The astrocyte-neuron lactate shuttle hypothesis posits that glial-generated lactate is transported to neurons to fuel metabolic processes required for long-term memory. Although studies in vertebrates have revealed that lactate shuttling is important for cognitive function, it is uncertain if this form of metabolic coupling is conserved in invertebrates or is influenced by age. Lactate dehydrogenase (Ldh) is a rate-limiting enzyme that interconverts lactate and pyruvate. Here we genetically manipulated expression of Drosophila melanogaster lactate dehydrogenase (dLdh) in neurons or glia to assess the impact of altered lactate metabolism on invertebrate aging and long-term courtship memory at different ages. We also assessed survival, negative geotaxis, brain-neutral lipids (the core component of lipid droplets), and whole head metabolites. Both upregulation and downregulation of dLdh in neurons resulted in decreased survival and memory impairment with age. Glial downregulation of dLdh expression caused age-related memory impairment without altering survival, while upregulated glial dLdh expression lowered survival without disrupting memory. Both neuronal and glial dLdh upregulation increased neutral lipid accumulation. We provide evidence that altered lactate metabolism with age affects the tricarboxylic acid (TCA) cycle, 2-hydroxyglutarate (2HG), and neutral lipid accumulation. Collectively, our findings indicate that the direct alteration of lactate metabolism in either glia or neurons affects memory and survival but only in an age-dependent manner. 

Survival, negative geotaxis climbing ability, and courtship conditioning were assessed in adult Drosophila melanogaster males of various ages and genotypes.

Extracts from heads of adult Drosophila melanogaster males of various ages and genotypes were used for western blot and metabolite analysis using gas chromatography-mass spectroscopy. 

Whole brains from adult Drosophila melanogaster males of various genotypes were used for Nile red staining and imaging fluorescence with confocal microscopy.

Full method details are provided in "Aging and memory are altered by genetically manipulating lactate dehydrogenase in the neurons or glia of flies."

Software used included GraphPad Prism version 9.3.1 (GraphPad Software, RRID:SCR_002798), Image Lab (Bio-Rad, RRID:SCR_014210), MassHunter Quantitative software( Agilent, RRID:SCR_015040), ZEN Digital Imaging for Light Microscopy (Zeiss,RRID:SCR_013672), and FIJI-ImageJ (RRID:SCR_002285).