The effects of ER morphology on synaptic structure and function in Drosophila melanogaster.

Hereditary spastic paraplegia (HSP) is a set of genetic diseases caused by mutations in one of 72 genes that results in age-dependent corticospinal axon degeneration accompanied by spasticity and paralysis. Two genes implicated in HSPs encode proteins that regulate endoplasmic reticulum (ER) morphology. Atlastin 1 (ATL1, also known as SPG3A) encodes an ER membrane fusion GTPase and reticulon 2 (RTN2, also known as SPG12) helps shape ER tube formation. Here, we use a new fluorescent ER marker to show that the ER within wild-type Drosophila motor nerve terminals forms a network of tubules that is fragmented and made diffuse upon loss of the atlastin 1 ortholog atl. atl or Rtnl1 loss decreases evoked transmitter release and increases arborization. Similar to other HSP proteins, Atl inhibits bone morphogenetic protein (BMP) signaling, and loss of atl causes age-dependent locomotor deficits in adults. These results demonstrate a crucial role for ER in neuronal function, and identify mechanistic links between ER morphology, neuronal function, BMP signaling and adult behavior.

Peroxisomes are required for lipid metabolism and muscle function in Drosophila melanogaster.

Peroxisomes are ubiquitous organelles that perform lipid and reactive oxygen species metabolism. Defects in peroxisome biogenesis cause peroxisome biogenesis disorders (PBDs). The most severe PBD, Zellweger syndrome, is characterized in part by neuronal dysfunction, craniofacial malformations, and low muscle tone (hypotonia). These devastating diseases lack effective therapies and the development of animal models may reveal new drug targets. We have generated Drosophila mutants with impaired peroxisome biogenesis by disrupting the early peroxin gene pex3, which participates in budding of pre-peroxisomes from the ER and peroxisomal membrane protein localization. pex3 deletion mutants lack detectible peroxisomes and die before or during pupariation. At earlier stages of development, larvae lacking Pex3 display reduced size and impaired lipid metabolism. Selective loss of peroxisomes in muscles impairs muscle function and results in flightless animals. Although, hypotonia in PBD patients is thought to be a secondary effect of neuronal dysfunction, our results suggest that peroxisome loss directly affects muscle physiology, possibly by disrupting energy metabolism. Understanding the role of peroxisomes in Drosophila physiology, specifically in muscle cells may reveal novel aspects of PBD etiology.

The metabotropic glutamate receptor activates the lipid kinase PI3K in Drosophila motor neurons through the calcium/calmodulin-dependent protein kinase II and the nonreceptor tyrosine protein kinase DFak.

Ligand activation of the metabotropic glutamate receptor (mGluR) activates the lipid kinase PI3K in both the mammalian central nervous system and Drosophila motor nerve terminal. In several subregions of the mammalian brain, mGluR-mediated PI3K activation is essential for a form of synaptic plasticity termed long-term depression (LTD), which is implicated in neurological diseases such as fragile X and autism. In Drosophila larval motor neurons, ligand activation of DmGluRA, the sole Drosophila mGluR, similarly mediates a PI3K-dependent downregulation of neuronal activity. The mechanism by which mGluR activates PI3K remains incompletely understood in either mammals or Drosophila. Here we identify CaMKII and the nonreceptor tyrosine kinase DFak as critical intermediates in the DmGluRA-dependent activation of PI3K at Drosophila motor nerve terminals. We find that transgene-induced CaMKII inhibition or the DFak(CG1) null mutation each block the ability of glutamate application to activate PI3K in larval motor nerve terminals, whereas transgene-induced CaMKII activation increases PI3K activity in motor nerve terminals in a DFak-dependent manner, even in the absence of glutamate application. We also find that CaMKII activation induces other PI3K-dependent effects, such as increased motor axon diameter and increased synapse number at the larval neuromuscular junction. CaMKII, but not PI3K, requires DFak activity for these increases. We conclude that the activation of PI3K by DmGluRA is mediated by CaMKII and DFak.