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Target identification is highly instructive in defining the biological roles of microRNAs. However, little is known about other small noncoding RNAs; for example, tRNA-derived RNA Fragments (tRFs). Some tRFs exhibit a gene-silencing mechanism distinctly different from that of typical microRNAs. We recently demonstrated that a respiratory syncytial virus (RSV)-induced tRF, called tRF5-GluCTC, promotes RSV replication. RSV is the single most important cause of lower respiratory tract infection in children. By using biochemical screening and bioinformatics analyses, we have identified apolipoprotein E receptor 2 (APOER2) as a target of tRF5-GluCTC. The 3′-portion of tRF5-GluCTC recognizes a target site in the 3′-untranslated region of APOER2 and suppresses its expression. We have also discovered that APOER2 is an anti-RSV protein whose suppression by tRF5-GluCTC promotes RSV replication. Our report represents the first identification of a natural target of a tRF and illustrates how a virus utilizes a host tRF to control a host gene to favor its replication.

Myasthenia gravis (MG) is an autoimmune disease caused by antibodies against the acetylcholine receptor (AChR), muscle-specific kinase (MuSK) or other AChR-related proteins in the postsynaptic muscle membrane. Localized or general muscle weakness is the predominant symptom and is induced by the antibodies. Patients are grouped according to the presence of antibodies, symptoms, age at onset and thymus pathology. Diagnosis is straightforward in most patients with typical symptoms and a positive antibody test, although a detailed clinical and neurophysiological examination is important in antibody-negative patients. MG therapy should be ambitious and aim for clinical remission or only mild symptoms with near-normal function and quality of life. Treatment should be based on MG subgroup and includes symptomatic treatment using acetylcholinesterase inhibitors, thymectomy and immunotherapy. Intravenous immunoglobulin and plasma exchange are fast-acting treatments used for disease exacerbations, and intensive care is necessary during exacerbations with respiratory failure. Comorbidity is frequent, particularly in elderly patients. Active physical training should be encouraged. Myasthenia gravis is an autoimmune disorder that is caused by autoantibodies against components of the neuromuscular junction. This Primer summarizes the epidemiology, mechanisms, diagnosis and treatment of myasthenia gravis and discusses the quality-of-life issues faced by patients.

Apolipoprotein E receptor 2 (ApoER2) and VLDL receptor belong to the low density lipoprotein receptor family and bind apolipoprotein E. These receptors interact with the clathrin machinery to mediate endocytosis of macromolecules but also interact with other adapter proteins to perform as signal transduction receptors. The best characterized signaling pathway in which ApoER2 and VLDL receptor (VLDLR) are involved is the Reelin pathway. This pathway plays a pivotal role in the development of laminated structures of the brain and in synaptic plasticity of the adult brain. Since Reelin and apolipoprotein E, are ligands of ApoER2 and VLDLR, these receptors are of interest with respect to Alzheimer’s disease. We will focus this review on the complex structure of ApoER2 and VLDLR and a recently characterized ligand, namely clusterin.

Induction of bone formation by Wnt ligands is inhibited when sclerostin (Scl), an osteocyte-produced antagonist, binds to its receptors, the low-density lipoprotein receptor-related proteins 5 or 6 (LRP5/6). Recently, it was shown that enhanced inhibition is achieved by Scl binding to the co-receptor LRP4. However, it is not clear if the binding of Scl to LRP4 facilitates Scl binding to LRP5/6 or inhibits the Wnt pathway in an LRP5/6-independent manner. Here, using the yeast display system, we demonstrate that Scl exhibits a stronger binding affinity for LRP4 than for LRP6. Moreover, we found stronger Scl binding to LRP6 in the presence of LRP4. We further show that a Scl mutant (Scl N93A ), which tightly binds LRP4 but not LRP6, does not inhibit the Wnt pathway on its own. We demonstrate that Scl N93A competes with Scl for a common binding site on LRP4 and antagonizes Scl inhibition of the Wnt signaling pathway in osteoblasts in vitro. Finally, we demonstrate that 2 weeks of bi-weekly subcutaneous injections of Scl N93A fused to the fragment crystallizable (Fc) domain of immunoglobulin (Scl N93A Fc), which retains the antagonistic activity of the mutant, significantly increases bone formation rate and enhances trabecular volumetric bone fraction, trabecular number, and bone length in developing mice. Our data show that LRP4 serves as an anchor that facilitates Scl–LRP6 binding and that inhibition of the Wnt pathway by Scl depends on its prior binding to LRP4. We further provide evidence that compounds that inhibit Scl–LRP4 interactions offer a potential strategy to promote anabolic bone functions.

Congenital myasthenia (CM) is a devastating neuromuscular disease, and mutations in DOK7, an adaptor protein that is crucial for forming and maintaining neuromuscular synapses, are a major cause of CM 1 , 2 . The most common disease-causing mutation ( DOK7 1124_1127 dup ) truncates DOK7 and leads to the loss of two tyrosine residues that are phosphorylated and recruit CRK proteins, which are important for anchoring acetylcholine receptors at synapses. Here we describe a mouse model of this common form of CM ( Dok7 CM mice) and a mouse with point mutations in the two tyrosine residues ( Dok7 2YF ). We show that Dok7 CM mice had severe deficits in neuromuscular synapse formation that caused neonatal lethality. Unexpectedly, these deficits were due to a severe deficiency in phosphorylation and activation of muscle-specific kinase (MUSK) rather than a deficiency in DOK7 tyrosine phosphorylation. We developed agonist antibodies against MUSK and show that these antibodies restored neuromuscular synapse formation and prevented neonatal lethality and late-onset disease in Dok7 CM mice. These findings identify an unexpected cause for disease and a potential therapy for both DOK7 CM and other forms of CM caused by mutations in AGRIN , LRP4 or MUSK , and illustrate the potential of targeted therapy to rescue congenital lethality. In a mouse model of congenital myasthenia caused by mutations in the Dok7 gene, agonist antibodies against MUSK restore synaptic function and survival.

The SORL1 protein plays a protective role against the secretion of the amyloid β peptide, a key event in the pathogeny of Alzheimer’s disease. We assessed the impact of SORL1 rare variants in early-onset Alzheimer’s disease (EOAD) in a case–control setting. We conducted a whole exome analysis among 484 French EOAD patients and 498 ethnically matched controls. After collapsing rare variants (minor allele frequency ≤1%), we detected an enrichment of disruptive and predicted damaging missense SORL1 variants in cases (odds radio (OR)=5.03, 95% confidence interval (CI)=(2.02–14.99), P =7.49.10 −5 ). This enrichment was even stronger when restricting the analysis to the 205 cases with a positive family history (OR=8.86, 95% CI=(3.35–27.31), P =3.82.10 −7 ). We conclude that predicted damaging rare SORL1 variants are a strong risk factor for EOAD and that the association signal is mainly driven by cases with positive family history.

SORL1 encodes the retromer-associated receptor SORLA that functions in endosomal recycling. Rare variants in SORL1 have been associated with Alzheimer's disease (AD) and rare pathogenic variants are estimated to occur in up to 2.75% of early onset AD patients and in 1.5% of unrelated late onset AD patients. While truncation mutations are observed almost exclusively in AD patients, it is currently unknown which among the hundreds of rare missense variants identified in SORL1, are pathogenic. Here we address this question by relying on SORLA's distinct molecular architecture. First, we completed a structure-guided sequence alignment for all the protein domains. Next, we identified proteins that contain domains homologous to those of SORLA, which include pathogenic variants for monogenic diseases. We identified the analogous domain positions of these variants in the SORLA protein sequence and showed that variants in these positions similarly impair SORL1, and lead to AD. Together, our findings represent a comprehensive study on SORLA protein variation and functional effects, which allowed us to prioritize SORL1 genetic variants into high or moderate priority mutations. We envision that our findings will be used by clinical geneticists for assessing variants they identify in patients, allowing further development of diagnostic procedures and patient counseling strategies. Ultimately, the identification of the most pathogenic variants will inform investigations into the molecular mechanisms of endosomal recycling which will support the development of therapeutic treatment strategies for SORL1 variant-carrying patients.

Emerging evidence suggests that osteoarthritis is associated with high cholesterol levels in some osteoarthritis patients. However, the specific mechanism under this metabolic osteoarthritis phenotype remains unclear. We find that cholesterol metabolism-related gene, LRP3 (low-density lipoprotein receptor-related protein 3) is significantly reduced in high-cholesterol diet mouse’s cartilage. By using Lrp3 −/− mice in vivo and LRP3 lentiviral-transduced chondrocytes in vitro, we identify that LRP3 positively regulate chondrocyte extracellular matrix metabolism, and its deficiency aggravate the degeneration of cartilage. Regardless of diet, LRP3 overexpression in cartilage attenuate anterior cruciate ligament transection induced osteoarthritis progression in rats and Lrp3 knockout-induced osteoarthritis progression in mice. LRP3 knockdown upregulate syndecan-4 by activating the Ras signaling pathway. We identify syndecan-4 as a downstream molecular target of LRP3 in osteoarthritis pathogenesis. These findings suggest that cholesterol-LRP3- syndecan-4 axis plays critical roles in osteoarthritis development, and LRP3 gene therapy may provide a therapeutic regimen for osteoarthritis treatment. This study demonstrates a role of cholesterol metabolism-related gene, Lrp3, in cartilage degeneration and osteoarthritis pathogenesis. LRP3 positively regulates cartilage extracellular matrix metabolism by targeting syndecan-4 via Ras signalling, implicating the cholesterol-LRP3-SDC4 axis in osteoarthritic cartilage degeneration.

Multiple signaling pathways, including Wnt signaling, participate in animal development, stem cell biology, and human cancer. Although many components of the Wnt pathway have been identified, unresolved questions remain as to the mechanism by which Wnt binding to its receptors Frizzled and Low-density lipoprotein receptor-related protein 6 (LRP6) triggers downstream signaling events. With live imaging of vertebrate cells, we show that Wnt treatment quickly induces plasma membrane-associated LRP6 aggregates. LRP6 aggregates are phosphorylated and can be detergent-solubilized as ribosome-sized multiprotein complexes. Phospho-LRP6 aggregates contain Wnt-pathway components but no common vesicular traffic markers except caveolin. The scaffold protein Dishevelled (Dvl) is required for LRP6 phosphorylation and aggregation. We propose that Wnts induce coclustering of receptors and Dvl in LRP6-signalosomes, which in turn triggers LRP6 phosphorylation to promote Axin recruitment and β-catenin stabilization.

Background Alzheimer’s disease (AD) is a progressive and age-associated neurodegenerative disorder that affects women disproportionally. However, the underlying mechanisms are poorly characterized. Moreover, while the interplay between sex and ApoE genotype in AD has been investigated, multi-omics studies to understand this interaction are limited. Therefore, we applied systems biology approaches to investigate sex-specific molecular networks of AD. Methods We integrated large-scale human postmortem brain transcriptomic data of AD from two cohorts (MSBB and ROSMAP) via multiscale network analysis and identified key drivers with sexually dimorphic expression patterns and/or different responses to APOE genotypes between sexes. The expression patterns and functional relevance of the top sex-specific network driver of AD were further investigated using postmortem human brain samples and gene perturbation experiments in AD mouse models. Results Gene expression changes in AD versus control were identified for each sex. Gene co-expression networks were constructed for each sex to identify AD-associated co-expressed gene modules shared by males and females or specific to each sex. Key network regulators were further identified as potential drivers of sex differences in AD development. LRP10 was identified as a top driver of the sex differences in AD pathogenesis and manifestation. Changes of LRP10 expression at the mRNA and protein levels were further validated in human AD brain samples. Gene perturbation experiments in EFAD mouse models demonstrated that LRP10 differentially affected cognitive function and AD pathology in sex- and APOE genotype-specific manners. A comprehensive mapping of brain cells in LRP10 over-expressed (OE) female E4FAD mice suggested neurons and microglia as the most affected cell populations. The female-specific targets of LRP10 identified from the single cell RNA-sequencing (scRNA-seq) data of the LRP10 OE E4FAD mouse brains were significantly enriched in the LRP10 -centered subnetworks in female AD subjects, validating LRP10 as a key network regulator of AD in females. Eight LRP10 binding partners were identified by the yeast two-hybrid system screening, and LRP10 over-expression reduced the association of LRP10 with one binding partner CD34. Conclusions These findings provide insights into key mechanisms mediating sex differences in AD pathogenesis and will facilitate the development of sex- and APOE genotype-specific therapies for AD.