Introduction: We previously demonstrated that astrocytes offer protection to neurons through paracrine signaling and sought to determine the neuroprotective agent in the astrocyte secretome. One candidate is lactate. Astrocyte Neuron Lactate Shuttle (ANLS) describes the transfer of lactate from astrocytes to neurons where it is used for energy. Astrocytic glycogen is converted into glucose by glycogen phosphorylase and is then metabolized during aerobic respiration or converted to lactate under anaerobic conditions. Lactate transportation is mediated by astrocytic monocarboxylic transporter (MCT) 1 and 4. Here we evaluated the role of astrocytic lactate in astrocyte-mediated neuron protection during ischemic-reperfusion injury (IRI). Methods: Neurons and astrocytes were grown from E16-18 and P1-2 rats, respectively. IRI was induced by oxygen glucose deprivation (OGD) in male and female primary astrocytes and neurons. OGD-induced astrocyte conditioned media (oACM) was generated from astrocytes treated with the MCT 1/4 inhibitor, syrosporine (SYR; 0.1, 10, 100 mM), glycogen phosphorylase inhibitor, 1,4-dideoxy-1,4-imino-D-arabinitol (DAB; 100, 250, 1500 mM) , or vehicle during 1h OGD. Neurons were then treated with oACM, SYR + oACM, DAB + oACM, L-lactate (0.1, 1, 10 mM), or vehicle for 2h OGD and reperfused for 20 hours. Lactate concentration in oACM was measured using a kit (n = 3/group). Neuronal cell viability was assessed with MTT assay (n = 6/group) and normalized to untreated healthy neuronal cultures. Results: All results are corrected to control values as a percent. Astrocytic lactate release is significantly increased after OGD (57.6% vs 100%, p < 0.01). This release is significantly reduced with SYR treatment (100% vs 51.5%, p < 0.001) but not significantly reduced with DAB treatment (100% vs 76.1%, p < 0.1). Unexpectedly, despite both conditions having reduced lactate content, SYR + oACM protected neurons as well as oACM (55% vs 49%) and DAB + oACM protected neuron cultures better than oACM (80% vs 115%, p < 0.001). Neurons treated with exogenous lactate did not show improved viability (23% vs 13.8%, p < 0.001). We found no significant sex differences. Conclusions: Our data demonstrate that the ANLS is not a significant contributor to astrocyte paracrine mediated neuroprotection in rats. Further research is needed to investigate other mechanisms of astrocyte-protection of neurons as this understanding will inform future therapeutics.
Brookshier et al. (Thu,) studied this question.