ADP-ribosylation of actin

Ophidian envenomation strategies and the role of purines

Snake envenomation employs three well integrated strategies: prey immobilization via hypotension, prey immobilization via paralysis, and prey digestion. Purines (adenosine, guanosine and inosine) evidently play a central role in the envenomation strategies of most advanced snakes. Purines constitute the perfect multifunctional toxins, participating simultaneously in all three envenomation strategies. Because they are endogenous regulatory compounds in all vertebrates, it is impossible for any prey organism to develop resistance to them. Purine generation from endogenous precursors in the prey explains the presence of many hitherto unexplained enzyme activities in snake venoms: 5'-nucleotidase, endonucleases (including ribonuclease), phosphodiesterase, ATPase, ADPase, phosphomonoesterase, and NADase. Phospholipases A(2), cytotoxins, myotoxins, and heparinase also participate in purine liberation, in addition to their better known functions. Adenosine contributes to prey immobilization by activation of neuronal adenosine A(1) receptors, suppressing acetylcholine release from motor neurons and excitatory neurotransmitters from central sites. It also exacerbates venom-induced hypotension by activating A(2) receptors in the vasculature. Adenosine and inosine both activate mast cell A(3) receptors, liberating vasoactive substances and increasing vascular permeability. Guanosine probably contributes to hypotension, by augmenting vascular endothelial cGMP levels via an unknown mechanism. Novel functions are suggested for toxins that act upon blood coagulation factors, including nitric oxide production, using the prey's carboxypeptidases. Leucine aminopeptidase may link venom hemorrhagic metalloproteases and endogenous chymotrypsin-like proteases with venom L-amino acid oxidase (LAO), accelerating the latter. The primary function of LAO is probably to promote prey hypotension by activating soluble guanylate cyclase in the presence of superoxide dismutase. LAO's apoptotic activity, too slow to be relevant to prey capture, is undoubtedly secondary and probably serves principally a digestive function. It is concluded that the principal function of L-type Ca(2+) channel antagonists and muscarinic toxins, in Dendroaspis venoms, and acetylcholinesterase in other elapid venoms, is to promote hypotension. Venom dipeptidyl peptidase IV-like enzymes probably also contribute to hypotension by destroying vasoconstrictive peptides such as Peptide YY, neuropeptide Y and substance P. Purines apparently bind to other toxins which then serve as molecular chaperones to deposit the bound purines at specific subsets of purine receptors. The assignment of pharmacological activities such as transient neurotransmitter suppression, histamine release and antinociception, to a variety of proteinaceous toxins, is probably erroneous. Such effects are probably due instead to purines bound to these toxins, and/or to free venom purines.

Reversible glutathionylation regulates actin polymerization in A431 cells

In response to growth factor stimulation, many mammalian cells transiently generate reactive oxygen species (ROS) that lead to the elevation of tyrosine-phosphorylated and glutathionylated proteins. While investigating EGF-induced glutathionylation in A431 cells, paradoxically we found deglutathionylation of a major 42-kDa protein identified as actin. Mass spectrometric analysis revealed that the glutathionylation site is Cys-374. Deglutathionylation of the G-actin leads to about a 6-fold increase in the rate of polymerization. In vivo studies revealed a 12% increase in F-actin content 15 min after EGF treatment, and F-actin was found in the cell periphery suggesting that in response to growth factor, actin polymerization in vivo is regulated by a reversible glutathionylation mechanism. Deglutathionylation is most likely catalyzed by glutaredoxin (thioltranferase), because Cd(II), an inhibitor of glutaredoxin, inhibits intracellular actin deglutathionylation at 2 microM comparable with its IC(50) in vitro. Moreover, mass spectral analysis showed efficient transfer of GSH from immobilized S-glutathionylated actin to glutaredoxin. Overall, this study revealed a novel physiological relevance of actin polymerization regulated by reversible glutathionylation of the penultimate cysteine mediated by growth factor stimulation.

cGMP-dependent ADP depolymerization of actin mediates estrogen increase in cervical epithelial permeability

Estrogen increases secretion of cervical mucus in women, and the effect depends on fragmentation of the cytoskeleton. The objective of the present study was to understand the molecular mechanism of estrogen action. Treatment of human cervical epithelial cells with 17beta-estradiol, sodium nitroprusside (SNP), or 8-bromoguanosine 3', 5'-cyclic monophosphate (8-Br-cGMP) increased cellular monomeric G-actin and decreased polymerized F-actin. The effects of estradiol were blocked by tamoxifen, by the guanylate cyclase inhibitor LY-83583, and by the cGMP-dependent protein kinase inhibitor KT-5823. The effects of SNP were blocked by LY-83583 and KT-5823, while the effects of 8-Br-cGMP were blocked only by KT-5823. Treatment with phalloidin decreased paracellular permeability and G-actin. Treatment with 17beta-estradiol, SNP, or 8-Br-cGMP attenuated SNP-induced phosphorylation of [(32)P]adenylate NAD in vitro: tamoxifen blocked the effect of estrogen; LY-83583 blocked the effect of SNP but not that of 8-Br-cGMP, while KT-5823 blocked effects of both SNP and 8-Br-cGMP. These results indicate that estrogen, nitric oxide (NO), and cGMP stimulate actin depolymerization. A possible mechanism is NO-induced, cGMP-dependent protein kinase augmentation of ADP-ribosylation of monomeric actin.

Nitric oxide as a signaling molecule in visual system development

The lateral geniculate nucleus (LGN) of the ferret is characterized by the readily discernible anatomical patterning of afferent terminations from the retina into both eye-specific layers and On/Off sublaminae. The eye-specific layers form during the first post-natal week, and On/Off sublaminae become apparent during the third to fourth post-natal weeks. The post-natal appearance of these patterns thus provides an advantageous model for the study of the mechanisms of activity-dependent development. The second phase of pattern formation, the appearance of On/Off sublaminae, involves the elaboration of appropriately placed axonal terminals and the restriction (or retraction) of inappropriately placed terminals. Previous work has demonstrated that this process is dependent on the activation of NMDA-receptors. Other studies have provided strong evidence that nitric oxide, a diffusible gas which is produced downstream of NMDA-receptor activation, acts as a retrograde messenger molecule to induce changes in pre-synaptic structures. In this article we review the evidence that nitric oxide plays a role in activity-dependent synaptic plasticity in the developing retinogeniculate pathway. The role of nitric oxide in other aspects of visual system development is also discussed.

Exogenous nitric oxide causes collapse of retinal ganglion cell axonal growth cones in vitro

We show that nitric oxide (NO) from applied NO-donating chemicals induces collapse of ganglion cell axonal growth cones extending from explants of tadpole retina in culture. Peroxynitrite, a neurotoxic product of NO and superoxide reaction, did not induce collapse, and oxyhemoglobin, which binds NO, blocked the highly effective collapsing activity of the NO donor S-nitrosocysteine. Membrane-permeable analogs of cyclic guanosine monophosphate had no collapsing activity. Inhibitors of NO synthase did not induce collapse. NO is a potential retrograde messenger through which postsynaptic neurons signal to their inputs to modify synaptic efficacy following NMDA receptor activation. Our results suggest a role for NO as such a messenger during development of the retinotectal projection.

A new model for the research into rhythmic contraction activity of cardiomyocytes in vitro

Heart cells continue to contract rhythmically after isolation and in culture in vitro. We describe a model of heart preparation in vitro that permits quantitative research on the frequency of contractions of cardiomyocytes. The chick embryo heart explants placed on a network of elastic glass fibers continued beating for months, recorded and analyzed with the methods of computer-assisted image analysis. The efficacy of this experimental model for the screening of effects of various agents on the frequency of contractions was examined by following the effects of nifedipine, caffeine, ethanol, and benzamide. The reversibility of the effects and the reproducibility of results were demonstrated quantitatively. The significance of a mechanical elastic load provided by glass fibers for the preservation of long-lasting contractile activity of cardiomyocytes is discussed and the common occurrence of oscillatory contraction processes in various eucaryotic cells is noted.

Actin and the actomyosin interface: a review

This review deals with the structure of the actin monomer, its assembly into filaments and the loci on F-actin involved in binding myosin. Two distinctly different arrangements of monomers have been suggested for actin filaments. One model proposed by Holmes et al. is well developed. It places the so-called 'large' domain close to the filament axis and the so-called 'small' domain out near the surface of the filament. A second, less-well developed, model proposed by Schutt et al. locates the 'small' domain close to the filament axis and they rotate the monomer so that 'bottom' of the 'large' domain is at the highest radius. We analyze the available evidence for the models of F-actin derived from X-ray diffraction, reconstructions from electron micrographs, fluorescence resonance energy transfer spectroscopy, chemical cross-linking, antibody probes, limited proteolysis, site-directed and natural mutations, nuclear magnetic resonance spectroscopy and other techniques. The result is an actin-centered view of the loci on actin which are probably involved in its interaction with the myosin 'head'. From these multiple contacts we speculate on the sequence of steps between the initial weak-binding state of S-1 to the actin filament through to the stable strong-binding state seen in the absence of free Mg-ATP, i.e., the rigor state.

Plants contain highly divergent actin isovariants

Actin protein isovariants have been identified in animals with distinct cytoplasmic or muscle specific patterns of expression. Analysis of vascular plant actin gene sequences suggests that an even greater diversity should exist within the plant actin protein families, but previous studies on plant proteins have not demonstrated the presence of multiple actin isovariants. Antibodies recognizing a conserved amino-terminal plant actin peptide, a family of plant actin peptides from a variable region, and two monoclonal antibodies to conserved epitopes within animal actins were used to identify isovariants of soybean actin resolved by two-dimensional isoelectric focusing (IEF) sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Approximately six to eight actin isovariants with pI values ranging from 5.1 to 5.8 have been identified from soybean hypocotyls, stems, leaves, and roots with varying amounts of most isovariants present in all four organs. Acidic isovariants were present in much higher levels in leaves and stems. Antisera with lambda-class actin specificity detected a subset of three isovariants in all organs examined. One monoclonal and one antipeptide antisera are shown to react well with a wide variety of plant actin isovariants. Similar patterns of actin isovariants were detected in the distant angiosperms, Arabidopsis, petunia, and maize. It is likely that many of these diverse classes of isovariants have been preserved throughout vascular plant evolution and reflect the ancient diversity within plant actin gene families. The extreme difference among isovariants implies the presence of a complex actin-based cytoskeletal system in plants.