Deletion of a single mevalonate kinase (Mvk) allele yields a murine model of hyper-IgD syndrome.

In the current study our objective was to develop a murine model of human hyper-IgD syndrome (HIDS) and severe mevalonic aciduria (MA), autoinflammatory disorders associated with mevalonate kinase deficiency (MKD). Deletion of one Mvk allele (Mvk (+/-)) yielded viable mice with significantly reduced liver Mvk enzyme activity; multiple matings failed to produce Mvk (-/-) mice. Cholesterol levels in tissues and blood, and isoprene end-products (ubiquinone, dolichol) in tissues were normal in Mvk (+/-) mice; conversely, mevalonate concentrations were increased in spleen, heart, and kidney yet normal in brain and liver. While the trend was for higher IgA levels in Mvk (+/-) sera, IgD levels were significantly increased (9-12-fold) in comparison to Mvk (+/+) littermates, in both young (<15 weeks) and older (>15 weeks) mice. Mvk (+/-) animals manifested increased serum TNF-alpha as compared to wild-type littermates, but due to wide variation in levels between individual Mvk (+/-) mice the difference in means was not statistically significant. Mvk (+/-) mice represent the first animal model of HIDS, and should prove useful for examining pathophysiology associated with this disorder.

In vivo assessment of local phosphodiesterase activity using tailored cyclic nucleotide-gated channels as cAMP sensors.

Phosphodiesterases (PDEs) catalyze the hydrolysis of the second messengers cAMP and cGMP. However, little is known about how PDE activity regulates cyclic nucleotide signals in vivo because, outside of specialized cells, there are few methods with the appropriate spatial and temporal resolution to measure cyclic nucleotide concentrations. We have previously demonstrated that adenovirus-expressed, olfactory cyclic nucleotide-gated channels provide real-time sensors for cAMP produced in subcellular compartments of restricted diffusion near the plasma membrane (Rich, T.C., K.A. Fagan, H. Nakata, J. Schaack, D.M.F. Cooper, and J.W. Karpen. 2000. J. Gen. Physiol. 116:147-161). To increase the utility of this method, we have modified the channel, increasing both its cAMP sensitivity and specificity, as well as removing regulation by Ca(2)+-calmodulin. We verified the increased sensitivity of these constructs in excised membrane patches, and in vivo by monitoring cAMP-induced Ca(2)+ influx through the channels in cell populations. The improved cAMP sensors were used to monitor changes in local cAMP concentration induced by adenylyl cyclase activators in the presence and absence of PDE inhibitors. This approach allowed us to identify localized PDE types in both nonexcitable HEK-293 and excitable GH4C1 cells. We have also developed a quantitative framework for estimating the K(I) of PDE inhibitors in vivo. The results indicate that PDE type IV regulates local cAMP levels in HEK-293 cells. In GH4C1 cells, inhibitors specific to PDE types I and IV increased local cAMP levels. The results suggest that in these cells PDE type IV has a high K(m) for cAMP, whereas PDE type I has a low K(m) for cAMP. Furthermore, in GH4C1 cells, basal adenylyl cyclase activity was readily observable after application of PDE type I inhibitors, indicating that there is a constant synthesis and hydrolysis of cAMP in subcellular compartments near the plasma membrane. Modulation of constitutively active adenylyl cyclase and PDE would allow for rapid control of cAMP-regulated processes such as cellular excitability.

A uniform extracellular stimulus triggers distinct cAMP signals in different compartments of a simple cell.

cAMP, the classical second messenger, regulates many diverse cellular functions. The primary effector of cAMP signals, protein kinase A, differentially phosphorylates hundreds of cellular targets. Little is known, however, about the spatial and temporal nature of cAMP signals and their information content. Thus, it is largely unclear how cAMP, in response to different stimuli, orchestrates such a wide variety of cellular responses. Previously, we presented evidence that cAMP is produced in subcellular compartments near the plasma membrane, and that diffusion of cAMP from these compartments to the bulk cytosol is hindered. Here we report that a uniform extracellular stimulus initiates distinct cAMP signals within different cellular compartments. By using cyclic nucleotide-gated ion channels engineered as cAMP biosensors, we found that prostaglandin E(1) stimulation of human embryonic kidney cells caused a transient increase in cAMP concentration near the membrane. Interestingly, in the same time frame, the total cellular cAMP rose to a steady level. The decline in cAMP levels near the membrane was prevented by pretreatment with phosphodiesterase inhibitors. These data demonstrate that spatially and temporally distinct cAMP signals can coexist within simple cells.