Low-Dose Microcystin-LR Elicits Sex-Dimorphic Transcriptomic Responses in Senescent <i>Nothobranchius furzeri</i> : Implications for Cyanotoxin Vulnerability in Aging Vertebrates

Microcystin-LR (MC-LR), a cyanobacterial toxin produced during harmful algal blooms, is an increasing environmental and public health concern as the frequency and intensity of harmful algal blooms continue to rise globally. While the effects of MC-LR have been extensively studied in young organisms, much less is known about how aging influences susceptibility to cyanotoxin exposure. Here, we used the naturally short-lived turquoise killifish, Nothobranchius furzeri , to investigate transcriptional responses to low-level MC-LR exposure in a senescent vertebrate. Approximately 8-month-old GRZ killifish were exposed to a low dose of 0.5 µg/L MC-LR, followed by whole-body RNA sequencing and sex-stratified differential expression analysis. Despite identical experimental conditions and exposure, males and females exhibited strikingly distinct transcriptional responses, with 313 differentially expressed genes (DEGs) in males and 263 in females and only 27 DEGs shared between the sexes. Among the shared responses, pck1 , a key regulator of gluconeogenesis, was strongly downregulated in both sexes, accompanied by altered expression of genes associated with mitochondrial function, metabolic regulation, extracellular matrix remodeling, and genome maintenance. Males exhibited prominent remodeling of skeletal muscle and contractile programs, supported by enrichment of sarcomeric, myofilament, and contractile-fiber-associated genes. In contrast, females showed pronounced alterations in reproductive and metabolic programs, including vitellogenin- and zona pellucida-associated transcripts. Cell/tissue associated marker-module analysis further revealed distinct sex-dependent shifts in structural, neural, immune, metabolic, and reproductive transcriptional signatures. Together, these findings demonstrate that MC-LR elicits a broad but strongly sex-dependent transcriptional response in senescent N. furzeri , involving responses in multiple physiological systems. Our study identifies biological sex as an important determinant of cyanotoxin responses in an aging context and establishes naturally aged N. furzeri as a tractable vertebrate model for investigating interactions between environmental exposure and biological aging.