The recycling of a secretory granule membrane protein

Preventing neurodegeneration by adrenergic astroglial excitation

Impairment of the main noradrenergic nucleus of the human brain, the locus coeruleus (LC), which has been discovered in 1784, represents one of defining factors of neurodegenerative diseases progression. Projections of LC neurons release noradrenaline/norepinephrine (NA), which stimulates astrocytes, homeostatic neuroglial cells enriched with adrenergic receptors. There is a direct correlation between the reduction in noradrenergic innervations and cognitive decline associated with ageing and neurodegenerative diseases. It is, therefore, hypothesized that the resilience of LC neurons to degeneration influences the neural reserve that in turn determines cognitive decline. Deficits in the noradrenergic innervation of the brain might be reversed or restrained by increasing the activity of existing LC neurons, transplanting noradrenergic neurons, and/or using drugs that mimic the activity of NA on astroglia. Here, these strategies are discussed with the aim to understand how astrocytes integrate neuronal network activity in the brain information processing in health and disease.

Exocytosis and endocytosis in neuroendocrine cells: inseparable membranes!

Although much has been learned concerning the mechanisms of secretory vesicle formation and fusion at donor and acceptor membrane compartments, relatively little attention has been paid toward understanding how cells maintain a homeostatic membrane balance through vesicular trafficking. In neurons and neuroendocrine cells, release of neurotransmitters, neuropeptides, and hormones occurs through calcium-regulated exocytosis at the plasma membrane. To allow recycling of secretory vesicle components and to preserve organelles integrity, cells must initiate and regulate compensatory membrane uptake. This review relates the fate of secretory granule membranes after full fusion exocytosis in neuroendocrine cells. In particular, we focus on the potential role of lipids in preserving and sorting secretory granule membranes after exocytosis and we discuss the potential mechanisms of membrane retrieval.

A nibbling mechanism for clathrin-mediated retrieval of secretory granule membrane after exocytosis

Clathrin-mediated endocytosis is the major pathway for recycling of granule membrane components after strong stimulation and high exocytotic rates. It resembles "classical" receptor-mediated endocytosis but has a trigger that is unique to secretion, the sudden appearance of the secretory granule membrane in the plasma membrane. The spatial localization, the relationship to individual fusion events, the nature of the cargo, and the timing and nature of the nucleation events are unknown. Furthermore, a size mismatch between chromaffin granules (∼300-nm diameter) and typical clathrin-coated vesicles (∼90 nm) makes it unlikely that clathrin-mediated endocytosis internalizes as a unit the entire fused granule membrane. We have used a combination of total internal reflection fluorescence microscopy of transiently expressed proteins and time-resolved quantitative confocal imaging of endogenous proteins along with a fluid-phase marker to address these issues. We demonstrate that the fused granule membrane remains a distinct entity and serves as a nucleation site for clathrin- and dynamin-mediated endocytosis that internalizes granule membrane components in small increments.

Retrieval and reuse of pituitary secretory granule proteins

The pituitary contains professional secretory cells, devoting a large fraction of their energy to the synthesis of hormones that are stored for secretion in response to a complex mixture of inputs. Ba2+, a substitute for Ca2+, and phorbol ester, a mimic for diacylglycerol, have a synergistic effect on exocytosis. By using these secretagogues, we developed a paradigm in which phorbol ester potentiation of Ba2+-evoked exocytosis produces a robust secretory response in multiple pituitary cell types. Because cells subjected to this stimulatory paradigm remain healthy despite their greatly reduced hormone content, we used this paradigm to study the fate of granule membrane proteins. We examined the turnover of peptidylglycine alpha-amidating monooxygenase (PAM), a membrane enzyme involved in the final maturation of many peptides, and VAMP2, a vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE). The stability of recently synthesized PAM was increased by sustained exocytosis. Biotinylation studies established that the appearance of integral membrane PAM at the plasma membrane was stimulated along with hormone secretion. PAM biotinylated on the cell surface undergoes cleavage to yield soluble peptidylglycine-alpha-hydroxylating monooxygenase that can then be secreted in a regulated fashion. Consistent with a kiss-and-run or cavicapture mode of secretion (Taraska, J. W., Perrais, D., Ohara-Imaizumi, M., Nagamatsu, S., and Almers, W. (2003) Proc. Natl. Acad. Sci. U. S. A. 100, 2070-2075), biotinylated prolactin was also retained by the cells and later released in response to secretagogues. Thus, pituitary cells can retrieve and reuse components of the machinery involved in the final stages of exocytosis (the SNAREs) as well as soluble and membrane granule proteins.

Secretagogue-triggered transfer of membrane proteins from neuroendocrine secretory granules to synaptic-like microvesicles

The membrane proteins of all regulated secretory organelles (RSOs) recycle after exocytosis. However, the recycling of those membrane proteins that are targeted to both dense core granules (DCGs) and synaptic-like microvesicles (SLMVs) has not been addressed. Since neuroendocrine cells contain both RSOs, and the recycling routes that lead to either organelle overlap, transfer between the two pools of membrane proteins could occur during recycling. We have previously demonstrated that a chimeric protein containing the cytosolic and transmembrane domains of P-selectin coupled to horseradish peroxidase is targeted to both the DCG and the SLMV in PC12 cells. Using this chimera, we have characterized secretagogue-induced traffic in PC12 cells. After stimulation, this chimeric protein traffics from DCGs to the cell surface, internalizes into transferrin receptor (TFnR)-positive endosomes and thence to a population of secretagogue-responsive SLMVs. We therefore find a secretagogue-dependent rise in levels of HRP within SLMVs. In addition, the levels within SLMVs of the endogenous membrane protein, synaptotagmin, as well as a green fluorescent protein-tagged version of vesicle-associated membrane protein (VAMP)/synaptobrevin, also show a secretagogue-dependent increase.

Sorting and storage during secretory granule biogenesis: looking backward and looking forward

Secretory granules are specialized intracellular organelles that serve as a storage pool for selected secretory products. The exocytosis of secretory granules is markedly amplified under physiologically stimulated conditions. While granules have been recognized as post-Golgi carriers for almost 40 years, the molecular mechanisms involved in their formation from the trans-Golgi network are only beginning to be defined. This review summarizes and evaluates current information about how secretory proteins are thought to be sorted for the regulated secretory pathway and how these activities are positioned with respect to other post-Golgi sorting events that must occur in parallel. In the first half of the review, the emerging role of immature secretory granules in protein sorting is highlighted. The second half of the review summarizes what is known about the composition of granule membranes. The numerous similarities and relatively limited differences identified between granule membranes and other vesicular carriers that convey products to and from the plasmalemma, serve as a basis for examining how granule membrane composition might be established and how its unique functions interface with general post-Golgi membrane traffic. Studies of granule formation in vitro offer additional new insights, but also important challenges for future efforts to understand how regulated secretory pathways are constructed and maintained.

Barium triggers rapid endocytosis in calf adrenal chromaffin cells

1. Changes in cell capacitance were monitored in whole-cell patch-clamp recordings from calf adrenal chromaffin cells using a software-based phase-tracking technique. Rapid endocytosis and exocytosis were observed in extracellular solutions containing either Ca2+ or Ba2+. 2. There was no significant difference in the magnitude or the time course of rapid endocytosis of cells stimulated in Ca2+ as compared to Ba2+. When cells were pretreated with caffeine and thapsigargin in order to deplete intracellular Ca2+ stores, rapid endocytosis in Ba2+ was not affected. This indicates that Ba2+ itself is capable of supporting rapid endocytosis. 3. The application of the calmodulin inhibitor calmidazolium via the intracellular pipette solution did not inhibit rapid endocytosis. Although our findings are inconsistent with an immediate requirement for calmodulin in rapid endocytosis, they do not rule out an involvement on a longer time scale. 4. While rapid endocytosis was not affected by the substitution of Ca2+ with Ba2+, the maximum rate of exocytosis was higher in cells stimulated in Ca2+ than in Ba2+. Since Ba2+ currents were much larger than Ca2+ currents during depolarizations to +10 mV (the test potential used in these experiments), Ba2+ appears to be less efficient at promoting exocytosis than Ca2+.

Overall lack of regulated secretion in a PC12 variant cell clone

A stable clone of PC12 neuroendocrine cells, named 27, known from previous studies to exhibit a defect of regulated secretion (lack of regulated secretory proteins, of synaptophysin, of dense granules and of catecholamine uptake and release; Clementi, E., Racchetti, G., Zacchetti, D., Panzeri, M. C., and Meldolesi, J. (1992) Eur. J. Neurosci. 4, 944-953) was characterized in detail to clarify the nature of its phenotype and the mechanisms of its establishment. The neuroendocrine nature of the PC12-27 phenotype was documented by specific markers: synapsins, neurofilament subunit H, neuronal kinesin, and alpha-latrotoxin receptor. Moreover, various intracellular membrane systems of PC12-27, including the endoplasmic reticulum and the Golgi complex, appeared similar to control PC12 in both morphology and marker expression. In contrast, all the investigated markers located either in dense granules (dopamine-beta-hydroxylase), in synaptic-like microvesicles (the acetylcholine transporter) or in both these regulated secretory organelles (VAMP2/synaptobrevin-2, synaptotagmin) were missing in PC12-27 cells, and the same was true also for the cytosolic and plasmalemma proteins involved in regulated exocytosis (Rab3, SNAP25, syntaxin). Pulse labeling and in vitro translation experiments revealed the defect to consist in a protein synthesis blockade that mRNA studies (reverse transcription-polymerase chain reaction, Northern blotting, and actinomycin D experiments) revealed to take place primarily at the transcriptional level. The secretion defect of PC12-27 cells was modified neither by various types of long term stimulation nor by nerve growth factor treatment. Moreover, when one of the missing regulated secretory proteins, chromogranin B, was expressed by cDNA transfection, it was secreted, however via the constitutive pathway. Our results demonstrate that PC12-27 cells are fully incompetent for both branches of regulated secretion, those of dense granules and synaptic-like microvesicles, possibly because of the impairment of a general expression control system that appears to operate independently of neuroendocrine cell differentiation.

Role of clathrin-coated vesicles in glycoprotein transport from the cell surface to the Golgi complex

Plasma membrane glycoproteins recycle to the Golgi complex, but the route followed by these proteins is not known. To elucidate the pathway of transport, the involvement of clathrin-coated vesicles was tested. This was accomplished by comparing the traffic of wild type low density lipoprotein receptor (LDLR) and FH 683, a mutant receptor whose endocytosis from the cell surface in coated vesicles is reduced by 90-95%. Wild type LDLR traveled from the cell surface to the sialyltransferase compartment of the Golgi with a half-time of 2.5 h in K562 human leukemia cells expressing receptor from a transfected cDNA. In contrast, FH 683 LDLR recycled to the Golgi at 33% of the wild type rate, suggesting that wild type LDLR is largely transported to the Golgi by a pathway that involves clathrin-coated vesicles. Moreover, because clathrin-coated vesicles that bud from the plasma membrane are transported to endosomes, surface-to-Golgi transport probably involves an endosomal intermediate. Finally, because there was substantial transport of mutant LDLR to the Golgi even though its endocytosis in coated vesicles was greatly reduced, there may be a second pathway of surface-to-Golgi traffic. Our results suggest that wild type LDLR may move from plasma membrane to Golgi by two routes. Two-thirds of the traffic proceeds via a coated vesicle-mediated pathway while the remainder may follow a clathrin-independent pathway.

Insulin secretory granule biogenesis and the proinsulin-processing endopeptidases

The insulin storage granule of the pancreatic beta cell is assembled within the trans Golgi network from around 50 or so gene products many of which are synthesized coordinately with the major component, proinsulin. An important contribution to our understanding of the regulation of this process has come from studies of the post-translational processing of proinsulin and of other proteins which are stored in the granule, particularly the processing enzymes themselves. The present review focusses on recent insights into the molecular nature of the processing machinery, and the granule Ca(2+)-dependent subtilisin-related endopeptidases which catalyse the initial rate-limiting step in the enzymic conversion of proinsulin.

Bovine chromaffin cells: studies on the biosynthesis of phospholipids in chromaffin granules

We have studied the biosynthesis of chromaffin granules by labelling primary cultures of bovine chromaffin cells with either [35S]methionine or various precursors for lipids. After labelling the cells were subjected to subcellular fractionation including density gradient centrifugation. After [35S]methionine significant label (mainly represented by labelled chromogranin A) was found in the soluble proteins of chromaffin granules, whereas the membranes were relatively little labelled. However incorporation into membrane bound dopamine beta-hydroxylase and cytochrome b-561 could be demonstrated. Neither of the used lipid precursors ([3H]glycerol, [3H]choline, [3H]palmitic acid or [3H]arachidonic acid) was incorporated to any significant extent into the soluble components of chromaffin granules. Thus there is no evidence that this secretory compartment contains any lipids or acylated proteins. Incorporation of lipid precursors into the membranes of chromaffin granules was apparently low. After short chases labelled lysolecithin was not present in these organelles. However with prolonged chase times labelled lysolecithin, apparently appeared in chromaffin granules irrespective of whether the cells were stimulated or not. We can conclude that the reusable membranes of chromaffin granules have a very low lipid turnover. Lysolecithin is not transferred into these organelles during biosynthesis but is formed in them during their long life span. This formation of lysolecithin is independent of stimulation of these cells and therefore unlikely to be involved in exocytosis.

Recycling of a secretory granule membrane protein after stimulated secretion

Recycling of a secretory granule membrane protein, dopamine-beta-hydroxylase, was examined in primary cultures of bovine adrenal chromaffin cells. Cells were stimulated to secrete in the presence of antibodies directed against the luminal domain of dopamine-beta-hydroxylase. The location of the antibodies after various times of reincubation and after a second secretory stimulus was assessed using immunofluorescence microscopy. Stimulation led to the exposure of dopamine-beta-hydroxylase at the plasma membrane, which could be detected by a polyclonal antibody in living and fixed cells. The plasma membrane dopamine-beta-hydroxylase, either alone or complexed with antibody, was rapidly internalized after removal of the secretagogue. Internalized protein-antibody complex remained stable for at least 24 hours of reculture. Twenty four hours after stimulation the cells with internalized antibody could respond to further stimulation and some of the antibody was re-exposed at the plasma membrane. These findings were confirmed using FACS analysis. This suggests that the antibody-protein complex had returned to secretory granules that could respond to further secretagogue stimulation.

Biogenesis of secretory granules

The biogenesis of secretory granules in endocrine, neuroendocrine, and exocrine cells is thought to involve a selective aggregation of the regulated secretory proteins into a dense-cored structure. The dense-core is then enveloped by membrane in the trans-Golgi network and buds, forming an immature secretory granule. The immature secretory granule then undergoes a maturation process which gives rise to the mature secretory granule. The recent data on the processes of aggregation, budding and maturation are summarized here. In addition, the current knowledge about the mature secretory granule is reviewed with emphasis on the biogenesis of the membrane of this organelle.

Translocation of the nuclear autoantigen La to cell surface: assembly and disassembly with the extracellular matrix

La (SS-B) protein is known as one major antigenic target for autoantibodies from patients with certain autoimmune diseases such as Sjogren's syndrome or Lupus Erythematosus. La protein belongs to the so called "extractable nuclear antigens". Here we report that La antigen is not restricted to the nucleus as one might deduce from the exclusive nuclear staining pattern of patient anti-La antibodies but after stimulation of serum-starved cells with 10% fetal calf serum (FCS) appears and stays for at least 45 min at the outer surface of CV-1 cells being available for binding of anti-La antibodies. In addition we found that a minor part of La antigen associates with the extracellular fibronectin network. After addition of 10% FCS to serum starved cells this extracellular autoantigen disassembled from the extracellular matrix and was taken up again by the cells. Incubation of serum starved cells with mercuric chloride, a known potent inducer of autoantibodies, also resulted in a detachment of the extracellular matrix associated La protein. From our studies it becomes likely that La protein itself is the antigen during autoimmunization. Moreover, once developed, anti-La antibodies might be able to bind to cell surface expressed La protein resulting in a damage of these cells leading to the inflammational events known to occur during disease.

Sorting and secretory pathways in exocrine cells

Exocrine secretory cells contain multiple post-Golgi pathways from protein secretion. The major pathway in pancreatic and parotid acinar cells involves protein sorting into storage granules that undergo exocytosis with or without stimulation by secretagogues. This route of release is paralleled by a minor nongranular (but vesicular) pathway that originates by budding from maturing secretory granules. The nongranular pathway carries the same polypeptides that undergo storage in the granules but in different relative amounts. These features indicate that sorting into the stimulus-regulated pathway reflects not only the deposition of secretory proteins into immature granules but may also involve selective aggregation of proteins along with exclusion and vesicle-mediated secretion of other polypeptides that are inefficiently retained. Storage granules represent a distinct compartment of the secretory pathway, as indicated by the specific composition of their limiting membranes. Little is known about processes that maintain the low content and limited diversity of integral proteins of the granule membrane as compared to the membranes with which it fuses during exocytosis and formation. Future studies will examine the role of the nongranular secretory pathway in acinar cells, the branchpoint of pathways that are directed to the apical or basolateral cell surfaces, the structural determinants of secretory sorting, and the distribution and function of specific granule membrane polypeptides.