Matrix-dependent local retention of secretory vesicle cargo in cortical neurons

High-throughput assay for regulated secretion of neuropeptides in mouse and human neurons

Neuropeptides are the largest group of chemical signals in the brain. More than 100 different neuropeptides modulate various brain functions and their dysregulation has been associated with neurological disorders. Neuropeptides are packed into dense core vesicles (DCVs), which fuse with the plasma membrane in a calcium-dependent manner. Here, we describe a novel high-throughput assay for DCV exocytosis using a chimera of Nanoluc luciferase and the DCV-cargo neuropeptide Y (NPY). The NPY-Nanoluc reporter colocalized with endogenous DCV markers in all neurons with little mislocalization to other cellular compartments. NPY-Nanoluc reported DCV exocytosis in both rodent and induced pluripotent stem cell-derived human neurons, with similar depolarization, Ca2+, RAB3, and STXBP1/MUNC18 dependence as low-throughput assays. Moreover, NPY-Nanoluc accurately reported modulation of DCV exocytosis by known modulators diacylglycerol analog and Ca2+ channel blocker and showed a higher assay sensitivity than a widely used single-cell low-throughput assay. Lastly, we showed that Nanoluc coupled to other secretory markers reports on constitutive secretion. In conclusion, the NPY-Nanoluc is a sensitive reporter of DCV exocytosis in mammalian neurons, suitable for pharmacological and genomic screening for DCV exocytosis genes and for mechanism-based treatments for central nervous system disorders.

Maximal Fusion Capacity and Efficient Replenishment of the Dense Core Vesicle Pool in Hippocampal Neurons

Neuropeptides and neurotrophins, stored in dense core vesicles (DCVs), are together the largest currently known group of chemical signals in the brain. Exocytosis of DCVs requires high-frequency or patterned stimulation, but the determinants to reach maximal fusion capacity and for efficient replenishment of released DCVs are unknown. Here, we systematically studied fusion of DCV with single vesicle resolution on different stimulation patterns in mammalian CNS neurons. We show that tetanic stimulation trains of 50-Hz action potential (AP) bursts maximized DCV fusion, with significantly fewer fusion event during later bursts of the train. This difference was omitted by introduction of interburst intervals but did not increase total DCV fusion. Interburst intervals as short as 5 s were sufficient to restore the fusion capacity. Theta burst stimulation (TBS) triggered less DCV fusion than tetanic stimulation, but a similar fusion efficiency per AP. Prepulse stimulation did not alter this. However, low-frequency stimulation (4 Hz) intermitted with fast ripple stimulation (200 APs at 200 Hz) produced substantial DCV fusion, albeit not as much as tetanic stimulation. Finally, individual fusion events had longer durations with more intense stimulation. These data indicate that trains of 50-Hz AP stimulation patterns triggered DCV exocytosis most efficiently and more intense stimulation promotes longer DCV fusion pore openings.SIGNIFICANCE STATEMENT Neuropeptides and neurotrophins modulate multiple regulatory functions of human body like reproduction, food intake or mood. They are packed into dense core vesicles (DCVs) that undergo calcium and action potential (AP) fusion with the plasma membrane. In order to study the fusion of DCVs in vitro, techniques like perfusion with buffer containing high concentration of potassium or electric field stimulation are needed to trigger the exocytosis of DCVs. Here, we studied the relationship between DCVs fusion properties and different electric field stimulation patterns. We used six different stimulation patterns and showed that trains of 50-Hz action potential bursts triggered DCV exocytosis most efficiently and more intense stimulation promotes longer DCV fusion pore openings.

Modulations of the neuronal trafficking of tissue-type plasminogen activator (tPA) influences glutamate release

The discovery of the neuronal expression of the serine protease tissue-type plasminogen activator (tPA) has opened new avenues of research, with important implications in the physiopathology of the central nervous system. For example, the interaction of tPA with synaptic receptors (NMDAR, LRP1, Annexin II, and EGFR) and its role in the maturation of BDNF have been reported to influence synaptic plasticity and neuronal survival. However, the mechanisms regulating the neuronal trafficking of tPA are unknown. Here, using high-resolution live cell imaging and a panel of innovative genetic approaches, we first unmasked the dynamic characteristics of the dendritic and axonal trafficking of tPA-containing vesicles under different paradigms of neuronal activation or inhibition. We then report a constitutive exocytosis of tPA- and VAMP2-positive vesicles, dramatically increased in conditions of neuronal activation, with a pattern which was mainly dendritic and thus post-synaptic. We also observed that the synaptic release of tPA led to an increase of the exocytosis of VGlut1 positive vesicles containing glutamate. Finally, we described alterations of the trafficking and exocytosis of neuronal tPA in cultured cortical neurons prepared from tau-22 transgenic mice (a preclinical model of Alzheimer's disease (AD)). Altogether, these data provide new insights about the neuronal trafficking of tPA, contributing to a better knowledge of the tPA-dependent brain functions and dysfunctions.

Vti1a/b support distinct aspects of TGN and cis-/medial Golgi organization

Retrograde trafficking towards the trans-Golgi network (TGN) is important for dense core vesicle (DCV) biogenesis. Here, we used Vti1a/b deficient neurons to study the impact of disturbed retrograde trafficking on Golgi organization and cargo sorting. In Vti1a/b deficient neurons, staining intensity of cis-/medial Golgi proteins (e.g., GM130 and giantin) was increased, while the intensity of two recycling TGN proteins, TGN38 and TMEM87A, was decreased and the TGN-resident protein Golgin97 was normal. Levels and localization of DCV cargo markers, LAMP1 and KDEL were also altered. This phenotype was not caused by reduced Golgi size or absence of a TGN compartment. The phenotype was partially phenocopied by disturbing sphingolipid homeostasis, but was not rescued by overexpression of sphingomyelin synthases or the sphingolipid synthesis inhibitor myriocin. We conclude that Vti1a/b are important for distinct aspects of TGN and cis-/medial Golgi organization. Our data underline the importance of retrograde trafficking for Golgi organization, DCV cargo sorting and the distribution of proteins of the regulated secretory pathway.

Functional Implications of Neurotransmitter Segregation

Here, we present and discuss the characteristics and properties of neurotransmitter segregation, a subtype of neurotransmitter cotransmission. We review early evidence of segregation and discuss its properties, such as plasticity, while placing special emphasis on its probable functional implications, either in the central nervous system (CNS) or the autonomic nervous system. Neurotransmitter segregation is a process by which neurons separately route transmitters to independent and distant or to neighboring neuronal processes; it is a plastic phenomenon that changes according to synaptic transmission requirements and is regulated by target-derived signals. Distant neurotransmitter segregation in the CNS has been shown to be related to an autocrine/paracrine function of some neurotransmitters. In retinal amacrine cells, segregation of acetylcholine (ACh) and GABA, and glycine and glutamate to neighboring terminals has been related to the regulation of the firing rate of direction-selective ganglion cells. In the rat superior cervical ganglion, segregation of ACh and GABA to neighboring varicosities shows a heterogeneous regional distribution, which is correlated to a similar regional distribution in transmission strength. We propose that greater segregation of ACh and GABA produces less GABAergic inhibition, strengthening ganglionic transmission. Segregation of ACh and GABA varies in different physiopathological conditions; specifically, segregation increases in acute sympathetic hyperactivity that occurs in cold stress, does not vary in chronic hyperactivity that occurs in hypertension, and rises in early ages of normotensive and hypertensive rats. Given this, we propose that variations in the extent of transmitter segregation may contribute to the alteration of neural activity that occurs in some physiopathological conditions and with age.

Munc18-1 Is Essential for Neuropeptide Secretion in Neurons

Neuropeptide secretion from dense-core vesicles (DCVs) controls many brain functions. Several components of the DCV exocytosis machinery have recently been identified, but the participation of a SEC1/MUNC18 (SM) protein has remained elusive. Here, we tested the ability of the three exocytic SM proteins expressed in the mammalian brain, MUNC18-1/2/3, to support neuropeptide secretion. We quantified DCV exocytosis at a single vesicle resolution on action potential (AP) train-stimulation in mouse CNS neurons (of unknown sex) using pHluorin-tagged and/or mCherry-tagged neuropeptide Y (NPY) or brain-derived neurotrophic factor (BDNF). Conditional inactivation of Munc18-1 abolished all DCV exocytosis. Expression of MUNC18-1, but not MUNC18-2 or MUNC18-3, supported DCV exocytosis in Munc18-1 null neurons. Heterozygous (HZ) inactivation of Munc18-1, as a model for reduced MUNC18-1 expression, impaired DCV exocytosis, especially during the initial phase of train-stimulation, when the release was maximal. These data show that neurons critically and selectively depend on MUNC18-1 for neuropeptide secretion. Impaired neuropeptide secretion may explain aspects of the behavioral and neurodevelopmental phenotypes that were observed in Munc18-1 HZ mice.SIGNIFICANCE STATEMENT Neuropeptide secretion from dense-core vesicles (DCVs) modulates synaptic transmission, sleep, appetite, cognition and mood. However, the mechanisms of DCV exocytosis are poorly characterized. Here, we identify MUNC18-1 as an essential component for neuropeptide secretion from DCVs. Paralogs MUNC18-2 or MUNC18-3 cannot compensate for MUNC18-1. MUNC18-1 is the first protein identified to be essential for both neuropeptide secretion and synaptic transmission. In heterozygous (HZ) Munc18-1 neurons, that have a 50% reduced MUNC18-1expression and model the human STXBP1 syndrome, DCV exocytosis is impaired, especially during the initial phase of train-stimulation, when the release is maximal. These data show that MUNC18-1 is essential for neuropeptide secretion and that impaired neuropeptide secretion on reduced MUNC18-1expression may contribute to the symptoms of STXBP1 syndrome.

Neuromodulator release in neurons requires two functionally redundant calcium sensors

Neuropeptides and neurotrophic factors secreted from dense core vesicles (DCVs) control many brain functions, but the calcium sensors that trigger their secretion remain unknown. Here, we show that in mouse hippocampal neurons, DCV fusion is strongly and equally reduced in synaptotagmin-1 (Syt1)- or Syt7-deficient neurons, but combined Syt1/Syt7 deficiency did not reduce fusion further. Cross-rescue, expression of Syt1 in Syt7-deficient neurons, or vice versa, completely restored fusion. Hence, both sensors are rate limiting, operating in a single pathway. Overexpression of either sensor in wild-type neurons confirmed this and increased fusion. Syt1 traveled with DCVs and was present on fusing DCVs, but Syt7 supported fusion largely from other locations. Finally, the duration of single DCV fusion events was reduced in Syt1-deficient but not Syt7-deficient neurons. In conclusion, two functionally redundant calcium sensors drive neuromodulator secretion in an expression-dependent manner. In addition, Syt1 has a unique role in regulating fusion pore duration.

Quantitative analysis of dense-core vesicle fusion in rodent CNS neurons

Neuropeptides are essential signaling molecules secreted by dense-core vesicles (DCVs). They contribute to information processing in the brain, controlling a variety of physiological conditions. Defective neuropeptide signaling is implicated in several psychiatric disorders. Here, we provide a protocol for the quantitative analysis of DCV fusion events in rodent neurons using pH-sensitive DCV fusion probes and custom-written analysis algorithms. This method can be used to study DCV fusion mechanisms and is easily adapted to investigate fusion principles of other secretory organelles. For complete details on the use and execution of this protocol, please refer to Persoon et al. (2019).

The physiology of regulated BDNF release

The neurotrophic factor BDNF is an important regulator for the development of brain circuits, for synaptic and neuronal network plasticity, as well as for neuroregeneration and neuroprotection. Up- and downregulations of BDNF levels in human blood and tissue are associated with, e.g., neurodegenerative, neurological, or even cardiovascular diseases. The changes in BDNF concentration are caused by altered dynamics in BDNF expression and release. To understand the relevance of major variations of BDNF levels, detailed knowledge regarding physiological and pathophysiological stimuli affecting intra- and extracellular BDNF concentration is important. Most work addressing the molecular and cellular regulation of BDNF expression and release have been performed in neuronal preparations. Therefore, this review will summarize the stimuli inducing release of BDNF, as well as molecular mechanisms regulating the efficacy of BDNF release, with a focus on cells originating from the brain. Further, we will discuss the current knowledge about the distinct stimuli eliciting regulated release of BDNF under physiological conditions.

Tetanus insensitive VAMP2 differentially restores synaptic and dense core vesicle fusion in tetanus neurotoxin treated neurons

The SNARE proteins involved in the secretion of neuromodulators from dense core vesicles (DCVs) in mammalian neurons are still poorly characterized. Here we use tetanus neurotoxin (TeNT) light chain, which cleaves VAMP1, 2 and 3, to study DCV fusion in hippocampal neurons and compare the effects on DCV fusion to those on synaptic vesicle (SV) fusion. Both DCV and SV fusion were abolished upon TeNT expression. Expression of tetanus insensitive (TI)-VAMP2 restored SV fusion in the presence of TeNT, but not DCV fusion. Expression of TI-VAMP1 or TI-VAMP3 also failed to restore DCV fusion. Co-transport assays revealed that both TI-VAMP1 and TI-VAMP2 are targeted to DCVs and travel together with DCVs in neurons. Furthermore, expression of the TeNT-cleaved VAMP2 fragment or a protease defective TeNT in wild type neurons did not affect DCV fusion and therefore cannot explain the lack of rescue of DCV fusion by TI-VAMP2. Finally, to test if two different VAMPs might both be required in the DCV secretory pathway, Vamp1 null mutants were tested. However, VAMP1 deficiency did not reduce DCV fusion. In conclusion, TeNT treatment combined with TI-VAMP2 expression differentially affects the two main regulated secretory pathways: while SV fusion is normal, DCV fusion is absent.

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.

General Principles of Neuronal Co-transmission: Insights From Multiple Model Systems

It is now accepted that neurons contain and release multiple transmitter substances. However, we still have only limited insight into the regulation and functional effects of this co-transmission. Given that there are 200 or more neurotransmitters, the chemical complexity of the nervous system is daunting. This is made more-so by the fact that their interacting effects can generate diverse non-linear and novel consequences. The relatively poor history of pharmacological approaches likely reflects the fact that manipulating a transmitter system will not necessarily mimic its roles within the normal chemical environment of the nervous system (e.g., when it acts in parallel with co-transmitters). In this article, co-transmission is discussed in a range of systems [from invertebrate and lower vertebrate models, up to the mammalian peripheral and central nervous system (CNS)] to highlight approaches used, degree of understanding, and open questions and future directions. Finally, we offer some outlines of what we consider to be the general principles of co-transmission, as well as what we think are the most pressing general aspects that need to be addressed to move forward in our understanding of co-transmission.

Dynamin 1 Restrains Vesicular Release to a Subquantal Mode In Mammalian Adrenal Chromaffin Cells

Dynamin 1 (dyn1) is required for clathrin-mediated endocytosis in most secretory (neuronal and neuroendocrine) cells. There are two modes of Ca²⁺-dependent catecholamine release from single dense-core vesicles: full-quantal (quantal) and subquantal in adrenal chromaffin cells, but their relative occurrences and impacts on total secretion remain unclear. To address this fundamental question in neurotransmission area using both sexes of animals, here we report the following: (1) dyn1-KO increased quantal size (QS, but not vesicle size/content) by ≥250% in dyn1-KO mice; (2) the KO-increased QS was rescued by dyn1 (but not its deficient mutant or dyn2); (3) the ratio of quantal versus subquantal events was increased by KO; (4) following a release event, more protein contents were retained in WT versus KO vesicles; and (5) the fusion pore size (d _p) was increased from ≤9 to ≥9 nm by KO. Therefore, Ca²⁺-induced exocytosis is generally a subquantal release in sympathetic adrenal chromaffin cells, implying that neurotransmitter release is generally regulated by dynamin in neuronal cells.SIGNIFICANCE STATEMENT Ca²⁺-dependent neurotransmitter release from a single vesicle is the primary event in all neurotransmission, including synaptic/neuroendocrine forms. To determine whether Ca²⁺-dependent vesicular neurotransmitter release is "all-or-none" (quantal), we provide compelling evidence that most Ca²⁺-induced secretory events occur via the subquantal mode in native adrenal chromaffin cells. This subquantal release mode is promoted by dynamin 1, which is universally required for most secretory cells, including neurons and neuroendocrine cells. The present work with dyn1-KO mice further confirms that Ca²⁺-dependent transmitter release is mainly via subquantal mode, suggesting that subquantal release could be also important in other types of cells.

Pool size estimations for dense-core vesicles in mammalian CNS neurons

Neuropeptides are essential signaling molecules transported and secreted by dense-core vesicles (DCVs), but the number of DCVs available for secretion, their subcellular distribution, and release probability are unknown. Here, we quantified DCV pool sizes in three types of mammalian CNS neurons in vitro and in vivo Super-resolution and electron microscopy reveal a total pool of 1,400-18,000 DCVs, correlating with neurite length. Excitatory hippocampal and inhibitory striatal neurons in vitro have a similar DCV density, and thalamo-cortical axons in vivo have a slightly higher density. Synapses contain on average two to three DCVs, at the periphery of synaptic vesicle clusters. DCVs distribute equally in axons and dendrites, but the vast majority (80%) of DCV fusion events occur at axons. The release probability of DCVs is 1-6%, depending on the stimulation. Thus, mammalian CNS neurons contain a large pool of DCVs of which only a small fraction can fuse, preferentially at axons.

The fusion pore, 60 years after the first cartoon

Neurotransmitter release occurs in the form of quantal events by fusion of secretory vesicles with the plasma membrane, and begins with the formation of a fusion pore that has a conductance similar to that of a large ion channel or gap junction. In this review, we propose mechanisms of fusion pore formation and discuss their implications for fusion pore structure and function. Accumulating evidence indicates a direct role of soluble N-ethylmaleimide-sensitive-factor attachment receptor proteins in the opening of fusion pores. Fusion pores are likely neither protein channels nor purely lipid, but are of proteolipidic composition. Future perspectives to gain better insight into the molecular structure of fusion pores are discussed.

Secretory vesicle trafficking in awake and anaesthetized mice: differential speeds in axons versus synapses

KEY POINTS

Despite their immense physiological and pathophysiological importance, we know very little about the biology of dense core vesicle (DCV) trafficking in the intact mammalian brain. DCVs are transported at similar average speeds in the anaesthetized and awake mouse brain compared to neurons in culture, yet maximal speed and pausing fraction of transport were higher. Microtubule plus (+)-end extension imaging visualized microtubular growth at 0.12 μm/s and revealed that DCVs were transported faster in the anterograde direction. DCV transport slowed down upon presynaptic bouton approach, possibly promoting synaptic localization and cargo release. Our work provides a basis to extrapolate DCV transport properties determined in cultured neurons to the intact mouse brain and reveals novel features such as slowing upon bouton approach and brain state-dependent trafficking directionality.

ABSTRACT

Neuronal dense core vesicles (DCVs) transport many cargo molecules like neuropeptides and neurotrophins to their release sites in dendrites or axons. The transport properties of DCVs in axons of the intact mammalian brain are unknown. We used viral expression of a DCV cargo reporter (NPY-Venus/Cherry) in the thalamus and two-photon in vivo imaging to visualize axonal DCV trafficking in thalamocortical projections of anaesthetized and awake mice. We found an average speed of 1 μm/s, maximal speeds of up to 5 μm/s and a pausing fraction of ∼11%. Directionality of transport differed between anaesthetized and awake mice. In vivo microtubule +-end extension imaging using MACF18-GFP revealed microtubular growth at 0.12 μm/s and provided positive identification of antero- and retrograde axonal transport. Consistent with previous reports, anterograde transport was faster (∼2.1 μm/s) than retrograde transport (∼1.4 μm/s). In summary, DCVs are transported with faster maximal speeds and lower pausing fraction in vivo compared to previous results obtained in vitro. Finally, we found that DCVs slowed down upon presynaptic bouton approach. We propose that this mechanism promotes synaptic localization and cargo release.

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

Chromogranin A and B (Cgs) are considered to be master regulators of cargo sorting for the regulated secretory pathway (RSP) and dense-core vesicle (DCV) biogenesis. To test this, we analyzed the release of neuropeptide Y (NPY)-pHluorin, a live RSP reporter, and the distribution, number, and appearance of DCVs, in mouse hippocampal neurons lacking expression of CHGA and CHGB genes. qRT-PCR analysis showed that expression of other granin family members was not significantly altered in CgA/B-/- neurons. As synaptic maturation of developing neurons depends on secretion of trophic factors in the RSP, we first analyzed neuronal development in standardized neuronal cultures. Surprisingly, dendritic and axonal length, arborization, synapse density, and synaptic vesicle accumulation in synapses were all normal in CgA/B-/- neurons. Moreover, the number of DCVs outside the soma, stained with endogenous marker Secretogranin II, the number of NPY-pHluorin puncta, and the total amount of reporter in secretory compartments, as indicated by pH-sensitive NPY-pHluorin fluorescence, were all normal in CgA/B-/- neurons. Electron microscopy revealed that synapses contained a normal number of DCVs, with a normal diameter, in CgA/B-/- neurons. In contrast, CgA/B-/- chromaffin cells contained fewer and smaller secretory vesicles with a smaller core size, as previously reported. Finally, live-cell imaging at single vesicle resolution revealed a normal number of fusion events upon bursts of action potentials in CgA/B-/- neurons. These events had normal kinetics and onset relative to the start of stimulation. Taken together, these data indicate that the two chromogranins are dispensable for cargo sorting in the RSP and DCV biogenesis in mouse hippocampal neurons.

The Action Radius of Oxytocin Release in the Mammalian CNS: From Single Vesicles to Behavior

The hypothalamic neuropeptide oxytocin (OT) has attracted the attention both of the scientific community and a general audience because of its prosocial effects in mammals, and OT is now seen as a facilitator of mammalian species propagation. Furthermore, OT is a candidate for the treatment of social deficits in several neuropsychiatric and neurodevelopmental conditions. Despite such possibilities and a long history of studies on OT behavioral effects, the mechanisms of OT actions in the brain remain poorly understood. In the present review, based on anatomical, biochemical, electrophysiological, and behavioral studies, we propose a novel model of local OT actions in the central nervous system (CNS) via focused axonal release, which initiates intracellular signaling cascades in specific OT-sensitive neuronal populations and coordinated brain region-specific behaviors.

CAPS-1 requires its C2, PH, MHD1 and DCV domains for dense core vesicle exocytosis in mammalian CNS neurons

CAPS (calcium-dependent activator protein for secretion) are multi-domain proteins involved in regulated exocytosis of synaptic vesicles (SVs) and dense core vesicles (DCVs). Here, we assessed the contribution of different CAPS-1 domains to its subcellular localization and DCV exocytosis by expressing CAPS-1 mutations in four functional domains in CAPS-1/-2 null mutant (CAPS DKO) mouse hippocampal neurons, which are severely impaired in DCV exocytosis. CAPS DKO neurons showed normal development and no defects in DCV biogenesis and their subcellular distribution. Truncation of the CAPS-1 C-terminus (CAPS Δ654-1355) impaired CAPS-1 synaptic enrichment. Mutations in the C2 (K428E or G476E) or pleckstrin homology (PH; R558D/K560E/K561E) domain did not. However, all mutants rescued DCV exocytosis in CAPS DKO neurons to only 20% of wild type CAPS-1 exocytosis capacity. To assess the relative importance of CAPS for both secretory pathways, we compared effect sizes of CAPS-1/-2 deficiency on SV and DCV exocytosis. Using the same (intense) stimulation, DCV exocytosis was impaired relatively strong (96% inhibition) compared to SV exocytosis (39%). Together, these data show that the CAPS-1 C-terminus regulates synaptic enrichment of CAPS-1. All CAPS-1 functional domains are required, and the C2 and PH domain together are not sufficient, for DCV exocytosis in mammalian CNS neurons.

Proteasome Activation by Small Molecules

Drugs that increase 26S proteasome activity have potential therapeutic applications in the treatment of neurodegenerative diseases. A chemical genetics screen of over 2,750 compounds using a proteasome activity probe as a readout in a high-throughput live-cell fluorescence-activated cell sorting-based assay revealed more than ten compounds that increase proteasome activity, with the p38 MAPK inhibitor PD169316 being one of the most potent ones. Genetic and chemical inhibition of either p38 MAPK, its upstream regulators, ASK1 and MKK6, and downstream target, MK2, enhance proteasome activity. Chemical activation of the 26S proteasome increases PROTAC-mediated and ubiquitin-dependent protein degradation and decreases the levels of both overexpressed and endogenous α-synuclein, without affecting the overall protein turnover. In addition, survival of cells overexpressing toxic α-synuclein assemblies is increased in the presence of p38 MAPK inhibitors. These findings highlight the potential of activation of 26S proteasome activity and that this can be achieved through multiple mechanisms by distinct molecules.

SNAP-25 gene family members differentially support secretory vesicle fusion

Neuronal dense-core vesicles (DCVs) transport and secrete neuropeptides necessary for development, plasticity and survival, but little is known about their fusion mechanism. We show that Snap-25-null mutant (SNAP-25 KO) neurons, previously shown to degenerate after 4 days in vitro (DIV), contain fewer DCVs and have reduced DCV fusion probability in surviving neurons at DIV14. At DIV3, before degeneration, SNAP-25 KO neurons show normal DCV fusion, but one day later fusion is significantly reduced. To test if other SNAP homologs support DCV fusion, we expressed SNAP-23, SNAP-29 or SNAP-47 in SNAP-25 KO neurons. SNAP-23 and SNAP-29 rescued viability and supported DCV fusion in SNAP-25 KO neurons, but SNAP-23 did so more efficiently. SNAP-23 also rescued synaptic vesicle (SV) fusion while SNAP-29 did not. SNAP-47 failed to rescue viability and did not support DCV or SV fusion. These data demonstrate a developmental switch, in hippocampal neurons between DIV3 and DIV4, where DCV fusion becomes SNAP-25 dependent. Furthermore, SNAP-25 homologs support DCV and SV fusion and neuronal viability to variable extents - SNAP-23 most effectively, SNAP-29 less so and SNAP-47 ineffectively.

Differential Maturation of the Two Regulated Secretory Pathways in Human iPSC-Derived Neurons

Neurons communicate by regulated secretion of chemical signals from synaptic vesicles (SVs) and dense-core vesicles (DCVs). Here, we investigated the maturation of these two secretory pathways in micro-networks of human iPSC-derived neurons. These micro-networks abundantly expressed endogenous SV and DCV markers, including neuropeptides. DCV transport was microtubule dependent, preferentially anterograde in axons, and 2-fold faster in axons than in dendrites. SV and DCV secretion were strictly Ca2+ and SNARE dependent. DCV secretion capacity matured until day in vitro (DIV) 36, with intense stimulation releasing 6% of the total DCV pool, and then plateaued. This efficiency is comparable with mature mouse neurons. In contrast, SV secretion capacity continued to increase until DIV50, with substantial further increase in secretion efficiency and decrease in silent synapses. These data show that the two secretory pathways can be studied in human neurons and that they mature differentially, with DCV secretion reaching maximum efficiency when that of SVs is still low.

The UPR reduces glucose metabolism via IRE1 signaling

Neurons are highly dependent on glucose. A disturbance in glucose homeostasis therefore poses a severe risk that is counteracted by activation of stress responses to limit damage and restore the energy balance. A major stress response that is activated under conditions of glucose deprivation is the unfolded protein response (UPR) that is aimed to restore proteostasis in the endoplasmic reticulum. The key signaling of the UPR involves the transient activation of a transcriptional program and an overall reduction of protein synthesis. Since the UPR is strategically positioned to sense and integrate metabolic stress signals, it is likely that - apart from its adaptive response to restore proteostasis - it also directly affects metabolic pathways. Here we investigate the direct role of the UPR in glucose homeostasis. O-GlcNAc is a post-translational modification that is highly responsive to glucose fluctuations. We find that UPR activation results in decreased O-GlcNAc modification, in line with reduced glucose metabolism. Our data indicate that UPR activation has no direct impact on the upstream processes in glucose metabolism; glucose transporter expression, glucose uptake and hexokinase activity. In contrast, prolonged UPR activation decreases glycolysis and mitochondrial metabolism. Decreased mitochondrial respiration is not accompanied by apoptosis or a structural change in mitochondria indicating that the reduction in metabolic rate upon UPR activation is a physiological non-apoptotic response. Metabolic decrease is prevented if the IRE1 pathway of the UPR is inhibited. This indicates that activation of IRE1 signaling induces a reduction in glucose metabolism, as part of an adaptive response.

Large dense-core vesicle exocytosis from mouse dorsal root ganglion neurons is regulated by neuropeptide Y

Peptidergic dorsal root ganglion (DRG) neurons transmit sensory and nociceptive information from the periphery to the central nervous system. Their synaptic activity is profoundly affected by neuromodulatory peptides stored and released from large dense-core vesicles (LDCVs). However, the mechanism of peptide secretion from DRG neurons is poorly understood. Using total internal reflection fluorescence microscopy (TIRFM), we visualized individual LDCVs loaded with fluorescent neuropeptide Y (NPY) and analyzed their stimulation-dependent release. We tested several protocols and found an overall low stimulation-secretion coupling that increased after raising intracellular Ca²⁺ concentration by applying a weak pre-stimulus. Interestingly, the stimulation protocol also influenced the mechanism of LDCV fusion. Depolarization of DRG neurons with a solution containing 60mM KCl triggered full fusion, kiss-and-run, and kiss-and-stay exocytosis with equal frequency. In contrast, field electrode stimulation primarily induced full fusion exocytosis. Finally, our results indicate that NPY can promote LDCV secretion. These results shed new light on the mechanism of NPY action during modulation of DRG neuron activity, an important pathway in the treatment of chronic pain.

Loss of MeCP2 disrupts cell autonomous and autocrine BDNF signaling in mouse glutamatergic neurons

Mutations in the MECP2 gene cause the neurodevelopmental disorder Rett syndrome (RTT). Previous studies have shown that altered MeCP2 levels result in aberrant neurite outgrowth and glutamatergic synapse formation. However, causal molecular mechanisms are not well understood since MeCP2 is known to regulate transcription of a wide range of target genes. Here, we describe a key role for a constitutive BDNF feed forward signaling pathway in regulating synaptic response, general growth and differentiation of glutamatergic neurons. Chronic block of TrkB receptors mimics the MeCP2 deficiency in wildtype glutamatergic neurons, while re-expression of BDNF quantitatively rescues MeCP2 deficiency. We show that BDNF acts cell autonomous and autocrine, as wildtype neurons are not capable of rescuing growth deficits in neighboring MeCP2 deficient neurons in vitro and in vivo. These findings are relevant for understanding RTT pathophysiology, wherein wildtype and mutant neurons are intermixed throughout the nervous system.

DOI:http://dx.doi.org/10.7554/eLife.19374.001

Rett syndrome is a progressive brain disorder. Individuals with the condition (who are typically girls) grow normally until they are 6-18 months old and then developmentally regress, with symptoms including anxiety, impaired coordination, seizures and breathing problems.

Rett syndrome is caused by mutations in the gene that encodes a protein called MeCP2. Researchers know that MeCP2 is vital for “excitatory” neurons in the brain to communicate with (and activate) their neighbors. Neurons that lack MeCP2 tend to make fewer of the connections across which they communicate – called synapses – with others.

Many researchers who study Rett syndrome use male mice that lack the MeCP2 protein. This mouse model mimics the symptoms seen in Rett patients, but at a faster and more severe rate. These studies have shown that restoring normal levels of the protein in neurons prevents the majority of Rett-like symptoms in these mice and reverses the disorder.

MeCP2 controls the activity of a number of other genes. These include the gene that produces a protein called Brain-Derived Neurotrophic Factor (BDNF), which helps neurons to grow. Sampathkumar et al. have now studied neurons from mouse models of Rett syndrome to investigate whether BDNF can overcome the defects seen in neurons that lack MeCP2. Viewed under a high-powered microscope, the Rett-like neurons appear smaller than healthy neurons and form fewer synapses. However, increasing the amount of BDNF in the diseased neurons restores normal growth and enables the cells to form more synapses.

Girls with Rett syndrome tend to have a mixture of healthy neurons and those that do not produce the right amount of MeCP2. To mimic this, Sampathkumar et al. grew a mixture of normal and Rett-like mouse neurons in a culture dish. The healthy neurons did not help the diseased neurons to form the correct number of synapses. However, increasing the levels of BDNF in the Rett-like neurons enhanced their ability to form synapses, and increased their cell size to match their healthy counterparts.

Further work is now required to uncover whether manipulating the gene that encodes BDNF – or other genes that MeCP2 controls the activity of – in the brain can reduce the symptoms and slow the progression of Rett syndrome.

DOI:http://dx.doi.org/10.7554/eLife.19374.002

Dual Function of NRP1 in Axon Guidance and Subcellular Target Recognition in Cerebellum

Subcellular target recognition in the CNS is the culmination of a multiple-step program including axon guidance, target recognition, and synaptogenesis. In cerebellum, basket cells (BCs) innervate the soma and axon initial segment (AIS) of Purkinje cells (PCs) to form the pinceau synapse, but the underlying mechanisms remain incompletely understood. Here, we demonstrate that neuropilin-1 (NRP1), a Semaphorin receptor expressed in BCs, controls both axonal guidance and subcellular target recognition. We show that loss of Semaphorin 3A function or specific deletion of NRP1 in BCs alters the stereotyped organization of BC axon and impairs pinceau synapse formation. Further, we identified NRP1 as a trans-synaptic binding partner of the cell adhesion molecule neurofascin-186 (NF186) expressed in the PC AIS during pinceau synapse formation. These findings identify a dual function of NRP1 in both axon guidance and subcellular target recognition in the construction of GABAergic circuitry.

Excitatory and Inhibitory Neurons in the Hippocampus Exhibit Molecularly Distinct Large Dense Core Vesicles

Hippocampal interneurons comprise a diverse family of inhibitory neurons that are critical for detailed information processing. Along with gamma-aminobutyric acid (GABA), interneurons secrete a myriad of neuroactive substances via secretory vesicles but the molecular composition and regulatory mechanisms remain largely unknown. In this study, we have carried out an immunohistofluorescence analysis to describe the molecular content of vesicles in distinct populations of hippocampal neurons. Our results indicate that phogrin, an integral protein of secretory vesicles in neuroendocrine cells, is highly enriched in parvalbumin-positive interneurons. Consistently, immunoelectron microscopy revealed phogrin staining in axon terminals of symmetrical synapses establishing inhibitory contacts with cell bodies of CA1 pyramidal neurons. Furthermore, phogrin is highly expressed in CA3 and dentate gyrus (DG) interneurons which are both positive for PV and neuropeptide Y. Surprisingly, chromogranin B a canonical large dense core vesicle marker, is excluded from inhibitory cells in the hippocampus but highly expressed in excitatory CA3 pyramidal neurons and DG granule cells. Our results provide the first evidence of phogrin expression in hippocampal interneurons and suggest the existence of molecularly distinct populations of secretory vesicles in different types of inhibitory neurons.

The Chemorepulsive Protein Semaphorin 3A and Perineuronal Net-Mediated Plasticity

During postnatal development, closure of critical periods coincides with the appearance of extracellular matrix structures, called perineuronal nets (PNN), around various neuronal populations throughout the brain. The absence or presence of PNN strongly correlates with neuronal plasticity. It is not clear how PNN regulate plasticity. The repulsive axon guidance proteins Semaphorin (Sema) 3A and Sema3B are also prominently expressed in the postnatal and adult brain. In the neocortex, Sema3A accumulates in the PNN that form around parvalbumin positive inhibitory interneurons during the closure of critical periods. Sema3A interacts with high-affinity with chondroitin sulfate E, a component of PNN. The localization of Sema3A in PNN and its inhibitory effects on developing neurites are intriguing features and may clarify how PNN mediate structural neural plasticity. In the cerebellum, enhanced neuronal plasticity as a result of an enriched environment correlates with reduced Sema3A expression in PNN. Here, we first review the distribution of Sema3A and Sema3B expression in the rat brain and the biochemical interaction of Sema3A with PNN. Subsequently, we review what is known so far about functional correlates of changes in Sema3A expression in PNN. Finally, we propose a model of how Semaphorins in the PNN may influence local connectivity.

Regulated Dynamic Trafficking of Neurexins Inside and Outside of Synaptic Terminals

Synapses depend on trafficking of key membrane proteins by lateral diffusion from surface populations and by exocytosis from intracellular pools. The cell adhesion molecule neurexin (Nrxn) plays essential roles in synapses, but the dynamics and regulation of its trafficking are unknown. Here, we performed single-particle tracking and live imaging of transfected, epitope-tagged Nrxn variants in cultured rat and mouse wild-type or knock-out neurons. We observed that structurally larger αNrxn molecules are more mobile in the plasma membrane than smaller βNrxns because αNrxns displayed higher diffusion coefficients in extrasynaptic regions and excitatory or inhibitory terminals. We found that well characterized interactions with extracellular binding partners regulate the surface mobility of Nrxns. Binding to neurexophilin-1 (Nxph1) reduced the surface diffusion of αNrxns when both molecules were coexpressed. Conversely, impeding other interactions by insertion of splice sequence #4 or removal of extracellular Ca(2+) augmented the mobility of αNrxns and βNrxns. We also determined that fast axonal transport delivers Nrxns to the neuronal surface because Nrxns comigrate as cargo on synaptic vesicle protein transport vesicles (STVs). Unlike surface mobility, intracellular transport of βNrxn(+) STVs was faster than that of αNrxns, but both depended on the microtubule motor protein KIF1A and neuronal activity regulated the velocity. Large spontaneous fusion of Nrxn(+) STVs occurred simultaneously with synaptophysin on axonal membranes mostly outside of active presynaptic terminals. Surface Nrxns enriched at synaptic terminals where αNrxns and Nxph1/αNrxns recruited GABAAR subunits. Therefore, our results identify regulated dynamic trafficking as an important property of Nrxns that corroborates their function at synapses.

SIGNIFICANCE STATEMENT

Synapses mediate most functions in our brains and depend on the precise and timely delivery of key molecules throughout life. Neurexins (Nrxns) are essential synaptic cell adhesion molecules that are involved in synaptic transmission and differentiation of synaptic contacts. In addition, Nrxns have been linked to neuropsychiatric diseases such as autism. Because little is known about the dynamic aspects of trafficking of neurexins to synapses, we investigated this important question using single-molecule tracking and time-lapse imaging. We identify distinct differences between major Nrxn variants both in surface mobility and during intracellular transport. Because their dynamic behavior is highly regulated, for example, by different binding activities, these processes have immediate consequences for the function of Nrxns at synapses.

Functional characterization of the PCLO p.Ser4814Ala variant associated with major depressive disorder reveals cellular but not behavioral differences

Genome-wide association studies have suggested a role for a genetic variation in the presynaptic gene PCLO in major depressive disorder (MDD). As with many complex traits, the PCLO variant has a small contribution to the overall heritability and the association does not always replicate. One variant (rs2522833, p.Ser4814Ala) is of particular interest given that it is a common, nonsynonymous exon variant near a calcium-sensing part of PCLO. It has been suggested that the molecular effects of such variations penetrate to a variable extent in the population due to phenotypic and genotypic heterogeneity at the population level. More robust effects may be exposed by studying such variations in isolation, in a more homogeneous context. We tested this idea by modeling PCLO variation in a mouse knock-in model expressing the Pclo(SA)(/)(SA) variant. In the highly homogeneous background of inbred mice, two functional effects of the SA-variation were observed at the cellular level: increased synaptic Piccolo levels, and 30% increased excitatory synaptic transmission in cultured neurons. Other aspects of Piccolo function were unaltered: calcium-dependent phospholipid binding, synapse formation in vitro, and synaptic accumulation of synaptic vesicles. Moreover, anxiety, cognition and depressive-like behavior were normal in Pclo(SA)(/)(SA) mice. We conclude that the PCLO p.Ser4814Ala missense variant produces mild cellular phenotypes, which do not translate into behavioral phenotypes. We propose a model explaining how (subtle) cellular phenotypes do not penetrate to the mouse behavioral level but, due to genetic and phenotypic heterogeneity and non-linearity, can produce association signals in human population studies.

SNAREs Controlling Vesicular Release of BDNF and Development of Callosal Axons

At presynaptic active zones, exocytosis of neurotransmitter vesicles (SVs) is driven by SNARE complexes that recruit Syb2 and SNAP25. However, it remains unknown which SNAREs promote the secretion of neuronal proteins, including those essential for circuit development and experience-dependent plasticity. Here we demonstrate that Syb2 and SNAP25 mediate the vesicular release of BDNF in axons and dendrites of cortical neurons, suggesting these SNAREs act in multiple spatially segregated secretory pathways. Remarkably, axonal secretion of BDNF is also strongly regulated by SNAP47, which interacts with SNAP25 but appears to be dispensable for exocytosis of SVs. Cell-autonomous ablation of SNAP47 disrupts the layer-specific branching of callosal axons of projection cortical neurons in vivo, and this phenotype is recapitulated by ablation of BDNF or its receptor, TrkB. Our results provide insights into the molecular mechanisms of protein secretion, and they define the functions of SNAREs in BDNF signaling and regulation of neuronal connectivity.

Synaptic neuropeptide release by dynamin-dependent partial release from circulating vesicles

Neurons release neuropeptides, enzymes, and neurotrophins by exocytosis of dense-core vesicles (DCVs). Peptide release from individual DCVs has been imaged in vitro with endocrine cells and at the neuron soma, growth cones, neurites, axons, and dendrites but not at nerve terminals, where peptidergic neurotransmission occurs. Single presynaptic DCVs have, however, been tracked in native terminals with simultaneous photobleaching and imaging (SPAIM) to show that DCVs undergo anterograde and retrograde capture as they circulate through en passant boutons. Here dynamin (encoded by the shibire gene) is shown to enhance activity-evoked peptide release at the Drosophila neuromuscular junction. SPAIM demonstrates that activity depletes only a portion of a single presynaptic DCV's content. Activity initiates exocytosis within seconds, but subsequent release occurs slowly. Synaptic neuropeptide release is further sustained by DCVs undergoing multiple rounds of exocytosis. Synaptic neuropeptide release is surprisingly similar regardless of anterograde or retrograde DCV transport into boutons, bouton location, and time of arrival in the terminal. Thus vesicle circulation and bidirectional capture supply synapses with functionally competent DCVs. These results show that activity-evoked synaptic neuropeptide release is independent of a DCV's past traffic and occurs by slow, dynamin-dependent partial emptying of DCVs, suggestive of kiss-and-run exocytosis.

CAPS-1 promotes fusion competence of stationary dense-core vesicles in presynaptic terminals of mammalian neurons

Neuropeptides released from dense-core vesicles (DCVs) modulate neuronal activity, but the molecules driving DCV secretion in mammalian neurons are largely unknown. We studied the role of calcium-activator protein for secretion (CAPS) proteins in neuronal DCV secretion at single vesicle resolution. Endogenous CAPS-1 co-localized with synaptic markers but was not enriched at every synapse. Deletion of CAPS-1 and CAPS-2 did not affect DCV biogenesis, loading, transport or docking, but DCV secretion was reduced by 70% in CAPS-1/CAPS-2 double null mutant (DKO) neurons and remaining fusion events required prolonged stimulation. CAPS deletion specifically reduced secretion of stationary DCVs. CAPS-1-EYFP expression in DKO neurons restored DCV secretion, but CAPS-1-EYFP and DCVs rarely traveled together. Synaptic localization of CAPS-1-EYFP in DKO neurons was calcium dependent and DCV fusion probability correlated with synaptic CAPS-1-EYFP expression. These data indicate that CAPS-1 promotes fusion competence of immobile (tethered) DCVs in presynaptic terminals and that CAPS-1 localization to DCVs is probably not essential for this role.

DOI:http://dx.doi.org/10.7554/eLife.05438.001

Our ability to think and act is due to the remarkable capacity of the brain to process complex information. This involves nerve cells (or neurons) communicating with each other in a rapid and precise manner by releasing synaptic vesicles containing neurotransmitters across the gaps—called synapses—between neurons. In addition to this fast neurotransmitter signalling, neurons can transmit signals by releasing chemical signals called neuropeptides. Neuropeptides are major regulators of human brain function, including mood, anxiety, and social interactions.

Neuropeptides and other neuromodulators such as serotonin and dopamine are normally packaged into bubble-like compartments called dense-core vesicles. Compared to synaptic vesicles we know much less about how dense-core vesicles are trafficked and released. Dense-core vesicles are generally mobile and move around the inside of cells to release neuropeptides where and when they are needed. However, some vesicles are stationary and may even be loosely tethered to the cell membrane. Most of the sites where dense-core vesicles can fuse with the cell membrane are at synapses.

Previous work has suggested that the protein CAPS-1 is important for moving dense-core vesicles to the correct sites on the cell membrane, and for releasing neuropeptides across the synapses of worms and flies. However, detailed insights into this process in mammalian neurons are lacking.

By examining neurons from both normal mice and mice lacking the CAPS-1 protein, Farina et al. have now analyzed the role CAPS-1 plays in releasing neuropeptides. In cells lacking CAPS-1 fewer dense-core vesicles merged with the cell membrane than in cells containing the protein. However, a new technique that tracks the movement of individual vesicles revealed that only stationary dense-core vesicles had difficulties fusing; mobile vesicles continued to fuse with the cell membrane in the normal manner. Introducing CAPS-1 into cells lacking this protein corrected the fusion defect experienced by the stationary vesicles.

Farina et al. also showed that CAPS-1 was present at most—but not all—synapses, and synapses that had more CAPS-1 released more neuropeptides. This work shows that CAPS proteins strongly influence the probability of dense-core vesicle release and that neurons can tune this probability at individual synapses by controlling the expression of CAPS. Future work will be aimed at understanding how neurons can achieve this and which protein domains in CAPS are required.

DOI:http://dx.doi.org/10.7554/eLife.05438.002

Dynamic mechanisms of neuroligin-dependent presynaptic terminal assembly in living cortical neurons

Background

Synapse formation occurs when synaptogenic signals trigger coordinated development of pre and postsynaptic structures. One of the best-characterized synaptogenic signals is trans-synaptic adhesion. However, it remains unclear how synaptic proteins are recruited to sites of adhesion. In particular, it is unknown whether synaptogenic signals attract synaptic vesicle (SV) and active zone (AZ) proteins to nascent synapses or instead predominantly function to create sites that are capable of forming synapses. It is also unclear how labile synaptic proteins are at developing synapses after their initial recruitment. To address these issues, we used long-term, live confocal imaging of presynaptic terminal formation in cultured cortical neurons after contact with the synaptogenic postsynaptic adhesion proteins neuroligin-1 or SynCAM-1.

Results

Surprisingly, we find that trans-synaptic adhesion does not attract SV or AZ proteins nor alter their transport. In addition, although neurexin (the presynaptic partner of neuroligin) typically accumulates over the entire region of contact between axons and neuroligin-1-expressing cells, SV proteins selectively assemble at spots of enhanced neurexin clustering. The arrival and maintenance of SV proteins at these sites is highly variable over the course of minutes to hours, and this variability correlates with neurexin levels at individual synapses.

Conclusions

Together, our data support a model of synaptogenesis where presynaptic proteins are trapped at specific axonal sites, where they are stabilized by trans-synaptic adhesion signaling.

Evolution of oxytocin pathways in the brain of vertebrates

The central oxytocin system transformed tremendously during the evolution, thereby adapting to the expanding properties of species. In more basal vertebrates (paraphyletic taxon Anamnia, which includes agnathans, fish and amphibians), magnocellular neurosecretory neurons producing homologs of oxytocin reside in the wall of the third ventricle of the hypothalamus composing a single hypothalamic structure, the preoptic nucleus. This nucleus further diverged in advanced vertebrates (monophyletic taxon Amniota, which includes reptiles, birds, and mammals) into the paraventricular and supraoptic nuclei with accessory nuclei (AN) between them. The individual magnocellular neurons underwent a process of transformation from primitive uni- or bipolar neurons into highly differentiated neurons. Due to these microanatomical and cytological changes, the ancient release modes of oxytocin into the cerebrospinal fluid were largely replaced by vascular release. However, the most fascinating feature of the progressive transformations of the oxytocin system has been the expansion of oxytocin axonal projections to forebrain regions. In the present review we provide a background on these evolutionary advancements. Furthermore, we draw attention to the non-synaptic axonal release in small and defined brain regions with the aim to clearly distinguish this way of oxytocin action from the classical synaptic transmission on one side and from dendritic release followed by a global diffusion on the other side. Finally, we will summarize the effects of oxytocin and its homologs on pro-social reproductive behaviors in representatives of the phylogenetic tree and will propose anatomically plausible pathways of oxytocin release contributing to these behaviors in basal vertebrates and amniots.

Use of pHluorin to assess the dynamics of axon guidance receptors in cell culture and in the chick embryo

During development, axon guidance receptors play a crucial role in regulating axons sensitivity to both attractive and repulsive cues. Indeed, activation of the guidance receptors is the first step of the signaling mechanisms allowing axon tips, the growth cones, to respond to the ligands. As such, the modulation of their availability at the cell surface is one of the mechanisms that participate in setting the growth cone sensitivity. We describe here a method to precisely visualize the spatio-temporal cell surface dynamics of an axon guidance receptor both in vitro and in vivo in the developing chick spinal cord. We took advantage of the pH-dependent fluorescence property of a green fluorescent protein (GFP) variant to specifically detect the fraction of the axon guidance receptor that is addressed to the plasma membrane. We first describe the in vitro validation of such pH-dependent constructs and we further detail their use in vivo, in the chick spinal chord, to assess the spatio-temporal dynamics of the axon guidance receptor of interest.

Semaphorin 3A binds to the perineuronal nets via chondroitin sulfate type E motifs in rodent brains

Chondroitin sulfate (CS) and the CS-rich extracellular matrix structures called perineuronal nets (PNNs) restrict plasticity and regeneration in the CNS. Plasticity is enhanced by chondroitinase ABC treatment that removes CS from its core protein in the chondroitin sulfate proteoglycans or by preventing the formation of PNNs, suggesting that chondroitin sulfate proteoglycans in the PNNs control plasticity. Recently, we have shown that semaphorin3A (Sema3A), a repulsive axon guidance molecule, localizes to the PNNs and is removed by chondroitinase ABC treatment (Vo, T., Carulli, D., Ehlert, E. M., Kwok, J. C., Dick, G., Mecollari, V., Moloney, E. B., Neufeld, G., de Winter, F., Fawcett, J. W., and Verhaagen, J. (2013) Mol. Cell. Neurosci. 56C, 186-200). Sema3A is therefore a candidate for a PNN effector in controlling plasticity. Here, we characterize the interaction of Sema3A with CS of the PNNs. Recombinant Sema3A interacts with CS type E (CS-E), and this interaction is involved in the binding of Sema3A to rat brain-derived PNN glycosaminoglycans, as demonstrated by the use of CS-E blocking antibody GD3G7. In addition, we investigate the release of endogenous Sema3A from rat brain by biochemical and enzymatic extractions. Our results confirm the interaction of Sema3A with CS-E containing glycosaminoglycans in the dense extracellular matrix of rat brain. We also demonstrate that the combination of Sema3A and PNN GAGs is a potent inhibitor of axon growth, and this inhibition is reduced by the CS-E blocking antibody. In conclusion, Sema3A binding to CS-E in the PNNs may be a mechanism whereby PNNs restrict growth and plasticity and may represent a possible point of intervention to facilitate neuronal plasticity.

Pre- and postsynaptic twists in BDNF secretion and action in synaptic plasticity

Overwhelming evidence collected since the early 1990's strongly supports the notion that BDNF is among the key regulators of synaptic plasticity in many areas of the mammalian central nervous system. Still, due to the extremely low expression levels of endogenous BDNF in most brain areas, surprisingly little data i) pinpointing pre- and postsynaptic release sites, ii) unraveling the time course of release, and iii) elucidating the physiological levels of synaptic activity driving this secretion are available. Likewise, our knowledge regarding pre- and postsynaptic effects of endogenous BDNF at the single cell level in mediating long-term potentiation still is sparse. Thus, our review will discuss the data currently available regarding synaptic BDNF secretion in response to physiologically relevant levels of activity, and will discuss how endogenously secreted BDNF affects synaptic plasticity, giving a special focus on spike timing-dependent types of LTP and on mossy fiber LTP. We will attempt to open up perspectives how the remaining challenging questions regarding synaptic BDNF release and action might be addressed by future experiments. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Munc13 controls the location and efficiency of dense-core vesicle release in neurons

Neuronal dense-core vesicles (DCVs) contain diverse cargo crucial for brain development and function, but the mechanisms that control their release are largely unknown. We quantified activity-dependent DCV release in hippocampal neurons at single vesicle resolution. DCVs fused preferentially at synaptic terminals. DCVs also fused at extrasynaptic sites but only after prolonged stimulation. In munc13-1/2-null mutant neurons, synaptic DCV release was reduced but not abolished, and synaptic preference was lost. The remaining fusion required prolonged stimulation, similar to extrasynaptic fusion in wild-type neurons. Conversely, Munc13-1 overexpression (M13OE) promoted extrasynaptic DCV release, also without prolonged stimulation. Thus, Munc13-1/2 facilitate DCV fusion but, unlike for synaptic vesicles, are not essential for DCV release, and M13OE is sufficient to produce efficient DCV release extrasynaptically.

Unfolded protein response activates glycogen synthase kinase-3 via selective lysosomal degradation

The unfolded protein response (UPR) is a stress response that is activated upon disturbed homeostasis in the endoplasmic reticulum. In Alzheimer's disease, as well as in other tauopathies, the UPR is activated in neurons that contain early tau pathology. A recent genome-wide association study identified genetic variation in a UPR transducer as a risk factor for tauopathy, supporting a functional connection between UPR activation and tau pathology. Here we show that UPR activation increases the activity of the major tau kinase glycogen synthase kinase (GSK)-3 in vitro via a selective removal of inactive GSK-3 phosphorylated at Ser(21/9). We demonstrate that this is mediated by the autophagy/lysosomal pathway. In brain tissue from patients with different tauopathies, lysosomal accumulations of pSer(21/9) GSK-3 are found in neurons with markers for UPR activation. Our data indicate that UPR activation increases the activity of GSK-3 by a novel mechanism, the lysosomal degradation of the inactive pSer(21/9) GSK-3. This may provide a functional explanation for the close association between UPR activation and early tau pathology in neurodegenerative diseases.

Hindered submicron mobility and long-term storage of presynaptic dense-core granules revealed by single-particle tracking

Dense-core granules (DCGs) are organelles found in neuroendocrine cells and neurons that house, transport, and release a number of important peptides and proteins. In neurons, DCG cargo can include the secreted neuromodulatory proteins tissue plasminogen activator (tPA) and/or brain-derived neurotrophic factor (BDNF), which play a key role in modulating synaptic efficacy in the hippocampus. This function has spurred interest in DCGs that localize to synaptic contacts between hippocampal neurons, and several studies recently have established that DCGs localize to, and undergo regulated exocytosis from, postsynaptic sites. To complement this work, we have studied presynaptically localized DCGs in hippocampal neurons, which are much more poorly understood than their postsynaptic analogs. Moreover, to enhance relevance, we visualized DCGs via fluorescence labeling of exogenous and endogenous tPA and BDNF. Using single-particle tracking, we determined trajectories of more than 150 presynaptically localized DCGs. These trajectories reveal that mobility of DCGs in presynaptic boutons is highly hindered and that storage is long-lived. We also computed mean-squared displacement curves, which can be used to elucidate mechanisms of transport. Over shorter time windows, most curves are linear, demonstrating that DCG transport in boutons is driven predominantly by diffusion. The remaining curves plateau with time, consistent with motion constrained by a submicron-sized corral. These results have relevance to recent models of presynaptic organization and to recent hypotheses about DCG cargo function. The results also provide estimates for transit times to the presynaptic plasma membrane that are consistent with measured times for onset of neurotrophin release from synaptically localized DCGs.

Dendritic position is a major determinant of presynaptic strength

Different regulatory principles influence synaptic coupling between neurons, including positional principles. In dendrites of pyramidal neurons, postsynaptic sensitivity depends on synapse location, with distal synapses having the highest gain. In this paper, we investigate whether similar rules exist for presynaptic terminals in mixed networks of pyramidal and dentate gyrus (DG) neurons. Unexpectedly, distal synapses had the lowest staining intensities for vesicular proteins vGlut, vGAT, Synaptotagmin, and VAMP and for many nonvesicular proteins, including Bassoon, Munc18, and Syntaxin. Concomitantly, distal synapses displayed less vesicle release upon stimulation. This dependence of presynaptic strength on dendritic position persisted after chronically blocking action potential firing and postsynaptic receptors but was markedly reduced on DG dendrites compared with pyramidal dendrites. These data reveal a novel rule, independent of neuronal activity, which regulates presynaptic strength according to dendritic position, with the strongest terminals closest to the soma. This gradient is opposite to postsynaptic gradients observed in pyramidal dendrites, and different cell types apply this rule to a different extent.

Coordinated trafficking of synaptic vesicle and active zone proteins prior to synapse formation

Background

The proteins required for synaptic transmission are rapidly assembled at nascent synapses, but the mechanisms through which these proteins are delivered to developing presynaptic terminals are not understood. Prior to synapse formation, active zone proteins and synaptic vesicle proteins are transported along axons in distinct organelles referred to as piccolo-bassoon transport vesicles (PTVs) and synaptic vesicle protein transport vesicles (STVs), respectively. Although both PTVs and STVs are recruited to the same site in the axon, often within minutes of axo-dendritic contact, it is not known whether or how PTV and STV trafficking is coordinated before synapse formation.

Results

Here, using time-lapse confocal imaging of the dynamics of PTVs and STVs in the same axon, we show that vesicle trafficking is coordinated through at least two mechanisms. First, a significant proportion of STVs and PTVs are transported together before forming a stable terminal. Second, individual PTVs and STVs share pause sites within the axon. Importantly, for both STVs and PTVs, encountering the other type of vesicle increases their propensity to pause. To determine if PTV-STV interactions are important for pausing, PTV density was reduced in axons by expression of a dominant negative construct corresponding to the syntaxin binding domain of syntabulin, which links PTVs with their KIF5B motor. This reduction in PTVs had a minimal effect on STV pausing and movement, suggesting that an interaction between STVs and PTVs is not responsible for enhancing STV pausing.

Conclusions

Our results indicate that trafficking of STVs and PTVs is coordinated even prior to synapse development. This novel coordination of transport and pausing might provide mechanisms through which all of the components of a presynaptic terminal can be rapidly accumulated at sites of synapse formation.

Neuromodulatory function of neuropeptides in the normal CNS

Neuropeptides are small protein molecules produced and released by discrete cell populations of the central and peripheral nervous systems through the regulated secretory pathway and acting on neural substrates. Inside the nerve cells, neuropeptides are selectively stored within large granular vesicles (LGVs), and commonly coexist in neurons with low-molecular-weight neurotransmitters (acetylcholine, amino acids, and catecholamines). Storage in LGVs is responsible for a relatively slow response to secretion that requires enhanced or repeated stimulation. Coexistence (i.e. the concurrent presence of a neuropeptide with other messenger molecules in individual neurons), and co-storage (i.e. the localization of two or more neuropeptides within individual LGVs in neurons) give rise to a complicated series of pre- and post-synaptic functional interactions with low-molecular-weight neurotransmitters. The typically slow response and action of neuropeptides as compared to fast-neurotransmitters such as excitatory/inhibitory amino acids and catecholamines is also due to the type of receptors that trigger neuropeptide actions onto target cells. Almost all neuropeptides act on G-protein coupled receptors that, upon ligand binding, activate an intracellular cascade of molecular enzymatic events, eventually leading to cellular responses. The latter occur in a time span (seconds or more) considerably longer (milliseconds) than that of low-molecular-weight fast-neurotransmitters, directly operating through ion channel receptors. As reviewed here, combined immunocytochemical visualization of neuropeptides and their receptors at the ultrastructural level and electrophysiological studies, have been fundamental to better unravel the role of neuropeptides in neuron-to-neuron communication.

Automated analysis of neuronal morphology, synapse number and synaptic recruitment

The shape, structure and connectivity of nerve cells are important aspects of neuronal function. Genetic and epigenetic factors that alter neuronal morphology or synaptic localization of pre- and post-synaptic proteins contribute significantly to neuronal output and may underlie clinical states. To assess the impact of individual genes and disease-causing mutations on neuronal morphology, reliable methods are needed. Unfortunately, manual analysis of immuno-fluorescence images of neurons to quantify neuronal shape and synapse number, size and distribution is labor-intensive, time-consuming and subject to human bias and error. We have developed an automated image analysis routine using steerable filters and deconvolutions to automatically analyze dendrite and synapse characteristics in immuno-fluorescence images. Our approach reports dendrite morphology, synapse size and number but also synaptic vesicle density and synaptic accumulation of proteins as a function of distance from the soma as consistent as expert observers while reducing analysis time considerably. In addition, the routine can be used to detect and quantify a wide range of neuronal organelles and is capable of batch analysis of a large number of images enabling high-throughput analysis.

Possible association of the semaphorin 3D gene (SEMA3D) with schizophrenia

Semaphorins are ligands of plexins, and the plexin-semaphorin signaling system is widely involved in many neuronal events including axon guidance, cell migration, axon pruning, and synaptic plasticity. The plexin A2 gene (PLXNA2) has been reported to be associated with schizophrenia. This finding prompted us to examine the possible association between the semaphorin 3D gene (SEMA3D) and schizophrenia in a Japanese population. We genotyped 9 tagging single nucleotide polymorphisms (SNPs) of SEMA3D including a non-synonymous variation, Lys701Gln (rs7800072), in a sample of 506 patients with schizophrenia and 941 healthy control subjects. The Gln701 allele showed a significant protective effect against the development of schizophrenia (p = 0.0069, odds ratio = 0.76, 95% confidence interval 0.63 to 0.93). Furthermore, the haplotype-based analyses revealed a significant association. The four-marker analysis (rs2190208-rs1029564-rs17159614-rs12176601), in particular, not including the Lys701Gln, revealed a highly significant association (p = 0.00001, global permutation), suggesting that there may be other functional polymorphisms within SEMA3D. Our findings provide strong evidence that SEMA3D confers susceptibility to schizophrenia, which could contribute to the neurodevelopmental impairments in the disorder.

Zinc is externalized rather than released during synaptic transmission

The synaptic vesicles of some glutamatergic terminals contain a high concentration of zinc that serves functions that remain obscure. In this publication we have used the membrane permeant zinc fluophore, ZnAF-2 to determine if zinc is released during the course of synaptic transmission. Stimulation of the slices with either high potassium or electrically, leads to an increase in fluorescence that long outlasts the stimulus and remains elevated for many minutes. We demonstrate that this response is inconsistent with the free release of zinc but is with the presentation of zinc coordinated to macromolecules within the exocytosed vesicles to the extracellular space; a process we term 'externalization'. Our data suggests a novel mechanism of synaptic transmission at zinc-rich glutamatergic terminals that distinguishes them from their metal free counterparts.

Duration of fusion pore opening and the amount of hormone released are regulated by myosin II during kiss-and-run exocytosis

Since the fusion pore of the secretory vesicle is resealed before complete dilation during 'kiss-and-run' exocytosis, their cargoes are not completely released. Although the transient fusion pore is kept open for several seconds, the precise mechanisms that control fusion pore maintenance, and their physiological significance, are not well understood. Using dual-colour TIRF (total internal reflection fluorescence) microscopy in neuroendocrine PC12 cells, we show that myosin II regulates the fusion pore dynamics during kiss-and-run exocytosis. The release kinetics of mRFP (monomeric red fluorescent protein)-tagged tPA (tissue plasminogen activator) and Venus-tagged BDNF (brain-derived neurotrophic factor), which show slower release kinetics than NPY (neuropeptide Y)-mRFP and insulin-mRFP, were prolonged by the overexpression of a wild-type form of the RLC (myosin II regulatory light chain). In contrast, overexpression of a dominant-negative form of RLC shortened the release kinetics. Using spH (synapto-pHluorin), a green fluorescent protein-based pH sensor inside the vesicles, we confirmed that the modulation of the release kinetics by myosin II is due to changes in the duration of fusion pore opening. In addition, we revealed that the amount of hormone released into the extracellular space upon stimulation was increased by overexpression of wild-type RLC. We propose that the duration of fusion pore opening is regulated by myosin II to control the amount of hormone released from a single vesicle.