Jeganathan Ramesh Babu

Sequestosome 1/p62 Is a Polyubiquitin Chain Binding Protein Involved in Ubiquitin Proteasome Degradation

ABSTRACT Herein, we demonstrate that the ubiquitin-associated (UBA) domain of sequestosome 1/p62 displays a preference for binding K63-polyubiquitinated substrates. Furthermore, the UBA domain of p62 was necessary for aggregate sequestration and cell survival. However, the inhibition of proteasome function compromised survival in cells with aggregates. Mutational analysis of the UBA domain reveals that the conserved hydrophobic patch MGF as well as the conserved leucine in helix 2 are necessary for binding polyubiquitinated proteins and for sequestration-aggregate formation. We report that p62 interacts with the proteasome by pull-down assay, coimmunoprecipitation, and colocalization. Depletion of p62 levels results in an inhibition of ubiquitin proteasome-mediated degradation and an accumulation of ubiquitinated proteins. Altogether, our results support the hypothesis that p62 may act as a critical ubiquitin chain-targeting factor that shuttles substrates for proteasomal degradation.

Sequestosome 1/p62 shuttles polyubiquitinated tau for proteasomal degradation

Inclusions isolated from several neurodegenerative diseases, including Alzheimers disease (AD), are characterized by ubiquitin‐positive proteinaceous aggregates. Employing confocal and immunoelectron microscopy, we find that the ubiquitin‐associating protein sequestosome1/p62, co‐localizes to aggregates isolated from AD but not control brain, along with the E3 ubiquitin ligase, TRAF6. This interaction could be recapitulated by co‐transfection in HEK293 cells. Employing both in vitro and in vivo approaches, tau was found to be a substrate of the TRAF6, possessing lysine 63 polyubiquitin chains. Moreover, tau recovered from brain of TRAF6 knockout mice, compared with wild type, was not ubiquitinated. Tau degradation took place through the ubiquitin–proteasome pathway and was dependent upon either the K63‐polyubiquitin chains or upon p62. In brain lysates of p62 knockout mice, tau fails to co‐interact with Rpt1, a proteasomal subunit, thereby indicating a requirement for p62 shuttling of tau to the proteasome. Our results demonstrate that p62 interacts with K63‐polyubiquitinated tau through its UBA domain and serves a novel role in regulating tau proteasomal degradation. We propose a model whereby either a decline in p62 expression or a decrease in proteasome activity may contribute to accumulation of insoluble/aggregated K63‐polyubiquitinated tau.

Neuroprotective effects of resveratrol in Alzheimer disease pathology

Alzheimer’s disease is a chronic neurodegenerative disorder characterized by a progressive loss of cognitive and behavioral abilities. Extracellular senile plaques and intracellular neurofibrillary tangles are hallmarks of AD. Researchers aim to analyze the molecular mechanisms underlying AD pathogenesis; however, the therapeutic options available to treat this disease are inadequate. In the past few years, several studies have reported interesting insights about the neuroprotective properties of the polyphenolic compound resveratrol (3, 5, 4′-trihydroxy-trans-stilbene) when used with in vitro and in vivo models of AD. The aim of this review is to focus on the neuroprotective and antioxidant effects of resveratrol on AD and its multiple potential mechanisms of action. In addition, because the naturally occurring forms of resveratrol have a very limited half-life in plasma, a description of potential analogs aimed at increasing the bioavailability in plasma is also discussed.

Signaling of the neurotrophin receptor p75 in relation to Alzheimer's disease

The cellular mechanism of neuronal apoptosis in Alzheimers disease (AD) is poorly understood. Many hypotheses have been put fourth to explain the underlying reason for neuro-degeneration in AD. Here, it is demonstrated that all neurotrophins that activated p75, without co-activation of the relevant Trk co-receptor, mediated apoptosis in hippocampal neurons. Thus, proneurotrophins and amyloid beta peptides (Abeta) can induce p75-mediated apoptosis in hippocampal neurons since they do not bind or activate Trk receptors. Based on the combined effects of aging, proneurotrophins, neurotrophins, and Abeta, a novel model of pathogenesis in AD is proposed. This mini-review explores the ligand and cell type based signaling pathways of the neurotrophin receptor p75 relating to Alzheimers disease.

Sequestosome 1/p62: across diseases

Sequestosome 1/p62 is a signal modulator or adaptor protein involved in receptor-mediated signal transduction. Sequestosome 1/p62 is gaining attention as it is involved in several diseases including Parkinson disease, Alzheimer disease, liver and breast cancer, Paget’s disease of bone, obesity and insulin resistance. In this review, we will focus on the most recent advances on the physiological function of p62 relevant to human diseases.

Resveratrol Protects the Brain of Obese Mice from Oxidative Damage

Resveratrol (3,5,4′-trihydroxy-trans-stilbene) is a polyphenolic phytoalexin that exerts cardioprotective, neuroprotective, and antioxidant effects. Recently it has been shown that obesity is associated with an increase in cerebral oxidative stress levels, which may enhance neurodegeneration. The present study evaluates the neuroprotective action of resveratrol in brain of obese (ob/ob) mice. Resveratrol was administered orally at the dose of 25 mg kg−1 body weight daily for three weeks to lean and obese mice. Resveratrol had no effect on body weight or blood glucose levels in obese mice. Lipid peroxides were significantly increased in brain of obese mice. The enzymatic antioxidants superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase, glucose-6-phosphate dehydrogenase and nonenzymatic antioxidants tocopherol, ascorbic acid, and glutathione were decreased in obese mice brain. Administration of resveratrol decreased lipid peroxide levels and upregulated the antioxidant activities in obese mice brain. Our findings indicate a neuroprotective effect of resveratrol by preventing oxidative damage in brain tissue of obese mice.

TRAF6 and p62 inhibit amyloid β-induced neuronal death through p75 neurotrophin receptor.

Amyloid β (Aβ) aggregates are the primary component of senile plaques in Alzheimer disease (AD) patients brain. Aβ is known to bind p75 neurotrophin receptor (p75(NTR)) and mediates Aβ-induced neuronal death. Recently, we showed that NGF leads to p75(NTR) polyubiquitination, which promotes neuronal cell survival. Here, we demonstrate that Aβ stimulation impaired the p75(NTR) polyubiquitination. TRAF6 and p62 are required for polyubiquitination of p75(NTR) on NGF stimulation. Interestingly, we found that overexpression of TRAF6/p62 restored p75(NTR) polyubiquitination upon Aβ/NGF treatment. Aβ significantly reduced NF-κB activity by attenuating the interaction of p75(NTR) with IKKβ. p75(NTR) increased NF-κB activity by recruiting TRAF6/p62, which thereby mediated cell survival. These findings indicate that TRAF6/p62 abrogated the Aβ-mediated inhibition of p75(NTR) polyubiquitination and restored neuronal cell survival.

Sequestosome 1/p62, a Scaffolding Protein, Is a Newly Identified Partner of IRS-1 Protein

Background: Sequestosome 1/p62 is a scaffolding protein that plays a critical role in receptor-mediated signal transduction. Results: p62 interacts with IRS-1 to enhance the Akt phosphorylation, GLUT4 translocation, and glucose uptake upon insulin stimulation. Conclusion: p62 participates in IRS-1 insulin signaling. Significance: p62 plays a role in insulin-signaling, which provides a potential mechanism for its connection to type 2 diabetes. Defects in the insulin-signaling pathway may lead to the development of skeletal muscle insulin resistance, which is one of the earliest abnormalities detected in individuals with the metabolic syndrome and predisposes them to develop type 2 diabetes. Previous studies have shown that deletion of the mouse sequestosome 1/p62 gene results in mature-onset obesity that progresses to insulin and leptin resistance and, ultimately, type 2 diabetes. Sequestosome 1/p62 is involved in receptor-mediated signal transduction and functions as an intracellular signal modulator or adaptor protein. Insulin receptor substrate-1 (IRS-1) plays a central role in transducing the insulin signal via phosphorylation, protein-protein interactions, and protein modifications. Mapping studies demonstrated that the SH2 domain at the amino terminus of sequestosome 1/p62 interacts with IRS-1 upon insulin stimulation. Further, IRS-1 interacts with p62 through its YMXM motifs at Tyr-608, Tyr-628, and/or Tyr-658 in a manner similar to its interaction with p85 of phosphoinositol 3-kinase. Overexpression of p62 increased phosphorylation of Akt, GLUT4 translocation, and glucose uptake, providing evidence that p62 participates in the insulin-signaling pathway through its interactions with IRS-1.

Noncoding RNAs in Neurodegenerative Diseases

Noncoding RNAs are widely known for their various essential roles in the development of central nervous system. It involves neurogenesis, neural stem cells generation, maintenance and maturation, neurotransmission, neural network plasticity, formation of synapses, and even brain aging and DNA damage responses. In this review, we will discuss the biogenesis of microRNA, various functions of noncoding RNAs specifically microRNAs (miRNAs) that act as the chief regulators of gene expression, and focus in particular on misregulation of miRNAs which leads to several neurodegenerative diseases as well as its therapeutic outcome. Recent evidences has shown that miRNAs expression levels are changed in patients with neurodegenerative diseases; hence, miRNA can be used as a potential diagnostic biomarker and serve as an effective therapeutic tool in overcoming various neurodegenerative disease processes.

High fat diet induces brain insulin resistance and cognitive impairment in mice.

High fat diet-induced obesity is associated with insulin resistance (IR) and other chronic, diet related illnesses, including dementia. Alzheimer disease is the most common form of dementia, and is characterized by the presence of amyloid plaques and neurofibrillary tangles in brain. This study was designed to determine whether diet-induced changes in peripheral insulin sensitivity could contribute to alterations in brain insulin signaling and cognitive functions. Six week old, male C57BL/6NHsd mice were randomly assigned a high fat diet (40% energy from fat) with 42g/L liquid sugar (HFS) added to the drinking water or a normal chow diet (12% energy from fat) for 14weeks. Metabolic phenotypes were characterized for energy expenditure, physical activity, and food intake, and glucose and insulin tolerance tests. In addition, we examined the changes in protein expression related to brain insulin signaling and cognitive function. Mice fed HFS exhibited a statistically significant increase in obesity, and lower glucose and insulin tolerance as compared to animals fed the normal chow diet. In brain, HFS elicited IR as evidenced by a significant decrease in tyrosine phosphorylation of insulin receptor and an increase serine phosphorylation of IRS-1. These changes were accompanied by inflammatory (NFκB, JNK) and stress responses (p38 MAPK, CHOP) in whole brain lysate. In addition, HFS mouse brain exhibited biochemical changes related to increased amyloid beta deposition and neurofibrillary tangle formation, and decreased synaptic plasticity. These results suggested changes in insulin sensitivity might contribute to cognitive impairment associated with the HFS diet in mice.