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.

Of the many inherited Charcot-Marie-Tooth peripheral neuropathies, type 2D (CMT2D) is caused by dominant point mutations in the gene GARS, encoding glycyl tRNA synthetase (GlyRS). Here we report a dominant mutation in Gars that causes neuropathy in the mouse. Importantly, both sensory and motor axons are affected, and the dominant phenotype is not caused by a loss of the GlyRS aminoacylation function. Mutant mice have abnormal neuromuscular junction morphology and impaired transmission, reduced nerve conduction velocities, and a loss of large-diameter peripheral axons, without defects in myelination. The mutant GlyRS enzyme retains aminoacylation activity, and a loss-of-function allele, generated by a gene-trap insertion, shows no dominant phenotype in mice. These results indicate that the CMT2D phenotype is caused not by reduction of the canonical GlyRS activity and insufficiencies in protein synthesis, but instead by novel pathogenic roles for the mutant GlyRS that specifically affect peripheral neurons.

Charcot-Marie-Tooth (CMT) diseases are the most common heritable peripheral neuropathy. At least 10 different mutant alleles of GARS (the gene for glycyl-tRNA synthetase) have been reported to cause a dominant axonal form of CMT (type 2D). A unifying connection between these mutations and CMT has been unclear. Here, mapping mutations onto the recently determined crystal structure of human GlyRS showed them within a band encompassing both sides of the dimer interface, with two CMT-causing mutations being at sites that are complementary partners of a ''kissing'' contact across the dimer interface. The CMT phenotype is shown here to not correlate with aminoacylation activity. However, most mutations affect dimer formation (to enhance or weaken). Seven CMT-causing variants and the wild-type protein were expressed in transfected neuroblastoma cells that sprout primitive neurites. Wild-type GlyRS distributed into the nascent neurites and was associated with normal neurite sprouting. In contrast, all mutant proteins were distribution-defective. Thus, CMT-causing mutations of GlyRS share a common defect in localization. This defect may be connected in some way to a change in the surfaces at the dimer interface.aminoacyl tRNA synthetase ͉ cellular localization ͉ crystal structure ͉ electrostatic surface potential ͉ inherited peripheral neuropathy

Aminoacyl transfer RNA (tRNA) synthetases establish the rules of the genetic code by catalyzing the aminoacylation of tRNAs. For some synthetases, accuracy depends critically on an editing function at a site distinct from the aminoacylation site. Mutants of Escherichia coli that incorrectly charge tRNA(Val) with cysteine were selected after random mutagenesis of the whole chromosome. All mutations obtained were located in the editing site of valyl-tRNA synthetase. More than 20% of the valine in cellular proteins from such an editing mutant organism could be replaced with the noncanonical aminobutyrate, sterically similar to cysteine. Thus, the editing function may have played a central role in restricting the genetic code to 20 amino acids. Disabling this editing function offers a powerful approach for diversifying the chemical composition of proteins and for emulating evolutionary stages of ambiguous translation.

Dominant-intermediate Charcot-Marie-Tooth neuropathy (DI-CMT