Capucin: a novel striatal marker down-regulated in rodent models of Huntington disease

Functional characterization of endocytic signals in the SynDIG/PRRT family members SynDIG1 and SynDIG4 in heterologous cells and neurons

The transmembrane protein Synapse Differentiation Induced Gene 4 (SynDIG4), also known as Proline-rich transmembrane protein 1 (PRRT1), is an AMPA-type glutamate receptor (AMPAR) auxiliary factor that is necessary for maintaining extra-synaptic pools of GluA1. Loss of SynDIG4, and the subsequent decrease in extra-synaptic GluA1, has been found to significantly impact synaptic plasticity in the hippocampus. However, how SynDIG4 establishes and maintains these pools is unclear. Previous studies suggested that endocytic machinery is important for maintaining a pool of mobile surface AMPARs, and that proteins associated with such cellular machinery are critical for proper protein trafficking and internalization. Given that SynDIG4 co-localizes with GluA1 in early and recycling endosomes in cultured hippocampal neurons, we sought to identify the sorting signals that target SynDIG4 to endosomes to further elucidate the role of SynDIG4 in GluA1 trafficking. In this study, we report that SynDIG4 possesses a YxxΦ sorting motif, 178-YVPV-181, responsible for binding to the AP-2 complex cargo-sorting subunit μ2. This motif appears critical for proper SynDIG4 internalization, as SynDIG4 mutant 178-AVPA-181, which disrupts binding to μ2, induces aberrant SynDIG4 accumulation at the plasma-membrane of heterologous cells and primary rat hippocampal neurons. We also show that SynDIG4 mutants lacking an endocytic signal co-localize with GluA1 but less so with GluA2 on the surface of heterologous cells. Furthermore, we show that another family member, SynDIG1, is enriched in the trans-Golgi network (TGN) and can traffic between the TGN and plasma membrane. We have identified a non-canonical μ2 binding sequence in SynDIG1 that induces aberrant accumulation at the plasma membrane of heterologous cells and primary rat hippocampal neurons, suggesting a conserved role for μ2-mediated endocytosis within the SynDIG family. These results provide important insight into the mechanisms by which SynDIG proteins are targeted to endosomal compartments as a step in understanding SynDIG-mediated regulation of AMPAR trafficking.

The 'dispanins' and related proteins in physiology and neurological disease

The dispanins are a family of 15 transmembrane proteins that have diverse and often unclear physiological functions. Many dispanins, including synapse differentiation induced gene 1 (SynDIG1), proline-rich transmembrane protein 1 (PRRT1)/SynDIG4, and PRRT2, are expressed in the central nervous system (CNS), where they are involved in the development of synapses, regulation of neurotransmitter release, and interactions with ion channels, including AMPA receptors (AMPARs). Others, including transmembrane protein 233 (TMEM233) and trafficking regulator of GLUT4-1 (TRARG1), are expressed in the peripheral nervous system (PNS); however, the function of these dispanins is less clear. Recently, a family of neurotoxins isolated from the giant Australian stinging tree was shown to target TMEM233 to modulate the function of voltage-gated sodium (NaV) channels, suggesting that the dispanins are inherently druggable. Here, we review current knowledge about the structure and function of the dispanins, in particular TMEM233 and its two most closely related homologs PRRT2 and TRARG1, which may be drug targets involved in neurological disease.

Emerging role of T3-binding protein μ-crystallin (CRYM) in health and disease

Thyroid hormones are essential metabolic and developmental regulators that exert a huge variety of effects in different organs. Triiodothyronine (T3) and thyroxine (T4) are synthesized in the thyroid gland and constitute unique iodine-containing hormones that are constantly regulated by a homeostatic feedback mechanism. T3/T4 activity in cells is mainly determined by specific transporters, cytosolic binding proteins, deiodinases (DIOs), and nuclear receptors. Modulation of intracellular T3/T4 level contributes to the maintenance of this regulatory feedback. μ-Crystallin (CRYM) is an important intracellular high-affinity T3-binding protein that buffers the amount of T3 freely available in the cytosol, thereby controlling its action. In this review, we focus on the molecular and pathological properties of CRYM in thyroid hormone signaling, with emphasis on its critical role in malignancies.

A Novel SCA3 Knock-in Mouse Model Mimics the Human SCA3 Disease Phenotype Including Neuropathological, Behavioral, and Transcriptional Abnormalities Especially in Oligodendrocytes

Spinocerebellar ataxia type 3 is the most common autosomal dominant inherited ataxia worldwide, caused by a CAG repeat expansion in the Ataxin-3 gene resulting in a polyglutamine (polyQ)-expansion in the corresponding protein. The disease is characterized by neuropathological, phenotypical, and specific transcriptional changes in affected brain regions. So far, there is no mouse model available representing all the different aspects of the disease, yet highly needed for a better understanding of the disease pathomechanisms. Here, we characterized a novel Ataxin-3 knock-in mouse model, expressing a heterozygous or homozygous expansion of 304 CAACAGs in the murine Ataxin-3 locus using biochemical, behavioral, and transcriptomic approaches. We compared neuropathological, and behavioral features of the new knock-in model with the in SCA3 research mostly used YAC84Q mouse model. Further, we compared transcriptional changes found in cerebellar samples of the SCA3 knock-in mice and post-mortem human SCA3 patients. The novel knock-in mouse is characterized by the expression of a polyQ-expansion in the murine Ataxin-3 protein, leading to aggregate formation, especially in brain regions known to be vulnerable in SCA3 patients, and impairment of Purkinje cells. Along these neuropathological changes, the mice showed a reduction in body weight accompanied by gait and balance instability. Transcriptomic analysis of cerebellar tissue revealed age-dependent differential expression, enriched for genes attributed to myelinating oligodendrocytes. Comparing these changes with those found in cerebellar tissue of SCA3 patients, we discovered an overlap of differentially expressed genes pointing towards similar gene expression perturbances in several genes linked to myelin sheaths and myelinating oligodendrocytes.

Beyond the AMPA receptor: Diverse roles of SynDIG/PRRT brain-specific transmembrane proteins at excitatory synapses

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors (AMPARs) are responsible for fast excitatory transmission in the brain. Deficits in synaptic transmission underlie a variety of neurological and psychiatric disorders. However, drugs that target AMPARs are challenging to develop, given the central role played in neurotransmission. Targeting AMPAR auxiliary factors offers an innovative approach for achieving specificity without altering baseline synaptic transmission. This review focuses on the SynDIG/proline-rich transmembrane protein (PRRT) family of AMPAR-associated transmembrane proteins. Although these factors are related based on sequence similarity, the proteins have evolved diverse actions at excitatory synapses that are not limited to the traditional role ascribed to an AMPAR auxiliary factor. SynDIG4/PRRT1 acts as a typical AMPAR auxiliary protein, while PRRT2 functions at presynaptic sites to regulate synaptic vesicle dynamics and is the causative gene for neurological paroxysmal disorders in humans. SynDIG/PRRT proteins are members of a larger superfamily that also include antiviral proteins known to restrict fusion between host and viral membranes and share some interesting characteristics.

Expression analysis and single-nucleotide polymorphisms of SYNDIG1L and UNC13C genes associated with thoracic vertebral numbers in sheep (Ovis aries)

The objective of the current study was to analyze expression levels of synapse differentiation inducing 1-like (SYNDIG1L) and unc-13 homolog C (UNC13C) genes in different tissues, while single-nucleotide polymorphisms (SNPs) of two genes were associated with multiple thoracic vertebrae traits in both Small-tailed Han sheep (STH) and Sunite sheep (SNT). The expression levels of SYNDIG1L and UNC13C were analyzed in the brain, cerebellum, heart, liver, spleen, lung, kidney, adrenal gland, uterine horn, longissimus muscle, and abdominal adipose tissues of two sheep breeds with different thoracic vertebral number (TVN) sheep (T13 groups and T14 groups) by real-time quantitative polymerase chain reaction (RT-qPCR). Meanwhile, the polymorphisms of UNC13C gene g.52919279C>T and SYNDIG1L gene g.82573325C>A in T14 and T13 were genotyped by the Sequenom MassARRAY^® SNP assay, and association analysis was performed with the TVN. The results demonstrated that UNC13C gene was extensively expressed in 11 tissues. The expression of UNC13C gene in longissimus muscle of T14 groups of STH was significantly higher than that of T13 groups (P<0.05). SYNDIG1L gene was overexpressed in brain and cerebellum tissues, and the expression level of UNC13C gene in the brain and cerebellum of T13 groups in SNT was significantly higher than that of T14 groups (P<0.01). Association analysis showed that SNPs found in the UNC13C gene had no significant effects on TVN for both two genes. The polymorphism of SYNDIG1L g.82573325C>A was significantly correlated with the TVN in both STH (P<0.05) and SNT (P<0.01). Taken together, the SYNDIG1L gene was related to thoracic vertebral development, and this variation may be potentially used as a molecular marker to select the multiple thoracic vertebrae in sheep.

Possible involvement of self-defense mechanisms in the preferential vulnerability of the striatum in Huntington's disease

HD is caused by a mutation in the huntingtin gene that consists in a CAG repeat expansion translated into an abnormal poly-glutamine (polyQ) tract in the huntingtin (Htt) protein. The most striking neuropathological finding in HD is the atrophy of the striatum. The regional expression of mutant Htt (mHtt) is ubiquitous in the brain and cannot explain by itself the preferential vulnerability of the striatum in HD. mHtt has been shown to produce an early defect in transcription, through direct alteration of the function of key regulators of transcription and in addition, more indirectly, as a result of compensatory responses to cellular stress. In this review, we focus on gene products that are preferentially expressed in the striatum and have down- or up-regulated expression in HD and, as such, may play a crucial role in the susceptibility of the striatum to mHtt. Many of these striatal gene products are for a vast majority down-regulated and more rarely increased in HD. Recent research shows that some of these striatal markers have a pro-survival/neuroprotective role in neurons (e.g., MSK1, A2A, and CB1 receptors) whereas others enhance the susceptibility of striatal neurons to mHtt (e.g., Rhes, RGS2, D2 receptors). The down-regulation of these latter proteins may be considered as a potential self-defense mechanism to slow degeneration. For a majority of the striatal gene products that have been identified so far, their function in the striatum is unknown and their modifying effects on mHtt toxicity remain to be experimentally addressed. Focusing on these striatal markers may contribute to a better understanding of HD pathogenesis, and possibly the identification of novel therapeutic targets.

IFITMs restrict the replication of multiple pathogenic viruses

The interferon-inducible transmembrane protein (IFITM) family inhibits a growing number of pathogenic viruses, among them influenza A virus, dengue virus, hepatitis C virus, and Ebola virus. This review covers recent developments in our understanding of the IFITM's molecular determinants, potential mechanisms of action, and impact on pathogenesis.

A further look at porcine chromosome 7 reveals VRTN variants associated with vertebral number in Chinese and Western pigs

The number of vertebrae is an economically important trait that affects carcass length and meat production in pigs. A major quantitative trait locus (QTL) for thoracic vertebral number has been repeatedly identified on pig chromosome (SSC) 7. To dissect the genetic basis of the major locus, we herein genotyped a large sample of animals from 3 experimental populations of Chinese and Western origins using 60K DNA chips. Genome-wide association studies consistently identified the locus across the 3 populations and mapped the locus to a 947-Kb region on SSC7. An identical-by-descent sharing assay refined the locus to a 100-Kb segment that harbors only two genes including VRTN and SYNDIG1L. Of them, VRNT has been proposed as a strong candidate of the major locus in Western modern breeds. Further, we resequenced the VRTN gene using DNA samples of 35 parental animals with known QTL genotypes by progeny testing. Concordance tests revealed 4 candidate causal variants as their genotypes showed the perfect segregation with QTL genotypes of the tested animals. An integrative analysis of evolutional constraints and functional elements supported two VRTN variants in a complete linkage disequilibrium phase as the most likely causal mutations. The promising variants significantly affect the number of thoracic vertebrae (one vertebra) in large scale outbred animals, and are segregating at rather high frequencies in Western pigs and at relatively low frequencies in a number of Chinese breeds. Altogether, we show that VRTN variants are significantly associated with the number of thoracic vertebrae in both Chinese and Western pigs. The finding advances our understanding of the genetic architecture of the vertebral number in pigs. Furthermore, our finding is of economical importance as it provides a robust breeding tool for the improvement of vertebral number and meat production in both Chinese indigenous pigs and Western present-day commercial pigs.

Capucin does not modify the toxicity of a mutant Huntingtin fragment in vivo

Genes selectively expressed in the striatum may be involved in the preferential vulnerability of striatal neurons to Huntington's disease (HD). Here, we investigated whether perturbations of Capucin expression, which is enriched in the striatum and downregulated in Huntington's disease models, could modify the neurotoxicity induced by the injection of a lentiviral vector encoding a short N-terminal fragment of mutant Huntingtin (mHtt) into the mouse striatum. Neither constitutive Capucin deficiency in knockout mice nor lentiviral vector-mediated Capucin overexpression in the striatum of adult wild type mice significantly modified vulnerability to the mHtt fragment in vivo, suggesting that Capucin has no impact on mHtt toxicity.

The dispanins: a novel gene family of ancient origin that contains 14 human members

The Interferon induced transmembrane proteins (IFITM) are a family of transmembrane proteins that is known to inhibit cell invasion of viruses such as HIV-1 and influenza. We show that the IFITM genes are a subfamily in a larger family of transmembrane (TM) proteins that we call Dispanins, which refers to a common 2TM structure. We mined the Dispanins in 36 eukaryotic species, covering all major eukaryotic groups, and investigated their evolutionary history using Bayesian and maximum likelihood approaches to infer a phylogenetic tree. We identified ten human genes that together with the known IFITM genes form the Dispanin family. We show that the Dispanins first emerged in eukaryotes in a common ancestor of choanoflagellates and metazoa, and that the family later expanded in vertebrates where it forms four subfamilies (A-D). Interestingly, we also find that the family is found in several different phyla of bacteria and propose that it was horizontally transferred to eukaryotes from bacteria in the common ancestor of choanoflagellates and metazoa. The bacterial and eukaryotic sequences have a considerably conserved protein structure. In conclusion, we introduce a novel family, the Dispanins, together with a nomenclature based on the evolutionary origin.

A serial analysis of gene expression profile of the Alzheimer's disease Tg2576 mouse model

Serial analysis of gene expression (SAGE), a technique that allows for the simultaneous detection of expression levels of the entire genome without a priori knowledge of gene sequences, was used to examine the transcriptional expression pattern of the Tg2576 mouse model of Alzheimer's disease (AD). Pairwise comparison between the Tg2576 and nontransgenic SAGE libraries identified a number of differentially expressed genes in the Tg2576 SAGE library, some of which were not previously revealed by the microarray studies. Real-time PCR was used to validate a panel of genes selected from the SAGE analysis in the Tg2576 mouse brain, as well as the hippocampus and temporal cortex of sporadic AD and normal age-matched controls. NADH dehydrogenase (ubiquinone) 1 alpha subcomplex 5 (NDUFA5) and FXYD domain-containing ion transport regulator 6 (FXYD6) were found to be significantly decreased in the Tg2576 mouse brain and AD hippocampus. PTEN-induced putative kinase 1 (PINK1), phosphatidylethanolamine binding protein (PEBP), crystalline mu (CRYM), and neurogranin (NRGN) were significantly decreased in AD tissues. The gene ontologies represented in the Tg2576 data were statistically analyzed and demonstrated a significant under-representation of genes involved with G-protein-coupled receptor signaling and odorant binding, while genes significantly over-represented were focused on cellular communication and cellular physiological processes. The novel approach of profiling the Tg2576 mouse brain using SAGE has identified different genes that could subsequently be examined for their potential as peripheral diagnostic and prognostic markers for Alzheimer's disease.

SynDIG1: an activity-regulated, AMPA- receptor-interacting transmembrane protein that regulates excitatory synapse development

During development of the central nervous system, precise synaptic connections between presynaptic and postsynaptic neurons are formed. While significant progress has been made in our understanding of AMPA receptor trafficking during synaptic plasticity, less is known about the molecules that recruit AMPA receptors to nascent synapses during synaptogenesis. Here we identify a type II transmembrane protein (SynDIG1) that regulates AMPA receptor content at developing synapses in dissociated rat hippocampal neurons. SynDIG1 colocalizes with AMPA receptors at synapses and at extrasynaptic sites and associates with AMPA receptors in heterologous cells and brain. Altered levels of SynDIG1 in cultured neurons result in striking changes in excitatory synapse number and function. SynDIG1-mediated synapse development is dependent on association with AMPA receptors via its extracellular C terminus. Intriguingly, SynDIG1 content in dendritic spines is regulated by neuronal activity. Altogether, we define SynDIG1 as an activity-regulated transmembrane protein that regulates excitatory synapse development.

Quantitative gene expression profiling of mouse brain regions reveals differential transcripts conserved in human and affected in disease models

Using serial analysis of gene expression, we collected quantitative transcriptome data in 11 regions of the adult wild-type mouse brain: the orbital, prelimbic, cingulate, motor, somatosensory, and entorhinal cortices, the caudate-putamen, the nucleus accumbens, the thalamus, the substantia nigra, and the ventral tegmental area. With >1.2 million cDNA tags sequenced, this database is a powerful resource to explore brain functions and disorders. As an illustration, we performed interregional comparisons and found 315 differential transcripts. Most of them are poorly characterized and 20% lack functional annotation. For 78 differential transcripts, we provide independent expression level measurements in mouse brain regions by real-time quantitative RT-PCR. We also show examples where we used in situ hybridization to achieve infrastructural resolution. For 30 transcripts, we next demonstrated that regional enrichment is conserved in the human brain. We then quantified the expression levels of region-enriched transcripts in the R6/2 mouse model of Huntington disease and the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson disease and observed significant alterations in the striatum, cerebral cortex, thalamus and substantia nigra of R6/2 mice and in the striatum of MPTP-treated mice. These results show that the gene expression data provided here for the mouse brain can be used to explore pathophysiological models and disclose transcripts differentially expressed in human brain regions.

Transcriptional signatures in Huntington's disease

While selective neuronal death has been an influential theme in Huntington's disease (HD), there is now a preponderance of evidence that significant neuronal dysfunction precedes frank neuronal death. The best evidence for neuronal dysfunction is the observation that gene expression is altered in HD brain, suggesting that transcriptional dysregulation is a central mechanism. Studies of altered gene expression began with careful observations of postmortem human HD brain and subsequently were accelerated by the development of transgenic mouse models. The application of DNA microarray technology has spurred tremendous progress with respect to the altered transcriptional processes that occur in HD, through gene expression studies of both transgenic mouse models as well as cellular models of HD. Gene expression profiles are remarkably comparable across these models, bolstering the idea that transcriptional signatures reflect an essential feature of disease pathogenesis. Finally, gene expression studies have been applied to human HD, thus not only validating the approach of using model systems, but also solidifying the idea that altered transcription is a key mechanism in HD pathogenesis. In the future, gene expression profiling will be used as a readout in clinical trials aimed at correcting transcriptional dysregulation in Huntington's disease.