Receptors for intercellular messenger molecules in microbes: similarities to vertebrate receptors and possible implications for diseases in man

Microbial Endocrinology in the Pathogenesis of Infectious Disease

Microbial endocrinology represents the intersection of two seemingly disparate fields, microbiology and neurobiology, and is based on the shared presence of neurochemicals that are exactly the same in host as well as in the microorganism. The ability of microorganisms to not only respond to, but also produce, many of the same neurochemicals that are produced by the host, such as during periods of stress, has led to the introduction of this evolutionary-based mechanism which has a role in the pathogenesis of infectious disease. The consideration of microbial endocrinology-based mechanisms has demonstrated, for example, that the prevalent use of catecholamine-based synthetic drugs in the clinical setting contributes to the formation of biofilms in indwelling medical devices. Production of neurochemicals by microorganisms most often employs the same biosynthetic pathways as those utilized by the host, indicating that acquisition of host neurochemical-based signaling system in the host may have been acquired due to lateral gene transfer from microorganisms. That both host and microorganism produce and respond to the very same neurochemicals means that there is bidirectionality contained with the theoretical underpinnings of microbial endocrinology. This can be seen in the role of microbial endocrinology in the microbiota-gut-brain axis and its relevance to infectious disease. Such shared pathways argue for a role of microorganism-neurochemical interactions in infectious disease.

Microbial endocrinology: Host-microbiota neuroendocrine interactions influencing brain and behavior

The ability of microorganisms, whether present as commensals within the microbiota or introduced as part of a therapeutic regimen, to influence behavior has been demonstrated by numerous laboratories over the last few years. Our understanding of the mechanisms that are responsible for microbiota-gut-brain interactions is, however, lacking. The complexity of the microbiota is, of course, a contributing factor. Nonetheless, while microbiologists approaching the issue of microbiota-gut-brain interactions in the behavior well recognize such complexity, what is often overlooked is the equal complexity of the host neurophysiological system, especially within the gut which is differentially innervated by the enteric nervous system. As such, in the search for common mechanisms by which the microbiota may influence behavior one may look for mechanisms which are shared by both host and microbiota. Such interkingdom signaling can be found in the shared production of neurochemical mediators that are found in both eukaryotes and prokaryotes. The study of the production and recognition of neurochemicals that are exactly the same in structure to those produced in the vertebrate organisms is known as microbial endocrinology. The examination of the microbiota from the vantage point of host-microbiota neuroendocrine interactions cannot only identify new microbial endocrinology-based mechanisms by which the microbiota can influence host behavior, but also lead to the design of interventions in which the composition of the microbiota may be modulated in order to achieve a specific microbial endocrinology-based profile beneficial to overall host behavior.

Protein-mediated interactions of pancreatic islet cells

The islets of Langerhans collectively form the endocrine pancreas, the organ that is soley responsible for insulin secretion in mammals, and which plays a prominent role in the control of circulating glucose and metabolism. Normal function of these islets implies the coordination of different types of endocrine cells, noticeably of the beta cells which produce insulin. Given that an appropriate secretion of this hormone is vital to the organism, a number of mechanisms have been selected during evolution, which now converge to coordinate beta cell functions. Among these, several mechanisms depend on different families of integral membrane proteins, which ensure direct (cadherins, N-CAM, occludin, and claudins) and paracrine communications (pannexins) between beta cells, and between these cells and the other islet cell types. Also, other proteins (integrins) provide communication of the different islet cell types with the materials that form the islet basal laminae and extracellular matrix. Here, we review what is known about these proteins and their signaling in pancreatic β -cells, with particular emphasis on the signaling provided by Cx36, given that this is the integral membrane protein involved in cell-to-cell communication, which has so far been mostly investigated for effects on beta cell functions.

Connexins: key mediators of endocrine function

The appearance of multicellular organisms imposed the development of several mechanisms for cell-to-cell communication, whereby different types of cells coordinate their function. Some of these mechanisms depend on the intercellular diffusion of signal molecules in the extracellular spaces, whereas others require cell-to-cell contact. Among the latter mechanisms, those provided by the proteins of the connexin family are widespread in most tissues. Connexin signaling is achieved via direct exchanges of cytosolic molecules between adjacent cells at gap junctions, for cell-to-cell coupling, and possibly also involves the formation of membrane "hemi-channels," for the extracellular release of cytosolic signals, direct interactions between connexins and other cell proteins, and coordinated influence on the expression of multiple genes. Connexin signaling appears to be an obligatory attribute of all multicellular exocrine and endocrine glands. Specifically, the experimental evidence we review here points to a direct participation of the Cx36 isoform in the function of the insulin-producing β-cells of the endocrine pancreas, and of the Cx40 isoform in the function of the renin-producing juxtaglomerular epithelioid cells of the kidney cortex.

A bacterial cytokine

Viable cells of Micrococcus luteus secrete a factor, which promotes the resuscitation and growth of dormant, nongrowing cells of the same organism. The resuscitation-promoting factor (Rpf) is a protein, which has been purified to homogeneity. In picomolar concentrations, it increases the viable cell count of dormant M. luteus cultures at least 100-fold and can also stimulate the growth of viable cells. Rpf also stimulates the growth of several other high G+C Gram-positive organisms, including Mycobacterium avium, Mycobacterium bovis (BCG), Mycobacterium kansasii, Mycobacterium smegmatis, and Mycobacterium tuberculosis. Similar genes are widely distributed among high G+C Gram-positive bacteria; genome sequencing has uncovered examples in Mycobacterium leprae and Mb. tuberculosis and others have been detected by hybridization in Mb. smegmatis, Corynebacterium glutamicum, and Streptomyces spp. The mycobacterial gene products may provide different targets for the detection and control of these important pathogens. This report is thus a description of a proteinaceous autocrine or paracrine bacterial growth factor or cytokine.

Norepinephrine-induced expression of the K99 pilus adhesin of enterotoxigenic Escherichia coli

This study examined whether the provision of norepinephrine, as would be encountered within the highly innervated gastrointestinal system, affected the growth rate of enterotoxigenic Escherichia coli (ETEC) and the expression of the K99 pilus adhesin virulence-related factor. The addition of norepinephrine to serum-containing medium resulted in a 3- to 7-fold increase in the growth rate of the K99+ ETEC strain B44 as compared to growth in vehicle supplemented medium or medium supplemented with normetanephrine, a norepinephrine metabolite that contains one more methyl group than norepinephrine. ELISA analysis revealed that K99 pilus adhesin expression was increased in norepinephrine supplemented culture as compared to normetanephrine and vehicle supplemented controls. This increase occurred from 9 to 15 hours of incubation which represented the exponential growth phase for the norepinephrine supplemented culture. These results indicate that addition of norepinephrine affects both ETEC growth and expression of a specific virulence factor.

Norepinephrine induced growth and expression of virulence associated factors in enterotoxigenic and enterohemorrhagic strains of Escherichia coli

The small intestine is richly innervated by the sympathetic nervous system. High concentrations of monoamines, most notably norepinephrine, are found throughout the various intestinal layers. In order to determine whether norepinephrine is capable of influencing bacterial pathogenesis, the growth and production of virulence factors in ETEC and EHEC were examined in a physiologically relevant medium utilizing very low initial bacterial inoculums to more closely mimie in vivo conditions. The growth of ETEC strain B44 and the production of the K99 pilus adhesin on a protein equivalent basis was greatly increased in the presence of norepinephrine. Growth of EHEC O157:H7 was also increased in norepinephrine containing medium as well as production of SLT-I and SLT-II. The ability of norepinephrine to increase both bacterial growth and expression of virulence factors was shown to be non-nutritional in nature. Given the abundant adrenergic innervation in the small intestine, these in vitro results suggest that the neurohumoral environment of the host may play a role in bacterial growth and expression of virulence factors.

The opiate system in invertebrates

The presence in diverse species of a similar mode of communication, that of a soluble messenger binding to a receptor, raises the question as to whether the specific components of this system are equally widespread. Do invertebrates use the same hormones and receptors as vertebrates do? Invertebrates ranging from unicellular organisms to insects have been shown to contain opiate-like peptides and binding sites, and they exhibit biological responses to opiates. However, critical genetic data are lacking. It is not known how signal systems arise phylogenetically, but it is conceivable that signal molecules that are already present cause the formation of their own receptors from membrane proteins.

External ions and direct membrane effects of enkephalins on Amoeba proteus

We have shown that amphiphilic hormones like vasopressins and endorphins increase water permeability and activate the contractile vacuole (CV) in Amoeba by direct action on the plasma membrane. Using our standard CV assay, the effects of nine opioids, morphine, naloxone and 7 enkephalin derivatives, have been compared in normal, ion-containing growth medium (Chalkley's) and in glass-distilled water. While the absence of external ions does not affect the activity of molecules with net positive charges, opioids with no net charge are devoid of action in glass-distilled water. This shows that, in addition to amphiphilicity, which permits insertion into the lipid core of the membrane, electrostatic interactions with ions and with negative charges of phospholipids membrane and glycocalyx, the thick glycoproteic cell coat of Amoeba, are important in the direct action of these compounds.

Specific, high-affinity binding sites for angiotensin II on Mycoplasma hyorhinis

Mycoplasmataceae are known to express various proteins that are similar to those present in mammals. We report a strain of Mycoplasma hyorhinis isolated from opossum kidney cells with specific, high-affinity binding sites for human angiotensin II (Kd = 5.1 +/- 1.9 nM). In contrast, two strains of M. hominis revealed no specific binding. These binding sites resembled mammalian angiotensin II receptors by their high affinity and by their sensitivity to dithiothreitol. However, they are different from mammalian angiotensin II receptors in that they bind angiotensin I with high affinity (Kd = 1.6 +/- 0.29 nM) but not angiotensin III (Kd approximately 330,000 nM). [125I]-angiotensin II binding was not inhibited by angiotensin receptor subtype antagonists DuP 753 and CGP 42112A but it was sensitive to bacitracin and aprotinin. Positions Asp1, Ile5, His6 and Pro7 were essential for binding to M. hyorhinis as deletion of these residues led to a more than 10,000-fold decrease in affinity.

Cytochemical demonstration of cholinergic, serotoninergic and peptidergic nerve elements in Gorgoderina vitelliloba (Trematoda: Digenea)

Standard enzyme cytochemical and indirect immunocytochemical techniques have been used in conjunction with light and confocal scanning laser microscopy (CSLM) to visualize cholinergic, serotoninergic and peptidergic nerve elements in whole-mount preparations of the amphibian urinary-bladder fluke, Gorgoderina vitelliloba. Cholinesterase (ChE) activity was localized in paired anterior ganglia, a connecting dorsal commissure and in the origins of the ventral nerve cords. Cholinergic ganglia were also evident in shelled embryos in the uterus. Serotonin-immunoreactivity (IR) was more extensive than ChE activity and was identified in both the central and peripheral nervous systems. Serotoninergic nerve fibres were associated with the somatic musculature and female reproductive ducts. Antisera to nine mammalian peptides and one invertebrate (FMRFamide) peptide have been used to investigate the peptidergic nervous system in the parasite. Immunoreactivity was obtained to five peptides, namely pancreatic polypeptide (PP), peptide YY (PYY), neuropeptide Y (NPY), substance P (SP) and FMRFamide. Peptidergic nerve fibres were found to be more abundant than demonstrable cholinergic or serotoninergic nerve fibres. NPY-IR was identified only in the main components of the central nervous system. However, PP- and PYY-IR occurred in the anterior ganglia, dorsal commissure, main nerve cords and in numerous small varicose fibres that ramified throughout the worm. Additionally, PP-immunoreactive nerve fibres were found to innervate the musculature of the female reproductive tracts. Six sites of IR were found in the acetabulum, using antisera directed towards the C-terminal end of PP and PYY, and these matched with the distribution of six non-ciliated rosette-like papillae observed by scanning electron microscopy. SP- and FMRFamide-IR were identified in the CNS, and FMRFamide-immunopositive nerve fibres were also evident in association with the gonopore cirrus region and with the terminal excretory pore. Results are discussed with respect to possible roles for each of the neurochemical types.

Evolutionary aspects of transmitter molecules, their receptors and channels

Classical transmitters are present in all phyla that have been studied; however, our detailed understanding of the process of neurotransmission in these phyla is patchy and has centred on those neurotransmitter receptor mechanisms which are amenable to study with the tools available at the time, for example, high-affinity ligands, tissues with high density of receptor protein, suitable electrophysiological recording systems. Studies also clearly show that many neurones exhibit co-localization of classical transmitters and neuropeptides. However, the physiological implications of this co-localization have yet to be elucidated in the vast majority of examples. The application of molecular biological techniques to the study of neurotransmitter receptors (to date mainly in vertebrates) is contributing to our understanding of the evolution of these proteins. Striking similarities in the structure of ligand-gated receptors have been revealed. Thus, although ligand-gated receptors differ markedly in terms of the endogenous ligands they recognize and the ion channels that they gate, the structural similarities suggest a strong evolutionary relationship. Pharmacological differences also exist between receptors that recognize the same neurotransmitter but in different phyla, and this may also be exploited to further the understanding of structure-function relationships for receptors. Thus, for instance, some invertebrate GABA receptors are similar to mammalian GABAA receptors but lack a modulatory site operated by benzodiazepines. Knowledge of the structure and subunit composition of these receptors and comparison with those that have already been elucidated for the mammalian nervous system might indicate the functional importance of certain amino acid residues or receptor subunits. These differences could also be exploited in the development of new agents to control agrochemical pests and parasites of medical importance. The study of the pharmacology of receptor proteins for neurotransmitters in invertebrates, together with the application of biochemical and molecular biological techniques to elucidate the structure of these molecules, is now gathering momentum. For certain receptors, e.g. the nicotinic receptor, we can expect to have fundamental information on the function of this receptor at the molecular level in both invertebrates and vertebrates in the near future.

Immunoglobulins of patients recovering from Yersinia enterocolitica infections exhibit Graves' disease-like activity in human thyroid membranes

Substantial evidence suggests a link between infections with Yersinia enterocolitica (YE) and Graves' disease. We have now examined the sera of 72 patients recovering from YE infection for immunoglobulins that interacted with the TSH receptor in human thyroid membranes. Compared with controls, in concentrations between 1 and 4 mg/mL, patient IgG produced a significant, concentration-dependent inhibition of TSH binding (p less than 0.001) and stimulation of adenylate cyclase activity (p less than 0.005-0.05). Whereas IgG from normal individuals caused no stimulation of adenylate cyclase, IgG from controls caused some concentration-dependent displacement of TSH, as previously reported. However, IgG from convalescents of YE infections was significantly more potent than normal IgG in reducing the binding of TSH to the membrane. Thus, at each examined concentration, YE patients' IgG displaced more TSH than IgG from normal controls. For each milligram per milliliter increment of IgG in the assay, patients' IgG caused a 10.2% inhibition of TSH binding (r -0.90, p less than 0.001), significantly greater than that seen with normal IgG (p less than 0.02). The present studies provide the first demonstration that IgG of patients recovering from YE infections react with the human TSH receptor. The antibodies presumably are produced against the TSH-binding protein present in YE. However, in view of lack of evidence for thyroid dysfunction in the sera of patients recovering from yersiniosis and the presence of TSH-binding proteins in other bacteria, we postulate that infection with YE is neither necessary nor sufficient to cause thyroid autoimmune disease.(ABSTRACT TRUNCATED AT 250 WORDS)

The evolution of hormonal signalling systems

1. A comparative analysis was made of chemosignalling systems responsible for the action of hormones, hormone-like substances, pheromones, etc. in vertebrates--multicellular invertebrates--unicellular eukaryotes. Many common features revealed in structural-functional organization of the above systems give evidence of their evolutionary conservatism. 2. It was shown that some molecular components as well as signal transduction mechanisms similar to those of higher eukaryote hormonal signalling systems are present in such early organisms as bacteria. This allowed a suggestion that the roots of chemosignalling systems are likely to be found in prokaryotes. 3. The evolution of hormonal signalling systems is discussed in terms of current theories of the origin of eukaryotic cell, its organelles and components. A hypothesis is put forward about endosymbiotic genesis of these signal transduction systems in eukaryotes. 4. A possible evolutionary scenario of the formation of hormonocompetent systems is proposed with hormone-sensitive adenylate cyclase complex taken as an example.

Specific binding sites for 125I-labelled human luteinizing hormone (hLH) in microsomal fractions from Drosophila

Microsomal fractions prepared from Drosophila flies and larvae bound 125I-labelled human luteinizing hormone (hLH) in a displaceable manner. Binding increased linearly with increasing microsome concentration, and was dependent on the pH and metal ion concentration of the medium, and on the temperature and duration of incubation. Scatchard plots of 125I-hLH binding demonstrated the presence of two distinct binding sites, one with high affinity and low capacity, the other with low affinity and high capacity. 125I-hLH binding was inhibited in a dose-dependent fashion by partially purified human gonadotropin preparations, but not by other proteins, hormones and peptides. Highly purified human chorionic gonadotropin (hCG) or hLH preparations were much less effective at inhibiting 125I-hLH binding to Drosophila microsomes than partially purified gonadotropins. This was due to the presence of a contaminant present in partially purified commercial hCG preparations. Chromatography of crude hCG preparations clearly resolved the LH-binding inhibitor from hCG. Moreover, whereas hCG (and epidermal growth factor-like factors present in crude hCG preparations) were not bound to CM-Sepharose, the Drosophila LH-binding inhibitor was adsorbed strongly. Fractionation of 125I-hLH tracer on concanavalin A-Sepharose suggested that Drosophila binding sites did not preferentially bind a subfraction of the hormone tracer. These data suggest that high affinity binding sites for hLH/hCG-like molecules are present in Drosophila microsomes.

Specific, high-affinity binding sites for human luteinizing hormone (hLH) and human chorionic gonadotrophin (hCG) in Candida species

The presence of specific binding sites for [125I]-labelled hLH and hCG is described in Candida species. Binding was present in three strains of Candida albicans, and in Candida tropicalis, and was greatest in microsomes, though binding was also present in cytosol fractions. hLH and hCG mutually competed for these binding sites. Other hormones did not bind and did not compete for hLH binding sites. Scatchard plots showed two classes of binding sites, one with high affinity, low capacity and the other with lower affinity, high capacity binding in both microsomes and cytosol. This is the first report of specific binding sites for mammalian peptide hormones in a yeast.

Bidirectional communication between the neuroendocrine and immune systems. Common hormones and hormone receptors

The results reviewed here support a molecular basis for bidirectional communication between the immune and neuroendocrine systems. The main findings can be summarized as follows: First, cells of the immune system can synthesize biologically active neuroendocrine peptide hormones. Second immune cells also possess receptors for many of these peptides. Third, these same neuroendocrine hormones can influence immune function; and fourth, lymphokines can influence neuroendocrine tissues. The interesting conceptual advance is the idea that the immune system may serve as a sensory organ. Thus, the immune system may sense stimuli that are not recognized by the central or peripheral nervous system. These stimuli are termed noncognitive and include bacteria, viruses, tumors and antigens. The recognition of such stimuli is converted into information in the form of lymphokines, monokines, and hormones and a message received by neuroendocrine tissues. On the other hand, nervous system recognition of stimuli can also be converted into chemical signals that can be relayed to immune cells resulting in physiological changes. On this basis, we predict that the pathophysiology associated with infectious agents may be related to the types and amounts of hormones produced by the immune system.

The effects of vasopressin and related peptides on osmoregulation in Amoeba proteus

We describe the effects of arginine-vasopressin (AVP) and five related peptides on the contractile vacuole, the osmoregulatory organelle of the fresh water Amoeba proteus. Arginine-vasopressin, lysine-vasopressin, and SKF 101926, a synthetic antagonist of vasopressin, cause a significant increase in the rate of output of the contractile vacuole. Deamino-vasopressin (dAVP), oxytocin, and arginine-vasotocin have no such activity, although dAVP interferes with the action of AVP when present in equimolar concentration. Relatively high concentrations are required and the effect of active peptides is readily reversible. When the normal, hypotonic medium (a synthetic pond water) is replaced by isotonic sucrose, the action of AVP on the vacuole is abolished. Thus vasopressin is believed to act by increasing permeability of the Amoeba plasma membrane to water.

Neuropeptides: conductors of the immune orchestra

There is increasing evidence for a bidirectional communications system between the immune system and the brain. Many of the substances involved in this communication appear to be neuropeptides. These findings have given biochemical validity to the clinical and epidemiological studies that have suggested that psychosocial factors can modulate the response to infections and neoplasms.