Phosphoinositides at the Neuromuscular Junction of Drosophila melanogaster: A Genetic Approach

De novo phosphoinositide synthesis in zebrafish is required for triad formation but not essential for myogenesis

Phosphoinositides (PIPs) and their regulatory enzymes are key players in many cellular processes and are required for aspects of vertebrate development. Dysregulated PIP metabolism has been implicated in several human diseases, including a subset of skeletal myopathies that feature structural defects in the triad. The role of PIPs in skeletal muscle formation, and particularly triad biogenesis, has yet to be determined. CDP-diacylglycerol-inositol 3-phosphatidyltransferase (CDIPT) catalyzes the formation of phosphatidylinositol, which is the base of all PIP species. Loss of CDIPT should, in theory, result in the failure to produce PIPs, and thus provide a strategy for establishing the requirement for PIPs during embryogenesis. In this study, we generated cdipt mutant zebrafish and determined the impact on skeletal myogenesis. Analysis of cdipt mutant muscle revealed no apparent global effect on early muscle development. However, small but significant defects were observed in triad size, with T-tubule area, inter terminal cisternae distance and gap width being smaller in cdipt mutants. This was associated with a decrease in motor performance. Overall, these data suggest that myogenesis in zebrafish does not require de novo PIP synthesis but does implicate a role for CDIPT in triad formation.

Molecular Remodeling of the Presynaptic Active Zone of Drosophila Photoreceptors via Activity-Dependent Feedback

Neural activity contributes to the regulation of the properties of synapses in sensory systems, allowing for adjustment to a changing environment. Little is known about how synaptic molecular components are regulated to achieve activity-dependent plasticity at central synapses. Here, we found that after prolonged exposure to natural ambient light the presynaptic active zone in Drosophila photoreceptors undergoes reversible remodeling, including loss of Bruchpilot, DLiprin-α, and DRBP, but not of DSyd-1 or Cacophony. The level of depolarization of the postsynaptic neurons is critical for the light-induced changes in active zone composition in the photoreceptors, indicating the existence of a feedback signal. In search of this signal, we have identified a crucial role of microtubule meshwork organization downstream of the divergent canonical Wnt pathway, potentially via Kinesin-3 Imac. These data reveal that active zone composition can be regulated in vivo and identify the underlying molecular machinery.

Synaptic PI(3,4,5)P3 is required for Syntaxin1A clustering and neurotransmitter release

PI(3,4,5)P3 is a low-abundance lipid thought to play a role in the regulation of synaptic activity; however, the mechanism remains obscure. We have constructed novel split Venus-based probes and used superresolution imaging to localize PI(3,4,5)P3 at Drosophila larval neuromuscular synapses. We find the lipid in membrane domains at neurotransmitter release sites, where it concentrates with Syntaxin1A, a protein essential for vesicle fusion. Reducing PI(3,4,5)P3 availability disperses Syntaxin1A clusters and increasing PI(3,4,5)P3 levels rescues this defect. In artificial giant unilamellar vesicles, PI(3,4,5)P3 also induces Syntaxin1A domain formation and this clustering, in vitro and in vivo, is dependent on positively charged residues in the Syntaxin1A-juxtamembrane domain. Functionally, reduced PI(3,4,5)P3 causes temperature-sensitive paralysis and reduced neurotransmitter release, a phenotype also seen in animals expressing a Syntaxin1A with a mutated juxtamembrane domain. Thus, our data indicate that PI(3,4,5)P3, based on electrostatic interactions, clusters Syntaxin1A at release sites to regulate neurotransmitter release.