Expression of mouse rod photoreceptor cGMP phosphodiesterase gamma subunit in bacteria

Ciprofloxacin-Induced Antibacterial Activity Is Attenuated by Phosphodiesterase Inhibitors

Background

Ciprofloxacin is a commonly used antibiotic for urinary tract infection that interacts with bacterial topoisomerases leading to oxidative radicals generation and bacterial cell death. Phosphodiesterase inhibitors (PDEis), on the other hand, are commonly used drugs for the management of erectile dysfunction. The group includes agents such as sildenafil, vardenafil, and tadalafil.

Objectives

We investigated whether PDEi could interfere with the antibacterial activity of ciprofloxacin.

Methods

PDEis were tested in several reference bacteria, including Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, Staphylococcus epidermidis, Acinetobacter baumannii, Proteus mirabilis, and Klebsiella pneumoniae utilizing a standard disc diffusion method and measuring both zones of inhibition and MIC.

Results

Results from both assays indicated that ciprofloxacin demonstrates potent activity against the tested reference bacteria. Additionally, when bacteria were treated with a combination of ciprofloxacin and sildenafil, tadalafil, or vardenafil, the zones of the combination inhibition were significantly reduced, whereas the MIC values were significantly greater than those of ciprofloxacin alone for all tested bacterial strains. In an attempt to examine the mechanism by which PDEis interfere with the action of ciprofloxacin, we utilized the in vitro E coli DNA gyrase cleavage assay. The results showed that PDEi drugs had no effect on ciprofloxacin’s inhibition of E coli gyrase activity.

Conclusions

Pretreatment of various reference bacterial cells with PDEis largely inhibited the antibacterial activity of ciprofloxacin.

The retinal cGMP phosphodiesterase gamma-subunit - a chameleon

Intrinsically disordered proteins (IDPs) represent an emerging class of proteins (or domains) that are characterized by a lack of ordered secondary and tertiary structure. This group of proteins has recently attracted tremendous interest primarily because of a unique feature: they can bind to different targets due to their structural plasticity, and thus fulfill diverse functions. The inhibitory gamma-subunit (PDEgamma) of retinal PDE6 is an intriguing IDP, of which unique protein properties are being uncovered. PDEgamma critically regulates the turn on as well as the turn off of visual signaling through alternate interactions with the PDE6 catalytic core, transducin, and the regulator of G protein signaling RGS9-1. The intrinsic disorder of PDEgamma does not compromise, but rather, optimizes its functionality. PDEgamma "curls up" when free in solution but "stretches out" when binding with the PDE6 catalytic core. Conformational changes of PDEgamma also likely occur in its C-terminal PDE6-binding region upon interacting with transducin during PDE6 activation. Growing evidence shows that PDEgamma is also a player in non-phototransduction pathways, suggesting additional protein targets. Thus, PDEgamma is highly likely to be adaptive in its structure and function, hence a "chameleon".

Regulation of photoreceptor phosphodiesterase (PDE6) by phosphorylation of its inhibitory gamma subunit re-evaluated

Phosphorylation of the inhibitory gamma subunit (Pgamma) of rod cGMP phosphodiesterase (PDE6) has been reported to turn off visual excitation without the requirement for inactivation of the photoreceptor G-protein transducin. We evaluated the significance of Pgamma phosphorylation for PDE6 regulation by preparing Pgamma stoichiometrically phosphorylated at Thr(22) or at Thr(35). Phosphorylation of Pgamma at either residue caused a minor decrease--not the previously reported increase--in the ability of Pgamma to inhibit catalysis at the active site of purified PDE6 catalytic dimers. Likewise, Pgamma phosphorylation had little effect on its potency to inhibit transducin-activated PDE6 depleted of its endogenous Pgamma subunits. The strength of Pgamma interaction with the regulatory GAF domain of PDE6 was reduced severalfold upon Pgamma phosphorylation at Thr(22) (but not Thr(35)), as judged by allosteric changes in cGMP binding to these noncatalytic sites on the enzyme (Mou, H., and Cote, R. H. (2001) J. Biol. Chem. 276, 27527-27534). In contrast, the effects of Pgamma phosphorylation on its interactions with activated transducin were much more pronounced. Phosphorylation of Pgamma at either Thr(22) or Thr(35) greatly diminished its ability to bind activated transducin, consistent with earlier work. In situ phosphorylation of Pgamma by endogenous rod outer segment kinases was enhanced severalfold upon light activation, but only approximately 10% of the endogenous Pgamma was phosphorylated. This is attributed to Pgamma being a poor substrate for protein kinases when associated with the PDE6 holoenzyme. We conclude that, contrary to previous reports, Pgamma phosphorylation at either Thr(22) or Thr(35) modestly weakens its direct interactions with PDE6. However, Pgamma phosphorylation subsequent to its dissociation from PDE6 is likely to abolish its binding to activated transducin and may serve to make phosphorylated Pgamma available to regulate other signal transduction pathways (e.g. mitogen-activated protein kinase; Wan, K. F., Sambi, B. S., Frame, M., Tate, R., and Pyne, N. J. (2001) J. Biol. Chem. 276, 37802-37808) in photoreceptor cells.

Unique retina cell phenotypes revealed by immunological analysis of recoverin expression in rat retina cells

Among retina-specific proteins, recoverin is unique with respect to its cellular regulation in that it is found in rods, cones, some bipolar cells, and a rare population of cells in the ganglion cell layer. Recoverin is a calcium-binding protein which inhibits rhodopsin kinase from phosphorylating rhodopsin. Because cells in the inner layers of the retina do not contain rhodopsin kinase, arrestin, or other phototransduction proteins, it seems likely that recoverin has a different function in those cell types. To study this protein more fully, antibodies were generated against the entire mouse recoverin protein, as well as against peptides from the amino and from the carboxyl termini. These antibodies confirmed the localization of recoverin in vivo and clearly demonstrated, in culture, cells which were recoverin positive and rhodopsin negative. Surprisingly, two unique cell phenotypes were seen in cell culture which are not found in vivo. These cells are [rhodopsin(+), recoverin(-)] and [arrestin(+), recoverin(-)]. These phenotypes appear to represent the same population of cells and suggest that the recoverin gene can be regulated independent of genes for other phototransduction proteins. This cell culture system will be useful for investigating environments and factors which participate in the expression of the recoverin gene, and may identify regulatory features of the recoverin gene which cause it to be illicitly expressed in small-cell lung carcinomas in cancer-associated retinopathy (CAR).

Photoreceptor phosphodiesterase: interaction of inhibitory gamma subunit and cyclic GMP with specific binding sites on catalytic subunits

The photoreceptor phosphodiesterase (PDE6) is the central effector enzyme in the phototransduction cascade of photoreceptor cells. It is the only known PDE isoform the activity of which is regulated by interaction with a heterotrimeric G protein. The rod PDE6 holoenzyme is a tetrameric protein consisting of two large catalytic alpha and beta subunits and two small gamma subunits, which serve as potent inhibitors of PDE6. In dark-adapted photoreceptors, the gamma subunits maintain PDE6 activity at a low level. When exposed to light the visual pigment rhodopsin activates the retinal G protein, transducin, leading to release of the inhibitory action of the gamma subunits. In addition to the active sites where cGMP is hydrolyzed, the alpha and beta catalytic subunits have high-affinity, noncatalytic cGMP binding sites. These noncatalytic sites do not directly regulate cGMP catalysis at the active site, but rather can modulate the affinity with which the gamma subunits bind to the catalytic subunits. This article describes a number of experimental approaches that have recently been developed for studying the interactions between catalytic and inhibitory subunits of PDE6, as well as the dynamics of cGMP binding to and dissociation from the PDE6 noncatalytic sites.

Guanylyl cyclase activating protein. A calcium-sensitive regulator of phototransduction

Guanylyl cyclase activating protein (GCAP1) has been proposed to act as a calcium-dependent regulator of retinal photoreceptor guanylyl cyclase (GC) activity. Using immunocytochemical and biochemical methods, we show here that GCAP1 is present in rod and cone photoreceptor outer segments where phototransduction occurs. Recombinant and native GCAP1 activate recombinant human retGC (outer segment-specific GC) and endogenous GC(s) in rod outer segment (ROS) membranes at low calcium. In addition, we isolate and clone a retinal homolog, termed GCAP2, that shows approximately 50% identity with GCAP1. Like GCAP1, GCAP2 activates photoreceptor GC in a calcium-dependent manner. Both GCAP1 and GCAP2 presumably act on GCs by a similar mechanism; however, GCAP1 specifically localizes to photoreceptor outer segments, while in these experiments GCAP2 was isolated from extracts of retina but not ROS. These results demonstrate that GCAP1 is an activator of ROS GC, while the finding of a second activator, GCAP2, suggests that a similar mechanism of GC regulation may be present in outer segments, other subcellular compartments of the photoreceptor, or other cell types.

Structure and functions of arrestins

Transmembrane signal transductions in a variety of cell types that mediate signals as diverse as those carried by neurotransmitters, hormones, and sensory signals share basic biochemical mechanisms that include: (1) an extracellular perturbation (neurotransmitter, hormone, odor, light); (2) specific receptors; (3) coupling proteins, such as G proteins; and (4) effector enzymes or ion channels. Parallel to these amplification reactions, receptors are precisely inactivated by mechanisms that involve protein kinases and regulatory proteins called arrestins. The structure and functions of arrestins are the focus of this review.

Molecular cloning and characterization of retinal photoreceptor guanylyl cyclase-activating protein

Guanylyl cyclase-activating protein (GCAP) is thought to mediate Ca(2+)-sensitive regulation of guanylyl cyclase (GC), a key event in recovery of the dark state of rod photoreceptors following light exposure. Here, we characterize GCAP from several vertebrate species by molecular cloning and provide evidence that GCAP contains a heterogeneously acylated N-terminal region that interacts with GC. Vertebrate GCAPs consist of 201-205 amino acids, and sequence analysis indicates the presence fo three EF hand Ca(2+)-binding motifs. These results establish that GCAP is a novel photoreceptor-specific member of a large family of Ca(2+)-binding proteins and suggest that it participates in the Ca(2+)-binding proteins and suggest that it participates in the Ca(2+)-sensitive activation of GC.