Homocysteine and cysteine - albumin binding in homocystinuria: assessment of cysteine status and implications for glutathione synthesis?

Summary. Measurement of plasma total cysteine rather than free dimeric cystine gives a better indication of cysteine status in homocystinuric patients. This is the result of displacement of cysteine from albumin by homocysteine and is related to the plasma homocysteine concentration. In control subjects the free/bound cyst(e)ine ratio was independent of albumin and total cysteine concentrations. In homocystinuric (HCU) patients both free and total cyst(e)ine values differed significantly from control values (P < 0.001) but whilst free cystine considerably overlapped control values the total cysteine concentrations were almost invariably lower. The possible consequences of this on glutathione synthesis was explored by assay of plasma total glutathione but no evidence for glutathione deficiency was found. Measurement of total cysteine, rather than free cystine, provides a better indication of cysteine status in HCU. Received February 1, 2001 Accepted November 13, 2001     

Sample processing alters glutathione and cysteine values in blood

The accurate assessment of glutathione status of blood is essential for its use as an index of health and aging. A major variable in glutathione analysis is sample processing, and identification of optimal standard conditions is needed. Thus our objective was to evaluate several methods to determine which one yields maximal levels of free and bound glutathione and cyst(e)ine in blood. Reduced glutathione (GSH), glutathione disulfide (GSSG), cysteine (Cys), and cystine were analyzed specifically by an HPLC-dual electrochemical method. The highest GSH levels were found in ultrafiltrates of hemolysates, which were 58% greater than those in acid extracts of whole blood, and accounted for 96% of the free and bound GSH in borohydride-reduced samples; GSSG was undetected. The next highest values were in acid extracts of hemolysates which were 13% greater than in extracts of whole blood; both extracts contained GSH and GSSG. Their GSSG contents expressed in GSH equivalents comprised 7-9% of GSH + GSSG. Cys levels were highest in ultrafiltrates which were 11-fold greater than in acid extracts of whole blood, accounting for 62% of the total cyst(e)ine pool. In summary, the results indicate that ultrafiltration of hemolysates is the blood processing method of choice to obtain maximal values of free and bound GSH and cyst(e)ine.     

Transsulphuration and methylation of homocysteine in control and mutant human fibroblasts

The ability of human skin-fibroblasts in monolayer culture to carry out transsulphuration and remethylation of homocysteine has been tested. The conversion of homocyst(e)ine to cyst(e)ine and methionine was studied in control and mutant cells by incubation for 16 h with l-[³⁵S]homocystine. Labelled cysteic acid and methionine sulphone were found in hydrolysates of oxidized cell proteins. The quantities found were dependent on the time of incubation and were used as a measure of cyst(e)ine and methionine formation, respectively. In control cells, labelled cyst(e)ine and labelled methionine were found. In cystathionine β-synthase-deficient cell lines, labelled cyst(e)ine formation was reduced, while labelled methionine formed was similar to that of controls, indicating the role of transsulphuration in the formation of cyst(e)ine observed in control cells. In a 5,10-methylenetetrahydrofolate reductase-deficient cell line, labelled methionine formation was reduced, indicating the role of N-5-methyltetrahydrofolate-requiring methylation of homocysteine in the formation of methionine observed in control cells.     

Cyst(e)ine Is the Transport Metabolite of Assimilated Sulfur from Bundle-Sheath to Mesophyll Cells in Maize Leaves

The intercellular distribution of the enzymes and metabolites of assimilatory sulfate reduction and glutathione synthesis was analyzed in maize (Zea mays L. cv LG 9) leaves. Mesophyll cells and strands of bundle-sheath cells from second leaves of 11-d-old maize seedlings were obtained by two different mechanical-isolation methods. Cross-contamination of cell preparations was determined using ribulose bisphosphate carboxylase (EC 4.1.1.39) and nitrate reductase (EC 1.6.6.1) as marker enzymes for bundle-sheath and mesophyll cells, respectively. ATP sulfurylase (EC 2.7.7.4) and adenosine 5′-phosphosulfate sulfotransferase activities were detected almost exclusively in the bundle-sheath cells, whereas GSH synthetase (EC 6.3.2.3) and cyst(e)ine, γ-glutamylcysteine, and glutathione were located predominantly in the mesophyll cells. Feeding experiments using [³⁵S]sulfate with intact leaves indicated that cyst(e)ine was the transport metabolite of reduced sulfur from bundle-sheath to mesophyll cells. This result was corroborated by tracer experiments, which showed that isolated bundle-sheath strands fed with [³⁵S]sulfate secreted radioactive cyst(e)ine as the sole thiol into the resuspending medium. The results presented in this paper show that assimilatory sulfate reduction is restricted to the bundle-sheath cells, whereas the formation of glutathione takes place predominantly in the mesophyll cells, with cyst(e)ine functioning as a transport metabolite between the two cell types.     

Cyst(e)ine residues of bovine white-matter proteolipid proteins. Role of disulphides in proteolipid conformation.

Cyst(e)ine residues of bovine white-matter proteolipid proteins were characterized in a highly purified preparation. From a total of 10.6 cyst(e)ine residues/molecule of protein, as determined by performic acid oxidation, 2.5-3 thiol groups were freely accessible to iodoacetamide, iodoacetic acid and 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB), when the proteins were solubilized in chloroform/methanol (C/M) (2:1, v/v). The presence of lipids had no effect on thiol-group exposure. One thiol group available to DTNB in C/M could not be detected when proteolipids were solubilized in the more polar solvent n-butanol. In a C/M solution of purified proteolipid proteins, SDS did not increase the number of reactive thiol groups, but the cleavage of one disulphide bridge made it possible to alkylate six more groups. C.d. and fluorescence studies showed that rupture of this disulphide bond changed the protein conformation, which was reflected in partial loss of helical structure and in a greater exposure to the solvent of at least one tryptophan residue. Cyst(e)ine residues were also characterized in the different components [PLP (principal proteolipid protein), DM20 and LMW (low-Mr proteins)] of the proteolipid preparation. Although the numbers of cyst(e)ine residues in PLP and DM20 were similar, in LMW fewer residues were alkylated under four different experimental conditions. The differences, however, are not simply related to differences in Mr.     

Reduction and fluorescent labeling of cyst(e)ine-containing proteins for subsequent structural analyses

Procedures which allow rapid, quantitative, and selective fluorescent labeling of protein cyst(e)ine residues prior to electrophoresis by reaction with 4-(aminosulfonyl)-7-fluoro-2,1,3-benzoxadiazole (ABD-F) under mild conditions are described. After labeling, the protein(s) of interest is easily monitored throughout electrophoresis and subsequent electroblotting or electroelution procedures. The stoichiometry of labeling and therefore the number of cysteine and/or half-cystine residues can be measured spectrophotometrically or fluorometrically and the derived cyst(e)ine adduct can also be quantitated by amino acid analysis and identified in protein sequencing. N-terminal blockage is not observed under the conditions utilized, nor are any other amino acid side chains modified. The procedures described allow complete, rapid, and facile reduction and alkylation of proteins with simultaneous incorporation of a fluorophore, permitting sensitive detection in subsequent manipulation of the proteins. Quantitative fluorescence prelabeling also allows the generation, purification, and sequencing of peptide fragments containing cyst(e)ine residues for determination of internal sequences and residues involved in disulfide bonds.     

Changes in the N-glycome, glycoproteins with Asn-linked glycans, of Giardia lamblia with differentiation from trophozoites to cysts

Giardia lamblia is present in the intestinal lumen as a binucleate, flagellated trophozoite or a quadranucleate, immotile cyst. Here we used the plant lectin wheat germ agglutinin (WGA), which binds to the disaccharide di-N-acetyl-chitobiose (GlcNAc₂), which is the truncated Asn-linked glycan (N-glycan) of Giardia, to affinity purify the N-glycomes (glycoproteins with N-glycans) of trophozoites and cysts. Fluorescent WGA bound to the perinuclear membranes, peripheral acidified vesicles, and plasma membranes of trophozoites. In contrast, WGA bound strongly to membranes adjacent to the wall of Giardia cysts and less strongly to the endoplasmic reticulum and acidified vesicles. WGA lectin-affinity chromatography dramatically enriched secreted and membrane proteins of Giardia, including proteases and acid phosphatases that retain their activities. With mass spectroscopy, we identified 91 glycopeptides with N-glycans and 194 trophozoite-secreted and membrane proteins, including 42 unique proteins. The Giardia oligosaccharyltransferase, which contains a single catalytic subunit, preferred N glycosylation sites with Thr to those with Ser in vivo but had no preference for flanking amino acids. The most-abundant glycoproteins in the N-glycome of trophozoites were lysosomal enzymes, folding-associated proteins, and unique transmembrane proteins with Cys-, Leu-, or Gly-rich repeats. We identified 157 secreted and membrane proteins in the Giardia cysts, including 20 unique proteins. Compared to trophozoites, cysts were enriched in Gly-rich repeat transmembrane proteins, cyst wall proteins, and unique membrane proteins but had relatively fewer Leu-rich repeat proteins, folding-associated proteins, and unique secreted proteins. In summary, there are major changes in the Giardia N-glycome with the differentiation from trophozoites to cysts.     

Exogenous cysteine and cystine promote cell proliferation in CaCo-2 cells

Abstract. Previous studies have shown that intracellular glutathione, a ubiquitous intracellular thiol, is related to cell proliferation and that cysteine or its disulphide form, cystine, also induces cell proliferation. Cysteine is a thiol containing amino acid and a rate-limiting precursor of glutathione. Therefore, it is still unresolved as to whether the proliferative effect of cysteine or cystine is entirely mediated by a change in the intracellular glutathione status. The objective of this study was to delineate the relationship among cysteine/cystine (thereafter referred to as cyst(e)ine), intracellular glutathione and cell proliferation in the human colon cancer CaCo-2 cell line. CaCo-2 cells were cultured in cyst(e)ine-free Dulbecco's Modified Eagle Medium without serum, and treated with 200 µ m cysteine and/or 200–400 µ m cystine for 24 h. In the presence of DL-buthionine-[S, R]-sulfoximine (BSO), a glutathione synthesis inhibitor, exogenously administered cyst(e)ine did not change the intracellular glutathione content, but increased the intracellular cysteine as well as cystine level. Addition of exogenous cyst(e)ine following 5 m m BSO treatment significantly increased cell proliferation as measured by ³H-thymidine incorporation and protein content. Cell cycle analyses revealed that cyst(e)ine promoted cell progression from the G₁ phase to the S phase. Correspondingly, cyst(e)ine treatment induced expression of cyclin D1 and phosphorylation of retinoblastoma protein (Rb). In conclusion, these data indicate that both cysteine and cystine have proliferative effects in CaCo-2 cells independent of an increase in intracellular glutathione. Induction of cyclin D1, phosphorylation of Rb, and subsequent facilitation of G₁-to-S phase transition were involved in the proliferative effect of exogenous cyst(e)ine.     

Glycogen phosphorylase in Acanthamoeba spp.: determining the role of the enzyme during the encystment process using RNA interference

Acanthamoeba infections are difficult to treat due to often late diagnosis and the lack of effective and specific therapeutic agents. The most important reason for unsuccessful therapy seems to be the existence of a double-wall cyst stage that is highly resistant to the available treatments, causing reinfections. The major components of the Acanthamoeba cyst wall are acid-resistant proteins and cellulose. The latter has been reported to be the major component of the inner cyst wall. It has been demonstrated previously that glycogen is the main source of free glucose for the synthesis of cellulose in Acanthamoeba, partly as glycogen levels fall during the encystment process. In other lower eukaryotes (e.g., Dictyostelium discoideum), glycogen phosphorylase has been reported to be the main tool used for glycogen breakdown in order to maintain the free glucose levels during the encystment process. Therefore, it was hypothesized that the regulation of the key processes involved in the Acanthamoeba encystment may be similar to the previously reported regulation mechanisms in other lower eukaryotes. The catalytic domain of the glycogen phosphorylase was silenced using RNA interference methods, and the effect of this phenomenon was assessed by light and electron microscopy analyses, calcofluor staining, expression zymogram assays, and Northern and Western blot analyses of both small interfering RNA-treated and control cells. The present report establishes the role of glycogen phosphorylase during the encystment process of Acanthamoeba. Moreover, the obtained results demonstrate that the enzyme is required for cyst wall assembly, mainly for the formation of the cell wall inner layer.     

Sulfur-containing Compounds in Lemna perpusilla 6746 Grown at a Range of Sulfate Concentrations

Lemna perpusilla 6746, grown photoautotrophically at a series of sulfate concentrations ranging from 0.32 to 1,000 μm, was labeled to radioisotopic equilibrium with ³⁵SO₄²⁻. Sulfur-containing compounds were isolated and purified from the colonies. Radioactivity in each compound was a measure of the amount of that compound present in the tissue. The following compounds were identified and quantitated: inorganic sulfate, glutathione, homocyst(e)ine, cyst(e)ine, methionine, S-methylmethionine sulfonium, S-adenosylmethionine, S-adenosylhomocysteine, cystathionine, chloroformsoluble (presumed to be sulfolipid), protein cyst(e)ine, and protein methionine. γ-Glutamylcyst(e)ine, erythro- and threo-thiothreonine, and S-methylcysteine were not detected. No volatile ³⁵S compounds were formed during plant growth at 1,000 μm sulfate, nor were significant amounts of ³⁵S compounds excreted into the medium.     

Beziehungen Zwischen Wollertrag Und Cyst(E)Ingehalt Im Blutplasma

Abstract

Durch Veröffentlichungen von Williams angeregt, sind Untersuchugen über die Bezichungen zwischen Wollertrag und Cyst(e)in-Konzentration im Blutplasma von Schafen vorgesehen. Dazu erfolgten zunächst methodische Versuche, bei denen zwei Methoden gut reproduzierbare Analysendaten ergaben. Dabei mußte das austauschbare Bindungsverhalten eines Teiles des freien Cystins im Blutplama mit den Plasmaproteinen und dessen Wirkung auf die Analysenergebnisse berücksichtigt werden. Die beiden Methoden und ihre Anwendung werden unter Beachtung der auf die Analysenergebnisse wirkenden Einflußfaktoren beschrieben, und die Schlußfolgerung gezogen, daß sie für den vorgesehenen Zweck geeignet sind.

RELATIONS BETWEEN WOOL YIELD AND CYST(E)INE CONCENTRATION IN BLOOD PLASMA. 1. DESCRIPTION OF THE PROBLEM AND ANALYSIS

Publications from Williams were the stimulation to study the relations between wool yield and cyst(e)ine concentration in blood plasma. Methodical experiments were carried out and the two methods used each provided clear results. At the same time the reversible binding of a part of the free cystine in the plasma with the plasma proteins and its effect on the analysis results might be considered. After an interpretation of the two methods their use is described taking into consideration the factors influencing the results of the analysis. It is concluded that the two methods are usable for the planned purpose.     

Identification of alpha 2-macroglobulin as a carrier protein for IL-6

In this report we demonstrate that alpha 2-macroglobulin (alpha 2M) is a carrier protein for IL-6. IL-6 was found to bind plasma proteins and an immunoblot analysis revealed that the complex between IL-6 and plasma proteins contains alpha 2M. Furthermore, purified alpha 2M bound IL-6. alpha 2M did not inhibit IL-6 activity or its binding to homologous receptor. IL-6 bound to alpha 2M retained its biologic activity and became resistant to treatment with proteases, although free IL-6 was easily degraded. These findings indicate that alpha 2M plays an important role as a carrier protein for IL-6 in serum and makes IL-6 produced at the local inflammatory site available to lymphocytes, hepatocytes, and hematopoietic stem cells, resulting in the induction of the coordinate systemic host defense reactions, such as immune response, acute phase reaction, and hematopoiesis.     

Changes in nuclear proteins of rat testis cells separated by velocity sedimentation.

The technique of velocity sedimentation at unit gravity has been used to separate rat testis cell suspensions into fractions enriched in particular cell types. Changes in the nuclear proteins from the various fractions have been characterized by polyacrylamide gel electrophoresis, and correlated with the changing morphology of the nucleus during spermatogenesis. The most striking alterations in both protein composition and nuclear morphology occur during spermatid maturation as both histone and non-histone proteins are replaced by highly basic, low molecular weight, spermatidal proteins. This replacement process is accompanied by a quantitative reduction in both histone and non-histone proteins. The synthesis of at least three basic proteins has been identified with late stage spermatids. One of these proteins is a highly basic sperm-specific protein containing high levels of cyst(e)ine and arginine. A second protein synthesized in late stage spermatids is lysine rich, while the third protein contains cyst(e)ine and co-migrates with histone F2a1 on acid-urea polyacrylamide gels. The changes in protein composition of rat testis nuclei after irradiation or hypophysectomy reflect the resulting changes in the cellular composition of the testis. After selective elimination of the germinal cells by irradiation, the electrophoretic pattern of acid-soluble proteins from the testis is very similar to that of somatic tissue. Thus, the cellular specificity of nuclear proteins demonstrated here using cell separation techniques is also apparent following treatments which selectively alter the cellular composition of the testis.     

The oxidation of cyst(e)ine by mast-cell tumour P815 in culture

1. Mast-cell tumour P815 cells oxidize [(35)S]cyst(e)ine to (35)SO(4) (2-). 2. Addition of cysteinesulphinate or sulphite decreases the formation of (35)SO(4) (2-); at the same time [(35)S]cysteinesulphinate or (35)SO(3) (2-) accumulates. 3. Extracts of P815 cells form sulphite from cysteinesulphinate and 2-oxoglutarate. The K(m) for cysteinesulphinate is 6.35mm and that for 2-oxoglutarate is 0.165mm. 4. Extracts oxidize sulphite to sulphate. 5. No formation of hydrogen sulphide from cyst(e)ine was detectable. 6. It is concluded that P815 cells oxidize cyst(e)ine to sulphate solely via cysteinesulphinate and sulphite. 7. The concentration of the enzymes catalysing this sequence is unaltered by several variations in the conditions of growth.     

Data handling for interactive metabolomics: tools for studying the dynamics of metabolome-macromolecule interactions

All published metabolomics studies investigate changes in either absolute or relative quantities of metabolites. However, blood plasma, one of the most commonly studied biofluids for metabolomics applications, is a complex, heterogeneous mixture of lipoproteins, proteins, small organic molecules and ions which together undergo a variety of possible molecular interactions including metal complexation, chemical exchange processes, micellular compartmentation of metabolites, enzyme-mediated biotransformations and small-molecule-macromolecule binding. In particular, many low molecular weight (MW) compounds (including drugs) can exist both ‘free’ in solution and bound to proteins or within organised aggregates of macromolecules. To study the effects of e.g. disease on these interactions we suggest that new approaches are needed. We have developed a technique termed ‘interactive metabolomics’ or i-metabolomics. i-metabolomics can be defined as: “The study of interactions between low MW biochemicals and macromolecules in heterogeneous biosamples such as blood plasma, without pre-selection of the components of interest”. Standard 1D NMR experiments commonly used in metabolomics allow metabolite concentration differences between samples to be investigated because the intensity of each peak depends on the concentration of the compound in question. On the other hand, the instrument can be set-up to measure molecular interactions by monitoring the diffusion coefficients of molecules. According to the Stokes–Einstein equation, the diffusion coefficient of a molecule is inversely proportional to its effective size, as represented by the hydrodynamic radius. Therefore, when low MW compounds are non-covalently bound to proteins, the observed diffusion coefficient for the compound will be intermediate between those of its free and bound forms. By measuring diffusion by NMR, the degree of protein binding can be estimated for either low MW endogenous biochemicals or xenobiotics. This type of experiment is referred to as either Diffusion-Ordered Spectroscopy (DOSY) or Diffusion-Edited Spectroscopy, depending on the type of post-acquisition data processing applied to the spectra. Results presented in this paper demonstrate approaches for the non-selective modelling of metabolite-macromolecule interactions (i-metabolomics), whilst additionally highlighting some of the all too frequently ignored issues associated with interpretation of data derived from profiling of blood plasma.     

Feeding pigs amino acids as protein-bound or in free form influences postprandial concentrations of amino acids,metabolites, and insulin

Dietary proteins need to be digested first while free amino acids (AAs) and small peptides are readily available for absorption and rapidly appear in the blood. The rapid postprandial appearance of dietary AA in the systemic circulation may result in inefficient AA utilisation for protein synthesis of peripheral tissues if other nutrients implicated in AA and protein metabolism are not available at the same time. The objective of this experiment was to compare the postprandial concentrations of plasma AA and other metabolites after the ingestion of a diet that provided AA either as proteins or as free AA and small peptides. Twenty-four male growing pigs (38.8 ± 2.67 kg) fitted with a jugular catheter were assigned to one of three diets that provided AA either in protein form (INT), free AA and small peptides (HYD), or as free AA (FAA). After an overnight fast and initial blood sampling, a small meal was given to each pig followed by serial blood collection for 360 min. Postprandial concentrations of plasma AA, glucose, insulin, and urea were then measured from the collected blood. Non-linear regression was used to summarise the postprandial plasma AA kinetics. Fasting concentrations of urea and some AA were higher (P < 0.05) while postprandial plasma insulin and glucose were lower (P < 0.01) for INT than for HYD and FAA. The area under the curve of plasma concentration after meal distribution was lower for INT for most AAs (P < 0.05), resulting in a flatter curve compared to HYD and FAA. This was the result of the slower appearance of dietary AA in the plasma when proteins are fed instead of free AA and small peptides. The flatter curve may also result from more AAs being metabolised by the intestine and liver when INT was fed. The metabolism of AA of the intestine and liver was higher for HYD than FAA. Providing AA as proteins or as free AA and small peptides affected the postprandial plasma kinetics of AA, urea, insulin, and glucose. Whether the flat kinetics when feeding proteins has a positive or negative effect on AA metabolism still needs to be explored.     

The dialysis of Ochratoxin A (OTA)

Standard dialysis did not result in a decrease of the OTA level in the blood serum of patients regularly treated by dialysis. Therefore, we examined the effect of dialysis on both OTA bound to the blood plasma proteins and free OTA. We carried out an in vivo experiment to determine OTA levels in the serum of patients in the terminal stage of chronic renal insufficiency (CHRI) before and after dialysis and also in the dialysate in which we did not find OTA. OTA bound to blood plasma proteins did not penetrate the dialysis membrane. In contrast, free OTA during an in vitro experiment with the identical dialyzer (as during the in vivo experiment), easily penetrated the same dialysis membrane.     

Glucosylceramide Transferase Activity Is Critical for Encystation and Viable Cyst Production by an Intestinal Protozoan,Giardia lamblia

The production of viable cysts by Giardia is essential for its survival in the environment and for spreading the infection via contaminated food and water. The hallmark of cyst production (also known as encystation) is the biogenesis of encystation-specific vesicles (ESVs) that transport cyst wall proteins to the plasma membrane of the trophozoite before laying down the protective cyst wall. However, the molecules that regulate ESV biogenesis and maintain cyst viability have never before been identified. Here, we report that giardial glucosylceramide transferase-1 (gGlcT1), an enzyme of sphingolipid biosynthesis, plays a key role in ESV biogenesis and maintaining cyst viability. We find that overexpression of this enzyme induced the formation of aggregated/enlarged ESVs and generated clustered cysts with reduced viability. The silencing of gGlcT1 synthesis by antisense morpholino oligonucleotide abolished ESV production and generated mostly nonviable cysts. Interestingly, when gGlcT1-overexpressed Giardia was transfected with anti-gGlcT1 morpholino, the enzyme activity, vesicle biogenesis, and cyst viability returned to normal, suggesting that the regulated expression of gGlcT1 is important for encystation and viable cyst production. Furthermore, the overexpression of gGlcT1 increased the influx of membrane lipids and fatty acids without altering the fluidity of plasma membranes, indicating that the expression of gGlcT1 activity is linked to lipid internalization and maintaining the overall lipid balance in this parasite. Taken together, our results suggest that gGlcT1 is a key player of ESV biogenesis and cyst viability and therefore could be targeted for developing new anti-giardial therapies.     

Sulphur-aromatic interactions in proteins

The geometry of sulphur-aromatic interactions in globular proteins has been analysed using crystallographic data derived from 36 proteins, solved to resolutions of 2 Å or better. About half of all sulphur atoms from cyst(e)ine and methionine residues are in contact ( 6 Å from ring centroid) with an aromatic ring (phenylalanine, tyrosine or tryptophan). Compared to carbon and nitrogen atoms the interacting sulphur atoms express an affinity towards the edge of the aromatic rings, and avoid the region above the ring in the vicinity of the π-electrons. This preference is similar to that previously found for oxygen atoms around phenylalanine rings, and may be electrostatic in origin.

Sulfw-aromatic interaction Protein Side-chain contact Cyst(e)ine Methionine Packing     

Phosphorylation, protein kinases and ADPKD

Autosomal dominant polycystic kidney disease (ADPKD) is a genetic disease characterized by renal cyst formation and caused by mutations in the PKD1 and PKD2 genes, which encode polycystin-1(PC-1) and -2 (PC-2) proteins, respectively. PC-1 is a large plasma membrane receptor involved in the regulation of several biological functions and signaling pathways including the Wnt cascade, AP-1, PI3kinase/Akt, GSK3β, STAT6, Calcineurin/NFAT and the ERK and mTOR cascades. PC-2 is a calcium channel of the TRP family. The two proteins form a functional complex and prevent cyst formation, but the precise mechanism(s) involved remains unknown. This article is part of a Special Issue entitled: Polycystic Kidney Disease.