Astrocytes actively participate in synaptic integration by releasing transmitter (glutamate) via a calcium-regulated, exocytosis-like process. Here we show that this process follows activation of the receptor CXCR4 by the chemokine stromal cell-derived factor 1 (SDF-1). An extraordinary feature of the ensuing signaling cascade is the rapid extracellular release of tumor necrosis factor-alpha (TNFalpha). Autocrine/paracrine TNFalpha-dependent signaling leading to prostaglandin (PG) formation not only controls glutamate release and astrocyte communication, but also causes their derangement when activated microglia cooperate to dramatically enhance release of the cytokine in response to CXCR4 stimulation. We demonstrate that altered glial communication has direct neuropathological consequences and that agents interfering with CXCR4-dependent astrocyte-microglia signaling prevent neuronal apoptosis induced by the HIV-1 coat glycoprotein, gp120IIIB. Our results identify a new pathway for glia-glia and glia-neuron communication that is relevant to both normal brain function and neurodegenerative diseases.

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Astrocytes emerge as key players in motor neuron degeneration in Amyotrophic Lateral Sclerosis (ALS). Whether astrocytes cause direct damage by releasing toxic factors or contribute indirectly through the loss of physiological functions is unclear. Here we identify in the hSOD1 G93A transgenic mouse model of ALS a degenerative process of the astrocytes, restricted to those directly surrounding spinal motor neurons. This phenomenon manifests with an early onset and becomes significant concomitant with the loss of motor cells and the appearance of clinical symptoms. Contrary to wild-type astrocytes, mutant hSOD1-expressing astrocytes are highly vulnerable to glutamate and undergo cell death mediated by the metabotropic type-5 receptor (mGluR5). Blocking mGluR5 in vivo slows down astrocytic degeneration, delays the onset of the disease and slightly extends survival in hSOD1 G93A transgenic mice. We propose that excitotoxicity in ALS affects both motor neurons and astrocytes, favouring their local interactive degeneration. This new mechanistic hypothesis has implications for therapeutic interventions.

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ATP, released by both neurons and glia, is an important mediator of brain intercellular communication. We find that selective activation of purinergic P2Y1 receptors (P2Y1R) in cultured astrocytes triggers glutamate release. By total internal fluorescence reflection imaging of fluorescence-labeled glutamatergic vesicles, we document that such release occurs by regulated exocytosis. The stimulus-secretion coupling mechanism involves Ca 2؉ release from internal stores and is controlled by additional transductive events mediated by tumor necrosis factor-␣ (TNF␣) and prostaglandins (PG). P2Y1R activation induces release of both TNF␣ and PGE 2 and blocking either one significantly reduces glutamate release. Accordingly, astrocytes from TNF␣-deficient (TNF ؊/؊ ) or TNF type 1 receptor-deficient (TNFR1 ؊/؊ ) mice display altered P2Y1R-dependent Ca 2؉ signaling and deficient glutamate release. In mixed hippocampal cultures, the P2Y1R-evoked process occurs in astrocytes but not in neurons or microglia. P2Y1R stimulation induces Ca 2؉ -dependent glutamate release also from acute hippocampal slices. The process in situ displays characteristics resembling those in cultured astrocytes and is distinctly different from synaptic glutamate release evoked by high K ؉ stimulation as follows: (a) it is sensitive to cyclooxygenase inhibitors; (b) it is deficient in preparations from TNF ؊/؊ and TNFR1 ؊/؊ mice; and (c) it is inhibited by the exocytosis blocker bafilomycin A1 with a different time course. No glutamate release is evoked by P2Y1R-dependent stimulation of hippocampal synaptosomes. Taken together, our data identify the coupling of purinergic P2Y1R to glutamate exocytosis and its peculiar TNF␣-and PG-dependent control, and we strongly suggest that this cascade operates selectively in astrocytes. The identified pathway may play physiological roles in glial-glial and glial-neuronal communication.

Despite indisputable progress in the molecular and genetic aspects of amyotrophic lateral sclerosis (ALS), a mechanistic comprehension of the neurodegenerative processes typical of this disorder is still missing and no effective cures to halt the progression of this pathology have yet been developed. Therefore, it seems that a substantial improvement of the outcome of ALS treatments may depend on a better understanding of the molecular mechanisms underlying neuronal pathology and survival as well as on the establishment of novel etiological therapeutic strategies. Noteworthy, a convergence of recent data from multiple studies suggests that, in cellular and animal models of ALS, a complex pathological interplay subsists between motor neurons and their non-neuronal neighbours, particularly glial cells. These observations not only have drawn attention to the physiopathological changes glial cells undergo during ALS progression, but they have moved the focus of the investigations from intrinsic defects and weakening of motor neurons to glia-neuron interactions. In this review, we summarize the growing body of evidence supporting the concept that different glial populations are critically involved in the dreadful chain of events leading to motor neuron sufferance and death in various forms of ALS. The outlined observations strongly suggest that glial cells can be the targets for novel therapeutic interventions in ALS.

Collective evidence indicates that motor neuron degeneration in amyotrophic lateral sclerosis (ALS) is non-cell-autonomous and requires the interaction with the neighboring astrocytes. Recently, we reported that a subpopulation of spinal cord astrocytes degenerates in the microenvironment of motor neurons in the hSOD1(G93A) mouse model of ALS. Mechanistic studies in vitro identified a role for the excitatory amino acid glutamate in the gliodegenerative process via the activation of its inositol 1,4,5-triphosphate (IP(3))-generating metabotropic receptor 5 (mGluR5). Since non-physiological formation of IP(3) can prompt IP(3) receptor (IP(3)R)-mediated Ca(2+) release from the intracellular stores and trigger various forms of cell death, here we investigated the intracellular Ca(2+) signaling that occurs downstream of mGluR5 in hSOD1(G93A)-expressing astrocytes. Contrary to wild-type cells, stimulation of mGluR5 causes aberrant and persistent elevations of intracellular Ca(2+) concentrations ([Ca(2+)](i)) in the absence of spontaneous oscillations. The interaction of IP(3)Rs with the anti-apoptotic protein Bcl-X(L) was previously described to prevent cell death by modulating intracellular Ca(2+) signals. In mutant SOD1-expressing astrocytes, we found that the sole BH4 domain of Bcl-X(L), fused to the protein transduction domain of the HIV-1 TAT protein (TAT-BH4), is sufficient to restore sustained Ca(2+) oscillations and cell death resistance. Furthermore, chronic treatment of hSOD1(G93A) mice with the TAT-BH4 peptide reduces focal degeneration of astrocytes, slightly delays the onset of the disease and improves both motor performance and animal lifespan. Our results point at TAT-BH4 as a novel glioprotective agent with a therapeutic potential for ALS.

Elevated cerebrospinal fluid (CSF), Neurofilament Light (NF-L) and phosphorylated Heavy (pNF-H) chain levels have been found in Amyotrophic Lateral Sclerosis (ALS), with studies reporting a correlation of both neurofilaments (NFs) with the disease progression. Here, we measured NF-L and pNF-H concentrations in the CSF of ALS patients from a single tertiary Center and investigated their relationship with disease-related variables. A total of 190 ALS patients (Bulbar, 29.9%; Spinal, 70.1%; M/F = 1.53) and 130 controls with mixed neurological diseases were recruited. Demographic and clinical variables were recorded, and ΔFS was used to rate the disease progression. Controls were divided into two cohorts: (1) patients with non-inflammatory neurological diseases (CTL-1); (2) patients with acute/subacute inflammatory diseases and tumors, expected to lead to significant axonal and tissue damage (CTL-2). For each patient and control, CSF was taken at the time of the diagnostic work-up and stored following the published guidelines. CSF NF-L and pNF-H were assayed with commercially available ELISA-based methods. Standard curves (from independent ELISA kits) were highly reproducible for both NFs, with a coefficient of variation < 20%. We found that CSF NF-L and pNF-H levels in ALS were significantly increased when compared to CTL-1 (NF-L: ALS, 4.7 ng/ml vs CTL-1, 0.61 ng/ml, p < 0.001; pNF-H: ALS, 1.7 ng/ml vs CTL-1, 0.03 ng/ml, p < 0.0001), but not to CTL-2. Analysis of different clinical and prognostic variables disclosed meaningful correlations with both NF-L and pNF-H levels. Our results, from a relatively large ALS cohort, confirm that CSF NF-L and pNF-H represent valuable diagnostic and prognostic biomarkers in ALS.