Extended synaptotagmin regulates membrane contact site structure and lipid transfer function in vivo

Abstract

Inter‐organelle communication between closely apposed membranes is proposed at membrane contact sites (MCS). However, the regulation of MCS structure and their functional relevance in vivo remain debated. The extended synaptotagmins (Esyt) are evolutionarily conserved proteins proposed to function at MCS. However, loss of all three Esyts in yeast or mammals shows minimal phenotypes questioning the functional importance of Esyt. We report that in Drosophila photoreceptors, MCS number is regulated by PLCβ activity. Photoreceptors of a null allele of Drosophila extended synaptotagmin (dEsyt) show loss of ER‐PM MCS. Loss of dEsyt results in mislocalization of RDGB, an MCS localized lipid transfer protein, required for photoreceptor structure and function, ultimately leading to retinal degeneration. dEsyt depletion enhanced the retinal degeneration, reduced light responses and slower rates of plasma membrane PIP2 resynthesis seen in rdgB mutants. Thus, dEsyt function and PLCβ signaling regulate ER‐PM MCS structure and lipid transfer in Drosophila photoreceptors.

ER-PM Contact Sites – SNARING Actors in Emerging Functions

“…In accordance with this role, Sac1 and E-Syt2 were shown to co-localise while E-Syt2 knockdown significantly reduced Sac1 puncta at MCSs coinciding with accumulation of PI4P and PIP 2 ( Dickson et al, 2016 ). A role of E-Syt2 in PM PI regulation fits well with the observation that E-Syts regulates growth as shown in Drosophila ( Nath et al, 2020 ) and mammalian neurons ( Gallo et al, 2020 ) because of the role of PIP 2 in PM dynamics ( Scholze et al, 2018 ). Furthermore, E-Syts were found to interact with FGFR1 ( Tremblay et al, 2015 ), a receptor that regulates PM PIP 2 ( Herdman and Moss, 2016 ).…”

“…Additionally, hetero- or homodimerisation of SMP domains forms a hydrophobic channel that can facilitate lipid transfer in vitro ( Schauder et al, 2014 ; Yu et al, 2016 ) ( Figures 1 , 2 ). Finally, the single E-Syt orthologue in Drosophila , catalysing lipid transfer in photoreceptors at the submicrovillar cisternae and the growth of the organism ( Nath et al, 2020 ), was also suggested to modulate synaptogenesis ( Kikuma et al, 2017 ). E-Syts have since been identified in mammalian neuronal cells but, while it is plausible that they can perform these functions ( Giordano et al, 2013 ; Fernández-Busnadiego et al, 2015 ) their physiological significance in neurons is so far unclear and may only be noticeable under specific conditions.…”

Endoplasmic Reticulum–Plasma Membrane Contact Sites: Regulators, Mechanisms, and Physiological Functions

“…However, E-Syts and tricalbins do have physiological effects. For example, E-Syts can maintain PM lipid homeostasis, promote nerve transmission and synaptic growth, modulate virus-induced membrane fusion, mediate the endocytosis of FGFR, and play roles in insulin secretion and diet-induced obesity development (Saheki et al, 2016 ; Tremblay et al, 2016 ; Kikuma et al, 2017 ; El Kasmi et al, 2018 ; Xie et al, 2019 ; Nath et al, 2020 ; Zhang et al, 2020 ). Tricalbins act in maintaining PM integrity (Toulmay and Prinz, 2012 ; Collado et al, 2019 ).…”

Tricalbin proteins regulate plasma membrane phospholipid homeostasis

“…Intriguingly, none of the strains tested showed significant changes in steady-state levels of any PI species ( Figure S2B ). Previous studies have implicated E-Syt family members in recycling diacylglycerol (DAG) from the PM to the ER and in the transfer of PI from the ER to the PM during the phosphoinositide cycle (Chang et al, 2013; Saheki et al, 2016; Nath et al, 2020). However, whilst PI homeostasis and PI4P synthesis at the PM apparently do not require the Tcb proteins, conversion of PI4P to PI(4,5)P 2 at the PM is impaired.…”