Zebrafish Offers New Perspective on Developmental Role of TOR Signaling.

Genetic studies on the molecular basis of growth control have converged on the target of rapamycin (TOR) pathway as a key regulator.1 When stimulated by nutrients (i.e. amino acids) or growth factors (i.e. insulin), TOR activates protein synthesis and other anabolic pathways to promote cell growth.1 Our knowledge of TOR's function in vivo is still rudimentary, particularly in the setting of vertebrate development. An important question is whether TOR functions as a constitutive regulator of growth in all cell types, or as a stage and organ specific regulator. Recently we employed the zebrafish as a vertebrate model system to study the developmental role of TOR signaling. We found that TOR signaling was required for a discrete step prior to epithelial differentiation. The results support the view that different organs may be reliant on TOR activity to differing degrees. In the case of the zebrafish, the digestive tract exhibits the greatest sensitivity to rapamycin, which may reflect its reliance on TOR signaling for normal growth. We suggest the hypothesis that TOR signaling may regulate the size of the intestine's absorptive surface area in response to systemic nutrient demand.