A lysosomal enigma CLN5 and its significance in understanding neuronal ceroid lipofuscinosis.

Neuronal Ceroid Lipofuscinosis (NCL), also known as Batten disease, is an incurable childhood brain disease. The thirteen forms of NCL are caused by mutations in thirteen CLN genes. Mutations in one CLN gene, CLN5, cause variant late-infantile NCL, with an age of onset between 4 and 7 years. The CLN5 protein is ubiquitously expressed in the majority of tissues studied and in the brain, CLN5 shows both neuronal and glial cell expression. Mutations in CLN5 are associated with the accumulation of autofluorescent storage material in lysosomes, the recycling units of the cell, in the brain and peripheral tissues. CLN5 resides in the lysosome and its function is still elusive. Initial studies suggested CLN5 was a transmembrane protein, which was later revealed to be processed into a soluble form. Multiple glycosylation sites have been reported, which may dictate its localisation and function. CLN5 interacts with several CLN proteins, and other lysosomal proteins, making it an important candidate to understand lysosomal biology. The existing knowledge on CLN5 biology stems from studies using several model organisms, including mice, sheep, cattle, dogs, social amoeba and cell cultures. Each model organism has its advantages and limitations, making it crucial to adopt a combinatorial approach, using both human cells and model organisms, to understand CLN5 pathologies and design drug therapies. In this comprehensive review, we have summarised and critiqued existing literature on CLN5 and have discussed the missing pieces of the puzzle that need to be addressed to develop an efficient therapy for CLN5 Batten disease.

Generation of megawatt peak power picosecond pulses from a divided-pulse fiber amplifier.

Control of nonlinearity is a challenge in fiber amplifiers designed to generate pulses of a few picoseconds duration, and as a result, picosecond fiber amplifiers have failed to reach peak power of 1MW. Divided-pulse amplification, combined with the use of circular polarization, allows the generation of 2.2 ps pulses with energy as high as 2.5 µJ and peak power of 1 MW.

Analyzing speckle contrast for HiLo microscopy optimization.

HiLo microscopy is a recently developed technique that provides both optical sectioning and fast imaging with a simple implementation and at a very low cost. The methodology combines widefield and speckled illumination images to obtain one optically sectioned image. Hence, the characteristics of such speckle illumination ultimately determine the quality of HiLo images and the overall performance of the method. In this work, we study how speckle contrast influence local variations of fluorescence intensity and brightness profiles of thick samples. We present this article as a guide to adjust the parameters of the system for optimizing the capabilities of this novel technology.

Intracellular distribution of lysosomal sialidase is controlled by the internalization signal in its cytoplasmic tail.

Sialidase (neuraminidase), encoded by the neu-1 gene in the major histocompatibility complex locus catalyzes the intralysosomal degradation of sialylated glycoconjugates. Inherited deficiency of sialidase results in sialidosis or galactosialidosis, both severe metabolic disorders associated with lysosomal storage of oligosaccharides and glycopeptides. Sialidase also plays an important role in cellular signaling and is specifically required for the production of cytokine interleukin-4 by activated T lymphocytes. In these cells, neu-1-encoded sialidase activity is increased on the cell surface, suggesting that a specific mechanism regulates sorting of this enzyme to the plasma membrane. We investigated that mechanism by first showing that sialidase contains the internalization signal found in lysosomal membrane proteins targeted to endosomes via clathrin-coated pits. The signal consists of a C-terminal tetrapeptide (412)YGTL(415), with Tyr(412) and Leu(415) essential for endocytosis of the enzyme. We further demonstrated that redistribution of sialidase from lysosomes to the cell surface of activated lymphocytes is accompanied by increased reactivity of the enzyme with anti-phosphotyrosine antibodies. We speculate that phosphorylation of Tyr(412) results in inhibition of sialidase internalization in activated lymphocytes.

Biogenesis of lysosomes in marshall cells and in cells of the male reproductive system.

UNLABELLED: The mechanism of plasma membrane trafficking and degradation is still poorly understood. This investigation deals with the biogenesis of lysosomes during endocytic flow in Marshall cells and in various cell types of the male reproductive system. Marshall cells were exposed to ammonium chloride (NH4Cl) and leupeptin after labeling with cationic ferritin. In some experiments, the treated cells were immunogold labeled with anti-prosaposin antibody. NH4Cl and leupeptin are lysosomotropic agents that affect the endosomal-lysosomal progression. Testes, efferent ducts and epididymis from mouse mutants with defects affecting plasma membrane degradation were also used to analyze this process. NH4Cl produced a retention of cationic ferritin in endosomes and hindered the endosomal/lysosomal progression. Leupeptin did not affect this process. NH4Cl decreased the labeling of prosaposin in endosomes and lysosomes, while leupeptin increased the labeling of prosaposin in lysosomes. The number of lysosomes per cytoplasmic area was higher in treated cells than in controls. These findings suggest that leupeptin affected lysosomes whereas NH4Cl affected both endosomes and lysosomes. The endosomal and lysosomal accumulation of prosaposin induced by the treatment with NH4Cl and leupeptin indicated that the site of entry of prosaposinwas both the lysosome and endosome. Electron microscopy (EM) of tissues from mouse mutants with defects affecting plasma membrane degradation substantiated these observations. The EM analysis revealed a selective accumulation of multivesicular bodies (MVBs) and the disappearance of lysosomes, in testicular fibroblasts, nonciliated cells of the efferent ducts and principal cells of the epididymis, suggesting that MVBs are precursors of lysosomes. IN CONCLUSION: (1) endosomes and MVBs are a required steps for degradation of membranes; (2) endosomes and MVBs are precursors of lysosomes; and (3) endosomes, MVBs, and lysosomes appear to be transient organelles.

Elevated levels of the polyadenylation factor CstF 64 enhance formation of the 1kB Testis brain RNA-binding protein (TB-RBP) mRNA in male germ cells.

The single copy mouse Testis Brain RNA-Binding Protein (TB-RBP) gene encodes three mRNAs of 3.0, 1.7, and 1.0 kb which only differ in their 3' UTRs. The 1 kb TB-RBP mRNA predominates in testis, while somatic cells preferentially express the 3.0 kb TB-RBP mRNA. Here we show that the 1 kb mRNA is translated several-fold more efficiently than the 3 kb TB-RBP in rabbit reticulocyte lysates and cells with elevated levels of the 1 kB TB-RBP mRNA express high levels of TB-RBP. To determine if the cleavage stimulatory factor CstF 64 can modulate the alternative splicing of the TB-RBP pre-mRNA and therefore TB-RBP expression, CstF 64 levels and binding to alternative polyadenylation sites were examined. CstF 64 is abundant in the testis and preferentially binds to a distal site in the TB-RBP pre-mRNA that produces the 3 kb TB-RBP. Moreover, upregulation or overexpression of CstF 64 increases the poly(A) site selection for the 1 kb TB-RBP mRNA. We propose that the level of the polyadenylation factor CstF 64 modulates the level of TB-RBP synthesis in male germ cells by an alternative processing of the TB-RBP pre-mRNA.

Mutations in sialidosis impair sialidase binding to the lysosomal multienzyme complex.

Sialidosis is an autosomal recessive disease caused by the genetic deficiency of lysosomal sialidase, which catalyzes the catabolism of sialoglycoconjugates. The disease is associated with progressive impaired vision, macular cherry-red spots, and myoclonus (sialidosis type I) or with skeletal dysplasia, Hurler-like phenotype, dysostosis multiplex, mental retardation, and hepatosplenomegaly (sialidosis type II). We analyzed the effect of the missense mutations G68V, S182G, G227R, F260Y, L270F, A298V, G328S, and L363P, which are identified in the sialidosis type I and sialidosis type II patients, on the activity, stability, and intracellular distribution of sialidase. We found that three mutations, F260Y, L270F, and A298V, which are clustered in the same region on the surface of the sialidase molecule, dramatically reduced the enzyme activity and caused a rapid intralysosomal degradation of the expressed protein. We suggested that this region might be involved in sialidase binding with lysosomal cathepsin A and/or beta-galactosidase in the multienzyme lysosomal complex required for the expression of sialidase activity. Transgenic expression of mutants followed by density gradient centrifugation of cellular extracts confirmed this hypothesis and showed that sialidase deficiency in some sialidosis patients results from disruption of the lysosomal multienzyme complex.

Divergent N-terminal sequences target an inducible testis deubiquitinating enzyme to distinct subcellular structures.

Ubiquitin-specific processing proteases (UBPs) presently form the largest enzyme family in the ubiquitin system, characterized by a core region containing conserved motifs surrounded by divergent sequences, most commonly at the N-terminal end. The functions of these divergent sequences remain unclear. We identified two isoforms of a novel testis-specific UBP, UBP-t1 and UBP-t2, which contain identical core regions but distinct N termini, thereby permitting dissection of the functions of these two regions. Both isoforms were germ cell specific and developmentally regulated. Immunocytochemistry revealed that UBP-t1 was induced in step 16 to 19 spermatids while UBP-t2 was expressed in step 18 to 19 spermatids. Immunoelectron microscopy showed that UBP-t1 was found in the nucleus while UBP-t2 was extranuclear and was found in residual bodies. For the first time, we show that the differential subcellular localization was due to the distinct N-terminal sequences. When transfected into COS-7 cells, the core region was expressed throughout the cell but the UBP-t1 and UBP-t2 isoforms were concentrated in the nucleus and the perinuclear region, respectively. Fusions of each N-terminal end with green fluorescent protein yielded the same subcellular localization as the native proteins, indicating that the N-terminal ends were sufficient for determining differential localization. Interestingly, UBP-t2 colocalized with anti-gamma-tubulin immunoreactivity, indicating that like several other components of the ubiquitin system, a deubiquitinating enzyme is associated with the centrosome. Regulated expression and alternative N termini can confer specificity of UBP function by restricting its temporal and spatial loci of action.

Role of prosaposin in the male reproductive system: effect of prosaposin inactivation on the testis, epididymis, prostate, and seminal vesicles.

SGP-1/prosaposin can be secreted or targeted to the lysosomes where it is processed into smaller saposins (A, B, C, and D) required for the hydrolysis of glycosphingolipids. The deficiency of saposins B and C results in variant forms of metachromatic leukodystrophy and Gaucher's disease, respectively, which are characterized by lysosomal storage of undegraded glycosphingolipids. In the nervous system, prosaposin presents trophic activity. A mouse model was recently developed by creating a null allele in embryonic stem cells through gene targeting to investigate the phenotypic diversity of prosaposin mutations and the involvement of this protein in lysosomal storage diseases, and for the development of therapeutic approaches. Mice homozygous mutants die at the age of 35-40 days and neurological disorders contribute to the early demise of the mutant mice. The male reproductive organs in homozygous mutants show several abnormalities, such as a decrease in testis size with reduced spermiogenesis and an involution of the prostate, seminal vesicles, and epididymis. In these animals, the blood levels of testosterone remain normal. In the prostate of homozygous mutants, only the basal epithelial cells appear to be present, while the secretory cells are absent. These findings suggest that prosaposin may be involved in the development and maintenance of the male reproductive organs, as well as, in cellular differentiation.

Biogenesis of lysosomes by endocytic flow of plasma membrane

The objective of this review is to examine the biogenesis of lysosomes during the endocytic flow of plasma membrane. Two models have been proposed to explain the formation of lysosomes by this process: the "maturational" and the "stationary" models. According to the former, pinocytotic vesicles fuse among themselves to yield endosomes, which in turn, transform first into multivesicular bodies (MVB) and then into mature lysosomes. Therefore, endosomes and lysosomes would be transient organelles. On the other hand, the "stationary" model proposes that the endocytic pathway is formed by functionally and physically distinct compartments. Cultured cells exposed to ammonium chloride (NH4Cl) and leupeptin after a pulse of cationic ferritin were recently used to freeze endosomes and lysosomes. NH4Cl produced a retention of cationic ferritin in endosomes, indicating that this agent interfered with the endosomal/lysosomal progression. Leupeptin did not affect this process. The number of lysosomes increased in cells treated with both lysosomotropic agents. Thus, NH4Cl affected the endosomal and lysosomal compartments, whereas leupeptin had a preferential effect on lysosomes. Mice mutants with defects of plasma membrane degradation, including a Tay-Sachs model, a Sandhoff disease model, as well as, mice with the inactivated prosaposin gene were used to analyze the biogenesis of lysosomes. Thin sections of mutant cells were examined under the electron microscope, and the analysis revealed a selective accumulation of MVBs and the disappearance of lysosomes, suggesting that the formation of MVBs is a required step in lysosomal maturation and that the intravesicular content of MVBs is necessary for the digestion of plasma membrane components. Taken together, these data indicate that endosomes and MVBs are preceding steps in lysosomal biogenesis and that endosomes, MVBs, and lysosomes are transient organelles

Biogenesis of lysosomes by endocytic flow of plasma membrane

The objective of this review is to examine the biogenesis of lysosomes during the endocytic flow of plasma membrane. Two models have been proposed to explain the formation of lysosomes by this process: the "maturational" and the "stationary" models. According to the former, pinocytotic vesicles fuse among themselves to yield endosomes, which in turn, transform first into multivesicular bodies (MVB) and then into mature lysosomes. Therefore, endosomes and lysosomes would be transient organelles. On the other hand, the "stationary" model proposes that the endocytic pathway is formed by functionally and physically distinct compartments. Cultured cells exposed to ammonium chloride (NH4Cl) and leupeptin after a pulse of cationic ferritin were recently used to freeze endosomes and lysosomes. NH4Cl produced a retention of cationic ferritin in endosomes, indicating that this agent interfered with the endosomal/lysosomal progression. Leupeptin did not affect this process. The number of lysosomes increased in cells treated with both lysosomotropic agents. Thus, NH4Cl affected the endosomal and lysosomal compartments, whereas leupeptin had a preferential effect on lysosomes. Mice mutants with defects of plasma membrane degradation, including a Tay-Sachs model, a Sandhoff disease model, as well as, mice with the inactivated prosaposin gene were used to analyze the biogenesis of lysosomes. Thin sections of mutant cells were examined under the electron microscope, and the analysis revealed a selective accumulation of MVBs and the disappearance of lysosomes, suggesting that the formation of MVBs is a required step in lysosomal maturation and that the intravesicular content of MVBs is necessary for the digestion of plasma membrane components. Taken together, these data indicate that endosomes and MVBs are preceding steps in lysosomal biogenesis and that endosomes, MVBs, and lysosomes are transient organelles

Role of sphingolipids in the transport of prosaposin to the lysosomes.

Prosaposin is the precursor of four lysosomal saposins that promote the degradation of glycosphingolipids (GSLs) by acidic hydrolases. GSLs contain a hydrophobic ceramide moiety, which acts as a membrane anchor, and a hydrophilic oligosaccharide chain that faces the lumen of the Golgi apparatus and extracellular spaces. By using fumonisin B1, PDMP and D609, we tested the hypothesis that sphingolipids mediate the transport of prosaposin to the lysosomes. Fumonisin B1 interferes with the synthesis of ceramide, PDMP blocks the formation of glucosylceramide and D609 blocks the formation of sphingomyelin. Fumonisin B1 produced a 59;-85% decrease in the density of gold particles in the lysosomes of CHO and NRK cells immunolabeled with anti-prosaposin antibody, and a 55% reduction in the lysosomes of CHO cells stably transfected with an expression vector containing a human prosaposin cDNA. To examine whether the mannose 6-phosphate receptor pathway was affected by this treatment, NRK and CHO cells treated or not with fumonisin B1 were labeled with anti-cathepsin A antibody. The results showed no significant differences in labeling of the lysosomes, suggesting that the effect of fumonisin B1 was specific. When fumonisin B1 and D609 were added to the media of transfected CHO cells, a decrease in immunofluorescence with anti-prosaposin antibody was observed by confocal microscopy. PDMP did not cause any reduction in immunoreactivity, indicating that sphingolmyelin appears to be involved in this process. In conclusion, our data support the hypothesis that sphingolipids, possibly sphingomyelin, are involved in the transport of prosaposin to the lysosomes.

Protein-protein interactions between the testis brain RNA-binding protein and the transitional endoplasmic reticulum ATPase, a cytoskeletal gamma actin and Trax in male germ cells and the brain.

Numerous functions have been proposed for the testis brain RNA-binding protein (TB-RBP) and its human homologue, Translin, ranging from mRNA transport and translational regulation to DNA rearrangement and repair. To gain insight into the likely functions of this 26 kDa protein, immunoprecipitation was used to identify proteins that interact with TB-RBP in mouse cytosolic extracts. Three proteins, the transitional endoplasmic reticulum ATPase, a cytoskeletal gamma actin, and Trax, were specifically immunoprecipitated with an affinity-purified antibody to recombinant mouse TB-RBP. In vitro binding assays with recombinant proteins and EM immunocytochemistry confirm that TB-RBP interacts with the TER ATPase in vitro and in vivo. Confocal microscopy has demonstrated that TB-RBP colocalizes with actin in the cytoplasm of male germ cells. The immunoprecipitation of Trax with TB-RBP confirms a published report demonstrating protein interactions between the two proteins in a yeast two-hybrid assay. These data support the hypothesis that TB-RBP serves as a link in attaching specific mRNAs to cytoskeletal structures and suggests an involvement for the ubiquitously expressed TER ATPase in intracellular and/or intercellular mRNA transport.

Activation of a UBC4-dependent pathway of ubiquitin conjugation during postnatal development of the rat testis.

During spermatogenesis, germ cells undergo mitotic and meiotic divisions to form haploid round spermatids which mature to functional elongated spermatozoa. During this process there occurs remodeling of cell structure and loss of most of the cytoplasm and a large fraction of cellular proteins. To evaluate the role of the ubiquitin proteolytic system in this protein loss, we measured levels of ubiquitinated proteins and rates of ubiquitin conjugation in extracts of testes from rats of different ages. Endogenous ubiquitin-protein conjugates increased till day 30 and then reached a plateau. In parallel, there was a progressive increase in the rate of conjugation of ubiquitin to proteins in testis extracts from these animals. To test the importance of two major ubiquitin conjugating enzyme families in the conjugation, immunoprecipitation of UBC2 or UBC4 from 10- and 30-day-old testis extracts was carried out and the remaining conjugation activity in supernatants was assayed. Depletion of either enzyme family resulted in decreased conjugation. However, most of the conjugation activity and, more importantly, the increased conjugation during development were UBC4-dependent. Immunocytochemistry demonstrated a marked increase in expression of UBC4 in spermatids, consistent with the UBC4-dependent activation of conjugation seen in vitro. In situ hybridization studies evaluated the contribution of various UBC4 isoforms to this induction. UBC4-1 mRNA was expressed in most cells. UBC4-2 mRNA was restricted to germ cells with high levels of expression in round and elongated spermatids. UBC4-testis had previously been shown to be expressed only in spermatids. Our data suggest that induction of various UBC4 isoforms activates overall conjugation and plays an important role in the cellular remodeling and protein loss occurring during spermatogenesis.

Expression and regulation of LRP-2/megalin in epithelial cells lining the efferent ducts and epididymis during postnatal development.

Low density lipoprotein receptor-related protein-2/megalin (LRP-2) is a receptor belonging to the low density lipoprotein receptor family that mediates endocytosis and lysosomal degradation of a variety of ligands including apolipoprotein J (Apo J)/clusterin/SGP-2. LRP-2 has been shown to be expressed regionally in the adult rat epididymis. In this study, we describe the pattern of expression of LRP-2 in the efferent ducts and epididymis during postnatal development of the rat and examine the role of testicular luminally derived substances on its expression. The expression of LRP-2 was analyzed immunocytochemically in tissues of normal animals ranging in age from postnatal day 7-90 and in 15-day-old efferent-duct-ligated animals sacrificed at later ages. In the efferent ducts, LRP-2 expression, appearing as a dense band on the apical surface of the nonciliated epithelial cells, was noted as early as day 7, well before the entry of sperm, Sertoli-cell-derived secretory products, and high levels of androgens. Efferent duct ligation studies further revealed that expression under this condition was comparable to controls at all later ages examined, suggesting that the factor regulating its expression was not a luminally derived testicular substance. In normal untreated animals, LRP-2 expression was not apparent at any of the ages examined in the proximal initial segment of the epididymis. By comparison, the distal initial segment, although having no LRP-2 expression from 7-15 days, showed expression in principal cells by day 21 which intensified at days 29 and 39. However, by day 49 and at later ages (56 and 90), LRP-2 immunoreactivity over principal cells became spotty or with weak or moderate reactivity in some cells and none in others. LRP-2 expression in the intermediate zone, proximal caput, corpus, and cauda regions also appeared in principal cells by day 21, intensified at days 29 and 39 and persisted as such at all later ages examined, correlating with high levels of androgens shown to occur by day 39. Although LRP-2 expression in the distal caput region was evident in principal cells at days 21 and 29, it became spotty with weak, moderate, or absent reactivity over principal cells at all later ages. These data suggest that LRP-2 expression is under the influence of both stimulatory and region-specific inhibitory factors. Analysis of 15-day-old efferent-duct-ligated animals at all later ages examined revealed that there was no change in LRP-2 expression along the entire epididymis, suggesting that both the stimulatory and inhibitory factors are not luminally derived testicular substances. The observed pattern of LRP-2 expression in all regions of the epididymis, except the distal caput region, was similar to that described for Apo J internalization by principal cells during postnatal development, showing a correlation between LRP-2 expression and its ligand, Apo J. In summary, LRP-2 expression in the epididymis undergoes region-specific changes during postnatal development and appears to be influenced by both stimulatory and inhibitory factors.

Biogenesis of lysosomes by endocytic flow of plasma membrane.

The objective of this review is to examine the biogenesis of lysosomes during the endocytic flow of plasma membrane. Two models have been proposed to explain the formation of lysosomes by this process: the "maturational" and the "stationary" models. According to the former, pinocytotic vesicles fuse among themselves to yield endosomes, which in turn, transform first into multivesicular bodies (MVB) and then into mature lysosomes. Therefore, endosomes and lysosomes would be transient organelles. On the other hand, the "stationary" model proposes that the endocytic pathway is formed by functionally and physically distinct compartments. Cultured cells exposed to ammonium chloride (NH4Cl) and leupeptin after a pulse of cationic ferritin were recently used to freeze endosomes and lysosomes. NH4Cl produced a retention of cationic ferritin in endosomes, indicating that this agent interfered with the endosomal/lysosomal progression. Leupeptin did not affect this process. The number of lysosomes increased in cells treated with both lysosomotropic agents. Thus, NH4Cl affected the endosomal and lysosomal compartments, whereas leupeptin had a preferential effect on lysosomes. Mice mutants with defects of plasma membrane degradation, including a Tay-Sachs model, a Sandhoff disease model, as well as, mice with the inactivated prosaposin gene were used to analyze the biogenesis of lysosomes. Thin sections of mutant cells were examined under the electron microscope, and the analysis revealed a selective accumulation of MVBs and the disappearance of lysosomes, suggesting that the formation of MVBs is a required step in lysosomal maturation and that the intravesicular content of MVBs is necessary for the digestion of plasma membrane components. Taken together, these data indicate that endosomes and MVBs are preceding steps in lysosomal biogenesis and that endosomes, MVBs, and lysosomes are transient organelles.

Biogenesis of lysosomes by endocytic flow of plasma membrane.

The objective of this review is to examine the biogenesis of lysosomes during the endocytic flow of plasma membrane. Two models have been proposed to explain the formation of lysosomes by this process: the [quot ]maturational[quot ] and the [quot ]stationary[quot ] models. According to the former, pinocytotic vesicles fuse among themselves to yield endosomes, which in turn, transform first into multivesicular bodies (MVB) and then into mature lysosomes. Therefore, endosomes and lysosomes would be transient organelles. On the other hand, the [quot ]stationary[quot ] model proposes that the endocytic pathway is formed by functionally and physically distinct compartments. Cultured cells exposed to ammonium chloride (NH4Cl) and leupeptin after a pulse of cationic ferritin were recently used to freeze endosomes and lysosomes. NH4Cl produced a retention of cationic ferritin in endosomes, indicating that this agent interfered with the endosomal/lysosomal progression. Leupeptin did not affect this process. The number of lysosomes increased in cells treated with both lysosomotropic agents. Thus, NH4Cl affected the endosomal and lysosomal compartments, whereas leupeptin had a preferential effect on lysosomes. Mice mutants with defects of plasma membrane degradation, including a Tay-Sachs model, a Sandhoff disease model, as well as, mice with the inactivated prosaposin gene were used to analyze the biogenesis of lysosomes. Thin sections of mutant cells were examined under the electron microscope, and the analysis revealed a selective accumulation of MVBs and the disappearance of lysosomes, suggesting that the formation of MVBs is a required step in lysosomal maturation and that the intravesicular content of MVBs is necessary for the digestion of plasma membrane components. Taken together, these data indicate that endosomes and MVBs are preceding steps in lysosomal biogenesis and that endosomes, MVBs, and lysosomes are transient organelles.