The C. elegans adult neuronal IIS/FOXO transcriptome reveals adult phenotype regulators

Abstract

Insulin/IGF-1 signaling (IIS) is a critical regulator of an organism’s most important biological decisions, from growth, development, and metabolism to reproduction and longevity. It primarily does so through the activity of the DAF-16/FOXO transcription factor, whose global targets were identified in C. elegans using whole-worm transcriptional analyses more than a decade ago1. IIS and FOXO also regulate important neuronal and adult behavioral phenotypes, such as the maintenance of memory2 and axon regeneration3 with age, in both mammals4 and C. elegans, but the neuron-specific IIS/FOXO targets that regulate these functions are still unknown. By isolating adult C. elegans neurons for transcriptional profiling, we identified both the wild-type and IIS/FOXO adult neuronal transcriptomes for the first time. IIS/FOXO neuron-specific targets are distinct from canonical IIS/FOXO-regulated longevity and metabolism targets, and are required for IIS/daf-2 mutants’ extended memory. We also discovered that the activity of the forkhead transcription factor FKH-9 in neurons is required for daf-2’s ability to regenerate axons with age, and its activity in non-neuronal tissues is required for daf-2’s long lifespan. Together, neuron-specific and canonical IIS/FOXO-regulated targets enable the coordinated extension of neuronal activities, metabolism, and longevity under low insulin-signaling conditions.

A Neuronal piRNA Pathway Inhibits Axon Regeneration in C. elegans

“…Transcriptional profiling of adult PLM neurons has detected expression of several genes in the piRNA pathway, including prde-1 and prg-1 (Kaletsky et al, 2016; Lockhead et al, 2016). Thus, we asked whether prde-1 mRNAs could be visualized in PLM neurons using smFISH (single-molecule fluorescence in situ hybridization), which allows detection of single mRNAs in cells (Raj et al, 2008).…”

“…Together, these results suggest that the function of PRG-1/PIWI in eliminating target mRNAs by endonucleolytic cleavage could be responsible for target gene silencing in somatic cells. By mining published TRN transcriptomes (Kaletsky et al, 2016; Lockhead et al, 2016) and piRNA target database (Bagijn et al, 2012), we identified numerous candidate target mRNAs for piRNA in TRN neurons (see STAR methods and Table S2). Further studies will be required to elucidate specific targets of the neuronal piRNA pathway critical for axon regeneration.…”

Fox transcription factors: from development to disease

“…In such times of nutrient shortage, expression of insulin receptor homologs such as DAF-2 is low. The sole C. elegans FoxO homolog, DAF-16, is therefore not phosphorylated and is thus able to enter the nucleus and transactivate its targets, a full list of which was recently published (Kaletsky et al, 2016). The effect of DAF-16 on longevity is primarily manifested in the intestine (Murphy et al, 2007;Libina et al, 2003), and its effects on lifespan are likely to be related to the regulation of genes involved in countering oxidative and thermal stress (Nemoto and Finkel, 2002).…”

“…A recent study delineated the neural targets of DAF-16 in C. elegans by comparing the transcriptomes of neurons that had been FACS sorted from wild type and insulin signaling-deficient (daf-16;daf-2) mutants (Kaletsky et al, 2016). Since many FoxO targets are conserved from C. elegans to humans (Webb et al, 2016), it has been proposed that understanding the mechanisms by which FoxO regulates aging in the nematode might provide a basis for extending human lifespan.…”

The Genetics of Axon Guidance and Axon Regeneration in Caenorhabditis elegans

“…The enhanced regeneration of daf-2 and daf-18 mutants requires the DLK-1 pathway, and DAF-16/FOXO itself may regulate dlk-1 transcription. Transcriptional profiling of neurons has also identified the forkhead factor fkh-9 as a target of DAF-16 required for daf-2-dependent adult axon regeneration (Kaletsky et al 2016). C. elegans should be a tractable model to explore how age affects axon regenerative capacity.…”