Tweek-dependent formation of ER-PM contact sites enables astrocyte phagocytic function and remodeling of neurons

Abstract

SummaryNeuronal remodeling generates an enormous amount of cellular debris, which is cleared from the nervous system by glia. At the larva-to-adult transition,Drosophilaastrocytes 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 pruning and Hox gene-mediated cell death in Beat-Va neurons

“…Beat-Va M neurite fragmentation and clearance occurs coordinately with the EcR-dependent transformation of larval astrocytes into phagocytic glia (Tasdemir-Yilmaz and Freeman 2014, Kang 2023). To determine what role astrocytes might play in driving Beat-Va M neuron remodeling, we generated a LexAop-EcR DN line, which we could drive with alrm-LexA to selectively block the transformation of astrocytes into phagocytes while allowing all other EcR-mediated neuronal signaling events to proceed normally.…”

“… A) Astrocytes labeled with GMR25H07-Gal4 (a strong astrocyte Gal4 line) crossed to CD8::GFP at wL3 showing their “wispy” morphology, B) 4APF showing the phagolysomes that astrocytes display (Kang 2023), and C) HE where astrocytes are normally only faintly detectable. D) Astrocytes expressing a UAS-EcR DN , labeled with mCD8::GFP at wL3 showing their “wispy” morphology, which is retained at E) 4APF, and F) HE, indicating a failure to transform.…”

Structural basis of bulk lipid transfer by bridge-like lipid transfer protein LPD-3

“…BLTP1 family members are found in all eukaryotes and have been demonstrated to localize to ER-plasma membrane (PM) contact sites in both yeast and worms 11,12 . Multiple lines of evidence support the notion that BLTP1 and its orthologs function as molecular bridges for lipid transport at ER-PM contact sites: in yeast, the BLTP1ortholog (Csf1) is important for growth at low temperatures and homeoviscous membrane adaptation 11,13,14 ; in Drosophila, the BLTP1 ortholog (Tweek) is required for development, synaptic vesicle recycling 15 , astrocyte-mediated phagocytosis of neuronal debris 16 ; and in C. elegans, the BLTP1 (which was not certified by peer review) is the author/funder. All rights reserved.…”

“…To investigate the possibility that these proteins functionally interact with LPD-3 complex in vivo (e.g. transiently or by binding to a subset), we assayed the ability of astrocytes to phagocytose neuronal debris during developmental neuronal remodeling, a recently discovered role for Drosophila Tweek (Tweek is the LPD-3 ortholog) 16 . We found that astrocyte-specific RNAi knockdown of 9 of the 12 additional proteins that co-purified with the LPD-3 complex resulted in significant defects in astrocyte phagocytic function (Extended Data Figure 3).…”

“…The copyright holder for this preprint this version posted June 22, 2024. ; https://doi.org/10.1101/2024.06. 21.600134 doi: bioRxiv preprint ortholog (LPD-3) is critical for animal development, efficient phagocytosis, and adaptation to cold stress 12,16 .…”