Polarized distribution of the trans-Golgi network marker TGN38 during the in vitro development of neocortical neurons: effects of nocodazole and brefeldin A

Synaptic activity and strength are reflected by changes in the post-synaptic secretory pathway

Neurons are highly asymmetric cells that span long distances and need to react promptly to local demands. Consequently, neuronal secretory pathway elements are distributed throughout neurites, specifically in post-synaptic compartments, to enable local protein synthesis and delivery. Whether and how changes in local synaptic activity correlate to post-synaptic secretory elements is still unclear. To assess this, we used STED nanoscopy and automated quantitative image analysis of post-synaptic markers of the endoplasmic reticulum, ER-Golgi intermediate compartment, trans-Golgi network, and spine apparatus. We found that the distribution of these proteins was dependent on pre-synaptic activity, measured as the amount of recycling vesicles. Moreover, their abundance correlated to both pre- and post-synaptic markers of synaptic strength. Overall, the results suggest that in small, low-activity synapses the secretory pathway components are tightly clustered in the synaptic area, presumably to enable rapid local responses, while bigger synapses utilise secretory machinery components from larger, more diffuse areas.

Specialization of biosynthetic membrane trafficking for neuronal form and function

Neuronal growth and synaptic transmission require the continuous production of adhesion molecules, neurotransmitter receptors, ion-channels, and secreted trophic factors, and thus critically relies on the secretory pathway-the series of intracellular organelles including the endoplasmic reticulum (ER) and the Golgi apparatus (GA), where membrane lipids and proteins are synthesized. Commensurate with the gigantic size of the neuronal membrane and its compartmentalization by thousands of synapses with distinct compositions and activities, the neuronal secretory pathway has evolved to both traffic synaptic components over very long distances, and locally control the composition of specified segments of dendrites. Here we review new insights into the distribution and dynamics of dendritic secretory organelles and their impact on postsynaptic compartments.

Building from the Ground up: Basement Membranes in Drosophila Development

Basement membranes (BMs) are sheetlike extracellular matrices found at the basal surfaces of epithelial tissues. The structural and functional diversity of these matrices within the body endows them with the ability to affect multiple aspects of cell behavior and communication; for this reason, BMs are integral to many developmental processes. The power of Drosophila genetics, as applied to the BM, has yielded substantial insight into how these matrices influence development. Here, we explore three facets of BM biology to which Drosophila research has made particularly important contributions. First, we discuss how newly synthesized BM proteins are secreted to and assembled exclusively on basal epithelial surfaces. Next, we examine how regulation of the structural properties of the BM mechanically supports and guides tissue morphogenesis. Finally, we explore how BMs influence development through the modulation of several major signaling pathways.

The dendritic SNARE fusion machinery involved in AMPARs insertion during long-term potentiation

Sorting endosomes carry α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors (AMPARs) from their maturation sites to their final destination at the dendritic plasma membrane through both constitutive and regulated exocytosis. Insertion of functional AMPARs into the postsynaptic membrane is essential for maintaining fast excitatory synaptic transmission and plasticity. Despite this crucial role in neuronal function, the machinery mediating the fusion of AMPAR-containing endosomes in dendrites has been largely understudied in comparison to presynaptic vesicle exocytosis. Increasing evidence suggests that similarly to neurotransmitter release, AMPARs insertion relies on the formation of a SNARE complex (soluble NSF-attachment protein receptor), whose composition in dendrites has just begun to be elucidated. This review analyzes recent findings of the fusion machinery involved in regulated AMPARs insertion and discusses how dendritic exocytosis and AMPARs lateral diffusion may work together to support synaptic plasticity.

Dendritic trafficking for neuronal growth and plasticity

Among the largest cells in the body, neurons possess an immense surface area and intricate geometry that poses many unique cell biological challenges. This morphological complexity is critical for neural circuit formation and enables neurons to compartmentalize cell-cell communication and local intracellular signalling to a degree that surpasses other cell types. The adaptive plastic properties of neurons, synapses and circuits have been classically studied by measurement of electrophysiological properties, ionic conductances and excitability. Over the last 15 years, the field of synaptic and neural electrophysiology has collided with neuronal cell biology to produce a more integrated understanding of how these remarkable highly differentiated cells utilize common eukaryotic cellular machinery to decode, integrate and propagate signals in the nervous system. The present article gives a very brief and personal overview of the organelles and trafficking machinery of neuronal dendrites and their role in dendritic and synaptic plasticity.

Reelin, neuronal polarity and process orientation of cortical neurons

Deficient reelin signaling leads to characteristic layering malformations in the cerebral cortex and causes polarity defects of cortical neurons. Since the discovery of reelin much has been learned about the molecular mechanisms that underlie the characteristic defects of layering defects in the reeler mutant. More recent studies provided insights in the crosstalk between reelin signaling and molecular pathways that control polarity development of radially migrating neurons. The present review summarizes and discusses recent findings on the role of reelin in modulating polarization and process orientation of neurons in the neocortex and hippocampus.

Monitoring of deiodinase deficiency based on transcriptomic responses in SH-SY5Y cells

Iodothyronine deiodinase types I, II, and III (D1, D2, and D3, respectively), which constitute a family of selenoenzymes, activate and inactivate thyroid hormones through the removal of specific iodine moieties from thyroxine and its derivatives. These enzymes are important in the biological effects mediated by thyroid hormones. The expression of activating and inactivating deiodinases plays a critical role in a number of cell systems, including the neuronal system, during development as well as in adult vertebrates. To investigate deiodinase-disrupting chemicals based on transcriptomic responses, we examined differences in gene expression profiles between T3-treated and deiodinase-knockdown SH-SY5Y cells using microarray analysis and quantitative real-time RT-PCR. A total of 1,558 genes, consisting of 755 upregulated and 803 downregulated genes, were differentially expressed between the T3-treated and deiodinase-knockdown cells. The expression levels of 10 of these genes (ID2, ID3, CCL2, TBX3, TGOLN2, C1orf71, ZNF676, GULP1, KLF9, and ITGB5) were altered by deiodinase-disrupting chemicals (2,3,7,8-tetrachlorodibenzo-p-dioxin, polychlorinated biphenyls, propylthiouracil, iodoacetic acid, methylmercury, β-estradiol, methimazole, 3-methylcholanthrene, aminotriazole, amiodarone, cadmium chloride, dimethoate, fenvalerate, octylmethoxycinnamate, iopanoic acid, methoxychlor, and 4-methylbenzylidene-camphor). These genes are potential biomarkers for detecting deiodinase deficiency and predicting their effects on thyroid hormone production.

Secretory outposts for the local processing of membrane cargo in neuronal dendrites

The large size and geometric complexity of neuronal dendrites necessitate specialized mechanisms to both deliver postsynaptic cargo over extended distances and regulate dendritic composition on a submicron scale. Despite the fundamental importance of membrane trafficking in dendrite growth, synapse formation and plasticity, the organelles and cellular rules governing postsynaptic trafficking are only now emerging. Here we review what is currently known about dendritic secretory organelles and their role in the development, maintenance and plasticity of postsynaptic compartments.

A clustered plasticity model of long-term memory engrams

Long-term memory and its putative synaptic correlates the late phases of both long-term potentiation and long-term depression require enhanced protein synthesis. On the basis of recent data on translation-dependent synaptic plasticity and on the supralinear effect of activation of nearby synapses on action potential generation, we propose a model for the formation of long-term memory engrams at the single neuron level. In this model, which we call clustered plasticity, local translational enhancement, along with synaptic tagging and capture, facilitates the formation of long-term memory engrams through bidirectional synaptic weight changes among synapses within a dendritic branch.

Developmental changes in the expression of ATP7A during a critical period in postnatal neurodevelopment

ATP7A is a P-type ATPase that transports copper from cytosol into the secretory pathway for loading onto cuproproteins or efflux. Mutations in Atp7a cause Menkes disease, a copper-deficiency disorder fatal in the postnatal period due to severe neurodegeneration. Early postnatal copper injections are known to diminish degenerative changes in some human patients and mice bearing mutations in Atp7a. In situ hybridization studies previously demonstrated that ATP7A transcripts are expressed widely in the brain. ATP7A-specific antibody was used to study the neurodevelopmental expression and localization of ATP7A protein in the mouse brain. Based on immunoblot analyses, ATP7A expression is most abundant in the early postnatal period, reaching peak levels at P4 in neocortex and cerebellum. In the developing and adult brain, ATP7A levels are greatest in the choroid plexus/ependymal cells of the lateral and third ventricles. ATP7A expression decreases in most neuronal subpopulations from birth to adulthood. In contrast, ATP7A expression increases in CA2 hippocampal pyramidal and cerebellar Purkinje neurons. ATP7A is expressed in a subset of astrocytes, microglia, oligodendrocytes, tanycytes and endothelial cells. ATP7A is largely localized to the trans-Golgi network, adopting the cell-specific and developmentally-regulated morphology of this organelle. The presence of ATP7A in the axons of postnatal, but not adult, optic nerve suggests stage-specific roles for this enzyme. In sum, the precisely-regulated neurodevelopmental expression of ATP7A correlates well with the limited therapeutic window for effective treatment of Menkes disease.

Rotenone induces aggregation of gamma-tubulin protein and subsequent disorganization of the centrosome: relevance to formation of inclusion bodies and neurodegeneration

Neurodegenerative disorders are characterized by progressive loss of specific neurons in the central nervous system. Although they have different etiologies and clinical manifestations, most of them share similar histopathologic characteristics such as the presence of inclusion bodies in both neurons and glial cells, which represent intracellular aggregation of misfolded or aberrant proteins. In Parkinson's disease, formation of inclusion bodies has been associated with the aggresome-related process and consequently with the centrosome. However, the significance of the centrosome in the neurodegenerative process remains obscure. In the present study, the morphological and functional changes in the centrosome induced by rotenone, a common insecticide used to produce experimental Parkinsonism, were examined both in vitro and in vivo. Aggregation of gamma-tubulin protein, which is a component of the centrosome matrix and recently identified in Lewy bodies of Parkinson's disease, was observed in primary cultures of mesencephalic cells treated with rotenone. Rotenone-treated neurons and astrocytes showed enlarged and multiple centrosomes. These centrosomes also displayed multiple aggregates of alpha-synuclein protein. Neurons with disorganized centrosomes exhibited neurite retraction and microtubule destabilization, and astrocytes showed disturbances of mitotic spindles. The Golgi apparatus, which is closely related to the centrosome, was dispersed in both rotenone-treated neuronal cells and the substantia nigra of rotenone-treated rats. Our findings suggested that recruitment of abnormal proteins in the centrosome contributed to the formation of inclusion bodies, and that rotenone markedly affected the structure and function of the centrosome with consequent induction of cytoskeleton disturbances, disassembly of the Golgi apparatus and collapse of neuronal cells.

Fragmentation of the Golgi apparatus induced by the overexpression of wild-type and mutant human tau forms in neurons

Tau is a microtubule-associated protein enriched in the axonal compartment. In several neurodegenerative diseases including Alzheimer's disease, hyperphosphorylated tau accumulates in the somatodendritic compartment, self-aggregates, and forms neurofibrillary tangles. A fragmentation of the neuronal Golgi apparatus (GA) was also observed in Alzheimer's disease. In the present study, we examined the effect of overexpressing human tau on the organization of the neuronal GA in rat hippocampal cultures and in JNPL3 mice expressing tau mutant P301L. GA fragmentation was noted in a significantly higher percentage of hippocampal neurons overexpressing wild-type human tau than in control neurons over-expressing green fluorescent protein (GFP) alone. Most importantly, in neurons overexpressing mutant forms of human tau (P301L, V337M, or R406W), the percentage of neurons with a fragmented GA was 10% higher than that of neurons overexpressing wild-type human tau. In JNPL3 mice, a significantly higher percentage of motor neurons presented a fragmented GA compared to control mice. Interestingly, fragmentation of the GA was more frequent in neurons containing an accumulation and aggregation of hyperphosphorylated tau in the cell body than in neurons without these features. In both primary hippocampal neurons and JNPL3 mice, the tau-induced GA fragmentation was not caused by apoptosis. The pre-sent results implicate tau in GA fragmentation and show that this event occurs before the formation of neurofibrillary tangles.

Heterogeneity of a fluorescent tegument component in single pseudorabies virus virions and enveloped axonal assemblies

The molecular mechanisms responsible for long-distance, directional spread of alphaherpesvirus infections via axons of infected neurons are poorly understood. We describe the use of red and green fluorescent protein (GFP) fusions to capsid and tegument components, respectively, to visualize purified, single extracellular virions and axonal assemblies after pseudorabies virus (PRV) infection of cultured neurons. We observed heterogeneity in GFP fluorescence when GFP was fused to the tegument component VP22 in both single extracellular virions and discrete puncta in infected axons. This heterogeneity was observed in the presence or absence of a capsid structure detected by a fusion of monomeric red fluorescent protein to VP26. The similarity of the heterogeneous distribution of these fluorescent protein fusions in both purified virions and in axons suggested that tegument-capsid assembly and axonal targeting of viral components are linked. One possibility was that the assembly of extracellular and axonal particles containing the dually fluorescent fusion proteins occurred by the same process in the cell body. We tested this hypothesis by treating infected cultured neurons with brefeldin A, a potent inhibitor of herpesvirus maturation and secretion. Brefeldin A treatment disrupted the neuronal secretory pathway, affected fluorescent capsid and tegument transport in the cell body, and blocked subsequent entry into axons of capsid and tegument proteins. Electron microscopy demonstrated that in the absence of brefeldin A treatment, enveloped capsids entered axons, but in the presence of the inhibitor, unenveloped capsids accumulated in the cell body. These results support an assembly process in which PRV capsids acquire a membrane in the cell body prior to axonal entry and subsequent transport.

Furin Directly Cleaves proMMP-2 in the trans-Golgi Network Resulting in a Nonfunctioning Proteinase

Proprotein convertases play an important role in tumorigenesis and invasiveness. Here, we report that a dibasic amino acid convertase, furin, directly cleaves proMMP-2 within the trans-Golgi network leading to an inactive form of matrix metalloproteinase-2 (MMP-2). Co-transfection of COS-1 cells with both proMMP-2 and furin cDNAs resulted in the cleavage of the N-terminal propeptide of proMMP-2. The molecular mass of cleaved MMP-2 (63 kDa), detected in both cell lysates and conditioned medium, is between the intermediate and fully activated forms of MMP-2 induced by membrane type 1-MMP. Furin-cleaved MMP-2 does not possess proteolytic activity as examined in a cell-free assay. Treatment of transfected cells with a furin inhibitor resulted in a dose-dependent inhibition of proMMP-2 cleavage; recombinant tissue inhibitor of metalloproteinase-2, which binds to the active site of membrane type 1-MMP, had no inhibitory effect. Site-directed mutagenesis of amino acids in the furin consensus recognition motif of proMMP-2(R69KPR72) prevented propeptide cleavage, thereby identifying the scissile bond and characterizing the basic amino acids required for cleavage. Other experimental observations were consistent with intracellular furin cleavage of proMMP-2 in the trans-Golgi network. The furin cleavage site in other proMMPs was examined. MMP-3, which contains the RXXR furin consensus sequence, was cleaved in furin co-transfected cells, whereas MMP-1, which lacks an RXXR consensus sequence, was not cleaved. In conclusion, we report the novel observation that furin can directly cleave the RXXR amino acid sequence in the propeptide domain of proMMP-2 leading to inactivation of the enzyme.

Rapid dendritic transport of TGN38, a putative cargo receptor

Protein transport to and from the postsynaptic plasma membrane is thought to be of central importance for synaptic plasticity. However, the molecular details of such processes are poorly understood. One mechanism by which membrane and secretory proteins may be transported to and from postsynaptic membranes is via cargo receptors. We studied the dendritic transport of TGN38, a putative cargo receptor thought to mediate protein transport between the trans-Golgi network (TGN), endosomes, and the plasma membrane. With fluorescence time-lapse imaging of neurons expressing a TGN38-green fluorescent protein fusion protein (GFP-TGN38), we observed rapid bidirectional dynamics of the protein in dendritic shafts. In addition, the protein was present on the surface and on intracellular membranes of dendrites and dendritic spines. Finally, GFP-TGN38 was found to cycle rapidly between the plasma membrane and intracellular membranes within dendrites, including those of spines. Together, our results suggest a role for TGN38 in facilitating rapid changes in the protein composition of postsynaptic membranes.

Endoplasmic reticulum export site formation and function in dendrites

The elongated and polarized characteristics of neurons render targeting of receptors to the plasma membrane of distal axonal projections and dendritic branches a major sorting task. Although the majority of biosynthetic cargo synthesis, transport, and sorting are believed to occur in the soma, local membrane protein translation and sorting has been reported recently to take place in dendrites and axons. We investigated where endoplasmic reticulum (ER) export occurs in dendrites using an in vitro permeabilized neuron system that enables us to specifically control the assembly of ER export sites. We show that ER export sites are assembled regularly throughout the entire dendritic tree by the regulated sequential recruitment of Sar1 and COPII (coat protein complex II). Moreover, activation of metabotropic glutamate receptors leads to the recruitment of the NMDA receptor subunit NR1 to remodeled ER export sites. We propose that regulation of receptor assembly and export from the ER in dendrites plays an important role in modulating receptor surface expression and neuronal function.

Subcellular post-transcriptional targeting: delivery of an intracellular protein to the extracellular leaflet of the plasma membrane using a glycosyl-phosphatidylinositol (GPI) membrane anchor in neurons and polarised epithelial cells

The effectiveness of viral vector-mediated gene transfer depends on the expression of therapeutic transgenes in the correct target cell types. So far, however, little attention has been given to targeted subcellular distribution of expressed transgenes. Targeting individual transgenes to particular subcellular compartments will provide various advantages in increasing the safety, efficacy, and specificity of viral vector-mediated gene delivery. Viruses normally hijack the cellular protein synthesis machinery for their own advantages. It is thus unknown whether cells infected with viral vectors will be able to target proteins to the correct subcellular organelles, or whether the subcellular targeting machinery would be selectively disrupted by viral infection. In this article we explored whether a herpes simplex virus type 1-derived vector could be used to deliver a transgene engineered to be targeted to the extracellular membrane of target cells. To do so we constructed a temperature-sensitive mutant HSV-1 vector, tsK-TT21 expressing a recombinant marker protein, tissue inhibitor of metalloproteinases (TIMP), linked to sequence encoding a signal for the addition of a glycosyl-phosphatidylinositol (GPI)-anchor within the endoplasmic reticulum. Our results demonstrate that HSV1-derived viral vectors can be used to target transgenes as GPI anchored proteins to the outside leaflet of plasma membranes, without disrupting the targeting machinery of host epithelial cells or neurons. This approach could then be used to target specific proteins to the cell membrane to modify cell-cell interactions, the function of specific plasma membrane proteins, or their interactions with other membrane proteins, and also to target a prodrug converting enzyme to the plasma membrane of target cells, therefore enhancing its cell killing effects.

Regulation of cytoplasmic dynein behaviour and microtubule organization by mammalian Lis1

Whereas total loss of Lis1 is lethal, disruption of one allele of the Lis1 gene results in brain abnormalities, indicating that developing neurons are particularly sensitive to a reduction in Lis1 dosage. Here we show that Lis1 is enriched in neurons relative to levels in other cell types, and that Lis1 interacts with the microtubule motor cytoplasmic dynein. Production of more Lis1 in non-neuronal cells increases retrograde movement of cytoplasmic dynein and leads to peripheral accumulation of microtubules. These changes may reflect neuron-like dynein behaviours induced by abundant Lis1. Lis1 deficiency produces the opposite phenotype. Our results indicate that abundance of Lis1 in neurons may stimulate specific dynein functions that function in neuronal migration and axon growth.

Subcellular RNA compartmentalization

The phenomenon of mRNA sorting to defined subcellular domains is observed in diverse organisms such as yeast and man. It is now becoming increasingly clear that specific transport of mRNAs to extrasomal locations in nerve cells of the central and peripheral nervous system may play an important role in nerve cell development and synaptic plasticity. Although the majority of mRNAs that are expressed in a given neuron are confined to the cell somata, some transcript species are specifically delivered to dendrites and/or, albeit less frequently, to the axonal domain. The physiological role and the molecular mechanisms of mRNA compartmentalization is now being investigated extensively. Even though most of the fundamental aspects await to be fully characterized, a few interesting data are emerging. In particular, there are a number of different subcellular distribution patterns of different RNA species in a given neuronal cell type and RNA compartmentalization may differ depending on the electrical activity of nerve cells. Furthermore, RNA transport is different in neurons of different developmental stages. Considerable evidence is now accumulating that mRNA sorting, at least to dendrites and the initial axonal segment, enables local synthesis of key proteins that are detrimental for synaptic function, nerve cell development and the establishment and maintenance of nerve cell polarity. The molecular determinants specifying mRNA compartmentalization to defined microdomains of nerve cells are just beginning to be unravelled. Targeting appears to be determined by sequence elements residing in the mRNA molecule to which proteins bind in a manner to direct these transcripts along cytoskeletal components to their site of function where they may be anchored to await transcriptional activation upon demand.

Dendritic and postsynaptic protein synthetic machinery

There is a growing body of evidence that local protein synthesis beneath synapses may provide a novel mechanism underlying plastic phenomena. In vivo and in vitro biochemical data show that dendrites can perform translation and glycosylation. Using antibodies directed against the eukaryotic protein synthetic machinery, we sought to identify the structures implicated in nonperinuclear translation, namely dendritic and postsynaptic protein synthesis. We performed a morphological and immunocytochemical analysis of ventromedial horn rat spinal cord neurons using both light and electron microscopy. We show at the cellular level that, in vivo, protein synthesis macrocomplexes (ribosomes and eIF-2) as well as the endomembranous system implicated in cotranslational and posttranslational modifications (endoplasmic reticulum and Golgi cisternae) penetrated some dendrites. Membrane-limited organelles of different shape and size are present close to the postsynaptic differentiations of most synapses, independently of their localization on the neuronal surface. We demonstrate (1) that some cisternae are immunoreactive for antibodies against ribosomal proteins and eIF-2, and (2) that markers of endoplasmic reticulum (BiP), intermediate compartment, and Golgi complex (rab1, CTR433, TGN38) label subsets of these subsynaptic organelles. Therefore, these findings indicate that synapses are equipped with the essential elements required for the synthesis and insertion of a well folded and glycosylated transmembrane protein.

Calcium-evoked dendritic exocytosis in cultured hippocampal neurons. Part II: mediation by calcium/calmodulin-dependent protein kinase II

Calcium-evoked dendritic exocytosis (CEDE), demonstrated in cultured hippocampal neurons, is a novel mechanism that could play a role in synaptic plasticity. A number of forms of neuronal plasticity are thought to be mediated by calcium/calmodulin-dependent protein kinase II (CaMKII). Here, we investigate the role of CaMKII in CEDE. We find that the developmental time course of CEDE parallels the expression of alphaCaMKII, a dominant subunit of CaMKII. An inhibitor of this enzyme, KN-62, blocks CEDE. Furthermore, 7 d in vitro neurons (which normally do not express alphaCaMKII nor show CEDE) can undergo CEDE when infected with a recombinant virus producing alphaCaMKII. Expression of a constitutively active CaMKII produces dendritic exocytosis in the absence of calcium stimulus, and this exocytosis is blocked by nocodazole, an inhibitor of microtubule polymerization that also blocks CEDE. These results indicate that CEDE is mediated by the activation of CaMKII, consistent with the view that CEDE plays a role in synaptic plasticity.

Translational machinery in dendrites of hippocampal neurons in culture

In neurons, several mRNAs are selectively delivered to dendritic domains where they are presumably translated by local protein synthetic machinery. Although electron microscopy has identified polyribosomes in dendrites, in particular in postsynaptic dendritic compartments, the functional composition of the local protein synthetic apparatus and the scope of its translational capacity have not been analyzed. To ascertain the translational competence of dendrites, we have probed hippocampal neurons in primary culture for various integral and associated factors of the translational apparatus. We report here that dendrites of such neurons are equipped with a spectrum of translational machinery components, including ribosomes, tRNAs, initiation and elongation factors, and elements of the cotranslational signal recognition mechanism. These components are differentially and nonuniformly distributed in dendritic arbors. Their dendritic location illustrates the soma-independent potential of dendrites to synthesize selected proteins in local domains.

Protein synthesis within dendrites: glycosylation of newly synthesized proteins in dendrites of hippocampal neurons in culture

There is increasing evidence that certain mRNAs are present in dendrites and can be translated there. The present study uses two strategies to evaluate whether dendrites also possess the machinery for protein glycosylation. First, precursor labeling techniques were used to conjunction with autoradiography to visualize glycosyltransferase activities that are characteristic of the rough endoplasmic reticulum (RER) (mannose) or the Golgi apparatus (GA) (galactose and fucose) in dendrites that had been separated from their cell bodies and in intact neurons treated with brefeldin A or low temperature. Second, immunocytochemical techniques were used to define the subcellular distribution of proteins that are considered markers of the RER (ribophorin I) and GA (p58, alpha-mannosidase II, galactosyltransferase, and TGN38/41). Autoradiographic analysis revealed that isolated dendrites incorporated sugar precursors in a tunicamycin-sensitive and protein synthesis-dependent manner. Moreover, when intact neurons were pulse-labeled with 3H-labeled sugars at low temperature or after treatment with brefeldin A, labeling was distributed over proximal and sometimes distal dendrites. Immunolabeling for RER markers was predominantly localized in cell bodies but extended for a considerable distance into dendrites of all neurons. Immunolabeling for GA markers was confined to the cell body in approximately 70% of the neurons, but in 30% of the neurons, the staining extended into proximal and middle dendrites. These results indicate that the machinery for glycosylation extends well into dendrites in many neurons.

Generation and characterization of an antiserum reactive with a proteolytic processing site within rat procorticotrophin-releasing hormone

In this paper we report the generation of an antibody specific for the cleavage site within procorticotrophin-releasing hormone (proCRH) at the N-terminus proCRH/CRH (1-41) junction. Using radioimmunoassay techniques were show that the antibody generated (781) cross-reacts specifically with the proCRH (137-150) Tyr fragment, corresponding to the cleavage site within the full length precursor molecule. The anti-cleavage site antibody does not crossreact with the endoproteolytic products originated from the CRH precursor molecule, i.e. CRH (1-41) or proCRH (125-151) or with any of the CRH-immunoreactive fragments tested i.e. CRH (36-41), CRH (1-20) and CRH (30-41). It also shows no cross-reactivity with CRH-related substances from other species, i.e. urotensin I (fish) and sauvagine (frog). The cleavage site antibody (781), recognizes the full length proCRH molecule in Western blotting and in liquid phase radioimmunoassay from transfected CHO-K1 cells expressing the full length pre-proCRH cDNA. Using immunofluorescence and immunoprecipitation techniques followed by SDS-PAGE and autoradiography, we confirm the presence of the intact CRH precursor molecule within the nucleus and the cytoplasm of stably transfected CHO-K1 cells expressing immunoreactive proCRH. The immunofluorescence studies using primary cultures of hypothalamic neurons, show that immunoreactive (IR) proCRH is localized within the perinuclear region and was also seen along the neuronal processes where it accumulates at their tips. Our results, therefore, show that this antibody will be an invaluable tool in the study of intracellular trafficking in relation to the endoproteolytic processing of the CRH precursor molecule.

The organization of the endoplasmic reticulum and the intermediate compartment in cultured rat hippocampal neurons

The boundaries of the organelles of the biosynthetic endomembrane system are still controversial. In this paper we take advantage of the unique architectural organization of neurons to investigate the localization of a spectrum of compartment-specific markers with the goal of defining the location of the rough endoplasmic reticulum (ER), smooth ER, intermediate compartment, and the Golgi complex. Markers of the rough ER (signal sequence receptor), Golgi complex (mannosidase II), and the trans Golgi network (TGN38) were essentially restricted to the cell body and the initial segment of one of the cell's dendrites. In contrast the cytochemical reaction product for glucose 6 phosphate, a classical ER marker, in addition to staining ER structures in the cell body also reacted with smooth ER elements that extended into both axons and dendrites. These peripheral smooth ER elements also reacted at the immunofluorescence level for ER marker 3-hydroxy-3-methylglutaryl-coenzyme A reductase, as well as for calnexin and protein disulfide isomerase. We also analyzed the location of rab1, rab2, p58, the KDEL receptor, and beta-subunit of coatomer. These intermediate compartment markers were found predominantly in the cell body but also extended to the proximal parts of the dendrites. Collectively, our data argue that the ER of hippocampal neurons consists of functionally and spatially distinct and separated domains, and they stress the power of the hippocampal neuron system for investigations of the organization of the ER by light microscopy.

Simultaneous detection of amplicon and HSV-1 helper encoded proteins reveals that neurons and astrocytoma cells do express amplicon-borne transgenes in the absence of synthesis of virus immediate early proteins

HSV-1 amplicon vectors were used to express either a cytoplasmic (beta-galactosidase) or a membrane targeted protein (TIMP-Thy1) in primary neuronal cultures, and a human astrocytoma cell line. Whereas some cells became infected by vector particles alone others were simultaneously infected by both vector and helper particles. Our results show that IEHCMV and HSV-1 IE3 promoters are able to direct transgene expression in these cells in the absence of synthesis of helper virus transacting proteins, and stress the need of monitoring expression from both partners of an amplicon population, in order to differentiate transgene expression in cells singly infected with amplicon particles, from those infected by both amplicon and helper particles.

Expression of biologically active procorticotrophin-releasing hormone (proCRH) in stably transfected CHO-K1 cells: characterization of nuclear proCRH

Corticotrophin-releasing hormone (CRH) is a 41 amino acid neuropeptide which is cleaved at a pair of dibasic amino acids from a larger precursor molecule (pre-proCRH) by the action of endopeptidases. In cells possessing a regulated secretory pathway, sorting of proneuropeptides and prohormones occurs within the trans-Golgi network, where they are finally packaged into secretory vesicles to be released in response to an external stimulus. Such cells also possess a constitutive secretory pathway, and neuropeptides are also translocated into this subcellular compartment. We have recently established stably transfected CHO-K1 cells expressing the rat pre-proCRH cDNA, and shown that proCRH was localized within the secretory pathway and the nucleus of transfected cells. Both the cytoplasmic and nuclear species of IR-CRH displayed an apparent molecular weight approximately 19 kDa, consistent with the size of the uncleaved CRH precursor molecule. In this paper, we further characterized the bitopological, i.e. nuclear and cytoplasmic localization of proCRH within transfected CHO-K1 cells. Immunoreactive nuclear CRH was not extractable using detergents (Triton X-100 and CHAPS), 10 mM salt washes or RNase digestion but could be abolished by digestion with DNase I. These results therefore suggest that nuclear proCRH is in close association with DNA/chromatin. Treatment of transfected cells with inhibitors of protein and RNA synthesis for up to 24 h had no effect upon immunoreactive nuclear CRH, indicating that it is very stable with a long half life. Brefeldin A treatment had no effect upon the nuclear translocation of newly synthesized proCRH, suggesting that late stages of the secretory pathway (i.e. post rough endoplasmic reticulum compartments) of the transfected cells do not play a role in proCRH nuclear transport. We also demonstrate that proCRH synthesized within stably transfected CHO-K1 cells is capable of stimulating ACTH release from primary cultures of anterior pituitary cells, therefore showing for the first time that the intact precursor is also biologically active and could act as an ACTH secretagogue in-vivo.

Synaptogenesis and distribution of presynaptic axonal varicosities in low density primary cultures of neocortex: an immunocytochemical study utilizing synaptic vesicle-specific antibodies, and an electrophysiological examination utilizing whole cell recording

Low-density primary cultures of neocortical neurons were utilized to examine: (i) early interactions of growing neurites with morphological characteristics of axons with other neuronal elements, and (ii) the distribution of presynaptic axonal varicosities closely apposed to MAP-2 immunoreactive, putatively postsynaptic, dendrites. At the light microscopical level axonal varicosities, presumably presynaptic terminals, were identified using immunocytochemistry incorporating antibodies specific for the synaptic vesicle antigens synaptophysin and synapsin. The presence of synaptophysin- and synapsin-immunoreactive swellings along axonal processes was first detected at 5 days post-plating and was also apparent in axons growing in isolation. At 5-7 days in vitro, immunolabelled axonal varicosities in close apposition to putative postsynaptic dendrites (MAP-2 immunoreactive) dendrites were detected. Electrophysiologically active synaptic contacts can also readily be detected at this stage. After 3 weeks in vitro presynaptic contacts do appear to be distributed heterogeneously along postsynaptic dendrites of many neurons in culture. As the culture matures a higher number of presynaptic profiles can be seen along dendrites, with a centrifugal distribution, e.g. a higher density of presynaptic axonal terminals in close apposition to more distal regions of larger dendrites, putatively considered to be apical dendrites of pyramidal-like neurons. In our cultures, the overall increase in the density and the pattern of distribution of presynaptic axon terminals immunoreactive for synaptic vesicle antigens closely apposed to putative post-synaptic structures mimics the general postnatal increase of synaptic density in the neocortex in vivo. Thus, low density primary cultures of neocortical neurons offer a valuable system to explore and manipulate (i) the molecular and cellular basis of neocortical synaptogenesis, and (ii) the pharmacology of neocortical synaptic transmission.