NMJ Analyser: a novel method to quantify neuromuscular junction morphology in zebrafish.

MOTIVATION: Neuromuscular junction (NMJ) structural integrity is crucial for transducing motor neuron signals that initiate skeletal muscle contraction. Zebrafish has emerged as a simple and efficient model to study NMJ structural morphology and function in the context of developmental neurobiology and neuromuscular diseases. However, methods to quantify NMJ morphology from voluminous data of NMJ confocal images accurately, rapidly, and reproducibly are lacking. RESULTS: We developed an ImageJ macro called "NMJ Analyser" to automatically and unbiasedly analyse NMJ morphology in zebrafish. From the Z-stack of a zebrafish hemisomite, both presynaptic and postsynaptic fluorescently labeled termini at NMJs are extracted from background signal, with larger clusters of termini being segmented into individual termini using an unbiased algorithm. The program then determines whether each presynaptic terminus is co-localized with a postsynaptic terminus and vice versa, or whether it is orphaned, and tabulates the number of orphan and co-localized pre- and postsynaptic termini. The usefulness of this ImageJ macro plugin will be helpful to quantify NMJ parameters in zebrafish, particularly during development and in disease models of neuromuscular diseases. It can enable high-throughput NMJ phenotypic screens in the drug discovery process for neuromuscular diseases. It could also be further applied to the investigation of NMJ of other developmental systems. AVAILABILITY AND IMPLEMENTATION: NMJ Analyser is available for download at https://github.com/PattenLab/NMJ-Analyser.git.

Physiological Functions of the Volume-Regulated Anion Channel VRAC/LRRC8 and the Proton-Activated Chloride Channel ASOR/TMEM206.

Volume-regulated anion channels (VRACs) and the acid-sensitive outwardly rectifying anion channel (ASOR) mediate flux of chloride and small organic anions. Although known for a long time, they were only recently identified at the molecular level. VRACs are heteromers consisting of LRRC8 proteins A to E. Combining the essential LRRC8A with different LRRC8 paralogues changes key properties of VRAC such as conductance or substrate selectivity, which is how VRACs are involved in multiple physiological functions including regulatory volume decrease, cell proliferation and migration, cell death, purinergic signalling, fat and glucose metabolism, insulin signalling, and spermiogenesis. VRACs are also involved in pathological conditions, such as the neurotoxic release of glutamate and aspartate. Certain VRACs are also permeable to larger, organic anions, including antibiotics and anti-cancer drugs, making them an interesting therapeutic target. ASOR, also named proton-activated chloride channel (PAC), is formed by TMEM206 homotrimers on the plasma membrane and on endosomal compartments where it mediates chloride flux in response to extracytosolic acidification and plays a role in the shrinking and maturation of macropinosomes. ASOR has been shown to underlie neuronal swelling which causes cell death after stroke as well as promoting the metastasis of certain cancers, making them intriguing therapeutic targets as well.

Missense mutations in CASK, coding for the calcium-/calmodulin-dependent serine protein kinase, interfere with neurexin binding and neurexin-induced oligomerization.

Mutations in the X-linked gene coding for the calcium-/calmodulin-dependent serine protein kinase (CASK) are associated with severe neurological disorders ranging from intellectual disability (in males) to mental retardation and microcephaly with pontine and cerebellar hypoplasia. CASK is involved in transcription control, in the regulation of trafficking of the post-synaptic NMDA and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, and acts as a presynaptic scaffolding protein. For CASK missense mutations, it is mostly unclear which of CASK's molecular interactions and cellular functions are altered and contribute to patient phenotypes. We identified five CASK missense mutations in male patients affected by neurodevelopmental disorders. These and five previously reported mutations were systematically analysed with respect to interaction with CASK interaction partners by co-expression and co-immunoprecipitation. We show that one mutation in the L27 domain interferes with binding to synapse-associated protein of 97 kDa. Two mutations in the guanylate kinase (GK) domain affect binding of CASK to the nuclear factors CASK-interacting nucleosome assembly protein (CINAP) and T-box, brain, 1 (Tbr1). A total of five mutations in GK as well as PSD-95/discs large/ZO-1 (PDZ) domains affect binding of CASK to the pre-synaptic cell adhesion molecule Neurexin. Upon expression in neurons, we observe that binding to Neurexin is not required for pre-synaptic localization of CASK. We show by bimolecular fluorescence complementation assay that Neurexin induces oligomerization of CASK, and that mutations in GK and PDZ domains interfere with the Neurexin-induced oligomerization of CASK. Our data are supported by molecular modelling, where we observe that the cooperative activity of PDZ, SH3 and GK domains is required for Neurexin binding and oligomerization of CASK.

Reduced C9orf72 function leads to defective synaptic vesicle release and neuromuscular dysfunction in zebrafish.

The most common genetic cause of amyotrophic lateral sclerosis (ALS) and fronto-temporal dementia (FTD) is a hexanucleotide repeat expansion within the C9orf72 gene. Reduced levels of C9orf72 mRNA and protein have been found in ALS/FTD patients, but the role of this protein in disease pathogenesis is still poorly understood. Here, we report the generation and characterization of a stable C9orf72 loss-of-function (LOF) model in the zebrafish. We show that reduced C9orf72 function leads to motor defects, muscle atrophy, motor neuron loss and mortality in early larval and adult stages. Analysis of the structure and function of the neuromuscular junctions (NMJs) of the larvae, reveal a marked reduction in the number of presynaptic and postsynaptic structures and an impaired release of quantal synaptic vesicles at the NMJ. Strikingly, we demonstrate a downregulation of SV2a upon C9orf72-LOF and a reduced rate of synaptic vesicle cycling. Furthermore, we show a reduced number and size of Rab3a-postive synaptic puncta at NMJs. Altogether, these results reveal a key function for C9orf72 in the control of presynaptic vesicle trafficking and release at the zebrafish larval NMJ. Our study demonstrates an important role for C9orf72 in ALS/FTD pathogenesis, where it regulates synaptic vesicle release and neuromuscular functions.

Functional analysis of CASK transcript variants expressed in human brain.

The calcium-/calmodulin dependent serine protein kinase (CASK) belongs to the membrane-associated guanylate kinases (MAGUK) family of proteins. It fulfils several different cellular functions, ranging from acting as a scaffold protein to transcription control, as well as regulation of receptor sorting. CASK functions depend on the interaction with a variety of partners, for example neurexin, liprin-α, Tbr1 and SAP97. So far, it is uncertain how these seemingly unrelated interactions and resulting functions of CASK are regulated. Here, we show that alternative splicing of CASK can guide the binding affinity of CASK isoforms to distinct interaction partners. We report seven different variants of CASK expressed in the fetal human brain. Four out of these variants are not present in the NCBI GenBank database as known human variants. Functional analyses showed that alternative splicing affected the affinities of CASK variants for several of the tested interaction partners. Thus, we observed a clear correlation of the presence of one splice insert with poor binding of CASK to SAP97, supported by molecular modelling. The alternative splicing and distinct properties of CASK variants in terms of protein-protein interaction should be taken into consideration for future studies.

Characterization of agonist-dependent somatostatin receptor subtype 2 trafficking in neuroendocrine cells.

BACKGROUND: Somatostatin (SOM) receptor subtype 2 (SSTR2) is the major receptor subtype mediating SOM effects throughout the neuraxis. We previously demonstrated that the non-selective agonist [D-Trp8]-SOM induces intracellular sequestration of SSTR2, whereas this receptor is maintained at the cell surface after treatment with the SSTR2-selective agonist L-779,976 in cells co-expressing SSTR2 and SSTR5. METHODS AND RESULTS: In this study, we knocked-out SSTR5 in AtT20 cells endogenously expressing both SSTR2 and SSTR5 and used immuno-labeling and confocal microscopy to investigate the effect of SSTR5 on regulation of SSTR2 trafficking. Our results indicate that unlike [D-Trp8]-SOM-induced intracellular sequestration, L-779,976 stimulation results in the maintenance of SSTR2 at the cell surface regardless of whether SSTR5 is present or not. We then examined the trafficking pathways of SSTR2 upon stimulation by either agonist. We found that both [D-Trp8]-SOM and L-779,976 induce SSTR2 internalization via transferrin-positive vesicles. However, SSTR2 internalized upon L-779,976 treatment undergoes rapid recycling to the plasma membrane, whereas receptors internalized by [D-Trp8]-SOM recycle slowly after washout of the agonist. Furthermore, [D-Trp8]-SOM stimulation induces degradation of a fraction of internalized SSTR2 whereas L-779,976-dependent, rapid SSTR2 recycling appears to protect internalized SSTR2 from degradation. In addition, Octreotide which has preferential SSTR2 affinity, induced differential effects on both SSTR2 trafficking and degradation. CONCLUSION: Our results indicate that the biased agonistic property of L-779,976 protects against SSTR2 surface depletion by rapidly initiating SSTR2 recycling while SSTR5 does not regulate L-779-976-dependent SSTR2 trafficking.

Regulated resurfacing of a somatostatin receptor storage compartment fine-tunes pituitary secretion.

The surfacing of the glucose transporter GLUT4 driven by insulin receptor activation provides the prototypic example of a homeostasis response dependent on mobilization of an intracellular storage compartment. Here, we generalize this concept to a G protein-coupled receptor, somatostatin receptor subtype 2 (SSTR2), in pituitary cells. Following internalization in corticotropes, SSTR2 moves to a juxtanuclear syntaxin-6-positive compartment, where it remains until the corticotropes are stimulated with corticotropin releasing factor (CRF), whereupon SSTR2 exits the compartment on syntaxin-6-positive vesicular/tubular carriers that depend on Rab10 for their fusion with the plasma membrane. As SSTR2 activation antagonizes CRF-mediated hormone release, this storage/resurfacing mechanism may allow for a physiological homeostatic feedback system. In fact, we find that SSTR2 moves from an intracellular compartment to the cell surface in pituitary gland somatotropes, concomitant with increasing levels of serum growth hormone (GH) during natural GH cycles. Our data thus provide a mechanism by which signaling-mediated plasma membrane resurfacing of SSTR2 can fine-tune pituitary hormone release.