by King’s College London
PreT-IR transcripts undergo cleavage/polyadenylation and neurite localisation in SFPQ-deprived neurons. a Correlation between RNAseq read slope gradient in null and number of cleavage sites in introns of sloping IR-transcripts. Two-tailed Pearson correlation coefficient, r: −0.3506, p < 0.0001 (n = 247 sloping IR-transcripts, FDR < 0.05). b Efficiency of cleavage site usage in classic (Left; (n = 107, FDR < 0.05)) and sloping (Middle; (n = 37, m < −0.1, FDR < 0.05)) IR-transcripts in sibling and null samples. (Right) Efficiency of cleavage site usage in retained introns between classic versus sloping IR-transcripts in null samples. Floating bar plots where the middle represents the median and the upper and lower limits represent the maximum and minimum values. Two-tailed Mann–Whitney tests. (Left) p = 0.21, (Middle) **p:0.0076, (Right) ****p < 0.0001 (c) Positions of 3’mRNAseq read clusters along igdcc3 in 24 hpf sibling null embryo RNA samples, indicating sites of cleavage and polyadenylation. Peak height indicates relative usage of each cleavage site among transcripts of that sample. Dashed lines demarcate intron-2. Black arrows indicate peaks targeted by 3’RACE. d 3’RACE validation of igdcc3 3’mRNAseq results in (c) in null neurites. Sequencing of products shows terminal intronic sequences containing PAS consensus sequences (red boxes) and the start of a PolyA tail. (Left) exons 1–2 + 6.5 kb intron-2 and polyA. (Right) exons 1–2 + 51 kb intron-2 and polyA. e, f Confocal z-projections (10 μm) of 1dpf (1 day post-fertilization/24hpf) sfpq sibling (e) & null (f) embryo axons, and igdcc3 IR and PreT-IR transcripts. Probe sets target either the 5’-most (red) or 3’-most (magenta) 10 kb of intron-2. Axons running longitudinally just anterior to the otic vesicle. Asterisks: axonal RNAs labeled by both intron-2 probe sets. Arrows: axonal RNAs labeled by 5’ intron-2 probe set only. RNA puncta are also present in neuronal cell bodies surrounding axons. Scale bars 10 μm. g, h Quantification of 5’ and 3’ intron-2 puncta in axons in (e, f). Each datapoint represents a different embryo. N = 10 sibling and 10 null embryos. Two-tailed unpaired t tests with Welch’s correction, ****p < 0.0001. Credit: Nature Communications (2022). DOI: 10.1038/s41467-022-34331-4
New research from the Institute of Psychiatry, Psychology & Neuroscience (IoPPN) at King’s College London has identified a new hallmark of the neurodegenerative disorder, amyotrophic lateral sclerosis (ALS), revealing a likely molecular trigger of the disease.
The study, published in Nature Communications, found that loss of the RNA processing protein, SFPQ, in motor neurons results in their “dying back.” This involves destruction of the axon (the part of the neuron responsible for connecting to and communicating with the rest of the body) and later, cell body death.
Using zebrafish, the investigators explored what happens inside SFPQ-depleted motor neurons before they degenerate. They found that draft messenger-RNAs (mRNAs) that are normally regulated by SFPQ are only partially edited and become truncated. To their surprise, these defective mRNAs are not degraded but instead stably localize to axons, where they accumulate and interfere with normal function. This is the first study to show the cascade of molecular events in axons triggered by loss of the protein.
As loss of SFPQ is strongly associated with ALS, the authors took their findings from the zebrafish and analyzed data from ALS patients. They found that the unusual defective mRNAs are enriched in patient neurons, pointing to these axonal mRNAs to be a cause for degeneration in the disease.
Corinne Houart, Professor of Developmental Neurobiology at King’s IoPPN and the study’s lead author said, “Finding the same mRNA abnormalities in zebrafish SFPQ mutant and in human ALS neurons is opening a new path in understanding the neurodegenerative process. Our findings as well as those of international colleagues show that controlling mRNA diversity in axons is essential of neuronal health. Uncovering the local changes in mRNA regulation in neurons affected by aging or neurological disorders will provide great progress in tackling these issues.”
It’s estimated that around 5,000 people in the UK live with ALS at any one time. The average survival time following diagnosis is between two to five years, and as yet there is no available cure. The findings open a possible new avenue of therapy in ALS patients, aimed at restoring SFPQ function in neurons. In doing so, one may be able to prevent their degeneration.
Dr. Richard Taylor, the study’s first author said, “We were surprised to find that these abnormal mRNAs, forming upon loss of SFPQ function, are not degraded in the nucleus. Instead, contrary to dogma they seem to escape and specifically localize to axons. Their effects either at the RNA level, or protein level if translated, are likely detrimental to axon integrity and may initiate the ‘dying back’ sequence of degeneration observed in ALS neurons”.
The researchers will now turn their focus to determining the mechanisms by which these defective mRNAs are toxic in ALS patient axons, and will investigate the effect of introducing normal SFPQ into patient iPSC-derived degenerating neurons.
Leave a Reply