ABSTRACT
The protein kinase inhibitor (PKI) family includes three genes encoding small, heat-stable inhibitors of the cyclic AMP-dependent kinase PKA. Each PKI isoform contains a PKA inhibitory domain and a nuclear export domain, enabling PKI to both inhibit PKA and remove it from the nucleus. The PKIbeta isoform, also known as testis PKI, is highly expressed in germ cells of the testis and is found at more modest levels in other tissues. In order to investigate its physiological role, we have generated PKIbeta knockout mice by gene targeting. These mice exhibit a partial loss of PKI activity in testis but remain fertile with normal testis development and function. PKIbeta knockout females also reproduce normally. The PKIbeta mutants were crossed with our previously derived PKIalpha mutants to obtain double-knockout mice. Remarkably, these mice are also viable and fertile with no obvious physiological defects in either males or females.
Subject(s)
Carrier Proteins/physiology , Cyclic AMP-Dependent Protein Kinases/antagonists & inhibitors , Intracellular Signaling Peptides and Proteins , Reproduction/physiology , Animals , Carrier Proteins/genetics , Female , Gene Expression , Gene Targeting , Male , Mice , Mice, Knockout , RNA, Messenger/genetics , RNA, Messenger/metabolism , Reproduction/genetics , Spermatogenesis/genetics , Spermatogenesis/physiology , Testis/metabolismABSTRACT
Protein kinase inhibitor (PKI) is a potent endogenous inhibitor of the cyclic AMP (cAMP)-dependent protein kinase (PKA). It functions by binding the free catalytic (C) subunit with a high affinity and is also known to export nuclear C subunit to the cytoplasm. The significance of these actions with respect to PKI's physiological role is not well understood. To address this, we have generated by homologous recombination mutant mice that are deficient in PKIalpha, one of the three isoforms of PKI. The mice completely lack PKI activity in skeletal muscle and, surprisingly, show decreased basal and isoproterenol-induced gene expression in muscle. Further examination revealed reduced levels of the phosphorylated (active) form of the transcription factor CREB (cAMP response element binding protein) in the knockouts. This phenomenon stems, at least in part, from lower basal PKA activity levels in the mutants, arising from a compensatory increase in the level of the RIalpha subunit of PKA. The deficit in gene induction, however, is not easily explained by current models of PKI function and suggests that PKI may play an as yet undescribed role in PKA signaling.