RNA Splicing Defects in Neurodegeneration

RNA Splicing Defects in Neurodegeneration

Overview

RNA Splicing Defects in Neurodegeneration describes a key molecular or cellular mechanism implicated in neurodegenerative disease. This page provides a detailed overview of the pathway components, signaling cascades, and their relevance to conditions such as Alzheimer's disease, Parkinson's disease, and related disorders.

RNA Splicing Defects in Neurodegeneration

Overview

RNA Splicing Defects in Neurodegeneration describes a key molecular or cellular mechanism implicated in neurodegenerative disease. This page provides a detailed overview of the pathway components, signaling cascades, and their relevance to conditions such as Alzheimer's disease, Parkinson's disease, and related disorders.

RNA splicing is a fundamental cellular process by which introns are removed from pre-messenger RNA (pre-mRNA) to generate mature mRNA transcripts. This process is catalyzed by the spliceosome, a large ribonucleoprotein complex composed of five small nuclear ribonucleoproteins (snRNPs) and numerous associated proteins. In neurodegenerative diseases, defects in RNA splicing have emerged as a critical pathogenic mechanism, contributing to protein dysregulation, neuronal dysfunction, and cell death["@liu2024"].

The Spliceosome and RNA Splicing Machinery

The spliceosome undergoes dynamic assembly and disassembly throughout the splicing cycle. The major (U2-dependent) spliceosome recognizes conserved sequence elements at the 5' splice site, branch point, and 3' splice site to catalyze intron removal with remarkable precision.

Core Spliceosomal Components

Small Nuclear Ribonucleoproteins (snRNPs):

• U1 snRNP recognizes the 5' splice site through base-pairing interactions
• U2 snRNP binds the branch point adenosine
• U4/U5/U6 tri-snRNP constitutes the catalytic core
• U5 snRNP contacts both splice site regions during catalysis

• Serine/arginine-rich (SR) proteins facilitate splice site recognition
• Heterogeneous nuclear ribonucleoproteins (hnRNPs) regulate alternative splicing
• PRP proteins (e.g., PRP8, PRP6) are essential for spliceosome assembly

Splicing Types

Constitutive splicing removes all introns from pre-mRNA to generate standard mRNA transcripts. Alternative splicing produces multiple mRNA variants from a single gene by selectively including or excluding specific exons or introns, greatly expanding proteomic diversity.

RNA Splicing Defects in Neurodegenerative Diseases

Alzheimer's Disease

In Alzheimer's disease (AD), RNA splicing defects contribute to disease pathogenesis through multiple mechanisms[@mills2024]:

Tau splicing alterations: The MAPT gene encoding tau protein undergoes aberrant alternative splicing in AD. Inclusion of exon 10 produces 3R (three repeat) tau isoforms, while exclusion produces 4R tau. An imbalance in 3R/4R ratio disrupts microtubule stability and promotes tau aggregation.

APP splicing dysregulation: Alternative splicing of APP (amyloid precursor protein) generates different isoforms with distinct amyloidogenic properties. Increased inclusion of exon 7 and exon 8 produces APP isoforms that may contribute to amyloid-beta generation.

Spliceosome component downregulation: Key spliceosomal proteins including SNRNP70, SNRNP116, and SF3B1 show reduced expression in AD brains, leading to widespread splicing abnormalities.

Intron retention: Recent studies reveal widespread intron retention in AD brains, affecting genes involved in synaptic function, mitochondrial metabolism, and stress responses.

Parkinson's Disease

RNA splicing defects in Parkinson's disease (PD) primarily affect genes involved in dopaminergic neuron survival and protein quality control[@bnfer2024]:

LRRK2 splicing: Mutations in LRRK2 (Leucine-Rich Repeat Kinase 2) are a common genetic cause of familial PD. Alternative splicing of LRRK2 produces variants with altered kinase activity and subcellular localization.

SNCA splicing: The gene encoding alpha-synuclein (SNCA) undergoes alternative splicing producing isoforms with different aggregation propensities. Increased inclusion of exon 3 (SNCA-126) or exon 5 (SNCA-140) may influence oligomerization and toxicity.

PARK2/PARKIN splicing: Mutations affecting splicing of the PARKIN gene lead to loss of functional protein and impaired mitophagy.

Splicing factor dysregulation: Splicing factors including HNRNPA2/B1, HNRNPA1, and SRSF2 show altered expression and localization in PD brains.

Amyotrophic Lateral Sclerosis

RNA splicing defects are particularly prominent in ALS, where they affect genes critical for motor neuron survival[@ferrari2024]:

TDP-43 pathology: TDP-43 (TAR DNA-binding protein 43) is an RNA-binding protein that regulates splicing of numerous target genes. In ALS, TDP-43 mislocalizes to cytoplasmic aggregates, leading to loss of its nuclear splicing function.

FUS splicing alterations: FUS (Fused in Sarcoma) is another RNA-binding protein mutated in familial ALS. FUS regulates splicing of genes involved in neuronal development and stress responses.

C9orf72 hexanucleotide repeat: The most common genetic cause of familial ALS involves a GGGGCC hexanucleotide repeat expansion in C9orf72. This repeat produces toxic RNAs that sequester splicing factors, leading to widespread splicing dysregulation.

NEFH splicing: Abnormal splicing of the neurofilament heavy chain (NEFH) gene produces a truncated protein that may contribute to axonal transport deficits.

Frontotemporal Dementia

Frontotemporal dementia (FTD) shares considerable overlap with ALS in terms of RNA splicing defects[@zhang2024]:

TDP-43 inclusion bodies: Like ALS, FTD-TDP shows TDP-43 pathology with consequent splicing abnormalities.

GRN splicing: Progranulin (GRN) gene mutations causing FTD affect splicing efficiency, with some mutations creating cryptic splice sites.

MAPT splicing: Certain MAPT mutations causing FTD alter exon 10 splicing, disrupting the 3R/4R tau ratio.

Huntington's Disease

RNA splicing defects in Huntington's disease (HD) affect genes involved in neuronal function and survival[@hodges2024]:

HTT splicing: The huntingtin gene (HTT) undergoes alternative splicing producing isoforms with different functional properties.

Splicing factor sequestration: Mutant huntingtin protein sequesters splicing factors including SF1 and U2AF, disrupting normal splicing patterns.

Exon skipping: Aberrant exon skipping events have been documented in HD brains, particularly affecting genes involved in synaptic function.

Molecular Mechanisms of Splicing Dysregulation

Loss of Nuclear RNA-Binding Proteins

Many neurodegenerative diseases involve aggregation of RNA-binding proteins in the cytoplasm, depleting their nuclear function[@lee2024]:

TDP-43 normally resides in the nucleus where it regulates splicing. In ALS/FTD, it mislocalizes to cytoplasmic inclusions, losing its nuclear function.

FUS similarly aggregates in the cytoplasm in some forms of familial ALS, disrupting its splicing regulatory activity.

hnRNPs including hnRNPA1, hnRNPA2/B1, and hnRNP C show altered localization and function in various neurodegenerative conditions.

Repeat-Associated Non-ATG Translation

Hexanucleotide repeat expansions in genes like C9orf72 undergo repeat-associated non-ATG (RAN) translation, producing toxic dipeptide repeat proteins that may disrupt splicing[@zu2023]:

Dipeptide repeat proteins can sequester splicing factors RNA foci formed by expanded repeats can sequester essential splicing proteins nucleocytoplasmic transport defects may affect splicing factor localization

Epigenetic and Transcriptional Dysregulation

Splicing is coupled to transcription and chromatin structure:

RNA polymerase II elongation rate influences splice site selection Chromatin modifications affect splice site recognition Transcription factor binding at regulatory elements can influence splicing patterns

Therapeutic Implications

Understanding RNA splicing defects in neurodegeneration has opened new therapeutic avenues[@rigo2024]:

Splice-Modulating Therapies

Antisense oligonucleotides (ASOs): ASOs can be designed to:

• Correct aberrant splicing patterns
• Promote exon skipping to restore reading frame
• Degrade toxic splice variants
• Restore normal splicing factor function

• Spliceosome inhibitors for certain cancers show potential for repurposing
• Small molecules that enhance or suppress specific splicing events

Gene Therapy Approaches

AAV-mediated delivery: Viral vectors can deliver:

• Wild-type splicing factor genes to compensate for loss-of-function
• ASOs targeting specific splicing defects
• CRISPR-Cas9 systems for precise splice site editing

Targets for Splice-Targeting Therapies

TDP-43 targets: ASOs designed to reduce TDP-43 aggregation or enhance its nuclear localization

FUS targets: Strategies to prevent FUS cytoplasmic aggregation

C9orf72 targets: ASOs targeting the expanded repeat to reduce toxic RNA and dipeptide repeat proteins

Research Frontiers

Current research directions in RNA splicing and neurodegeneration include[@guo2024]:

• Single-molecule splicing assays to characterize spliceosome dynamics
• CRISPR screening to identify splicing factors critical for neuronal survival
• Patient-derived neurons to model splicing defects
• Proteomic studies of spliceosome composition in disease
• Epitranscriptomic modifications affecting splicing regulation

Conclusion

RNA splicing defects represent a common pathogenic mechanism across multiple neurodegenerative diseases. The loss of normal splicing function due to protein aggregation, genetic mutations, or transcriptional dysregulation leads to widespread abnormalities in mRNA processing. These defects affect genes critical for neuronal survival, synaptic function, and protein quality control, creating a feed-forward loop of neurodegeneration. Understanding these mechanisms offers promising therapeutic targets for disease-modifying treatments.

Alternative Splicing and Neuronal Diversity

Alternative splicing is critical for generating the molecular diversity required for complex neuronal functions. In the brain, hundreds of genes undergo neuron-specific alternative splicing to produce protein isoforms with distinct functional properties[@ding2024].

Neuron-Specific Exon Programs

Neurexins and neuroligins are synapse-associated cell adhesion molecules with extensive alternative splicing. Multiple cassette exons in these genes produce isoforms that determine synaptic specificity and function. Dysregulation of neurexin/neuologin splicing contributes to synaptic dysfunction in neurodegeneration.

Ion channels including sodium channels (SCN8A, SCN2A), calcium channels (CACNA1A, CACNA1C), and potassium channels undergo alternative splicing to generate isoforms with distinct electrophysiological properties. Splicing alterations in these channels may contribute to neuronal hyperexcitability in ALS and other disorders.

Actin cytoskeleton regulators including ABRA, SMARCA2, and CDC42EP3 undergo alternative splicing that affects dendritic spine morphology and synaptic plasticity.

Activity-Dependent Splicing

Neuronal activity influences alternative splicing through:

Calcium-dependent splicing factors: Calmodulin-dependent kinases and calcineurin regulate splicing factor localization and activity

Neural activity-regulated splicing: Activity-dependent exon inclusion in genes like NR2A (GRIN2A) and PKM (PKM) influences synaptic plasticity

Activity-dependent splice switching: Neuronal depolarization can rapidly alter splicing patterns through phosphorylation of splicing regulators

Spliceosome Assembly and Regulation

The assembly of the spliceosome is a highly regulated process that occurs in distinct stages[@wahl2024]:

Stage 1: Complex E Formation

• U1 snRNP binds the 5' splice site
• U2AF binds the 3' splice site
• SF1 binds the branch point
• These factors recruit additional proteins to form the early complex (E)

Stage 2: Complex A Formation

• U2 snRNP displaces SF1 and binds the branch point
• ATP hydrolysis drives this transition
• U2AF remains associated with the 3' splice site

Stage 3: Complex B Formation

• U4/U5/U6 tri-snRNP joins the complex
• This requires ATP hydrolysis
• The complex undergoes conformational changes

Stage 4: Complex B* Activation

• Extensive rearrangements occur
• U1 and U4 snRNPs are released
• The catalytic center is activated
• This is the rate-limiting step in splicing

Stage 5: Catalytic Steps

• First transesterification: 5' splice site cleavage
• Second transesterification: exon ligation
• Lariat formation and release

Stage 6: Complex Disassembly

• Post-catalytic spliceosome disassembles
• snRNPs are recycled for new rounds of splicing

Splicing Factor Phosphorylation

Splicing factor phosphorylation is a key regulatory mechanism:

SR protein phosphorylation:

• SRPKs (SR protein kinases) phosphorylate serine residues in RS domains
• Dephosphorylation by phosphatases regulates spliceosome recycling
• Kinase inhibitors show therapeutic potential in neurodegeneration

• Casein kinase 2 (CK2) and protein kinase C (PKC) phosphorylate hnRNPs
• Altered phosphorylation affects RNA binding and localization

• Phosphorylation affects subcellular localization
• Hyperphosphorylation can cause aggregation

Splicing and Mitochondrial Function

Mitochondrial dysfunction is a hallmark of neurodegeneration, and splicing defects contribute to this dysfunction[@gao2024]:

Mitochondrial RNA Processing

• Mitochondria have their own splicing machinery
• Defects in mitochondrial splicing affect oxidative phosphorylation
• Some neurodegeneration-associated proteins localize to mitochondria

Splicing of Mitochondrial Proteins

Nuclear-encoded mitochondrial proteins undergo splicing:

• Alternative splicing produces isoforms with different mitochondrial targeting sequences
• Splicing defects can affect protein import and function

Mitophagy and Splicing

Genes involved in mitophagy undergo splicing:

• PINK1 and PARK2 (PARKIN) splicing alterations in PD
• Effects on mitochondrial quality control

Splicing and Proteostasis

The proteostasis network is affected by splicing defects:

Unfolded Protein Response

Splicing alterations affect ER stress responses:

• XBP1 splicing generates the transcription factor XBP1s
• Dysregulated XBP1 splicing affects protein folding capacity

Autophagy

Autophagy-related genes undergo alternative splicing:

• ATG proteins have alternatively spliced isoforms
• Splicing alterations affect autophagic flux

Protein Quality Control

Molecular chaperones show splicing alterations:

• Hsp70 and Hsp90 isoforms are differentially expressed
• Splicing affects protein refolding capacity

Genetic Factors in Splicing Dysregulation

ALS/FTD Genetic Factors

C9orf72:

• Hexanucleotide repeat expansion is the most common genetic cause of ALS/FTD
• RNA foci sequester splicing factors
• Dipeptide repeat proteins disrupt splicing

• Over 50 mutations cause familial ALS
• Most mutations affect splicing regulation
• Loss-of-function affects splicing of numerous targets

• Mutations cluster in the C-terminal region affecting localization
• FUS regulates splicing of neuronal genes

Parkinson's Disease Genetic Factors

LRRK2:

• Mutations are the most common cause of familial PD
• LRRK2 splicing variants affect function
• Splicing factor involvement in PD pathogenesis

• Alpha-synuclein alternative splicing affects aggregation
• Splicing variants as therapeutic targets

Diagnostic and Prognostic Implications

Splicing biomarkers offer diagnostic potential:

Fluid Biomarkers

• Spliced vs. unspliced ratios in cerebrospinal fluid
• Exon junction complexes as markers of splicing efficiency
• Splicing factor levels in blood or CSF

Imaging Biomarkers

• Correlation between splicing patterns and neuroimaging findings
• Use of splicing PET tracers under development

Prognostic Value

• Splicing patterns may predict disease progression
• Response to therapeutic interventions

The Role of Spliceosome Mutations in Neurodegeneration

Several neurodegenerative diseases are caused by mutations in spliceosome components[@ferrari2024]:

Spliceosomal Protein Mutations

SRSF2 mutations:

• SRSF2 (serine/arginine-rich splicing factor 2) mutations cause familial ALS
• Mutations affect RNA binding and splicing regulation
• Altered splicing of neuronal genes

• U2AF1 (U2AF small subunit 1) mutations associated with ALS
• Affected in 1-2% of cases
• Alters 3' splice site recognition

• SF3B1 (splicing factor 3b subunit 1) mutations in certain dementias
• Affect early spliceosome assembly
• Lead to widespread splicing defects

Mutations Affecting Splicing Regulation

HNRNPA1/HNRNPA2B1:

• Mutations cause ALS and multisystem proteinopathy
• Affect prion-like domain aggregation
• Disrupt splicing regulation

• Over 50 ALS-causing mutations
• Most affect splicing regulation
• Loss-of-function mechanisms

Post-Transcriptional RNA Processing

Beyond splicing, other RNA processing steps are affected in neurodegeneration:

RNA Editing

ADAR-mediated editing:

• Adenosine-to-inosine editing affected in AD
• Alters coding potential of edited transcripts
• Affects glutamate receptor editing

• Cytosine-to-uracil editing in neurodegeneration
• May generate neoantigens

RNA Stability

mRNA decay factors:

• ALS/FTD proteins affect mRNA stability
• Staufen and TTP family proteins involved
• Altered mRNA half-life in disease

RNA Transport

Transport granules:

• Neuronal RNA transport affects local translation
• Granule components affected in ALS/FTD
• Disrupted local protein synthesis

Splicing and Neurodevelopment

Early neurodevelopmental splicing programs may influence adult neurodegeneration:

Developmental Splicing Transitions

• Neonatal splicing patterns differ from adults
• Some disease-associated splicing changes recapitulate development
• Early splicing factor expression patterns

Splicing Factor Expression During Development

Developmental regulation of splicing factors:

• Expression levels change during brain development
• Some splicing factors are neuron-specific
• Developmental splicing patterns in disease

Splicing Therapeutic Targets

Multiple therapeutic approaches target splicing[@rigo2024]:

Antisense Oligonucleotide Strategies

Splice-switching oligonucleotides:

• Bind pre-mRNA to alter splice site recognition
• Can promote exon inclusion or skipping
• Deliverable to CNS via intrathecal administration

• ASO targeting C9orf72 repeat expansion
• ASO to restore normal TDP-43 splicing
• ASO for tau exon 10 splicing

Small Molecule Modulators

Spliceosome-targeting drugs:

• SF3B1 modulators in clinical trials
• Natural compounds affecting splicing
• Repurposing opportunities

CRISPR-Based Approaches

Splice site editing:

• CRISPR to correct disease-causing splice mutations
• Base editing of splice sites
• Prime editing for larger changes

Splicing Defects in Specific Neurodegenerative Diseases

Spinal Muscular Atrophy

SMN (survival motor neuron) deficiency caused by SMN1 loss of function leads to spinal muscular atrophy. SMN2, a paralog, undergoes aberrant splicing that produces truncated protein. This understanding led to splice-targeting therapies:

Spinraza (nusinersen): ASO promoting SMN2 exon 7 inclusion Onasemnogene abeparvovec: Gene therapy delivering SMN1 Risdiplam: Small molecule splice modulator

Retinitis Pigmentosa

splicing defects in photoreceptor genes cause retinitis pigmentosa. Over 100 mutations affect splicing of genes including RHO, USH2A, and PRPF31. Therapeutic approaches include ASOs and CRISPR.

Charcot-Marie-Tooth Disease

Alternative splicing of PMP22 (peripheral myelin protein 22) causes the most common form of CMT1A. Understanding this splicing defect has informed therapeutic development.

Systems-Level Views of Splicing Dysregulation

Transcriptome-Wide Analyses

RNA-seq studies reveal:

• Widespread splicing changes in neurodegeneration
• Specific exon skipping/inclusion patterns
• Intron retention events

• Cell type-specific splicing changes
• Heterogeneity in splicing defects
• Glial vs. neuronal patterns

Network Analysis

Splicing regulatory networks:

• Transcription factor-splicing factor interactions
• RNA binding protein networks
• Disease-specific network perturbations

Cross-Disease Comparisons

Shared splicing defects across diseases:

• Common pathways affected
• Disease-specific signatures
• Convergence on core splicing machinery

Future Research Directions

Several critical questions remain[@guo2024]:

Mechanistic Questions

• What triggers initial splicing defects?
• How do different genetic causes converge on similar splicing patterns?
• What determines neuronal vulnerability to splicing dysregulation?

Therapeutic Questions

• Can splicing defects be corrected in vivo?
• What is the optimal timing for intervention?
• How can delivery to relevant neurons be improved?

Biomarker Questions

• Can splicing patterns serve as biomarkers?
• Can patient stratification be improved?
• Can treatment response be monitored?

See Also

• [Alzheimer's Disease](/diseases/alzheimers-disease)
• [Parkinson's Disease](/diseases/parkinsons-disease)

External Links

• [PubMed](https://pubmed.ncbi.nlm.nih.gov/)
• [KEGG Pathways](https://www.genome.jp/kegg/pathway.html)

References