Insulin receptor substrate protein p53 localization in rats suggests mechanism for specific polyglutamine neurodegeneration

Multivariate analysis of subjective responses to d-amphetamine in healthy volunteers finds novel genetic pathway associations

RATIONALE

Researchers studying behavioral and physiologic effects of d-amphetamine have explored individual response differences to the drug. Concurrently, genome-wide analyses have identified several single-nucleotide polymorphisms (SNPs) associated with these traits. Univariate methods can identify SNPs associated with behavioral and physiological traits, but multivariate analyses allow identification of clusters of related biologically relevant SNPs and behavioral components.

OBJECTIVES

The aim of the study was to identify clusters of related biologically relevant SNPs and behavioral components in the responses of healthy individuals to d-amphetamine using multivariate analysis.

METHODS

Individuals (N = 375) without substance abuse histories completed surveys and detailed cardiovascular monitoring during randomized, blinded sessions: d-amphetamine (10 and 20 mg) and placebo. We applied parallel independent component analysis (Para-ICA) to data previously analyzed with univariate approaches, revealing new associations between genes and behavioral responses to d-amphetamine.

RESULTS

Three significantly associated (p < .001) phenotype-genotype pairs emerged. The first component included physiologic measures of systolic and diastolic blood pressure (BP) and mean arterial pressure (MAP) along with SNPs in calcium and glutamatergic signaling pathways. The second associated components included the "Anger" items from the Profile of Mood States (POMS) questionnaire and the marijuana effects from the Addiction Research Center Inventory (Cuyas, Verdejo-Garcia et al.), with enriched genetic pathways involved in cardiomyopathy and MAPK signaling. The final pair included "Anxious," "Fatigue," and "Confusion" items from the POMS questionnaire, plus functional pathways related to cardiac muscle contraction and cardiomyopathy.

CONCLUSIONS

Multifactorial genetic networks related to calcium signaling, glutamatergic and dopaminergic synapse function, and amphetamine addiction appear to mediate common behavioral and cardiovascular responses to d-amphetamine.

A large scale Huntingtin protein interaction network implicates Rho GTPase signaling pathways in Huntington disease

Huntington disease (HD) is an inherited neurodegenerative disease caused by a CAG expansion in the HTT gene. Using yeast two-hybrid methods, we identified a large set of proteins that interact with huntingtin (HTT)-interacting proteins. This network, composed of HTT-interacting proteins (HIPs) and proteins interacting with these primary nodes, contains 3235 interactions among 2141 highly interconnected proteins. Analysis of functional annotations of these proteins indicates that primary and secondary HIPs are enriched in pathways implicated in HD, including mammalian target of rapamycin, Rho GTPase signaling, and oxidative stress response. To validate roles for HIPs in mutant HTT toxicity, we show that the Rho GTPase signaling components, BAIAP2, EZR, PIK3R1, PAK2, and RAC1, are modifiers of mutant HTT toxicity. We also demonstrate that Htt co-localizes with BAIAP2 in filopodia and that mutant HTT interferes with filopodial dynamics. These data indicate that HTT is involved directly in membrane dynamics, cell attachment, and motility. Furthermore, they implicate dysregulation in these pathways as pathological mechanisms in HD.

Insulin, synaptic function, and opportunities for neuroprotection

A steadily growing number of studies have begun to establish that the brain and insulin, while traditionally viewed as separate, do indeed have a relationship. The uptake of pancreatic insulin, along with neuronal biosynthesis, provides neural tissue with the hormone. As well, insulin acts upon a neuronal receptor that, although a close reflection of its peripheral counterpart, is characterized by unique structural and functional properties. One distinction is that the neural variant plays only a limited part in neuronal glucose transport. However, a number of other roles for neural insulin are gradually emerging; most significant among these is the modulation of ligand-gated ion channel (LGIC) trafficking. Notably, insulin has been shown to affect the tone of synaptic transmission by regulating cell-surface expression of inhibitory and excitatory receptors. The manner in which insulin regulates receptor movement may provide a cellular mechanism for insulin-mediated neuroprotection in the absence of hypoglycemia and stimulate the exploration of new therapeutic opportunities.

Enhanced NMDA receptor-mediated synaptic transmission, enhanced long-term potentiation, and impaired learning and memory in mice lacking IRSp53

IRSp53 is an adaptor protein that acts downstream of Rac and Cdc42 small GTPases and is implicated in the regulation of membrane deformation and actin filament assembly. In neurons, IRSp53 is an abundant postsynaptic protein and regulates actin-rich dendritic spines; however, its in vivo functions have not been explored. We characterized transgenic mice deficient of IRSp53 expression. Unexpectedly, IRSp53(-/-) neurons do not show significant changes in the density and ultrastructural morphologies of dendritic spines. Instead, IRSp53(-/-) neurons exhibit reduced AMPA/NMDA ratio of excitatory synaptic transmission and a selective increase in NMDA but not AMPA receptor-mediated transmission. IRSp53(-/-) hippocampal slices show a markedly enhanced long-term potentiation (LTP) with no changes in long-term depression. LTP-inducing theta burst stimulation enhances NMDA receptor-mediated transmission. Spatial learning and novel object recognition are impaired in IRSp53(-/-) mice. These results suggest that IRSp53 is involved in the regulation of NMDA receptor-mediated excitatory synaptic transmission, LTP, and learning and memory behaviors.

The insulin receptor substrate of 53 kDa (IRSp53) limits hippocampal synaptic plasticity

IRSp53 is an essential intermediate between the activation of Rac and Cdc42 GTPases and the formation of cellular protrusions; it affects cell shape by coupling membrane-deforming activity with the actin cytoskeleton. IRSp53 is highly expressed in neurons where it is also an abundant component of the postsynaptic density (PSD). Here we analyze the physiological function of this protein in the mouse brain by generating IRSp53-deficient mice. Neurons in the hippocampus of young and adult knock-out (KO) mice do not exhibit morphological abnormalities in vivo. Conversely, primary cultured neurons derived from IRSp53 KO mice display retarded dendritic development in vitro. On a molecular level, Eps8 cooperates with IRSp53 to enhance actin bundling and interacts with IRSp53 in developing neurons. However, postsynaptic Shank proteins which are expressed at high levels in mature neurons compete with Eps8 to block actin bundling. In electrophysiological experiments the removal of IRSp53 increases synaptic plasticity as measured by augmented long term potentiation and paired-pulse facilitation. A primarily postsynaptic role of IRSp53 is underscored by the decreased size of the PSDs, which display increased levels of N-methyl-d-aspartate receptor subunits in IRSp53 KO animals. Our data suggest that the incorporation of IRSp53 into the PSD enables the protein to limit the number of postsynaptic glutamate receptors and thereby affect synaptic plasticity rather than dendritic morphology. Consistent with altered synaptic plasticity, IRSp53-deficient mice exhibit cognitive deficits in the contextual fear-conditioning paradigm.

Cytoplasmic polyadenylation element binding protein 1-mediated mRNA translation in Purkinje neurons is required for cerebellar long-term depression and motor coordination

The ability of neurons to modify synaptic connections is critical for proper brain development and function in the adult. It is now clear that changes in synaptic strength are often accompanied by changes in synaptic morphology. This synaptic plasticity can be maintained for varying lengths of time depending on the type of neuronal activity that first induced the changes. Long-term synaptic plasticity requires the synthesis of new proteins, and one mechanism for the regulation of experience-induced protein synthesis in neurons involves cytoplasmic polyadenylation element binding protein (CPEB1). CPEB1 can bidirectionally regulate mRNA translation, first repressing translation, and then activating translation after the phosphorylation of two critical residues (T171 and S177). To determine the full extent of CPEB1-mediated protein synthesis in synaptic function, we engineered a line of mice expressing CPEB1 with these phosphorylation sites mutated to alanines (mCPEB1-AA) exclusively in cerebellar Purkinje neurons (PNs). Thus, mRNAs bound by mCPEB1-AA would be held in a translationally dormant state. We show that mCPEB1-AA localizes to synapses in cerebellum and resulted in a loss of protein synthesis-dependent phase of parallel fiber-PN long-term depression. This was accompanied by a change in spine number and spine length that are likely attributable in part to the dysregulation of IRSp53, a protein known to play a role in synaptic structure. Finally, mCPEB1-AA mice displayed a significant impairment of motor coordination and a motor learning delay.

Deep brain stimulation in a rat model modulates TH, CaMKIIa and Homer1 gene expression

High-frequency stimulation (HFS) of subthalamic nucleus (STN) is a therapy for late-stage Parkinson's disease. Its mechanisms of action are not yet fully understood. In the present study, gene expression analyses were performed in a rat model of Parkinson's disease, i.e. striatal 6-hydroxydopamine (6-OHDA) lesion. Using microarrays, gene expression was analysed in 1-mm-thick sagittal brain slices, including basal ganglia of five groups of male Wistar rats. These were unmanipulated rats (group A), unlesioned rats with implanted electrode but without stimulation (group B), unlesioned, stimulated rats (group C), 6-OHDA-lesioned rats with implanted electrode but without stimulation (group D), and finally 6-OHDA-lesioned and stimulated rats (group E). A statistically significant downregulation of tyrosine hydroxylase (TH) mRNA expression induced by 6-OHDA lesion and an HFS-induced TH upregulation in 6-OHDA-lesioned rats could be detected. It could be hypothesized that the HFS-induced upregulation of TH is the result of neuronal STN modulation and mediated via projections from STN to substantia nigra pars compacta. Furthermore, a downregulation of calcium/calmodulin-dependent protein kinase type IIA and Homer1 was observed. This downregulation could result in a reduced sensitivity towards glutamate in basal ganglia downstream of STN.

Striatal specificity of gene expression dysregulation in Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by an expanded CAG repeat region in exon 1 of the HD gene. This mutation results in the presence of an abnormally long polyglutamine tract in the encoded protein, huntingtin (htt). A major question in this field is how the mutant htt protein, which is expressed ubiquitously throughout the brain and body, causes severe neuropathologic changes predominantly in the striatum. The mechanisms accounting for this specificity are unknown. The role of transcriptional dysregulation in the pathophysiology of HD has gained much attention in recent years, however, this theory has been unable to explain the specificity of dysfunction and degeneration in HD. Microarray studies have showed hundreds of gene expression changes in mouse models of HD and in post-mortem brain samples from HD subjects. Among the genes whose expression levels are preferentially altered are those that exhibit enriched expression in the striatum, which we have argued are the most relevant to disease pathology. These "striatal-enriched" genes are associated with several systems previously implicated in HD pathology, especially disturbances in transcriptional processes and calcium homeostasis. Large-scale changes in striatal gene expression in this manner would likely have particularly devastating effects to normal striatal function and could explain the specificity of striatal dysfunction and ultimate neurodegeneration observed in HD.

Selective deficits in the expression of striatal-enriched mRNAs in Huntington's disease

We have identified and cataloged 54 genes that exhibit predominant expression in the striatum. Our hypothesis is that such mRNA molecules are likely to encode proteins that are preferentially associated with particular physiological processes intrinsic to striatal neurons, and therefore might contribute to the regional specificity of neurodegeneration observed in striatal disorders such as Huntington's disease (HD). Expression of these genes was measured simultaneously in the striatum of HD R6/1 transgenic mice using Affymetrix oligonucleotide arrays. We found a decrease in expression of 81% of striatum-enriched genes in HD transgenic mice. Changes in expression of genes associated with G-protein signaling and calcium homeostasis were highlighted. The most striking decrement was observed for a newly identified subunit of the sodium channel, beta 4, with dramatic decreases in expression beginning at 8 weeks of age. A subset of striatal genes was tested by real-time PCR in caudate samples from human HD patients. Similar alterations in expression were observed in human HD and the R6/1 model for the striatal genes tested. Expression of 15 of the striatum-enriched genes was measured in 6-hydroxydopamine-lesioned rats to determine their dependence on dopamine innervation. No changes in expression were observed for any of these genes. These findings demonstrate that mutant huntingtin protein causes selective deficits in the expression of mRNAs responsible for striatum-specific physiology and these may contribute to the regional specificity of degeneration observed in HD.

Novel brain 14-3-3 interacting proteins involved in neurodegenerative disease

We isolated two novel 14-3-3 binding proteins using 14-3-3 zeta as bait in a yeast two-hybrid screen of a human brain cDNA library. One of these encoded the C-terminus of a neural specific armadillo-repeat protein, delta-catenin (neural plakophilin-related arm-repeat protein or neurojungin). delta-Catenin from brain lysates was retained on a 14-3-3 affinity column. Mutation of serine 1072 in the human protein and serine 1094 in the equivalent site in the mouse homologue (in a consensus binding motif for 14-3-3) abolished 14-3-3 binding to delta-catenin in vitro and in transfected cells. delta-catenin binds to presenilin-1, encoded by the gene most commonly mutated in familial Alzheimer's disease. The other clone was identified as the insulin receptor tyrosine kinase substrate protein of 53 kDa (IRSp53). Human IRSp53 interacts with the gene product implicated in dentatorubral-pallidoluysian atrophy, an autosomal recessive disorder associated with glutamine repeat expansion of atrophin-1.

Standardized quantitative in situ hybridization using radioactive oligonucleotide probes for detecting relative levels of mRNA transcripts verified by real-time PCR

In situ hybridization (ISH) is an essential technique for mapping gene expression in the brain. Although many ISH protocols provide for quantitative analysis of individual mRNAs in different brain regions or across experimental conditions, this technique has lacked the necessary standardization for quantitative comparisons between different mRNA transcripts. We have developed a standardized quantitative ISH (SQuISH) protocol that utilizes multiple radioactive oligonucleotide probes, providing for increased sensitivity, decreased background and accurate comparison of relative mRNA levels. We evaluated the SQuISH protocol against a riboprobe-based ISH procedure by comparing the mRNA expression levels in the brain for two transcripts, insulin receptor substrate p53 (IRSp53) and Calsenilin. The results of these two methods were then validated by real-time quantitative PCR. Both protocols exhibited identical mRNA expression patterns for IRSp53 and Calsenilin. In three brain regions analyzed, the levels of IRSp53 mRNA expression were approximately 1.5-fold higher with the riboprobe-based ISH than with the SQuISH procedure, although the relative abundance in regional expression levels was similar between the two methods. In contrast, the levels of Calsenilin mRNA expression were 10-17-fold higher with the riboprobe-based ISH than with the SQuISH procedure and the relative abundance in regional expression levels was different. When compared to the real-time PCR results, the SQuISH trade mark method showed almost identical relative levels of IRSp53 to Calsenilin mRNA in all three brain regions analyzed, while the riboprobe-based procedure showed a completely opposite trend. These results support the accuracy of the SQuISH protocol for determining relative mRNA levels in the brain.

Insulin and neurodegenerative disease: shared and specific mechanisms

Insulin has functions in the brain and dysregulation of these functions may contribute to the expression of late-life neurodegenerative disease. We provide a brief summary of research on the influence of insulin on normal brain function. We then review evidence that perturbation of this role may contribute to the symptoms and pathogenesis of various neurodegenerative disorders, such as Alzheimer's disease, vascular dementia, Parkinson's disease, and Huntington's disease. We conclude by considering whether insulin dysregulation contributes to neurodegenerative disorders through disease-specific or general mechanisms.

Novel espin actin-bundling proteins are localized to Purkinje cell dendritic spines and bind the Src homology 3 adapter protein insulin receptor substrate p53

We identified a group of actin-binding-bundling proteins that are expressed in cerebellar Purkinje cells (PCs) but are not detected in other neurons of the CNS. These proteins are novel isoforms of the actin-bundling protein espin that arise through the use of a unique site for transcriptional initiation and differential splicing. Light and electron microscopic localization studies demonstrated that these espin isoforms are enriched in the dendritic spines of PCs. They were detected in the head and neck and in association with the postsynaptic density (PSD) of dendritic spines in synaptic contact with parallel or climbing fibers. They were also highly enriched in PSD fractions isolated from cerebellum. The PC espins efficiently bound and bundled actin filaments in vitro, and these activities were not inhibited by Ca2+. When expressed in transfected neuronal cell lines, the PC espins colocalized with actin filaments and elicited the formation of coarse cytoplasmic actin bundles. The insulin receptor substrate p53 (IRSp53), an Src homology 3 (SH3) adapter protein and regulator of the actin cytoskeleton, was identified as an espin-binding protein in yeast two-hybrid screens. Cotransfection studies and pull-down assays showed that this interaction was direct and required the N-terminal proline-rich peptide of the PC espins. Thus, the PC espins exhibit the properties of modular actin-bundling proteins with the potential to influence the organization and dynamics of the actin cytoskeleton in PC dendritic spines and to participate in multiprotein complexes involving SH3 domain-containing proteins, such as IRSp53.

Novel isoform of insulin receptor substrate p53/p58 is generated by alternative splicing in the CRIB/SH3-binding region

Insulin receptor substrate p53/p58 (IRSp53) is involved in cytoskeletal dynamics and is a candidate disease sensor in polyglutamine expansion neurodegeneration. It is widely expressed throughout the body, but its levels are dramatically elevated in forebrain regions. IRSp53 functions as a signal transducing adaptor between activated Rho family GTPases and their effectors. There are four known alternatively spliced isoforms of IRSp53 that vary by the identity of the 3'-terminal exon. We report here that there is a fifth alternatively spliced isoform, IRSp53-B, which lacks 40 amino acids abutting the CRIB/SH3 (Cdc42/Rac-interactive binding/Src homology 3)-binding site. We speculate that the novel form has an altered function related to the mechanism of autoinactivation. IRSp53-B has an odd history in the mammalian lineage, which may complicate the use of rodent models to study cytoskeletal reorganization.

Distinctive tissue distribution and phosphorylation of IRSp53 isoforms

An insulin-receptor substrate of 53-kDa protein (IRSp53) is an adapter protein, which interacts with the Rho-family of GTPases and mediates neurite outgrowth. It also binds to DRPLA protein, a product of the gene responsible for a polyglutamine disease, dentatorubral-pallidoluysian atrophy (DRPLA). Isoforms of human IRSp53 have been reported, each with a unique amino acid sequence at the C-terminal end. Here we report the distinctive tissue distribution and phosphorylation of three isoforms (L, S, and T-forms). Western blotting analyses with isoform-specific antibodies demonstrated that the L and S-forms were expressed in the brain, whereas the T-form was not present in any tissues examined, but was found in a cancer cell line. The L and S-forms were phosphorylated upon stimulation with insulin, and the T-form with IGF-I. Since phospho-acceptor sites were localized to the common portion, the difference in phosphorylation seems to be due to the unique C-terminal sequence.