Kodeeswaran Parameshwaran

Amyloid beta peptides and glutamatergic synaptic dysregulation

Alzheimers disease (AD) is a major neurodegenerative disorder in which overproduction and accumulation of amyloid beta (Abeta) peptides result in synaptic dysfunction. Recent reports strongly suggest that in the initial stages of AD glutamate receptors are dysregulated by Abeta accumulation resulting in disruption of glutamatergic synaptic transmission which parallels early cognitive deficits. In the presence of Abeta, 2-amino-3-(3-hydoxy-5-methylisoxazol-4-yl) propionic acid (AMPA) glutamate receptor function is disrupted and the surface expression is reduced. Abeta has also been shown to modulate N-methyl-d-aspartate receptors (NMDARs) and metabotropic glutamate receptors. The Abeta mediated glutamate receptor modifications can lead to synaptic dysfunction resulting in excitotoxic neurodegeneration during the progression of AD. This review discusses the recent findings that glutamatergic signaling could be compromised by Abeta induced modulation of synaptic glutamate receptors in specific brain regions.

Central insulin resistance and synaptic dysfunction in intracerebroventricular-streptozotocin injected rodents

To better understand the role of insulin signaling in the development of Alzheimers disease (AD), we utilized an animal model (intracerebroventricular injection of streptozotocin-ic-streptozotocin (STZ)) that displays insulin resistance only in the brain and exhibits AD pathology. In this model, deficits in hippocampal synaptic transmission and long-term potentiation (LTP) were observed. The decline in LTP correlated with decreased expression of NMDAR subunits NR2A and NR2B. The deficits in LTP were accompanied by changes in the expression and function of synaptic AMPARs. In ic-STZ animals, an alteration in integrin-linked kinase (ILK)-glycogen synthase kinase 3 beta (GSK-3-β) signaling was identified (p < 0.05). Similarly, there was decreased expression (p < 0.05) of brain derived neurotropic factor (BDNF) and stargazin, an AMPAR auxiliary subunit; both are required for driving AMPA receptors to the surface of the postsynaptic membrane. Our data illustrate that altered ILK-GSK-3β signaling due to impaired insulin signaling may decrease the trafficking and function of postsynaptic glutamate receptors; thereby, leading to synaptic deficits contributing to memory loss.

D-Galactose Effectiveness in Modeling Aging and Therapeutic Antioxidant Treatment in Mice

Accumulating evidence suggests that mitochondrial dysfunction and oxidative stress play major roles in aging. Chronic administration of D-galactose has been reported to cause deterioration of cognitive and motor skills that are similar to symptoms of aging and, therefore, is regarded as a model of accelerated aging. Because enhancing endogenous antioxidants is now widely regarded as an attractive therapy for conditions associated with mitochondrial oxidative stress, in the present study the effects of α-lipoic acid, L-carnitine, and PMX-500F on D-galactose treated mice were tested. Female mice were injected with (100 mg/kg) D-(+)-galactose for 6 weeks and some groups were treated with a daily dose of α-lipoic acid (5 mg/kg), L-carnitine (3.9 mg/kg), PMX-500F (11.9 mg/kg), or the vehicle (0.1 M Tris, pH 7.4). Control mice were treated with physiological saline. An accelerating Rota-Rod, open field test, and Y-maze test were performed, and serum lactate concentrations were analyzed. These analyses did not identify impairment in motor coordination, open-field activity, or spatial memory (p > 0.05). Similarly, serum lactate concentrations in D-galactose-treated mice were not elevated when compared to controls (p > 0.05). Treatment with the antioxidant compounds at the given concentrations did not result in any changes in the behavioral parameters tested. In conclusion, results of this study illustrate that chronic, short-term D-galactose treatment may not represent a suitable model for inducing readily detectable age-related neurobehavioral symptoms in mice.

Central activation of PPAR-gamma ameliorates diabetes induced cognitive dysfunction and improves BDNF expression.

Diabetes and Alzheimers disease share pathologic links toward cognitive deficits. Pharmacologic agonist of the nuclear receptor, peroxisomal proliferator-activating receptor gamma (PPARγ), that is, rosiglitazone (rosi), are insulin sensitizing agents that improve memory in Alzheimers disease. However, direct molecular signaling targets that improve memory by PPARγ in the hippocampus have not been investigated. We compared outcomes from oral versus intracerebroventricular (ICV) administration of rosi on memory and changes in synaptic plasticity in type 2 diabetic (db/db) mice. Db/db mice treated with rosi (ICV) showed significant improvement in memory, long-term potentiation, and post-tetanic potentiation but did not improve peripheral insulin sensitivity. Gene and protein analysis revealed increased brain-derived neurotrophic factor (BDNF) in db/db mice treated with rosi (ICV). Transcriptional activation of exon IX as determined by luciferase assays confirmed PPARγ regulation of BDNF promoter activity. Transient transfection of constitutively active PPARγ plasmid in hippocampal neuronal cells induced increased BDNF, AMPA, and NMDA receptors expression and spine formation. Findings from the present study implicate a novel PPARγ-BDNF molecular signaling mechanism as a potential therapeutic target for cognitive impairment.

AMPA receptor trafficking and synaptic plasticity require SQSTM1/p62.

SQSTM1/p62 is a multidomain/scaffold for the atypical protein kinase Cs (aPKC). Phosphorylation of AMPA receptors by PKC has been shown to regulate their insertion in the postsynaptic membrane. Here, we directly tested whether p62 could interact with AMPA receptor subunits and influence their trafficking and phosphorylation. GluR1 receptor intracellular loop L2–3 and the ZZ‐type zinc finger domain of p62 are essential for the interaction between these two proteins. In this context, both p62 and aPKC‐mediated phosphorylation were necessary for surface delivery of the receptor. Our findings reveal that p62 is the first protein identified that interacts with a region of the GluR receptor other than the C‐terminal tail. Furthermore, mice deficient in p62 displayed impaired hippocampal CA1 long‐term potentiation (LTP), along with diminished surface expression of GluR1 and phosphorylation of S818. Lastly, we identify a conserved sequence (ISExSL) shared by all p62 interacting‐aPKC substrates. These findings support a model where p62 interaction and aPKC phosphorylation act together to mediate AMPA receptor trafficking and long‐term synaptic plasticity in the hippocampus.

Mouse models of mitochondrial complex I dysfunction

Diseases of the mitochondria generally affect cells with high-energy demand, and appear to most profoundly affect excitatory cells that have localized high energy requirements, such as neurons and cardiac and skeletal muscle cells. Complex I of the mammalian mitochondrial respiratory chain is a very large, 45 subunit enzyme, and functional deficiency of complex I is the most frequently observed cause of oxidative phosphorylation (OXPHOS) disorders. Impairment of complex I results in decreased cellular energy production and is responsible for a variety of human encephalopathies, myopathies and cardiomyopathies. Complex I deficiency may be caused by mutations in any of the seven mitochondrial or 38 nuclear genes that encode complex I subunits or by mutations in various other nuclear genes that affect complex I assembly or function. Mouse models that faithfully mimic human complex I disorders are needed to better understand the role of complex I in health and disease and for evaluation of potential therapies for mitochondrial diseases. In this review we discuss existing mouse models of mitochondrial complex I dysfunction, focusing on those with similarities to human mitochondrial disorders. We also discuss some of the noteworthy murine genetic models in which complex I genes are not disrupted, but complex I dysfunction is observed, along with some of the more popular chemical compounds that inhibit complex I function and are useful for modeling complex I deficiency in mice. This article is part of a Directed Issue entitled: Bioenergetic dysfunction, adaptation and therapy.

Ebselen effects on MPTP-induced neurotoxicity.

We evaluated the effect of ebselen on human SH-SY5Y dopaminergic neuronal cells and determined whether ebselen, a glutathione peroxidase-mimetic, protected against MPTP-induced dopamine depletion in mice. Ebselen (10-100 microM) inhibited the proliferation of SH-SY5Y cells dose-dependently. Ebselen did not induce any behavioral changes and did not block MPTP-induced tremor and akinesia. Ebselen had no effect on the monoamine oxidase activity and did not protect against MPTP-induced dopamine depletion in striatum.

Protection by an antioxidant of rotenone-induced neuromotor decline, reactive oxygen species generation and cellular stress in mouse brain

Exposure to environmental toxins, including rotenone, results in central nervous system and systemic toxicity. Rotenone is a widely used pesticide and a mitochondrial complex I (CI) inhibitor. This study reports effectiveness of a synthetic lipoylcarnitine antioxidant compound, lipoylcarnitine methyl ester iodide (PMX-500F), for treatment of chronic rotenone induced neurological deficits in mice. Mice (C57BL/6NTac; two months of age) received oral administration of rotenone (30 mg/kg/day) or vehicle, preceded by intraperitoneal injection of PMX-500F (19 mg/kg) or vehicle for four weeks. In the Rota-rod test, rotenone treatment had no effect (P>0.05); however, PMX-500F treatment improved locomotor coordination and endurance (latency to fall time; P<0.05). For neuromuscular strength (wire hang test), rotenone treated mice showed reduced latency to fall compared to control mice (P<0.05). PMX-500F treatment improved the outcome in both control and rotenone exposed mice (P<0.05). Rotenone administration increased ROS generation in the forebrain and midbrain regions, but not in the cerebellum (P<0.05). Co-treatment with PMX-500F normalized the ROS in forebrain and midbrain regions to that of the control concentrations. In rotenone administered mice, activated stress-activated protein kinase/c-Jun NH2-terminal kinase (pSAPK/JNK) was higher in forebrain and midbrain lysates than in control mice (P<0.05) and treatment with PMX-500F reduced pSAPK/JNK to control levels. The pSAPK/JNK levels in the cerebellum were similar in all four groups (P>0.05). Total SAPK/JNK was not altered by either rotenone or PMX-500F treatment (P>0.05). These results illustrate that an antioxidant, here PMX-500F, provides protection against rotenone induced decline in neuromotor function, reactive oxygen species (ROS) generation and cellular stress.

Lysosomal dysfunction produces distinct alterations in synaptic α-amino -3 -hydroxy -5 -methylisoxazolepropionic acid and N-Methyl-D-Aspartate receptor currents in hippocampus

The early processes that lead to synaptic dysfunction during aging are not clearly understood. Dysregulation of &agr;-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors may cause age-related cognitive decline. Using hippocampal slice cultures exhibiting lysosomal dysfunction, an early marker of brain aging that is linked to pr otein accumulation, we identified alterations to AMPA and NMDA receptor-mediated synaptic currents. The miniature and spontaneous excitatory postsynaptic currents that were examined after 3, 6, and 9 days of lysosomal disruption showed progressive changes in amplitude, frequency, and rise and decay kinetics. To investigate whether modifications in specific channel properties of single synaptic receptors contributed to changes in the amplitude and time course of synaptic currents, we examined the single channel properties of synaptic AMPA and NMDA receptors. The channel open probability and the mean open times showed decreases in both receptor populations, whereas the closed times were increased without any change in the channel conductance. The Western blot analysis revealed a progressive decline in synaptic markers including glutamate receptor subunits. These results indicate that lysosomal dysfunction leads to progressive functional perturbation of AMPA and NMDA receptors in this slice model of protein accumulation, suggesting that age-related cognitive decline could result from altered glutamate receptor function before reductions in synaptic density.

Ampakine CX516 ameliorates functional deficits in AMPA receptors in a hippocampal slice model of protein accumulation.

AMPAkines are positive modulators of AMPA receptors, and previous work has shown that these compounds can facilitate synaptic plasticity and improve learning and memory in both animals and humans; thus, their role in the treatment of cognitive impairment is worthy of investigation. In this study, we have utilized an organotypic slice model in which chloroquine-induced lysosomal dysfunction produces many of the pathogenic attributes of Alzheimers disease. Our previous work demonstrated that synaptic AMPA receptor function is impaired in hippocampal slice cultures exhibiting lysosomal dysfunction leading to protein accumulation. The present study investigated the effect of the AMPAkine CX516 on AMPAR-mediated synaptic transmission as well as the CX516 induced modification of single channel AMPA receptor properties in this organotypic slice-culture model. In whole cell recordings from CA1 pyramidal neurons in chloroquine-treated slices we observed a significant decrease in AMPAR-mediated mEPSC frequency and amplitude indicating synaptic dysfunction. Following application of CX516, these parameters returned to nearly normal levels. Similarly, we report chloroquine-induced impairment of AMPAR single channel properties (decreased probability of opening and mean open time), and significant recovery of these properties following CX516 administration. These results suggest that AMPA receptors may be potential pharmaceutical targets for the treatment of neurodegenerative diseases, and highlights AMPAkines, in particular, as possible therapeutic agents.