Kismet/CHD7/CHD8 and Amyloid Precursor Protein-like Regulate Synaptic Levels of Rab11 at the Drosophila Neuromuscular Junction.

The transmembrane protein ß-amyloid precursor protein (APP) is central to the pathophysiology of Alzheimer's disease (AD). The ß-amyloid hypothesis posits that aberrant processing of APP forms neurotoxic ß-amyloid aggregates, which lead to the cognitive impairments observed in AD. Although numerous additional factors contribute to AD, there is a need to better understand the synaptic function of APP. We have found that Drosophila APP-like (APPL) has both shared and non-shared roles at the synapse with Kismet (Kis), a chromatin helicase binding domain (CHD) protein. Kis is the homolog of CHD7 and CHD8, both of which are implicated in neurodevelopmental disorders including CHARGE Syndrome and autism spectrum disorders, respectively. Loss of function mutations in kis and animals expressing human APP and BACE in their central nervous system show reductions in the glutamate receptor subunit, GluRIIC, the GTPase Rab11, and the bone morphogenetic protein (BMP), pMad, at the Drosophila larval neuromuscular junction (NMJ). Similarly, processes like endocytosis, larval locomotion, and neurotransmission are deficient in these animals. Our pharmacological and epistasis experiments indicate that there is a functional relationship between Kis and APPL, but Kis does not regulate appl expression at the larval NMJ. Instead, Kis likely influences the synaptic localization of APPL, possibly by promoting rab11 transcription. These data identify a potential mechanistic connection between chromatin remodeling proteins and aberrant synaptic function in AD.

The CHD Protein Kismet Restricts the Synaptic Localization of Cell Adhesion Molecules at the Drosophila Neuromuscular Junction.

The appropriate expression and localization of cell surface cell adhesion molecules must be tightly regulated for optimal synaptic growth and function. How neuronal plasma membrane proteins, including cell adhesion molecules, cycle between early endosomes and the plasma membrane is poorly understood. Here we show that the Drosophila homolog of the chromatin remodeling enzymes CHD7 and CHD8, Kismet, represses the synaptic levels of several cell adhesion molecules. Neuroligins 1 and 3 and the integrins αPS2 and ßPS are increased at kismet mutant synapses but Kismet only directly regulates transcription of neuroligin 2. Kismet may therefore regulate synaptic CAMs indirectly by activating transcription of gene products that promote intracellular vesicle trafficking including endophilin B (endoB) and/or rab11. Knock down of EndoB in all tissues or neurons increases synaptic FasII while knock down of EndoB in kis mutants does not produce an additive increase in FasII. In contrast, neuronal expression of Rab11, which is deficient in kis mutants, leads to a further increase in synaptic FasII in kis mutants. These data support the hypothesis that Kis influences the synaptic localization of FasII by promoting intracellular vesicle trafficking through the early endosome.

The CD63 homologs, Tsp42Ee and Tsp42Eg, restrict endocytosis and promote neurotransmission through differential regulation of synaptic vesicle pools.

The Tetraspanin (Tsp), CD63, is a transmembrane component of late endosomes and facilitates vesicular trafficking through endosomal pathways. Despite being widely expressed in the human brain and localized to late endosomes, CD63's role in regulating endo- and exocytic cycling at the synapse has not been investigated. Synaptic vesicle pools are highly dynamic and disruptions in the mobilization and replenishment of these vesicle pools have adverse neuronal effects. We find that the CD63 homologs, Tsp42Ee and Tsp42Eg, are expressed at the Drosophila neuromuscular junction to regulate synaptic vesicle pools through both shared and unique mechanisms. Tsp42Ee and Tsp42Eg negatively regulate endocytosis and positively regulate neurotransmitter release. Both tsp mutants show impaired locomotion, reduced miniature endplate junctional current frequencies, and increased endocytosis. Expression of human CD63 in Drosophila neurons leads to impaired endocytosis suggesting the role of Tsps in endocytosis is conserved. We further show that Tsps influence the synaptic cytoskeleton and membrane composition by regulating Futsch loop formation and synaptic levels of SCAR and PI(4,5)P2. Finally, Tsp42Ee and Tsp42Eg influence the synaptic localization of several vesicle-associated proteins including Synapsin, Synaptotagmin, and Cysteine String Protein. Together, our results present a novel function for Tsps in the regulation of vesicle pools and provide insight into the molecular mechanisms of Tsp-related synaptic dysfunction.