HERP, a New Primary Target of Notch Regulated by Ligand Binding

Notch signaling dictates cell fate and critically influences cell proliferation, differentiation, and apoptosis in metazoans. Ligand binding initiates the signal through regulated intramembrane proteolysis of a transmembrane Notch receptor which releases the signal-transducing Notch intracellular domain (NICD). The HES/E(spl) gene family is a primary target of Notch and thus far the only known Notch effector. A newly isolated HERP family, a HES-related basic helix-loop-helix protein family, has been proposed as a potential target of Notch, based on its induction following NICD overexpression. However, NICD is physiologically maintained at an extremely low level that typically escapes detection, and therefore, nonregulated overexpression of NICD—as in transient transfection—has the potential of generating cellular responses of little physiological relevance. Indeed, a constitutively active NICD indiscriminately up-regulates expression of both HERP1 and HERP2 mRNAs. However, physiological Notch stimulation through ligand binding results in the selective induction of HERP2 but not HERP1 mRNA and causes only marginal up-regulation of HES1 mRNA. Importantly, HERP2 is an immediate target gene of Notch signaling since HERP2 mRNA expression is induced even in the absence of de novo protein synthesis. HERP2 mRNA induction is accompanied by specific expression of HERP2 protein in the nucleus. Furthermore, using RBP-Jk-deficient cells, we show that an RBP-Jk protein, a transcription factor that directly activates HES/E(spl) transcription, also is essential for HERP2 mRNA expression and that expression of exogenous RBP-Jk is sufficient to rescue HERP2 mRNA expression. These data establish that HERP2 is a novel primary target gene of Notch that, together with HES, may effect diverse biological activities of Notch.