Diversity and stability of the gut microbiome of naked mole-rat (Heterocephalus glaber), the longest-lived rodent

1. Problem Statement

The naked mole-rat (NMR) is a unique subterranean rodent known for its exceptional longevity (up to 40 years), negligible senescence, cancer resistance, and hypoxia tolerance. Despite its status as a premier model for aging research, the functional potential and genomic diversity of its gut microbiome remain poorly characterized. Previous studies were limited to:

• Taxonomic descriptions based on culturing or 16S rRNA gene sequencing.
• Lack of functional data: The metabolic capabilities of the microbiota, particularly regarding the digestion of their low-protein, high-fiber tuber diet, were unknown.
• Missing non-bacterial components: Archaea, fungi, and protozoa were largely overlooked.
• Comparative gaps: There was a lack of evolutionary context comparing NMRs to other rodents (e.g., mice, squirrels) and lagomorphs (rabbits, hares) to understand how their unique physiology shapes microbial communities.

2. Methodology

The study employed a multi-omics approach combining 16S rRNA gene sequencing and whole-metagenome shotgun sequencing (WMS) to characterize the gut microbiota of laboratory-bred NMRs.

• Sample Collection:
  • NMRs: 24 individuals (ages up to 15 years) from Kumamoto University colonies.
  • Comparative Species: Samples from 6 other species were analyzed, including Damaraland mole-rats (DMR), lesser blind mole-rats, Siberian flying squirrels, multiple mouse strains (C57BL/6, MSM/Ms, FVB/N), European brown hares, and European rabbits. Some data were sourced from public repositories.
• 16S rRNA Sequencing:
  • Targeted the V3–V4 region.
  • Analyzed using QIIME2 (v2024.2) with DADA2 for denoising and SILVA 138.1 for taxonomy.
  • Alpha and Beta diversity analyses were performed using rarefied data and centered log-ratio (rclr) transformations.
• Whole Metagenome Sequencing (WMS):
  • Performed on 11 NMR individuals (ages 2–16 years).
  • Preprocessing: Reads were decontaminated against host genomes (NMR, human, mouse, DMR) and adapters using Bowtie2 and Cutadapt.
  • Taxonomic Classification: Kraken2 and Bracken were used for species-level abundance estimation.
  • Metagenome-Assembled Genomes (MAGs): Reads were assembled with MEGAHIT, binned with MetaBAT2, and filtered (CheckM2) to yield high-quality MAGs (≥90% completeness, ≤5% contamination). Taxonomy was assigned via GTDB-Tk.
  • Functional Annotation:
    • CAZymes: Predicted using dbCAN2 to identify carbohydrate-active enzymes.
    • KEGG Orthology (KO): Predicted using KofamScan.
    • Thermometry: Gut temperature was inferred using the Metagenomic Thermometer (MetaThermo) based on amino acid composition.

3. Key Contributions

• First Comprehensive Metagenomic Analysis: This is the first study to apply shotgun metagenomics and MAG reconstruction to the NMR gut, moving beyond 16S limitations.
• Discovery of Non-Bacterial Microbiota: The study identified significant populations of Archaea (methanogens), Fungi, and Protozoa (flagellates and ciliates) that are typically missed in bacterial-focused studies.
• Functional Resolution of Fermentation: The study elucidated a "tiered" degradation model for plant polysaccharides involving specific bacterial lineages and non-bacterial partners.
• Evolutionary Context: By comparing NMRs with diverse rodents and lagomorphs, the study establishes the distinctiveness of the NMR microbiome in an evolutionary framework.

4. Key Results

A. Diversity and Stability

• High Diversity: NMRs exhibited the highest microbial richness (number of genera) among all compared rodents, even after rarefaction. Approximately 29% of genera remained unclassified, suggesting a high degree of novelty.
• Age and Sex Stability: Unlike mice and humans, where microbiota shifts significantly with age, the NMR gut microbiota remained taxonomically stable across ages (0–15 years) and between sexes. This stability is likely maintained by coprophagy (fecal consumption) and a shared colony environment.
• Distinctiveness: Principal Component Analysis (PCA) clearly separated NMRs from other rodents. Key distinguishing taxa included Treponema, Fibrobacter, and specific unclassified Prevotellaceae and Erysipelotrichaceae genera.

B. Metabolic Potential and Tiered Fermentation

• CAZyme Enrichment: The NMR microbiome was heavily enriched in Carbohydrate-Active Enzymes (CAZymes) targeting plant cell-wall polysaccharides (cellulose, hemicellulose, pectin), resembling ruminants more than mice.
• Tiered Degradation Model:
  1. Primary Degraders: Fibrobacter (and related MAGs) possess enzymes (GH45, GH11, CBM30) for breaking down crystalline cellulose and lignocellulose.
  2. Secondary Degraders: Treponema and Prevotellaceae utilize enzymes (GH5, GH13) to process partially depolymerized glucans.
  3. Terminal Processing: Erysipelotrichaceae (e.g., Allobaculum) and Paludibacteraceae handle ester removal and terminal sugar release.
• Hydrogen Metabolism: The system relies on sulfate reduction (by Desulfovibrio) and methanogenesis (by Methanobacteriaceae) as hydrogen sinks. This suggests a low-methane fermentation pathway compared to true ruminants, adapted to the hypoxic burrow environment.
• NMR-Specific Genes: Core KOs specific to NMRs were enriched in anaerobic metabolism, ferredoxin-dependent enzymes, and redox balancing, reflecting adaptation to low-oxygen conditions.

C. Non-Bacterial Components

• Archaea: Methanogens (Methanobacteriaceae) were abundant, confirming active methanogenesis.
• Protozoa: The study detected flagellates (Trichomonadidae) and ciliates (Balantidium, Giardia). These likely contribute to cellulose digestion and protein recycling, similar to roles in termites and ruminants.
• Fungi: Ascomycota (e.g., Pichiaceae, Aspergillaceae) were present.
• Elusimicrobiota: The genus Avelusimicrobium, typically associated with termites, was consistently recovered in NMRs, suggesting a convergent evolutionary adaptation to fiber-rich diets.

D. Environmental Adaptation

• Thermal Signature: The predicted gut temperature based on microbial amino acid composition was 28.78 ± 1.59°C, closely matching the experimentally measured cecal temperature (28.9 ± 1.3°C). This confirms the cecum acts as a heat sink, cooler than the core body temperature, which is unique among mammals.

5. Significance

• Mechanism of Longevity: The study provides a mechanistic link between the NMR's unique gut microbiome and its "healthy" longevity. The stable, diverse, and highly efficient fermentation system allows for optimal energy extraction from a low-quality diet while maintaining redox homeostasis in a hypoxic environment.
• Model for Aging: The resistance of the NMR microbiome to age-related dysbiosis (unlike in mice/humans) suggests that microbial stability may be a biomarker or contributor to negligible senescence.
• Biotechnological Potential: The discovery of novel CAZymes and tiered degradation pathways in NMRs offers potential applications in biofuel production and biomass conversion.
• Ecological Insight: The presence of termite-associated bacteria (Avelusimicrobium) and protozoa highlights how subterranean, fiber-rich lifestyles drive convergent evolution in gut microbiomes across vastly different taxa (rodents vs. insects).

In conclusion, this paper establishes the NMR gut microbiome as a distinct, stable, and highly specialized ecosystem that supports the host's extreme longevity and subterranean lifestyle through efficient, tiered polysaccharide degradation and unique redox balancing mechanisms.