TDP-43 loss drives premature aging and regenerative failure in skeletal muscle stem cells

Technical Summary: TDP-43 Loss Drives Premature Aging and Regenerative Failure in Skeletal Muscle Stem Cells

Problem Statement
TAR DNA-binding protein 43 (TDP-43) dysfunction is a defining pathological feature of amyotrophic lateral sclerosis (ALS) and related neurodegenerative disorders, characterized by the mislocalization of TDP-43 from the nucleus to the cytoplasm and the formation of aggregates. While ALS has traditionally been viewed as a motor neuron-driven disease, emerging evidence suggests intrinsic muscle abnormalities contribute to pathology. However, the specific role of TDP-43 in skeletal muscle stem cells (satellite cells, or SCs)—which are essential for lifelong muscle regeneration and homeostasis—remains poorly understood. It is unclear whether ALS-associated TDP-43 mutations are sufficient to impair SC function or if complete loss of TDP-43 activity is required to compromise muscle regeneration and stem cell maintenance. Furthermore, the relationship between TDP-43 loss and the age-dependent decline of muscle regenerative capacity is not defined.

Methodology
The study employed a multi-faceted approach using mouse models and in vitro/ex vivo assays to dissect the role of TDP-43 in satellite cells:

1. Genetic Models:
   • ALS-associated Mutants: The authors utilized Knock-In (KI) mice expressing ALS-associated gain-of-function (GOF) Tardbp^Q331K and loss-of-function (LOF) Tardbp^F210I mutations. Due to embryonic lethality in homozygous F210I mice, heterozygous adults were analyzed.
   • Conditional Knockout (cKO): An inducible, SC-specific TDP-43 knockout model (Pax7^CreERT2/+;Tardbp^fl/fl) was generated to achieve complete ablation of TDP-43 specifically in PAX7-expressing satellite cells upon tamoxifen administration.
2. Phenotypic Characterization:
   • Homeostasis and Injury: Muscle architecture, fiber size, composition (MyHC isoforms), fibrosis, and fat infiltration were assessed via histology (H&E, Sirius Red, Oil Red O) in uninjured and cardiotoxin (CTX)-injured muscles (TA, soleus, EDL, gastrocnemius) at various time points (7–30 days post-injury).
   • Stem Cell Analysis: Satellite cells were isolated via Fluorescence-Activated Cell Sorting (FACS) using markers (CD31^negCD45^negSca1^negIntegrin-α7⁺CD34⁺). SC abundance, clonogenic capacity, and differentiation potential were quantified.
   • Ex Vivo Culture: Single myofibers were cultured to monitor SC activation, proliferation, and differentiation dynamics over 72 hours using immunostaining for PAX7, MYOD, KI67, and MYOG.
3. Molecular Analysis:
   • Transcriptomics: Bulk RNA sequencing (RNA-seq) was performed on FACS-sorted SCs from adult and aged (2-year-old) WT and cKO mice. Differential expression analysis and Gene Set Enrichment Analysis (GSEA) were used to compare cKO signatures against physiological aging signatures.
4. Sexual Dimorphism: All experiments were conducted in both male and female mice to assess sex-specific effects.

Key Results

• ALS Mutations Do Not Impair SC Function: Mice carrying ALS-associated Tardbp^Q331K (GOF) or heterozygous Tardbp^F210I (LOF) mutations displayed normal muscle homeostasis, fiber composition, and SC abundance under both homeostatic and regenerative (post-CTX injury) conditions. These mutations were insufficient to impair SC clonogenicity or differentiation.
• Complete TDP-43 Loss Causes SC Depletion and Functional Failure:
  • Sexual Dimorphism: Complete loss of TDP-43 in SCs led to a significant reduction in the SC pool in female mice (73% decrease in TA, 54% in soleus) but had a minimal effect on males under homeostatic conditions.
  • Shift in Stemness: TDP-43 deficiency caused a shift from a CD34^high (stem-like, quiescent) state to a CD34^low (primed) state, indicating a loss of the most primitive stem cell compartment.
  • Proliferation and Differentiation Defects: In vitro and ex vivo assays revealed that TDP-43-deficient SCs failed to clonally expand, exhibited impaired survival, and could not initiate terminal differentiation (absence of Myogenin expression), despite retaining the ability to activate (MYOD expression) and enter the cell cycle (KI67).
• Regenerative Collapse: Following muscle injury, TDP-43 cKO mice failed to regenerate muscle tissue. Injured muscles showed severe atrophy, extensive fibrosis, and fat infiltration. Crucially, the SC pool was not replenished post-injury; by 30 days, the residual SC pool was nearly exhausted, leading to a "regenerative collapse."
• Premature Aging Signature: Transcriptomic analysis of TDP-43-deficient SCs revealed the upregulation of stress response, mitochondrial dysfunction, and altered TGFβ signaling pathways. Gene Set Enrichment Analysis (GSEA) demonstrated a significant overlap between the transcriptomic signature of young adult TDP-43 cKO SCs and physiologically aged WT SCs. This indicates that TDP-43 loss drives the premature acquisition of an aging-like state.
• Aging Exacerbates Depletion: In aged (2-year-old) cKO mice, the depletion of the SC pool was further exacerbated compared to young adults, and the expansion of the CD34^low primed population was more pronounced. However, the transcriptomic differences between adult and aged cKO SCs were minimal, suggesting the aging-like program is established early upon TDP-43 loss.

Significance and Claims
The paper claims that TDP-43 is a critical regulator of satellite cell stemness and regenerative competence, distinct from its role in maintaining mature myofibers (which remain preserved under homeostatic conditions even with TDP-43 loss). The authors establish that:

1. TDP-43 is Essential for SC Maintenance: While ALS-associated point mutations do not impair SC function, complete loss of TDP-43 is catastrophic for the stem cell pool, leading to depletion and regenerative failure.
2. Mechanism of Aging: TDP-43 deficiency induces a molecular program in SCs that mimics physiological aging, characterized by stress responses and loss of stemness. This suggests that TDP-43 safeguards SCs against premature aging.
3. Sexual Dimorphism: The requirement for TDP-43 in maintaining the SC pool is more pronounced in females, highlighting a sex-specific vulnerability in muscle stem cell biology.
4. Disease Relevance: These findings suggest that impaired TDP-43 activity within muscle stem cells may be a previously underappreciated component of ALS and related TDP-43 proteinopathies, contributing directly to muscle deterioration beyond the effects of denervation.

The study concludes that TDP-43 protects the muscle stem cell compartment from premature aging and is indispensable for the expansion and differentiation required for muscle repair, positioning SC dysfunction as a potential therapeutic target in TDP-43-associated diseases.