Glial metabolism versatility regulates mushroom body-driven behavioral output in Drosophila

Abstract

Providing metabolic support to neurons is now recognized as a major function of glial cells that is conserved from invertebrates to vertebrates. However, research in this field has focused for more than two decades on the relevance of lactate and glial glycolysis for neuronal energy metabolism, while overlooking many other facets of glial metabolism and their impact on neuronal physiology, circuit activity, and behavior. Here, we review recent work that has unveiled new features of glial metabolism, especially in Drosophila, in the modulation of behavioral traits involving the mushroom bodies (MBs). These recent findings reveal that spatially and biochemically distinct modes of glucose-derived neuronal fueling are implemented within the MB in a memory type-specific manner. In addition, cortex glia are endowed with several antioxidant functions, whereas astrocytes can serve as pro-oxidant agents that are beneficial to redox signaling underlying long-term memory. Finally, glial fatty acid oxidation seems to play a dual fail-safe role: first, as a mode of energy production upon glucose shortage, and, second, as a factor underlying the clearance of excessive oxidative load during sleep. Altogether, these integrated studies performed in Drosophila indicate that glial metabolism has a deterministic role on behavior.

Figure 1.

Anatomy and localization of the five types of glial cells in the Drosophila central brain illustrated on the mushroom body (MB). The MB is a bilaterally symmetrical structure in the adult Drosophila brain. The cell bodies of the intrinsic MB neurons, called Kenyon cells (KCs), cluster on the posterior part of the brain. The axons project toward the anterior part of the brain and ramify dorsally to form the vertical α and α′ lobes, and medially to form the horizontal β, β′, and γ lobes (Tanaka et al. 2008). Five main types of adult glia are shown. Together, the perineural and subperineural glia form the surface glia. The cortex glia surround the neuronal cell bodies. Ensheathing glia surround the neuropil and separate the distinct MB lobes. Finally, astrocyte-like glia (ALG) branch into the axons and are associated with the synapses (Awasaki et al. 2008; Freeman 2015; Ou et al. 2016; Kremer et al. 2017).

Figure 2.

Different memory phases involve distinct modes of metabolic recruitment of glial cells by Kenyon cells (KCs). Similar to Figure 1, cortex glia are shown in yellow, astrocyte-like glia (ALG) are depicted in red, and KCs are in blue. (A) A single cycle of associative aversive conditioning leads to the formation of middle-term memory (MTM). The cortex glia import glucose via glug transporters; glucose is then glycolyzed into pyruvate, which in turn is converted into l-alanine by the alanine transaminase (ALAT) enzyme. This l-alanine is transported into the mushroom body (MB) neurons at the level of the soma, where it is reconverted into pyruvate. It then enters the mitochondria through the Mpc1 transporter and is broken down into acetyl-CoA (ACoA) by pyruvate dehydrogenase (PDH), which fuels the mitochondrial tricarboxylic acid (TCA) cycle to produce energy for MTM formation through oxidative phosphorylation (Rabah et al. 2023a). (B) Spaced training drives energy-expensive long-term memory (LTM) consolidation (Plaçais et al. 2017). ALAT-mediated conversion of pyruvate into l-alanine occurs in the ALG surrounding the local KC axons, where it is involved in up-regulating neuronal mitochondrial oxidative phosphorylation (Rabah et al. 2023a). Spaced training also triggers cholinergic excitation of cortex glia via the action of acetylcholine (ACh) on α7 nicotinic receptor (nAChRα7), which induces the release of insulin-like peptide 4 (Ilp4) and the autocrine stimulation of the insulin receptor (InR) in the cortex glia. This results in increased glucose uptake through nebu, a separate type of cortex glia glucose transporter, which is shuttled into the neurons via their Glut1 transporters. This glucose fuels the pentose phosphate pathway (PPP) to produce molecules required for gene expressions, such as ribulose-6-phosphate (used in nucleotide biosynthesis) and NADPH (a reducing agent) (De Tredern et al. 2021). At the axonal level, cholinergic activation of astrocytic nAChRα7 results in a Ca²⁺ surge; along with NADPH sourced from astrocytic PPP, this activates the NADPH oxidase (NOX) enzyme, finally producing O₂°⁻. This O₂°⁻ is converted into H₂O₂ by extracellular superoxide dismutase 3 (SOD3) expressed by ALG. H₂O₂ is imported into the α/β lobes via aquaporin (AQP) and drives the oxidation of proteins involved in a redox signaling cascade, resulting in LTM formation. NADPH produced at the cortex glia level travels to the axons and also participates in protein reduction (Rabah et al. 2023b). (C) Starvation induces the formation of a unique type of consolidated memory called ketone body (KB)-dependent aversive memory (K-AM) (Silva et al. 2022). Lipid droplet stores of cortex glia are mobilized into fatty acids (FAs) by Brummer (Bmm) lipase activity. FAs are imported into mitochondria via the CPT1 transporter, where they undergo β-oxidation into ACoA, which is subsequently catalyzed by the HMGS enzyme to produce KBs. These KBs are exported from cortex glia through the Chaski transporters and imported into MB neurons via the Silnoon transporters. They then enter the neuronal mitochondria where they are converted by the ACAT1 enzyme into ACoA, which enters the TCA cycle.