Human iPSC-derived hepatocytes in 2D and 3D suspension culture for cryopreservation and in vitro toxicity studies.

Hepatocytes are of special interest in biomedical research for disease modelling, drug screening and in vitro toxicology. Human induced pluripotent stem cell (hiPSC)-derived hepatocytes could complement primary human hepatocytes due to their capability for large-scale expansion. In this study, we present an optimized protocol for the generation of hepatocyte-like cells (HLCs) from hiPSC in monolayer (2D) and suspension culture (3D) for production of organoids. A protocol was initially optimized in 2D using a gene edited CYP3A4 Nanoluciferase reporter hiPSC line for monitoring the maturity of HLCs and cryopreservation of definitive endoderm (DE) cells. The protocol was optimized for microwell cultures for high-throughput screening to allow for a sensitive and fast readout of drug toxicity. To meet the increasing demand of hepatic cells in biomedical research, the differentiation process was furthermore translated to scalable suspension-based bioreactors for establishment of hepatic organoids. In pilot studies, the technical settings have been optimized by adjusting the initial seeding density, rotation speed, inoculation time, and medium viscosity to produce homogeneous hepatic organoids and to maximize the biomass yield (230 organoids/ml). To speed up the production process, cryopreservation approaches for the controlled freezing of organoids were analysed with respect to cell recovery and marker expression. The results showed that cryopreserved organoids maintained their phenotype only when derived from hepatocyte progenitors (HPs) at day 8 but not from more mature stages. The establishment of robust protocols for the production of large batches of hepatocytes and hepatic organoids could substantially boost their use in biomedical and toxicology studies.

Serotonin 2B Receptor by Interacting with NMDA Receptor and CIPP Protein Complex May Control Structural Plasticity at Glutamatergic Synapses.

The serotonin 2B (5-HT2B) receptor coupled to Gq-protein contributes to the control of neuronal excitability and is implicated in various psychiatric disorders. The mechanisms underlying its brain function are not fully described. Using peptide affinity chromatography combined with mass spectrometry, we found that the PDZ binding motif of the 5-HT2B receptor located at its C-terminal end interacts with the scaffolding protein channel interacting PDZ protein (CIPP). We then showed, in COS-7 cells, that the association of the 5-HT2B receptor to CIPP enhanced receptor-operated inositol phosphate (IP) production without affecting its cell surface and intracellular levels. Co-immunoprecipitation experiments revealed that CIPP, the 5-HT2B receptor, and the NR1 subunit of the NMDA receptor form a macromolecular complex. CIPP increased 5-HT2B receptor clustering at the surface of primary cultured hippocampal neurons and prevented receptor dispersion following agonist stimulation, thus potentiating IP production and intracellular calcium mobilization in dendrites. CIPP or 5-HT2B receptor stimulation in turn dispersed NR1 clusters colocalized with 5-HT2B receptors and increased the density and maturation of dendritic spines. Collectively, our results suggest that the 5-HT2B receptor, the NMDA receptor, and CIPP may form a signaling platform by which serotonin can influence structural plasticity of excitatory glutamatergic synapses.

Filling the drug discovery gap: is high-content screening the missing link?

In recent years, questions about the sustainability of the current drug discovery process have triggered a revival of interest in phenotypic drug discovery approaches. This trend has clearly been amplified by the emergence of multiple cell-based assay technologies enabling a higher degree of translatability between in vitro conditions and physio-pathological situations, including induced pluripotent stem cells, three-dimensional models, co-culture and organ-on-a-chip systems, complemented by advances in gene editing technologies. Progress in High-Content Screening technology has also contributed to the recent excitement for phenotypic drug discovery approaches, bringing image-capture and processing, and data-analysis, to a level of content and throughput fully compatible with large scale drug discovery efforts. Nevertheless, implementation of HCS in discovery projects must be carefully considered, to ensure optimal performance and the generation of relevant data to enable the discovery of first-in-class medicines.

Increased DNA methylation in the suicide brain.

Clinical studies find that childhood adversity and stressful life events in adulthood increase the risk for major depression and for suicide. The predispositions to either major depression or suicide are thought to depend on genetic risk factors or epigenetic effects. We investigated DNA methylation signatures postmortem in brains of suicides with diagnosis of major depressive disorder. DNA methylation levels were determined at single C-phosphate-G (CpG) resolution sites within ventral prefrontal cortex of 53 suicides and nonpsychiatric controls, aged 16 to 89 years. We found that DNA methylation increases throughout the lifespan. Suicides showed an 8-fold greater number of methylated CpG sites relative to controls (P < 2.2 x 10(-16)), with greater DNA methylation changes over and above the increased methylation observed in normal aging. This increased DNA methylation may be a significant contributor to the neuropathology and psychopathology underlying the risk of suicide in depression.

Los estudíos clínicos han encontrado que la niñez adversa y los acontecimientos vitales estresantes en la adultez aumentan el riesgo para la depresión mayor y para el suícídío. Se piensa que la predísposición tanto para la depresión como para el suicidio depende de factores de ríesgo genético o de efectos epígenéticos. Se investigaron los signos de metilación del ADN en cerebros postmortem de pacíentes suícídas con díagnóstíco de depresíón mayor. Los níveles de metílacíón del ADN se determínaron en resolución única de Cítosinafosfato-Guanina (CfG) en la corteza prefrontal ventral de 53 pacíentes suicidas y de controles no psiquíátricos con edades entre 16 y 89 años. Se encontró que la metílación del ADN aumenta a lo largo de la vída. Los pacientes suícidas mostraron un número 8 veces mayor de sitios CfG metílados en relacíón con los controles (p<2,2x10-16), con cambíos mayores en la metilación del ADN, por sobre lo observado en el envejecímíento normal. Este aumento en la metílación del ADN puede contribuir de manera signifícativa a la neuropatología y a la psicopatología que están a la base del riesgo de suicidio en la depresión.

D'après des études cliniques, le malheur dans l'enfance et les événements de vie stressant à l'âge adulte augmentent le risque de dépression caractérisée (majeure) et de suicide. Les prédispositions à la dépression caractérisée ou au suicide dépendent probablement de facteurs de risque génétiques ou d'effets épigénétiques. Nous avons analysé des signatures postmortem de la méthylation de l'ADN dans des cerveaux de suicidés diagnostiqués comme ayant eu un épisode dépressif caractérisé. Nous avons déterminé les taux de méthylation de l'ADN avec une résolution CpG (C-phosphate-G) à site unique dans le cortex préfrontal ventral de 53 suicidés et témoins non psychiatriques âgés de 16 à 89 ans. Nous avons trouvé que la méthylation de l'ADN augmente tout au long de la vie. Le nombre de sites CpG méthylés étaient multipliés par 8 chez les suicidés par rapport aux témoins (p<2,2x10-16), avec des changements de méthylation de l'ADN plus importants, allant au-delà de l'accroissement de la méthylation observé dans le vieillissement normal. Cette augmentation de la méthylation de l'ADN peut participer significativement à la neuropathologie et à la psychopathologie qui sous-tend le risque de suicide dans la dépression.

Calcineurin interacts with the serotonin transporter C-terminus to modulate its plasma membrane expression and serotonin uptake.

Homeostasis of serotonergic transmission critically depends on the rate of serotonin reuptake via its plasma membrane transporter (SERT). SERT activity is tightly regulated by multiple mechanisms, including physical association with intracellular proteins and post-translational modifications, such as phosphorylation, but these mechanisms remain partially understood. Here, we show that SERT C-terminal domain recruits both the catalytic and regulatory subunits of the Ca(2+)-activated protein phosphatase calcineurin (CaN) and that the physical association of SERT with CaN is promoted by CaN activity. Coexpression of constitutively active CaN with SERT increases SERT cell surface expression and 5-HT uptake in HEK-293 cells. It also prevents the reduction of 5-HT uptake induced by an acute treatment of cells with the protein kinase C activator ß-PMA and concomitantly decreases PMA-elicited SERT phosphorylation. In addition, constitutive activation of CaN in vivo favors 5-HT uptake in the adult mouse brain, whereas CaN inhibition reduces cerebral 5-HT uptake. Constitutive activation of CaN also decreases immobility in the forced swim test, indicative of an antidepressant-like effect of CaN. These results identify CaN as an important regulator of SERT activity in the adult brain and provide a novel molecular substrate of clinical interest for the understanding of increased risk of mood disorders in transplanted patients treated with immunosuppressive CaN inhibitors.

S32212, a novel serotonin type 2C receptor inverse agonist/α2-adrenoceptor antagonist and potential antidepressant: I. A mechanistic characterization.

Although most antidepressants suppress serotonin (5-HT) and/or noradrenaline reuptake, blockade of 5-HT(2C) receptors and α(2)-adrenoceptors likewise enhances monoaminergic transmission. These sites are targeted by the urea derivative N- [4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-1,2-dihydro-3-H-benzo[e]indole-3-carboxamide (S32212). S32212 was devoid of affinity for monoamine reuptake sites, yet displayed pronounced affinity (pK(i), 8.2) for constitutively active human 5-HT(2CINI) (h5-HT(2CINI)) receptors, behaving as an inverse agonist in reducing basal Gα(q) activation, [(3)H]inositol-phosphate production, and the spontaneous association of h5-HT(2CINI)-Renilla luciferase receptors with ß-arrestin2-yellow fluorescent protein. Furthermore, upon 18-h pretreatment, S32212 enhanced the plasma membrane expression of h5-HT(2CINI) receptors as visualized by confocal microscopy and quantified by enzyme-linked immunosorbent assay. Its actions were prevented by the neutral antagonist 6-chloro-5-methyl-N-[6-(2-methylpyridin-3-yloxy)pyridin-3-yl]indoline-1-carboxamide (SB242,084), which also impeded the induction by long-term exposure to S32212 of otherwise absent Ca(2+) mobilization in mouse cortical neurones. In vivo, S32212 blunted the inhibitory influence of the 5-HT(2C) agonist 2-(3-chlorobenzyloxy)-6-(1-piperazinyl)pyrazine (CP809,101) on ventrotegmental dopaminergic neurones. S32212 also blocked 5-HT-induced Gα(q) and phospholipase C activation at the h5-HT(2A) and, less potently, h5-HT(2B) receptors and suppressed the discriminative stimulus properties of the 5-HT(2A) agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane in rats. S32212 manifested marked affinity for human α(2A)- (pK(i) 7.2), α(2B)- (pK(i) 8.2), and α(2C)- (pK(i) 7.4) adrenoceptors, at which it abolished noradrenaline-induced recruitment of Gα(i3), Gα(o), adenylyl cyclase, and extracellular-regulated kinase1/2. Moreover, S32212 dose-dependently abolished the discriminative stimulus effects of the α(2)-adrenoceptor agonist (S)-spiro[(1-oxa-2-amino-3-azacyclopent-2-ene)-4,2'-(1',2',3',4'-tetrahydronaphthalene)] (S18616). Finally, S32212 displayed negligible affinity for α(1A)-adrenoceptors, histamine H(1) receptors, and muscarinic M(1) receptors. In conclusion, S32212 behaves as an inverse agonist at h5-HT(2C) receptors and as an antagonist at human α(2)-adrenoceptors (and h5-HT(2A) receptors). Its promising profile in preclinical models potentially relevant to the treatment of depression is described in J Pharmacol Exp Ther 340:765-780, 2012.

MethylomeDB: a database of DNA methylation profiles of the brain.

MethylomeDB (http://epigenomics.columbia.edu/methylomedb/index.html) is a new database containing genome-wide brain DNA methylation profiles. DNA methylation is an important epigenetic mark in the mammalian brain. In human studies, aberrant DNA methylation alterations have been associated with various neurodevelopmental and neuropsychiatric disorders such as schizophrenia, and depression. In this database, we present methylation profiles of carefully selected non-psychiatric control, schizophrenia, and depression samples. We also include data on one mouse forebrain sample specimen to allow for cross-species comparisons. In addition to our DNA methylation data generated in-house, we have and will continue to include published DNA methylation data from other research groups with the focus on brain development and function. Users can view the methylation data at single-CpG resolution with the option of wiggle and microarray formats. They can also download methylation data for individual samples. MethylomeDB offers an important resource for research into brain function and behavior. It provides the first source of comprehensive brain methylome data, encompassing whole-genome DNA methylation profiles of human and mouse brain specimens that facilitate cross-species comparative epigenomic investigations, as well as investigations of schizophrenia and depression methylomes.

Role of CpG context and content in evolutionary signatures of brain DNA methylation.

DNA methylation is essential in brain function and behavior; therefore, understanding the role of DNA methylation in brain-based disorders begins with the study of DNA methylation profiles in normal brain. Determining the patterns and scale of methylation conservation and alteration in an evolutionary context enables the design of focused but effective methylation studies of disease states. We applied an enzymatic-based approach, Methylation Mapping Analysis by Paired-end Sequencing (Methyl-MAPS), which utilizes second-generation sequencing technology to provide an unbiased representation of genome-wide DNA methylation profiles of human and mouse brains. In this large-scale study, we assayed CpG methylation in cerebral cortex of neurologically and psychiatrically normal human postmortem specimens, as well as mouse forebrain specimens. Cross-species human-mouse DNA methylation conservation analysis shows that DNA methylation is not correlated with sequence conservation. Instead, greater DNA methylation conservation is correlated with increasing CpG density. In addition to CpG density, these data show that genomic context is a critical factor in DNA methylation conservation and alteration signatures throughout mammalian brain evolution. We identify key genomic features that can be targeted for identification of epigenetic loci that may be developmentally and evolutionarily conserved and wherein aberrations in DNA methylation patterns can confer risk for disease.

The melatonergic agonist and clinically active antidepressant, agomelatine, is a neutral antagonist at 5-HT(2C) receptors.

The novel antidepressant, agomelatine, behaves as an agonist at melatonergic receptors, and as an antagonist at edited, human serotonin2C(VSV) receptors [h5-HT2C(VSV)Rs]. However, its actions at constitutively active 5-HT2CRs have yet to be characterized, an issue addressed herein. At unedited h5-HT2C(INI)Rs expressed in HEK-293 cells, 5-HT enhanced [35S]GTPγS binding to Gαq, whereas the inverse agonists SB206,553 and S32006 inhibited binding and, by analogy to the neutral antagonist, SB242,084, agomelatine exerted no effect alone. Mirroring these observations, 5-HT stimulated, whereas SB206,553 and S32006 inhibited, [3H]inositol phosphate formation. Both the agonist actions of 5-HT and the inverse agonist actions of SB206,553 and S32006 were abolished by agomelatine and SB242,084. As demonstrated by bioluminescence resonance energy transfer, 5-HT enhanced, whereas SB206,553 and S32006 decreased, association of 'h5-HT2C(INI)-Rluc-tagged' receptors with yellow-fluorescence-protein-coupled ß-arrestin2. These actions of 5-HT, SB206,553 and S32006 were prevented by agomelatine and SB242,084 were ineffective alone. As shown by ELISA and confocal microscopy, prolonged (18 h) exposure to SB206,553 or S32006 enhanced cell surface expression of N-terminal Flag-tagged h5-HT2C(INI)Rs: these effects were blocked by agomelatine and SB242,084, which were inactive alone. Finally, following pre-exposure to SB206,553 or S32006 for 18 h, 5-HT triggered 5-HT2CR-mediated elevations in cytosolic Ca2+ in primary cultures of mice cortical neurons. Agomelatine and SB242,084, inactive alone, prevented these actions of SB206,553 and S32006. In conclusion, agomelatine behaves as a neutral antagonist at constitutively active h5-HT2C(INI)Rs and native, cortical 5-HT2CRs. It will be of interest to determine whether the neutral antagonist properties of agomelatine are related to its favourable clinical profile of antidepressant properties with few side-effects and no discontinuation syndrome.

Genome-wide divergence of DNA methylation marks in cerebral and cerebellar cortices.

BACKGROUND: Emerging evidence suggests that DNA methylation plays an expansive role in the central nervous system (CNS). Large-scale whole genome DNA methylation profiling of the normal human brain offers tremendous potential in understanding the role of DNA methylation in brain development and function. METHODOLOGY/SIGNIFICANT FINDINGS: Using methylation-sensitive SNP chip analysis (MSNP), we performed whole genome DNA methylation profiling of the prefrontal, occipital, and temporal regions of cerebral cortex, as well as cerebellum. These data provide an unbiased representation of CpG sites comprising 377,509 CpG dinucleotides within both the genic and intergenic euchromatic region of the genome. Our large-scale genome DNA methylation profiling reveals that the prefrontal, occipital, and temporal regions of the cerebral cortex compared to cerebellum have markedly different DNA methylation signatures, with the cerebral cortex being hypermethylated and cerebellum being hypomethylated. Such differences were observed in distinct genomic regions, including genes involved in CNS function. The MSNP data were validated for a subset of these genes, by performing bisulfite cloning and sequencing and confirming that prefrontal, occipital, and temporal cortices are significantly more methylated as compared to the cerebellum. CONCLUSIONS: These findings are consistent with known developmental differences in nucleosome repeat lengths in cerebral and cerebellar cortices, with cerebrum exhibiting shorter repeat lengths than cerebellum. Our observed differences in DNA methylation profiles in these regions underscores the potential role of DNA methylation in chromatin structure and organization in CNS, reflecting functional specialization within cortical regions.

Chromatin and sequence features that define the fine and gross structure of genomic methylation patterns.

Abnormalities of genomic methylation patterns are lethal or cause disease, but the cues that normally designate CpG dinucleotides for methylation are poorly understood. We have developed a new method of methylation profiling that has single-CpG resolution and can address the methylation status of repeated sequences. We have used this method to determine the methylation status of >275 million CpG sites in human and mouse DNA from breast and brain tissues. Methylation density at most sequences was found to increase linearly with CpG density and to fall sharply at very high CpG densities, but transposons remained densely methylated even at higher CpG densities. The presence of histone H2A.Z and histone H3 di- or trimethylated at lysine 4 correlated strongly with unmethylated DNA and occurred primarily at promoter regions. We conclude that methylation is the default state of most CpG dinucleotides in the mammalian genome and that a combination of local dinucleotide frequencies, the interaction of repeated sequences, and the presence or absence of histone variants or modifications shields a population of CpG sites (most of which are in and around promoters) from DNA methyltransferases that lack intrinsic sequence specificity.

Inverse agonist and neutral antagonist actions of antidepressants at recombinant and native 5-hydroxytryptamine2C receptors: differential modulation of cell surface expression and signal transduction.

Despite the importance of 5-hydroxytryptamine (5-HT)(2C) (serotonin) receptors in the control of depressive states, actions of antidepressants at these receptors remain poorly characterized. This issue was addressed both in human embryonic kidney (HEK)-293 cells coexpressing unedited human 5-HT(2CINI) receptors and Galpha(q) protein and in cultured mouse cortical neurons. Indicative of constitutive activity, the inverse agonist SB206,553 decreased basal inositol phosphate (IP) production in HEK-293 cells. The tetracyclic antidepressants mirtazapine and mianserin likewise suppressed basal IP formation. Conversely, the tricyclics amitriptyline and clomipramine, the m-chlorophenylpiperazine derivatives trazodone and nefazodone, and the 5-HT reuptake inhibitors fluoxetine and citalopram were inactive alone, although they blocked 5-HT-induced IP production. Inverse agonist actions of 5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2,3-f]indole (SB206,553) and mirtazapine were abolished by the neutral antagonist 6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy)pyridin-3-ylcarbamoyl]indoline (SB242,084), which was inactive alone. As assessed by confocal microscopy and enzyme-linked immunosorbent assay, prolonged treatment of HEK-293 cells with SB206,553, mirtazapine, or mianserin, but not the other antidepressants, enhanced cell surface expression of 5-HT(2C) receptors: 5-HT-induced IP production was also increased, and both these actions were blocked by SB242,084. Cortical neurons were shown by reverse transcription-polymerase chain reaction to predominantly express constitutively active 5-HT(2C) receptor isoforms. Prolonged pretreatment with SB206,553 or mirtazapine triggered an otherwise absent 5-HT-induced elevation in cytosolic Ca(2+) concentrations. SB242,084, which was inactive alone, abolished these effects of SB206,553 and mirtazapine. In conclusion, the tetracyclic antidepressants mirtazapine and mianserin, but not other clinically established antidepressants, suppress constitutive activity at recombinant and native 5-HT(2C) receptors. The clinical significance of inverse agonist versus neutral antagonist properties both during and after drug administration will be of interest to elucidate.

PACAP type I receptor transactivation is essential for IGF-1 receptor signalling and antiapoptotic activity in neurons.

Insulin-like growth factor-1 (IGF-1) and pituitary adenylyl cyclase activating polypeptide (PACAP) are both potent neurotrophic and antiapoptotic factors, which exert their effects via phosphorylation cascades initiated by tyrosine kinase and G-protein-coupled receptors, respectively. Here, we have adapted a recently described phosphoproteomic approach to neuronal cultures to characterize the phosphoproteomes generated by these neurotrophic factors. Unexpectedly, IGF-1 and PACAP increased the phosphorylation state of a common set of proteins in neurons. Using PACAP type 1 receptor (PAC1R) null mice, we showed that IGF-1 transactivated PAC1Rs constitutively associated with IGF-1 receptors. This effect was mediated by Src family kinases, which induced PAC1R phosphorylation on tyrosine residues. PAC1R transactivation was responsible for a large fraction of the IGF-1-associated phosphoproteome and played a critical role in the antiapoptotic activity of IGF-1. Hence, in contrast to the general opinion that the trophic activity of IGF-1 is solely mediated by tyrosine kinase receptor-associated signalling, we show that it involves a more complex signalling network dependent on the PAC1 Gs-protein-coupled receptor in neurons.

Molecular and functional characterization of proteins interacting with the C-terminal domains of 5-HT2 receptors: emergence of 5-HT2 "receptosomes".

Many cellular functions are carried out by multiprotein complexes. The last five years of research have revealed that many G-protein coupled receptor (GPCR) functions that are not mediated by G proteins involve protein networks, which interact with their intracellular domains. This review focuses on one family of GPCRs activated by serotonin, the 5-HT(2) receptor family, which comprises three closely related subtypes, the 5-HT(2A), the 5-HT(2B) and the 5-HT(2c) receptors. These receptors still raise particular interest, because a large number of psychoactive drugs including hallucinogens, anti-psychotics, anxiolytics and anti-depressants, mediate their action, at least in part, through activation of 5-HT(2) receptors. Recent studies based on two-hybrid screens, proteomic, biochemical and cell biology approaches, have shown that the C-terminal domains of 5-HT(2) receptors interact with intracellular proteins. To date, the protein network associated with the C-terminus of the 5-HT(2C) receptor has been the most extensively characterized, using a proteomic approach combining affinity chromatography, mass spectrometry and immunoblotting. It includes scaffolding proteins containing one or several PDZ domains, signalling proteins and proteins of the cytoskeleton. Data indicating that the protein complexes interacting with 5-HT(2) receptor C-termini tightly control receptor trafficking and receptor-mediated signalling will also be reviewed.

The serotonin 5-HT2A and 5-HT2C receptors interact with specific sets of PDZ proteins.

The 5-hydroxytryptamine type 2A (5-HT(2A)) receptor and the 5-HT(2C) receptor are closely related members of the G-protein-coupled receptors activated by serotonin that share very similar pharmacological profiles and cellular signaling pathways. These receptors express a canonical class I PDZ ligand (SXV) at their C-terminal extremity. Here, we have identified proteins that interact with the PDZ ligand of the 5-HT(2A) and 5-HT(2C) receptors by a proteomic approach associating affinity chromatography using immobilized synthetic peptides encompassing the PDZ ligand and mass spectrometry. We report that both receptor C termini interact with specific sets of PDZ proteins in vitro. The 5-HT(2C) receptor but not the 5-HT(2A) receptor binds to the Veli-3.CASK.Mint1 ternary complex and to SAP102. In addition, the 5-HT(2C) receptor binds more strongly to PSD-95 and MPP-3 than the 5-HT(2A) receptor. In contrast, a robust interaction between the 5-HT(2A) receptor and the channel-interacting PDZ protein CIPP was found, whereas CIPP did not significantly associate with the 5-HT(2C) receptor. We also show that residues located at the -1 position and upstream the PDZ ligand in the C terminus of the 5-HT(2A) and 5-HT(2C) receptors are major determinants in their interaction with specific PDZ proteins. Immunofluorescence and electron microscopy studies strongly suggested that these specific interactions also take place in living cells and that the 5-HT(2) receptor-PDZ protein complexes occur in intracellular compartments. The interaction of the 5-HT(2A) and the 5-HT(2C) receptor with specific sets of PDZ proteins may contribute to their different signal transduction properties.