Chaski, a novel Drosophila lactate/pyruvate transporter required in glia cells for survival under nutritional stress

Abstract

The intercellular transport of lactate is crucial for the astrocyte-to-neuron lactate shuttle (ANLS), a model of brain energetics according to which neurons are fueled by astrocytic lactate. In this study we show that the Drosophila chaski gene encodes a monocarboxylate transporter protein (MCT/SLC16A) which functions as a lactate/pyruvate transporter, as demonstrated by heterologous expression in mammalian cell culture using a genetically encoded FRET nanosensor. chaski expression is prominent in the Drosophila central nervous system and it is particularly enriched in glia over neurons. chaski mutants exhibit defects in a high energy demanding process such as synaptic transmission, as well as in locomotion and survival under nutritional stress. Remarkably, locomotion and survival under nutritional stress defects are restored by chaski expression in glia cells. Our findings are consistent with a major role for intercellular lactate shuttling in the brain metabolism of Drosophila.The function of the nervous system requires a large supply of energy, as exemplified by the fact that in the mammalian brain, 50-60% of total ATP produced in the brain is used to support ion transport 1 ; in flies, retinal photoreceptor cells consume about 10% of the ATP production of the whole animal 2 . Our current view about the field of brain energetics has evolved from a one centered in neurons into a one in which astrocytes and neurons play complementary roles to support the high demand of excitability and synaptic activity. In this view, the metabolic communication between neurons and glia is crucial to sustain brain function, relevating the need to unravel the mechanisms that underly this communication.Within cells, several glucose-derived metabolic intermediates can subsequently be oxidized for energy production (i.e. lactate, pyruvate, glutamate, or acetate) 3 , while ketone bodies are mainly used during development and starvation 4,5 . In mammals, it has been estimated that over 10% of glucose entering the brain is converted to lactate despite normal oxygen levels, a metabolic process known as aerobic glycolysis 6 . Lactate production through aerobic glycolysis is a metabolic feature of astrocytes 7,8 . The high-energy demand as the result of glutamatergic synaptic activity is thought to stimulate aerobic glycolysis in astrocytes 9 producing lactate that is secreted and used by neurons as energy source. This metabolic interaction has been termed the astrocyte-neuron lactate shuttle (ANLS) hypothesis 9,10 . Recent evidence from invertebrates supports that metabolic compartmentalization and coupling of neurons and glial cells is a conserved, fundamental feature of bilaterian nervous systems independent of their size 11 . Moreover, the lack of an apparent detrimental effect of glycolytic enzyme deletion in Drosophila neurons suggests that insects may have evolved an extreme version of ANLS, in which neurons would be fueled by lactate and/or alanine produced by glial cells 11 . In vertebrates, lactate is co-transported with proton...

Neuronal lactate levels depend on glia‐derived lactate during high brain activity in Drosophila

“…Solutes can then be transported to internal glial cells such as cortex‐glia (CG) that surround neuronal bodies, AG surrounding the neuropile and ensheathing glia (EG) covering axons (Figure a). Unpublished data from our group indicates that all glial cells express several genes predicted to be d MCTs, including chaski (Delgado et al, ). Thus, given the diversity of glial cell subtypes, we sought to determine if other types of glia also show lactate transport.…”

“…Previously we characterized Chaski (Chk) as a d MCT with a role in synaptic activity at the NMJ of larvae as well as in locomotion and in the adaptive response to nutritional restriction in the adult Drosophila (Delgado et al, ) Unpublished data from our laboratory shows that chk mutants are not able to survive mild starvation or stressful temperature during the larval stage, suggesting a more important role than previously thought. To determine the role of Chaski in this ex vivo system, wild type (W 1118 ) or chk mutants ( chk CRISPR ) brains expressing Laconic in glial cells ( repo > Laconic ) or motor neurons ( C380 > Laconic ) were exposed to increasing lactate concentrations.…”

“…One explanation for these results is that pyruvate is first converted into lactate within glial cells, and then shuttled into neurons. To test this hypothesis, we repeated the trans‐acceleration experiment using the non‐metabolized pyruvate analog oxamate (Delgado et al, ). Oxamate, in addition to not being metabolized by mitochondria, inhibits the LDH enzyme blocking the conversion of pyruvate to lactate.…”

“…Recent evidence has shown the existence of a metabolic coupling between glial cells and neurons in the nervous system of Drosophila , where the production and transport of lactate and pyruvate play important roles in synaptic activity, energy production and adaptation to nutritional stress (Delgado et al, ). However, the mechanisms and characteristics that underlie the transport of monocarboxylates in vivo have not been fully addressed.…”

“…The gene CG8271, called silnoon ( sln ), was identified as a d MCT and shown to be involved in apoptosis induced by extracellular butyrate (Jang, Lee, & Chung, ). Our group identified Chaski (CG3409) as a d MCT for lactate/pyruvate (Delgado et al, ). In the brain, chaski plays an important role in synaptic activity, locomotion, and in the adaptive response to nutritional restriction; the latter being rescued by the expression of chaski in glial cells.…”