The impact of anti-U1-RNP positivity: systemic lupus erythematosus versus mixed connective tissue disease.

Anti-U1-RNP positivity remains mandatory for the mixed connective tissue disease (MCTD) diagnosis, reason for which anti-U1-RNP occurrence in patients with lupus clinical features might determine diagnostic issues. Thus, the prevalence of 25-30% for anti-RNP was reported in John Hopkins and LUMINA lupus cohorts and also 13% prevalence for the anti-U1-RNP in Euro-Lupus cohort. Presence of anti-U1-RNP antibodies in patients fulfilling SLE criteria (but not the MCTD ones) was associated with manifestations such as Raynaud phenomenon, musculoskeletal and lung impairment or nail fold capillaroscopy changes, some clinical features frequently encountered in MCTD patients and only rarely described in lupus population. The use of more specific markers such as 70 kDa anti-U1-RNP or anti-Sm-D was proposed for discriminating between SLE and MCTD. In addition, the IgM serotype of anti-U1-RNP seems more frequently expressed in SLE, while the IgG serotype alone in MCTD. Better acknowledgement of possible clinical involvements in lupus subsets, such as the peculiarities related to the anti-U1-RNP positivity, could provide access to early diagnosis of rather rare but possible severe lupus organ impairments (e.g. pulmonary arterial hypertension).

The splicing factor U1-70K interacts with the SMN complex and is required for nuclear gem integrity.

The nuclear SMN complex localizes to specific structures called nuclear gems. The loss of gems is a cellular marker for several neurodegenerative diseases. Here, we identify that the U1-snRNP-specific protein U1-70K localizes to nuclear gems, and we show that U1-70K is necessary for gem integrity. Furthermore, we show that the interaction between U1-70K and the SMN complex is RNA independent, and we map the SMN complex binding site to the unstructured N-terminal tail of U1-70K. Consistent with these results, the expression of the U1-70K N-terminal tail rescues gem formation. These findings show that U1-70K is an SMN-complex-associating protein, and they suggest a new function for U1-70K in the formation of nuclear gems.

Integrated approaches for analyzing U1-70K cleavage in Alzheimer's disease.

The accumulation of pathologic protein fragments is common in neurodegenerative disorders. We have recently identified in Alzheimer's disease (AD) the aggregation of the U1-70K splicing factor and abnormal RNA processing. Here, we present that U1-70K can be cleaved into an N-terminal truncation (N40K) in ∼50% of AD cases, and the N40K abundance is inversely proportional to the total level of U1-70K. To map the cleavage site, we compared tryptic peptides of N40K and stable isotope labeled U1-70K by liquid chromatography-tandem mass spectrometry (MS), revealing that the proteolysis site is located in a highly repetitive and hydrophilic domain of U1-70K. We then adapted Western blotting to map the cleavage site in two steps: (i) mass spectrometric analysis revealing that U1-70K and N40K share the same N-termini and contain no major modifications; (ii) matching N40K with a series of six recombinant U1-70K truncations to define the cleavage site within a small region (Arg300 ± 6 residues). Finally, N40K expression led to substantial degeneration of rat primary hippocampal neurons. In summary, we combined multiple approaches to identify the U1-70K proteolytic site and found that the N40K fragment might contribute to neuronal toxicity in Alzheimer's disease.

Expression and Purification of Human U1-70K (snRNP70) and its BAD Domains Using an E. coli Expression System.

U1-70K (snRNP70) serves as an indispensable protein component within the U1 complex, assuming a pivotal role in both constitutive and alternative RNA splicing processes. Notably, U1-70K engages in interactions with SR proteins, instigating the assembly of the spliceosome. This protein undergoes regulation through phosphorylation at multiple sites. Of significant interest, U1-70K has been implicated in Alzheimer's disease, in which it tends to form detergent-insoluble aggregates. Even though it was identified more than three decades ago, our understanding of U1-70K remains notably constrained, primarily due to challenges such as low levels of recombinant expression, susceptibility to protein degradation, and insolubility. In endeavoring to address these limitations, we devised a multifaceted approach encompassing codon optimization, strategic purification, and a solubilization protocol. This methodology has enabled us to achieve a high yield of full-length, soluble U1-70K, paving the way for its comprehensive biophysical and biochemical characterization. Furthermore, we provide a detailed protocol for the preparation of phosphorylated U1-70K. This set of protocols promises to be a valuable resource for scientists exploring the intricate web of U1-70K-related mechanisms in the context of RNA splicing and its implications in neurodegenerative disorders and other disorders and biological processes. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Expression and purification of full-length U1-70K from E. coli Support Protocol 1: Making chemically competent BL21 Star pRARE/pBB535 cells Basic Protocol 2: Phosphorylation of full-length U1-70K using SRPK1 Support Protocol 2: Purification of SRPK1 Basic Protocol 3: Expression and purification of U1-70K BAD1 from E. coli Basic Protocol 4: Phosphorylation of U1-70K BAD1 using SRPK1 Basic Protocol 5: Expression and purification of U1-70K BAD2 from E. coli.

The U1-70K and SRSF1 interaction is modulated by phosphorylation during the early stages of spliceosome assembly.

In eukaryotes, pre-mRNA splicing is vital for RNA processing and orchestrated by the spliceosome, whose assembly starts with the interaction between U1-70K and SR proteins. Despite the significance of the U1-70K/SR interaction, the dynamic nature of the complex and the challenges in obtaining soluble U1-70K have impeded a comprehensive understanding of the interaction at the structural level for decades. We overcome the U1-70K solubility issues, enabling us to characterize the interaction between U1-70K and SRSF1, a representative SR protein. We unveil specific interactions: phosphorylated SRSF1 RS with U1-70K BAD1, and SRSF1 RRM1 with U1-70K RRM. The RS/BAD1 interaction plays a dominant role, whereas the interaction between the RRM domains further enhances the stability of the U1-70K/SRSF1 complex. The RRM interaction involves the C-terminal extension of U1-70K RRM and the conserved acid patches on SRSF1 RRM1 that is involved in SRSF1 phase separation. Our circular dichroism spectra reveal that BAD1 adapts an α-helical conformation and RS is intrinsically disordered. Intriguingly, BAD1 undergoes a conformation switch from α-helix to ß-strand and random coil upon RS binding. In addition to the regulatory mechanism via SRSF1 phosphorylation, the U1-70K/SRSF1 interaction is also regulated by U1-70K BAD1 phosphorylation. We find that U1-70K phosphorylation inhibits the U1-70K and SRSF1 interaction. Our structural findings are validated through in vitro splicing assays and in-cell saturated domain scanning using the CRISPR method, providing new insights into the intricate regulatory mechanisms of pre-mRNA splicing.

Tau-Mediated Disruption of the Spliceosome Triggers Cryptic RNA Splicing and Neurodegeneration in Alzheimer's Disease.

In Alzheimer's disease (AD), spliceosomal proteins with critical roles in RNA processing aberrantly aggregate and mislocalize to Tau neurofibrillary tangles. We test the hypothesis that Tau-spliceosome interactions disrupt pre-mRNA splicing in AD. In human postmortem brain with AD pathology, Tau coimmunoprecipitates with spliceosomal components. In Drosophila, pan-neuronal Tau expression triggers reductions in multiple core and U1-specific spliceosomal proteins, and genetic disruption of these factors, including SmB, U1-70K, and U1A, enhances Tau-mediated neurodegeneration. We further show that loss of function in SmB, encoding a core spliceosomal protein, causes decreased survival, progressive locomotor impairment, and neuronal loss, independent of Tau toxicity. Lastly, RNA sequencing reveals a similar profile of mRNA splicing errors in SmB mutant and Tau transgenic flies, including intron retention and non-annotated cryptic splice junctions. In human brains, we confirm cryptic splicing errors in association with neurofibrillary tangle burden. Our results implicate spliceosome disruption and the resulting transcriptome perturbation in Tau-mediated neurodegeneration in AD.

Characterization of Detergent Insoluble Proteome in Chronic Traumatic Encephalopathy.

Quantitative proteomics of postmortem human brain can identify dysfunctional proteins that contribute to neurodegenerative disorders like Alzheimer disease (AD) and frontotemporal dementia. Similar studies in chronic traumatic encephalopathy (CTE) are limited, therefore we hypothesized that proteomic sequencing of CTE frontal cortex brain homogenates from varying CTE pathologic stages may provide important new insights into this disorder. Quantitative proteomics of control, CTE and AD brains was performed to characterize differentially expressed proteins, and we identified over 4000 proteins in CTE brains, including significant enrichment of the microtubule associated protein tau. We also found enrichment and pathologic aggregation of RNA processing factors as seen previously in AD, supporting the previously recognized overlap between AD and CTE. In addition to these similarities, we identified CTE-specific enrichment of proteins which increase with increasing severity of CTE pathology. NADPH dehydrogenase quinone 1 (NQO1) was one of the proteins which showed significant enrichment in CTE and also correlated with increasing CTE stage. NQO1 demonstrated neuropathologic correlation with hyperphosphorylated tau in glial cells, mainly astrocytes. These results demonstrate that quantitative proteomic analysis of CTE postmortem human brain can identify disease relevant findings and novel cellular pathways involved in CTE pathogenesis.

U1 small nuclear RNA variants differentially form ribonucleoprotein particles in vitro.

The U1 small nuclear (sn)RNA participates in splicing of pre-mRNAs by recognizing and binding to 5' splice sites at exon/intron boundaries. U1 snRNAs associate with 5' splice sites in the form of ribonucleoprotein particles (snRNPs) that are comprised of the U1 snRNA and 10 core components, including U1A, U1-70K, U1C and the 'Smith antigen', or Sm, heptamer. The U1 snRNA is highly conserved across a wide range of taxa; however, a number of reports have identified the presence of expressed U1-like snRNAs in multiple species, including humans. While numerous U1-like molecules have been shown to be expressed, it is unclear whether these variant snRNAs have the capacity to form snRNPs and participate in splicing. The purpose of the present study was to further characterize biochemically the ability of previously identified human U1-like variants to form snRNPs and bind to U1 snRNP proteins. A bioinformatics analysis provided support for the existence of multiple expressed variants. In vitro gel shift assays, competition assays, and immunoprecipitations (IPs) revealed that the variants formed high molecular weight assemblies to varying degrees and associated with core U1 snRNP proteins to a lesser extent than the canonical U1 snRNA. Together, these data suggest that the human U1 snRNA variants analyzed here are unable to efficiently bind U1 snRNP proteins. The current work provides additional biochemical insights into the ability of the variants to assemble into snRNPs.