notum

Notum is a term with multiple uses in biology. In arthropod anatomy, notum refers to the dorsal plate of a thoracic segment. The thorax typically comprises prothorax, mesothorax, and metathorax, with a notum on the dorsal surface of each segment. The notum serves as structural protection and as an attachment site for flight muscles in winged species; in some insects it can bear additional plates, such as the scutellum, and its size and shape can vary among species, contributing to comparative anatomy and evolution studies.

In molecular biology, Notum (capital N) denotes a secreted carboxylesterase enzyme encoded by the NOTUM gene in humans and many vertebrates. Notum functions as a negative regulator of Wnt signaling by catalytically removing a lipid modification, palmitoleate, from Wnt proteins. This deacylation reduces Wnt proteins’ ability to activate Frizzled receptors, influencing gradient formation during development and maintaining tissue homeostasis. Dysregulation of Notum has been linked to developmental disorders and cancer in various studies. The enzyme is secreted into the extracellular space and can act in autocrine or paracrine fashion.

Etymology for both uses traces to Latin notum, meaning back or dorsum, reflecting the dorsal nature of the anatomical plate and the dorsal-facing activity of the secreted enzyme in the context of signaling regulation.

See also: Wnt signaling, tergite, sternite, scutellum, NOTUM gene.