Extracellular alpha-synuclein and S100A9 species cause a shift in microglial energy metabolism

Loss of dopaminergic neurons during Parkinson‘s disease (PD) is often associated with the abnormal aggregation and accumulation of the protein alpha-synuclein (α-syn) which amongst other components form structures known as Lewy bodies, however the direct mechanism of α-syn neurotoxicity is not entirely clear. In addition, recent post mortem evidence from PD patients show that α-syn may co-aggregate with another pro-inflammatory and amyloidogenic protein S100A9 [1]. Microglial activation by α-syn as well as S100A9 has been demonstrated [2,3], however, their effect on the bioenergetics of microglia has not been studied. The bioenergetic profile of microglia is closely associated with their functional phenotype therefore it is important to know how these pathological stimuli influence microglial energy metabolism. We investigated the effects of recombinant and pre-aggregated α-syn and S100A9 on the glycolytic capacity and oxidative phosphorylation of murine microglia (BV-2 cell line) and compared them to the effects caused by LPS. BV-2 cells were incubated with either LPS, α-syn or S100A9 for 24 h. Oxidative phosphorylation was evaluated by high resolution respirometry in permeabilized BV-2 cells using Oroboros O2k oxygraph while glycolytic activity was measured by monitoring proton flux in intact BV-2 cells using the O2k pH-ISE module. LPS induced a decrease in phosphorylating respiration when oxidizing pyruvate + malate (PM) as well as pyruvate + malate + succinate (PMS) while the glycolytic activity of BV-2 cells after exposure to LPS markedly increased. Microglia incubated with S100A9 showed a similar bioenergetic response – decreased phosphorylating respiration with PM and PMS and increased glycolytic activity. After exposure to α-syn the phosphorylating respiration with PM significantly decreased, however respiration with PMS did not signifficantly differ from control. The glycolytic activity of BV-2 cells exposed to α-syn also markedly increased when compared to control. Amyloidogenic proteins α-syn and S100A9 cause a decrease in mitochondrial respiration of BV-2 cells with complex I substrates, however only S100A9 decreases maximal respiration when complex I and complex II substrates are supplied together. Both α-syn and S100A9 cause an increase in the glycolytic activity of BV-2 cells.