SAM-synthase specific effects on metabolism and mitochondria underlie resistance to heat stress in C. elegans

S-adenosylmethionine (SAM), produced by SAM synthases, is critical for various cellular regulatory pathways and the synthesis of diverse metabolites. Studies have often equated the effects of knocking down one synthase with broader SAM-dependent outcomes such as histone methylation or phosphatidylcholine (PC) production. However, many organisms express multiple synthase genes including humans and Caenorhabditis elegans. Evidence in C. elegans, which possesses an expanded family of SAM synthases, suggest that the enzymatic source of SAM impacts its function. For instance, loss of sams-1 leads to enhanced heat shock survival and increased lifespan, whereas reducing sams-4 adversely affects heat stress survival. Here, we reveal that loss of sams-1 exerts age-dependent effects on nuclear-encoded mitochondrial gene expression, mitochondrial metabolites, and mitophagy. Notably, we find that that SAMS-1 exerts synthase-specific effects on PC production. We propose a mechanistic framework wherein the reduced SAM from SAMS-1 acts through PC to impact mitochondrial fission and mitophagy, thereby enhancing survival during heat stress. This study highlights multifaceted roles of SAM across metabolic pathways and synthase-specific SAM functions. Overall design: Lysis of young adult C. elegans was performed in 0.5% SDS, 5% ß-ME, 10 mM EDTA, 10 mM Tris-HCl ph7.4, 0.5 mg/ml Proteinase K, before purification of RNA by TRI-Reagent (Sigma). cDNA was produced with Transcriptor First-strand cDNA kits (Roche), and RT-PCR was performed using Kappa SYBR Green 2X Mastermix. RNA for sequencing was purified using RNAeasy columns (Qiagen). RNA sequencing (including library construction was performed by BGI (Hong Kong). Reads were analyzed through the NextFlow analysis platform (https://github.com/DanHUMassMed/RNA-Seq-Nextflow), using DeSeq2 to identify differentially expressed genes. Gene set enrichment was performed using WormCat (http://www.wormcat.com).