Microtubule-associated protein 1A is a modifier of tubby hearing (moth1)

Glutamate receptor-interacting protein 1 protein binds to the microtubule-associated protein

To identify the interaction proteins for the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunit glutamate receptor-interacting protein 1 (GRIP1), GRIP1 interactions with microtubule-associated protein (MAP)-1B light chain (LC) were investigated. GRIP1 interacts with MAP-1A and MAP-1B in the yeast two-hybrid assay, as is indicated also by glutathione S-transferase (GST) pull-down and coimmunoprecipitation with MAP-1B LC antibody in brain fractions. These results suggest a novel mechanism for localizing AMPA receptors to synaptic sites.

Microtubule-associated protein light chain 2 is a stargazin-AMPA receptor complex-interacting protein in vivo

The ataxic mutant mouse stargazer is a null mutant for stargazin, a protein involved in the regulation of cell surface trafficking and synaptic targeting of AMPA receptors. The extreme C terminus of stargazin (sequence, -TTPV), confers high affinity for PDZ domain-containing proteins e.g. PSD-95. Interaction with PDZ proteins enables stargazin to fulfill its role as an AMPA receptor synaptic targeting molecule but is not essential for its ability to influence AMPA receptor trafficking to the neuronal cell surface. Using the yeast-two hybrid approach we screened for proteins that interact with the intracellular C-terminal tail of stargazin. Positive interactors included PDZ domain-containing proteins e.g. SAP97, SAP102, and PIST. Interestingly, light chain 2 of microtubule-associated protein 1 (LC2), which does not contain a PDZ domain, was also a strong interactor. This was shown to be a direct interaction that occurred upstream of the -TTPV sequence of stargazin. Immunoprecipitations of Triton X-100 soluble cerebellar extracts revealed that LC2 is pulled down not only by anti-stargazin antibodies but also anti-GluR2 antibodies suggesting that stargazin and AMPA receptor subunits associate with LC2. Immunopurified full-length, native stargazin was shown to co-associate not only with GluR2 in vivo but also with full-length, native LC2. Indeed, LC2 co-associates with stargazin when part of a tripartite complex comprising LC2-stargazin-GluR2. Since this complex was extracted using Triton X-100 and was devoid of PSD95, SAP97, and actin we postulate that LC2 is involved in trafficking of AMPA receptors in cerebellar neurons before they are anchored at the synapse.

Abnormal cholinergic and GABAergic vascular innervation in the hypothalamic arcuate nucleus of obesetub/tubmice

Tubby and tubby-like proteins (TULPs) are encoded by members of a small gene family. An autosomal recessive mutation in the mouse tub gene leads to blindness, deafness, and maturity-onset obesity. The mechanisms by which the mutation causes the obesity syndrome has not been established. We compared obese tub/tub mice and their lean littermates in order to find abnormalities within the mediobasal hypothalamus, a region intimately associated with the regulation of body weight. Using an antiserum to the vesicular acetylcholine transporter (VAChT), a marker for cholinergic neurons, many unusually large VAChT-immunoreactive (-ir) nerve terminals, identified by colocalization with the synaptic vesicle protein synaptophysin, were demonstrated in the hypothalamic arcuate nucleus of obese tub/tub mice. Double-labeling showed that VAChT-ir nerve endings also contained glutamic acid decarboxylase (GAD), a marker for gamma-aminobutyric acid (GABA) neurons. The VAChT- and GAD-ir nerve terminals were in close contact with blood vessels, identified with antisera to platelet endothelial cell adhesion molecule-1 (PECAM; also called CD31), laminin, smooth muscle actin (SMA), and glucose transporter-1 (GLUT1). Such large cholinergic and GABAergic nerve terminals surrounding blood vessels were not seen in the arcuate nucleus of lean tub/+ mice. The presence of abnormal cholinergic/GABAergic vascular innervation in the arcuate nucleus suggests that alterations in this region, which contains neurons that receive information from the periphery and which relays information about the energy status to other parts of the brain, may be central in the development of the obese phenotype in animals with an autosomal recessive mutation in the tub gene.

Les surdités héréditaires : génétique moléculaire

This article outlines recent advances in explaining hereditary deafness in molecular terms, focusing on isolated (i.e. nonsyndromic) hearing loss. The number of genes identified (36 to date) is growing rapidly. However, difficulties inherent in genetic linkage analysis, coupled with the possible involvement of environmental causes, have so far prevented the characterization of the main genes causative or predisposing to the late-onset forms of deafness.

Deafness genes and their diagnostic applications

Hearing impairment (HI) is clinically and genetically very heterogeneous, and auditory genes are discovered at a very rapid pace. The identification of deafness genes is enabling us to understand the molecular process of hearing, and it offers prospects for DNA testing of HI. However, the routine application of these tests is hampered by the large number of genes involved in HI and by the fact that molecular screening of these genes is often quite expensive and time consuming. An important gene that should be considered in congenital or childhood onset autosomal recessive HI is GJB2 since mutations in this gene account for at least 50% of this type of HI. In the present review, we describe the known deafness genes and we provide an overview of the current, routinely used diagnostic DNA tests.

Tubby proteins: the plot thickens

The tubby mouse, which shows late-onset obesity and neurosensory deficits, arises from a mutation in the Tub gene. Tub shares homology with the genes for tubby-like proteins Tulp1, Tulp2 and Tulp3. Ablation of Tub, Tulp1 or Tulp3 causes disease phenotypes that are indicative of their importance in nervous-system function and development. Despite this importance, the biochemical functions of tubby-like proteins are only now beginning to be understood. At present, data indicate that tubby-like proteins might function as heterotrimeric-G-protein-responsive intracellular signalling factors, although an array of data also implicates them in other processes.

The nature and identification of quantitative trait loci: a community's view

This white paper by eighty members of the Complex Trait Consortium presents a community's view on the approaches and statistical analyses that are needed for the identification of genetic loci that determine quantitative traits. Quantitative trait loci (QTLs) can be identified in several ways, but is there a definitive test of whether a candidate locus actually corresponds to a specific QTL?

Functional complexity of intermediate filament cytoskeletons: from structure to assembly to gene ablation

The cell biology of intermediate filament (IF) proteins and their filaments is complicated by the fact that the members of the gene family, which in humans amount to at least 65, are differentially expressed in very complex patterns during embryonic development. Thus, different tissues and cells express entirely different sets and amounts of IF proteins, the only exception being the nuclear B-type lamins, which are found in every cell. Moreover, in the course of evolution the individual members of this family have, within one species, diverged so much from each other with regard to sequence and thus molecular properties that it is hard to envision a unifying kind of function for them. The known epidermolytic diseases, caused by single point mutations in keratins, have been used as an argument for a role of IFs in mechanical "stress resistance," something one would not have easily ascribed to the beaded chain filaments, a special type of IF in the eye lens, or to nuclear lamins. Therefore, the power of plastic dish cell biology may be limited in revealing functional clues for these structural elements, and it may therefore be of interest to go to the extreme ends of the life sciences, i.e., from the molecular properties of individual molecules including their structure at the atomic level to targeted inactivation of their genes in living animals, mouse, and worm to define their role more precisely in metazoan cell physiology.

Nonsyndromic hearing loss

The past decade has seen extremely rapid progress in the field of hereditary hearing loss. To date, 80 loci for nonsyndromic hearing loss have been mapped to the human genome. Furthermore, 30 genes have been identified. These genes belong to a wide variety of protein classes: from myosins and other cytoskeletal proteins, over channel and gap junction components, to transcription factors, extracellular matrix proteins and genes with an unknown function. The identification of these genes has enabled geneticists to offer DNA diagnostic tests for some types of nonsyndromic hearing loss. Moreover, it holds the promise to significantly improve the molecular knowledge on the auditory and vestibular organs and on the pathological mechanisms leading to hearing loss. This opens perspectives for future therapeutic and/or preventive measures for hearing loss. This review attempts to give an overview of the current knowledge of the genes responsible for nonsyndromic hearing loss, their expression and functions in the cochlea.

A genetic screen for mouse mutations with defects in serotonin responsiveness

The serotonergic system plays a key role in regulating basic behaviors. Deficits in serotonergic neurotransmission have been implicated in psychiatric disorders, such as schizophrenia and depression. Here we have optimized a behavioral screen and performed a small scale genetic screen to identify genes involved in serotonin responsiveness in the mouse. Treatment of mice with serotonin, serotonin precursors, or serotonin agonists results in a quantifiable head twitch response (HTR), which is drug dosage-dependent and dependent on the 5-HT2A receptor system. This assay can uncover variation in serotonin responsiveness as shown by our identification of inbred strains with high, medium, and low head twitch responses to administration of the serotonin agonist DOI (+-1-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane). We chose C57Bl/6J mice for our mutagenesis screen, because of their robust HTR and because of the availability of their complete genomic sequence. We optimized this assay by examining dose and age dependence of DOI-induced HTR in 6-week and 3-month-old C57BL/6J mice. HTR decreases only slightly in 3-month-old mice, and a substantial but submaximal HTR is induced by 0.75-1 mg/kg of DOI. We assayed HTR in response to DOI of 247 G1 C57BL/6J progeny from C57BL/6J males, which had been mutagenized with ethylnitrososurea (ENU), and recovered one provisionally heritable hyper-responsive mutation. This and future mutations recovered via this protocol may provide ideal subjects for the study of human psychiatric disorders, such as depression and schizophrenia, and thereby aid in the development of better therapeutic strategies for these disorders. Thus, it is well worth expanding on this genetic screen in its current form and by addition of further pharmacologic assays in the future.

Modifier genes and protective alleles in humans and mice

Interest in modifier genes is growing rapidly because of their ability to modulate the phenotype of individuals with monogenic and multigenic traits and diseases. A neglected class of modifiers is protective alleles that can suppress disease in otherwise susceptible individuals. Together these modifier genes and protective alleles provide important glimpses into the molecular and cellular basis for the functional networks that provide robustness and homeostasis in complex biological systems.

Genetic background determines phenotypic severity of the Plp rumpshaker mutation

The rumpshaker mutation of the proteolipid protein (Plp) gene causes dysmyelination in man and mouse. We show that the phenotype in the mouse depends critically on the genetic background in which the mutation is expressed. On the C3H background there is normal longevity whereas changing to a C57BL/6 strain results in seizures and death at around postnatal day 30. The more severe phenotype is associated with less myelin and reduced levels of major myelin proteins. There are also more apoptotic cells, including oligodendrocytes, increased numbers of proliferating cells, increased numbers of NG2+ oligodendrocyte progenitors and increased microglia compared to the milder phenotype. The number of mature oligodendrocytes is similar to wild-type in both strains of mutant, however, suggesting that increased oligodendrocyte death is matched by increased generation from progenitors. The dichotomy of phenotype probably reflects the influence of modifying loci. The localization of these putative modifying genes and their mode of action remain to be determined.

Finding genes that underlie complex traits

Phenotypic variation among organisms is central to evolutionary adaptations underlying natural and artificial selection, and also determines individual susceptibility to common diseases. These types of complex traits pose special challenges for genetic analysis because of gene-gene and gene-environment interactions, genetic heterogeneity, low penetrance, and limited statistical power. Emerging genome resources and technologies are enabling systematic identification of genes underlying these complex traits. We propose standards for proof of gene discovery in complex traits and evaluate the nature of the genes identified to date. These proof-of-concept studies demonstrate the insights that can be expected from the accelerating pace of gene discovery in this field.

Beyond Mendel: an evolving view of human genetic disease transmission

Methodological and conceptual advances in human genetics have led to the identification of an impressive number of human disease genes. This wealth of information has also revealed that the traditional distinction between Mendelian and complex disorders might sometimes be blurred. Genetic and mutational data on an increasing number of disorders have illustrated how phenotypic effects can result from the combined action of alleles in many genes. In this review, we discuss how an improved understanding of the genetic basis of multilocus inheritance is catalysing the transition from a segmented view of human genetic disease to a conceptual continuum between Mendelian and complex traits.

Mouse models for human deafness: current tools for new fashions

Mouse models are one of the major tools used for discovery and characterization of genes for non-syndromic deafness in humans. The similarities between the mouse and human genomes, and between the physiology and morphology of their auditory systems, are striking. This article describes the latest mouse models, including spontaneous, 'knockout' and ENU (N-ethyl-N-nitrosourea)-induced mutants, and the recent discovery of modifier genes that are involved in mouse deafness; this discovery is leading the search for genetic modifiers for human disorders.