P38 MAPK is involved in enhanced NMDA receptor-dependent excitotoxicity in YAC transgenic mouse model of Huntington disease

Ethyl pyruvate ameliorates 3-nitropropionic acid-induced striatal toxicity through anti-neuronal cell death and anti-inflammatory mechanisms

The potential neuroprotective value of ethyl pyruvate (EP) for the treatment of the striatal toxicity is largely unknown. We investigated whether EP promotes the survival of striatal neurons in a 3-nitropropionic acid (3-NP)-induced mouse model of Huntington's disease (HD). EP (5, 10, 20, and 40mg/kg/day, i.p.) was daily injected from 30min before 3-NP intoxication (pretreatment) and from onset/progression/peak point of neurological impairment by 3-NP intoxication. EP produced a neuroprotective effect in dose- and time-dependant manners. EP pretreatment of 40mg/kg/day produced the best neuroprotective effect among other conditions. Pretreatment of EP significantly attenuated neurological impairment and lethality and prevented formation of lesion area and neuronal loss in the striatum after 3-NP intoxication. This neuroprotection afforded by EP was associated with the suppression of succinate dehydrogenase activity, apoptosis, and microglial activation. The suppressive effect of EP corresponded to the down-regulation of mitogen-activated protein kinases (MAPKs) and nuclear factor-kappa B (NF-κB) signal pathways, and mRNA expression of inflammatory mediators including tumor necrosis factor-alpha, interleukin (IL)-1β, IL-6, inducible nitric oxide synthase, and cyclooxygenase-2 in the striatum after 3-NP intoxication. Interestingly, the intrathecal introduction of inhibitors MAPKs and NF-κB into control mice decreased the lethality after 3-NP intoxication. Our findings indicate that EP may effectively alleviate 3-NP-induced striatal toxicity by inhibition of the MAPKs and NF-κB pathways in the striatum, and that EP has a wide therapeutic window, suggesting that EP may have therapeutic value in the treatment of aspects of HD's disease related to inflammation.

Alterations in STriatal-Enriched protein tyrosine Phosphatase expression, activation, and downstream signaling in early and late stages of the YAC128 Huntington's disease mouse model

Striatal neurodegeneration and synaptic dysfunction in Huntington's disease are mediated by the mutant huntingtin (mHtt) protein. MHtt disrupts calcium homeostasis and facilitates excitotoxicity, in part by altering NMDA receptor (NMDAR) trafficking and function. Pre-symptomatic (excitotoxin-sensitive) transgenic mice expressing full-length human mHtt with 128 polyglutamine repeats (YAC128 Huntington's disease mice) show increased calpain activity and extrasynaptic NMDAR (Ex-NMDAR) localization and signaling. Furthermore, Ex-NMDAR stimulation facilitates excitotoxicity in wild-type cortical neurons via calpain-mediated cleavage of STriatal-Enriched protein tyrosine Phosphatase 61 (STEP61). The cleavage product, STEP33, cannot dephosphorylate p38 mitogen-activated protein kinase (MAPK), thereby augmenting apoptotic signaling. Here, we show elevated extrasynaptic calpain-mediated cleavage of STEP61 and p38 phosphorylation, as well as STEP61 inactivation and reduced extracellular signal-regulated protein kinase 1/2 phosphorylation (ERK1/2) in the striatum of 6-week-old, excitotoxin-sensitive YAC128 mice. Calpain inhibition reduced basal and NMDA-induced STEP61 cleavage. However, basal p38 phosphorylation was normalized by a peptide disrupting NMDAR-post-synaptic density protein-95 (PSD-95) binding but not by calpain inhibition. In 1-year-old excitotoxin-resistant YAC128 mice, STEP33 levels were not elevated, but STEP61 inactivation and p38 and ERK 1/2 phosphorylation levels were increased. These results show that in YAC128 striatal tissue, enhanced NMDAR-PSD-95 interactions contributes to elevated p38 signaling in early, excitotoxin-sensitive stages, and suggest that STEP61 inactivation enhances MAPK signaling at late, excitotoxin-resistant stages. The YAC128 Huntington's disease mouse model shows early, enhanced susceptibility to NMDA receptor-mediated striatal apoptosis, progressing to late-stage excitotoxicity resistance. This study shows that elevated NMDA receptor-PSD-95 interactions as well as decreased extrasynaptic STriatal-Enriched protein tyrosine Phosphatase 61 (STEP61) activation may contribute to early enhanced apoptotic signaling. In late-stage YAC128 mice, reduced STEP61 levels and activity correlate with elevated MAPK signaling, consistent with excitotoxicity resistance. Solid and dotted arrows indicate conclusions drawn from the current study and other literature, respectively.

HTT-lowering reverses Huntington's disease immune dysfunction caused by NFκB pathway dysregulation

Huntington's disease is an inherited neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin gene. The peripheral innate immune system contributes to Huntington's disease pathogenesis and has been targeted successfully to modulate disease progression, but mechanistic understanding relating this to mutant huntingtin expression in immune cells has been lacking. Here we demonstrate that human Huntington's disease myeloid cells produce excessive inflammatory cytokines as a result of the cell-intrinsic effects of mutant huntingtin expression. A direct effect of mutant huntingtin on the NFκB pathway, whereby it interacts with IKKγ, leads to increased degradation of IκB and subsequent nuclear translocation of RelA. Transcriptional alterations in intracellular immune signalling pathways are also observed. Using a novel method of small interfering RNA delivery to lower huntingtin expression, we show reversal of disease-associated alterations in cellular function-the first time this has been demonstrated in primary human cells. Glucan-encapsulated small interfering RNA particles were used to lower huntingtin levels in human Huntington's disease monocytes/macrophages, resulting in a reversal of huntingtin-induced elevated cytokine production and transcriptional changes. These findings improve our understanding of the role of innate immunity in neurodegeneration, introduce glucan-encapsulated small interfering RNA particles as tool for studying cellular pathogenesis ex vivo in human cells and raise the prospect of immune cell-directed HTT-lowering as a therapeutic in Huntington's disease.

Differential Changes in Postsynaptic Density Proteins in Postmortem Huntington's Disease and Parkinson's Disease Human Brains

NMDA and AMPA-type glutamate receptors and their bound membrane-associated guanylate kinases (MAGUKs) are critical for synapse development and plasticity. We hypothesised that these proteins may play a role in the changes in synapse function that occur in Huntington's disease (HD) and Parkinson's disease (PD). We performed immunohistochemical analysis of human postmortem brain tissue to examine changes in the expression of SAP97, PSD-95, GluA2 and GluN1 in human control, and HD- and PD-affected hippocampus and striatum. Significant increases in SAP97 and PSD-95 were observed in the HD and PD hippocampus, and PSD95 was downregulated in HD striatum. We observed a significant increase in GluN1 in the HD hippocampus and a decrease in GluA2 in HD and PD striatum. Parallel immunohistochemistry experiments in the YAC128 mouse model of HD showed no change in the expression levels of these synaptic proteins. Our human data show that major but different changes occur in glutamatergic proteins in HD versus PD human brains. Moreover, the changes in human HD brains differ from those occurring in the YAC128 HD mouse model, suggesting that unique changes occur at a subcellular level in the HD human hippocampus.

Neuroprotective effects of a novel single compound 1-methoxyoctadecan-1-ol isolated from Uncaria sinensis in primary cortical neurons and a photothrombotic ischemia model

We identified a novel neuroprotective compound, 1-methoxyoctadecan-1-ol, from Uncaria sinensis (Oliv.) Havil and investigated its effects and mechanisms in primary cortical neurons and in a photothrombotic ischemic model. In primary rat cortical neurons against glutamate-induced neurotoxicity, pretreatment with 1-methoxyoctadecan-1-ol resulted in significantly reduced neuronal death in a dose-dependent manner. In addition, treatment with 1-methoxyoctadecan-1-ol resulted in decreased neuronal apoptotic death, as assessed by nuclear morphological approaches. To clarify the neuroprotective mechanism of 1-methoxyoctadecan-1-ol, we explored the downstream signaling pathways of N-methyl-D-aspartate receptor (NMDAR) with calpain activation. Treatment with glutamate leads to early activation of NMDAR, which in turn leads to calpain-mediated cleavage of striatal-enriched protein tyrosine phosphatase (STEP) and subsequent activation of p38 mitogen activated protein kinase (MAPK). However, pretreatment with 1-methoxyoctadecan-1-ol resulted in significantly attenuated activation of GluN2B-NMDAR and a decrease in calpain-mediated STEP cleavage, leading to subsequent attenuation of p38 MAPK activation. We confirmed the critical role of p38 MAPK in neuroprotective effects of 1-methoxyoctadecan-1-ol using specific inhibitor SB203580. In the photothrombotic ischemic injury in mice, treatment with 1-methoxyoctadecan-1-ol resulted in significantly reduced infarct volume, edema size, and improved neurological function. 1-methoxyoctadecan-1-ol effectively prevents cerebral ischemic damage through down-regulation of calpain-mediated STEP cleavage and activation of p38 MAPK. These results suggest that 1-methoxyoctadecan-1-ol showed neuroprotective effects through down-regulation of calpain-mediated STEP cleavage with activation of GluN2B-NMDAR, and subsequent alleviation of p38 MAPK activation. In addition, 1-methoxyoctadecan-1-ol might be a useful therapeutic agent for brain disorder such as ischemic stroke.

Bidirectional control of postsynaptic density-95 (PSD-95) clustering by Huntingtin

Huntington disease is associated with early alterations in corticostriatal synaptic function that precede cell death, and it is postulated that ameliorating such changes may delay clinical onset and/or prevent neurodegeneration. Although many of these synaptic alterations are thought to be attributable to a toxic gain of function of the mutant huntingtin protein, the role that nonpathogenic huntingtin (HTT) plays in synaptic function is relatively unexplored. Here, we compare the immunocytochemical localization of a major postsynaptic scaffolding protein, PSD-95, in striatal neurons from WT mice and mice overexpressing HTT with 18 glutamine repeats (YAC18, nonpathogenic). We found that HTT overexpression resulted in a palmitoylation- and BDNF-dependent increase in PSD-95 clustering at synaptic sites in striatal spiny projection neurons (SPNs) co-cultured with cortical neurons. Surprisingly, the latter effect was mediated presynaptically, as HTT overexpression in cortical neurons alone was sufficient to increase PSD-95 clustering in the postsynaptic SPNs. In contrast, antisense oligonucleotide knockdown of HTT in WT co-cultures resulted in a significant reduction of PSD-95 clustering in SPNs. Notably, despite these bidirectional changes in PSD-95 clustering, we did not observe an alteration in basal electrophysiological measures of AMPA and NMDA receptors. Thus, unlike in previous studies in the hippocampus, enhanced or decreased PSD-95 clustering alone was insufficient to drive AMPA or NMDA receptors into or out of SPN synapses. In all, our results demonstrate that nonpathogenic HTT can indeed influence synaptic protein localization and uncover a novel role of HTT in PSD-95 distribution.

An increase in the association of GluN2B containing NMDA receptors with membrane scaffolding proteins was related to memory declines during aging

The NMDA receptor is an important component of spatial working and reference memory. The receptor is a heterotetramer composed of a family of related subunits. The GluN2B subunit of the NMDA receptor appears to be essential for some forms of memory and is particularly vulnerable to change with age in both the hippocampus and cerebral cortex. GluN2B expression is particularly reduced in frontal cortex synaptic membranes. The current study examined the relationship between spatial cognition and protein-protein interactions of GluN2B-containing NMDA receptors in frontal cortex crude synaptosome from 3, 12, and 26-month-old C57BL/6 mice. Aged mice showed a significant decline in spatial reference memory and reversal learning from both young and middle-aged mice. Coimmunoprecipitation of GluN2B subunits revealed an age-related increase in the ratio of both postsynaptic density-95 (PSD-95) and the GluN2A subunit to the GluN2B subunit. Higher ratios of PSD-95/GluN2B and GAIP-interacting protein C-terminus (GIPC)/GluN2B were associated with poorer learning index scores across all ages. There was a significant correlation between GIPC/GluN2B and PSD-95/GluN2B ratios, but PSD-95/GluN2B and GluN2A/GluN2B ratios did not show a relationship. These results suggest that there were more triheteromeric (GluN2B/GluN2A/GluN1) NMDA receptors in older mice than in young adults, but this did not appear to impact spatial reference memory. Instead, an increased association of GluN2B-containing NMDA receptors with synaptic scaffolding proteins in aged animals may have contributed to the age-related memory declines.

Modulation of NMDA receptor at the synapse: promising therapeutic interventions in disorders of the nervous system

There is general agreement that excessive activation of N-methyl-D-aspartate (NMDA) receptors plays a key role in mediating at least some aspects of synaptic dysfunction in several central nervous system disorders. On this view, in the last decades, research focused on the discovery of different compounds able to reduce NMDA receptor activity, such as classical and/or subunit-specific antagonists. However, the increasing body of knowledge on specific signaling pathways downstream NMDA receptors led to the identification of new pharmacological targets for NMDA receptor-related pathological conditions. Moreover, besides over-activation, several studies indicated that also abnormal NMDA receptor trafficking, resulting in the modification of the receptor subunit composition at the synapse, has a major role in the pathogenesis of several brain disorders. For this reason, the discovery of the molecular mechanisms regulating the abundance of synaptic versus extra-synaptic NMDA receptors as well as the activation of the specific signaling pathways downstream the different NMDA receptor subtypes is needed for the development of novel therapeutic approaches for NMDA receptor-dependent synaptic dysfunction.

Age-dependent dystonia in striatal Gγ7 deficient mice is reversed by the dopamine D2 receptor agonist pramipexole

Gγ7 is enriched in striatum and forms a heterotrimeric complex with Gαolf /Gβ, which is coupled to D1 receptor (D1R). Here, we attempted to characterize the pathophysiological, neurochemical, and pharmacological features of mice deficient of Gγ7 gene. Gγ7 knockout mice exhibited age-dependent deficiency in rotarod behavior and increased dystonia-like clasping reflex without loss of striatal neurons. The neurochemical basis for the motor manifestations using immunoblot analysis revealed increased levels of D1R, ChAT and NMDA receptor subunits (NR1 and NR2B) concurrent with decreased levels of D2R and Gαolf , possibly because of the secondary changes of decreased Gαolf /Gγ7-mediated D1R transmission. These behavioral and neurochemical changes are closely related to those observed in Huntington's disease (HD) human subjects and HD model mice. Taking advantage of the finding of D2R down-regulation in Gγ7 knockout mice and the dopamine-mediated synergistic relationship in the control of locomotion between D2R-striatopallidal and D1R-stritonigral neurons, we hypothesized that D2-agonist pramipexole would reverse behavioral dyskinesia caused by defective D1R/Gαolf signaling. Indeed, the rotarod deficiency and clasping reflex were reversed by pramipexole treatment under chronic administration. These findings suggest that Gγ7 knockout mice could be a new type of movement disorders, including HD and useful for the evaluation of therapeutic candidates.

Korean Red Ginseng Extract Attenuates 3-Nitropropionic Acid-Induced Huntington's-Like Symptoms

Korean red ginseng (KRG) possesses neuroprotective activity. However, the potential neuroprotective value of KRG for the striatal toxicity is largely unknown. We investigated whether KRG extract (KRGE) could have a neuroprotective effect in a 3-nitropropionic acid- (3-NP) induced (i.p.) Huntington's disease (HD) model. KRGE (50, 100, and 250 mg/kg/day, p.o.) was administrated 10 days before 3-NP injection (pre-administration), from the same time with 3-NP injection (co-administration), or from the peak point of neurological impairment by 3-NP injection (post-administration). Pre-administration of KRGE produced the greatest neuroprotective effect in this model. Pre-administration of KRGE significantly decreased 3-NP-induced neurological impairment, lethality, lesion area, and neuronal loss in the 3-NP-injected striatum. KRGE attenuated microglial activation and phosphorylation of mitogen-activated protein kinases (MAPKs) and nuclear factor-kappa B (NF-κB) signal pathway. KRGE also reduced the level of mRNA expression of tumor necrosis factor-alpha, interleukin- (IL-) 1β, IL-6, inducible nitric oxide synthase, and OX-42. Interestingly, the intrathecal administration of SB203580 (a p38 inhibitor) or PD98059 (an inhibitor of MAPK Kinase, MEK) increased the survival rate in the 3-NP-induced HD model. Pre-administration of KRGE may effectively inhibit 3-NP-induced striatal toxicity via the inhibition of the phosphorylation of MAPKs and NF-κB pathways, indicating its therapeutic potential for suppressing Huntington's-like symptoms.

The intersections of NMDAR-dependent synaptic plasticity and cell survival

The discovery of a requirement for N-methyl d-aspartate receptor (NMDAR) activation in long-term potentiation (LTP) set off an explosion of interest in the mechanisms of NMDAR-dependent synaptic plasticity. Meanwhile other research has advanced our understanding of how NMDAR activation regulates neuronal death and survival. Surprisingly, there have been few attempts to correlate these important areas of research. Here we review current knowledge of the various mechanisms of NMDAR-dependent synaptic plasticity that are shared with neuronal survival and death, while drawing comparisons with the proneurotrophin/neurotrophin receptor and intracellular signaling systems. Our conclusion is that NMDAR-dependent LTP and long-term depression (LTD) share many common mechanisms with cell survival and cell death, respectively. The intersections of plasticity and cell survival may represent novel avenues for neuroprotection. This article is part of the Special Issue entitled 'Glutamate Receptor-Dependent Synaptic Plasticity'.

Pathophysiology of Huntington's disease: time-dependent alterations in synaptic and receptor function

Huntington's disease (HD) is a progressive, fatal neurological condition caused by an expansion of CAG (glutamine) repeats in the coding region of the Huntington gene. To date, there is no cure but great strides have been made to understand pathophysiological mechanisms. In particular, genetic animal models of HD have been instrumental in elucidating the progression of behavioral and physiological alterations, which had not been possible using classic neurotoxin models. Our groups have pioneered the use of transgenic HD mice to examine the excitotoxicity hypothesis of striatal neuronal dysfunction and degeneration, as well as alterations in excitation and inhibition in striatum and cerebral cortex. In this review, we focus on synaptic and receptor alterations of striatal medium-sized spiny (MSNs) and cortical pyramidal neurons in genetic HD mouse models. We demonstrate a complex series of alterations that are region-specific and time-dependent. In particular, many changes are bidirectional depending on the degree of disease progression, that is, early vs. late, and also on the region examined. Early synaptic dysfunction is manifested by dysregulated glutamate release in striatum followed by progressive disconnection between cortex and striatum. The differential effects of altered glutamate release on MSNs originating the direct and indirect pathways is also elucidated, with the unexpected finding that cells of the direct striatal pathway are involved early in the course of the disease. In addition, we review evidence for early N-methyl-D-aspartate receptor (NMDAR) dysfunction leading to enhanced sensitivity of extrasynaptic receptors and a critical role of GluN2B subunits. Some of the alterations in late HD could be compensatory mechanisms designed to cope with early synaptic and receptor dysfunctions. The main findings indicate that HD treatments need to be designed according to the stage of disease progression and should consider regional differences.