Peroxisomes in sebaceous glands. V. Complex peroxisomes in the mouse preputial gland: serial sectioning and three-dimensional reconstruction studies

Primary cilia are critical for tracheoesophageal septation

INTRODUCTION

Primary cilia play pivotal roles in the patterning and morphogenesis of a wide variety of organs during mammalian development. Here we examined murine foregut septation in the cobblestone mutant, a hypomorphic allele of the gene encoding the intraflagellar transport protein IFT88, a protein essential for normal cilia function.

RESULTS

We reveal a crucial role for primary cilia in foregut division, since their dramatic decrease in cilia in both the foregut endoderm and mesenchyme of mutant embryos resulted in a proximal tracheoesophageal septation defects and in the formation of distal tracheo(broncho)esophageal fistulae similar to the most common congenital tracheoesophageal malformations in humans. Interestingly, the dorsoventral patterning determining the dorsal digestive and the ventral respiratory endoderm remained intact, whereas Hedgehog signaling was aberrantly activated.

CONCLUSIONS

Our results demonstrate the cobblestone mutant to represent one of the very few mouse models that display both correct endodermal dorsoventral specification but defective compartmentalization of the proximal foregut. It stands exemplary for a tracheoesophageal ciliopathy, offering the possibility to elucidate the molecular mechanisms how primary cilia orchestrate the septation process. The plethora of malformations observed in the cobblestone embryo allow for a deeper insight into a putative link between primary cilia and human VATER/VACTERL syndromes.

Simultaneous live-imaging of peroxisomes and the ER in plant cells suggests contiguity but no luminal continuity between the two organelles

Transmission electron micrographs of peroxisomes in diverse organisms, including plants, suggest their close association and even luminal connectivity with the endoplasmic reticulum (ER). After several decades of debate de novo peroxisome biogenesis from the ER is strongly favored in yeasts and mammals. Unfortunately many of the proteins whose transit through the ER constitutes a major evidence for peroxisome biogenesis from the ER do not exhibit a similar localization in plants. Consequently, at best the ER acts as a membrane source for peroxisome in plants. However, in addition to their de novo biogenesis from the ER an increase in peroxisome numbers also occurs through fission of existing peroxisomes. In recent years live-imaging has been used to visualize peroxisomes and the ER but the precise spatio-temporal relationship between the two organelles has not been well-explored. Here we present our assessment of the peroxisome-ER relationship through imaging of living Arabidopsis thaliana plants simultaneously expressing different color combinations of fluorescent proteins targeted to both organelles. Our observations on double transgenic wild type and a drp3a/apm1 mutant Arabidopsis plants suggest strong correlations between the dynamic behavior of peroxisomes and the neighboring ER. Although peroxisomes and ER are closely aligned there appears to be no luminal continuity between the two. Similarly, differentially colored elongated peroxisomes of a drp3a mutant expressing a photoconvertible peroxisomal matrix protein are unable to fuse and share luminal protein despite considerable intermingling. Substantiation of our observations is suggested through 3D iso-surface rendering of image stacks, which shows closed ended peroxisomes enmeshed among ER tubules possibly through membrane contact sites (MCS). Our observations support the idea that increase in peroxisome numbers in a plant cell occurs mainly through the fission of existing peroxisomes in an ER aided manner.

III. Cellular ultrastructures in situ as key to understanding tumor energy metabolism: biological significance of the Warburg effect

Despite the universality of metabolic pathways, malignant cells were found to have their metabolism reprogrammed to generate energy by glycolysis even under normal oxygen concentrations (the Warburg effect). Therefore, the pathway energetically 18 times less efficient than oxidative phosphorylation was implicated to match increased energy requirements of growing tumors. The paradox was explained by an abnormally high rate of glucose uptake, assuming unlimited availability of substrates for tumor growth in vivo. However, ultrastructural analysis of tumor vasculature morphogenesis showed that the growing tissue regions did not have continuous blood supply and intermittently depended on autophagy for survival. Erythrogenic autophagy, and resulting ATP generation by glycolysis, appeared critical to initiating vasculature formation where it was missing. This study focused on ultrastructural features that reflected metabolic switch from aerobic to anaerobic. Morphological differences between and within different types of cells were evident in tissue sections. In cells undergoing nucleo-cytoplasmic conversion into erythrosomes (erythrogenesis), gradual changes led to replacing mitochondria with peroxisomes, through an intermediate form connected to endoplasmic reticulum. Those findings related to the issue of peroxisome biogenesis and to the phenomenon of hemogenic endothelium. Mitochondria were compacted also during mitosis. In vivo, cells that lost and others that retained capability to use oxygen coexisted side-by-side; both types were important for vasculature morphogenesis and tissue growth. Once passable, the new vasculature segment could deliver external oxygen and nutrients. Nutritional and redox status of microenvironment had similar effect on metabolism of malignant and non-malignant cells demonstrating the necessity to maintain structure-energy equivalence in all living cells. The role of glycolysis in initiating vasculature formation, and in progression of cell cycle through mitosis, indicated that Warburg effect had a fundamental biological significance extending to non-malignant tissues. The approach used here could facilitate integration of accumulated cyber knowledge on cancer metabolism into predictive science.

Multiple essential roles for primary cilia in heart development

Background

The primary cilium is a microtubule-based, plasma membrane-ensheathed protrusion projecting from the basal bodies of almost all cell types in the mammalian body. In the past several years a plethora of papers has indicated a crucial role for primary cilia in the development of a wide variety of organs. We have investigated heart development in cobblestone, a hypomorphic allele of the gene encoding the intraflagellar transport protein Ift88, and uncovered a number of the most common congenital heart defects seen in newborn humans.

Methods

We generated serial sections of mutant cobblestone and wild type embryos in the region encompassing the heart and the cardiac outflow tract. The sections were further processed to generate three-dimensional reconstructions of these structures, and immunofluorescence confocal microscopy, transmission electron microscopy, and in situ hybridization were used to examine signal transduction pathways in the relevant areas. Whole mount in situ hybridization was also employed for certain developmental markers.

Results

In addition to an enlarged pericardium and failure of both ventricular and atrial septum formation, the cobblestone mutants displayed manifold defects in outflow tract formation, including persistent truncus arteriosus, an overriding aorta, and abnormal transformation of the aortic arches. To discern the basis of these anomalies we examined both the maintenance of primary cilia as well as endogenous and migratory embryonic cell populations that contribute to the outflow tract and atrioventricular septa. The colonization of the embryonic heart by cardiac neural crest occurred normally in the cobblestone mutant, as did the expression of Sonic hedgehog. However, with the loss of primary cilia in the mutant hearts, there was a loss of both downstream Sonic hedgehog signaling and of Islet 1 expression in the second heart field, a derivative of the pharyngeal mesoderm. In addition, defects were recorded in development of atrial laterality and ventricular myocardiogenesis. Finally, we observed a reduction in expression of Bmp4 in the outflow tract, and complete loss of expression of both Bmp2 and Bmp4 in the atrioventricular endocardial cushions. Loss of BMP2/4 signaling may result in the observed proliferative defect in the endocardial cushions, which give rise to both the atrioventricular septa as well as to the septation of the outflow tract.

Conclusions

Taken together, our results potentially identify a novel link between Sonic hedgehog signaling at the primary cilium and BMP-dependent effects upon cardiogenesis. Our data further point to a potential linkage of atrioventricular septal defects, the most common congenital heart defects, to genes of the transport machinery or basal body of the cilia.

Peroxisome dynamics in cultured mammalian cells

Despite the identification and characterization of various proteins that are essential for peroxisome biogenesis, the origin and the turnover of peroxisomes are still unresolved critical issues. In this study, we used the HaloTag technology as a new approach to examine peroxisome dynamics in cultured mammalian cells. This technology is based on the formation of a covalent bond between the HaloTag protein--a mutated bacterial dehalogenase which is fused to the protein of interest--and a synthetic haloalkane ligand that contains a fluorophore or affinity tag. By using cell-permeable ligands of distinct fluorescence, it is possible to image distinct pools of newly synthesized proteins, generated from a single genetic HaloTag-containing construct, at different wavelengths. Here, we show that peroxisomes display an age-related heterogeneity with respect to their capacity to incorporate newly synthesized proteins. We also demonstrate that these organelles do not exchange their protein content. In addition, we present evidence that the matrix protein content of pre-existing peroxisomes is not evenly distributed over new organelles. Finally, we show that peroxisomes in cultured mammalian cells, under basal growth conditions, have a half-life of approximately 2 days and are mainly degraded by an autophagy-related mechanism. The implications of these findings are discussed.

A crucial role for primary cilia in cortical morphogenesis

Primary cilia are important sites of signal transduction involved in a wide range of developmental and postnatal functions. Proteolytic processing of the transcription factor Gli3, for example, occurs in primary cilia, and defects in intraflagellar transport (IFT), which is crucial for the maintenance of primary cilia, can lead to severe developmental defects and diseases. Here we report an essential role of primary cilia in forebrain development. Uncovered by N-ethyl-N-nitrosourea-mutagenesis, cobblestone is a hypomorphic allele of the IFT gene Ift88, in which Ift88 mRNA and protein levels are reduced by 70-80%. cobblestone mutants are distinguished by subpial heterotopias in the forebrain. Mutants show both severe defects in the formation of dorsomedial telencephalic structures, such as the choroid plexus, cortical hem and hippocampus, and also a relaxation of both dorsal-ventral and rostral-caudal compartmental boundaries. These defects phenocopy many of the abnormalities seen in the Gli3 mutant forebrain, and we show that Gli3 proteolytic processing is reduced, leading to an accumulation of the full-length activator isoform. In addition, we observe an upregulation of canonical Wnt signaling in the neocortex and in the caudal forebrain. Interestingly, the ultrastructure and morphology of ventricular cilia in the cobblestone mutants remains intact. Together, these results indicate a critical role for ciliary function in the developing forebrain.

Effects of laparoscopic division of spermatic vessels on histological changes of testes: long-term observation in the model of prepubertal rat

Laparoscopic Fowler-Stephens and Palomo procedures are now commonly performed in children with high positioned intra-abdominal cryptorchidism and varicocele, respectively. During the procedures, the spermatic vessels are ligated and therefore the question of risk related to testicular atrophy is often raised. The long-term follow-up of the histology after the procedures is rare. In this study, we simulated a laparoscopic spermatic vessels clipping and division (SVCD) in a prepubertal rat model, and examined the histological alterations of the testes with regard to spermatogenic arrest between prepuberty and middle age. Thirty-day-old Wistar rats divided randomly into three groups underwent laparoscopic sham operation, unilateral SVCD and unilateral SVCD with additional contralateral orchiectomy, respectively. Histological investigations observed on semithin and paraffin sections were performed at seven different postoperative intervals between day 9 and day 540. We defined partial, most and complete spermatogenic arrest of the seminiferous tubules to correspond with mild, severe spermatogenic arrest and atrophy, respectively. Laparoscopic SVCD induced testicular spermatogenic arrest in a total of 85% of the operated testes with different severity; 27% of operated testes with mild or severe spermatogenic arrest were seen between puberty and middle age (day 45-540 postoperative), and their size was only slightly reduced. Of the operated testes, 51% showed atrophic signs with a striking decrease in size, and their contralateral testes revealed in all cases mild or severe spermatogenic arrest started as early as day 45 postoperatively. Parallel to the spermatogenic arrest, Leydig cell hyperplasia developed frequently in impaired testes, especially in those without contralateral testes, finally reaching a typical adenoma size. Laparoscopic SVCD in prepubertal rats could disturb spermatogenesis with differing severity in most cases. This impairment could persist from peripuberty to middle age, and even involve the contralateral testes, in the case of operated testes and show complete spermatogenic arrest. This study showed that laparoscopic SVCD may have high risk in compromising the operated testis.

Shared components of mitochondrial and peroxisomal division

Mitochondria and peroxisomes are ubiquitous subcellular organelles, which are highly dynamic and display large plasticity. Recent studies have led to the surprising finding that both organelles share components of their division machinery, namely the dynamin-related protein DLP1/Drp1 and hFis1, which recruits DLP1/Drp1 to the organelle membranes. This review addresses the current state of knowledge concerning the dynamics and fission of peroxisomes, especially in relation to mitochondrial morphology and division in mammalian cells.

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.

Peroxisome biogenesis and the role of protein import

Peroxisomes are metabolic organelles with enzymatic content that are found in virtually all cells and are involved in beta-oxidation of fatty acids, hydrogen peroxide-based respiration and defence against oxidative stress. The steps of their biogenesis involves "peroxins", proteins encoded by PEX genes. Peroxins are involved in three key stages of peroxisome development: (1). import of peroxisomal membrane proteins; (2). import of peroxisomal matrix proteins and (3). peroxisome proliferation. Of these three areas, peroxisomal matrix-protein import is by far the best understood and accounts for most of the available published data on peroxisome biogenesis. Defects in peroxisome biogenesis result in peroxisome biogenesis disorders (PBDs), which although rare, have no known cure to-date. This review explores current understanding of each key area in peroxisome biogenesis, paying particular attention to the role of protein import.

Inactivation of ether lipid biosynthesis causes male infertility, defects in eye development and optic nerve hypoplasia in mice

Although known for almost 80 years, the physiological role of plasmalogens (PLs), the major mammalian ether lipids (ELs), is still enigmatic. Humans that lack ELs suffer from rhizomelic chondrodysplasia punctata (RCDP), a peroxisomal disorder usually resulting in death in early childhood. In order to learn more about the functions of ELs, we generated a mouse model for RCDP by a targeted disruption of the dihydroxyacetonephosphate acyltransferase gene. The mutant mice revealed multiple abnormalities, such as male infertility, defects in eye development, cataract and optic nerve hypoplasia, some of which were also observed in RCDP. Mass spectroscopic analysis demonstrated the presence of highly unsaturated fatty acids including docosahexaenoic acid (DHA) in brain PLs and the occurrence of PLs in lipid raft microdomains (LRMs) isolated from brain myelin. In mutants, PLs were completely absent and the concentration of brain DHA was reduced. The marker proteins flotillin-1 and F3/contactin were found in brain LRMs in reduced concentrations. In addition, the gap junctional protein connexin 43, known to be recruited to LRMs and essential for lens development and spermatogenesis, was down-regulated in embryonic fibroblasts of the EL-deficient mice. Free cholesterol, an important constituent of LRMs, was found in these fibroblasts to be accumulated in a perinuclear compartment. These data suggest that the EL-deficient mice allow the identification of new phenotypes not related so far to EL-deficiency (male sterility, defects in myelination and optic nerve hypoplasia) and indicate that PLs are required for the correct assembly and function of LRMs.

Human peroxisomal disorders

Peroxisomes are single membrane-bound cell organelles performing numerous metabolic functions. The present article aims to give an overview of our current knowledge about inherited peroxisomal disorders in which these organelles are lacking or one or more of their functions are impaired. They are multiorgan disorders and the nervous system is implicated in most. After a summary of the historical names and categories, each having distinct symptoms and prognosis, microscopic pathology is reviewed in detail. Data from the literature are added to experience in the authors' laboratory with 167 liver biopsy and autopsy samples from peroxisomal patients, and with a smaller number of chorion samples for prenatal diagnosis, adrenal-, kidney-, and brain samples. Various light and electron microscopic methods are used including enzyme- and immunocytochemistry, polarizing microscopy, and morphometry. Together with other laboratory investigations and clinical data, this approach continues to contribute to the diagnosis and further characterization of peroxisomal disorders, and the discovery of novel variants. When liver specimens are examined, three main groups including 9 novel variants (33 patients) are distinguished: (1) absence or (2) presence of peroxisomes, and (3) mosaic distribution of cells with and without peroxisomes (10 patients). Renal microcysts, polarizing trilamellar inclusions, and insoluble lipid in macrophages in liver, adrenal cortex, brain, and in interstitial cells of kidney are also valuable for classification. On a genetic basis, complementation of fibroblasts has classified peroxisome biogenesis disorders into 12 complementation groups. Peroxisome biogenesis genes (PEX), knock-out-mice, and induction of redundant genes are briefly reviewed, including some recent results with 4-phenylbutyrate. Finally, regulation of peroxisome expression during development and in cell cultures, and by physiological factors is discussed.

Tubulo-reticular clusters of peroxisomes in living COS-7 cells: dynamic behavior and association with lipid droplets

We characterized more complex peroxisomal structures, i.e., tubulo-reticular peroxisomal clusters, in greater detail under in vivo conditions in COS-7 cells that were transfected with a GFP-PTS1 fusion protein. Live cell imaging revealed the dynamic nature of peroxisomal clusters and allowed a detailed analysis of the motile properties of a heterogeneous peroxisome population. Furthermore, peroxisomal clusters were found to be associated with lipid droplets. The frequency of peroxisomal clusters correlated with an increase in cell density and in the size of lipid droplets. These data provide further evidence for the dynamic nature of the peroxisomal compartment and indicate that peroxisomal clusters have a function in lipid metabolism.

Real time imaging reveals a peroxisomal reticulum in living cells

We have directly imaged the dynamic behavior of a variety of morphologically different peroxisomal structures in HepG2 and COS-7 cells transfected with a construct encoding GFP bearing the C-terminal peroxisomal targeting signal 1. Real time imaging revealed that moving peroxisomes interacted with each other and were engaged in transient contacts, and at higher magnification, tubular peroxisomes appeared to form a peroxisomal reticulum. Local remodeling of these structures could be observed involving the formation and detachment of tubular processes that interconnected adjacent organelles. Inhibition of cytoplasmic dynein based motility by overexpression of the dynactin subunit, dynamitin (p50), inhibited the movement of peroxisomes in vivo and interfered with the reestablishment of a uniform distribution of peroxisomes after recovery from nocodazole treatment. Isolated peroxisomes moved in vitro along microtubules in the presence of a microtubule motor fraction. Our data reveal that peroxisomal behavior in vivo is significantly more dynamic and interactive than previously thought and suggest a role for the dynein/dynactin motor in peroxisome motility.

Peroxisome distribution along the crypt-villus axis of the guinea pig small intestine

Peroxisomes and peroxisomal enzyme expression were investigated biochemically and morphometrically in guinea pig intestinal epithelial cells at different stages of their migration along the crypt-villus axis. Epithelial cells were sequentially isolated along the axis and the specific activities of the peroxisomal enzymes catalase and acyl-CoA oxidase were found to be significantly higher in differentiated and mature cells situated at the villus tip and stem than in the crypt. Conversely, 1-alk-1'enyl, 2-acyl phospholipid (plasmalogen) concentration in the crypt and middle villus was significantly higher than in villus tip cells. Assay of alkyl DHAP synthase and fatty acyl CoA reductase (enzymes responsible for the production of plasmalogen precursors) showed no correlating activity gradient with plasmalogen concentration. Morphometric analysis revealed that peroxisomes were present even in the most immature stem cells, however, their number and volume and surface densities increased as the epithelial cell developed as did the proportion of elongated and vermiform peroxisomes to spherical structures. Senescent cells at the tip of the villus, however, showed a dramatic decrease in number of peroxisomes per cell possibly due to cellular degradation. We conclude that the peroxisomal compartment of the guinea pig small intestinal epithelial cell develops as a function of cell development possibly reflecting adaptation to maximise its metabolic capacity.

Synthesis of plasmalogens in eye lens epithelial cells

The present paper describes cloning and sequencing of the mouse cDNA encoding dihydroxyacetonephosphate acyltransferase (DAPAT), the peroxisomal key enzyme of plasmalogen (PM) biosynthesis. Using monospecific antibodies, we localized DAPAT and alkyl dihydroxyacetonephosphate synthase to peroxisomes of mouse lens epithelial cells (LECs) and determined their enzymatic activity. By electrospray ionization mass spectrometry of mouse lens lipid extracts, we identified phosphatidyl ethanolamine including plasmenyl ethanolamine species as major constituents. Our data demonstrate the capacity of LECs to synthesize PMs and the high coincidence between deficiency of PM and early manifestation of cataract in patients with peroxisomal disorders suggests that ether-bonded lipids may play an important role in maintaining lens transparency.

Tubular peroxisomes in HepG2 cells: selective induction by growth factors and arachidonic acid

We showed recently the plasticity of the peroxisomal compartment in the human hepatoblastoma cell line HepG2 as evidenced by the presence of elongated tubular peroxisomes measuring up to 5 microm next to much smaller spherical or rod-shaped ones (0.1-0.3 microm). Since the occurrence of tubular peroxisomes in a given cell in culture is synchronized, with neighboring cells containing either small spherical or elongated tubular peroxisomes, cell counting of immunofluorescence preparations stained for catalase was used for the quantitative assessment of the dynamics of the peroxisomal compartment and the factors regulating this process. Initial studies revealed that the formation of tubular peroxisomes is primarily influenced by the cell density as well as by lipid- and protein-factors in fetal calf serum, being independent of an intact microtubular network. Biochemical studies showed that the occurrence of tubular peroxisomes correlated with the expression of the mRNA for 70 kDa peroxisomal membrane protein (PMP70), but not with that of matrix proteins. By cultivation of cells in serum- and protein-free media specific factors were identified which influenced the formation of tubular peroxisomes. Among several growth factors tested, nerve growth factor (NGF) was the most potent one inducing tubular peroxisomes and its effect was blocked by K252b, a specific inhibitor of neurotrophin receptor pathway, suggesting the involvement of signal transduction in this process. Furthermore, from several polyunsaturated fatty acids (PUFA) which all induced tubular peroxisomes, the arachidonic acid (AA) was the most potent one. Our observations suggest that tubular peroxisomes are transient structures in the process of rapid expansion of the peroxisomal compartment which are induced either by specific growth factors or by polyunsaturated fatty acids both of which are involved in intracellular signaling.

Isolation of rat hepatic peroxisomes by means of immune free flow electrophoresis

Rat hepatic peroxisomes (PO) were separated from other cell organelles by free flow electrophoresis (FFE) in combination with immunocomplexing PO prior to FFE with an antibody directed against the cytoplasmic aspect of the peroxisomal membrane protein PMP 70. This novel approach is based on a method termed antigen-specific electrophoretic cell separation (ASECS) which was originally introduced for the isolation of human T and B lymphocyte subpopulations by Hansen and Hannig (J. Immunol. Methods 1982, 51, 197-208). We adapted this technique to PO isolation from a crude peroxisomal fraction, streamlining it by the following modifications: (i) The sandwich-technique recommended to further lower a negative surface charge was renounced. (ii) Instead, the pH of the electrophoresis buffer was raised from 7.2 to 8.0, thus minimizing the electrophoretic mobility of the particles immunocomplexed due to the fact that the isoelectric point (pI) of IgG molecules is close to pH 8.0. PO isolated by this modification, referred to as immune free flow electrophoresis (IFFE), are as pure, intact, and structurally well-preserved as are highly purified PO obtained by density gradient centrifugation. The technique is currently applied for the isolation of peroxisomal subpopulations that are difficult to obtain by means of density gradient centrifugation.

Effects of ethylene glycol on the ultrastructure of hepatocytes

The ultrastructure of rat hepatocytes after acute experimental ethylene glycol poisoning was examined. On the 1st, 5th and 14th days after poisoning the material from the centrolobular zone (zone III) was collected. Proliferation and enlargement of SER at the early period of poisoning and evidence of mitochondrial damage both at the early and late time after ethylene glycol intoxication were found. In the liver extensive capillary deposits surrounded by membranes were seen, filled with flocculent material of middle electron density. The results show destruction of the cytoplasmic organelles, especially mitochondria, on the 1st and 5th days after ethylene glycol intoxication, and symptoms of damage removing together with regeneration on the 15th day of the experiment.

Novel peroxisomal populations in subcellular fractions from rat liver. Implications for peroxisome structure and biogenesis

According to current concepts, new peroxisomes are formed by division of pre-existing peroxisomes or by budding from a peroxisomal reticulum. Recent cytochemical and biochemical data indicate that protein content in peroxisomes are heterogenous and that import of newly synthesized proteins may be restricted to certain protein import-competent peroxisomal subcompartments (Yamamoto, K., and Fahimi, H. D. (1987) J. Cell Biol. 105, 713-722; Heinemann, P., and Just, W. W. (1992) FEBS Lett. 300, 179-182; Lüers, G., Hashimoto, T., Fahimi, H. D., and Völkl, A. (1993) J. Cell Biol. 121, 1271-1280). We have observed that substantial amounts of peroxisomal proteins are found together with "microsomes" (100,000 x g pellet) after subcellular fractionation of rat liver homogenates. In this study we have investigated the origin of these peroxisomal proteins by modified gradient centrifugation procedures in Nycodenz and by analysis of enzyme activity distributions, Western blotting, and immunoelectron microscopy. It is concluded that much of this material is confined to novel populations of "peroxisomes." Immunocytochemistry on gradient fractions showed that some vesicles were enriched in acyl-CoA oxidase and peroxisomal multifunctional enzyme ("catalase-negative") whereas others were enriched in catalase and thiolase ("acyl-CoA oxidase-negative"). Double immunolabeling experiments verified the strong heterogeneity in the protein contents of these vesicles and also identified peroxisomes varying in size from about 0.5 microns ("normal peroxisomes") to extremely small vesicles of less than 100 nm in diameter. The possibility that these vesicles may be related to different subcompartments of a larger peroxisomal structure involved in protein import and biogenesis will be discussed.

Giant peroxisomes in oleic acid-induced Saccharomyces cerevisiae lacking the peroxisomal membrane protein Pmp27p

We have purified peroxisomal membranes from Saccharomyces cerevisiae after induction of peroxisomes in oleic acid-containing media. About 30 distinct proteins could be discerned among the HPLC- and SDS-PAGE-separated proteins of the high salt-extracted peroxisomal membranes. The most abundant of these, Pmp27p, was purified and the corresponding gene PMP27 was cloned and sequenced. Its primary structure is 32% identical to PMP31 and PMP32 of the yeast Candida biodinii (Moreno, M., R. Lark, K. L. Campbell, and M. J. Goodman. 1994. Yeast. 10:1447-1457). Immunoelectron microscopic localization of Pmp27p showed labeling of the peroxisomal membrane, but also of matrix-less and matrix containing tubular membranes nearby. Electronmicroscopical data suggest that some of these tubular extensions might interconnect peroxisomes to form a peroxisomal reticulum. Cells with a disrupted PMP27 gene (delta pmp27) still grew well on glucose or ethanol, but they failed to grow on oleate although peroxisomes were still induced by transfer to oleate-containing media. The induced peroxisomes of delta pmp27 cells were fewer but considerably larger than those of wild-type cells, suggesting that Pmp27p may be involved in parceling of peroxisomes into regular quanta. delta pmp27 cells cultured in oleate-containing media form multiple buds, of which virtually all are peroxisome deficient. The growth defect of delta pmp27 cells on oleic acid appears to result from the inability to segregate the giant peroxisomes to daughter cells.

Ultrastructural aspects of the biogenesis of peroxisomes in rat liver

A model summarizing our current concepts on the ultrastructural basis of the biogenesis of peroxisomes is presented. Accordingly, the initial stage of de novo build-up of peroxisomes is characterized by the formation of myelin-like figures and membranous attachments onto the surface of pre-existing peroxisomes. Such membranous structures may provide the appropriate lipid environment for the incorporation of peroxisomal membrane proteins and subsequently become the preferential sites for import of newly synthesized matrix proteins. After the import the membranous structures develop into small peroxisomes which may remain attached briefly to the larger particles but eventually separate to become new peroxisomes. Whereas some matrix proteins such as catalase are distributed in all newly formed peroxisomes, other ones like urate oxidase and D-amino acid oxidase are compartmentalized only in some of them, giving rise to heterogeneity of peroxisomes.

Metabolic pathways in mammalian peroxisomes

This article summarizes our current knowledge of the metabolic pathways present in mammalian peroxisomes. Emphasis is placed on those aspects that are not covered by other articles in this issue: peroxisomal enzyme content and topology; the peroxisomal beta-oxidation system; substrates of peroxisomal beta-oxidation such as very-long-chain fatty acids, branched fatty acids, dicarboxylic fatty acids, prostaglandins and xenobiotics; the role of peroxisomes in the metabolism of purines, polyamines, amino acids, glyoxylate and reactive oxygen products such as hydrogen peroxide, superoxide anions and epoxides.

Development of the yeast Pichia pastoris as a model organism for a genetic and molecular analysis of peroxisome assembly

We describe the isolation of mutants of the yeast Pichia pastoris that are deficient in peroxisome assembly (pas). These mutants of P. pastoris can be identified solely by their inability to grow on methanol and oleic acid, the utilization of which requires peroxisomal enzymes, and are defined by the absence of normal peroxisomes as judged by electron microscopy and biochemical fractionation experiments. These mutants are the result of genetic defects at single loci and represent at least eight different complementation groups. The isolation of pas mutants of P. pastoris by a simple screen for mutants unable to use methanol and oleic acid represents a significantly more efficient method for identification of pas mutants than is possible in other organisms. To exploit this advantage fully we also developed new reagents for the genetic and molecular manipulation of P. pastoris. These include a set of auxotrophic strains with an essentially wild-type genetic background, plasmids that act as Escherichia coli-P. pastoris shuttle vectors, and genomic DNA libraries for isolation of P. pastoris genes by functional complementation of mutants or by nucleic acid hybridization. The availability of numerous pas mutants and the reagents necessary for their molecular analysis should lead to the isolation and characterization of genes involved in peroxisome assembly.

Subcellular fractionation evidence for a putative peroxisome-mitochondrion attachment in the liver of normal and genetically obese (ob/ob and db/db) mice

1. Liver post-nuclear supernatants (PNS) from several mouse strains were fractionated by zonal centrifugation and fractions analysed by marker-enzyme estimations+electron microscopy. 2. Rate-dependent banding of PNS yielded peroxisome-enriched (PER) and mitochondrion-enriched (MER) regions. 3. Density-dependent banding of PER yielded peroxisomes (approximately 1.22 g/ml) well separated from mitochondria (approximately 1.8 g/ml). 4. Density-dependent banding of MER yielded peroxisomes that co-distributed with mitochondria and electron microscopy revealed close proximity of the two organelles. 5. Experiments demonstrated that co-distribution was not due to weak binding of proteins or to agglutination of organelles. 6. The results indicate in vivo attachment of some mitochondria and peroxisomes.

Peroxisomes in guinea pig liver: their peculiar morphological features may reflect certain aspects of lipoprotein metabolism in this species

We have studied the ultrastructural characteristics and the distribution of peroxisomes in guinea pig liver using electron-microscopic cytochemistry for catalase and morphometry. By light microscopy, peroxisomes appear as dark 0.2-0.5 microns granules in the cytoplasm of liver parenchymal cells, often forming large clusters that measure up to 5 microns across. Rows of single peroxisomes or their aggregates line the sinusoidal surface of hepatocytes. Electron microscopy reveals that clusters of up to 25 individual peroxisomes are usually located in the subsinusoidal region of parenchymal cells. The mean diameter and the volume density of peroxisomes are larger in pericentral than in periportal regions of the liver lobule. Whereas large amounts of lipoprotein particles with a mean diameter of 160 nm (chylomicrons) are present in the Disse space, the cytoplasm of parenchymal cells contains multivesicular bodies and abundant lipid droplets. In addition, the Golgi complexes show distended lipoprotein-filled vesicles suggesting active biosynthesis of lipoproteins. We propose that the unique features of peroxisomes in guinea pig liver, such as cluster formation and alignment along the sinusoidal surface, may be related to the high levels of lipoproteins in the portal circulation and their hepatic catabolism in this species.

Ultrastructural evidence of mature Leydig cells and Leydig cell regression in the neonatal human testis

The neonatal period in male development is characterized by an acute rise in serum testosterone, which peaks at 2 to 3 months of age. The purpose of this study is to examine the neonatal human testicular interstitium at 4 months for evidence of Leydig cell maturation, as well as any morphological criteria relating to the fate of Leydig cells during this period, specifically, for signs of cell regression. Leydig cells are described with impressive development of the steroid secreting apparatus, which are consistent with the mature Leydig cells found during early fetal development and in the adult. The outstanding feature of these cells is the "organelle association" of extensive, anastamosing tubules of smooth endoplasmic reticulum (SER), pleomorphic mitochondria with a component of tubular cristae, and abundant microperoxisomes associated with the SER. Well-developed Golgi elements, regionalized RER, and diverse cell inclusions are also characteristics of these cells. Reinke crystals and paracrystalline inclusions are absent. Gap junctions are common in this system and are notable in the asymmetric nature of the adjacent cytoplasmic components. These findings provide a morphologic correlate to the reported neonatal phase of testosterone production in man. Intermediate forms of Leydig cells are described with "organelle associations" including decreased SER with increased lipid droplets, and decreased SER with prominent cytoplasmic filaments and/or dramatic mitochondrial changes supportive of mitochondrial involution. Cells consistent with immature Leydig cells are also present. The rather impressive diversity in cell morphology present during this time frame of 4 months, slightly past the peak in testosterone production, provides evidence of Leydig cell regression and a continuity of the mature neonatal Leydig cells with the immature Leydig cells of childhood (Prince, 1984). There is also some evidence of cell degeneration. Although the developmental history of Leydig cells has been described for years as biphasic, it is time to view Leydig cell development in man as a triphasic event, fetal, neonatal, and pubertal.

Topogenesis of peroxisomal proteins

Molecular and biochemical analysis of the biogenesis of peroxisomes has made rapid progress in recent years. Research on the mechanism of targeting of peroxisomal proteins has revealed that many, but not all, peroxisomal proteins have a conserved tripeptide motif in their carboxy-terminal portions which is required for entry into peroxisomes; the topogenic signal mechanism thus differs in these instances from those employed in mitochondria and endoplasmic reticulum. Other factors involved in peroxisome biogenesis are also coming to light.

Three-dimensional and histochemical studies of peroxisomes in cultured hepatocytes by quick-freezing and deep-etching method

Primary cultured mouse hepatocytes were treated with clofibric acid to induce peroxisome proliferation. They were briefly fixed with paraformaldehyde and centrifuged to prepare pellets. The hepatocytes were split open to remove cytoplasmic soluble proteins and fixed with glutaraldehyde. They were routinely incubated for catalase enzyme cytochemistry and fixed in osmium tetroxide. The specimens were quickly frozen, deeply etched and rotary shadowed by platinum and carbon. Peroxisomes were identified as osmium reaction products on replica membranes. In treated hepatocytes, the number of peroxisomes was considerably increased and smooth membranous structures resembling sER (referred to as 'peroxisome-forming sheets') showed hypertrophy. The rER associated with intermediate filaments was significantly decreased and 'peroxisome-forming sheets' appeared, being accompanied with budding and fragmentation of peroxisomes.

Formation of the Dictyostelium spore coat

The spore coat forms as a rigid extracellular wall around each spore cell during culmination. Coats purified from germinated spores contain multiple protein species and an approximately equal mass of polysaccharide, consisting mostly of cellulose and a galactose/N-acetylgalactosamine polysaccharide (GPS). All but the cellulose are prepackaged during prespore cell differentiation in a regulated secretory compartment, the prespore vesicle. The morphology of this compartment resembles an anastomosing, tubular network rather than a spherical vesicle. The molecules of the prespore vesicles are not uniformly mixed but are segregated into partially overlapping domains. Although lysosomal enzymes have been found in the prespore vesicle, this compartment does not function as a lysosome because it is not acidic, and a common antigen associated with acid hydrolases is found in another, acidic vesicle population. All the prespore vesicle profiles disappear at the time of appearance of their contents outside of the cell; this constitutes an early stage in spore coat formation, which can be detected both by microscopy and flow cytometry. As an electron-dense layer, the future outer layer of the coat, condenses, cellulose can be found and is located immediately beneath this outer layer. Certain proteins and the GPS become associated with either the outer or inner layers surrounding this middle cellulose layer. Assembly of the inner and outer layers occurs in part from a pool of glycoproteins that is shared between spores, and unincorporated molecules loosely reside in the interspore matrix, a location from which they can be easily washed away. When the glycosylation of several major protein species is disrupted by mutation, the coat is assembled, but differences are found in its porosity and the extractibility of certain proteins. In addition, the retention or loss of proteolytic fragments in the mutants indicates regions of spore coat proteins that are required for association with the coat. Comparative examination of the macrocyst demonstrates that patterns of molecular distributions are not conserved between the macrocyst and spore coats. Thus spore coat assembly is characterized by highly specific intermolecular interactions, leading to saturable associations of individual glycoproteins with specific layers and the exclusion of excess copies to the interspore space.

Biogenesis of peroxisomes: immunocytochemical investigation of peroxisomal membrane proteins in proliferating rat liver peroxisomes and in catalase-negative membrane loops

Treatment of rats with a new hypocholesterolemic drug BM 15766 induces proliferation of peroxisomes in pericentral regions of the liver lobule with distinct alterations of the peroxisomal membrane (Baumgart, E., K. Stegmeier, F. H. Schmidt, and H. D. Fahimi. 1987. Lab. Invest. 56:554-564). We have used ultrastructural cytochemistry in conjunction with immunoblotting and immunoelectron microscopy to investigate the effects of this drug on peroxisomal membranes. Highly purified peroxisomal fractions were obtained by Metrizamide gradient centrifugation from control and treated rats. Immunoblots prepared from such peroxisomal fractions incubated with antibodies to 22-, 26-, and 70-kD peroxisomal membrane proteins revealed that the treatment with BM 15766 induced only the 70-kD protein. In sections of normal liver embedded in Lowicryl K4M, all three membrane proteins of peroxisomes could be localized by the postembedding technique. The strongest labeling was obtained with the 22-kD antibody followed by the 70-kD and 26-kD antibodies. In treated animals, double-membraned loops with negative catalase reaction in their lumen, resembling smooth endoplasmic reticulum segments as well as myelin-like figures, were noted in the proximity of some peroxisomes. Serial sectioning revealed that the loops seen at some distance from peroxisomes in the cytoplasm were always continuous with the peroxisomal membranes. The double-membraned loops were consistently negative for glucose-6-phosphatase, a marker for endoplasmic reticulum, but were distinctly labeled with antibodies to peroxisomal membrane proteins. Our observations indicate that these membranous structures are part of the peroxisomal membrane system. They could provide a membrane reservoir for the proliferation of peroxisomes and the expansion of this intracellular compartment.

Peroxisome targeting signal of rat liver acyl-coenzyme A oxidase resides at the carboxy terminus

To identify the topogenic signal of peroxisomal acyl-coenzyme A oxidase (AOX) of rat liver, we carried out in vitro import experiments with mutant polypeptides of the enzyme. Full-length AOX and polypeptides that were truncated at the N-terminal region were efficiently imported into peroxisomes, as determined by resistance to externally added proteinase K. Polypeptides carrying internal deletions in the C-terminal region exhibited much lower import activities. Polypeptides that were truncated or mutated at the extreme C terminus were totally import negative. When the five amino acid residues at the extreme C terminus were attached to some of the import-negative polypeptides, the import activities were rescued. Moreover, the C-terminal 199 and 70 amino acid residues of AOX directed fusion proteins with two bacterial enzymes to peroxisomes. These results are interpreted to mean that the peroxisome targeting signal of AOX residues at the C terminus and the five or fewer residues at the extreme terminus have an obligatory function in targeting. The C-terminal internal region also has an important role for efficient import, possibly through a conformational effect.

Peroxisome targeting signal of rat liver acyl-coenzyme A oxidase resides at the carboxy terminus

To identify the topogenic signal of peroxisomal acyl-coenzyme A oxidase (AOX) of rat liver, we carried out in vitro import experiments with mutant polypeptides of the enzyme. Full-length AOX and polypeptides that were truncated at the N-terminal region were efficiently imported into peroxisomes, as determined by resistance to externally added proteinase K. Polypeptides carrying internal deletions in the C-terminal region exhibited much lower import activities. Polypeptides that were truncated or mutated at the extreme C terminus were totally import negative. When the five amino acid residues at the extreme C terminus were attached to some of the import-negative polypeptides, the import activities were rescued. Moreover, the C-terminal 199 and 70 amino acid residues of AOX directed fusion proteins with two bacterial enzymes to peroxisomes. These results are interpreted to mean that the peroxisome targeting signal of AOX residues at the C terminus and the five or fewer residues at the extreme terminus have an obligatory function in targeting. The C-terminal internal region also has an important role for efficient import, possibly through a conformational effect.

Peroxisomal oxidases: cytochemical localization and biological relevance

(1) alpha-HAOX has a broad substrate specificity. In rat kidney, the enzyme reacts with aliphatic and aromatic alpha-hydroxy acids, in rat liver, however, only with aliphatic ones. (2) The best substrate for the demonstration of alpha-HAOX activity in rat and human liver is glycolate. (3) alpha-hydroxy butyric acid is the best substrate in the luminometric assay for the demonstration of alpha-HAOX activity in the rat kidney, whereas glycolate is not catalysed by the enzyme. (4) In the proximal tubulus epithelial cells of the rat kidney alpha-HAOX is concentrated in the peripheral matrix of the peroxisomes.

Three-dimensional reconstruction of a peroxisomal reticulum in regenerating rat liver: evidence of interconnections between heterogeneous segments

The three-dimensional (3-D) form and the interrelationship of peroxisomes (Po) in the model of regenerating rat liver after partial hepatectomy were studied by computer-assisted 3-D reconstruction of serial ultrathin sections. Po were labeled cytochemically for either catalase, which stains them all uniformly, or for D-amino acid oxidase (DAA-OX), which gives a heterogeneous reaction with lightly and darkly stained PO. In regenerating rat liver, Po exhibit marked pleomorphism with some budding forms and dumbbell-shaped ones. The 3-D reconstruction revealed many single spherical Po measuring 0.15-0.8 micron in diameter. In addition, two to five Po were found interconnected with each other via narrow 30-50-nm hourglass-shaped bridges forming a reticulum. Such aggregates of Po measured 1.5-2.5 microns across. Whereas all segments of this reticulum stained homogeneously for catalase, they exhibited a marked difference in the intensity of the DAA-OX reaction. These observations are consistent with the view of peroxisomal proliferation by budding or fragmentation from preexisting ones. Under such conditions of rapid growth as in regenerating liver, Po may be interconnected forming a reticulum. The interconnections between Po with differing DAA-OX activities suggest that they originate from the same parent organelle.

Neuronal lipidosis and neuroaxonal dystrophy in cerebro-hepato-renal (Zellweger) syndrome

Neuropathological examination of three males with cerebro-hepato-renal (Zellweger) syndrome (CHRS) revealed selective neuronal lipidosis and neuroaxonal dystrophy of the dorsal nucleus of Clarke and lateral cuneate nucleus. This lipidotic alteration was visualized as perikaryal or axonal enlargements with cytoplasmic striations. With the light microscope, the striated material was birefringent and resistant to traditional lipid stains; ultrastructurally, it was composed of lipid clefts, lamellae and lamellar-lipid profiles; biochemically, the affected region contained large amounts of cholesterol esterified to very long-chain fatty acids, both saturated and monounsaturated. This metabolic lesion, though localized to specific sensory neurons, suggests that a more generalized defect in neuronal fatty acid metabolism may be operative in CHRS.

In vitro synthesis of peroxisomal membrane polypeptides

Peroxisomal membranes containing predominantly integral peroxisome membrane polypeptides were obtained from a highly purified peroxisomal fraction. Following sodium dodecylsulfate polyacrylamide gel electrophoresis three polypeptides with apparent molecular weights of 69, 36, and 22 kDa were isolated and used to raise antibodies in rabbits. Cell-free synthesis of these polypeptides was carried out in an in vitro translational system derived from rabbit reticulocytes. By subjecting peroxisomal membranes to reductive methylation [14C]-radiolabeled mature membrane polypeptides were obtained. The comparison of the three mature integral peroxisome membrane polypeptides with their corresponding in vitro synthesis products revealed no size differences indicating the lack of recognizable presequences for these peroxisomal membrane polypeptides.

Serial section analysis of mouse hepatic peroxisomes

The ultrastructure and organization of mouse hepatic peroxisomes were investigated using serial thin sections and the alkaline diaminobenzidine technique for visualization of the peroxidatic activity of catalase. Mouse periportal hepatocytes exhibit three classes of peroxisomes which display morphological and cytochemical heterogeneity: 1) large, circular to ovoid organelles containing a crystalline nucleoid, 2) small, circular to elongate, anucleoid particles, and 3) tail-like extensions which are devoid of both catalase activity (only traces of reaction deposits) and a crystalline core. Serial section analysis reveals that these profiles correspond to three diverse interconnecting peroxisomal segments which constitute a highly complex organelle. In particular, the large nucleoid-containing peroxisomal segment exhibits an intimate relationship to the endoplasmic reticulum. However, direct membrane continuities between the two compartments are never observed. With respect to the complex structure of the organelle the following conclusions can be drawn concerning biochemical studies on liver peroxisomes: 1) During homogenization and subcellular fractionation procedures, fragmentation of peroxisomes into particles of different size classes should be expected. 2) These peroxisomal fragments are inhomogeneous with respect to their matrix contents and possess at least one rupture site on their membrane surface. 3) Soluble matrix and, to a lesser degree, membrane components of peroxisomes contribute to the soluble fraction. 4) Crude microsomal fractions are regularly contaminated by peroxisomal membrane fragments.

Marginal plates in hepatic peroxisomes of Ichthyophis glutinosus (Amphibia: Gymnophiona). A cytochemical study

The ultrastructure of hepatic peroxisomes was investigated in Ichthyophis glutinosus (Amphibia: Gymnophiona), employing perfusion fixation and the diaminobenzidine (DAB) technique for the visualization of catalase. The majority of peroxisomes is circular or rod-shaped, although elongated particles occasionally occur. They contain a finely granular matrix, lightly stained after the DAB procedure. Their mean diameter is approximately 0.25 micron. Serial sections reveal that the circular and rod-shaped peroxisomal profiles are cross and oblique sections of highly tortuous, tubular organelles exceeding 2 micron in length. In addition to tubular profiles, elongated, rectangular particles, as well as straight dumbbell-shaped organelles with distinct marginal plates are observed. They range from 900 to 1650 nm in length (mean = 1200 nm). In the flattened, thin central portion of the dumbbell-shaped particle, the peroxisomal membranes form a cisterna enclosing one or two uniformly thick marginal plates, which display a definite substructure with a periodicity of 10 nm. These findings indicate that peroxisomes in the liver of Ichthyophis exhibit a complex organization. It is suggested that the organelles undergo a specific differentiation process, morphologically characterized by the formation of enlarged segments of unusual shape.