Spliceosome malfunction causes neurodevelopmental disorders with overlapping features

Pre-mRNA splicing is a highly coordinated process. While its dysregulation has been linked to neurological deficits, our understanding of the underlying molecular and cellular mechanisms remains limited. We implicated pathogenic variants in U2AF2 and PRPF19, encoding spliceosome subunits in neurodevelopmental disorders (NDDs), by identifying 46 unrelated individuals with 23 de novo U2AF2 missense variants (including 7 recurrent variants in 30 individuals) and 6 individuals with de novo PRPF19 variants. Eight U2AF2 variants dysregulated splicing of a model substrate. Neuritogenesis was reduced in human neurons differentiated from human pluripotent stem cells carrying two U2AF2 hyper-recurrent variants. Neural loss of function (LoF) of the Drosophila orthologs U2af50 and Prp19 led to lethality, abnormal mushroom body (MB) patterning, and social deficits, which were differentially rescued by wild-type and mutant U2AF2 or PRPF19. Transcriptome profiling revealed splicing substrates or effectors (including Rbfox1, a third splicing factor), which rescued MB defects in U2af50-deficient flies. Upon reanalysis of negative clinical exomes followed by data sharing, we further identified 6 patients with NDD who carried RBFOX1 missense variants which, by in vitro testing, showed LoF. Our study implicates 3 splicing factors as NDD-causative genes and establishes a genetic network with hierarchy underlying human brain development and function.

Rbfox1 up-regulation impairs BDNF-dependent hippocampal LTP by dysregulating TrkB isoform expression levels

Brain-derived neurotrophic factor (BDNF) is a potent modulator of brain synaptic plasticity. Signaling defects caused by dysregulation of its Ntrk2 (TrkB) kinase (TrkB.FL) and truncated receptors (TrkB.T1) have been linked to the pathophysiology of several neurological and neurodegenerative disorders. We found that upregulation of Rbfox1, an RNA binding protein associated with intellectual disability, epilepsy and autism, increases selectively hippocampal TrkB.T1 isoform expression. Physiologically, increased Rbfox1 impairs BDNF-dependent LTP which can be rescued by genetically restoring TrkB.T1 levels. RNA-seq analysis of hippocampi with upregulation of Rbfox1 in conjunction with the specific increase of TrkB.T1 isoform expression also shows that the genes affected by Rbfox1 gain of function are surprisingly different from those influenced by Rbfox1 deletion. These findings not only identify TrkB as a major target of Rbfox1 pathophysiology but also suggest that gain or loss of function of Rbfox1 regulate different genetic landscapes.

SorCS2-mediated NR2A trafficking regulates motor deficits in Huntington's disease

Motor dysfunction is a prominent and disabling feature of Huntington's disease (HD), but the molecular mechanisms that dictate its onset and progression are unknown. The N-methyl-D-aspartate receptor 2A (NR2A) subunit regulates motor skill development and synaptic plasticity in medium spiny neurons (MSNs) of the striatum, cells that are most severely impacted by HD. Here, we document reduced NR2A receptor subunits on the dendritic membranes and at the synapses of MSNs in zQ175 mice that model HD. We identify that SorCS2, a vacuolar protein sorting 10 protein-domain (VPS10P-domain) receptor, interacts with VPS35, a core component of retromer, thereby regulating surface trafficking of NR2A in MSNs. In the zQ175 striatum, SorCS2 is markedly decreased in an age- and allele-dependent manner. Notably, SorCS2 selectively interacts with mutant huntingtin (mtHTT), but not WT huntingtin (wtHTT), and is mislocalized to perinuclear clusters in striatal neurons of human HD patients and zQ175 mice. Genetic deficiency of SorCS2 accelerates the onset and exacerbates the motor coordination deficit of zQ175 mice. Together, our results identify SorCS2 as an interacting protein of mtHTT and demonstrate that impaired SorCS2-mediated NR2A subunit trafficking to dendritic surface of MSNs is, to our knowledge, a novel mechanism contributing to motor coordination deficits of HD.

Gene targeting in mouse embryonic stem cells

The scientific value of a mouse model with a targeted mutation depends greatly upon how carefully the mutation has been engineered. Until recently, our ability to alter the mouse genome has been limited by both the lack of technologies to conditionally target a locus and by conventional cloning. The "cre/loxP" and "recombineering" technologies have overcome some of these limitations and have greatly enhanced our ability to manipulate the mouse genome in a sophisticated way. However, there are still some practical aspects that need to be considered to successfully target a specific genetic locus. Here, we describe the process to engineer a targeted mutation to generate a mouse model. We include a tutorial using the publicly available informatic tools that can be downloaded for processing the genetic information needed to generate a targeting vector.

The scaffold protein Cybr is required for cytokine-modulated trafficking of leukocytes in vivo

Trafficking and cell adhesion are key properties of cells of the immune system. However, the molecular pathways that control these cellular behaviors are still poorly understood. Cybr is a scaffold protein highly expressed in the hematopoietic/immune system whose physiological role is still unknown. In vitro studies have shown it regulates LFA-1, a crucial molecule in lymphocyte attachment and migration. Cybr also binds cytohesin-1, a guanine nucleotide exchange factor for the ARF GTPases, which affects actin cytoskeleton remodeling during cell migration. Here we show that expression of Cybr in vivo is differentially modulated by type 1 cytokines during lymphocyte maturation. In mice, Cybr deficiency negatively affects leukocytes circulating in blood and lymphocytes present in the lymph nodes. Moreover, in a Th1-polarized mouse model, lymphocyte trafficking is impaired by loss of Cybr, and Cybr-deficient mice with aseptic peritonitis have fewer cells than controls present in the peritoneal cavity, as well as fewer leukocytes leaving the bloodstream. Mutant mice injected with Moloney murine sarcoma/leukemia virus develop significantly larger tumors than wild-type mice and have reduced lymph node enlargement, suggesting reduced cytotoxic T-lymphocyte migration. Taken together, these data support a role for Cybr in leukocyte trafficking, especially in response to proinflammatory cytokines in stress conditions.

A kinase-deficient TrkC receptor isoform activates Arf6-Rac1 signaling through the scaffold protein tamalin

Neurotrophins play an essential role in mammalian development. Most of their functions have been attributed to activation of the kinase-active Trk receptors and the p75 neurotrophin receptor. Truncated Trk receptor isoforms lacking the kinase domain are abundantly expressed during development and in the adult; however, their function and signaling capacity is largely unknown. We show that the neurotrophin-3 (NT3) TrkCT1-truncated receptor binds to the scaffold protein tamalin in a ligand-dependent manner. Moreover, NT3 initiation of this complex leads to activation of the Rac1 GTPase through adenosine diphosphate-ribosylation factor 6 (Arf6). At the cellular level, NT3 binding to TrkCT1–tamalin induces Arf6 translocation to the membrane, which in turn causes membrane ruffling and the formation of cellular protrusions. Thus, our data identify a new signaling pathway elicited by the kinase-deficient TrkCT1 receptor. Moreover, we establish NT3 as an upstream regulator of Arf6.

Cdc25b phosphatase is required for resumption of meiosis during oocyte maturation

In a wide variety of animal species, oocyte maturation is arrested temporarily at prophase of meiosis I (ref. 1). Resumption of meiosis requires activation of cyclin-dependent kinase-1 (CDK1, p34cdc2), one component of maturation-promoting factor (MPF). The dual specificity phosphatases Cdc25a, Cdc25b and Cdc25c are activators of cyclin-dependent kinases; consequently, they are postulated to regulate cell-cycle progression in meiosis and mitosis as well as the DNA-damage response. We generated Cdc25b-deficient (Cdc25b-/-) mice and found that they are viable. As compared with wildtype cells, fibroblasts from Cdc25b-/- mice grew vigorously in culture and arrested normally in response to DNA damage. Female Cdc25b-/- mice were sterile, and Cdc25b-/- oocytes remained arrested at prophase with low MPF activity. Microinjection of wildtype Cdc25b mRNA into Cdc25b-/- oocytes caused activation of MPF and resumption of meiosis. Thus, Cdc25b-/- female mice are sterile because of permanent meiotic arrest resulting from the inability to activate MPF. Cdc25b is therefore essential for meiotic resumption in female mice. Mice lacking Cdc25b provide the first genetic model for studying the mechanisms regulating prophase arrest in vertebrates.

Dissection of NT3 functions in vivo by gene replacement strategy

The development of the peripheral nervous system is governed in part by a family of neurotrophic factors that signal through Trk tyrosine kinase receptors. Neurotrophin 3 (NT3) ablation in mice causes a more severe neuronal phenotype than deletion of its receptor TrkC, suggesting that NT3 acts also through other non-preferred Trk receptors. To study the role of low-affinity ligand receptor interactions in vivo, we have replaced the Nt3 gene with the gene for brain-derived neurotrophic factor (BDNF), a TrkB ligand. As in NT3 and TrkC null mice, the proprioception system of these mutants failed to assemble. However, sensory fiber projections in the embryonic spinal cord suggest chemotropic effects of BDNF in vivo. In the dorsal root ganglia, the developmental dynamic of neuron numbers demonstrates that NT3 is required for activation of TrkB during neurogenesis and that TrkA is required during target tissue innervation. In the inner ear, the ectopic BDNF rescued the severe neuronal deficits caused by NT3 absence, indicating that TrkB and TrkC activate equivalent pathways to promote survival of cochlear neurons. However, specific increased innervation densities suggest unique functions for BDNF and NT3 beyond promoting neuronal survival. This mouse model has allowed the dissection of specific spatiotemporal Trk receptor activation by NT3. Our analysis provides examples of how development can be orchestrated by complex high- and low-affinity interactions between ligand and receptor families.

Evidence for a role of truncated trkC receptor isoforms in mouse development

The trkC locus encodes several receptors for neurotrophin-3, including the well studied full-length tyrosine kinase isoform, in addition to receptor isoforms lacking the kinase active domain. TrkC receptors are widely expressed throughout mouse development in many different organs. To investigate the function of truncated receptors in vivo and to identify cell types that are biologically responsive to this gene product, we have overexpressed a physiological truncated trkC isoform in the mouse. Mice overexpressing this receptor develop to term but die in the first postnatal days. High levels of transgene expression result in severe developmental defects in the peripheral nervous system and in the heart. The severity of neuronal losses observed in these animals suggests that truncated receptors may act by sequestering neurotrophin, thus, closely relating this mouse model to the neurotrophin-3-deficient one. Lower levels of exogenous truncated receptor in transgenic mice result in a more modest phenotype and, in some neuronal populations, do not cause neural deficits. Taken together, these data suggest that truncated trkC receptor isoforms may have modulatory functions in development.

Targeted deletion of all isoforms of the trkC gene suggests the use of alternate receptors by its ligand neurotrophin-3 in neuronal development and implicates trkC in normal cardiogenesis

We have generated null mutant mice that lack expression of all isoforms encoded by the trkC locus. These mice display a behavioral phenotype characterized by a loss of proprioceptive neurons. Neuronal counts of sensory ganglia in the trkC mutant mice reveal less severe losses than those in NT-3 null mutant mice, strongly suggesting that NT-3, in vivo, may signal through receptors other than trkC. Mice lacking either NT-3 or all trkC receptor isoforms die in the early postnatal period. Histological examination of trkC-deficient mice reveals severe cardiac defects such as atrial and ventricular septal defects, and valvular defects including pulmonic stenosis. Formation of these structures during development is dependent on cardiac neural crest function. The similarities in cardiac defects observed in the trkC and NT-3 null mutant mice indicate that the trkC receptor mediates most NT-3 effects on the cardiac neural crest.

Absence of sensory neurons before target innervation in brain-derived neurotrophic factor-, neurotrophin 3-, and TrkC-deficient embryonic mice

Gene-targeting experiments of Trk receptors and neurotrophins has confirmed the expectation that embryonic sensory and sympathetic neurons require neurotrophin function for survival. They have further revealed correlation between a specific neurotrophin requirement and eventual sensory modality. We have analyzed embryonic and neonatal mice with mutations in the BDNF, neurotrophin 3 (NT-3), and TrkC genes. Our data confirm an unexpectedly high proportion of sensory neuron losses in NT-3 (>70%), BDNF (>20%), and TrkC (>30%) mutants, which encompass populations thought to be NGF-dependent. Direct comparison of TrkC and NT-3 mutants indicates that only a subset of the NT-3-dependent neurons also requires TrkC. The observed losses in our TrkC mutant, which is null for all proteins encoded by the gene, are more severe than those previously reported for the kinase-negative TrkC mutation, implicating additional and important functions for the truncated receptors. Our data further indicate that mature NGF-requiring neurons undergo precocious and transitory requirements for NT-3 and/or BDNF. We suggest that neurotrophins may function in creating early heterogeneity that would enable ganglia to compensate for diverse modality requirements before the period of naturally occurring death.

Targeted mutation in the neurotrophin-3 gene results in loss of muscle sensory neurons

Neurotrophin 3 (NT-3) is one of four related polypeptide growth factors that share structural and functional homology to nerve growth factor (NGF). NT-3 and its receptor, called neurotrophic tyrosine kinase receptor type 3 (Ntrk3; also called TrkC), are expressed early and throughout embryogenesis. We have inactivated the NT-3 gene in embryonic stem (ES) cells by homologous recombination. The mutated allele has been transmitted through the mouse germ line, and heterozygote intercrosses have yielded homozygous mutant newborn pups. The NT-3-deficient mutants fail to thrive and exhibit severe neurological dysfunction. Analysis of mutant embryos uncovers loss of Ntrk3/TrkC-expressing sensory neurons and abnormalities at early stages of sensory neuronal development. NT-3-deficient mice will permit further study of the role of this neurotrophin in neural development.

A Report on the Evaluation of Exhibits-Mediscope 1961

Thirty-two educational exhibits presented by the Ontario Medical Association at the 1961 Canadian National Exhibition in Toronto in the exhibit known as "Mediscope 1961" were subjected to an evaluative study. Applying the criteria of educational effectiveness to each exhibit, relative ratings for each exhibit as well as the educational value of Mediscope as a whole were obtained. Quantitative data indicated that this venture in health education was a highly successful endeavour, as 80% of the criteria for educational effectiveness were met by all exhibits. In addition, the study emphasized the potential of educational exhibits in the field of public health education as well as education of specific groups.The desirability of similar studies is stressed. In addition to quantitative assessment of educational exhibits, such studies would disclose the impact of health information on the attitudes and behavioural changes on the part of the public.