In summary activated A AR exacerbated the

In summary, activated A2AR exacerbated the reverse transport function of endothelial EAATs through a direct or indirect pathway depending on PKA and glutamate levels in response to OGD in vitro, but A2AR inhibition quickly restored the normal transport function. Moreover, the key mechanisms by which A2AR regulates endothelial EAATs were also verified in a mouse TBI model. Based on our results, the important role of A2AR in regulating glutamate homeostasis in the brain was closely related to its ability to regulate endothelial EAAT function. Additionally, the early intervention in the function of endothelial A2AR after brain injury may represent a very promising treatment.
Acknowledgements We thank Prof. Jiang-Fan Chen (Boston University) for kindly providing the A2AR KO mice.
Introduction Glutamate is the primary excitatory neurotransmitter in the central nervous system (CNS), where it initiates rapid signal transmission and is involved in learning, memory and synaptic plasticity (Parkin et al., 2018, Willard and Koochekpour, 2013). Following its synaptic release, glutamate is taken up into surrounding astrocytes and the glutamate gradient returns to resting levels (Sulkowski et al., 2014). As high levels of extracellular glutamate are associated with excitotoxic neuronal death, glutamate concentration is optimally maintained via the removal of glutamate from the synapse by astrocytic glutamate transporters after impulse transmission (Fig. 1A) (Jia et al., 2015, Karki et al., 2015b). Astrocytic glutamate transporters, also referred to as excitatory amino Milnacipran HCl receptor transporters (EAATs) in humans, play a primary role in the rapid termination of glutamate signaling and the maintenance of extracellular glutamate levels (Shigeri et al., 2004). Excess levels of synaptic glutamate result in the overstimulation of postsynaptic glutamate receptors, leading to excitotoxic neuronal death (Karki et al., 2018). An increasing body of evidence reveals that excitotoxicity is associated with neurological disorders, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), Parkinson's disease (PD), manganism, ischemia, schizophrenia, epilepsy, and autism (Fig. 1B) (Bristot Silvestrin et al., 2013, Garcia-Esparcia et al., 2018, Mironova et al., 2018, Petr et al., 2013a). While the mechanisms of excitotoxicity are not well understood, the dysregulation of EAATs may greatly influence glutamate excitotoxicity and the resulting neuropathology. In particular, EAAT1 and EAAT2, the primary glutamate transporters in the CNS, may significantly impact glutamate excitotoxicity (Karki et al., 2013a). Glutamate-aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1) are homologs (displaying >70% homology) of EAAT1 and EAAT2 in rodents, respectively, and thus can be used interchangeably (Jimenez et al., 2014).
Glutamate transporters There are five EAAT subtypes identified in humans, referred to as EAAT1-5 (Bridges and Esslinger, 2005). EAAT1 and 2 are predominantly expressed in astrocytes (Karki et al., 2013a), although they are also expressed in other types of glial cells, including microglia and oligodendrocytes (Parkin et al., 2018). Once taken into astrocytes, glutamate is converted to glutamine by glutamine synthase. The newly generated glutamine is subsequently available for transport back to presynaptic neurons, a process referred to as glutamate-glutamine cycling (Shen et al., 2009). EAAT3 is primarily found in neurons, particularly at the post-synaptic terminals (He and Casaccia-Bonnefil, 2008). Other glutamate transporter subtypes such as EAAT4 and 5 are also expressed in the human CNS (Amara and Fontana, 2002). EAAT4, encoded by the human SLC1A6 gene, is expressed predominantly in the Purkinje cells in the cerebellum, while EAAT5, encoded by the human SLC1A7 gene, is expressed in the photoreceptor cells of the retina (Amara and Fontana, 2002). This indicates the important roles of EAAT4 and 5 in glutamate neurotransmission in specific regions of the brain (Amara and Fontana, 2002, Perkins et al., 2018, Yamashita et al., 2006).