Ultracytochemistry of cholera-toxin binding sites in ciliary processes

Neutralization of cholera toxin by Rosaceae family plant extracts

BACKGROUND

Cholera is one of the most deadly diarrheal diseases that require new treatments. We investigated the neutralization of cholera toxin by five plant extracts obtained from the Rosaceae family that have been traditionally used in Poland to treat diarrhea (of unknown origin).

METHODS

Hot water extracts were prepared from the dried plant materials and lyophilized before phytochemical analysis and assessment of antimicrobial activity using microdilution assays. The ability of the plant extracts to neutralize cholera toxin was analyzed by measurement of cAMP levels in cell cultures, enzyme-linked immunosorbent assay and electrophoresis, as well as flow cytometry and fluorescence microscopy studies of fluorescent-labeled cholera toxins with cultured human fibroblasts.

RESULTS

The antimicrobial assays displayed modest bacteriostatic potentials. We found that the plant extracts modulate the effects of cholera toxin on intracellular cAMP levels. Three plant extracts (Agrimonia eupatoria L., Rubus fruticosus L., Fragaria vesca L.) suppressed the binding of subunit B of cholera toxin to the cell surface and immobilized ganglioside GM1 while two others (Rubus idaeus L., Rosa.canina L.) interfered with the toxin internalization process.

CONCLUSIONS

The traditional application of the Rosaceae plant infusions for diarrhea appears relevant to cholera, slowing the growth of pathogenic bacteria and either inhibiting the binding of cholera toxin to receptors or blocking toxin internalization. The analyzed plant extracts are potential complements to standard antibiotic treatment and Oral Rehydration Therapy for the treatment of cholera.

Endotoxins in cholera and pertussis toxins interfere with in vivo responses to these agents in the albino rabbit eye

Intravitreal injections of cholera or pertussis toxin (CTX or PTX, 0.5-1 microgram/eye) decreased intraocular pressure (IOP) up to 50% in the albino rabbit eye, which lasted up to six days. Both toxins were active on G-proteins as determined by in vitro and in vivo effects on ciliary process adenylyl cyclase activity and by ADP ribosylation of G-protein alpha-subunits with 32P-NAD. However, forty-two hours after toxin injection aqueous humor proteins increased from control levels of 0.8-1.2 mg/ml to 8-25 mg/ml. Both toxins contained 1-3 parts per thousand endotoxin sufficient to cause the IOP and aqueous humor protein responses observed. We conclude that the in vivo responses to intraocular CTX or PTX obtained from commercial sources may not provide unequivocal evidence for the role(s) of G-proteins in aqueous humor dynamics, and must be interpreted with caution.

Role of G-proteins in ciliary process adenylyl cyclase responses of the albino rabbit eye

After intravitreal injections of cholera or pertussis toxin (CTX or PTX, 0.5 -1 microgram/eye) into the albino rabbit eye, the in vitro responses of ciliary process adenylyl cyclase (AC) to isoproterenol, vasoactive intestinal peptide (VIP), and forskolin (FSK) were increased 21-40% for PTX, but for CTX-injected eyes AC responses to fluoroaluminate, VIP and FSK decreased 70-50%. The increased responses after PTX suggests that this toxin blocked an inhibitory Gi control of AC that is present in the control tissue. However, prolonged (> 24 hr) in vivo exposure to CTX appears to down-regulate the AC enzyme. In contrast to the in vivo findings, AC responsiveness was unaffected by PTX pre-treatment of membranes in vitro, while CTX pre-treatment increased basal activity (+600%), and the FSK response (+30%), but decreased responsiveness to fluoroaluminate, VIP and isoproterenol by 88-56%. Treatment of ciliary process membranes with 32P-NAD and CTX or PTX followed by SDS-PAGE autoradiography of labelled proteins gave two bands for the G-protein alpha-subunits of Gs (45, 56 kDa) and one broad band centered at 40 kDa for Gi-type subunits respectively. Western blots using specific antibodies showed the presence of Gi type I or III, but no detectable Gi type II or Go in rabbit ciliary processes. We conclude that the changes in adenylyl cyclase enzyme responses after intraocular CTX or PTX may not correlate with cAMP levels and intraocular pressure effects. However, the in vitro biochemical data on AC responses and on G-proteins provide evidence for dual regulation of ciliary process AC by activating and inhibitory G-proteins.

Localization of the GM1 ganglioside in the vestibular system using cholera toxin

Cholera toxin is an ubiquitous activator of intracellular adenylate cyclase and is divided in two major components: A and B. The B-component consists of several subunits that specifically bind to the external cell membrane. The receptor for the toxin, the GM1 ganglioside, is concentrated in nervous tissues. The B subunit of the cholera toxin, conjugated to different molecules (i.e., choleragenoid) is therefore a sensitive anatomical tracer and has been used to detect the presence of GM1 in mammalian tissues. Using choleragenoid, unlabeled and labeled with FITC, we have determined the distribution of the GM1 ganglioside in the vestibular system of the chinchilla. Vestibular tissues were fixed in 4% paraformaldehyde in phosphate buffer, decalcified in 10% EDTA and prepared as either whole-mount, surface-preparations, or for radial cryosections. Positive control tissue consisted of binding to normal brain tissues. Negative controls consisted of several treatments: masking of the GM1 receptors with unlabeled choleragenoid, tissue extraction of GM1 using ethanol, and preabsorbing the choleragenoid with bovine GM1. In addition, to exclude staining of glycoproteins that may have a carbohydrate structure similar to GM1, tissues were digested with trypsin prior to choleragenoid exposure. In the vestibular system, a strongly positive reaction was observed in: the sensory stereocilia and supporting cells of the maculae and cristae, epithelial cells of the planum semilunatum, and polygonal cells of the semicircular canal. Positive but less strong reactivity was observed in the sensory cell body of maculae and cristae, nerve fibers, epithelial cells of utricle and ampulla walls and flattened epithelial cells of the semicircular canals. No reactivity was present in the supporting connective tissue cells and fibrils, blood vessels, gelatinous cupula of the cristae ampullaris and statoconial membranes. Brain tissue showed strong choleragenoid reactivity. The negative controls showed no or greatly reduced reactivity to choleragenoid. Trypsin digestion did not decrease reactivity to choleragenoid.

Radioautographic localization of 125I-atrial natriuretic factor (ANF) in rat tissues

Rats were injected either with synthetic 125I-Arg 101-Tyr 126 atrial natriuretic factor (ANF) or with 125I-ANF together with an excess of cold Arg 101-Tyr 126 ANF. Binding sites in various tissues were accepted depending on two criteria: displacement of radioactivity by cold ANF and absence of localization of silver grains on putative target cells in the presence of cold ANF. Binding sites were localized on zona glomerulosa cells and on adrenergic and noradrenergic cells of adrenal medulla, on hepatocytes, on the base of mature epithelial cells of villi in the small intestine, on smooth muscle cells of the muscularis layer of the colon and on the base of epithelial cells of the ciliary bodies. In addition, binding sites were localized in the vasculature of kidney, adrenal cortex, lung and liver. Binding sites were particularly numerous on renal glomerular endothelial cells. These results indicate that ANF may have important hemodynamic effects in kidney, lung, liver and adrenal cortex, may regulate water and ion transport in small intestine and ciliary bodies and may have metabolic effects in the liver. The presence of binding sites on the zona glomerulosa is in agreement with the important inhibitory effect of the peptide on aldosterone secretion.

Human ciliary epithelia in monolayer culture

The ciliary epithelia of human (one to 12 months old) ciliary processes were isolated by trypsin and EDTA, cultured in Dulbecco's Modified Eagle Medium (DMEM) with 5% fetal calf serum and examined by phase and electron microscopy. The primary cultures were maintained for three to four months. Only a few non-pigmented epithelia adhered and none of them proliferated. After the first passage the culture seemed to consist of only the pigmented epithelia. Most cells were densely pigmented at first, then became less pigmented during successive proliferations. Half of the cells remained densely pigmented after the first subculture, another half remained less pigmented. The cells started to lose their pigment granules at four to six weeks in culture. After three months of culture, the cell sheets became entirely unpigmented. In thin section, most of the pigment granules in the cells at two weeks in culture were pre-melanosomes, and half of them were at the earliest stage of pre-melanosomes. Monolayer cells possessed basement membranes. At 14 weeks in culture, most cells established an apparent polarity, contained well-developed Golgi apparatus and rough endoplasmic reticulum and intermediate filaments, but no pigment granules. A bundle of intermediate filaments was found in the perinuclear cytoplasm. Multilayer cells presented a typical apex-to-apex and base-to-base configuration , and the extracellular material was detected only in the base-to-base intercellular spaces. Our culture system provided differentiated cells derived from the pigment epithelia of human ciliary processes.

A major pathway for the regulation of intraocular pressure

There has been a suspicion on the part of many clinicians and research scientists that intraocular pressure can be regulated by neural and/or humoral influences upon the rate of aqueous humor formation. It has been difficult, if not impossible, to separate specific influences of the central nervous system upon intraocular pressure from vascular induced or other secondary alterations. The past two decades have witnesses a great deal of study of the role of the adrenergic nervous system upon the regulation of intraocular pressure. From the investigations it is possible to formulate an integrated concept that can place years of work and speculation on a firm molecular foundation. The secretory tissue of the eye, the ciliary processes, contain an enzyme receptor complex, comprised by receptor complex, comprised by receptor bound membrane proteins, the catalytic moiety of the enzyme, a guanyl nucleotide regulatory protein (or N protein) and other features. The enzyme can be activated by well known neurohumoral or humoral agents that consist of catecholamines, glycoprotein hormones produced by the hypothalamic pituitary axis, and other related compounds, including placental gonadotropin. These compounds cause the ciliary epithelia to produce cyclic AMP at an accelerated rate. Cyclic AMP, as a second messenger, causes, either directly or indirectly, a decrease in the rate of aqueous humor formation that may be modulated by cofactors. Clinical syndromes fit the experimental data so that an integrated explanation can be given for the reduced intraocular pressure witnessed under certain central nervous system and adrenergic influences. The molecular biology of this concept provides important leads for future investigations that bear directly both upon the regulation of intraocular pressure and upon glaucoma.