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

Abstract

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.

Beneficial effects of rapamycin in a Drosophila model for hereditary spastic paraplegia

“…It has been previously shown that muscle Tor is activated in Drosophila larvae when synaptic transmission is attenuated by deleting one of two alternate subunits of the muscle glutamate receptor GluRII (Penney et al, 2012). We have recently shown that neuronal atl loss decreases evoked transmitter release from motor neurons (Summerville et al, 2016), suggesting that this decreased synaptic transmission is responsible for the muscle Tor activation. In this perspective, our observation that the locomotor and viability deficits are accelerated by expressing the TrpA1 channel raises the possibility that increasing the frequency of this sub-normal synaptic transmission further increases muscle Tor activation.…”

“…It is unknown how muscle Atl might regulate Tor activity in a cell-autonomous manner. However, as an ER fusion protein, Atl controls ER morphology, and atl loss disrupts ER morphology in multiple cell types (Orso et al, 2009;Summerville et al, 2016). Disruption of ER morphology within the muscle could alter localization and trafficking through the endocytic system of Tor-activating receptors such as the insulin receptor.…”

“…We have previously reported that pan-neuronal, RNAi-mediated atl knockdown (elav>atl-RNAi) causes agedependent locomotor deficits in adult males (Summerville et al, 2016). Following on from this, we determined that these progressive locomotor deficits, manifested initially as paralysis and then ultimately as death, were accelerated when males were aged in vials containing single females (Fig.…”

“…Because pan-neuronal expression of atl + causes embryonic lethality (Summerville et al, 2016), we introduced an atl + transgene with codons deoptimized for translation in Drosophila. We found that expression of this deoptimized atl + transgene, but not of the neutral transgene GFP, significantly delayed paralysis (P<0.001, Fig.…”

“…It is possible that other HSP genes might behave in a similar way to atlastin. In Drosophila, orthologs of several HSP genes, including Reticulon (Rtnl1, SPG12), spastin (SPG4), spartin (SPG20) and spichthyin (SPG6) have been identified and analyzed previously (Lee et al, 2008;Nahm et al, 2013;O'Sullivan et al, 2012;Sherwood et al, 2004;Summerville et al, 2016;Wang et al, 2007). These studies have demonstrated that mutations or RNAi knockdown of any of these genes confer a remarkably uniform set of behavioral and nervous system phenotypes, including progressive age-dependent locomotor deficits, decreased evoked neurotransmitter release from larval motor nerve terminals, increases in retrograde (muscle to neuron) signaling mediated by a ligand of the BMP family and increases in the number of synaptic boutons at the larval neuromuscular junction (note that not all phenotypes have been reported for all genes listed).…”