Ribosome Dysregulation in Huntington's Disease - A Mitochondrial Protein Synthesis Crisis

Huntington's disease (HD), a fatal neurodegenerative disorder, is marked by progressive striatal neuron loss and severe motor dysfunction. While mutant huntingtin (mHTT) toxicity has long been linked to mitochondrial defects, recent studies spotlight ribosome-mediated translation collapse as a critical driver of HD pathology. By integrating ribosome profiling (Ribo-Seq) and proteomics, researchers have uncovered striking ribosome stalling and impaired protein synthesis in mitochondria, reshaping our understanding of HD's molecular underpinnings.

Mitochondrial Ribosomes: Unique Translational Machinery Under Siege

Mitochondria rely on specialized ribosomes (mitoribosomes) to synthesize essential oxidative phosphorylation (OXPHOS) subunits encoded by mitochondrial DNA (mtDNA). Unlike cytosolic ribosomes, mitoribosomes feature distinct structural adaptations, including reduced rRNA content and increased protein components, enabling them to decode mt-mRNAs with minimal tRNA diversity. These ribosomes are particularly sensitive to disruptions in translation factors, tRNA modifications, or membrane potential—all processes compromised in HD.

Notably, mitoribosome assembly requires precise coordination between nuclear-encoded ribosomal proteins (e.g., MRPS22) and mitochondrial rRNA. In HD striatal cells, mHTT aggregates disrupt this process, leading to fragmented mitoribosomes and impaired initiation complex formation. Ribosome profiling data further revealed abnormal ribosome footprints on mt-mRNAs, suggesting defective decoding or elongation. Such structural and functional ribosome anomalies directly impair OXPHOS biogenesis, fueling neuronal energy deficits.

Fig 1 Mitochondrial protein synthesis is diminished in HD cells.¹

Ribosome Profiling Exposes Paradoxical Stalling in HD Mitochondria

Ribo-Seq, a technique mapping ribosome-protected mRNA fragments, uncovered a hallmark of HD: increased ribosome occupancy on mtDNA-encoded OXPHOS transcripts despite reduced protein output. For example, mt-Co1 mRNA showed 2.1-fold higher ribosome density in HD models, yet COX1 protein levels dropped by 50%. This paradox points to ribosome stalling—a phenomenon where ribosomes pause mid-translation due to tRNA shortages, aberrant mRNA structures, or mitoribosome malfunction.

Stalling hotspots were identified near rare mitochondrial codons (e.g., AGA for arginine), which require nuclear-encoded tRNA modifiers for accurate decoding. In HD, mHTT disrupts nuclear-mitochondrial communication, depriving mitoribosomes of these critical modifiers. Consequently, ribosomes accumulate at problematic codons, triggering premature termination or ribosome collision events. These findings align with proteomics data showing elevated ubiquitination of mitochondrial proteins, indicative of co-translational quality control failure.

Ribosome Occupancy vs. Protein Yield: A Mitochondrial Paradox Decoded

While ribosome occupancy typically correlates with translation activity, HD mitochondria defy this rule. Tandem mass tag (TMT) proteomics revealed stark declines in mtDNA-encoded proteins (e.g., 60% reduction in ND1) despite stable or elevated ribosome binding to their mRNAs. This disconnect highlights two HD-specific defects:

1. Inefficient Ribosome Recycling: Prolonged ribosome occupancy on mt-mRNAs limits availability of free ribosomes for new translation rounds.
2. Post-Translational Degradation: Newly synthesized proteins are rapidly tagged for degradation due to misfolding or oxidative damage.

Ribosome stalling exacerbates these issues. Stalled ribosomes obstruct subsequent translating ribosomes, forming "traffic jams" that reduce overall output. Meanwhile, collision-induced ribosome quality control (RQC) pathways, which normally rescue stalled complexes, are overwhelmed in HD mitochondria. This dual failure creates a feedforward loop of translational inefficiency and proteotoxic stress.

Fig 2 TMT-MS/MS proteomics of isolated mitochondria from control and HD cells.¹

Ribosome Stalling and OXPHOS Collapse: A Vicious Cycle

Mitochondrial ribosome dysfunction directly impairs OXPHOS complex assembly. For instance, ribosome stalling on mt-ATP6 mRNA reduces ATP synthase production, slashing ATP output by 40% in HD cells. This energy crisis further destabilizes mitoribosomes, which require ATP for tRNA charging and elongation factor function. Compromised OXPHOS also elevates reactive oxygen species (ROS), damaging mtDNA and creating mutation-prone regions that worsen ribosome decoding errors.

Notably, nuclear-encoded OXPHOS subunits (e.g., SDHA) escape ribosome-level dysregulation, as their cytosolic translation remains intact. This selective vulnerability of mitochondrial ribosomes explains why striatal neurons—rich in mitochondria—succumb earliest in HD.

Therapeutic Strategies: Rescuing Mitochondrial Ribosome Function

Emerging HD therapies aim to reboot mitochondrial translation:

• tRNA Supplementation: Delivering modified tRNAs (e.g., mt-AGA-Arg tRNA) via AAV vectors could alleviate ribosome stalling at rare codons.
• Ribosome-Stabilizing Compounds: Small molecules like EMETA bind mitoribosomes, preventing disassembly under oxidative stress.
• ROS Scavengers: MitoTEMPOL, a mitochondrial-targeted antioxidant, reduces ribosome stalling by protecting tRNA pools from oxidation.

Preclinical studies show promise. In HD mice, tRNA overexpression improved mitochondrial protein synthesis by 30% and delayed motor symptom onset. Similarly, enhancing ribosome biogenesis via PGC-1α activation restored OXPHOS function in patient-derived neurons.

Future Directions: Mapping Ribosome Dynamics in HD

Key unanswered questions include:

1. Single-Ribosome Resolution: How do individual mitoribosomes behave in live neurons? Advanced imaging (e.g., ribo-HiLiTE) could track real-time translation.
2. Stalling Triggers: Are specific mRNA regions or mHTT-RNA interactions responsible for ribosome pauses?
3. Cross-Tissue Variability: Do ribosome defects occur in non-neuronal tissues, contributing to systemic HD symptoms?

Technologies like cryo-EM of patient mitoribosomes and Ribo-Seq at single-cell resolution will deepen mechanistic insights. Meanwhile, Technologies like cryo-EM of patient mitoribosomes and Ribo-Seq at single-cell resolution will deepen mechanistic insights. Meanwhile, systematic genetic screens targeting ribosome biogenesis factors may uncover novel therapeutic nodes.

Conclusion

Ribosome dysregulation in HD mitochondria represents a linchpin connecting genetic mutation to neuronal death. By stalling translation and destabilizing OXPHOS, defective mitoribosomes ignite a self-reinforcing cycle of energy failure and proteotoxicity. While challenges remain, therapies targeting ribosome stability, tRNA availability, or collision resolution offer hope for breaking this cycle. As research unravels the intricate dance between ribosomes and mitochondrial health, a new era of HD treatment—one centered on rescuing protein synthesis—may soon dawn.

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Reference

1. Dagar, Sunayana, et al. "Ribosome profiling and Mass Spectrometry reveal widespread mitochondrial translation defects in a Striatal Cell Model of Huntington Disease." Molecular & Cellular Proteomics 23.4 (2024). https://doi.org/10.1016/j.mcpro.2024.100746. Distributed under the Open Access license CC BY 4.0, without modification.