Dense-core vesicle biogenesis and exocytosis in neurons lacking chromogranins A and B

Endosomal protein DENND10 promotes developmental competence of neurite extension

A distinguishing feature of neurons is the presence of long neurites that enable far-reaching communication. Establishing this complex morphology requires precise regulation of intracellular transport and signaling. Our study identifies DENND10, an ancient endosomal protein, as a crucial factor in shaping neuron morphology. DENND10 is a potential regulator of Rab GTPase signaling and interacts with the CCC/Retriever endosomal complex. Loss of DENND10 in a neuronal cell culture model resulted in shortened neurites. Quantitative proteomics revealed two distinct processes of neurite outgrowth: differentiation-induced biochemical changes and a pre-existing vesicular transport system modulated by DENND10. Mechanistically, both Rab27 and CCC complex subunit CCDC22 act downstream of DENND10 to support neurite extension. In primary cortical neurons, loss of DENND10 or CCDC22 led to shortened dendrites and impaired axon development. These findings provide a conceptual framework for neuronal morphogenesis during differentiation and highlight the critical role of DENND10/CCC in neurite extension.

Molecular requirements of chromogranin B for the long-sought anion shunter of regulated secretion

All eukaryotes utilize regulated secretion to release molecular signals packaged in secretory granules for local and remote signaling. An anion shunt conductance was first suggested in secretory granules of bovine chromaffin cells nearly five decades ago. Biochemical identity of this conductance remains undefined. CLC-3, an intracellular Cl-/H+ exchanger, was proposed as a candidate sixteen years ago, which, however, was contested experimentally. Here, we show that chromogranin B (CHGB) makes the kernel of the long-sought anion shunter in cultured and primary neuroendocrine cells and its channel functions are essential to proper granule maturation. Intragranular pH measurements and cargo maturation assays revealed that normal granular acidification, proinsulin-insulin conversion, and dopamine-loading in neuroendocrine cells all rely on functional CHGB+ channels. Primary β-cells from Chgb-/- mice exhibited persistent granule deacidification, which suffices to uplift plasma proinsulin level, diminish glucose-induced 2nd-phase insulin secretion and dwindle monoamine content in chromaffin granules from the knockout mice. Data from targeted genetic manipulations, dominant negativity of a deletion mutant lacking channel-forming parts and tests of CLC-3/5 and ANO-1/2 all exclude CHGB-less channels from anion shunting in secretory granules. The highly conserved CHGB+ channels thus function in regulated secretory pathways in neuronal, endocrine, exocrine and stem cells of probably all vertebrates.

Rab10 regulates neuropeptide release by maintaining Ca2+ homeostasis and protein synthesis

Dense core vesicles (DCVs) transport and release various neuropeptides and neurotrophins that control diverse brain functions, but the DCV secretory pathway remains poorly understood. Here, we tested a prediction emerging from invertebrate studies about the crucial role of the intracellular trafficking GTPase Rab10, by assessing DCV exocytosis at single-cell resolution upon acute Rab10 depletion in mature mouse hippocampal neurons, to circumvent potential confounding effects of Rab10's established role in neurite outgrowth. We observed a significant inhibition of DCV exocytosis in Rab10-depleted neurons, whereas synaptic vesicle exocytosis was unaffected. However, rather than a direct involvement in DCV trafficking, this effect was attributed to two ER-dependent processes, ER-regulated intracellular Ca2+ dynamics, and protein synthesis. Gene Ontology analysis of differentially expressed proteins upon Rab10 depletion identified substantial alterations in synaptic and ER/ribosomal proteins, including the Ca2+ pump SERCA2. In addition, ER morphology and dynamics were altered, ER Ca2+ levels were depleted, and Ca2+ homeostasis was impaired in Rab10-depleted neurons. However, Ca2+ entry using a Ca2+ ionophore still triggered less DCV exocytosis. Instead, leucine supplementation, which enhances protein synthesis, largely rescued DCV exocytosis deficiency. We conclude that Rab10 is required for neuropeptide release by maintaining Ca2+ dynamics and regulating protein synthesis. Furthermore, DCV exocytosis appeared more dependent on (acute) protein synthesis than synaptic vesicle exocytosis.

Neuronal network inactivity potentiates neuropeptide release from mouse cortical neurons

Neurons adapt to chronic activity changes by modifying synaptic properties, including neurotransmitter release. However, whether neuropeptide release via dense core vesicles (DCVs)-a distinct regulated secretory pathway-undergoes similar adaptation remains unclear. Here, we demonstrate that 24-hour action potential blockade leads to significant DCV accumulation in primary mouse cortical neurons of both sexes. Reactivation with action potential trains induced enhanced Ca2+-influx and 700% more DCV exocytosis compared to control neurons. Notably, total DCV cargo protein levels were unchanged, while mRNA levels of corresponding genes were reduced. Blocking neurotransmitter release with Tetanus toxin induced DCV accumulation, similar to that induced by network silencing with TTX. Hence, chronic network silencing triggers increased DCV accumulation due to reduced exocytosis during silencing. These accumulated DCVs can be released upon reactivation resulting in a massive potentiation of DCV exocytosis, possibly contributing to homeostatic mechanisms.Significance Statement This study addresses an unexplored area - how dense core vesicles (DCVs) exocytosis adapts to chronic changes in activity - and demonstrates accumulation of DCVs and a massive upregulation of DCV exocytosis in response to 24h inactivity. The potentiation of neuropeptide release might contribute to homeostatic regulation of neuronal networks in the brain.

Action potential-independent spontaneous microdomain Ca2+ transients-mediated continuous neurotransmission regulates hyperalgesia

Neurotransmitters and neuromodulators can be released via either action potential (AP)-evoked transient or AP-independent continuous neurotransmission. The elevated AP-evoked neurotransmission in the primary sensory neurons plays crucial roles in hyperalgesia. However, whether and how the AP-independent continuous neurotransmission contributes to hyperalgesia remains largely unknown. Here, we show that primary sensory dorsal root ganglion (DRG) neurons exhibit frequent spontaneous microdomain Ca2+ (smCa) activities independent of APs across the cell bodies and axons, which are mediated by the spontaneous opening of TRPA1 channels and trigger continuous neurotransmission via the cyclic adenosine monophosphate-protein kinase A signaling pathway. More importantly, the frequency of smCa activity and its triggered continuous neurotransmission in DRG neurons increased dramatically in mice experiencing inflammatory pain, inhibition of which alleviates hyperalgesia. Collectively, this work revealed the AP-independent continuous neurotransmission triggered by smCa activities in DRG neurons, which may serve as a unique mechanism underlying the nociceptive sensitization in hyperalgesia and offer a potential target for the treatment of chronic pain.

Dominant Expression of Chromogranin B in Pituitary Corticotrophs and Its Putative Role in Interaction With Secretogranin III

SummaryPrevious studies have suggested that chromogranin A (CgA) is a partner molecule of secretogranin III (SgIII). In mouse pituitary corticotroph-derived AtT-20 cells, SgIII plays a role in sorting CgA/hormone aggregates into secretory granules (SGs). Although CgA expression is equivocal, CgB is clearly detectable in the rat pituitary corticotrophs. Therefore, we hypothesized that CgB shares a function with CgA in pituitary corticotrophs. In the binding assays, CgB, similar to CgA, showed binding activity to SgIII under weakly acidic conditions and in the presence of Ca2+. Considering the differences in animal species, the different abilities of antibodies, and the conditions of tissue fixation and thin sectioning in immunofluorescence histochemistry, we found that CgA was expressed in a small population (approximately 10%), and its expression intensity was weaker than that of CgB (>98%) in rodent pituitary corticotrophs. In addition, similar to CgA, CgB and SgIII were colocalized in adrenocorticotropic hormone (ACTH) granules. The labeling of CgA and CgB was not completely consistent, and CgB colocalized with SgIII in many granules. These results suggest that there are multiple sorting systems for ACTH granules in pituitary corticotrophs and that the SgIII/CgB complex behaves more dominantly than the SgIII/CgA complex, which has somewhat different properties.

Molecular requirements of chromogranin B for the long-sought anion shunter of regulated secretion

All eukaryotes utilize regulated secretion to release molecular signals packaged in secretory granules for local and remote signaling. An anion shunt conductance was first suggested in secretory granules of bovine chromaffin cells nearly five decades ago. Biochemical identity of this conductance remains undefined. CLC-3, an intracellular Cl - /H + exchanger, was proposed as a candidate sixteen years ago, which, however, was contested experimentally. Here, we show that chromogranin B (CHGB) makes the kernel of the long-sought anion shunter in cultured and primary neuroendocrine cells and its channel functions are essential to proper granule maturation. Intragranular pH measurements and cargo maturation assays revealed that normal granular acidification, proinsulin-insulin conversion, and dopamine-loading in neuroendocrine cells all rely on functional CHGB+ channels. Primary β-cells from Chgb-/- mice exhibited persistent granule deacidification, which suffices to uplift plasma proinsulin level, diminish glucose-induced 2 nd -phase insulin secretion and dwindle monoamine content in chromaffin granules from the knockout mice. Data from targeted genetic manipulations, dominant negativity of a deletion mutant lacking channel-forming parts and tests of CLC-3/5 and ANO-1/2 all exclude CHGB -less channels from anion shunting in secretory granules. The highly conserved CHGB+ channels thus function in regulated secretory pathways in neuronal, endocrine, exocrine and stem cells of probably all vertebrates.

HIGHLIGHTS

Loss of CHGB channel functions impairs secretory granule acidification in neuroendocrine cells, which necessitates anion shunt conduction. CHGBΔMIF, a mutant unable to form a functional Cl - channel, exerts negative dominance on endogenous CHGB and results in granule deacidification in cultured cells. Neither CLC-3 & -5 nor ANO-1 & -2 participate in the CHGB-mediated granule acidification. Clcn3 knockout effects on regulated secretion can be attributed to its functions in endosomal and endolysosomal compartments. Primary Chgb-/- β-cells exhibit persistent granule deacidification, presenting a unifying mechanism for disparate mouse phenotypes: hyperproinsulinemia, near abrogation of 2 nd phase insulin release after glucose challenge and diminution of monoamine contents in chromaffin granules.

Global Gene Expression Profiling and Bioinformatics Analysis Reveal Downregulated Biomarkers as Potential Indicators for Hepatocellular Carcinoma

Objective: The study aimed to elucidate the significance of CLEC4G, CAMK2β, SLC22A1, CBFA2T3, and STAB2 in the prognosis of hepatocellular carcinoma (HCC) patients and their associated molecular biological characteristics. Additionally, the research sought to identify new potential biomarkers with therapeutic and diagnostic relevance for clinical applications. Methods and Materials: We utilized a publicly available high throughput phosphoproteomics and proteomics data set of HCC to focus on the analysis of 12 downregulated phosphoproteins in HCC. Our approach integrates bioinformatic analysis with pathway analysis, encompassing gene ontology (GO) analysis, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, and the construction of a protein-protein interaction (PPI) network. Results: In total, we quantified 11547 phosphorylation sites associated with 4043 phosphoproteins from a cohort of 159 HCC patients. Within this extensive data set, our specific focus was on 19 phosphorylation sites displaying significant downregulation (log2 FC ≤ -2 with p-values < 0.0001). Remarkably, our investigation revealed distinct pathways exhibiting differential regulation across multiple dimensions, including the genomic, transcriptomic, proteomic, and phosphoproteomic levels. These pathways encompass a wide range of critical cellular processes, including cellular component organization, cell cycle control, signaling pathways, transcriptional and translational control, and metabolism. Furthermore, our bioinformatics analysis unveiled noteworthy insights into the subcellular localizations, biological processes, and molecular functions associated with these proteins and phosphoproteins. Within the context of the PPI network, we identified 12 key genes CLEC4G, STAB2, ADH1A, ADH1B, CAMK2B, ADH4, CHGB, PYGL, ADH1C, AKAP12, CBFA2T3, and SLC22A1 as the top highly interconnected hub genes. Conclusions: The findings related to CLEC4G, ADH1B, SLC22A1, CAMK2β, CBFA2T3, and STAB2 indicate their reduced expression in HCC, which is associated with an unfavorable prognosis. Furthermore, the results of KEGG and GO pathway analyses suggest that these genes may impact liver cancer by engaging various targets and pathways, ultimately promoting the progression of hepatocellular carcinoma. These results underscore the significant potential of CLEC4G, ADH1B, SLC22A1, CAMK2β, CBFA2T3, and STAB2 as key contributors to HCC development and advancement. This insight holds promise for identifying therapeutic targets and charting research avenues to enhance our understanding of the intricate molecular mechanisms underlying hepatocellular carcinoma.

Immunohistochemical field parcellation of the human hippocampus along its antero-posterior axis

The primate hippocampus includes the dentate gyrus, cornu ammonis (CA), and subiculum. CA is subdivided into four fields (CA1-CA3, plus CA3h/hilus of the dentate gyrus) with specific pyramidal cell morphology and connections. Work in non-human mammals has shown that hippocampal connectivity is precisely patterned both in the laminar and longitudinal axes. One of the main handicaps in the study of neuropathological semiology in the human hippocampus is the lack of clear laminar and longitudinal borders. The aim of this study was to explore a histochemical segmentation of the adult human hippocampus, integrating field (medio-lateral), laminar, and anteroposterior longitudinal patterning. We provide criteria for head-body-tail field and subfield parcellation of the human hippocampus based on immunodetection of Rabphilin3a (Rph3a), Purkinje-cell protein 4 (PCP4), Chromogranin A and Regulation of G protein signaling-14 (RGS-14). Notably, Rph3a and PCP4 allow to identify the border between CA3 and CA2, while Chromogranin A and RGS-14 give specific staining of CA2. We also provide novel histological data about the composition of human-specific regions of the anterior and posterior hippocampus. The data are given with stereotaxic coordinates along the longitudinal axis. This study provides novel insights for a detailed region-specific parcellation of the human hippocampus useful for human brain imaging and neuropathology.

Distinct Alterations in Dendritic Spine Morphology in the Absence of β-Neurexins

Dendritic spines are essential for synaptic function because they constitute the postsynaptic compartment of the neurons that receives the most excitatory input. The extracellularly shorter variant of the presynaptic cell adhesion molecules neurexins, β-neurexin, has been implicated in various aspects of synaptic function, including neurotransmitter release. However, its role in developing or stabilizing dendritic spines as fundamental computational units of excitatory synapses has remained unclear. Here, we show through morphological analysis that the deletion of β-neurexins in hippocampal neurons in vitro and in hippocampal tissue in vivo affects presynaptic dense-core vesicles, as hypothesized earlier, and, unexpectedly, alters the postsynaptic spine structure. Specifically, we observed that the absence of β-neurexins led to an increase in filopodial-like protrusions in vitro and more mature mushroom-type spines in the CA1 region of adult knockout mice. In addition, the deletion of β-neurexins caused alterations in the spine head dimension and an increase in spines with perforations of their postsynaptic density but no changes in the overall number of spines or synapses. Our results indicate that presynaptic β-neurexins play a role across the synaptic cleft, possibly by aligning with postsynaptic binding partners and glutamate receptors via transsynaptic columns.

Estrogen-dependent expression and function of secretogranin 2a in female-specific peptidergic neurons

Secretogranin 2 (Scg2) is a member of the secretogranin/chromogranin family of proteins that is involved in neuropeptide and hormone packaging to secretory granules and serves as a precursor for several secreted pleiotropic peptides. A recent study in zebrafish showed that the teleost Scg2 orthologs, scg2a and scg2b, play an important role in mating behavior, but its modes of action and regulatory mechanisms remain unclear. In this study, we identify scg2a in another teleost species, medaka, by transcriptomic analysis as a gene that is expressed in an ovarian secretion-dependent manner in a group of neurons relevant to female sexual receptivity, termed FeSP neurons. Investigation of scg2a expression in the FeSP neurons of estrogen receptor (Esr)-deficient medaka revealed that it is dependent on estrogen signaling through Esr2b, the major determinant of female-typical mating behavior. Generation and characterization of scg2a-deficient medaka showed no overt changes in secretory granule packaging in FeSP neurons. This, along with the observation that Scg2a and neuropeptide B, a major neuropeptide produced by FeSP neurons, colocalize in a majority of secretory granules, suggests that Scg2a mainly serves as a precursor for secreted peptides that act in conjunction with neuropeptide B. Further, scg2a showed sexually biased expression in several brain nuclei implicated in mating behavior. However, we found no significant impact of scg2a deficiency on the performance of mating behavior in either sex. Collectively, our results indicate that, although perhaps not essential for mating behavior, scg2a acts in an estrogen/Esr2b signaling-dependent manner in neurons that are relevant to female sexual receptivity.

Cell-specific secretory granule sorting mechanisms: the role of MAGEL2 and retromer in hypothalamic regulated secretion

Intracellular protein trafficking and sorting are extremely arduous in endocrine and neuroendocrine cells, which synthesize and secrete on-demand substantial quantities of proteins. To ensure that neuroendocrine secretion operates correctly, each step in the secretion pathways is tightly regulated and coordinated both spatially and temporally. At the trans-Golgi network (TGN), intrinsic structural features of proteins and several sorting mechanisms and distinct signals direct newly synthesized proteins into proper membrane vesicles that enter either constitutive or regulated secretion pathways. Furthermore, this anterograde transport is counterbalanced by retrograde transport, which not only maintains membrane homeostasis but also recycles various proteins that function in the sorting of secretory cargo, formation of transport intermediates, or retrieval of resident proteins of secretory organelles. The retromer complex recycles proteins from the endocytic pathway back to the plasma membrane or TGN and was recently identified as a critical player in regulated secretion in the hypothalamus. Furthermore, melanoma antigen protein L2 (MAGEL2) was discovered to act as a tissue-specific regulator of the retromer-dependent endosomal protein recycling pathway and, by doing so, ensures proper secretory granule formation and maturation. MAGEL2 is a mammalian-specific and maternally imprinted gene implicated in Prader-Willi and Schaaf-Yang neurodevelopmental syndromes. In this review, we will briefly discuss the current understanding of the regulated secretion pathway, encompassing anterograde and retrograde traffic. Although our understanding of the retrograde trafficking and sorting in regulated secretion is not yet complete, we will review recent insights into the molecular role of MAGEL2 in hypothalamic neuroendocrine secretion and how its dysregulation contributes to the symptoms of Prader-Willi and Schaaf-Yang patients. Given that the activation of many secreted proteins occurs after they enter secretory granules, modulation of the sorting efficiency in a tissue-specific manner may represent an evolutionary adaptation to environmental cues.

Chromogranin B (CHGB) is dimorphic and responsible for dominant anion channels delivered to cell surface via regulated secretion

Regulated secretion is conserved in all eukaryotes. In vertebrates granin family proteins function in all key steps of regulated secretion. Phase separation and amyloid-based storage of proteins and small molecules in secretory granules require ion homeostasis to maintain their steady states, and thus need ion conductances in granule membranes. But granular ion channels are still elusive. Here we show that granule exocytosis in neuroendocrine cells delivers to cell surface dominant anion channels, to which chromogranin B (CHGB) is critical. Biochemical fractionation shows that native CHGB distributes nearly equally in soluble and membrane-bound forms, and both reconstitute highly selective anion channels in membrane. Confocal imaging resolves granular membrane components including proton pumps and CHGB in puncta on the cell surface after stimulated exocytosis. High pressure freezing immuno-EM reveals a major fraction of CHGB at granule membranes in rat pancreatic β-cells. A cryo-EM structure of bCHGB dimer of a nominal 3.5 Å resolution delineates a central pore with end openings, physically sufficient for membrane-spanning and large single channel conductance. Together our data support that CHGB-containing (CHGB+) channels are characteristic of regulated secretion, and function in granule ion homeostasis near the plasma membrane or possibly in other intracellular processes.

Tandem mass tag-based quantitative proteomic analysis of effects of multiple sevoflurane exposures on the cerebral cortex of neonatal and adult mice

Introduction

Sevoflurane is the most commonly used general anesthetic in pediatric surgery, but it has the potential to be neurotoxic. Previous research found that long-term or multiple sevoflurane exposures could cause cognitive deficits in newborn mice but not adult mice, whereas short-term or single inhalations had little effect on cognitive function at both ages. The mechanisms behind these effects, however, are unclear.

Methods

In the current study, 6- and 60-day-old C57bl mice in the sevoflurane groups were given 3% sevoflurane plus 60% oxygen for three consecutive days, each lasting 2 hours, while those in the control group only got 60% oxygen. The cortex tissues were harvested on the 8th or 62nd day. The tandem mass tags (TMT)pro-based quantitative proteomics combined with liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis, Golgi staining, and western blotting analysis were applied to analyze the influences of multiple sevoflurane anesthesia on the cerebral cortex in mice with various ages. The Morris water maze (MWM) test was performed from postnatal day (P)30 to P36 or P84 to P90 after control or multiple sevoflurane treatment. Sevoflurane anesthesia affected spatial learning and memory and diminished dendritic spines primarily in newborn mice, whereas mature animals exhibited no significant alterations.

Results

A total of 6247 proteins were measured using the combined quantitative proteomics methods of TMTpro-labeled and LC-MS/MS, 443 of which were associated to the age-dependent neurotoxic mechanism of repeated sevoflurane anesthesia. Furthermore, western blotting research revealed that sevoflurane-induced brain damage in newborn mice may be mediated by increasing the levels of protein expression of CHGB, PTEN, MAP2c, or decreasing the level of SOD2 protein expression.

Conclusion

Our findings would help to further the mechanistic study of age-dependent anesthetic neurotoxicity and contribute to seek for effective protection in the developing brain under general anesthesia.

Pushing the frontiers: tools for monitoring neurotransmitters and neuromodulators

Neurotransmitters and neuromodulators have a wide range of key roles throughout the nervous system. However, their dynamics in both health and disease have been challenging to assess, owing to the lack of in vivo tools to track them with high spatiotemporal resolution. Thus, developing a platform that enables minimally invasive, large-scale and long-term monitoring of neurotransmitters and neuromodulators with high sensitivity, high molecular specificity and high spatiotemporal resolution has been essential. Here, we review the methods available for monitoring the dynamics of neurotransmitters and neuromodulators. Following a brief summary of non-genetically encoded methods, we focus on recent developments in genetically encoded fluorescent indicators, highlighting how these novel indicators have facilitated advances in our understanding of the functional roles of neurotransmitters and neuromodulators in the nervous system. These studies present a promising outlook for the future development and use of tools to monitor neurotransmitters and neuromodulators.

Deletion of β-Neurexins in Mice Alters the Distribution of Dense-Core Vesicles in Presynapses of Hippocampal and Cerebellar Neurons

Communication between neurons through synapses includes the release of neurotransmitter-containing synaptic vesicles (SVs) and of neuromodulator-containing dense-core vesicles (DCVs). Neurexins (Nrxns), a polymorphic family of cell surface molecules encoded by three genes in vertebrates (Nrxn1–3), have been proposed as essential presynaptic organizers and as candidates for cell type-specific or even synapse-specific regulation of synaptic vesicle exocytosis. However, it remains unknown whether Nrxns also regulate DCVs. Here, we report that at least β-neurexins (β-Nrxns), an extracellularly smaller Nrxn variant, are involved in the distribution of presynaptic DCVs. We found that conditional deletion of all three β-Nrxn isoforms in mice by lentivirus-mediated Cre recombinase expression in primary hippocampal neurons reduces the number of ultrastructurally identified DCVs in presynaptic boutons. Consistently, colabeling against marker proteins revealed a diminished population of chromogranin A- (ChrgA-) positive DCVs in synapses and axons of β-Nrxn-deficient neurons. Moreover, we validated the impaired DCV distribution in cerebellar brain tissue from constitutive β-Nrxn knockout (β-TKO) mice, where DCVs are normally abundant and β-Nrxn isoforms are prominently expressed. Finally, we observed that the ultrastructure and marker proteins of the Golgi apparatus, responsible for packaging neuropeptides into DCVs, seem unchanged. In conclusion, based on the validation from the two deletion strategies in conditional and constitutive KO mice, two neuronal populations from the hippocampus and cerebellum, and two experimental protocols in cultured neurons and in the brain tissue, this study presented morphological evidence that the number of DCVs at synapses is altered in the absence of β-Nrxns. Our results therefore point to an unexpected contribution of β-Nrxns to the organization of neuropeptide and neuromodulator function, in addition to their more established role in synaptic vesicle release.

In Vivo Imaging of Axonal Organelle Transport in the Mouse Brain

Visualization and analysis of axonal organelle transport has been mostly conducted in vitro, using primary neuronal cell cultures, although more recently, intravital organelle imaging has been established in model organisms such as drosophila, zebrafish, and mouse. In this chapter, we describe a method to visualize axonal transport of cellular organelles such as dense core vesicles or mitochondria in the living mouse brain in order to study organelle transport in its native environment. We achieve this goal by injecting adeno-associated viruses expressing fluorescently tagged marker proteins into thalamic nuclei of mice, thereby transducing neurons that project to the surface of the brain. Axonal projections and trafficking of organelles can be imaged with a 2-photon microscope through a chronically implanted window in the mouse skull in anesthetized as well as awake mice.

An increase in VGF expression through a rapid, transcription-independent, autofeedback mechanism improves cognitive function

The release of neuropeptides from dense core vesicles (DCVs) modulates neuronal activity and plays a critical role in cognitive function and emotion. The granin family is considered a master regulator of DCV biogenesis and the release of DCV cargo molecules. The expression of the VGF protein (nonacronymic), a secreted neuropeptide precursor that also belongs to the extended granin family, has been previously shown to be induced in the brain by hippocampus-dependent learning, and its downregulation is mechanistically linked to neurodegenerative diseases such as Alzheimer's disease and other mood disorders. Currently, whether changes in translational efficiency of Vgf and other granin mRNAs may be associated and regulated with learning associated neural activity remains largely unknown. Here, we show that either contextual fear memory training or the administration of TLQP-62, a peptide derived from the C-terminal region of the VGF precursor, acutely increases the translation of VGF and other granin proteins, such as CgB and Scg2, via an mTOR-dependent signaling pathway in the absence of measurable increases in mRNA expression. Luciferase-based reporter assays confirmed that the 3'-untranslated region (3'UTR) of the Vgf mRNA represses VGF translation. Consistently, the truncation of the endogenous Vgf mRNA 3'UTR results in substantial increases in VGF protein expression both in cultured primary neurons and in brain tissues from knock in mice expressing a 3'UTR-truncation mutant encoded by the modified Vgf gene. Importantly, Vgf 3'UTR-truncated mice exhibit enhanced memory performance and reduced anxiety- and depression-like behaviors. Our results therefore reveal a rapid, transcription-independent induction of VGF and other granin proteins after learning that are triggered by the VGF-derived peptide TLQP-62. Our findings suggest that the rapid, positive feedforward increase in the synthesis of granin family proteins might be a general mechanism to replenish DCV cargo molecules that have been released in response to neuronal activation and is crucial for memory function and mood stability.

Measurement of Exocytosis in Genetically Manipulated Mast Cells

The hallmark of mast cell activation is secretion of immune mediators by regulated exocytosis. Measurements of mediator secretion from mast cells that are genetically manipulated by transient transfections provide a powerful tool for deciphering the underlying mechanisms of mast cell exocytosis. However, common methods to study regulated exocytosis in bulk culture of mast cells suffer from the drawback of high signal-to-noise ratio because of their failure to distinguish between the different mast cell populations, that is, genetically modified mast cells versus their non-transfected counterparts. In particular, the low transfection efficiency of mast cells poses a significant limitation on the use of conventional methodologies. To overcome this hurdle, we developed a method, which discriminates and allows detection of regulated exocytosis of transfected cells based on the secretion of a fluorescent secretory reporter. We used a plasmid encoding for Neuropeptide Y (NPY) fused to a monomeric red fluorescent protein (NPY-mRFP), yielding a fluorescent secretory granule-targeted reporter that is co-transfected with a plasmid encoding a gene of interest. Upon cell trigger, NPY-mRFP is released from the cells by regulated exocytosis, alongside the endogenous mediators. Therefore, using NPY-mRFP as a reporter for mast cell exocytosis allows either quantitative, via a fluorimeter assay, or qualitative analysis, via confocal microscopy, of the genetically manipulated mast cells. Moreover, this method may be easily modified to accommodate studies of regulated exocytosis in any other type of cell.

Secretogranin III upregulation is involved in parkinsonian toxin-mediated astroglia activation

Environmental neurotoxins such as paraquat (PQ), manganese, and 1-1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) are associated with a higher risk of Parkinson's disease (PD). These parkinsonian toxins exert certain common toxicological effects on astroglia; however, their role in the regulatory functions of astroglial secretory proteins remains unclear. In a previous study, we observed that secretogranin II (SCG2) and secretogranin III (SCG3), which are important components of the regulated secretory pathway, were elevated in PQ-activated U118 astroglia. In the current study, we used the parkinsonian toxins dopamine (DA), active metabolite of MPTP (MPP⁺), MnCl₂, and lipopolysaccharide (LPS) as inducers, and studied the potential regulation of SCG2 and SCG3. Our results showed that all the parkinsonian toxins except LPS affected astroglial viability but did not cause apoptosis. Exposure to DA, MPP⁺, and MnCl₂ upregulated glial fibrillary acidic protein (GFAP), a marker for astrocyte activation, and stimulated the levels of several astrocytic-derived factors. Further, DA, MPP⁺, and MnCl₂ exposure impeded astroglial cell cycle progression. Moreover, the expression of SCG3 was elevated, while its exosecretion was inhibited in astroglia activated by parkinsonian toxins. The level of SCG2 remained unchanged. In combination with our previous findings, the results of this study indicate that SCG3 may act as a cofactor in astrocyte activation stimulated by various toxins, and the regulation of SCG3 could be involved in the toxicological mechanism by which parkinsonian toxins affect astroglia.

Associations of serum chromogranin A with depressive symptoms in men with unipolar depressive disorder

OBJECTIVE

The aim of the present study was to determine the changes of serum chromogranin A (CgA) levels upon depression by investigating the relationship between serum CgA levels and the depressive symptoms assessed by 24-item Hamilton Rating Scale for Depression (HRSD-24).

METHOD

Serum CgA levels were measured by enzyme-linked immunosorbent assay in 133 male patients with major depressive disorder (MDD) and were compared with those of 47 healthy controls. Then generalized linear regression, logistic regression and restricted cubic spline models were performed to examine the association between serum CgA levels and depressive symptoms.

RESULTS

Serum CgA levels were lower in MDD patients than in controls (P < 0.001) and were inversely associated with scores on HRSD-24 in unadjusted, age, smoking, alcohol consumption, traumatic life events and family history of depression-adjusted and fully adjusted linear regression model. The fully adjusted regression coefficient with 95% confidence intervals was -0.028 (-0.045, -0.010) for serum CgA levels and HRSD-24 score. Serum CgA levels were inversely associated with depressive symptoms (HRSD ≥20) in each logistic regression model.

CONCLUSION

Serum CgA decrease was noted in male patients of MDD and may be inversely associated with depressive symptoms.

Chromogranin A in the early steps of the neurosecretory pathway

Chromogranin A (CgA) is a soluble glycoprotein stored with hormones and neuropeptides in secretory granules (SG) of most (neuro)endocrine cells and neurons. Since its discovery in 1967, many studies have reported its structural characteristics, biological roles, and mechanisms of action. Indeed, CgA is both a precursor of various biologically active peptides and a granulogenic protein regulating the storage and secretion of hormones and neuropeptides. This review emphasizes the findings and theoretical concepts around the CgA-linked molecular machinery controlling hormone/neuropeptide aggregation and the interaction of CgA-hormone/neuropeptide aggregates with the trans-Golgi membrane to allow hormone/neuropeptide targeting and SG biogenesis. We will also discuss the intriguing alteration of CgA expression and secretion in various neurological disorders, which could provide insights to elucidate the molecular mechanisms underlying these pathophysiological conditions.

The RAB3-RIM Pathway Is Essential for the Release of Neuromodulators

Secretion principles are conserved from yeast to humans, and many yeast orthologs have established roles in synaptic vesicle exocytosis in the mammalian brain. Surprisingly, SEC4 orthologs and their effectors, the exocyst, are dispensable for synaptic vesicle exocytosis. Here, we identify the SEC4 ortholog RAB3 and its neuronal effector, RIM1, as essential molecules for neuropeptide and neurotrophin release from dense-core vesicles (DCVs) in mammalian neurons. Inactivation of all four RAB3 genes nearly ablated DCV exocytosis, and re-expression of RAB3A restored this deficit. In RIM1/2-deficient neurons, DCV exocytosis was undetectable. Full-length RIM1, but not mutants that lack RAB3 or MUNC13 binding, restored release. Strikingly, a short N-terminal RIM1 fragment only harboring RAB3- and MUNC13-interacting domains was sufficient to support DCV exocytosis. We propose that RIM and MUNC13 emerged as mammalian alternatives to the yeast exocyst complex as essential RAB3/SEC4 effectors and organizers of DCV fusion sites by recruiting DCVs via RAB3.

Chromogranin B regulates early-stage insulin granule trafficking from the Golgi in pancreatic islet β-cells

Chromogranin B (CgB, also known as CHGB) is abundantly expressed in dense core secretory granules of multiple endocrine tissues and has been suggested to regulate granule biogenesis in some cell types, including the pancreatic islet β-cell, though the mechanisms are poorly understood. Here, we demonstrate a critical role for CgB in regulating secretory granule trafficking in the β-cell. Loss of CgB impairs glucose-stimulated insulin secretion, impedes proinsulin processing to yield increased proinsulin content, and alters the density of insulin-containing granules. Using an in situ fluorescent pulse-chase strategy to track nascent proinsulin, we show that loss of CgB impairs Golgi budding of proinsulin-containing secretory granules, resulting in a substantial delay in trafficking of nascent granules to the plasma membrane with an overall decrease in total plasma membrane-associated granules. These studies demonstrate that CgB is necessary for efficient trafficking of secretory proteins into the budding granule, which impacts the availability of insulin-containing secretory granules for exocytic release.This article has an associated First Person interview with the first author of the paper.

Costorage of High Molecular Weight Neurotransmitters in Large Dense Core Vesicles of Mammalian Neurons

It is today widely accepted that several types of high molecular weight (MW) neurotransmitters produced by neurons are synthesized at the cell body, selectively stored within large dense core vesicles (LDCVs) and anterogradely transported to terminals where they elicit their biological role(s). Among these molecules there are neuropeptides and neurotrophic factors, the main focus of this perspective article. I here first provide a brief resume of the state of art on neuronal secretion, with primary emphasis on the molecular composition and mechanism(s) of filling and release of LDCVs. Then, I discuss the perspectives and future directions of research in the field as regarding the synthesis and storage of multiple high MW transmitters in LDCVs and the possibility that a selective sorting of LDCVs occurs along different neuronal processes and/or their branches. I also consider the ongoing discussion that diverse types of neurons may contain LDCVs with different sets of integral proteins or dial in a different fashion with LDCVs containing the same cargo. In addition, I provide original data on the size of LDCVs in rat dorsal root ganglion neurons and their central terminals in the spinal cord after immunogold labeling for calcitonin gene-related peptide (CGRP), neuropeptide K, substance P, neurokinin A or somatostatin. These data corroborate the idea that, similarly to endocrine cells, LDCVs undergo a process of maturation which involves a homotypic fusion followed by a reduction in size and condensation of cargo. They also give support to the conjecture that release at terminals occurs by cavicapture, a process of partial fusion of the vesicle with the axolemma, accompanied by depletion of cargo and diminution of size.

Imaging FITC-dextran as a Reporter for Regulated Exocytosis

Regulated exocytosis is a process by which cargo, which is stored in secretory granules (SGs), is released in response to a secretory trigger. Regulated exocytosis is fundamental for intercellular communication and is a key mechanism for the secretion of neurotransmitters, hormones, inflammatory mediators, and other compounds, by a variety of cells. At least three distinct mechanisms are known for regulated exocytosis: full exocytosis, where a single SG fully fuses with the plasma membrane, kiss-and-run exocytosis, where a single SG transiently fuses with the plasma membrane, and compound exocytosis, where several SGs fuse with each other, prior to or after SG fusion with the plasma membrane. The type of regulated exocytosis undertaken by a cell is often dictated by the type of secretory trigger. However, in many cells, a single secretory trigger can activate multiple modes of regulated exocytosis simultaneously. Despite their abundance and importance across cell types and species, the mechanisms that determine the different modes of secretion are largely unresolved. One of the main challenges in investigating the different modes of regulated exocytosis, is the difficulty in distinguishing between them as well as exploring them separately. Here we describe the use of fluorescein isothiocyanate (FITC)-dextran as an exocytosis reporter, and live cell imaging, to differentiate between the different pathways of regulated exocytosis, focusing on compound exocytosis, based on the robustness and duration of the exocytic events.