Altered secretion and accumulation of kidney glycosphingolipids by mouse pigmentation mutants with lysosomal dysfunctions

The endolysosomal system in conventional and unconventional protein secretion

Most secreted proteins are transported through the "conventional" endoplasmic reticulum-Golgi apparatus exocytic route for their delivery to the cell surface and release into the extracellular space. Nonetheless, formative discoveries have underscored the existence of alternative or "unconventional" secretory routes, which play a crucial role in exporting a diverse array of cytosolic proteins outside the cell in response to intrinsic demands, external cues, and environmental changes. In this context, lysosomes emerge as dynamic organelles positioned at the crossroads of multiple intracellular trafficking pathways, endowed with the capacity to fuse with the plasma membrane and recognized for their key role in both conventional and unconventional protein secretion. The recent recognition of lysosomal transport and exocytosis in the unconventional secretion of cargo proteins provides new and promising insights into our understanding of numerous physiological processes.

What’s special about secretory lysosomes?

Lysosomes are organelles specialised for their role in intracellular protein degradation. A small number of cell types also use their lysosomes as regulated secretory organelles. These secretory lysosomes package additional secretory products, respond to extracellular stimuli and fuse with the plasma membrane to release their contents. Recent research has identified unique components of the secretory machinery in these cells. However, studies on conventional lysosomes in non-secretory cells reveal that even their lysosomes can fuse with the plasma membrane in response to membrane damage. What then is special about secretory lysosomes?

Abnormal Expression and Subcellular Distribution of Subunit Proteins of the AP-3 Adaptor Complex Lead to Platelet Storage Pool Deficiency in the Pearl Mouse

The pearl mouse is a model for Hermansky Pudlak Syndrome (HPS), whose symptoms include hypopigmentation, lysosomal abnormalities, and prolonged bleeding due to platelet storage pool deficiency (SPD). The gene for pearl has recently been identified as the beta3A subunit of the AP-3 adaptor complex. The objective of these experiments was to determine if the expression and subcellular distribution of the AP-3 complex were altered in pearl platelets and other tissues. The beta3A subunit was undetectable in all pearl cells and tissues. Also, expression of other subunit proteins of the AP-3 complex was decreased. The subcellular distribution of the remaining AP-3 subunits in platelets, macrophages, and a melanocyte-derived cell line of pearl mice was changed from the normal punctate, probably endosomal, pattern to a diffuse cytoplasmic pattern. Ultrastructural abnormalities in mutant lysosomes were likewise apparent in mutant kidney and a cultured mutant cell line. Genetically distinct mouse HPS models had normal expression of AP-3 subunits. These and related experiments strongly suggest that the AP-3 complex regulates the biogenesis/function of organelles of platelets and other cells and that abrogation of expression of the AP-3 complex leads to platelet SPD.

Mouse models of Hermansky Pudlak syndrome: a review

Hermansky Pudlak Syndrome (HPS) is a recessively inherited disease affecting the contents and/or the secretion of several related subcellular organelles including melanosomes, lysosomes, and platelet dense granules. It presents with disorders of pigmentation, prolonged bleeding, and ceroid deposition, often accompanied by severe fibrotic lung disease and colitis. In the mouse, the disorder is clearly multigenic, caused by at least 14 distinct mutations. Studies on the mouse mutants have defined the granule abnormalities of HPS and have shown that the disease is associated with a surprising variety of phenotypes affecting many tissues. This is an exciting time in HPS research because of the recent molecular identification of the gene causing a major form of human HPS and the expected identifications of several mouse HPS genes. Identifications of mouse HPS genes are expected to increase our understanding of intracellular vesicle trafficking, lead to discovery of new human HPS genes, and suggest diagnostic and therapeutic approaches toward the more severe clinical consequences of the disease.

A fusion pore phenotype in mast cells of the ruby-eye mouse

Using patch-clamp capacitance and amperometric techniques, we have identified an exocytotic phenotype that affects the function of the fusion pore, the molecular structure that connects the lumen of a secretory vesicle with the extracellular environment during exocytosis. Direct observation of individual exocytotic events in mast cells from the ruby-eye mouse (ru/ru) showed a 3-fold increase in the fraction and duration of transient fusion events with respect to wild-type mice. The fraction of the total fusion events that were transient increased from 0.22 +/- 0.02 (wild type) to 0.65 +/- 0.02 (ru/ru), and the average duration of these events increased from 418 +/- 32 ms (wild type) to 1207 +/- 89 ms (ru/ru). We also show that this phenotype can reduce and delay an evoked secretory response by causing the fusion of vesicles that have been previously emptied by repeated cycles of transient fusion. The exocytotic phenotype that we describe here may be a cause of diseases like platelet storage pool deficiency and prolonged bleeding times for which the ruby-eye mouse serves as an animal model. Furthermore, the identification of the gene causing the fusion pore phenotype reported here will illuminate the molecular mechanisms regulating exocytotic fusion.

Expression of glycosphingolipids in serum-free primary cultures of mouse kidney cells: male-female differences and androgen sensitivity

The expression of neutral glycosphingolipids was examined in primary kidney cell cultures derived from adult male and female beige mutant mice (C57BL/6J;bgj/bgj) with enrichment for proximal tubule cells during preparation of explants and using defined serum-free medium for the culture conditions. Cells proliferated for 7 days in vitro to provide confluent or nearly confluent monolayers of epithelial-type growth indicative of proximal tubule cells. The male vs female differences in neutral glycosphingolipids seen in the kidney in vivo were retained in these 7 day cultures. Cultures derived from males contained galacto- and digalactosylceramides whereas those from females did not express these types of glycolipids. Also, male cells had higher ratios of sphingosine: phytosphingosine containing species in Nfa (non-hydroxy fatty acid) globotriaosylceramide and in glucosylceramide than females. The shift in sphingosine: phytosphingosine to male ratios in Nfa globotriaosylceramide and in glucosylceramide could be stimulated in female kidney cells by treatment with 10(-5) M testosterone or 5 alpha-dihydrotestosterone. The male-specific expression of neutral glycosphingolipids, then, appears to be stable character of male-type differentiation in mouse kidney that is passed on during proliferation in culture. Female kidney cells retain an ability to respond to androgens with specific changes in neutral glycosphingolipid expression during 7 days of growth in vitro in serum-free conditions, but do not respond with the induction of the male-specific glycolipids galacto- and digalactosylceramides as seen in vivo.

The accumulation and metabolism of glycosphingolipids in primary kidney cell cultures from beige mice

In the normal C57BL/6J male mouse a specific subset of the kidney glycosphingolipids which is associated with multilamellar bodies of lysosomal origin and represents about 10% of the total kidney glycolipids, is excreted into the urine each day. This excretion is blocked and glycosphingolipids accumulate in the kidney of bgJ/bgJ mutants of this strain. To examine this process in vitro, glycosphingolipid metabolism and excretion were studied in beige mouse kidney cell cultures. Primary kidney cell cultures from male C57BL/6J control and bgJ/bgJ beige mutants were grown in D-valine medium and glycosphingolipids labeled with [3H]palmitate. As we have shown previously, the giant lysosomes of altered morphology were maintained in cultures of the beige kidney cells. Beige-J and control cells synthesized the same types of glycosphingolipids, but the mutant cells had quantitatively higher levels of these compounds than control cells, as determined by high performance liquid chromatography. Beige-J cells incorporated more [3H]palmitate into glycosphingholipids than control cells on a cpm/mg protein basis and the specific activity (cpm/pmole glycosphingolipid) was lower in beige cells. Medium from beige-J cells accumulated more glycosphingolipids than that from control cells in a 24 h period. The glycosphingolipids released into the medium as determined by HPLC were primarily non-lysosomal types and both control and mutant cells retained the glycosphingolipids associated with lysosomal multilamellar bodies excreted in vivo. The elevated levels of lysosomal glycosphingolipids and the dysmorphic lysosomes in primary cultures of beige cells, then, are not caused by a mutant block in secretion of lysosomes.

Rapid kidney changes resulting from glycosphingolipid depletion by treatment with a glucosyltransferase inhibitor

The ceramide analog, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol, inhibits the glucosylation of ceramide and thus, by virtue of the normal catabolism of the higher glucosphingolipids, leads to a general depletion of cellular glucolipids. In a previous study with chronic administration of this inhibitor in mice, it was found that the kidneys and liver, particularly the former, grew more poorly than the organs of control mice. This study shows that the inhibitor produces rapid decreases in glucolipid concentration in kidney which are maintained for at least 5 days without noticeable harm. The changes were enhanced by inclusion of L-cycloserine in the injection scheme. Cycloserine blocks ketosphinganine synthase and thus slows the synthesis of all sphingolipids. However, sphingomyelin levels did not drop significantly in this study. The glucosyltransferase inhibitor also produced a small decrease in kidney beta-D-glucuronidase and distinct increases in the levels of glucocerebrosidase, galactocerebrosidase and sphingomyelinase. It also produced a small but distinct decrease in the level of glucosyltransferase, after a delay of a few hours, possibly because the inhibitor was metabolized to a covalently inactivating product. Comparison with kidney, liver and brain showed that the kidney was more sensitive to the action of the morpholino inhibitor.

Forms and intracellular distribution of alpha-D-mannosidases in murine liver and spleen

1. The intracellular distribution of alpha-D-mannosidase in homogenates of murine liver and spleen was investigated by differential and gradient density centrifugation. 2. In both tissues an enzyme with a neutral pH optimum was found in the cytosol together with an alpha-D-mannosidase with optimal activity between pH 5.5 and 6.0 which was also partially membrane-bound. 3. In liver the acidic alpha-D-mannosidase was obtained almost entirely in a particulate form distributed equally between a heterogeneous low density region and heavy density lysosomes. 4. The lysosomal form of the liver enzyme was purified to electrophoretic homogeneity and shown to be a glycoprotein composed of four identical subunits of molecular weight 65 kDa. 5. Antibody raised against the purified liver alpha-D-mannosidase immunoprecipitated a polypeptide from spleen which had the same molecular size. This acidic enzyme was the predominant type of alpha-D-mannosidase in spleen, but in contrast to liver, it was obtained mainly in a cytosoluble form, the remaining activity being present in the heterogeneous light density compartment. 6. Although both tissues contain the same molecular form of the acidic alpha-D-mannosidase, in murine spleen this enzyme does not appear to be associated with stable heavy density lysosomes.

Actin polymerization and ATP hydrolysis

F-actin is the major component of muscle thin filaments and, more generally, of the microfilaments of the dynamic, multifunctional cytoskeletal systems of nonmuscle eukaryotic cells. Polymeric F-actin is formed by reversible noncovalent self-association of monomeric G-actin. To understand the dynamics of microfilament systems in cells, the dynamics of polymerization of pure actin must be understood. The following model has emerged from recent work. During the polymerization process, adenosine 5'-triphosphate (ATP) that is bound to G-actin is hydrolyzed to adenosine 5'-diphosphate (ADP) that is bound to F-actin. The hydrolysis reaction occurs on the F-actin subsequent to the polymerization reaction in two steps: cleavage of ATP followed by the slower release of inorganic phosphate (Pi). As a result, at high rates of filament growth a transient cap of ATP-actin subunits exists at the ends of elongating filaments, and at steady state a stabilizing cap of ADP.Pi-actin subunits exists at the barbed ends of filaments. Cleavage of ATP results in a highly stable filament with bound ADP.Pi, and release of Pi destabilizes the filament. Thus these two steps of the hydrolytic reaction provide potential mechanisms for regulating the monomer-polymer transition.

Evaluation of hepatic and renal function in cats with chediak-higashi syndrome

Cats with the Chediak-Higashi Syndrome (CHS) have partial oculocutaneous albinism, a bleeding tendency, and enlarged cytoplasmic granules in many cell types including those in the liver and kidney. Hepatic and renal function was evaluated in six CHS and six age-matched control cats to determine if the functions of these organs were compromised by the CHS trait. Serum concentrations of alanine aminotransferase, alkaline phosphatase, and total bilirubin were determined to assess the status of the liver. Sulfobromophthalein retention tests were also performed. Renal function was evaluated by determination of (14)C-inulin clearance; blood urea nitrogen and serum creatinine concentrations; 24-hour protein/creatinine ratios, percent clearance ratios of calcium, phosphorus, sodium, potassium and chloride; and urinalysis values. The CHS cats were not significantly (P > 0.05) different from the control cats in any of the above tests. Use of a non-parametric statistical test did reveal a mild difference (P = 0.047) in 24-hour protein excretion between CHS and control cats. Complete blood counts were performed, and the packed cell volume and hemoglobin concentrations were significantly lower (P< 0.05) in the CHS cats than in the control cats.

Animal model: renal lesions in cats with Chediak-Higashi-Steinbrinck syndrome

In this study designed to characterize the renal lesions in cats with the autosomal recessive Chediak-Higashi-Steinbrinck syndrome (CHS), the renal tubular epithelial cells of CHS cats were examined by light microscopy. Numerous cells of proximal convoluted and straight tubules, thin loop of Henle, distal convoluted and straight tubules, and collecting tubules contained enlarged cytoplasmic granules morphologically consistent with lysosomes. In general, the enlarged lysosomes were larger and more numerous in proximal convoluted and straight tubular cells and were generally more massive in older cats. The lesions observed were similar to those in the renal epithelial cells of other species with CHS and were consistent with those reported previously in other tissues of CHS cats. It is concluded that CHS cats are an appropriate model in which to study the effects of the CHS renal lesions on renal function in this syndrome.

Glycosphingolipid patterns in primary mouse kidney cultures

Primary kidney cultures from C57BL/6J mice, 6 weeks of age or older, were produced using D-valine medium to select for epithelial cell growth. After allowing the cells to attach and proliferate for 1 week following plating, medium was changed once per week. Cells formed nearly confluent monolayers during the second week of culture. The cultured cells contained all of the glycosphingolipids seen in the adult kidney, analyzed by high performance liquid chromatography as their perbenzoyl derivatives. Glucosylceramide, however, was highly predominant in the cultured cells, whereas dihexosyl- and trihexosylceramides predominate in the intact kidney. Sex differences in glycolipid contents found in the intact kidney were also apparent in these cultured cells: The concentration of neutral glycolipids, in general, was higher in male cells than in those derived from females, and the male-specific glycolipid nonhydroxy fatty acid digalactosylceramide was high in male cells but very low in female cells. Neutral glycosphingolipids were labeled in 2-week-old cultures using [3H]palmitate. The [3H]palmitate was incorporated into all of the glycolipids within 2 hr of labeling. Hence, adult mouse kidney cells in D-valine medium retain their differentiated characteristics for a sufficient period of time to allow investigation of glycolipid syntheses in monolayer cultures of epithelial cells derived from this organ.