Vesicular Glutamate Transporters in Astrocytes as Potential New Therapeutic Targets: Astrocyte-targeted Viral Vectors Expressing Inhibitory Nanobodies

Astrocytes, the main Central Nervous System (CNS) glial cell type, actively release transmitters, including glutamate, and thereby participate in physiological brain information processing. However, dysregulated transmitter release from astrocytes can contribute to CNS disease pathogenesis and progression. Therefore, targeting astrocyte glutamate release is a promising new therapeutic strategy in hyper-glutamatergic brain conditions, as it does not directly block glutamatergic neurotransmission. Basing on the evidence that astrocytes express Vesicular Glutamate Transporters (VGLUT), in collaboration with other NCCR TransCure partners, we developed an innovative approach for astrocyte-selective delivery of nanobodies inhibiting VGLUT. We inserted the anti-VGLUT nanobody constructs in astrocyte-directed viral vectors that were administered peripherally, crossed the blood-brain-barrier and led to successful cell-specific CNS-wide expression of the nanobodies

Peroxynitrite Inhibits Glutamate Transporter Subtypes

The reuptake of glutamate in neurons and astrocytes terminates excitatory signals and prevents the persistence of excitotoxic levels of glutamate in the synaptic cleft. This process is inhibited by oxygen radicals and hydrogen peroxide (H2O2). Here we show that another biological oxidant, peroxynitrite (ONOO-), formed by combination of superoxide (O2-) and nitric oxide (NO), potently inhibits glutamate uptake by purified or recombinant high affinity glutamate transporters reconstituted in liposomes. ONOO- reduces selectively the Vmax of transport; its action is fast (reachingor = 90% within 20 s), dose-dependent (50% inhibition at 50 microM), persistent upon ONOO- (or by product) removal, and insensitive to the presence of the lipid antioxidant vitamin E in the liposomal membranes. Therefore, it likely depends on direct interaction of ONOO- with the glutamate transporters. Three distinct recombinant glutamate transporters from the rat brain, GLT1, GLAST, and EAAC1, exhibit identical sensitivity to ONOO . H2O2 also inhibits reconstituted transport, and its action matches that of ONOO- on all respects; however, this is observed only with 5-10 mM H202 and after prolonged exposure (10 min) in highly oxygenated buffer. NO, released from NO donors (up to 10 mM), does not modify reconstituted glutamate uptake, although in parallel conditions it promotes cGMP formation in synaptosomal cytosolic fraction. Overall, our results suggest that the glutamate transporters contain conserved sites in their structures conferring vulnerability to ONOO- and other oxidants

The BH4 domain of Bcl-XL rescues astrocyte degeneration in amyotrophic lateral sclerosis by modulating intracellular calcium signals

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 hSOD1G93A 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 (IP3)-generating metabotropic receptor 5 (mGluR5). Since non-physiological formation of IP3 can prompt IP3 receptor (IP3R)-mediated Ca2+ release from the intracellular stores and trigger various forms of cell death, here we investigated the intracellular Ca2+ signaling that occurs downstream of mGluR5 in hSOD1G93A-expressing astrocytes. Contrary to wild-type cells, stimulation of mGluR5 causes aberrant and persistent elevations of intracellular Ca2+ concentrations ([Ca2+]i) in the absence of spontaneous oscillations. The interaction of IP3Rs with the anti-apoptotic protein Bcl-XL was previously described to prevent cell death by modulating intracellular Ca2+ signals. In mutant SOD1-expressing astrocytes, we found that the sole BH4 domain of Bcl-XL, fused to the protein transduction domain of the HIV-1 TAT protein (TAT-BH4), is sufficient to restore sustained Ca2+ oscillations and cell death resistance. Furthermore, chronic treatment of hSOD1G93A 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 AL

Astrocyte function from information processing to cognition and cognitive impairment

Astrocytes serve important roles that affect recruitment and function of neurons at the local and network levels. Here we review the contributions of astrocyte signaling to synaptic plasticity, neuronal network oscillations, and memory function. The roles played by astrocytes are not fully understood, but astrocytes seem to contribute to memory consolidation and seem to mediate the effects of vigilance and arousal on memory performance. Understanding the role of astrocytes in cognitive processes may also advance our understanding of how these processes go awry in pathological conditions. Indeed, abnormal astrocytic signaling can cause or contribute to synaptic and network imbalances, leading to cognitive impairment. We discuss evidence for this from animal models of Alzheimer's disease and multiple sclerosis and from animal studies of sleep deprivation and drug abuse and addiction. Understanding the emerging roles of astrocytes in cognitive function and dysfunction will open up a large array of new therapeutic opportunities

Astrocytes: powering memory.

Creating long-term memory requires a cellular program in neurons involving gene expression, protein synthesis, and formation of new synaptic connections. Suzuki et al. (2011) show that astrocytes, glial cells of the brain, play a necessary role in this program by converting glycogen to lactate and transporting it to neurons

Defective tumor necrosis factor-alpha-dependent control of astrocyte glutamate release in a transgenic mouse model of Alzheimer disease.

The cytokine tumor necrosis factor-alpha (TNFalpha) induces Ca2+-dependent glutamate release from astrocytes via the downstream action of prostaglandin (PG) E2. By this process, astrocytes may participate in intercellular communication and neuromodulation. Acute inflammation in vitro, induced by adding reactive microglia to astrocyte cultures, enhances TNFalpha production and amplifies glutamate release, switching the pathway into a neurodamaging cascade (Bezzi, P., Domercq, M., Brambilla, L., Galli, R., Schols, D., De Clercq, E., Vescovi, A., Bagetta, G., Kollias, G., Meldolesi, J., and Volterra, A. (2001) Nat. Neurosci. 4, 702-710). Because glial inflammation is a component of Alzheimer disease (AD) and TNFalpha is overexpressed in AD brains, we investigated possible alterations of the cytokine-dependent pathway in PDAPP mice, a transgenic model of AD. Glutamate release was measured in acute hippocampal and cerebellar slices from mice at early (4-month-old) and late (12-month-old) disease stages in comparison with age-matched controls. Surprisingly, TNFalpha-evoked glutamate release, normal in 4-month-old PDAPP mice, was dramatically reduced in the hippocampus of 12-month-old animals. This defect correlated with the presence of numerous beta-amyloid deposits and hypertrophic astrocytes. In contrast, release was normal in cerebellum, a region devoid of beta-amyloid deposition and astrocytosis. The Ca2+-dependent process by which TNFalpha evokes glutamate release in acute slices is distinct from synaptic release and displays properties identical to those observed in cultured astrocytes, notably PG dependence. However, prostaglandin E2 induced normal glutamate release responses in 12-month-old PDAPP mice, suggesting that the pathology-associated defect involves the TNFalpha-dependent control of secretion rather than the secretory process itself. Reduced expression of DENN/MADD, a mediator of TNFalpha-PG coupling, might account for the defect. Alteration of this neuromodulatory astrocytic pathway is described here for the first time in relation to Alzheimer disease