Phenotype of Mrps5-Associated Phylogenetic Polymorphisms Is Intimately Linked to Mitoribosomal Misreading.

We have recently identified point mutation V336Y in mitoribosomal protein Mrps5 (uS5m) as a mitoribosomal ram (ribosomal ambiguity) mutation conferring error-prone mitochondrial protein synthesis. In vivo in transgenic knock-in animals, homologous mutation V338Y was associated with a discrete phenotype including impaired mitochondrial function, anxiety-related behavioral alterations, enhanced susceptibility to noise-induced hearing damage, and accelerated metabolic aging in muscle. To challenge the postulated link between Mrps5 V338Y-mediated misreading and the in vivo phenotype, we introduced mutation G315R into the mouse Mrps5 gene as Mrps5 G315R is homologous to the established bacterial ram mutation RpsE (uS5) G104R. However, in contrast to bacterial translation, the homologous G → R mutation in mitoribosomal Mrps5 did not affect the accuracy of mitochondrial protein synthesis. Importantly, in the absence of mitochondrial misreading, homozygous mutant MrpS5G315R/G315R mice did not show a phenotype distinct from wild-type animals.

Mutant MRPS5 affects mitoribosomal accuracy and confers stress-related behavioral alterations.

The 1555 A to G substitution in mitochondrial 12S A-site rRNA is associated with maternally transmitted deafness of variable penetrance in the absence of otherwise overt disease. Here, we recapitulate the suggested A1555G-mediated pathomechanism in an experimental model of mitoribosomal mistranslation by directed mutagenesis of mitoribosomal protein MRPS5. We first establish that the ratio of cysteine/methionine incorporation and read-through of mtDNA-encoded MT-CO1 protein constitute reliable measures of mitoribosomal misreading. Next, we demonstrate that human HEK293 cells expressing mutant V336Y MRPS5 show increased mitoribosomal mistranslation. As for immortalized lymphocytes of individuals with the pathogenic A1555G mutation, we find little changes in the transcriptome of mutant V336Y MRPS5 HEK cells, except for a coordinated upregulation of transcripts for cytoplasmic ribosomal proteins. Homozygous knock-in mutant Mrps5 V338Y mice show impaired mitochondrial function and a phenotype composed of enhanced susceptibility to noise-induced hearing damage and anxiety-related behavioral alterations. The experimental data in V338Y mutant mice point to a key role of mitochondrial translation and function in stress-related behavioral and physiological adaptations.

Loss of Nogo-A, encoded by the schizophrenia risk gene Rtn4, reduces mGlu3 expression and causes hyperexcitability in hippocampal CA3 circuits.

Recent investigations of Nogo-A, a well characterized protein inhibitor of neurite outgrowth in the brain, have revealed additional functions including a role in neuropsychiatric disorders such as schizophrenia. Here we examined Nogo-A functions in mouse CA3 hippocampal circuitry. Patch clamp recordings showed that the absence of Nogo-A results in a hyperactive network. In addition, mGlu3 metabotropic glutamate receptors, which exhibit mutations in certain forms of schizophrenia, were downregulated specifically in the CA3 area. Furthermore, Nogo-A-/- mice showed disordered theta oscillations with decreased incidence and frequency, similar to those observed in mGlu3-/- mice. As disruptions in theta rhythmicity are associated with impaired spatial navigation, we tested mice using modified Morris water maze tasks. Mice lacking Nogo-A exhibited altered search strategies, displaying greater dependence on global as opposed to local reference frames. This link between Nogo-A and mGlu3 receptors may provide new insights into mechanisms underlying schizophrenia.

Distinct in vivo roles of secreted APP ectodomain variants APPsα and APPsß in regulation of spine density, synaptic plasticity, and cognition.

Increasing evidence suggests that synaptic functions of the amyloid precursor protein (APP), which is key to Alzheimer pathogenesis, may be carried out by its secreted ectodomain (APPs). The specific roles of APPsα and APPsß fragments, generated by non-amyloidogenic or amyloidogenic APP processing, respectively, remain however unclear. Here, we expressed APPsα or APPsß in the adult brain of conditional double knockout mice (cDKO) lacking APP and the related APLP2. APPsα efficiently rescued deficits in spine density, synaptic plasticity (LTP and PPF), and spatial reference memory of cDKO mice. In contrast, APPsß failed to show any detectable effects on synaptic plasticity and spine density. The C-terminal 16 amino acids of APPsα (lacking in APPsß) proved sufficient to facilitate LTP in a mechanism that depends on functional nicotinic α7-nAChRs. Further, APPsα showed high-affinity, allosteric potentiation of heterologously expressed α7-nAChRs in oocytes. Collectively, we identified α7-nAChRs as a crucial physiological receptor specific for APPsα and show distinct in vivo roles for APPsα versus APPsß. This implies that reduced levels of APPsα that might occur during Alzheimer pathogenesis cannot be compensated by APPsß.

Similar reliability and equivalent performance of female and male mice in the open field and water-maze place navigation task.

Although most nervous system diseases affect women and men differentially, most behavioral studies using mouse models do not include subjects of both sexes. Many researchers worry that data of female mice may be unreliable due to the estrous cycle. Here, we retrospectively evaluated sex effects on coefficient of variation (CV) in 5,311 mice which had performed the same place navigation protocol in the water-maze and in 4,554 mice tested in the same open field arena. Confidence intervals for Cohen's d as measure of effect size were computed and tested for equivalence with 0.2 as equivalence margin. Despite the large sample size, only few behavioral parameters showed a significant sex effect on CV. Confidence intervals of effect size indicated that CV was either equivalent or showed a small sex difference at most, accounting for less than 2% of total group to group variation of CV. While female mice were potentially slightly more variable in water-maze acquisition and in the open field, males tended to perform less reliably in the water-maze probe trial. In addition to evaluating variability, we also directly compared mean performance of female and male mice and found them to be equivalent in both water-maze place navigation and open field exploration. Our data confirm and extend other large scale studies in demonstrating that including female mice in experiments does not cause a relevant increase of data variability. Our results make a strong case for including mice of both sexes whenever open field or water-maze are used in preclinical research.

Mutations in NONO lead to syndromic intellectual disability and inhibitory synaptic defects.

The NONO protein has been characterized as an important transcriptional regulator in diverse cellular contexts. Here we show that loss of NONO function is a likely cause of human intellectual disability and that NONO-deficient mice have cognitive and affective deficits. Correspondingly, we find specific defects at inhibitory synapses, where NONO regulates synaptic transcription and gephyrin scaffold structure. Our data identify NONO as a possible neurodevelopmental disease gene and highlight the key role of the DBHS protein family in functional organization of GABAergic synapses.