner. This reduction of body weight contrasts sharply with the adipogenic response observed with thiazolidinediones, another family of insulin-sensitizing agents. In young cynomolgus monkeys at a dosage of 5mg/kg per day and more, K-111 induced an up to three-fold increase in lipid beta-oxidation enzymes with an 1.5- to 2-fold increase in peroxisome volume density. This moderate increase in peroxisomal activity by K-111 in monkeys is consistent with its role as an PPARalpha activator and corresponds to the observations with fibrates in other low responder mammalian species. The increase in beta-oxidation may explain, at least in part, the lipid modulating effect as well as the antidiabetic potency of K-111. This pharmacological profile makes K-111 a highly promising drug candidate for clinical applications in the treatment of type 2 diabetes, dyslipidaemia, obesity and the metabolic syndrome.

Expression of peroxisome proliferator-activated receptors in the liver of gray mullet (Mugil cephalus).

During the last decade, peroxisome proliferation has emerged as a novel biomarker of exposure to certain organic chemical pollutants in aquatic organisms. Peroxisome proliferation is mediated by nuclear receptors, peroxisome proliferator-activated receptors (PPARs). Three PPAR subtypes have been described in mammals: PPAR alpha, PPAR beta and PPAR gamma. PPARs have also been discovered in several fish species. The aim of the present study was to investigate the expression of PPAR subtypes and their cellular distribution patterns in the liver of gray mullet Mugil cephalus, a fish species widely distributed in estuaries and coastal areas in Europe and used as sentinel of environmental pollution. For this purpose, antibodies were generated against the three subtypes of mouse PPARs and different protocols of antigen retrieval were used. In western blots, main bands were detected of approximately 44 kDa for PPAR alpha, two bands of 44 and 58 kDa for PPAR beta and a single band of 56 kDa for PPAR gamma. Similar results were obtained in mouse liver and may indicate antibody recognition of two forms of the protein in certain cases. PPAR alpha was the subtype most markedly expressed in gray mullet liver, being expressed mainly in melanomacrophages, nuclei of hepatocytes and sinusoidal cells and connective tissue surrounding bile ducts. PPAR beta was expressed in the same cell types but immunolabeling was generally weaker than for PPAR alpha. PPAR gamma showed very weak expression; positivity was mainly found in melanomacrophages and connective tissue surrounding bile ducts. Our results demonstrate that all the three PPAR subtypes are expressed in gray mullet liver but in different intensities. The cellular distribution patterns of PPAR subtypes in gray mullet liver resembled partly those found in mouse liver with PPAR alpha as the main subtype expressed in hepatocytes. The fact that melanomacrophages, cells of the immune system in fish, show strong expression of both PPAR alpha and PPAR beta whereas PPAR gamma expression is almost restricted to this cell type suggest a significant role of PPAR-mediated regulation of cell function in melanomacrophages.

Differential cellular and subcellular localization of heme-binding protein 23/peroxiredoxin I and heme oxygenase-1 in rat liver.

Heme-binding protein 23 (HBP23), also termed peroxiredoxin (Prx) I, and heme oxygenase-1 (HO-1) are distinct antioxidant stress proteins that are co-ordinately induced by oxidative stress. HBP23/Prx I has thioredoxin-dependent peroxidase activity with high binding affinity for the pro-oxidant heme, while HO-1 is the inducible isoform of the rate-limiting enzyme of heme degradation. We investigated the cellular and subcellular localization of both proteins in rat liver. Whereas by immunohistochemistry (IHC) a uniformly high level of HBP23/Prx I expression was observed in liver parenchymal and different sinusoidal cells, HO-1 expression was restricted to Kupffer cells. By immunoelectron microscopy using the protein A-gold technique, HBP23/Prx I immunoreactivity was detected in cytoplasm, nuclear matrix, mitochondria, and peroxisomes of parenchymal and non-parenchymal liver cell populations. In contrast, the secretory pathway, i.e., the endoplasmic reticulum and Golgi complex, was free of label. As determined by immunocytochemical (ICC) studies in liver cell cultures and by Western and Northern blotting analysis, HBP23/Prx I was highly expressed in cultures of isolated hepatocytes and Kupffer cells. In contrast, HO-1 was constitutively expressed only in Kupffer cell cultures but was also inducible in hepatocytes. These data suggest that HBP23/Prx I and HO-1 may have complementary antioxidant functions in different cell populations in rat liver.

A review of morphological techniques for detection of peroxisomal (and mitochondrial) proteins and their corresponding mRNAs during ontogenesis in mice: application to the PEX5-knockout mouse with Zellweger syndrome.

In the era of application of molecular biological gene-targeting technology for the generation of knockout mouse models to study human genetic diseases, the availability of highly sensitive and reliable methods for the morphological characterization of the specific phenotypes of these mice is of great importance. In the first part of this report, the role of morphological techniques for studying the biology and pathology of peroxisomes is reviewed, and the techniques established in our laboratories for the localization of peroxisomal proteins and corresponding mRNAs in fetal and newborn mice are presented and discussed in the context of the international literature. In the second part, the literature on the ontogenetic development of the peroxisomal compartment in mice, with special emphasis on liver and intestine is reviewed and compared with our own data reported recently. In addition, some recent data on the pathological alterations in the liver of the PEX5(-/-) mouse with a peroxisomal biogenesis defect are briefly discussed. Finally, the methods developed during these studies for the localization of mitochondrial proteins (respiratory chain complexes and MnSOD) are presented and their advantages and pitfalls discussed. With the help of these techniques, it is now possible to identify and distinguish unequivocally peroxisomes from mitochondria, two classes of cell organelles giving by light microscopy a punctate staining pattern in microscopical immunohistochemical preparations of paraffin-embedded mouse tissues.

Peroxisomal motility and interaction with microtubules.

Recent in vivo observations have revealed that peroxisomes are more dynamic and interactive than previously assumed. The growing recognition of the tubular and reticular morphology of peroxisomes in living cells, their association with microtubules, and the dynamic movements of peroxisomes in vivo and in vitro have inspired the query into the investigation of the cellular machinery that mediates such a complex behaviour. The characterisation of the underlying molecular components of this machinery is providing insight into the mechanisms regulating peroxisomal morphology and intracellular distribution.

Expression of catalase mRNA and protein in adult rat brain: detection by nonradioactive in situ hybridization with signal amplification by catalyzed reporter deposition (ISH-CARD) and immunohistochemistry (IHC)/immunofluorescence (IF).

Catalase, the classical peroxisomal marker enzyme, decomposes hydrogen peroxide and is involved in the antioxidant defense mechanisms of mammalian cells. In addition, catalase can oxidize, by means of its peroxidatic activity, a variety of substrates such as methanol and ethanol, producing the corresponding aldehydes. The involvement of brain catalase in the oxidation of ethanol is well established, and severe afflictions of the CNS in hereditary peroxisomal diseases (e.g., Zellweger syndrome) are well known. Whereas the distribution of catalase in the CNS has been investigated by enzyme histochemistry and immunohistochemistry (IHC), very little is known about the exact localization of catalase mRNA in brain. Here we report the application of a tyramine/CARD (catalyzed reporter deposition)-enhanced nonradioactive in situ hybridization (ISH) protocol for detection of catalase mRNA in sections of perfusion-fixed, paraffin-embedded rat brain. Catalase mRNA could be demonstrated in a large number of neurons throughout the rat brain as a distinct cytoplasmic staining signal with excellent morphological resolution. Compared to our standard ISH protocol, the CARD-enhanced protocol for catalase mRNA detection in rat brain showed higher sensitivity and significantly better signal-to-noise ratio. In parallel IHC experiments, using an antigen retrieval method consisting of combined trypsin digestion and microwave treatment of paraffin sections, the catalase antigen was found as distinct cytoplasmic granules in most catalase mRNA-positive neurons. In addition, catalase-positive granules, presumably peroxisomes, were found by confocal laser scanning microscopy in glial cells, which were identified by double labeling immunofluorescence for GFAP and CNPase for astroglial cells and oligodentrocytes, respectively. The excellent preservation of morphology and sensitive detection of both mRNA and protein in our preparations warrant the application of the protocols described here for systematic studies of catalase and other peroxisomal proteins in diverse pathological conditions such as Alzheimer's disease and aging.

Expression of peroxisome proliferator-activated receptors in zebrafish (Danio rerio).

Peroxisomes increase in size and number in responsive animals ranging from mammals to marine mussels and fish species when treated with certain compounds named peroxisome proliferators. This phenomenon, known as peroxisome proliferation, is mediated by nuclear receptors termed peroxisome proliferator-activated receptors (PPARs). Three PPAR subtypes have been described (alpha, beta, and gamma) and in mammals PPARalpha is mainly expressed in tissues that catabolize fatty acids, PPARbeta is ubiquitously distributed, and PPARgamma is mainly expressed in the adipose tissue and immune system. The aim of this study was to analyze the tissue distribution of different PPAR subtypes in zebrafish Danio rerio using commercially available antibodies against PPARalpha, PPARbeta, and PPARgamma. In western blots, specific bands were detected at about 58 kDa for PPARalpha and PPARbeta. For PPARgamma the band was detected at 56 kDa. Similar results were obtained in mouse liver homogenates used as positive control, indicating the specificity of the antibodies. Immunohistochemistry was performed in paraformaldehyde-fixed tissue using either microwave or microwave plus trypsin pretreatment for antigen retrieval. In zebrafish, PPARalpha was expressed mainly in liver parenchymal cells, proximal tubules of kidney, enterocytes, and pancreas. PPARbeta showed a widespread distribution and was expressed in the liver, proximal and distal tubules and glomeruli of the kidney, pancreas, enterocytes and smooth muscle of the intestine, skin epithelium, lymphocytes, and male and female gonads. PPARgamma expression was weak in pancreatic cells, intestine, and gonads for both pretreatments. Most of the signal detected was cytoplasmic; only in the cases of PPARalpha and PPARbeta was some nuclear labeling detected in the liver. In mouse tissues, the distribution of PPAR subtypes was similar to that described previously for rats. Our results demonstrate that all three distinct PPAR subtypes are present in zebrafish. The tissue and cellular distribution of PPAR subtypes in zebrafish resembled partly that described before in mammals. Further studies are needed to decipher the functions of PPAR subtypes in zebrafish and other aquatic organisms and particularly their role in regulation of metabolic responses to xenobiotic exposure.

Cellular and subcellular distribution of D-aspartate oxidase in human and rat brain.

The unusual amino acid D-aspartate is present in significant amounts in brain and endocrine glands and is supposed to be involved in neurotransmission and neurosecretion (Wolosker et al. [2000] Neuroscience 100:183-189). D-aspartate oxidase is the only enzyme known to metabolize D-aspartate and could regulate its level in different regions of the brain. We examined the cellular and subcellular distribution of this enzyme and its mRNA in human and rat brain by immunohistochemistry, in situ hybridization, and immunoelectron microscopy. D-aspartate oxidase protein and mRNA are ubiquitous. The protein shows a granular pattern, particularly within neurons and to a significantly lesser extent in astrocytes and oligodendrocytes. No evidence for a synaptic association was observed. Whereas between most positive neurons only gradual differences were observed, in the hypothalamic paraventricular nucleus, neurons with high enzyme content were found next to others with no labeling. cDNA cloning of D-aspartate oxidase corroborates an inherent targeting signal sequence for protein import into peroxisomes. Immunoelectron microscopy showed that the protein is localized in single membrane-bound organelles, apparently peroxisomes.