Light and scanning electron microscopy of the tongue of the sand lizard (Lacerta agilis)

BACKGROUND

Despite the fact that numerous reptile species are widely studied by the researchers, information describing the detailed structure of particular organs in many reptiles is missing.

MATERIALS AND METHODS

The tongue of the sand lizard (Lacerta agilis) was examined under the light and scanning electron microscope. It is divided into bifurcated apex, corpus and bifurcated radix. The tip of the lingual apex is devoid of lingual papillae.

RESULTS

The remaining dorsal surface of the tongue bears either fused papillae in the form of caudally directed ridges or individual papillae represented by mu- shroom-like or semilunar prominences (lingual apex) or fish scale-like papillae (lingual corpus) and horizontally laid ridges extending in the form of lobulated prominences (lingual corpus, lingual radix). Regardless of the shape, lingual papillae contain numerous muscle fibres and they are all considered to be mechanical. The lingual epithelium changes from the simple squamous into stratified squamous in the caudal direction. No salivary glands or sensory structures were recognised.

CONCLUSIONS

This description is to be used mainly for comparative studies. It could also help to understand how different lizards capture the pray.

Alterations in G-protein-regulated transmembrane signalling induced in murine myocardium by coxsackievirus B3 infection

OBJECTIVE

Cardiomyopathy is usually associated with marked alterations in myocardial transmembrane signalling. Although acute viral myocarditis may result in chronic cardiomyopathy in some cases, the possible consequences of viral infection on function of the myocardial signal-transducing complex have not been explored. Therefore, the present study was designed to investigate the G-protein-regulated adenylyl cyclase signalling system in murine myocardium during myocarditis induced by coxsackievirus B3 (CVB3) infection.

METHODS

We examined the functional characteristics of adenylyl cyclase as well as the function and distribution of beta-adrenoceptors, m-cholinoceptors and G-proteins in myocardial plasma membranes isolated from the hearts of mice with acute (7 days pi) or late phase (21 days pi) myocarditis and the obtained results were compared with the corresponding data determined in age-matched controls.

RESULTS

While the basal adenylyl cyclase activity was not significantly altered, the ability of forskolin, sodium fluoride and GTP gamma S to activate adenylyl cyclase was lowered by about 20% in samples from virus-infected animals. The level of Gs alpha in myocardial plasma membranes as well as the functional activity of Gs alpha was not affected by viral infection, but the Gi alpha content was increased by about 20%. The total number of beta-adrenoceptors in myocardial plasma membranes increased by about 12-15% due to higher content of the beta 2-adrenoceptor subtype. Although the agonist-binding parameters of beta-adrenoceptors were not significantly altered, the ability of these receptors to mediate stimulation of adenylate cyclase was markedly diminished (by 56-80%). The total number of m-cholinoceptors in samples derived from virus-infected mice increased considerably (by 29-59%) and a significant proportion of the receptors shifted to a higher affinity status, but their ability to transduce agonist signals was impaired.

CONCLUSIONS

These data are the first to demonstrate that several different sites of the myocardial G-protein-regulated adenylyl cyclase signalling complex are significantly altered in acute as well as in late phase of CVB3-induced myocarditis.

Prolonged exposure of hamsters to cold changes the levels of G proteins in brown adipose tissue plasma membranes

The levels of G proteins in plasma membranes prepared from brown adipose tissue of control and cold-exposed hamsters were determined by quantitative immunoblotting and competitive ELISA. Prolonged (four weeks) exposure of hamsters to cold decreased significantly the total content of the alpha subunits of the stimulatory (Gs alpha) as well as inhibitory (Gi alpha (1,2)) G proteins. Interestingly, the reduction in the Gs alpha content was solely due to a large reduction in the content of the short (45 kDa) isoform of Gs alpha, while the level of the long (52 kDa) isoform of Gs alpha remained unchanged. The level of the beta subunit of G protein was decreased comparably to the reduction in the total content of the alpha subunits. Cold-induced alterations in the G protein network associated with plasma membranes of brown adipose tissue were accompanied by changed characteristics of AlF(4-)-sensitive adenylyl cyclase activity.

Isoproterenol-induced subcellular redistribution of G-protein beta subunits in S49 lymphoma cells demonstrated by a novel competitive ELISA

A novel competitive ELISA has been developed for the determination of levels of the beta subunit of guanine-nucleotide-binding protein (G-protein) using antipeptide antibodies directed against the amino terminus of the beta subunit. Because beta subunits form highly hydrophobic.heterodimeric complexes with gamma subunits of G-proteins, specific assay conditions were required. Optimal concentrations of antibodies, detergents, Mg2+ as well as ionic strength were determined. In addition, we found that an effective binding of the used antibodies to the beta subunit was ensured only after denaturation of the beta gamma complexes. Subsequently, this ELISA was used for quantitation of the beta subunit in subcellular fractions of S49 lymphoma cells during isoproterenol-mediated desensitization of beta-adrenergic controlled transmembrane signalling system. A 10 min as well as 60 min treatment of the cells with isoproterenol (1 nmol/ml) resulted in a significant shift of G-protein beta subunits (presumably as beta gamma complexes) from the plasma membrane fractions to low-density microsomal fractions. No significant change was detected after the hormone action in the distribution of plasma membrane constitutive enzymes. In conclusion, the developed ELISA helped us to reveal that beta-adrenergic stimulation can induce redistribution of the beta gamma dimer from plasma membranes to low-density microsomes.

The short and long forms of the alpha subunit of the stimulatory guanine-nucleotide-binding protein are unequally redistributed during (-)-isoproterenol-mediated desensitization of intact S49 lymphoma cells

We report here that desensitization of the beta-adrenergic receptor-triggered transmembrane signalling in S49 wild-type lymphoma cells, induced by (-)-isoproterenol (1 microM), results in unequal intracellular redistribution of the splicing variants of the alpha subunit of the stimulatory guanine-nucleotide-binding regulatory (Gs alpha) protein (Gs alpha-short and Gs alpha-long) and alters the functional characteristics of the membrane-associated signal transduction complex. We found that two cellular pools of membranes, light-density membranes and plasma membranes prepared by sucrose-density-gradient centrifugation of cell homogenates differed in their content of Gs alpha splicing subforms and, moreover, that prolonged activation of the beta-adrenergic pathway induced intermembrane redistribution of the splicing variants of Gs alpha. Short (10 min) as well as prolonged (1 h) (-)-isoproterenol treatment of the cells shifted Gs alpha-short from light-density membranes to plasma membranes and increased the total amount of light-density membrane-bound Gs alpha-long; in parallel, the maximal (-)-isoproterenol-stimulated or AlF4(-)-stimulated adenylyl cyclase activities measured in the plasma membrane pools prepared from treated cells decreased. The functional characteristics of the membrane-bound Gs alpha pools were examined by a cyc(-)-reconstitutive adenylyl cyclase assay where extracts of the plasma membrane and light-density-membrane pools, respectively, were mixed with plasma membranes derived from the mutant S49 cell line, cyc-, lacking Gs alpha. The maximal cyc(-)-reconstitutive activities of the extracts prepared from light-density membranes of short-term as well as long-term desensitized cells increased compared to control cells. These findings may indicate differences in the functioning of the splicing variants of Gs alpha.

Adenovirus infection of myocardial cells induces an enhanced sensitivity to beta-adrenergic agonists by increasing the concentration of the stimulatory G-protein

Neonatal rat cardiocytes were infected with a recombinant adenovirus type 5 containing the SV40 early promoter-Gsa fusion gene in order to evaluate the presumed role of the stimulatory G-protein (Gs) in hypertrophy of myocardial cells. In vitro infection of myocardial cells with the recombinant adenovirus induced a 79-fold increase in Gs alpha mRNA and a 5-fold increase in Gs alpha protein, which was accompanied by a pronounced cell hypertrophy but not cell proliferation. Interestingly, adenovirus-infected cells displayed features of cell hypertrophy, an increase in sodium fluoride-stimulatible membrane-bound activity of adenylyl cyclase, and an enhanced beta-adrenergic sensitivity irrespective of the presence or absence of the SV40 early promoter-Gs alpha fusion gene in the virus. While the recombinant adenovirus induced a 5-fold versus 3-fold increase for plain adenovirus in cellular Gs alpha, membrane-bound Gs alpha was increased about 2-fold in both instances, which can explain similar increase in the G-protein-modulated adenylyl cyclase activity determined in membranes derived from myocardial cells infected with both types of the virus. It is concluded that adenovirus infection per se can lead to overexpression of Gs alpha and myocardial hypertrophy and thus may be of importance in the pathogenesis of virus-induced cardiomyopathy.

Coxsackievirus B3 entry into the host cell interferes with G-protein-mediated transmembrane signalling

In the present work we used various cell lines in order to study the possible effect of coxsackievirus B3 (CVB3) entry on the adenylyl cyclase transmembrane signalling system. A significant decrease (by about 10-20%) was found in forskolin-augmented as well as in A1F-4- and GTP gamma S-sensitive adenylyl cyclase activity in plasma membranes isolated from HeLa, HEp-2, Vero and green monkey kidney cells shortly (up to 60 min) preincubated with CVB3 (5 PFU/cell). Moreover, the ability of G-proteins derived from plasma membranes of infected cells to reconstitute AC activity in the cyc- mutant of S49 cells was also reduced. Content of G-protein subunits, however, remained unchanged after CVB3 attachment. Functional alterations in the G-protein-mediated adenylyl cyclase signalling system were accompanied by a marked decrease (by about 20-40%) of intracellular cAMP levels in virus-affected cells. These findings demonstrate clearly that CVB3 may affect functioning of the G-protein regulated adenylyl cyclase transmembrane signalling system in virus-sensitive cells as early as during the first period of its contact with the cellular plasma membrane.

Activated Gs alpha but not Gi alpha prevents the thermal inactivation of adenylyl cyclase in plasma membranes derived from S49 lymphoma cells

The thermal inactivation of adenylyl cyclase was studied in plasma membranes isolated from wild-type and the mutant cell strain cyc- of S49 lymphoma. The half-life of adenylyl cyclase activity at 30 degrees C was decreased from 14.2 min to 3.4 min by the presence of detergents. ATP as well as forskolin prevented the adenylyl cyclase inactivation in a dose-response manner independent of the utilized type of cell membranes. Activation of G-proteins by GTP gamma S or by AlF-4 in wild-type membranes but not in cyc- membranes partially prevented adenylyl cyclase inactivation. Adenylyl cyclase activity in cyc- membranes was preserved in the presence of GTP gamma S or AlF-4 from the observed detergent-induced inactivation by complementation of these membranes with an extract from wild-type membranes. ADP-ribosylation of Gi alpha in cyc- membranes did not influence the kinetics of the inactivation process of adenylyl cyclase, whereas ADP-ribosylated Gs alpha protein protected adenylyl cyclase more effectively than non-ribosylated Gs alpha in wild-type plasma membranes when GTP was used as an activator.

Plasma-membrane-independent pool of the alpha subunit of the stimulatory guanine-nucleotide-binding regulatory protein in a low-density-membrane fraction of S49 lymphoma cells

We report that compartmentalisation of the stimulatory guanine-nucleotide-binding regulatory protein (Gs) exists in S49 lymphoma cells. In addition to the previously reported cytosolic form of the alpha subunit of Gs (Gs alpha) [Ransnäs, L. A., Svoboda P., Jasper, J. R. & Insel, P. A. (1989) Proc. Natl Acad. Sci. USA 86, 7900-7903], three membrane-bound forms of Gs alpha were identified through rate-zonal centrifugation in sucrose density gradients, Gs alpha-specific anti-peptide serum and an adenylate cyclase complementation assay. The sedimentation profile of the first pool of Gs alpha in the high-density portion of the gradient (1.13-1.16 g/cm3) is identical with that of beta-adrenergic-receptor binding, Na/K-ATPase and adenylate cyclase activity, and may therefore be identified as plasma-membrane fragments. The second pool, which was recovered in the middle portion of the gradient (1.09-1.11 g/cm3), contains a much lower total amount of Gs alpha and correlates with the endoplasmic reticulum (microsomal) enzyme markers, NADPH-cytochrome-c reductase and glucose-6-phosphatase. The identity of the third pool of Gs alpha located at the top of the gradient (1.06-1.08 g/cm3), is unknown. The Golgi apparatus marker, UDPgalactose:N-acetylglucosamine glycosyltransferase, was partially recovered in this area; however, this enzyme was also present in the high-density portion of the gradient. Complete absence of specific adenylate cyclase and Na/K-ATPase activity indicates that this low-density (light) membrane form of Gs alpha is distinct from any plasma-membrane fragments. Furthermore, sedimentation at 100,000 x g proves its particulate (membrane) character. The light membrane form of Gs alpha subunit is functionally active in an adenylate cyclase complementation assay using cyc- membranes devoid of Gs alpha. Overall, our data indicates that a substantial portion of Gs alpha is localized in membrane pools other than plasma membrane.