Progressive protein aggregation in PRPF31 patient retinal pigment epithelium cells: the mechanism and its reversal through activation of autophagy
Authors: Georgiou Maria , Yang Chunbo , Atkinson Robert , Pan Kuan-Ting , Buskin Adriana , Molina Marina Moya , Collin Joseph , Al-Aama Jumana , Goertler Franziska , Ludwig Sebastian E. J. , Davey Tracey , Lührmann Reinhard , Nagaraja-Grellscheid Sushma , Johnson Colin , Ali Robin , Armstrong Lyle , Korolchuk Viktor , Urlaub Henning , Mozaffari-Jovin Sina , Lako Majlinda 
Published: October 2021

Mutations in pre-mRNA processing factor 31 (PRPF31), a core protein of the spliceosomal tri-snRNP complex, cause autosomal-dominant retinitis pigmentosa (adRP). It has remained an enigma why mutations in ubiquitously expressed tri-snRNP proteins result in retina-specific disorders, and so far, the underlying mechanism of splicing factors-related RP is poorly understood. Here, we used iPSC technology to generate retinal organoids and RPE models from three patients with severe and very severe PRPF31-adRP, normal individuals and a CRISPR/Cas9-corrected isogenic control. To fully assess the impacts of PRPF31 mutations, quantitative proteomics analyses of retinal organoids and RPE cells was carried out showing RNA splicing, autophagy and lysosome, unfolded protein response (UPR) and visual cycle-related pathways to be significantly affected. Strikingly, the patient-derived RPE and retinal cells were characterised by the presence of large amounts of cytoplasmic aggregates containing the mutant PRPF31 and misfolded, ubiquitin-conjugated proteins including key visual cycle proteins, which accumulated progressively with time. Mutant PRPF31 variant was not incorporated into splicing complexes, but reduction of PRPF31 wildtype levels led to tri-snRNP assembly defects in Cajal bodies of PRPF31 patient retinal cells with reduced U4/U6 snRNPs and accumulation of U5, smaller nuclear speckles and reduced formation of active spliceosomes giving rise to global splicing dysregulation. Moreover, the impaired waste disposal mechanisms further exacerbated aggregate formation, and targeting these by activating the autophagy pathway using Rapamycin resulted in reduction of cytoplasmic aggregates and improved cell survival. Our data demonstrate that it is the progressive aggregate accumulation that overburdens the waste disposal machinery rather than direct PRPF31-initiated mis-splicing, and thus relieving the RPE cells from insoluble cytoplasmic aggregates presents a novel therapeutic strategy that can be combined with gene therapy studies to fully restore RPE and retinal cell function in PRPF31-adRP patients.

Highlights

PRPF31 RP mutations lead to formation of insoluble aggregates containing the mutant PRPF31 and misfolded, ubiquitin conjugated proteins including key visual cycle proteins (e.g. RLBP1) in RPE cells, which accumulate progressively with time and affect tight junctions and cell survival.

Mutant PRPF31 is predominantly localised in cytoplasmic aggregates of patient specific RPE and retinal cells and is not able to be incorporated into splicing complexes to cause direct mis-splicing.

High-throughput quantitative proteomics identifies significantly altered RNA splicing, visual perception, retinoid metabolism, waste disposal and unfolded protein response pathways in patient RPE cells, and autophagy and lysosome, unfolded protein response (UPR) and visual cycle-related pathways in photoreceptor cells.

Accumulation of PRPF31 mutant variant as cytoplasmic aggregates reduces wildtype PRPF31 in the nucleus leading to tri-snRNP assembly defects, characterised by accumulation of U5 and reduction of U4/U6 snRNPs in Cajal bodies, altered morphology of nuclear speckles and consequently downregulation of active spliceosomes (Bact and C complexes) in PRPF31 patient RPE and retinal cells.

Proteomic study of insoluble aggregates identifies other RP-linked splicing factors and multiple key retinal-specific proteins, whose variants are linked to retinitis pigmentosa, within the aggregates of patient RPE cells.

PRPF31 patient RPE cells have impaired waste disposal and proteasome mediated degradation, which together with the impaired autophagy pathway, further exacerbate aggregate formation.

Phagocytosis of photoreceptor outer segment fragments (POS) shed daily by RPE cells accelerates aggregation of key proteins indicating enhanced cytoplasmic aggregate formation under physiological conditions in patient RPE cells.

Activation of autophagy via administration of rapamycin results in reduction of cytoplasmic aggregates in RPE cells, correct localisation of mislocated and misfolded proteins to the nucleus, thereby improving cell survival.