Misfolded proteins are associated with several pathological conditions including neurodegeneration. Although some of these abnormally folded proteins result from mutations in genes encoding disease-associated proteins (for example, repeat-expansion diseases), more general mechanisms that lead to misfolded proteins in neurons remain largely unknown. Here we demonstrate that low levels of mischarged transfer RNAs (tRNAs) can lead to an intracellular accumulation of misfolded proteins in neurons. These accumulations are accompanied by upregulation of cytoplasmic protein chaperones and by induction of the unfolded protein response. We report that the mouse sticky mutation, which causes cerebellar Purkinje cell loss and ataxia, is a missense mutation in the editing domain of the alanyl-tRNA synthetase gene that compromises the proofreading activity of this enzyme during aminoacylation of tRNAs. These findings demonstrate that disruption of translational fidelity in terminally differentiated neurons leads to the accumulation of misfolded proteins and cell death, and provide a novel mechanism underlying neurodegeneration.

No abstract

A novel spontaneous neurological mutation, scrambler (scm), appeared in the inbred mouse strain DC/Le (dancer) in 1991. Mice homozygous for this recessive mutation are recognized by an unstable gait and whole-body tremor. The cerebella of 30-day-old scrambler homozygotes are hypoplastic and devoid of folia; however, neither seizures nor abnormal brain wave patterns have been observed. Homozygous scrambler mutants have an ataxic gait which in the male may be a contributory factor in the failure to mate. Female homozygotes mate and breed. Life span is not reduced in either sex. Scrambler is similar to the reeler mutation in phenotype and pathology and, like reeler, probably results from defective neuronal migration. We mapped the scrambler mutation to Chromosome (Chr) 4, proving that it is distinct from the recently cloned reeler gene on Chr 5. We also determined the map position of the agrin gene, Agrn, on Chr 4, and on this basis eliminated it as a candidate for scm. Currently there is no known homology of scrambler with human lissencephalies or other human disorders caused by abnormal neuronal migration.

Mouse deafness mutations provide valuable models of human hearing disorders and entry points into molecular pathways important to the hearing process. A newly discovered mouse mutation named hurry-scurry (hscy) causes deafness and vestibular dysfunction. Scanning electron microscopy of cochleae from 8-dayold mutants revealed disorganized hair bundles, and by 50 days of age, many hair cells are missing. To positionally clone hscy, 1,160 F 2 mice were produced from an intercross of (C57BL͞6-hscy ؋ CAST͞EiJ) F 1 hybrids, and the mutation was localized to a 182-kb region of chromosome 17. A missense mutation causing a critical cysteine to phenylalanine codon change was discovered in a previously undescribed gene within this candidate interval. The gene is predicted to encode an integral membrane protein with four transmembrane helices. A synthetic peptide designed from the predicted protein was used to produce specific polyclonal antibodies, and strong immunoreactivity was observed on hair bundles of both inner and outer hair cells in cochleae of newborn ؉͞؉ controls and ؉͞hscy heterozygotes but was absent in hscy͞ hscy mutants. Accordingly, the gene was given the name ''tetraspan membrane protein of hair cell stereocilia,'' symbol Tmhs. Two related proteins (>60% amino acid identity) are encoded by genes on mouse chromosomes 5 and 6 and, together with the Tmhsencoded protein (TMHS), comprise a distinct tetraspan subfamily. Our localization of TMHS to the apical membrane of inner ear hair cells during the period of stereocilia formation suggests a function in hair bundle morphogenesis.mouse ͉ hair cell ͉ stereocilia ͉ tetraspan H earing loss is the most prevalent sensory disorder in human populations, occurring in Ϸ0.2-0.3% of all live births. More than half of these childhood cases are thought to be genetic (1). The development and maintenance of the intricate structures and complex mechanisms of the mammalian inner ear require the proper functioning and concerted interactions of hundreds of genes and their products. To date, Ͼ100 forms of human nonsyndromic deafness have been genetically mapped (Hereditary Hearing Loss Homepage (http:͞͞dnalab-www.uia.ac.be͞ dnalab͞hhh), and many of the genes responsible have been identified and characterized (2). More than 200 mouse mutations are known to affect hearing and balance, and mouse models have been developed for Ͼ50 human hearing disorders (Hearing Impairment in Mice, www.jax.org͞hmr͞index.html). Because of the complex nature of the ear, it is likely that many more deafness-related genes remain to be discovered.The inner ear is comprised of the vestibular region, which controls balance, and the cochlea, which is important in detecting, amplifying, and transmitting auditory information to the brain. In the organ of Corti of the cochlea, two distinct types of sensory cells, inner and outer hair cells, are essential for the transduction of sound into nerve impulses. Stereocilia are modified microvilli that project from the apical membranes of inner ear hair cells. The acti...

The neurologic mutant mouse, oscillator, is characterized by a fine motor tremor and muscle spasms that begin at 2 weeks of age and progressively worsen, resulting in death by 3 weeks of age. We report the localization of the oscillator mutation to the central region of mouse Chr 11, and demonstrate its allelism with spasmodic, a recessive viable neurological mutation which displays excessive startle. Oscillator is caused by a microdeletion in the gene coding for the alpha 1 subunit of the adult glycine receptor (Glra1). Glra1 assembles into a pentameric complex with the beta subunit of the glycine receptor (3 alpha (1)2 beta 5) to form a glycine-gated chloride channel. This receptor is the major adult glycine receptor, and the site of action of the poison strychnine. The oscillator deletion causes a frameshift resulting in loss of the highly conserved third cytoplasmic loop and fourth transmembrane domain of the protein. Membranes isolated from oscillator homozygote spinal cords display a 90% reduction in glycine-displaceable strychnine binding. This lack of ligand binding function confirms that oscillator is a complete loss of function allele. The oscillator mutation provides evidence that although at least four different alpha subunits exist for the glycine receptor, none of the other subunits can compensate for the loss of alpha 1 function. Mutations which impair GLRA1 function in humans have been shown to cause dominant familial startle disease. The identification of the oscillator mutation suggests that severe loss of function alleles in humans would result in prenatal or neonatal lethality.