Ribosomal biogenesis defects trigger subunit specific developmental checkpoints via TOR signaling and gap junction in...

Ribosome biogenesis is critical for postembryonic development progression in Caenorhabditis elegans. Although maternally supplied ribosomes allow null mutants of ribosomal protein genes to complete embryogenesis, subsequent larval stages arrest if de novo ribosome production is compromised. Here, we compared null mutants in large (rpl-5, rpl-33) and small (rps-10, rps-23) ribosomal subunit genes with mutants defective in rRNA synthesis (rpoa-2 and rDNA loci). By tracking divisions of the mesoblast (M) cell, we discovered that large subunit mutations cause a stringent arrest in M cell proliferation, distinctly more severe than the partial arrests observed in small subunit and rRNA synthesis mutants. Unlike nutrient-deprived (starvation) L1 diapause, this arrest does not activate the cyclin-dependent kinase inhibitor CKI-1, suggesting a CKI-1-independent checkpoint. Gene expression analyses revealed that rpl-5(0) and rDNA(0) mutants share overexpression of genes involved in ribosomal RNA processing and ribosome assembly, whereas larvae depleted of the RNA polymerase I subunit RPOA-2 uniquely overexpress lipid metabolism genes. Tissue-specific manipulations previously confirmed that ribosomal insufficiency in a single tissue can impose a whole-organism developmental block. Genetic analyses further implicated the gap junction protein INX-14 and the TORC2 component SINH-1 as partial suppressors of the M cell arrest in small ribosomal subunit mutants (rps-23(0)), but not in large ribosomal subunit mutants (rpl-5(0)). Introducing null mutations in downstream TORC1/TORC2 kinases to a tissue-specific RPOA-2 depletion background similarly modulated growth arrest, suggesting that gap junction communication and TOR pathways converge upon a ribosomal stress checkpoint. Collectively, our findings highlight a unique, CKI-1-independent arrest driven by large ribosomal subunit gene loss and reveal how distinct signaling pathways coordinate postembryonic development in response to ribosome biogenesis defects.