Chemokines as neuromodulators: Regulation of glutamatergic transmission by cxcr4-mediated glutamate release from astrocytes

Chemokines and their receptors are expressed both, in the developing humans and in the adult central nervous system (CNS). Numbers of papers showed that the CXCL12 and CXCR4 receptor are differentially expressed; CXCL12 is highly expressed in glial cells (mostly on astrocytes) and CXCR4 in both glial (astrocytes and microglia) and neuronal cells. The pattern of distribution supports the idea that the CXCL12/CXCR4 system is involved in the fast bidirectional communication system between astrocytes and neurons. Infact, local application of CXCL12 in cultured and in situ astrocytes induces calcium-dependent glutamate release through a direct activation of the G-protein coupled receptor (GPCRs) CXCR4. The chemokine-mediated glutamate release process in astrocytes seems to involve a long chain of intracellular and extracellular events related to the release of two chemical mediators, the tumor necrosis factor-alpha (TNFα) and prostaglandins (PGs). The mechanisms of glutamate release from astrocytes have been extensively studied during the last years. The first evidence for the existence of a calcium-dependent exocytosis pathway had been provided few years ago when by complementing the ultrastructural studies in situ with dynamic total internal reflection fluorescence (TIRF) real-time imaging studies in cultured astrocytes it has been shown that astrocytes express glutammatergic vesicles able to undergo regulated exocytosis. In our recent paper we investigated the physiological role of chemokines by studying whether the CXCL12/CXCR4 system triggers release of glutamate by regulated exocytosis. The activation of CXCR4 receptor induces a burst of exocytosis that occurs in the order of a few hundred milliseconds and is mediated by the release of calcium from internal stores. The exocytotic burst unfolds in a time scale much shorter than that of dense-core granules in neurosecretory cells, which are governed by voltage-gated calcium channels. Taking into account that astrocytes are electrically non-excitable and that their release rely only on the activation of GPCRs, the rapid exocytotic pathway is of a difficult interpretation without considering the existence of a local morphological and functional interaction between vesicles and site of calcium release. Indeed, by looking at the spatial organization of endoplasmic reticulum tubules and sites of vesicles docking with TIRF illumination, one can find that endoplasmic reticulum tubules, together with plasma membrane and docked vesicles form complex and peculiar structures of sub-micrometer spaces that provide the structural basis for local calcium signaling. Upon stimulation of group I of mGluR, sub-micrometer localized calcium elevations (hot spots) are generated in the sub-plasma membrane domains of ER. Interestingly, in most cases the sub-membrane calcium events occurred at/near sites where SLMVs underwent exocytosis (interdistance: ≤280 nm), and were in strict temporal correlation with the fusion events. Recent works have shown that glutamate released from astrocytes during physiological synaptic activity targets neuronal receptors in either axonal terminals or dendrites, exerting a neuromodulatory action. In particular in a recent paper it has been shown that at the perforant path-granule cell (PP-GC) synapses in the hippocampal dentate gyrus, astrocytes of the outer molecular layer sense synaptic activity, elevate their intracellular calcium and release glutamate via exocytosis of glutammatergic vesicles. Astrocytic glutamate is released at presynaptic level in close proximity of NR2B-containing NMDA receptors; the activation of such receptors results in an increased synaptic transmitter release and in the strengthening of synaptic transmission. The appreciation that brain activity involves interactive signaling between neurons and glia opens new perspectives for understanding the pathogenesis of brain diseases with strong inflammatory components (such as Alzheimer's disease and AIDS dementia complex). In particular, in the presence of inflammatory cytokines such as interleukin-1β (IL-1β) and TNFα, changes in the expression of junctional proteins, in propagation of intracellular calcium waves and glutamate release, have been observed. Thus, when local inflammatory reaction is triggered in the brain, microglial cells rapidly migrate to the site of injury become activated and start releasing a number of mediators such as PGs and TNFα, deeply altering the properties of calcium-dependent glutamate release from astrocytes.