Ribosome biogenesis

Ribosome biogenesis produces the ribosomal subunits required for protein-synthesis. For human cytosolic ribosomes, RNA polymerase I transcribes the 47S precursor that is processed into 18S, 5.8S and 28S rRNAs; 5S rRNA is produced separately. rRNA modification and assembly with ribosomal proteins generate precursor 40S and 60S subunits in the nucleolus/nucleus, followed by export and late cytoplasmic maturation ¹. Mitochondrial ribosome biogenesis uses distinct rRNAs and proteins and should not be inferred from cytosolic 18S/28S measurements.

Direct machinery includes RRN3/TIF-IA and UBTF at RNA-polymerase-I transcription and snoRNP-associated processing factors such as fibrillarin (FBL). mtor and c-myc are upstream growth regulators rather than ribosomal structural components; 4ebp1 primarily controls translation initiation and is not a core ribosome-biogenesis factor. In a primary mammalian study, rapamycin-sensitive mTOR signaling regulated Pol-I initiation through TIF-IA ².

Reproductive-aging evidence

Li et al. reported age-associated enrichment of cytosolic and mitochondrial ribosomal transcripts in human oocytes and cumulus cells, increased 18S/28S rRNA and altered nucleolar morphology in cumulus cells, alongside higher nascent-protein labeling and mTOR activity. Several ribosomal-gene loci showed lower promoter methylation or H3K9me3 in very small omics subsets ³. See li-2025-ribosome-age-related-infertility.

Increased ribosomal transcripts, rRNA abundance and nucleolar area are compatible with increased biogenesis, but none alone establishes the number, composition or fidelity of functional ribosomes. The evidence is reproductive-tissue-specific and does not establish increased ribosome biogenesis as a universal direction of aging. The age-associated transition near 34 years was data-derived and is not a validated biological threshold. Oocyte methylomes used two pooled libraries per age group; cumulus-cell methylomes used two younger and four nominally older donors; CUT&Tag used two donors per age group.

Intervention interpretation

Rapamycin reduced translation, mTOR target phosphorylation and selected senescence/aggregate readouts in cultured human cumulus cells and during mouse oocyte maturation. Because rapamycin also changes autophagy, metabolism and stress responses, these experiments do not isolate ribosome biogenesis as the causal mediator. gap/no-mechanism

Druggability

The process is aging-context tier 1 indirectly through clinical mTOR inhibitors: rapamycin engages an upstream regulator and has human aging-relevant trial exposure, including the Li IVF study. The tier does not imply a process-selective drug or that global ribosome shutdown is safe. Rapamycin also changes autophagy, metabolism and stress responses, so its effects cannot identify ribosome biogenesis as the sole mediator.

Footnotes

Footnotes

1. Reactome R-HSA-73762, RNA polymerase I promoter opening, and R-HSA-72312, rRNA processing (Homo sapiens), reviewed canonical pathways, current through 2026-08-09 ↩

2. doi:10.1101/gad.285504 · Mayer C et al. · Genes Dev 2004;18:423–434 · primary mammalian-cell study linking rapamycin-sensitive mTOR signaling to RNA-polymerase-I initiation through TIF-IA ↩

3. li-2025-ribosome-age-related-infertility · doi:10.1016/j.xcrm.2025.102424 · Li J et al. · Cell Reports Medicine 2025;6:102424 · human oocyte/cumulus multi-omics, culture perturbations, mouse oocyte maturation and a small human sirolimus IVF trial ↩