Subcellular localization of SV2 and other secretory vesicle components in PC12 cells by an efficient method of preembedding EM immunocytochemistry for cell cultures

ERG30, a VAP-33-related protein, functions in protein transport mediated by COPI vesicles

Intracellular transport of newly synthesized and mature proteins via vesicles is controlled by a large group of proteins. Here we describe a ubiquitous rat protein-endoplasmic reticulum (ER) and Golgi 30-kD protein (ERG30)-which shares structural characteristics with VAP-33, a 33-kD protein from Aplysia californica which was shown to interact with the synaptic protein VAMP. The transmembrane topology of the 30-kD ERG30 corresponds to a type II integral membrane protein, whose cytoplasmic NH(2) terminus contains a predicted coiled-coil motif. We localized ERG30 to the ER and to pre-Golgi intermediates by biochemical and immunocytochemical methods. Consistent with a role in vesicular transport, anti-ERG30 antibodies specifically inhibit intra-Golgi transport in vitro, leading to significant accumulation of COPI-coated vesicles. It appears that ERG30 functions early in the secretory pathway, probably within the Golgi and between the Golgi and the ER.

Redistribution of F-actin and large dense-cored vesicles in the human neuroblastoma SH-SY5Y in response to secretagogues and protein kinase Calpha activation

Our previous studies have shown that noradrenaline release is enhanced by activation of protein kinase Calpha in SH-SY5Y cells. In the present study, we report that activation of protein kinase Calpha leads to (a) partial redistribution of the F-actin cytoskeleton and (b) a 2.5-fold increase in the number of large dense-cored vesicles within 100 nm of the plasma membrane. This redistribution can be prevented by down-regulation of protein kinase Calpha by up to 48 h exposure to phorbol dibutyrate. Treatment with the secretagogues 100 mM KCl, the Ca2+ ionophore A23187 (20 microM) and 1 mM carbachol also leads to a partial disassembly of the F-actin cytoskeleton. This is accompanied by an increase in the number of large dense cored vesicles at the plasma membrane following exposure to KCl and A23187 but not following exposure to carbachol. These results are discussed in relation to the hypothesis that a key step in the enhancement of noradrenaline release following activation of protein kinase Calpha and elevation of intracellular calcium is the movement of large dense cored vesicles to the plasma membrane following partial disassembly of the F-actin cytoskeleton.

Neuroendocrine differentiation and nerves in human adrenal cortex and cortical lesions

Neuroendocrine differentiation and nerve distribution were studied in sections from human cortex (n=11) and cortical lesions (hyperplasias, n=9; adenomas, n=13; carcinomas, n=14) with four markers, namely chromogranin A(CgA), synaptophysin (SYN), neuron-specific enolase (NSE), protein gene product (PGP) 9.5 and small synaptic vesicle protein (SV)2. All but two cases expressed neuroendocrine differentiation. NSE was the most commonly occurring marker and the NSE immunoreactive cells were detected in normal cortex, mainly in zona glomerulosa, as well as in adenomas and carcinomas. SYN and PGP 9.5 immunoreactive cells were especially prominent in the carcinomas, while SV2 immunoreactive cells were seen mainly in normal cortex. The difference in distribution pattern of the neuroendocrine markers between adenomas and carcinomas was not so distinct that it can be used for histopathological diagnosis. The significance of neuroendocrine differentiation in cortex and cortical lesions is uncertain, but may reflect an involvement in special hormonal functions. No obvious relationship was found between the clinical syndromes and the degree of neuroendocrine differentiation. Three of the neuroendocrine markers also visualized nerve structures. PGP 9.5, which is regarded as the most 'general' nerve marker, visualized more nerve structures than did the other markers. Normal cortex contained most immunoreactive nerves, whereas they were less numerous in hyperplasias and sparse or even absent in the neoplasms. The nerves appeared among the parenchymal cells but were particularly prominent around vessels. The results suggest that the cortical nerves influence not only the regulation of the blood supply but also the hormonal regulation at the cellular level.

Pre-embedding immunolabeling for electron microscopy: an evaluation of permeabilization methods and markers

For scarce antigens or antigens which are embedded in a dense macromolecular structure, on-section labeling, the first method of choice, is not always successful. Often, the antigen can be localized by immunofluorescence microscopy, usually by a pre-embedding labeling method. Most of these methods lead to loss of ultrastructural details and, hence, labeling at electron microscope resolution does not add essential information. The scope of this paper is to compare five permeabilization methods for pre-embedding labelling for electron microscopy. We aim for a method that is easy to use and suitable for routine investigations. For our ongoing work, special attention is given to labeling of the cell nucleus. Accessibility of cytoplasmic and nuclear antigens is monitored with a set of different marker antibodies. From this investigation, we suggest that prefixation with formaldehyde/glutaraldehyde is necessary to stabilize the ultrastructure before using a detergent (Triton X-100 or Brij 58) to permeabilize or remove the membranes. The experimental conditions for labeling should be checked first with fluorescence or fluorescence-gold markers by fluorescence microscopy. Then either ultrasmall gold particles (with or without fluorochrome) with silver enhancement or, if the ultrasmall gold particles are obstructed, peroxidase markers are advised. The most promising technique to localize scarce antigens with good contrast is the combination of a pre-embedding peroxidase/tyramide-FITC or -biotin labeling followed by an on-section colloidal gold detection.

The K+ channel, Kv2.1, is apposed to astrocytic processes and is associated with inhibitory postsynaptic membranes in hippocampal and cortical principal neurons and inhibitory interneurons

A variety of voltage-gated ion channels are expressed on principal cell dendrites and have been proposed to play a pivotal role in the regulation of dendritic excitability. Previous studies at the light microscopic level demonstrated that the K+ channel subunit Kv2.1 expression was polarized to the cell soma and dendrites of principal neurons throughout the central nervous system. Here, using double immunostaining we now show that Kv2.1 protein is similarly expressed in the majority of cortical and hippocampal parvalbumin, calbindin and somatostatin-containing inhibitory interneurons. At the electron microscopic level Kv2.1 immunoreactivity was primarily observed on the plasma membrane of the somata and proximal dendrites of both principal neurons and inhibitory interneurons; expression was low on smaller dendritic branches, and absent on axons and presynaptic terminals. Kv2.1 subunit expression was highly concentrated on the cell surface membrane immediately facing astrocytic processes. Kv2.1 expression was also concentrated in specific cytoplasmic compartments and on the subsurface cisterns underlying the plasma membrane facing astrocytes. In addition, Kv2.1 subunit immunoreactivity was associated with postsynaptic densities of a fraction of inhibitory symmetric synapses; while expression at asymmetric synapses was rare. These data demonstrate that channels formed by Kv2.1 subunits are uniquely positioned on the soma and principal dendrites of both pyramidal cells and inhibitory interneurons at sites immediately adjacent to astrocytic processes. This close apposition to astrocytes will ensure a rapid removal and limit the influence of K+ released into the extracellular space. This expression pattern suggests that channels formed by Kv2.1 are poised to provide a role in the regulation of neuronal dendritic excitability.

The cytoplasmic tail of the vesicular acetylcholine transporter contains a synaptic vesicle targeting signal

The human homologue of the vesicular acetylcholine transporter (hVAChT) and the neuronal isoform of the vesicular monoamine transporter (hVMAT2) are differentially targeted to two populations of regulated secretory organelles when expressed in PC12 cells. Western blot analysis of subcellular fractions from sucrose equilibrium density gradients and glycerol velocity gradients of homogenates from stably transfected cells revealed hVAChT immunoreactivity in fractions that contain synaptophysin, a marker of synaptic vesicles, while hVMAT2 immunoreactivity was confined to heavy fractions containing chromogranin B, a marker of large dense core vesicles. In cells treated with nerve growth factor, hVAChT immunoreactivity alone co-localized with synaptophysin and was abundantly expressed on synaptic vesicle clusters. Chimeras between hVMAT2 and hVAChT were utilized to identify the domain of hVAChT required for its expression on synaptic vesicles and which would shift the expression of hVMAT2 from large dense core vesicles to synaptic vesicles. Biochemical, immunocytochemical, and electron microscopic analyses revealed that a chimera in which the cytoplasmic tail of hVMAT2 was replaced with hVAChT sequences was now preferentially targeted to synaptic vesicles. In addition, hVAChT expression on synaptic vesicles was nearly abolished when the hVMAT2 cytoplasmic tail was present. Thus, structural information resides within the terminal cytoplasmic domain of VAChT, which specifically targets it to synaptic vesicles.

Vesicular neurotransmitter transporters. Potential sites for the regulation of synaptic function

Neurotransmission depends on the regulated release of chemical transmitter molecules. This requires the packaging of these substances into the specialized secretory vesicles of neurons and neuroendocrine cells, a process mediated by specific vesicular transporters. The family of genes encoding the vesicular transporters for biogenic amines and acetylcholine have recently been cloned. Direct comparison of their transport characteristics and pharmacology provides information about vesicular transport bioenergetics, substrate feature recognition by each transporter, and the role of vesicular amine storage in the mechanism of action of psychopharmacologic and neurotoxic agents. Regulation of vesicular transport activity may affect levels of neurotransmitter available for neurosecretion and be an important site for the regulation of synaptic function. Gene knockout studies have determined vesicular transport function is critical for survival and have enabled further evaluation of the role of vesicular neurotransmitter transporters in behavior and neurotoxicity. Molecular analysis is beginning to reveal the sites involved in vesicular transporter function and the sites that determine substrate specificity. In addition, the molecular basis for the selective targeting of these transporters to specific vesicle populations and the biogenesis of monoaminergic and cholinergic synaptic vesicles are areas of research that are currently being explored. This information provides new insights into the pharmacology and physiology of biogenic amine and acetylcholine vesicular storage in cardiovascular, endocrine, and central nervous system function and has important implications for neurodegenerative disease.