LIN-39 functions as a neuron-specific developmental determinant of longevity in Caenorhabditis elegans with reduced insulin/IGF-like signaling [ATAC-seq]
The nuclear chromatin landscape changes with age – a phenomenon observed across many species, including humans. Notably, perturbing this landscape can alter the rate of aging, indicating at least a partial role in controlling this process. This raises the question of whether chromatin alterations also distinguish animals that age at unusual rates. To explore this, we focused on insulin/IGF-like signaling (IIS) in Caenorhabditis elegans, a well-characterized signaling pathway whose impairment drastically slows aging and extends lifespan of these animals. Specifically, we conducted ATAC- and mRNA-seq in animals with reduced IIS (i.e., daf-2 mutants). Interestingly, we found that enhancer regions tend to close and become transcriptionally repressed with age, but under reduced IIS, they tend to open and become transcriptionally active, suggesting their potential relevance for lifespan regulation. By screening transcription factors (TFs) predicted to bind these regions, we identified the HOX gene LIN-39 as a novel aging-preventive TF required for the longevity of daf-2 mutants. LIN-39 performs this role specifically during development and in neurons. Further investigation showed that LIN-39 was acting around the L3 larval stage and in the hermaphrodite-specific VC class of cholinergic motor neurons – at a time when they undergo maturation. Finally, this LIN-39-dependent longevity required DAF-16/FOXO, a well-established aging-preventive TF that acts downstream of DAF-2, which may synergize with LIN-39 in VC neurons to open enhancer the regions. We ultimately propose a model whereby longevity of daf-2 mutant hermaphrodites requires a longevity-promoting signal emitted by VC neurons – a signal that relies on correct VC neuron maturation around the L3 stage, which in turn is assured by the actions of LIN-39 and resulting chromatin and gene expression changes. This renders LIN-39 a rare example of a developmental determinant of longevity. And given the conservation of IIS, HOX and FOXO TFs, and cholinergic neuron function across species, the here described mechanism may reflect a general principle by which developmental transcriptional programs shape adult longevity. Overall design: ATAC-Seq (Assay for Transposase-Accessible Chromatin using sequencing) in Caenorhabditis elegans: for daf-2(e1370), daf-2(e1370); daf-16 RNAi, daf-2(e1370); lin-39 RNAi and wild-type animals at day 1 of adulthood, and for germline-deficient glp-4 mutant animals at day 1, 3, and 7 of adulthood.