Regulable expression of inhibin A in wild-type and inhibin alpha null mice

Exogenous regulation of protein expression creates the potential to examine the consequences of homeostatic Dysregulation in many physiological system s and, when used in transgenic mice, provides the capability of restoring a gene product to its knockout background without antigenicity issues. In th is study, we used a mifeprisone-inducible system (the GeneSwitch system) to regulate the expression of inhibin A from the liver of mice. Inhibin is a heterodimeric protein (alpha/beta) wherein one of its subunits (beta) is ca pable of homodimerizing to form its physiological antagonist, activin (beta /beta). Inhibin is also expressed in two forms, A and B, as determined by t he subtype of beta-subunit that dimerizes with the alpha-subunit (alpha/bet a A or alpha/beta B). To utilize the GeneSwitch system, transgenic transact ivator mice with liver-specific expression of a mifepristone-activated chim eric nuclear receptor (GLVP) were crossed with transgenic target mice conta ining a GVLP-responsive promoter upstream of poliovirus IRES (internal ribo some entry site)-linked sequences coding for the alpha- and beta-subunits o f inhibin A. This Intercross produced "bigenic" mice capable of regulable e xpression of inhibin A from the liver. Overexpression of inhibin A in wild- type mice produced a phenotype wherein males had decreased testis size and females had a block in folliculogenesis at the early antral stage, findings similar to activin type IIA receptor (ActRIIA) null mice. These phenotypes were most likely due to suppressed serum FSH, confirming that the liver-de rived inhibin A was secreted into the serum to down-regulate pituitary FSH levels. Furthermore, the generation of bigenic mice in the inhibin alpha nu ll background allowed for the induction of inhibin A in inhibin alpha null male mice with subsequent rescue of these mice from their gonadal tumor-ind uced lethal phenotype, This work demonstrates the in vivo production of a h eterodimeric hormone from a single inducible promoter to study its therapeu tic and physiological effects. In addition, these studies are the first exa mple of an inducible system being used to prevent a lethal knockout phenoty pe in an animal model.