Integrative role of CTPS cytoophidia in polyploid tissue growth and nutrient adaptation.

Tissue growth and development are fundamental to organismal survival, requiring precise coordination of metabolic processes, nutrient availability, and signaling pathways. Cytidine triphosphate synthase (CTPS) is a rate-limiting enzyme in nucleotide biosynthesis and assembles filamentous cytoophidia, conserved across species. Despite increasing interest in cytoophidia, how CTPS filaments integrate metabolic and signaling cues to drive cell size and tissue growth remains incompletely understood. Using RNA interference and clustered regularly interspaces short palindromic repeats (CRISPR) / CRISPR-associate nuclease 9 gene editing, we generated CTPS-knockdown and point-mutated mutants to investigate the role of cytoophidia in cell growth. Specifically, we introduced the H355A mutation, which disrupts CTPS filament formation without affecting its enzymatic activity. Our findings revealed that CTPS depletion or filament disruption significantly impairs growth in polyploid organs, such as the fat body and salivary glands, underscoring the pivotal role of CTPS cytoophidia in cell growth regulation. Mutants lacking cytoophidia exhibited reduced DNA replication activity and smaller cell sizes compared to wild-type controls. Mechanistically, we found that nutrient-sensing pathways, particularly insulin-PI3K-Akt signaling pathway, regulate CTPS expression and cytoophidia formation in response to nutrient availability. Activation of the sterol regulatory element-binding protein partially rescued the growth defects caused by CTPS depletion. These findings provide new insights into the molecular mechanisms of the regulation of CTPS filaments, highlighting their role as critical mediators of tissue growth by integrating environmental demands, metabolism, and signaling pathways to regulate cell size and nutrient adaptation.