Gabriella Racchetti

Regulated exocytosis: a novel, widely expressed system

Electrophysiological studies in some secretory and non-secretory cells have identified an extensive form of calcium-induced exocytosis that is rapid (hundreds of milliseconds), insensitive to tetanus toxin and distinct from regulated secretion. We have now identified a marker of the process, desmoyokin-AHNAK, in a clonal derivative of the neuronal cell line, PC12. In resting cells, desmoyokin-AHNAK is localized within the lumen of specific vesicles, but appears on the cell surface during stimulation. Desmoyokin-AHNAK-positive vesicles exist in a variety of cells and tissues and are distinct from the endoplasmic reticulum, Golgi, trans-Golgi, endosomes and lysosomes, and from Glut4 and constitutive secretion vesicles. They seem to be involved in two models of plasmalemma enlargement: differentiation and membrane repair. We therefore propose that these vesicles should be called enlargosomes.

Cytosolic Ca2+ Binding Proteins during Rat Brain Ageing: Loss of Calbindin and Calretinin in the Hippocampus, with no Change in the Cerebellum

The expression of two cytosolic, high affinity Ca2+‐binding proteins, calbindin‐28 and calretinin, has been investigated in the cerebellum and hippocampus of young and old rats (from 12 days to 30 months) by combining immunofluorescence and Western blotting. Three markers, calreticulin (the major Ca2+ binding protein within the lumen of the endoplasmic reticulum), MAP‐2 (a microtubule binding protein concentrated in neuronal dendrites) and synaptophysin (an integral protein of synaptic vesicles), were studied in parallel. In the cerebellar cortex a rise from 12 to 60 days was observed with calbindin‐28 and, especially, calretinin, concentrated in the Purkinje and granule neurons, respectively. The level of expression of the two proteins subsequently remained high and the distribution was unchanged, even in the cerebellum of old animals. A completely different pattern was observed in the hippocampus. Here calretinin, present especially in fibres and interneurons, was abundant in the young, decreased in the adult and reached low values in the old rats. Calbindin‐28 accumulated during growth, especially in a subpopulation of CA1 pyramidal cells and in the mossy fibres of CA3, then declined, although irregularily, during ageing. These changes of the two proteins were more marked in the dorsal and central parts than in the ventral part of the hippocampus. In the same brain areas the levels of expression of the three additional markers and their distribution within neurons and synapses were unchanged by ageing. These last results demonstrate that ageing is not accompanied by a marked loss of hippocampal neurons, yet most of the latter cells appear to modify their general Ca2+ homeostasis as a consequence of the decreased level of cytosolic Ca2+ binding proteins. These events could contribute to the changes in neuronal functioning that have been reported in the aged hippocampus, whereas those of the cerebellum appear to be sustained by other mechanisms.

Enlargeosome Traffic: Exocytosis Triggered by Various Signals Is Followed by Endocytosis, Membrane Shedding or Both

Enlargeosomes are cytoplasmic organelles discharged by regulated exocytosis, identified by immunofluorescence of their membrane marker, desmoyokin/Ahnak, but never revealed at the ultrastructural level. Among the numerous enlargeosome‐positive cells, the richest and most extensively characterized are those of a PC12 clone, PC12‐27, defective of classical neurosecretion. By using ultrastructural immunoperoxidase labeling of formaldehyde‐fixed, Triton‐X‐100‐permeabilized PC12‐27 cells, we have now identified the enlargeosomes as small vesicles scattered in the proximity of, but never docked to, the plasma membrane. Upon stimulation, these vesicles undergo exocytosis [rapid after the Ca2+ ionophore, ionomycin, much slower after either the phorbol ester, phorbol myristate acetate (PMA), or ATP, working through a P2Y receptor], with appearance in the plasma membrane of typical desmoyokin/Ahnak (d/A)‐positive, Ω‐shaped and open profiles evolving into flat patches. Postexocytic removal of the exocytized d/A‐positive membrane occurs by two processes: generation of endocytic vesicles, predominant after ionomycin and ATP 100–500 μM; and shedding of membrane‐bound cytoplasmic bodies, predominant after PMA and 1 mM ATP, containing little or no trace of endoplasmic reticulum, Golgi, endo/lysosomes and also of a plasma membrane marker. Depending on the stimulation, therefore, the cell‐surface expansion by enlargeosome exocytosis is not always recycled but can induce release of specific membranes, possibly important in the pericellular environment.

Astrocyte stellation, a process dependent on Rac1 is sustained by the regulated exocytosis of enlargeosomes†

Cultured astrocytes exhibit a flat/epitelioid phenotype much different from the star‐like phenotype of tissue astrocytes. Upon exposure to treatments that affect the small GTPase Rho and/or its effector ROCK, however, flat astrocytes undergo stellation, with restructuring of cytoskeleton and outgrowth of processes with lamellipodia, assuming a phenotype closer to that exhibited in situ. The mechanisms of this change are known only in part. Using the ROCK blocker drug Y27632, which induces rapid (tens of min), dose‐dependent and reversible stellations, we focused on two specific aspects of the process: its dependence on small GTPases and the large surface expansion of the cells. Contrary to previous reports, we found stellation to be governed by the small G protein Rac1, up to disappearance of the process when Rac1 was downregulated or blocked by a specific drug. In contrast cdc42, the other G‐protein often involved in phenotype changes, appeared not involved. The surface expansion concomitant to cytoskeleton restructuring, also dependent on Rac1, was found to be at least partially sustained by the exocytosis of enlargeosomes, small vesicles distinct from classical cell organelles, which are abundant in astrocytes. Exhaustion of stellation induced by repeated administrations of Y27632 correlated with the decrease of the enlargeosome pool. A whole‐cell process like stellation of cultured astrocytes might be irrelevant in the brain tissue. However, local restructuring of the cytoskeleton coordinate with surface expansion, occurring at critical cell sites and sustained by mechanisms analogous to those of stellation, might be of importance in both astrocyte physiology and pathology.

Differential Expression of Markers and Activities in a Group of PC12 Nerve Cell Clones.

Sixteen clones, recently isolated from the PC12 nerve cell line, were analysed for a variety of markers and activities. Two endoplasmic reticulum (ER) luminal markers, the chaperone protein BiP and the major Ca2+ storage protein calreticulin, as well as the 40‐kD rough ER membrane marker and the plus‐end‐directed mirotubule motor protein, kinesin, were found to be expressed at similar levels. These results suggest that the size of the ER, the function of microtubules and the capacity of the rapidly exchanging Ca2+ store do not change substantially among the clones. Other proteins expressed at comparable levels were synapsin I and IIa, members of a nerve cell‐specific protein family known to bind synaptic vesicles to the cytoskeleton. In contrast, another ER membrane protein, calnexin, and the markers of secretory organelles were found to vary markedly. One clone (clone 27) completely lacked both chromogranin B and secretogranin II, the proteins contained within dense granules, and synaptophysin, a marker of clear vesicles. Other clones expressed these markers to variable and apparently mutually unrelated levels. Marked variability was observed also in the uptake of exogenous catecholamines, in their release both at rest and after stimulation, and in nerve growth factor‐induced differentiation. These results provide indirect information about the mechanisms that regulate the expression of structures and activities in PC12 cells. Of particular interest is clone 27, which appears globally incompetent for regulated secretion and might therefore be a valuable tool for the study of this activity in a nerve cell.

Neurosecretory cells without neurosecretion: evidence of an independently regulated trait of the cell phenotype.

Neurosecretion competence is a fundamental property that enables differentiated neurones and professional neurosecretory cells to store neurotransmitters and hormones in specialized organelles, the synaptic‐like vesicles and dense granules, and to release them by regulated exocytosis. In our laboratory, the study of rat phaeochromocytoma (PC12) clones that fail to express the above organelles or any other components involved in neurosecretion, whilst maintaining most of the general markers of the parental population, has served to demonstrate that this trait is controlled independently from the rest of the phenotype. The present review focuses on recent advances in elucidating the molecular mechanisms governing neurosecretion competence. Moreover, the opportunities that such neurosecretion‐defective PC12 clones offer for the investigation of new aspects of regulated exocytosis and the localization of its components are summarized.

Cytosolic Ca2+ buffering, a cell property that in some neurons markedly decreases during aging, has a protective effect against NMDA/nitric oxide-induced excitotoxicity.

In order to clarify the role of cytosolic Ca2+ buffering, a property that in living cells is sustained primarily by high affinity binding proteins, in NMDA receptor-sustained neuron excitotoxicity, cultures of the neuroblastoma line CHP 100 (which is known to express the receptor) were loaded with the chelator BAPTA by incubation with various concentrations (0.03-1 microM) of its acetoxymethylester derivative. The effectiveness of the loading in terms of cytosolic buffering was confirmed by fura-2 measurement experiments in which the [Ca2+]i transients induced by cell exposure to ATP were blunted in the initial peak (up to -75%) and also in the following plateau. When the BAPTA-loaded neuroblastoma cells were exposed to NMDA (1 mM), excitotoxicity was reduced dose-dependently up to almost 70%, while the generation of cGMP was inhibited up to completion. The latter result suggested the possible involvement of nitric oxide in the NMDA-induced excitoxicity, a mechanism confirmed by the dose-dependent inhibitory effect induced by the nitric oxide synthase blocker, L-N-(1-iminoethyl)-ornithine, which protected the cells completely when administered at 300 microM. Flow cytometry analysis of DNA revealed that the mechanism of excitotoxicity in CHP100 cells does not involve apoptosis. We conclude that cytosolic Ca2+ buffering, a property known to vary considerably among neuronal cells and to change in some neurons also during ageing, has a general protective effect. Such a protection appears to take place via the blunting of the glutamate-induced [Ca2+]i responses mediated by the NMDA receptor, with prevention of the ensuing overactivation of nitric oxide synthase and of the irreversible derangement of the ionic homeostasis of the cell.

A New Form of Neurite Outgrowth Sustained by the Exocytosis of Enlargeosomes Expressed under the Control of REST

In neurons and neurosecretory (nerve) cells, neurite outgrowth requires surface enlargement sustained by exocytosis of specific but poorly characterized vesicles. A canonical, relatively slow form of outgrowth is known to require the v‐SNARE Ti‐VAMP. Recently, we have identified a new, rapid form, triggered by activation of Rac1 and sustained by the exocytosis of enlargeosomes (v‐SNARE: VAMP4). By parallel study of various pheochromocytoma PC12 cell clones exhibiting either a single or both forms of outgrowth, we show that expression of enlargeosomes, their exocytosis at growth cones and their form of neurite outgrowth are positively governed by the RE‐1 silencing transcription factor (REST), a repressor of many nerve cell‐specific genes. Using a high REST/enlargeosome‐rich PC12 clone transfected with TrkA, we found (i) that nerve growth factor (NGF) can increase the expression of both REST and the enlargeosome maker, Ahnak; and (ii) that outgrowth triggered by NGF, independent from the form triggered by Rac1 and supported mostly by exocytic, Ti‐VAMP‐positive organelles distinct from enlargeosomes, occurs at slow or fast rates depending on the strength of the TrkA signaling. These results confirm the duality of the outgrowth forms sustained by the two types of exocytic vesicles, reveal their distinct properties and identify new aspects of the REST impact in nerve cell specificity/function.

Transduction signals induced in rat brain cortex astrocytes by the HIV-1 gp120 glycoprotein

Cultures of rat brain cortex astrocytes were exposed to 10−10−10−9 M of the HIV‐1 envelope glycoprotein, gp120. No specific binding was revealed by the iodinated protein, suggesting expression of only a few sites onto the cells. In contrast, two transduction signals were rapidly induced by gp120: increased tyrosine phosphorylation of a ∼56 kDa protein and increased [Ca2+]i. This latter effect, present in of the investigated astrocytes, consisted in: discrete or biphasic peaks; slowly rising plateaus; and various types of oscillations. Moreover, in apparently unresponsive cells [Ca2+]i rose slowly (45 min) to double the resting levels. Rat brain cortex astrocytes thus appear highly sensitive to gp120. The induced array of signals might contribute to neurotoxicity during HIV infection.

The Ca2+-dependent exocytosis of enlargeosomes is greatly reinforced by genistein via a non-tyrosine kinase-dependent mechanism

Studies carried out by immunofluorescence, patch‐clamping and FM dye fluorescence consistently showed that the Ca2+‐induced exocytosis of enlargeosomes, specific vesicles expressed by many cell types, is strongly reinforced by pre‐treatment of the cells with genistein, a wide spectrum blocker of tyrosine kinases, which also induces many additional effects. Various other blockers of tyrosine kinases, however, were ineffective, and the same occurred with drugs mimicking most of the rapid, non‐tyrosine kinase‐dependent effects of genistein. The reinforcement of enlargeosome‐regulated exocytosis, therefore, is a new effect of genistein and a peculiar property of the enlargeosome exocytosis, not shared by analogous processes.