Sumoylation of the astroglial glutamate transporter EAAT2 governs its intracellular compartmentalization.

EAAT2 is a predominantly astroglial glutamate transporter responsible for the majority of synaptic glutamate clearance in the mammalian central nervous system (CNS). Its dysfunction has been linked with many neurological disorders, including amyotrophic lateral sclerosis (ALS). Decreases in EAAT2 expression and function have been implicated in causing motor neuron excitotoxic death in ALS. Nevertheless, increasing EAAT2 expression does not significantly improve ALS phenotype in mouse models or in clinical trials. In the SOD1-G93A mouse model of inherited ALS, the cytosolic carboxy-terminal domain is cleaved from EAAT2, conjugated to SUMO1, and accumulated in astrocytes where it triggers astrocyte-mediated neurotoxic effects as disease progresses. However, it is not known whether this fragment is sumoylated after cleavage or if full-length EAAT2 is already sumoylated prior to cleavage as part of physiological regulation. In this study, we show that a fraction of full-length EAAT2 is constitutively sumoylated in primary cultures of astrocytes in vitro and in the CNS in vivo. Furthermore, the extent of sumoylation of EAAT2 does not change during the course of ALS in the SOD1-G93A mouse and is not affected by the expression of ALS-causative mutant SOD1 proteins in astrocytes in vitro, indicating that EAAT2 sumoylation is not driven by pathogenic mechanisms. Most interestingly, sumoylated EAAT2 localizes to intracellular compartments, whereas non-sumoylated EAAT2 resides on the plasma membrane. In agreement, promoting desumoylation in primary astrocytes causes increased EAAT2-mediated glutamate uptake. These findings could have implications for optimizing therapeutic approaches aimed at increasing EAAT2 activity in the dysfunctional or diseased CNS.

The nuclear import receptor Kapß2 modifies neurotoxicity mediated by poly(GR) in C9orf72-linked ALS/FTD.

Expanded intronic G4C2 repeats in the C9ORF72 gene cause amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). These intronic repeats are translated through a non-AUG-dependent mechanism into five different dipeptide repeat proteins (DPRs), including poly-glycine-arginine (GR), which is aggregation-prone and neurotoxic. Here, we report that Kapß2 and GR interact, co-aggregating, in cultured neurons in-vitro and CNS tissue in-vivo. Importantly, this interaction significantly decreased the risk of death of cultured GR-expressing neurons. Downregulation of Kapß2 is detrimental to their survival, whereas increased Kapß2 levels mitigated GR-mediated neurotoxicity. As expected, GR-expressing neurons displayed TDP-43 nuclear loss. Raising Kapß2 levels did not restore TDP-43 into the nucleus, nor did alter the dynamic properties of GR aggregates. Overall, our findings support the design of therapeutic strategies aimed at up-regulating Kapß2 expression levels as a potential new avenue for contrasting neurodegeneration in C9orf72-ALS/FTD.

C9orf72 poly(PR) mediated neurodegeneration is associated with nucleolar stress.

The ALS/FTD-linked intronic hexanucleotide repeat expansion in the C9orf72 gene is translated into dipeptide repeat proteins, among which poly-proline-arginine (PR) displays the most aggressive neurotoxicity in-vitro and in-vivo . PR partitions to the nucleus when expressed in neurons and other cell types. Using drosophila and primary rat cortical neurons as model systems, we show that by lessening the nuclear accumulation of PR, we can drastically reduce its neurotoxicity. PR accumulates in the nucleolus, a site of ribosome biogenesis that regulates the cell stress response. We examined the effect of nucleolar PR accumulation and its impact on nucleolar function and determined that PR caused nucleolar stress and increased levels of the transcription factor p53. Downregulating p53 levels, either genetically or by increasing its degradation, also prevented PR-mediated neurotoxic phenotypes both in in-vitro and in-vivo models. We also investigated whether PR could cause the senescence phenotype in neurons but observed none. Instead, we found induction of apoptosis via caspase-3 activation. In summary, we uncovered the central role of nucleolar dysfunction upon PR expression in the context of C9-ALS/FTD.

Glucose Hypometabolism Prompts RAN Translation and Exacerbates C9orf72-related ALS/FTD Phenotypes.

The most prevalent genetic cause of both amyotrophic lateral sclerosis and frontotemporal dementia is a (GGGGCC)n nucleotide repeat expansion (NRE) occurring in the first intron of the C9orf72 gene (C9). Brain glucose hypometabolism is consistently observed in C9-NRE carriers, even at pre-symptomatic stages, although its potential role in disease pathogenesis is unknown. Here, we identified alterations in glucose metabolic pathways and ATP levels in the brain of asymptomatic C9-BAC mice. We found that, through activation of the GCN2 kinase, glucose hypometabolism drives the production of dipeptide repeat proteins (DPRs), impairs the survival of C9 patient-derived neurons, and triggers motor dysfunction in C9-BAC mice. We also found that one of the arginine-rich DPRs (PR) can directly contribute to glucose metabolism and metabolic stress. These findings provide a mechanistic link between energy imbalances and C9-ALS/FTD pathogenesis and support a feedforward loop model that opens several opportunities for therapeutic intervention.

C9orf72 poly(PR) mediated neurodegeneration is associated with nucleolar stress.

The ALS/FTD-linked intronic hexanucleotide repeat expansion in the C9orf72 gene is aberrantly translated in the sense and antisense directions into dipeptide repeat proteins, among which poly proline-arginine (PR) displays the most aggressive neurotoxicity in-vitro and in-vivo. PR partitions to the nucleus when heterologously expressed in neurons and other cell types. We show that by lessening the nuclear accumulation of PR, we can drastically reduce its neurotoxicity. PR strongly accumulates in the nucleolus, a nuclear structure critical in regulating the cell stress response. We determined that, in neurons, PR caused nucleolar stress and increased levels of the transcription factor p53. Downregulating p53 levels also prevented PR-mediated neurotoxicity both in in-vitro and in-vivo models. We investigated if PR could induce the senescence phenotype in neurons. However, we did not observe any indications of such an effect. Instead, we found evidence for the induction of programmed cell death via caspase-3 activation.

Rats expressing human cytosolic copper-zinc superoxide dismutase transgenes with amyotrophic lateral sclerosis: associated mutations develop motor neuron disease.

Some cases of familial amyotrophic lateral sclerosis (ALS) are caused by mutations in the gene encoding cytosolic, copper-zinc superoxide dismutase (SOD1). We report here that rats that express a human SOD1 transgene with two different ALS-associated mutations (G93A and H46R) develop striking motor neuron degeneration and paralysis. As in the human disease and transgenic ALS mice, pathological analysis demonstrates selective loss of motor neurons in the spinal cords of these transgenic rats. In spinal cord tissues, this is accompanied by activation of apoptotic genes known to be activated by mutant SOD1 protein in vitro and in vivo. These animals provide additional support for the proposition that motor neuron death in SOD1-related ALS reflects one or more acquired, neurotoxic properties of the mutant SOD1 protein. The larger size of this rat model as compared with the ALS mice will facilitate studies involving manipulations of spinal fluid (implantation of intrathecal catheters for chronic therapeutic studies; CSF sampling) and spinal cord (e.g., direct administration of viral- and cell-mediated therapies).

Survival in transgenic ALS mice does not vary with CNS glutathione peroxidase activity.

OBJECTIVE: Transgenic mice that overexpress a human gene encoding mutant cytosolic superoxide dismutase (SOD1) develop a progressive motor neuron loss that resembles human ALS. Why mutant SOD1 initiates motor neuron death is unknown. One hypothesis proposes that the mutant molecule has enhanced peroxidase activity, reducing hydrogen peroxide (H2O2) to form toxic hydroxyl adducts on critical targets. To test this hypothesis, the authors generated transgenic ALS mice with altered levels of glutathione peroxidase (GSHPx), the major soluble enzyme that detoxifies H2O2. METHODS: SOD1(G93A) ALS mice were bred with mice bearing a murine GSHPx transgene that have a four-fold elevation in brain GSHPx levels and with mice having targeted inactivation of the GSHPx gene and reduced brain GSHPx activity. RESULTS: Survival was not prolonged in ALS mice with elevated brain GSHPx activity (p = 0.09). ALS mice with decreased GSHPx brain activity (20% of normal) showed no acceleration of the disease course (p = 0.89). The age at disease onset in the ALS mice was unaffected by brain GSHPx activity. CONCLUSION: The level of GSHPx activity in the CNS of transgenic ALS mice does not play a critical role in the development of motor neuron disease.

Long-term potentiation and synaptic protein phosphorylation.

Long-term potentiation (LTP) is a well known experimental model for studying the activity-dependent enhancement of synaptic plasticity, and because of its long duration and its associative properties, it has been proposed as a system to investigate the molecular mechanisms of memory formation. At present, there are several lines of evidence that indicate that pre- and postsynaptic kinases and their specific substrates are involved in molecular mechanisms underlying LTP. Many studies focus on the involvement of protein kinase C (PKC). One way to investigate the role of PKC in long-term potentiation is to determine the degree of phosphorylation of its substrates after in situ phosphorylation in hippocampal slices. Two possible targets are the presynaptic membrane-associated protein B-50 (a.k.a. GAP 43, neuromodulin and F1), which has been implicated in different forms of synaptical plasticity in the brain such as neurite outgrowth, hippocampal LTP and neurotransmitter release, and the postsynaptic protein neurogranin (a.k.a. RC3, BICKS and p17) which function remains to be determined. This review will focus on the protein kinase C activity in pre- and postsynaptic compartment during the early phase of LTP and the possible involvement of its substrates B-50 and neurogranin.

Activation of pre- and postsynaptic protein kinase C during tetraethylammonium-induced long-term potentiation in the CA1 field of the hippocampus.

Tetraethylammonium (TEA) induces a form of long-term potentiation (LTP) that is independent on N-methyl-D-aspartate (NMDA) receptor activation (LTP(K)). LTP(K) may be a suitable chemical model to study molecular mechanisms underlying LTP. We monitored the phosphorylation state of two identified neural-specific protein kinase C (PKC) substrates (the presynaptic protein GAP-43/B-50 and postsynaptic protein RC3) after different chemical depolarisations. TEA induced a long-lasting increase in synaptic efficacy in the CA1 field of the hippocampus and increased the phosphorylation of both GAP-43/B-50 and RC3 (51 and 56.1%, respectively). These effects were blocked by the voltage-dependent calcium channel antagonist nifedipine, but not by the NMDA receptor antagonist AP5. These data show that in LTP(K) the in situ phosphorylation of pre-and postsynaptic PKC substrates is increased, indicating that NMDA receptor-dependent and NMDA receptor-independent LTP share common Ca(2+)-dependent expression mechanisms, including activation of pre- and postsynaptic PKC.

Protein kinase C in synaptic plasticity: changes in the in situ phosphorylation state of identified pre- and postsynaptic substrates.

1. Long-term potentiation and its counterpart long-term depression are two forms of activity dependent synaptic plasticity, in which protein kinases and protein phosphatases are essential. 2. B-50/GAP-43 and RC3/neurogranin are two defined neuronal PKC substrates with different synaptic localization. B-50/GAP-43 is a presynaptic protein and RC3/neurogranin is only found at the postsynaptic site. Measuring their phosphorylation state in hippocampal slices, allows us to simultaneously monitor changes in pre- and postsynaptic PKC mediated phosphorylation. 3. Induction of LTP in the CA1 field of the hippocampus is accompanied with an increase in the in situ phosphorylation of both B-50/GAP-43 and RC3/neurogranin, during narrow, partially overlapping, time windows. 4. Pharmacological data show that mGluR stimulation results in an increase in the in situ phosphorylation of B-50/GAP-43 and RC3/neurogranin.

Caspase-1 and -3 are sequentially activated in motor neuron death in Cu,Zn superoxide dismutase-mediated familial amyotrophic lateral sclerosis.

Familial amyotrophic lateral sclerosis-linked mutations in copper-zinc superoxide dismutase cause motor neuron death through one or more acquired toxic properties. An early event in the mechanism of toxicity from such mutants is now demonstrated to be activation of caspase-1. Neuronal death, however, follows only after months of chronic caspase-1 activation concomitantly with activation of the executioner caspase-3 as the final step in the toxic cascade. Thus, a common toxicity of mutant SOD1 is a sequential activation of at least two caspases, caspase-1 that acts slowly as a chronic initiator and caspase-3 acting as the final effector of cell death.

Antibodies to postsynaptic PKC substrate neurogranin prevent long-term potentiation in hippocampal CA1 neurons.

Both pre- and postsynaptic protein kinase C have been implicated in long-term potentiation. Neurogranin (also known as BICKs and RC3) is a neuronal postsynaptic protein kinase C substrate. In the present study we injected monoclonal IgGs that recognize the protein kinase C phosphorylation site in neurogranin and B-50 (GAP-43), and that have been shown to inhibit protein kinase C-mediated B-50 phosphorylation, through a whole-cell clamp pipette into CA1 pyramidal neurons in rat hippocampal slices. Injection of neurogranin IgGs, but not of control IgGs, prevented the induction of tetanus-induced long-term potentiation without affecting post-tetanic potentiation. Our results suggest that neurogranin is involved in mechanisms of activity-dependent synaptic plasticity.

Decreased intracellular superoxide levels activate Sindbis virus-induced apoptosis.

Infection of many cultured cell types with Sindbis virus (SV), an alphavirus, triggers apoptosis through a commonly utilized caspase activation pathway. However, the upstream signals by which SV activates downstream apoptotic effectors, including caspases, remain unclear. Here we report that in AT-3 prostate carcinoma cells, SV infection decreases superoxide (O-2) levels within minutes of infection as monitored by an aconitase activity assay. This SV-induced decrease in O-2 levels appears to activate or modulate cell death, as a recombinant SV expressing the O-2 scavenging enzyme, copper/zinc superoxide dismutase (SOD), potentiates SV-induced apoptosis. A recombinant SV expressing a mutant form of SOD, which has reduced SOD activity, has no effect. The potentiation of SV-induced apoptosis by wild type SOD is because of its ability to scavenge intracellular O-2 rather than its ability to promote the generation of hydrogen peroxide. Pyruvate, a peroxide scavenger, does not affect the ability of wild type SOD to potentiate cell death; and increasing the intracellular catalase activity via a recombinant SV vector has no effect on SV-induced apoptosis. Moreover, increasing intracellular O-2 by treatment of 3T3 cells with paraquat protects them from SV-induced death. Altogether, our results suggest that SV may activate apoptosis by reducing intracellular superoxide levels and define a novel redox signaling pathway by which viruses can trigger cell death.

Caspase-1 is activated in neural cells and tissue with amyotrophic lateral sclerosis-associated mutations in copper-zinc superoxide dismutase.

The mechanism by which mutations in the superoxide dismutase (SOD1) gene cause motor neuron degeneration in familial amyotrophic lateral sclerosis (ALS) is unknown. Recent reports that neuronal death in SOD1-familial ALS is apoptotic have not documented activation of cell death genes. We present evidence that the enzyme caspase-1 is activated in neurons expressing mutant SOD1 protein. Proteolytic processing characteristic of caspase-1 activation is seen both in spinal cords of transgenic ALS mice and neurally differentiated neuroblastoma (line N2a) cells with SOD1 mutations. This activation of caspase-1 is enhanced by oxidative challenge (xanthine/xanthine oxidase), which triggers cleavage and secretion of the interleukin 1beta converting enzyme substrate, pro-interleukin 1beta, and induces apoptosis. This N2a culture system should be an instructive in vitro model for further investigation of the proapoptotic properties of mutant SOD1.

Amyotrophic lateral sclerosis-linked glutamate transporter mutant has impaired glutamate clearance capacity.

We have investigated the functional impact of a naturally occurring mutation of the human glutamate transporter GLT1 (EAAT2), which had been detected in a patient with sporadic amyotrophic lateral sclerosis. The mutation involves a substitution of the putative N-linked glycosylation site asparagine 206 by a serine residue (N206S) and results in reduced glycosylation of the transporter and decreased uptake activity. Electrophysiological analysis of N206S revealed a pronounced reduction in transport rate compared with wild-type, but there was no alteration in the apparent affinities for glutamate and sodium. In addition, no change in the sensitivity for the specific transport inhibitor dihydrokainate was observed. However, the decreased rate of transport was associated with a reduction of the N206S transporter in the plasma membrane. Under ionic conditions, which favor the reverse operation mode of the transporter, N206S exhibited an increased reverse transport capacity. Furthermore, if coexpressed in the same cell, N206S manifested a dominant negative effect on the wild-type GLT1 activity, whereas it did not affect wild-type EAAC1. These findings provide evidence for a role of the N-linked glycosylation in both cellular trafficking and transport function. The resulting alteration in glutamate clearance capacity likely contributes to excitotoxicity that participates in motor neuron degeneration in amyotrophic lateral sclerosis.

Temporal differences in the phosphorylation state of pre- and postsynaptic protein kinase C substrates B-50/GAP-43 and neurogranin during long-term potentiation.

The phosphorylation state of two identified neuralspecific protein kinase C substrates (the presynaptic protein B-50 and the postsynaptic protein neurogranin) was monitored after the induction of long term potentiation in the CA1 field of rat hippocampus slices by quantitative immunoprecipitation following 32Pi labeling in the recording chamber. B-50 phosphorylation was increased from 10 to 60 min, but no longer at 90 min after long term potentiation had been induced, neurogranin phosphorylation only at 60 min. Increased phosphorylation was not found when long term potentiation was blocked with the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovalerate, when only low frequency stimulation was applied or tetanic stimulation failed to induce long term-potentiation. Our data show that both B-50 and neurogranin phosphorylation are increased following the induction of long term potentiation, thus providing strong evidence for pre- and postsynaptic protein kinase C activation during narrow, partially overlapping, time windows after the induction of long term potentiation.

Phosphorylation of the casein kinase II domain of B-50 (GAP-43) in rat cortical growth cones.

Growth-associated phosphoprotein B-50 is a neural protein kinase C (PKC) substrate enriched in nerve growth cones that has been implicated in growth cone plasticity. Here we investigated whether B-50 is a physiological substrate for casein kinase II (CKII) in purified rat cortical growth cone preparations. Using site-specific proteolysis and known modulators of PKC, in combination with immunoprecipitation, mass spectrometry, and phosphoamino acid analysis, we demonstrate that endogenous growth cone B-50 is phosphorylated at multiple sites, on both serine and threonine residues. Consistent with previous reports, stimulation of PKC activity increased the phosphorylation of only those proteolytic fragments containing Ser41. Under basal conditions, however, phosphorylation was predominantly associated with fragments not containing Ser41. Mass spectrometry of tryptic digests of B-50, which had been immunoprecipitated from untreated growth cones, revealed that in situ phosphorylation occurs within peptides B-50(181-198) and B-50(82-98). These peptides contain the major and minor in vitro CKII phosphosites, respectively. In addition, cyanogen bromide digestion of immunoprecipitated chick B-50 generated a 4-kDa C-terminal B-50 phosphopeptide, confirming that phosphorylation of the CKII domain occurs across evolutionary diverse species. We conclude that B-50 in growth cones is not only a substrate for PKC, but also for CKII.