Neuronal SNAREs and their key regulators together drive synaptic vesicle exocytosis and synaptic transmission as a single integrated membrane fusion machine. Human pathogenic mutations have now been ...reported for all eight core components, but patients are diagnosed with very different neurodevelopmental syndromes. We propose to unify these syndromes, based on etiology and mechanism, as “SNAREopathies.” Here, we review the strikingly diverse clinical phenomenology and disease severity and the also remarkably diverse genetic mechanisms. We argue that disease severity generally scales with functional redundancy and, conversely, that the large effect of mutations in some SNARE genes is the price paid for extensive integration and exceptional specialization. Finally, we discuss how subtle differences in components being rate limiting in different types of neurons helps to explain the main symptoms.
Verhage and Sørensen review the diversity in genetic mechanisms and clinical manifestations of syndromes caused by mutations in the presynaptic fusion machinery. They argue that the redundancy and phenotypic heterogeneity among neurons help to explain this diversity.
The molecular mechanisms for calcium‐triggered membrane fusion have long been sought for, and detailed models now exist that account for at least some of the functions of the many proteins involved ...in the process. Key players in the fusion reaction are a group of proteins that, upon binding to calcium, trigger the merger of cargo‐filled vesicles with the plasma membrane. Low‐affinity, fast‐kinetics calcium sensors of the synaptotagmin family – especially synaptotagmin‐1 and synaptotagmin‐2 – are the main calcium sensors for fast exocytosis triggering in many cell types. Their functions extend beyond fusion triggering itself, having been implicated in the calcium‐dependent vesicle recruitment during activity, docking of vesicles to the plasma membrane and priming, and even in post‐fusion steps, such as fusion pore expansion and endocytosis. Furthermore, synaptotagmin diversity imparts distinct properties to the release process itself. Other calcium‐sensing proteins such as Munc13s and protein kinase C play important, but more indirect roles in calcium‐triggered exocytosis. Because of their higher affinity, but intrinsic slower kinetics, they operate on longer temporal and spatial scales to organize assembly of the release machinery. Finally, the high‐affinity synaptotagmin‐7 and Doc2 (Double C2‐domain) proteins are able to trigger membrane fusion in vitro, but cellular measurements in different systems show that they may participate in either fusion or vesicle priming. Here, we summarize the properties and possible interplay of (some of) the major C2‐domain containing calcium sensors in calcium‐triggered exocytosis.
This article is part of a mini review series: “Synaptic Function and Dysfunction in Brain Diseases”.
C2‐domain containing calcium sensors are involved in releasing the vesicular content in secretory cells. Some sensors with a low calcium affinity and fast kinetics are optimally suited for triggering the release itself, whereas others work in the priming of vesicles. The interplay of several kinds of sensors give rise to the complex release kinetics found in secretory cells.
This article is part of a mini review series: “Synaptic Function and Dysfunction in Brain Diseases”.
Exocytosis requires formation of SNARE soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor complexes between vesicle and target membranes. Recent assessments in reduced model ...systems have produced divergent estimates of the number of SNARE complexes needed for fusion. Here, we used a titration approach to answer this question in intact, cultured chromaffin cells. Simultaneous expression of wild-type SNAP-25 and a mutant unable to support exocytosis progressively altered fusion kinetics and fusion-pore opening, indicating that both proteins assemble into heteromeric fusion complexes. Expressing different wild-type:mutant ratios revealed a third-power relation for fast (synchronous) fusion and a near-linear relation for overall release. Thus, fast fusion typically observed in synapses and neurosecretory cells requires at least three functional SNARE complexes, whereas slower release might occur with fewer complexes. Heterogeneity in SNARE-complex number may explain heterogeneity in vesicular release probability.
The SNAP-25 Protein Family Kádková, Anna; Radecke, Julika; Sørensen, Jakob B.
Neuroscience,
11/2019, Letnik:
420
Journal Article
Recenzirano
•We review the expression, function and regulation of the SNAP-25 protein family.•SNAP-25 and SNAP-23 are specialized for regulated exocytosis.•SNAP-25 couples to Ca2+-sensors and drives ...Ca2+-triggered exocytosis in neuronal cells.•SNAP-29 is involved in constitutive exocytosis and autophagosome fusion.•SNAP-47 drives AMPA-receptor trafficking and autophagosome fusion.
SNARE-complexes drive the fusion of membrane-bound vesicles with target membranes or with each other (homotypic fusion). The SNARE-proteins are subdivided into Qa, Qb, Qc and R-SNAREs depending on their position in the four-helical SNARE-bundle. Here, we review the SNAP-25 protein sub-family, which includes both the Qb and Qc SNARE-domains within a single protein. In vertebrates, this sub-family consists of SNAP-25, SNAP-23, SNAP-29 and SNAP-47, named for their apparent molecular weights. SNAP-25 and SNAP-23 are specialized for driving regulated exocytosis. SNAP-25 performs this function in the nervous system, and in neuroendocrine cells, where fast Ca2+-dependent triggering is required in order to synchronize release with an electrical signal, whereas SNAP-23 drives regulated exocytosis in most other cases that have been studied, e.g. platelet exocytosis or glucose transporter trafficking. SNAP-25 is regulated by alternative splicing, phosphorylation and by G-protein binding, and it regulates Ca2+-channels, neuronal survival and postsynaptic spine development. SNAP-23 is primarily regulated by phosphorylation within the linker connecting Qb to Qc. Cross-rescue experiments show that SNAP-25 and SNAP-23 can (at least partly) substitute for each other, whereas SNAP-29 and SNAP-47 cannot. SNAP-29 is present on intracellular membranes and performs functions in autophagosome-to-lysosome fusion, among others. An overlapping function for SNAP-47 was described; in addition, SNAP-47 mediates postsynaptic AMPA-receptor insertion. Overall, the presence of two SNARE-domains confers members of this family the ability to associate to different Qa and R-SNAREs and drive diverse membrane fusion reactions; one member of the family, SNAP-25, has been devoted entirely to Ca2+-triggered fusion and has taken on a number of additional, regulatory roles.
Owing to the worrying increase in carbon dioxide concentrations in the atmosphere, there is a need to electrify fossil-fuel–powered chemical processes such as the Haber-Bosch ammonia synthesis. ...Lithium-mediated electrochemical nitrogen reduction has shown preliminary promise but still lacks sufficient faradaic efficiency and ammonia formation rate to be industrially relevant. Here, we show that oxygen, previously believed to hinder the reaction, actually greatly improves the faradaic efficiency and stability of the lithium-mediated nitrogen reduction when added to the reaction atmosphere in small amounts. With this counterintuitive discovery, we reach record high faradaic efficiencies of up to 78.0 ± 1.3% at 0.6 to 0.8 mole % oxygen in 20 bar of nitrogen. Experimental x-ray analysis and theoretical microkinetic modeling shed light on the underlying mechanism.
Vesicle Docking in Regulated Exocytosis Verhage, Matthijs; Sørensen, Jakob B
Traffic (Copenhagen, Denmark),
September 2008, Letnik:
9, Številka:
9
Journal Article
Recenzirano
Odprti dostop
In electron micrographs, many secretory and synaptic vesicles are found 'docked' at the target membrane, but it is unclear why and how. It is generally assumed that docking is a necessary first step ...in the secretory pathway before vesicles can acquire fusion competence (through 'priming'), but recent studies challenge this. New biophysical methods have become available to detect how vesicles are tethered at the target membrane, and genetic manipulations have implicated many genes in tethering, docking and priming. However, these studies have not yet led to consistent working models for these steps. In this study, we review recent attempts to characterize these early steps and the cellular factors to orchestrate them. We discuss whether assays for docking, tethering and priming report on the same phenomena and whether all vesicles necessarily follow the same linear docking-priming-fusion pathway. We conclude that most evidence to date is consistent with such a linear pathway assuming several refinements that imply that some vesicles can be nonfunctionally docked ('dead-end' docking) or, conversely, that the linear pathway can be greatly accelerated (crash fusion).
The number of fully sequenced fungal genomes is rapidly increasing. Since genetic tools are poorly developed for most filamentous fungi, it is currently difficult to employ genetic engineering for ...understanding the biology of these fungi and to fully exploit them industrially. For that reason there is a demand for developing versatile methods that can be used to genetically manipulate non-model filamentous fungi. To facilitate this, we have developed a CRISPR-Cas9 based system adapted for use in filamentous fungi. The system is simple and versatile, as RNA guided mutagenesis can be achieved by transforming a target fungus with a single plasmid. The system currently contains four CRISPR-Cas9 vectors, which are equipped with commonly used fungal markers allowing for selection in a broad range of fungi. Moreover, we have developed a script that allows identification of protospacers that target gene homologs in multiple species to facilitate introduction of common mutations in different filamentous fungi. With these tools we have performed RNA-guided mutagenesis in six species of which one has not previously been genetically engineered. Moreover, for a wild-type Aspergillus aculeatus strain, we have used our CRISPR Cas9 system to generate a strain that contains an AACU_pyrG marker and demonstrated that the resulting strain can be used for iterative gene targeting.
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