Non-coding RNAs in Neurodegeneration

Non-coding RNAs in Neurodegeneration

Introduction

Non-coding RNAs (ncRNAs) represent a vast class of RNA molecules that are not translated into protein but serve critical regulatory functions in gene expression, chromatin remodeling, and cellular homeostasis. In the central nervous system, ncRNAs are expressed at particularly high levels and exhibit brain-region-specific patterns, reflecting the transcriptional complexity required for neuronal function. Dysregulation of ncRNAs—including microRNAs (miRNAs), long non-coding RNAs (lncRNAs), circular RNAs (circRNAs), and piwi-interacting RNAs (piRNAs)—has emerged as a central feature of Alzheimer's disease, Parkinson's disease, ALS, Huntington's disease, FTD, and other neurodegenerative diseases[@noncoding].

These molecules regulate critical pathological processes including amyloid-beta production, tau phosphorylation, neuroinflammation, oxidative stress, autophagy, and synaptic dysfunction, making them promising therapeutic targets and biomarkers. The study of ncRNAs has revealed novel disease mechanisms and opened new therapeutic avenues for neurodegenerative disorders.

MicroRNAs (miRNAs)

Biogenesis and Function

Non-coding RNAs in Neurodegeneration

Introduction

Non-coding RNAs (ncRNAs) represent a vast class of RNA molecules that are not translated into protein but serve critical regulatory functions in gene expression, chromatin remodeling, and cellular homeostasis. In the central nervous system, ncRNAs are expressed at particularly high levels and exhibit brain-region-specific patterns, reflecting the transcriptional complexity required for neuronal function. Dysregulation of ncRNAs—including microRNAs (miRNAs), long non-coding RNAs (lncRNAs), circular RNAs (circRNAs), and piwi-interacting RNAs (piRNAs)—has emerged as a central feature of Alzheimer's disease, Parkinson's disease, ALS, Huntington's disease, FTD, and other neurodegenerative diseases[@noncoding].

These molecules regulate critical pathological processes including amyloid-beta production, tau phosphorylation, neuroinflammation, oxidative stress, autophagy, and synaptic dysfunction, making them promising therapeutic targets and biomarkers. The study of ncRNAs has revealed novel disease mechanisms and opened new therapeutic avenues for neurodegenerative disorders.

MicroRNAs (miRNAs)

Biogenesis and Function

MicroRNAs are small (~22 nucleotide) single-stranded RNAs that regulate gene expression post-transcriptionally by binding to complementary sequences in the 3' untranslated regions (3'UTRs) of target mRNAs. This binding—mediated through the RNA-induced silencing complex (RISC) and Argonaute proteins—leads to mRNA degradation or translational repression. A single miRNA can regulate hundreds of target mRNAs, and more than 60% of human protein-coding genes contain conserved miRNA binding sites. In the brain, miRNAs are essential for neuronal differentiation, synaptic plasticity, and neuroimmune regulation[@bartel2018].

The biogenesis of miRNAs involves multiple steps: primary miRNA (pri-miRNA) transcription by RNA polymerase II, processing by the Drosha-DGCR8 microprocessor complex in the nucleus, export to the cytoplasm, and final processing by Dicer to generate mature miRNA duplexes. Any step in this process can be disrupted in neurodegeneration, leading to altered miRNA expression and function.

Key miRNAs in Alzheimer's Disease

miR-132: One of the most consistently downregulated miRNAs in Alzheimer's disease brain tissue, particularly in the hippocampus and prefrontal cortex. miR-132 normally promotes neuronal survival and dendritic morphogenesis. Its loss leads to upregulation of inositol 1,4,5-trisphosphate 3-kinase B (ITPKB), which activates BACE1 and enhances tau phosphorylation via GSK3β, thereby intensifying both amyloid plaque burden and neurofibrillary tangle formation. Circulating miR-132 levels correlate with Braak staging and cognitive decline, supporting its utility as a fluid biomarker[@hernandezrapp2016].

miR-146a: A key regulator of the innate immune response in the brain. miR-146a is upregulated in microglia in AD brain, where it targets complement factor H (CFH) and interleukin-1 receptor-associated kinase 1 (IRAK1), modulating NLRP3 inflammasome and NF-κB signaling. While initially neuroprotective by dampening toll-like receptor responses, chronic miR-146a elevation may paradoxically drive neuroinflammatory pathology by suppressing CFH and impairing complement regulation[@lukiw2008].

miR-155: A pro-inflammatory miRNA elevated in AD brain, cerebrospinal fluid, and plasma. miR-155 directly represses SOCS1 (suppressor of cytokine signaling 1) and CFH, amplifying neuroinflammation. Genetic deletion of miR-155 in AD mouse models reduces microglial activation and amyloid burden. miR-155 is also elevated in Parkinson's disease and ALS, suggesting a shared neuroinflammatory mechanism[@guedes2014].

miR-125b: Upregulated in AD brain and cerebrospinal fluid. Promotes tau phosphorylation by targeting the phosphatases DUSP6 and PPP1CA, and enhances neuroinflammation by activating NF-κB signaling in astrocytes[@banzhafstrathmann2014].

miR-29a/b/c cluster: Downregulated in sporadic AD brain. miR-29a and miR-29b-1 directly target BACE1/Aβ production. The miR-29 family also regulates DNA methyltransferases (DNMTs), linking ncRNA dysfunction to epigenetic alterations in AD[@hbert2008].

Key miRNAs in Parkinson's Disease

miR-7: Highly enriched in dopaminergic neurons of the substantia nigra. miR-7 directly suppresses alpha-synuclein expression and protects against oxidative stress and mitochondrial dysfunction. Loss of miR-7, often mediated by decreased ciRS-7/CDR1as (its circular RNA sponge), contributes to α-synuclein accumulation and dopaminergic neuron vulnerability[@junn2009].

miR-34b/c: Downregulated early in PD, even in premotor stages. miR-34b/c targets DJ-1 and Parkin, proteins essential for mitophagy and mitochondrial quality control. Their deficiency impairs Complex I activity and increases oxidative stress[@mionesmoyano2011].

miR-133b: Specifically enriched in midbrain dopaminergic neurons, miR-133b regulates dopaminergic neuron maturation and function via the transcription factor Pitx3. miR-133b is deficient in PD midbrain, contributing to impaired dopamine neurotransmission[@kim2007].

miRNAs in ALS and Huntington's Disease

In ALS, miR-206 is upregulated at the neuromuscular junction and promotes reinnervation, while miR-9 and miR-105 are downregulated, leading to aberrant neurofilament expression and axonal degeneration. In Huntington's disease, REST/NRSF—normally sequestered by wild-type huntingtin—is released by mutant huntingtin and represses neural miRNAs including miR-9, miR-9*, miR-29b, and miR-124a, driving transcriptional dysregulation[@packer2008].

Long Non-coding RNAs (lncRNAs)

Biogenesis and Function

Long non-coding RNAs are transcripts exceeding 200 nucleotides that do not encode proteins but exert diverse regulatory functions: acting as molecular scaffolds for chromatin-modifying complexes, guides for transcription factors, decoys that sequester proteins or miRNAs, and enhancers of gene expression. The human brain expresses more lncRNAs than any other organ, with many showing exquisite cell-type and brain-region specificity.

BACE1-AS

BACE1-AS is a conserved lncRNA transcribed from the opposite strand of the BACE1 gene locus. BACE1-AS forms an RNA duplex with BACE1 mRNA, stabilizing it and increasing both BACE1 mRNA and protein levels. This elevates β-secretase activity and amyloid-beta production. BACE1-AS is markedly upregulated in AD brain, particularly in the hippocampus and entorhinal cortex, and its levels correlate with Aβ42 concentrations. BACE1-AS also sponges miR-214-3p, further derepressing BACE1 expression. Knockdown of BACE1-AS reduces Aβ40 and Aβ42 levels in vitro, establishing it as a potential therapeutic target[@faghihi2008].

NEAT1

Nuclear Enriched Abundant Transcript 1 (NEAT1) is essential for the formation and maintenance of nuclear paraspeckles, subnuclear bodies involved in RNA processing and gene expression regulation. NEAT1 is significantly upregulated in AD brain and in amyloid-beta-treated neuronal cultures. It modulates amyloid-beta metabolism through the miR-124/BACE1 axis and interferes with PINK1-dependent mitophagy, promoting mitochondrial dysfunction and amyloid accumulation. Paradoxically, NEAT1 knockdown also increases p-tau levels via the FZD3/GSK3β pathway, suggesting it serves as a fine-tuner of multiple AD pathways[@zhao2019].

MALAT1

Metastasis-Associated Lung Adenocarcinoma Transcript 1 (MALAT1) is a highly conserved lncRNA enriched in neurons. In neurodegenerative contexts, MALAT1 is neuroprotective: it reduces neuronal apoptosis, inhibits neuroinflammation, and promotes neurite outgrowth. MALAT1 is decreased in Aβ1-42-treated neurons and in AD brain. It modulates miR-125b expression, suppressing neuronal apoptosis and inflammatory signaling. In Parkinson's disease, MALAT1 regulates α-synuclein expression and microglial activation[@zhang2017].

HOTAIR

HOX Transcript Antisense Intergenic RNA (HOTAIR) recruits the Polycomb Repressive Complex 2 (PRC2) to specific genomic loci, catalyzing histone H3K27 trimethylation and gene silencing. HOTAIR is elevated in AD brain, where it promotes neuronal apoptosis by repressing neuroprotective gene networks. It may also contribute to epigenetic dysregulation of genes involved in synaptic function and neuronal survival.

Other Neurodegeneration-Associated lncRNAs

• BDNF-AS: Antisense transcript that represses BDNF expression. Elevated in AD and HD, contributing to reduced neurotrophic support.
• MEG3: Maternally expressed gene 3, an imprinted lncRNA that regulates p53-mediated apoptosis. Dysregulated in AD, PD, and HD.
• SNHG1: Small nucleolar RNA host gene 1, elevated in PD brain, promotes α-synuclein expression and microglial activation via miRNA sponging.
• NKILA: NF-κB-interacting lncRNA that modulates neuroinflammatory signaling in microglia.

Circular RNAs (circRNAs)

Circular RNAs are particularly abundant in the brain, accumulate with aging, and show enrichment at synapses, where they may regulate local translation and synaptic plasticity.

CDR1as/ciRS-7

Cerebellar degeneration-related protein 1 antisense (CDR1as), also called circular RNA sponge for miR-7 (ciRS-7), is the most extensively studied circRNA in neurodegeneration. CDR1as contains over 70 conserved binding sites for miR-7 and acts as a potent miRNA sponge. By sequestering miR-7, CDR1as indirectly derepresses miR-7 targets including α-synuclein, BACE1, and ubiquitin-conjugating enzyme UBE2A[@lukiw2013].

CDR1as is significantly reduced in sporadic AD brain, particularly in the hippocampal CA1 region. This reduction releases miR-7 from its sponge, paradoxically allowing miR-7 to suppress UBE2A, impairing ubiquitin-mediated clearance of amyloid-beta and contributing to senile plaque deposition. In mouse models, genetic deletion of the CDR1as locus causes miR-7 and miR-671 deregulation, leading to synaptic and neuronal dysfunction[@piwecka2017].

Other Neurodegeneration-Associated circRNAs

• circHDAC9: Downregulated in AD, regulates HDAC9 expression and tau protein acetylation.
• circDLGAP4: Reduced in stroke and neurodegeneration models, modulates blood-brain barrier integrity via miR-143 sponging.
• circSNCA: Sponges miR-7, promoting α-synuclein accumulation in Parkinson's disease.
• circRNA_0079670: Elevated in cerebrospinal fluid of ALS patients, potential biomarker.

Piwi-Interacting RNAs (piRNAs)

Piwi-interacting RNAs are small RNAs (26-31 nucleotides) that silence transposable elements and regulate epigenetic modifications through the PIWI-piRNA pathway. Although initially characterized in germline cells, piRNAs are also expressed in post-mitotic neurons. Dysregulation of piRNAs in AD brain correlates with retrotransposon activation, suggesting that loss of transposon silencing may contribute to genomic instability and neuronal death. piR-61648 and piR-34393 are altered in AD brain and may serve as fluid biomarkers[@qiu2017].

ncRNAs as Biomarkers for Neurodegeneration

Non-coding RNAs are attractive biomarkers due to their stability in biofluids (protected within extracellular vesicles or bound to proteins), disease-specific expression patterns, and detectability by sensitive PCR-based assays.

Blood-Based miRNA Biomarkers

| miRNA | Disease | Direction | Clinical Utility |
|-------|---------|-----------|-----------------|
| miR-132 | AD | ↓ | Correlates with cognitive decline, Braak staging |
| miR-146a | AD | ↑ | Reflects neuroinflammatory activity |
| miR-155 | AD, PD, ALS | ↑ | Pan-neurodegenerative inflammation marker |
| miR-29a/b | AD | ↓ | Associated with BACE1 elevation and Aβ burden |
| miR-34b/c | PD | ↓ | Early PD marker (premotor stage) |
| miR-7 | PD | ↓ | Reflects dopaminergic vulnerability |
| miR-206 | ALS | ↑ | Correlates with denervation and disease progression |
| miR-9 | HD | ↓ | Reflects REST derepression |

CSF and Exosomal ncRNA Biomarkers

Cerebrospinal fluid ncRNA profiles offer closer proximity to CNS pathology. Exosome-encapsulated miRNAs are of particular interest as they cross the blood-brain barrier and reflect their cell of origin. Neural-derived exosomal miR-132 and miR-212 are reduced in preclinical AD, potentially years before symptom onset. lncRNAs such as BACE1-AS and circRNAs like CDR1as are also detectable in CSF and may improve diagnostic accuracy in combination panels.

Therapeutic Strategies Targeting ncRNAs

Antisense Oligonucleotides (ASOs)

ASOs can specifically degrade pathogenic ncRNAs (e.g., BACE1-AS) or modulate RNA splicing. The success of nusinersen (Spinraza) for spinal muscular atrophy demonstrates the clinical viability of ASO therapeutics for neurodegenerative disease.

miRNA Mimics and Anti-miRs

Restoring depleted miRNAs (miR-132 mimics for AD) or inhibiting overexpressed miRNAs (anti-miR-155 for neuroinflammation) are active preclinical strategies. Locked nucleic acid (LNA)-modified anti-miRs improve stability and CNS penetration. Challenges include off-target effects, delivery across the blood-brain barrier, and the promiscuity of miRNA-target interactions.

RNA-Based Therapeutics in Clinical Trials

While most ncRNA-targeted therapeutics for neurodegeneration remain preclinical, several platforms are advancing: lipid nanoparticle-encapsulated miRNA mimics, adeno-associated virus (AAV)-delivered ncRNA regulators, and conjugate-based delivery (e.g., transferrin receptor antibody-conjugates for brain).

Cross-Disease Shared ncRNA Signatures

Several ncRNAs are dysregulated across multiple neurodegenerative diseases, suggesting convergent regulatory mechanisms:

• miR-155, miR-146a: Shared neuroinflammatory signatures across AD, PD, ALS, and MS
• miR-9, miR-124: Pan-neuronal miRNAs repressed by REST/NRSF derepression in HD, AD, and aging
• NEAT1: Elevated across AD, PD, ALS, and HD, implicating nuclear paraspeckle dysfunction as a shared mechanism
• CDR1as: Reduced in AD and PD, linking circular RNA dysregulation to protein aggregation

Connections to Other Mechanisms

| Pathway | Interaction |
|---------|-------------|
| [Alzheimer's Disease](/diseases/alzheimers-disease) | miRNAs regulate APP processing, BACE1, and tau phosphorylation |
| [Parkinson's Disease](/diseases/parkinsons-disease) | miR-7 targets alpha-synuclein; miR-34b/c modulates mitophagy |
| [Neuroinflammation](/mechanisms/neuroinflammation) | miR-155, miR-146a regulate microglial activation |
| [Autophagy](/mechanisms/autophagy) | lncRNAs and circRNAs modulate autophagic flux |
| [Synaptic dysfunction](/mechanisms/synaptic-dysfunction) | miRNAs regulate synaptic protein expression |

See Also

• [Alzheimer's Disease](/diseases/alzheimers-disease)
• [Parkinson's Disease](/diseases/parkinsons-disease)
• [Huntington's Disease](/mechanisms/huntington-pathway)
• [Amyotrophic Lateral Sclerosis](/diseases/amyotrophic-lateral-sclerosis)
• [Neuroinflammation](/mechanisms/neuroinflammation)

Non-coding RNAs in Neuroinflammation

Neuroinflammation is a hallmark of neurodegenerative diseases, and ncRNAs play crucial roles in modulating the inflammatory response. Microglia, the resident immune cells of the brain, express specific miRNA signatures that determine their activation state.

The miR-155/ miR-146a balance is particularly important in neuroinflammation. While miR-155 promotes pro-inflammatory signaling by targeting SOCS1 and CFH, miR-146a serves as a feedback inhibitor of NF-κB signaling. In neurodegenerative diseases, this balance is disrupted, leading to chronic neuroinflammation. Therapeutic modulation of these miRNAs represents a promising approach to dampening neuroinflammation.

lncRNAs also contribute to neuroinflammation. NEAT1, which we discussed earlier in the context of AD pathogenesis, also regulates inflammatory gene expression through its role in paraspeckle formation. The NF-κB-interacting lncRNA (NKILA) directly binds to NF-κB/IκB complexes, sequestering them in the cytoplasm and preventing inflammatory gene transcription. Dysregulation of NKILA contributes to excessive neuroinflammation in multiple neurodegenerative conditions.

Epigenetic Regulation by ncRNAs

Beyond their direct gene regulatory functions, ncRNAs play important roles in epigenetic regulation. lncRNAs like HOTAIR recruit chromatin-modifying complexes to specific genomic loci, altering histone modifications and DNA methylation patterns. In neurodegeneration, these epigenetic functions are often dysregulated, leading to aberrant gene expression patterns.

The REST/CoREST complex, which we encountered in the context of HD, is regulated by multiple ncRNAs. miR-9 and miR-9* target REST mRNA, while the lncRNA LOC100507053 competes for REST binding. These interactions form a complex network that controls neuronal gene expression programs.

ncRNAs in Glial Cells

While much research has focused on neuronal ncRNAs, glial cells also express unique ncRNA profiles that influence neurodegenerative processes. Astrocyte-specific miRNAs regulate neurotoxicity and inflammatory responses, while oligodendrocyte miRNAs are essential for myelination.

In ALS, miR-218 is downregulated in motor neurons and astrocytes, leading to increased glutamate excitotoxicity. Restoring miR-218 expression through viral delivery has shown promise in preclinical models. Similarly, in MS and other demyelinating disorders, miR-219 promotes oligodendrocyte differentiation and myelination.

Future Directions

The field of ncRNA research in neurodegeneration is rapidly evolving. Single-cell RNA sequencing is revealing cell-type-specific ncRNA expression patterns, while new technologies like spatial transcriptomics are mapping ncRNA function in the intact brain. Long-read sequencing is enabling the discovery of novel ncRNA species, and comparative genomics is revealing evolutionarily conserved regulatory networks.

Key future directions include:

• Development of brain-penetrant ncRNA therapeutics
• Identification of ncRNA combination signatures for improved biomarker panels
• Understanding ncRNA function in glia and non-neuronal cells
• Mapping ncRNA networks at single-cell resolution

Conclusion

Non-coding RNAs represent a critical layer of gene regulation in the nervous system. Their dysregulation contributes to multiple pathological processes in neurodegeneration, from protein aggregation to neuroinflammation. The Promise of ncRNA-based therapeutics lies in their ability to target multiple disease pathways simultaneously. As delivery technologies improve and our understanding of ncRNA biology deepens, these molecules may become central to our therapeutic arsenal against neurodegenerative diseases.

ncRNA-Targeted Drug Delivery Challenges

Developing ncRNA-based therapeutics for CNS diseases faces several key challenges. The blood-brain barrier restricts delivery of large nucleic acid molecules to the brain. Viral vectors like AAV can transduce neurons but have limited cargo capacity for some ncRNAs. Non-viral approaches using lipid nanoparticles or polymeric vectors offer safer alternatives but typically have lower transduction efficiency.

Current delivery strategies under investigation include:

• Conjugation to brain-targeting ligands (transferrin receptor antibodies, angiopep peptides)
• Exosome-mediated delivery leveraging natural CNS tropism
• Focused ultrasound for temporary BBB disruption
• Intranasal delivery to bypass the blood-brain barrier

Clinical Translation Pipeline

While no ncRNA-targeted therapy has yet received regulatory approval for neurodegenerative disease, the pipeline is growing. Several miRNA modulators are in preclinical development, and early-phase clinical trials are exploring miRNA-based approaches in other neurological conditions. The success of nusinersen for spinal muscular atrophy and the antisense oligonucleotide tofersen for SOD1-associated ALS demonstrate the clinical feasibility of RNA-targeting approaches in the CNS.

Summary

Non-coding RNAs have emerged as critical regulators of neurodegenerative disease pathogenesis. From miRNAs that fine-tune amyloid processing and tau phosphorylation to lncRNAs that scaffold epigenetic machinery, these molecules influence every aspect of neuronal dysfunction. Their accessibility in biofluids makes them attractive biomarkers, while their therapeutic targeting offers the promise of multi-pathology modulation. Continued investment in understanding ncRNA biology and developing delivery technologies will accelerate the translation of these insights into clinical benefits for patients with neurodegenerative diseases.

The integration of systems biology approaches with traditional molecular biology has revealed the complexity of ncRNA regulatory networks in neurodegeneration. These networks span multiple scales—from transcriptional regulation by lncRNAs to post-translational control by miRNAs—and multiple cell types. Understanding these network-level interactions will be essential for predicting off-target effects and optimizing therapeutic targeting.
The coming decade will likely see significant advances as these challenges are addressed through innovative chemistry, novel delivery systems, and deeper mechanistic understanding.

References