Aminoacyl‐transfer RNA (tRNA) synthetases ligate amino acids to specific tRNAs and are essential for protein synthesis. Although alanyl‐tRNA synthetase (AARS) is a synthetase implicated in a wide ...range of neurological disorders from Charcot‐Marie‐Tooth disease to infantile epileptic encephalopathy, there have been limited data on their pathogenesis. Here, we report loss‐of‐function mutations in AARS in two siblings with progressive microcephaly with hypomyelination, intractable epilepsy, and spasticity. Whole‐exome sequencing identified that the affected individuals were compound heterozygous for mutations in AARS gene, c.2067dupC (p.Tyr690Leufs*3) and c.2738G>A (p.Gly913Asp). A lymphoblastoid cell line developed from one of the affected individuals showed a strong reduction in AARS abundance. The mutations decrease aminoacylation efficiency by 70%–90%. The p.Tyr690Leufs*3 mutation also abolished editing activity required for hydrolyzing misacylated tRNAs, thereby increasing errors during aminoacylation. Our study has extended potential mechanisms underlying AARS‐related disorders to include destabilization of the protein, aminoacylation dysfunction, and defective editing activity.
Mutations in alanyl‐tRNA synthetase (AARS) are implicated in neurological disorders, but there have been limited data on the pathogenesis. We identified loss‐of‐function mutations in AARS in two siblings with progressive microcephaly with hypomyelination, intractable epilepsy and spasticity. A lymphoblastoid cell line derived from an affected indivual showed a strong reduction in AARS protein abundance. Functional analysis of the mutations broadened the spectrum of potential mechanisms underlying AARS‐related disorders to include destabilization of the protein, aminoacylation dysfunction, and defective editing activity.
Changes in dendritic morphology in response to activity have long been thought to be a critical component of how neural circuits develop to properly encode sensory information. Ventral-preferring ...direction-selective ganglion cells (vDSGCs) have asymmetric dendrites oriented along their preferred direction, and this has been hypothesized to play a critical role in their tuning. Here we report the surprising result that visual experience is critical for the alignment of vDSGC dendrites to their preferred direction. Interestingly, vDSGCs in dark-reared mice lose their inhibition-independent dendritic contribution to direction-selective tuning while maintaining asymmetric inhibitory input. These data indicate that different mechanisms of a cell’s computational abilities can be constructed over development through divergent mechanisms.
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•Visual deprivation prevents vDSGC dendrite orientation toward the ventral axis•Ventral direction tuning is maintained despite altered dendritic orientation•Inhibition-independent DS tuning is reduced in visually deprived vDSGCs•Spatially offset, null direction inhibition is maintained in visually deprived vDSGCs
El-Quessny et al. investigate the structure-function relationship of a ventrally-tuned directionally-selective ganglion cell (vDSGC), which has ventrally oriented dendrites during adulthood. They find that visual experience orients the vDSGC dendrites ventrally, affecting dendritic mechanisms, while sparing circuit mechanisms, for direction computations.
Exome sequencing identified homozygous loss-of-function variants in DIAPH1 (c.2769delT; p.F923fs and c.3145C>T; p.R1049X) in four affected individuals from two unrelated consanguineous families. The ...affected individuals in our report were diagnosed with postnatal microcephaly, early-onset epilepsy, severe vision impairment, and pulmonary symptoms including bronchiectasis and recurrent respiratory infections. A heterozygous DIAPH1 mutation was originally reported in one family with autosomal dominant deafness. Recently, however, a homozygous nonsense DIAPH1 mutation (c.2332C4T; p.Q778X) was reported in five siblings in a single family affected by microcephaly, blindness, early onset seizures, developmental delay, and bronchiectasis. The role of DIAPH1 was supported using parametric linkage analysis, RNA and protein studies in their patients' cell lines and further studies in human neural progenitors cells and a diap1 knockout mouse. In this report, the proband was initially brought to medical attention for profound metopic synostosis. Additional concerns arose when his head circumference did not increase after surgical release at 5 months of age and he was diagnosed with microcephaly and epilepsy at 6 months of age. Clinical exome analysis identified a homozygous DIAPH1 mutation. Another homozygous DIAPH1 mutation was identified in the research exome analysis of a second family with three siblings presenting with a similar phenotype. Importantly, no hearing impairment is reported in the homozygous affected individuals or in the heterozygous carrier parents in any of the families demonstrating the autosomal recessive microcephaly phenotype. These additional families provide further evidence of the likely causal relationship between DIAPH1 mutations and a neurodevelopmental disorder.
During nervous system development, neurons choose synaptic partners with remarkable specificity; however, the cell-cell recognition mechanisms governing rejection of inappropriate partners remain ...enigmatic. Here, we show that mouse retinal neurons avoid inappropriate partners by using the FLRT2-uncoordinated-5 (UNC5) receptor-ligand system. Within the inner plexiform layer (IPL), FLRT2 is expressed by direction-selective (DS) circuit neurons, whereas UNC5C/D are expressed by non-DS neurons projecting to adjacent IPL sublayers. In vivo gain- and loss-of-function experiments demonstrate that FLRT2-UNC5 binding eliminates growing DS dendrites that have strayed from the DS circuit IPL sublayers. Abrogation of FLRT2-UNC5 binding allows mistargeted arbors to persist, elaborate, and acquire synapses from inappropriate partners. Conversely, UNC5C misexpression within DS circuit sublayers inhibits dendrite growth and drives arbors into adjacent sublayers. Mechanistically, UNC5s promote dendrite elimination by interfering with FLRT2-mediated adhesion. Based on their broad expression, FLRT-UNC5 recognition is poised to exert widespread effects upon synaptic partner choices across the nervous system.
Abstract
Throughout the nervous system, the organization of excitatory and inhibitory synaptic inputs within a neuron’s receptive field shapes its output computation. In some cases, multiple motifs ...of synaptic organization can contribute to a single computation. Here, we compare two of these mechanisms performed by two morphologically distinct retinal direction-selective ganglion cells (DSGCs): directionally tuned inhibition and spatially offset inhibition. Using drifting stimuli, we found that DSGCs that have asymmetric dendrites exhibited stronger directionally tuned inhibition than symmetric DSGCs. Using stationary stimuli to map receptive fields, we found that DSGCs with both symmetric and asymmetric dendrites exhibited similar spatially offset inhibition. Interestingly, we observed that excitatory and inhibitory synapses for both cell types were locally correlated in strength. This result indicates that in the mouse retina, dendritic morphology influences the amount of tuned inhibition attained through asymmetric wiring but does not dictate the synaptic organization of excitation relative to inhibition.
Despite recent advances in understanding the genetic bases of microcephaly, a large number of cases of microcephaly remain unexplained, suggesting that many microcephaly syndromes and associated ...genes are yet to be identified. Here we report mutations in PYCR2, which encodes an enzyme in the proline biosynthesis pathway, as the cause of a unique syndrome characterized by postnatal microcephaly, hypomyelination, and reduced cerebral white matter volume. Linkage mapping and whole-exome sequencing identified homozygous mutations in PYCR2 (c.355C>T p.Arg119Cys and c.751C>T p.Arg251Cys) in the affected individuals of two consanguineous families. A lymphoblastoid cell line from one affected individual showed a strong reduction in PYCR2 level. When mutant cDNAs were transfected into HEK293FT cells, the mutant protein retained normal mitochondrial localization but the level was lower than the wild-type protein, suggesting that mutant protein is less stable. A PYCR2-deficient HEK293FT cell line generated by clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 genome-editing showed that PYCR2 loss of function led to decreased mitochondrial membrane potential and increased susceptibility to apoptosis under oxidative stress. Morpholino-based knockdown of the zebrafish PYCR2 homolog, pycr1b, recapitulated the human microcephaly phenotype, which was rescued by wild-type human PYCR2 mRNA, but not by mutant mRNAs, further supporting the pathogenicity of the identified variants. Hypomyelination and the absence of lax, wrinkly skin distinguishes this condition from that caused by previously reported mutations in the gene encoding PYCR2’s isozyme, PYCR1, suggesting a unique and indispensable role for PYCR2 in the human central nervous system during development.
Retinal ganglion cells (RGCs) represent the output computations of the visual world. In the mouse, they represent > 30 parallel channels of visual processing, encoding both image-forming and ...non-image forming features. The dendrites of RGCs are considered essential in ensuring that the downstream visual system receives a full depiction of the sampled visual field. Different retinal ganglion cell types have evolved a multitude of dendritic arbor architectures that allow them to efficiently encode the spatial and temporal features of the visual scene that they are tuned to. Despite this fundamental role of dendritic morphology, there are relatively few studies that directly test whether different dendritic morphologies lead to variations in the wiring and resulting computations within a neural circuit.This main body of this work has aimed to address this question in a well-defined neural circuit in the retina that is responsible for our ability to detect the direction an object moves in the visual world. Direction selectivity is a neural computation where a directionally selective retinal ganglion cell (DSGC), one of the output neurons of the retina whose axons comprise the optic nerve, fire more action potentials in response to motion in one direction, versus motion in the opposite direction. Multiple presynaptic mechanisms involving the specific wiring of excitatory and inhibitory inputs onto DSGC dendrites have been postulated to contribute to the direction selectivity computation. Additionally, postsynaptic computations within the DSGC dendrites have been postulated to sharpen their directional tuningHere, we explore how DSGC dendrites contribute to both the presynaptic organization of excitatory and inhibitory inputs, and the postsynaptic contribution to directional tuning. First, we show that visual experience influences the dendritic orientation in a population of asymmetric ventral preferring DSGCs (vDSGCs) whose dendrites point ventrally. By comparing the tuning of normally versus dark-reared vDSGCs, we find that their tuning to ventral motion is preserved regardless of their dendritic orientation. This is due to the persistence of asymmetric wiring of inhibition in dark-reared vDSGCs. However, we find that the ventral orientation of vDSGC dendrites is necessary for their postsynaptic directional computation, which occurs in the absence of inhibitory input. Hence, in vDSGCs, dendritic morphology dictates postsynaptic but not presynaptic mechanisms for directional computation.Second, we show that dendritic morphology, across two distinct populations of DSGCs, does not determine the organization of presynaptic inputs. By comparing the excitatory and inhibitory receptive fields across two morphologically distinct DSGC subtypes, we find that although asymmetric DSGCs exhibit greater tuning of their inhibitory input in response to a moving stimulus, compared to symmetric DSGCs, the synaptic organization of excitation relative to inhibition is comparable across cell types. Hence, DSGC dendritic morphology does not dictate the organization of excitatory and inhibitory synaptic inputs relative to each other.
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