Acsl, the Drosophila ortholog of intellectual-disability-related ACSL4, inhibits synaptic growth by altered lipids

Abstract

Nervous system development and function are tightly regulated by metabolic processes, including the metabolism of lipids such as fatty acids. Mutations in long-chain acyl-CoA synthetase 4 (ACSL4) are associated with non-syndromic intellectual disabilities. We previously reported that Acsl, the Drosophila ortholog of mammalian ACSL3 and ACSL4, inhibits neuromuscular synapse growth by suppressing bone morphogenetic protein (BMP) signaling. Here, we report that Acsl regulates the composition of fatty acids and membrane lipids, which in turn affects neuromuscular junction (NMJ) synapse development. Acsl mutant brains had a decreased abundance of C16:1 fatty acyls; restoration of Acsl expression abrogated NMJ overgrowth and the increase in BMP signaling. A lipidomic analysis revealed that Acsl suppressed the levels of three lipid raft components in the brain, including mannosyl glucosylceramide (MacCer), phosphoethanolamine ceramide and ergosterol. The MacCer level was elevated in Acsl mutant NMJs and, along with sterol, promoted NMJ overgrowth, but was not associated with the increase in BMP signaling in the mutants. These findings suggest that Acsl inhibits NMJ growth by stimulating C16:1 fatty acyl production and concomitantly suppressing raft-associated lipid levels.

The glycosphingolipid MacCer promotes synaptic bouton formation in Drosophila by interacting with Wnt

“…It has been previously shown that the enzymatic activity of Egh is reduced in egh 62d18 and egh 7 mutants ( Wandall et al, 2005 ). Here we show that both homozygous egh 62d18 and hetero-allelic egh 62d18 / egh 7 mutants also displayed fewer and larger synaptic boutons ( Figure 2,D,O and Figure 2—figure supplement 1 ; Huang et al, 2016 ). In contrast, neuronal (presynaptic) overexpression of Egh driven by the pan-neuronal nSyb-Gal4 or motor neuronal specific OK6-Gal4 led to synaptic overgrowth with more and smaller boutons ( Figure 2, D–H and O and Figure 2—figure supplement 1 ), indicating that Egh promotes bouton formation.…”

“…We observed more boutons and satellite boutons in homozygous brn fs.107 and trans-allelic brn 1.6P6 / brn fs.107 mutants; neuronal knockdown of brn by an RNAi under the control of nSyb-Gal4 or OK6-Gal4 also resulted in NMJ overgrowth. Conversely, neuronal overexpression of Brn driven by nSyb-Gal4 or OK6-Gal4 led to fewer and larger boutons ( Figure 2, I–M and P ; Huang et al, 2016 ), recapitulating the phenotype of egh mutants. Furthermore, overexpression and knockdown of brn in muscles by C57-Gal4 or in glia by Repo-Gal4 did not affect NMJ growth ( Figure 2—figure supplement 2 ).…”

“…Nevertheless, the molecular and cellular functions of GSLs in development are poorly understood, partially due to the complexity of GSL metabolism and the variety of GSL structures in vertebrates. Because of the comparatively simple metabolic pathways and the power of genetic studies in invertebrates ( Bellen and Yamamoto, 2015 ; Zhu and Han, 2014 ), a mechanistic understanding of the role of GSLs in development and function of the nervous system is beginning to be established ( Dahlgaard et al, 2012 ; Huang et al, 2016 ; Kniazeva et al, 2015 ; Yonamine et al, 2011 ).…”

“…Specifically, GSLs interact with raft-associated signaling proteins, such as epidermal growth factor receptor (EGFR) and Notch, thereby facilitating signal transduction ( Coskun et al, 2011 ; Hamel et al, 2010 ; Wang and Yu, 2013 ). Our previous study uncovered that the GSL mannosyl glucosylceramide (MacCer) promotes NMJ overgrowth ( Huang et al, 2016 ). However, the mechanisms by which GSLs mediate in vivo neural development remain elusive.…”

Target‐dependent retrograde signaling mediates synaptic plasticity at theDrosophilaneuromuscular junction

“…Synaptic pMad is likely part of a complex system involving receptor dynamics and endosomal processing/shuttling, that ‘tags’ axonal branches for growth, maturation, and plasticity. The molecular mechanisms known to regulate local BMP signaling in presynaptic boutons are diverse: the dynamics of nuclear importins, interactions with other trans‐synaptic and cytoplasmic signaling systems, recycling of BMP receptors through endocytic and endosomal regulation, RNA binding protein activity, phospholipid composition, and cytoskeletal regulation (Banerjee & Riordan, ; Banerjee, Venkatesan, & Bhat, ; Deshpande et al, ; Eaton & Davis, ; Fuentes‐Medel et al, ; Heo et al, ; Higashi‐Kovtun et al, ; Huang et al, ; Kim et al, ; Li et al, ; Liu et al, ; Merino et al, ; Piccioli & Littleton, ; Politano et al, ; Sulkowski et al, ; Summerville et al, ; Vanlandingham et al, ; Zhang et al, ; Zhao et al, ). Local, retrograde effects are also possible through muscle‐specific regulation of glutamate receptor composition and Gbb production/release (this study; also see Halstead et al, ; Sulkowski et al, ).…”