A Drosophila model to study retinitis pigmentosa pathology associated with mutations in the core splicing factor Prp8

Addressing the tissue specificity of U5 snRNP spliceosomopathies

Precursor mRNA (pre-mRNA) must undergo splicing to remove intron sequences and join exons. This splicing process is catalysed by an RNA/protein complex called the spliceosome. At the centre of the catalytic spliceosome is the U5 small nuclear ribonucleoprotein (snRNP). Pathogenic variants in U5 snRNP core proteins are associated with various diseases commonly known as spliceosomopathies. Variants in TXNL4A and EFTUD2 manifest in craniofacial malformations while variants in PRPF8 and SNRNP200 manifest in retinitis pigmentosa. This perspective highlights research addressing how these specific manifestations come about as the spliceosome is required in all cells and at all developmental stages. Cell and animal models can replicate the human clinical specificity providing explanations for the specificity of the disorders. We propose that future research could benefit from models originating from patient-derived induced pluripotent stem cells (iPSCs) and isogenic controls to compare the coding and non-coding transcriptomic perturbations. Analysis of spliceosomal protein complexes and their interactome could also uncover novel insights on molecular pathogenesis. Finally, as studies highlight changes in metabolic processes, metabolomic studies could become a new venture in studying the consequences of U5 snRNP variants.

Xrp1 governs the stress response program to spliceosome dysfunction

Co-transcriptional processing of nascent pre-mRNAs by the spliceosome is vital to regulating gene expression and maintaining genome integrity. Here, we show that the deficiency of functional U5 small nuclear ribonucleoprotein particles (snRNPs) in Drosophila imaginal cells causes extensive transcriptome remodeling and accumulation of highly mutagenic R-loops, triggering a robust stress response and cell cycle arrest. Despite compromised proliferative capacity, the U5 snRNP-deficient cells increased protein translation and cell size, causing intra-organ growth disbalance before being gradually eliminated via apoptosis. We identify the Xrp1-Irbp18 heterodimer as the primary driver of transcriptional and cellular stress program downstream of U5 snRNP malfunction. Knockdown of Xrp1 or Irbp18 in U5 snRNP-deficient cells attenuated JNK and p53 activity, restored normal cell cycle progression and growth, and inhibited cell death. Reducing Xrp1-Irbp18, however, did not rescue the splicing defects, highlighting the requirement of accurate splicing for cellular and tissue homeostasis. Our work provides novel insights into the crosstalk between splicing and the DNA damage response and defines the Xrp1-Irbp18 heterodimer as a critical sensor of spliceosome malfunction and mediator of the stress-induced cellular senescence program.

Retinitis pigmentosa-associated mutations in mouse Prpf8 cause misexpression of circRNAs and degeneration of cerebellar granule cells

A subset of patients with retinitis pigmentosa (RP) carry mutations in several spliceosomal components including the PRPF8 protein. Here, we established two alleles of murine Prpf8 that genocopy or mimic aberrant PRPF8 found in RP patients-the substitution p.Tyr2334Asn and an extended protein variant p.Glu2331ValfsX15. Homozygous mice expressing the aberrant Prpf8 variants developed within the first 2 mo progressive atrophy of the cerebellum because of extensive granule cell loss, whereas other cerebellar cells remained unaffected. We further show that a subset of circRNAs were deregulated in the cerebellum of both Prpf8-RP mouse strains. To identify potential risk factors that sensitize the cerebellum for Prpf8 mutations, we monitored the expression of several splicing proteins during the first 8 wk. We observed down-regulation of all selected splicing proteins in the WT cerebellum, which coincided with neurodegeneration onset. The decrease in splicing protein expression was further pronounced in mouse strains expressing mutated Prpf8. Collectively, we propose a model where physiological reduction in spliceosomal components during postnatal tissue maturation sensitizes cells to the expression of aberrant Prpf8 and the subsequent deregulation of circRNAs triggers neuronal death.

Ecd promotes U5 snRNP maturation and Prp8 stability

Pre-mRNA splicing catalyzed by the spliceosome represents a critical step in the regulation of gene expression contributing to transcriptome and proteome diversity. The spliceosome consists of five small nuclear ribonucleoprotein particles (snRNPs), the biogenesis of which remains only partially understood. Here we define the evolutionarily conserved protein Ecdysoneless (Ecd) as a critical regulator of U5 snRNP assembly and Prp8 stability. Combining Drosophila genetics with proteomic approaches, we demonstrate the Ecd requirement for the maintenance of adult healthspan and lifespan and identify the Sm ring protein SmD3 as a novel interaction partner of Ecd. We show that the predominant task of Ecd is to deliver Prp8 to the emerging U5 snRNPs in the cytoplasm. Ecd deficiency, on the other hand, leads to reduced Prp8 protein levels and compromised U5 snRNP biogenesis, causing loss of splicing fidelity and transcriptome integrity. Based on our findings, we propose that Ecd chaperones Prp8 to the forming U5 snRNP allowing completion of the cytoplasmic part of the U5 snRNP biogenesis pathway necessary to meet the cellular demand for functional spliceosomes.

First person – Dimitrije Stanković

First Person is a series of interviews with the first authors of a selection of papers published in Disease Models & Mechanisms, helping early-career researchers promote themselves alongside their papers. Dimitrije Stanković is first author on ‘A Drosophila model to study retinitis pigmentosa pathology associated with mutations in the core splicing factor Prp8’, published in DMM. Dimitrije is a PhD student in the lab of Mirka Uhlirova at CECAD Research Center in Cologne, Germany, investigating the functional consequences of aberrant pre-mRNA splicing in the context of regulation of gene expression, activation of stress signaling pathways and DNA damage.