Structural basis of lipid transfer by a bridge-like lipid-transfer protein
Kang Y, Lehmann K, Vanegas J, Long H, Jefferson A, Freeman M, Clark S
Abstract
Bridge-like lipid-transport proteins (BLTPs) are an evolutionarily conserved family of proteins that localize to membrane-contact sites and are thought to mediate the bulk transfer of lipids from a donor membrane, typically the endoplasmic reticulum, to an acceptor membrane, such as that of the cell or an organelle1. Although BLTPs are fundamentally important for a wide array of cellular functions, their architecture, composition and lipid-transfer mechanisms remain poorly characterized. Here we present the subunit composition and the cryogenic electron microscopy structure of the native LPD-3 BLTP complex isolated from transgenic Caenorhabditis elegans. LPD-3 folds into an elongated, rod-shaped tunnel of which the interior is filled with ordered lipid molecules that are coordinated by a track of ionizable residues that line one side of the tunnel. LPD-3 forms a complex with two previously uncharacterized proteins, one of which we have named Spigot and the other of which remains unnamed. Spigot interacts with the N-terminal end of LPD-3 where lipids are expected to enter the tunnel, and experiments in multiple model systems indicate that Spigot has a conserved role in BLTP function. Our LPD-3 complex structural data reveal protein-lipid interactions that suggest a model for how the native LPD-3 complex mediates bulk lipid transport and provides a foundation for mechanistic studies of BLTPs.
Tweek-dependent formation of ER-PM contact sites enables astrocyte phagocytic function and remodeling of neurons
Kang Y, JeffersonA, SheehanA, De La Torre R,JayT, ChiaoL, HulegaardA, CortyM, BaconguisI, ZhouZ, FreemanM
Abstract
Neuronal remodeling generates an enormous amount of cellular debris, which is cleared from the nervous system by glia. At the larva-to-adult transition, Drosophila astrocytes transform into phagocytes and engulf degenerating larval synapses, axonal and dendritic debris. Here we show Tweek, a member of the bridge-like lipid transfer protein family, is upregulated in astrocytes as they ramp up their phagocytic function early in metamorphosis, and is essential for internalization and degradation of neuronal debris. Tweek forms a bridge between the endoplasmic reticulum (ER) and plasma membrane (PM), and loss of Tweek disrupts ER-PM contact formation and membrane lipid distribution. Patient-identified mutations in the human homolog associated with Alkuraya-Kucinskas syndrome resulted in similar defects in neuronal remodeling, indicating these are loss of function mutations. We propose Tweek helps establish and maintain ER-PM contacts during astrocyte phagocytic function and drives bulk lipid transfer to the plasma membrane for continued efficient internalization and degradation of neuronal debris.
Astrocyte-dependent local neurite Beat-Va neurons
Lehmann K, Hupp M, Abalde-Atristain L, Cheng Y, Jefferson A, Sheehan A, Kang Y**, Freeman M**
** Co-corresponding author
Abstract
Developmental neuronal remodeling is extensive and mechanistically diverse across the nervous system. We sought to identify Drosophila pupal neurons that underwent mechanistically new types of neuronal remodeling and describe remodeling Beat-VaM and Beat-VaL neurons. We show that Beat-VaM neurons produce highly branched neurites in the CNS during larval stages that undergo extensive local pruning. Surprisingly, although the ecdysone receptor (EcR) is essential for pruning in all other cell types studied, Beat-VaM neurons remodel their branches extensively despite cell autonomous blockade EcR or caspase signaling. Proper execution of local remodeling in Beat-VaM neurons instead depends on extrinsic signaling from astrocytes converging with intrinsic and less dominant EcR-regulated mechanisms. In contrast, Beat-VaL neurons undergo steroid hormone-dependent, apoptotic cell death, which we show relies on the segment-specific expression of the Hox gene Abd-B. Our work provides new cell types in which to study neuronal remodeling, highlights an important role for astrocytes in activating local pruning in Drosophila independent of steroid signaling, and defines a Hox gene-mediated mechanism for segment-specific cell elimination.
A protein tunnel that shuttles lipids around the cell
Kang Y, Clark S.
An in vivo genetic screen identifies Crq as a potent glial regulator of synapse elimination in development and aging
Taylor J, Kang Y, Ouellet-Massicotte V, Kristine Micael M, Lacuros-Perkins V, Chen J, Sheehan A, Freeman M
Astrocyte growth during morphogenesis is driven by the Tre1/S1pr1 phospholipid-binding G protein-coupled receptor
Chen J, Stork T, Kang Y, Sheehan A, Paton C, Monk K, & Freeman M
Large-scale growth of C. elegans and isolation of membrane protein complexes
Clark S, Jeong H, Goehring A, Kang Y, Gouaux E
An ELISA-based method for rapid genetic screens in Drosophila
Jay T*, Kang Y*, Jefferson A, Freeman M
* Contributed equally
Injury-Induced Inhibition of Bystander Neurons Requires dSarm and Signaling from Glia
Hsu J, Kang Y, Corty M, Mathieson D, Peters O, Freeman M