Using a Combination of Indirect Calorimetry, Infrared Thermography, and Blood Glucose Levels to Measure Brown Adipose Tissue Thermogenesis in Humans

In mammals, brown adipose tissue (BAT) is activated rapidly in response to cold in order to maintain body temperature. Although BAT has been studied greatly in small animals, it is difficult to measure the activity of BAT in humans. Therefore, little is known about the heat-generating capacity and physiological significance of BAT in humans, including the degree to which components of the diet can activate BAT. This is due to the limitations in the currently most used method to assess the activation of BAT-radiolabeled glucose (fluorodeoxyglucose or ¹⁸FDG) measured by positron emission tomography-computerized tomography (PET-CT). This method is usually performed in fasted subjects, as feeding induces glucose uptake by the muscles, which can mask the glucose uptake into the BAT. This paper describes a detailed protocol for quantifying total-body human energy expenditure and substrate utilization from BAT thermogenesis by combining indirect calorimetry, infrared thermography, and blood glucose monitoring in carbohydrate-loaded adult males. To characterize the physiological significance of BAT, measures of the impact of BAT activity on human health are critical. We demonstrate a protocol to achieve this by combining carbohydrate loading and indirect calorimetry with measurements of supraclavicular changes in temperature. This novel approach will help to understand the physiology and pharmacology of BAT thermogenesis in humans.

5-HT3 Receptors on Mitochondria Influence Mitochondrial Function

The 5-hydroxytryptamine 3 (5-HT₃) receptor belongs to the pentameric ligand-gated cation channel superfamily. Humans have five different 5-HT₃ receptor subunits: A to E. The 5-HT₃ receptors are located on the cell membrane, but a previous study suggested that mitochondria could also contain A subunits. In this article, we explored the distribution of 5-HT₃ receptor subunits in intracellular and cell-free mitochondria. Organelle prediction software supported the localization of the A and E subunits on the inner membrane of the mitochondria. We transiently transfected HEK293T cells that do not natively express the 5-HT₃ receptor with an epitope and fluorescent protein-tagged 5HT3A and 5HT3E subunits. Fluorescence microscopy and cell fractionation indicated that both subunits, A and E, localized to the mitochondria, while transmission electron microscopy revealed the location of the subunits on the mitochondrial inner membrane, where they could form heteromeric complexes. Cell-free mitochondria isolated from cell culture media colocalized with the fluorescent signal for A subunits. The presence of A and E subunits influenced changes in the membrane potential and mitochondrial oxygen consumption rates upon exposure to serotonin; this was inhibited by pre-treatment with ondansetron. Therefore, it is likely that the 5-HT₃ receptors present on mitochondria directly impact mitochondrial function and that this may have therapeutic implications.

Corrigendum: Both caffeine and Capsicum annuum fruit powder lower blood glucose levels and increase brown adipose tissue temperature in healthy adult males

[This corrects the article DOI: 10.3389/fphys.2022.870154.].

Both caffeine and Capsicum annuum fruit powder lower blood glucose levels and increase brown adipose tissue temperature in healthy adult males

Using a combination of respiratory gas exchange, infrared thermography, and blood glucose (BGL) analysis, we have investigated the impact of Capsicum annuum (C. annuum) fruit powder (475 mg) or caffeine (100 mg) on metabolic activity in a placebo controlled (lactose, 100 mg) double-blinded three-way cross-over-design experiment. Metabolic measurements were made on day 1 and day 7 of supplementation in eight adult male participants (22.2 ± 2 years of age, BMI 23 ± 2 kg/m², x̅ ± SD). Participants arrived fasted overnight and were fed a high carbohydrate meal (90 g glucose), raising BGL from fasting baseline (4.4 ± 0.3 mmol/L) to peak BGL (8.5 ± 0.3 mmol/L) 45 min after the meal. Participants consumed the supplement 45 min after the meal, and both caffeine and C. annuum fruit powder restored BGL (F _(8,178) = 2.2, p = 0.02) to near fasting levels within 15 min of supplementation compared to placebo (120 min). In parallel both supplements increased energy expenditure (F _{(2, 21)} = 175.6, p < 0.001) over the 120-min test period (caffeine = 50.74 ± 2 kcal/kg/min, C. annuum fruit = 50.95 ± 1 kcal/kg/min, placebo = 29.34 ± 1 kcal/kg/min). Both caffeine and C. annuum fruit powder increased supraclavicular fossa temperature (F _(2,42) = 32, p < 0.001) on both day 1 and day 7 of testing over the 120-min test period. No statistical difference in core temperature or reference point temperature, mean arterial pressure or heart rate was observed due to supplementation nor was any statistical difference seen between day 1 and day 7 of intervention. This is important for implementing dietary ingredients as potential metabolism increasing supplements. Together the results imply that through dietary supplements such as caffeine and C. annuum, mechanisms for increasing metabolism can be potentially targeted to improve metabolic homeostasis in people.

A systematic review and meta-analyses of interleukin-1 receptor associated kinase 3 (IRAK3) action on inflammation in in vivo models for the study of sepsis

Background

Interleukin-1 receptor associated kinase 3 (IRAK3) is a critical modulator of inflammation and is associated with endotoxin tolerance and sepsis. Although IRAK3 is known as a negative regulator of inflammation, several studies have reported opposing functions, and the temporal actions of IRAK3 on inflammation remain unclear. A systematic review and meta-analyses were performed to investigate IRAK3 expression and its effects on inflammatory markers (TNF-α and IL-6) after one- or two-challenge interventions, which mimic the hyperinflammatory and immunosuppression phases of sepsis, respectively, using human or animal in vivo models.

Methods

This systematic review and meta-analyses has been registered in the Open Science Framework (OSF) (Registration DOI: 10.17605/OSF.IO/V39UR). A systematic search was performed to identify in vivo studies reporting outcome measures of expression of IRAK3 and inflammatory markers. Meta-analyses were performed where sufficient data was available.

Results

The search identified 7778 studies for screening. After screening titles, abstracts and full texts, a total of 49 studies were included in the systematic review. The review identified significant increase of IRAK3 mRNA and protein expression at different times in humans compared to rodents following one-challenge, whereas the increases of IL-6 and TNF-α protein expression in humans were similar to rodent in vivo models. Meta-analyses confirmed the inhibitory effect of IRAK3 on TNF-α mRNA and protein expression after two challenges.

Conclusions

A negative correlation between IRAK3 and TNF-α expression in rodents following two challenges demonstrates the association of IRAK3 in the immunosuppression phase of sepsis. Species differences in underlying biology affect the translatability of immune responses of animal models to human, as shown by the dissimilarity in patterns of IRAK3 mRNA and protein expression between humans and rodents following one challenge that are further influenced by variations in experimental procedures.

Intrinsic and extrinsic apoptosis responses in leukaemia cells following daunorubicin treatment

BACKGROUND

Daunorubicin is used clinically in the treatment of myeloma, acute lymphatic and myelocytic leukaemia. The toxic lesions caused by daunorubicin induce various modes of cell death, including apoptosis. Apoptosis is highly regulated programmed cell death that can be initiated mainly via two pathways, through death receptors (extrinsic) or involvement of the mitochondria (intrinsic). Induction of apoptosis via these pathways has been alluded following treatment with daunorubicin, but never compared in acute lymphoblastic leukaemia over a time course.

METHODS

This study investigated the mechanisms of daunorubicin induced apoptosis in the treatment of CCRF-CEM, MOLT-4 (acute T-lymphoblastic leukaemia) and SUP-B15 (acute B-lymphoblastic leukaemia) cells. Cells were treated with daunorubicin for 4 h, and then placed in recovery medium (without daunorubicin) for 4 h, 12 h and 24 h. Apoptotic response was analysing using annexin-V expression, caspase activity, mitochondrial membrane potential change and an array to detect 43 apoptotic proteins.

RESULTS

Daunorubicin induced apoptosis in all leukemic cell lines, but with different levels and duration of response. Both apoptosis levels and caspase activity increased after four hours recovery then declined in CCRF-CEM and MOLT-4 cells. However, SUP-B15 cells displayed initially comparable levels but remained elevated over the 24 h assessment period. Changes in mitochondrial membrane potential occurred in both MOLT-4 and CCRF-CEM cells but not in SUP-B15 cells. Expression of apoptotic proteins, including Bcl-2, Bax, caspase 3 and FADD, indicated that daunorubicin potentially induced both extrinsic and intrinsic apoptosis in both CCRF-CEM and MOLT-4 cells, but only extrinsic apoptosis in SUP-B15 cells.

CONCLUSIONS

This study describes variations in sensitivities and timing of apoptotic responses in different leukaemia cell lines. These differences could be attributed to the lack of functional p53 in coordinating the cells response following cytotoxic treatment with daunorubicin, which appears to delay apoptosis and utilises alternative signalling mechanisms that need to be further explored.

Effects of Caffeine on Brown Adipose Tissue Thermogenesis and Metabolic Homeostasis: A Review

The impact of brown adipose tissue (BAT) metabolism on understanding energy balance in humans is a relatively new and exciting field of research. The pathogenesis of obesity can be largely explained by an imbalance between caloric intake and energy expenditure, but the underlying mechanisms are far more complex. Traditional non-selective sympathetic activators have been used to artificially elevate energy utilization, or suppress appetite, however undesirable side effects are apparent with the use of these pharmacological interventions. Understanding the role of BAT, in relation to human energy homeostasis has the potential to dramatically offset the energy imbalance associated with obesity. This review discusses paradoxical effects of caffeine on peripheral adenosine receptors and the possible role of adenosine in increasing metabolism is highlighted, with consideration to the potential of central rather than peripheral mechanisms for caffeine mediated BAT thermogenesis and energy expenditure. Research on the complex physiology of adipose tissue, the embryonic lineage and function of the different types of adipocytes is summarized. In addition, the effect of BAT on overall human metabolism and the extent of the associated increase in energy expenditure are discussed. The controversy surrounding the primary β-adrenoceptor involved in human BAT activation is examined, and suggestions as to the lack of translational findings from animal to human physiology and human in vitro to in vivo models are provided. This review compares and distinguishes human and rodent BAT effects, thus developing an understanding of human BAT thermogenesis to aid lifestyle interventions targeting obesity and metabolic syndrome. The focus of this review is on the effect of BAT thermogenesis on overall metabolism, and the potential therapeutic effects of caffeine in increasing metabolism via its effects on BAT.

Innervation of supraclavicular adipose tissue: A human cadaveric study

Functional brown adipose tissue (BAT) was identified in adult humans only in 2007 with the use of fluorodeoxyglucose positron emission tomography imaging. Previous studies have demonstrated a negative correlation between obesity and BAT presence in humans. It is proposed that BAT possesses the capacity to increase metabolism and aid weight loss. In rodents it is well established that BAT is stimulated by the sympathetic nervous system with the interscapular BAT being innervated via branches of intercostal nerves. Whilst there is evidence to suggest that BAT possesses beta-3 adrenoceptors, no studies have identified the specific nerve branch that carries sympathetic innervation to BAT in humans. The aim of this study was to identify and trace the peripheral nerve or nerves that innervate human BAT in the supraclavicular region. The posterior triangle region of the neck of cadaveric specimens were dissected in order to identify any peripheral nerve branches piercing and/or terminating in supraclavicular BAT. A previously undescribed branch of the cervical plexus terminating in a supraclavicular adipose depot was identified in all specimens. This was typically an independent branch of the plexus, from the third cervical spinal nerve, but in one specimen was a branch of the supraclavicular nerve. Histological analysis revealed the supraclavicular adipose depot contained tyrosine hydroxylase immunoreactive structures, which likely represent sympathetic axons. This is the first study that identifies a nerve branch to supraclavicular BAT-like tissue. This finding opens new avenues for the investigation of neural regulation of fat metabolism in humans.

The capacity for oestrogen to influence obesity through brown adipose tissue thermogenesis in animal models: A systematic review and meta-analysis

Pharmacological interventions to aid weight loss have historically targeted either appetite suppression or increased metabolic rate. Brown adipose tissue (BAT) possesses the capacity to expend energy in a futile cycle, thus increasing basal metabolic rate. In animal models, oestrogen has been implicated in the regulation of body weight, and it is hypothesized that oestrogen is acting by modulating BAT metabolism. A systematic search was performed, to identify research articles implementing in vivo oestrogen-related interventions and reporting outcome measures that provide direct or indirect measures of BAT metabolism. Meta-analyses were conducted where sufficient data were available. The final library of 67 articles were predominantly in rodent models and provided mostly indirect measures of BAT metabolism. Results of this review found that oestrogen's effects on body weight, in rats and possibly mice, are likely facilitated by both metabolic and appetitive mechanisms but are largely only found in ovariectomized models. There is a need for further studies to clarify the potential effects of oestrogen on BAT metabolism in gonad-intact and castrated male animal models.

IRAK3 modulates downstream innate immune signalling through its guanylate cyclase activity

Interleukin-1 receptor associated kinase 3 (IRAK3) is a cytoplasmic homeostatic mediator of inflammatory responses and is potentially useful as a prognostic marker in inflammation. IRAK3 inhibits signalling cascades downstream of myddosome complexes associated with toll like receptors. IRAK3 contains a death domain that interacts with other IRAK family members, a pseudokinase domain and a C-terminus domain involved with tumour necrosis factor receptor associated factor 6 (TRAF6). Previous bioinformatic studies revealed that IRAK3 contained a guanylate cyclase centre in its pseudokinase domain but its role in IRAK3 action is unresolved. We demonstrate that wildtype IRAK3 is capable of producing cGMP. Furthermore, we show that a specific point mutation in the guanylate cyclase centre reduced cGMP production. Cells containing toll like receptor 4 and a nuclear factor kappa-light-chain-enhancer of activated B cells (NFĸB) reporter system were transfected with IRAK3 or mutant IRAK3 proteins. Cell-permeable cGMP treatment of untransfected control cells suppresses downstream signalling through modulation of the NFĸB in the presence of lipopolysaccharides. Cells transfected with wildtype IRAK3 also suppress lipopolysaccharide induced NFĸB activity in the absence of exogenous cGMP. Lipopolysaccharide induced NFĸB activity was not suppressed in cells transfected with the IRAK3 mutant with reduced cGMP-generating capacity. Whereas in the presence of exogenously applied cell-permeable cGMP the IRAK3 mutant was able to retain its function by suppressing lipopolysaccharide induced NFĸB activity. Furthermore, increasing the amount of membrane permeable cGMP did not affect IRAK3's ability to reduce NFĸB activity. These results suggest that cGMP generated by IRAK3 may be involved in regulatory function of the protein where the presence of cGMP may selectively affect downstream signalling pathway(s) by modulating binding and/or activity of nearby proteins that interact in the inflammatory signalling cascade.

Determination of the DNA repair pathways utilised by acute lymphoblastic leukaemia cells following daunorubicin treatment

Objective

DNA double strand breaks (DNA-DSBs) are among the most lethal DNA lesions leading to genomic instability and repaired by either homologous recombination (HR) or the non-homologous end joining (NHEJ) mechanisms. The purpose of this study was to assess the importance and the level of activation of non-homologous end joining (NHEJ) and homologous recombination (HR) DNA repair pathways in three cell lines, CCRF-CEM and MOLT-4 derived from T lymphocytes and SUP-B15 derived from B lymphocytes following treatment with chemotherapy agent daunorubicin.

Results

The Gamma histone H2AX (γH2AX) assay was used assess the effects of DNA-PK inhibitor NU7026 and RAD51 inhibitor RI-2 on repair of DNA-DSB following treatment with daunorubicin. In all cell lines, the NHEJ DNA repair pathway appeared more rapid and efficient. MOLT-4 and CCFR-CEM cells utilised both NHEJ and HR pathways for DNA-DSB repair. Whereas, SUP-B15 cells utilised only NHEJ for DSB repair, suggestive of a deficiency in HR repair pathways.

Time dependent response of daunorubicin on cytotoxicity, cell cycle and DNA repair in acute lymphoblastic leukaemia

BACKGROUND

Daunorubicin is commonly used in the treatment of acute lymphoblastic leukaemia (ALL). The aim of this study was to explore the kinetics of double strand break (DSB) formation of three ALL cell lines following exposure to daunorubicin and to investigate the effects of daunorubicin on the cell cycle and the protein kinases involved in specific checkpoints following DNA damage and recovery periods.

METHODS

Three ALL cell lines CCRF-CEM and MOLT-4 derived from T lymphocytes and SUP-B15 derived from B lymphocytes were examined following 4 h treatment with daunorubicin chemotherapy and 4, 12 and 24 h recovery periods. Cell viability was measured via MTT (3-(4,5-dimethylthiazol-2-yl)-2-5 diphenyltetrazolium bromide) assay, reactive oxygen species (ROS) production by flow cytometry, double stranded DNA breaks by detecting γH2AX levels while stages of the cell cycle were detected following propidium iodide staining and flow cytometry. Western blotting was used to detect specific proteins while RNA was extracted from all cell lines and converted to cDNA to sequence Ataxia-telangiectasia mutated (ATM).

RESULTS

Daunorubicin induced different degrees of toxicity in all cell lines and consistently generated reactive oxygen species. Daunorubicin was more potent at inducing DSB in MOLT-4 and CCRF-CEM cell lines while SUP-B15 cells showed delays in DSB repair and significantly more resistance to daunorubicin compared to the other cell lines as measured by γH2AX assay. Daunorubicin also causes cell cycle arrest in all three cell lines at different checkpoints at different times. These effects were not due to mutations in ATM as sequencing revealed none in any of the three cell lines. However, p53 was phosphorylated at serine 15 only in CCRF-CEM and MOLT-4 but not in SUP-B15 cells. The lack of active p53 may be correlated to the increase of SOD2 in SUP-B15 cells.

CONCLUSIONS

The delay in DSB repair and lower sensitivity to daunorubicin seen in the B lymphocyte derived SUP-B15 cells could be due to loss of function of p53 that may be correlated to increased expression of SOD2 and lower ROS production.

High Molecular Weight DNA Enrichment with Peptide Nucleic Acid Probes

Here, we describe how peptide nucleic acid (PNA) probes can be used to enrich genomic DNA fractions to facilitate downstream analysis, such as the haplotype phasing of the isolated genomic pieces. This method enriches for polymorphic regions of fragmented chromosomes by physically separating the desired sequence and flanking regions. The PNA probes used for enrichment are novel synthetic nucleic acids with highly specific targeting and hybridization properties. Using a enrichment technique, we capture high molecular weight genomic DNA using nothing more than a simple modification to standard genomic DNA extraction from blood.

Effect of the 5-HT4 receptor agonist tegaserod on the expression of GRK2 and GRK6 in the rat gastrointestinal tract

Objective

Tegaserod is a 5-hydroxytryptamine type 4 (5-HT₄) receptor agonist, formerly used in treating constipation predominant irritable bowel syndrome, which desensitizes 5-HT₄ receptors in rat oesophagus and colon in vitro. Desensitization of 5-HT₄ receptors is regulated by G-protein coupled receptor kinases. This study was designed to assess the effect of 5-HT₄ receptor activation on the expression of GRK2 and GRK6 in the rat oesophagus and distal colon by acute administration of tegaserod.

Results

Rats were treated with a single dose of tegaserod (5 mg/kg) and tissue samples of the oesophagus and distal colon were prepared and level of GRK2 and GRK6 protein expression was determined using western blotting. The immunodensity of GRK2 and GRK6 was normalized against the loading control β-actin and compared with control animals. Acute administration of tegaserod for 1, 2, 3, 4, 6, and 8 h did not change significantly the immunodensity of GRK2 or GRK6 in the oesophagus or GRK2 in the distal colon when compared with control animals. This may indicate that the basal level of GRK2 and GRK6 expression is sufficient to regulate the desensitization of 5-HT₄ receptors in acute drug treatment.

Moonlighting Proteins and Their Role in the Control of Signaling Microenvironments, as Exemplified by cGMP and Phytosulfokine Receptor 1 (PSKR1)

Signal generating and processing complexes and changes in concentrations of messenger molecules such as calcium ions and cyclic nucleotides develop gradients that have critical roles in relaying messages within cells. Cytoplasmic contents are densely packed, and in plant cells this is compounded by the restricted cytoplasmic space. To function in such crowded spaces, scaffold proteins have evolved to keep key enzymes in the correct place to ensure ordered spatial and temporal and stimulus-specific message generation. Hence, throughout the cytoplasm there are gradients of messenger molecules that influence signaling processes. However, it is only recently becoming apparent that specific complexes involving receptor molecules can generate multiple signal gradients and enriched microenvironments around the cytoplasmic domains of the receptor that regulate downstream signaling. Such gradients or signal circuits can involve moonlighting proteins, so called because they can enable fine-tune signal cascades via cryptic additional functions that are just being defined. This perspective focuses on how enigmatic activity of moonlighting proteins potentially contributes to regional intracellular microenvironments. For instance, the proteins associated with moonlighting proteins that generate cyclic nucleotides may be regulated by cyclic nucleotide binding directly or indirectly. In this perspective, we discuss how generation of cyclic nucleotide-enriched microenvironments can promote and regulate signaling events. As an example, we use the phytosulfokine receptor (PSKR1), discuss the function of its domains and their mutual interactions and argue that this complex architecture and function enhances tuning of signals in microenvironments.

Intramolecular crosstalk between catalytic activities of receptor kinases

Signal modulation is important for the growth and development of plants and this process is mediated by a number of factors including physiological growth regulators and their associated signal transduction pathways. Protein kinases play a central role in signaling, including those involving pathogen response mechanisms. We previously demonstrated an active guanylate cyclase (GC) catalytic center in the brassinosteroid insensitive receptor (AtBRI1) within an active intracellular kinase domain resulting in dual enzymatic activity. Here we propose a novel type of receptor architecture that is characterized by a functional GC catalytic center nested in the cytosolic kinase domain enabling intramolecular crosstalk. This may be through a cGMP-AtBRI1 complex forming that may induce a negative feedback mechanism leading to desensitisation of the receptor, regulated through the cGMP production pathway. We further argue that the comparatively low but highly localized cGMP generated by the GC in response to a ligand is sufficient to modulate the kinase activity. This type of receptor therefore provides a molecular switch that directly and/or indirectly affects ligand dependent phosphorylation of downstream signaling cascades and suggests that subsequent signal transduction and modulation works in conjunction with the kinase in downstream signaling.

The brassinosteroid receptor BRI1 can generate cGMP enabling cGMP-dependent downstream signaling

The brassinosteroid receptor brassinosteroid insensitive 1 (BRI1) is a member of the leucine-rich repeat receptor-like kinase family. The intracellular kinase domain of BRI1 is an active kinase and also encapsulates a guanylate cyclase catalytic centre. Using liquid chromatography tandem mass spectrometry, we confirmed that the recombinant cytoplasmic domain of BRI1 generates pmol amounts of cGMP per μg protein with a preference for magnesium over manganese as a co-factor. Importantly, a functional BRI1 kinase is essential for optimal cGMP generation. Therefore, the guanylate cyclase activity of BRI1 is modulated by the kinase while cGMP, the product of the guanylate cyclase, in turn inhibits BRI1 kinase activity. Furthermore, we show using Arabidopsis root cell cultures that cGMP rapidly potentiates phosphorylation of the downstream substrate brassinosteroid signaling kinase 1 (BSK1). Taken together, our results suggest that cGMP acts as a modulator that enhances downstream signaling while dampening signal generation from the receptor.

Haplotyping the human leukocyte antigen system from single chromosomes

We describe a method for determining the parental HLA haplotypes of a single individual without recourse to conventional segregation genetics. Blood samples were cultured to identify and sort chromosome 6 by bivariate flow cytometry. Single chromosome 6 amplification products were confirmed with a single nucleotide polymorphism (SNP) array and verified by deep sequencing to enable assignment of both alleles at the HLA loci, defining the two haplotypes. This study exemplifies a rapid and efficient method of haplotyping that can be applied to any chromosome pair, or indeed all chromosome pairs, using a single sorting operation. The method represents a cost-effective approach to complete phasing of SNPs, which will facilitate a deeper understanding of the links between SNPs, gene regulation and protein function.

Distribution of 5-HT3, 5-HT4, and 5-HT7 Receptors Along the Human Colon

Background/Aims

Several disorders of the gastrointestinal tract are associated with abnormal serotonin (5-HT) signaling or metabolism where the 5-HT₃ and 5-HT₄ receptors are clinically relevant. The aim was to examine the distribution of 5-HT₃, 5-HT₄, and 5-HT₇ receptors in the normal human colon and how this is associated with receptor interacting chaperone 3, G protein coupled receptor kinases, and protein LIN-7 homologs to extend previous observations limited to the sigmoid colon or the upper intestine.

Methods

Samples from ascending, transverse, descending, and sigmoid human colon were dissected into 3 separate layers (mucosa, longitudinal, and circular muscles) and ileum samples were dissected into mucosa and muscle layers (n = 20). Complementary DNA was synthesized by reverse transcription from extracted RNA and expression was determined by quantitative or end point polymerase chain reaction.

Results

The 5-HT₃ receptor subunits were found in all tissues throughout the colon and ileum. The A subunit was detected in all samples and the C subunit was expressed at similar levels while the B subunit was expressed at lower levels and less frequently. The 5-HT₃ receptor E subunit was mainly found in the mucosa layers. All splice variants of the 5-HT₄ and 5-HT₇ receptors were expressed throughout the colon although the 5-HT₄ receptor d, g, and i variants were expressed less often.

Conclusions

The major differences in 5-HT receptor distribution within the human colon are in relation to the mucosa and muscular tissue layers where the 5-HT₃ receptor E subunit is predominantly found in the mucosal layer which may be of therapeutic relevance.

Conserved Functional Motifs and Homology Modeling to Predict Hidden Moonlighting Functional Sites

Moonlighting functional centers within proteins can provide them with hitherto unrecognized functions. Here, we review how hidden moonlighting functional centers, which we define as binding sites that have catalytic activity or regulate protein function in a novel manner, can be identified using targeted bioinformatic searches. Functional motifs used in such searches include amino acid residues that are conserved across species and many of which have been assigned functional roles based on experimental evidence. Molecules that were identified in this manner seeking cyclic mononucleotide cyclases in plants are used as examples. The strength of this computational approach is enhanced when good homology models can be developed to test the functionality of the predicted centers in silico, which, in turn, increases confidence in the ability of the identified candidates to perform the predicted functions. Computational characterization of moonlighting functional centers is not diagnostic for catalysis but serves as a rapid screening method, and highlights testable targets from a potentially large pool of candidates for subsequent in vitro and in vivo experiments required to confirm the functionality of the predicted moonlighting centers.

Plant natriuretic peptides induce proteins diagnostic for an adaptive response to stress

In plants, structural and physiological evidence has suggested the presence of biologically active natriuretic peptides (PNPs). PNPs are secreted into the apoplast, are systemically mobile and elicit a range of responses signaling via cGMP. The PNP-dependent responses include tissue specific modifications of cation transport and changes in stomatal conductance and the photosynthetic rate. PNP also has a critical role in host defense responses. Surprisingly, PNP-homologs are produced by several plant pathogens during host colonization suppressing host defense responses. Here we show that a synthetic peptide representing the biologically active fragment of the Arabidopsis thaliana PNP (AtPNP-A) induces the production of reactive oxygen species in suspension-cultured A. thaliana (Col-0) cells. To identify proteins whose expression changes in an AtPNP-A dependent manner, we undertook a quantitative proteomic approach, employing tandem mass tag (TMT) labeling, to reveal temporal responses of suspension-cultured cells to 1 nM and 10 pM PNP at two different time-points post-treatment. Both concentrations yield a distinct differential proteome signature. Since only the higher (1 nM) concentration induces a ROS response, we conclude that the proteome response at the lower concentration reflects a ROS independent response. Furthermore, treatment with 1 nM PNP results in an over-representation of the gene ontology (GO) terms "oxidation-reduction process," "translation" and "response to salt stress" and this is consistent with a role of AtPNP-A in the adaptation to environmental stress conditions.

Calcium is the switch in the moonlighting dual function of the ligand-activated receptor kinase phytosulfokine receptor 1

BACKGROUND

A number of receptor kinases contain guanylate cyclase (GC) catalytic centres encapsulated in the cytosolic kinase domain. A prototypical example is the phytosulfokine receptor 1 (PSKR1) that is involved in regulating growth responses in plants. PSKR1 contains both kinase and GC activities however the underlying mechanisms regulating the dual functions have remained elusive.

FINDINGS

Here, we confirm the dual activity of the cytoplasmic domain of the PSKR1 receptor. We show that mutations within the guanylate cyclase centre modulate the GC activity while not affecting the kinase catalytic activity. Using physiologically relevant Ca2+ levels, we demonstrate that its GC activity is enhanced over two-fold by Ca2+ in a concentration-dependent manner. Conversely, increasing Ca2+ levels inhibits kinase activity up to 500-fold at 100 nM Ca2+.

CONCLUSIONS

Changes in calcium at physiological levels can regulate the kinase and GC activities of PSKR1. We therefore propose a functional model of how calcium acts as a bimodal switch between kinase and GC activity in PSKR1 that could be relevant to other members of this novel class of ligand-activated receptor kinases.

Calcium is the switch in the moonlighting dual function of the ligand-activated receptor kinase phytosulfokine receptor 1

Background

A number of receptor kinases contain guanylate cyclase (GC) catalytic centres encapsulated in the cytosolic kinase domain. A prototypical example is the phytosulfokine receptor 1 (PSKR1) that is involved in regulating growth responses in plants. PSKR1 contains both kinase and GC activities however the underlying mechanisms regulating the dual functions have remained elusive.

Findings

Here, we confirm the dual activity of the cytoplasmic domain of the PSKR1 receptor. We show that mutations within the guanylate cyclase centre modulate the GC activity while not affecting the kinase catalytic activity. Using physiologically relevant Ca²⁺ levels, we demonstrate that its GC activity is enhanced over two-fold by Ca²⁺ in a concentration-dependent manner. Conversely, increasing Ca²⁺ levels inhibits kinase activity up to 500-fold at 100 nM Ca²⁺.

Conclusions

Changes in calcium at physiological levels can regulate the kinase and GC activities of PSKR1. We therefore propose a functional model of how calcium acts as a bimodal switch between kinase and GC activity in PSKR1 that could be relevant to other members of this novel class of ligand-activated receptor kinases.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-014-0060-z) contains supplementary material, which is available to authorized users.

Human leukocyte antigen haplotype phasing by allele-specific enrichment with peptide nucleic acid probes

Targeted capture of large fragments of genomic DNA that enrich for human leukocyte antigen (HLA) system haplotypes has utility in haematopoietic stem cell transplantation. Current methods of HLA matching are based on inference or familial studies of inheritance; and each approach has its own inherent limitations. We have designed and tested a probe–target-extraction method for capturing specific HLA haplotypes by hybridization of peptide nucleic acid (PNA) probes to alleles of the HLA-DRB1 gene. Short target fragments contained in plasmids were initially used to optimize the method followed by testing samples of genomic DNA from human subjects with preselected HLA haplotypes and obtained approximately 10% enrichment for the specific haplotype. When performed with high-molecular-weight genomic DNA, 99.0% versus 84.0% alignment match was obtained for the specific haplotype probed. The allele-specific target enrichment that we obtained can facilitate the elucidation of haplotypes between the 65 kb separating the HLA-DRB1 and the HLA-DQA1 genes, potentially spanning a total distance of at least 130 kb. Allele-specific target enrichment with PNA probes is a straightforward technique that has the capability to improve the resolution of DNA and whole genome sequencing technologies by allowing haplotyping of enriched DNA and crucially, retaining the DNA methylation profile.

A cyclic nucleotide sensitive promoter reporter system suitable for bacteria and plant cells

BACKGROUND

Cyclic AMP (cAMP) and cyclic GMP (cGMP) have roles in relaying external signals and modifying gene expression within cells in all phyla. Currently there are no reporter systems suitable for bacteria and plant cells that measure alterations in downstream gene expression following changes in intracellular levels of cyclic nucleotides. As the plant protein OLIGOPEPTIDE TRANSPORTER X (OPTX) is upregulated by cGMP, we fused the OPTX promoter to a luciferase reporter gene (OPTX:LUC) to develop a plant cell reporter of cGMP-induced gene expression. We prepared a second construct augmented with three mammalian cGMP response elements (OPTXcGMPRE:LUC) and a third construct containing five gibberellic acid response elements (OPTXGARE:LUC). All three constructs were tested in bacteria and isolated plant protoplasts.

RESULTS

Membrane permeable cGMP enhanced luciferase activity of OPTX:LUC and OPTXGARE:LUC in protoplasts. Treatment with the plant hormone gibberellic acid which acts via cGMP also generated downstream luciferase activity. However, membrane permeable cAMP induced similar responses to cGMP in protoplasts. Significantly increased luciferase activity occurred in bacteria transformed with either OPTXcGMPRE:LUC or OPTXGARE:LUC in response to membrane permeable cAMP and cGMP. Bacteria co-transformed with OPTXcGMPRE:LUC or OPTXGARE:LUC and the soluble cytoplasmic domain of phytosulfokine receptor1 (PSKR1; a novel guanylate cyclase) had enhanced luciferase activity following induction of PSKR1 expression.

CONCLUSIONS

We have developed promoter reporter systems based on the plant OPTX promoter that can be employed in bacteria and isolated plant cells. We have shown that it can be used in bacteria to screen recombinant proteins for guanylate cyclase activity as increases in intracellular cGMP levels result in altered gene transcription and luciferase activity.

A critical review of the role of Fc gamma receptor polymorphisms in the response to monoclonal antibodies in cancer

Antibody-dependent cellular cytotoxicity (ADCC) is a major mechanism of action of therapeutic monoclonal antibodies (mAbs) such as cetuximab, rituximab and trastuzumab. Fc gamma receptors (FcgR) on human white blood cells are an integral part of the ADCC pathway. Differential response to therapeutic mAbs has been reported to correlate with specific polymorphisms in two of these genes: FCGR2A (H131R) and FCGR3A (V158F). These polymorphisms are associated with differential affinity of the receptors for mAbs. This review critically examines the current evidence for genotyping the corresponding single nucleotide polymorphisms (SNPs) to predict response to mAbs in patients with cancer.

Molecular methods for the study of signal transduction in plants

Novel and improved analytical methods have led to a rapid increase in our understanding of the molecular mechanism underlying plant signal transduction. Progress has been made both at the level of single-component analysis and in vivo imaging as well as at the systems level where transcriptomics and particularly phosphoproteomics afford a window into complex biological responses. Here we review the role of the cyclic nucleotides cAMP and cGMP in plant signal transduction as well as the discovery and biochemical and biological characterization of an increasing number of complex multi-domain nucleotide cyclases that catalyze the synthesis of cAMP and cGMP from ATP and GTP, respectively.

Structural and functional characterization of receptor kinases with nucleotide cyclase activity

There has been an increase in the identification and characterization of plant receptor kinases possessing nucleotide cyclase activity. This has necessitated the development of robust methodologies for the structural and functional characterization of this biologically important family of proteins. Here we outline some of the techniques that can be effectively used in the characterization of this bifunctional family of proteins.

Assessment of the pharmacological properties of 5-methoxyindole derivatives at 5-HT4 receptors

Objectives

The aim was to examine the biological activity of 5-methoxytryptamine derivatives at the 5-hydroxytryptamine (5-HT)4 receptor to explore the effect of substitution on the aliphatic amine of the 5-methoxyamine scaffold.

Methods

Three compounds were tested for affinity at the 5-HT4 receptor by radioligand binding and functional activity using guinea-pig ileum and human colon circular muscle preparations and also in the mouse whole gut transit test.

Key findings

The three compounds all had agonist properties at the 5-HT4 receptor but their efficacy differed in the different functional tests. Compound 3 had the highest affinity for the 5-HT4 receptor and was a full agonist at relaxing human colon circular muscle with efficacy closest to 5-HT. Compounds 1 and 2 were partial agonists in this assay with lower efficacies; compound 2 was a full agonist in the guinea-pig ileum assay whereas compound 3 was a partial agonist. Compounds 1 and 2 also showed activity in the mouse gut transit assay while compound 3 had no activity.

Conclusions

Of the compounds tested, compound 3 was the most promising 5-HT4 receptor agonist and the results highlight the value of using human tissue in functional tests when assessing compounds for potential activity.

Moonlighting kinases with guanylate cyclase activity can tune regulatory signal networks

Guanylate cyclase (GC) catalyzes the formation of cGMP and it is only recently that such enzymes have been characterized in plants. One family of plant GCs contains the GC catalytic center encapsulated within the intracellular kinase domain of leucine rich repeat receptor like kinases such as the phytosulfokine and brassinosteroid receptors. In vitro studies show that both the kinase and GC domain have catalytic activity indicating that these kinase-GCs are examples of moonlighting proteins with dual catalytic function. The natural ligands for both receptors increase intracellular cGMP levels in isolated mesophyll protoplast assays suggesting that the GC activity is functionally relevant. cGMP production may have an autoregulatory role on receptor kinase activity and / or contribute to downstream cell expansion responses. We postulate that the receptors are members of a novel class of receptor kinases that contain functional moonlighting GC domains essential for complex signaling roles.

Plant natriuretic peptides: control of synthesis and systemic effects

Plant natriuretic peptides (PNPs) are signalling molecules that are secreted into the apoplast particularly under conditions of biotic and abiotic stress.  At the local level, PNPs modulate their own expression via feed forward and feedback loops to enable tuning of the response at the transcript and protein level and to prevent over-expression.  PNPs also employ a systemic signal, possibly electrical, to rapidly alter photosynthesis and respiration not only in treated leaves but also in upper and lower leaves thereby modulating and integrating physiological responses at the level of the whole plant. 

The phytosulfokine (PSK) receptor is capable of guanylate cyclase activity and enabling cyclic GMP-dependent signaling in plants

Phytosulfokines (PSKs) are sulfated pentapeptides that stimulate plant growth and differentiation mediated by the PSK receptor (PSKR1), which is a leucine-rich repeat receptor-like kinase. We identified a putative guanylate cyclase (GC) catalytic center in PSKR1 that is embedded within the kinase domain and hypothesized that the GC works in conjunction with the kinase in downstream PSK signaling. We expressed the recombinant complete kinase (cytoplasmic) domain of AtPSKR1 and show that it has serine/threonine kinase activity using the Ser/Thr peptide 1 as a substrate with an approximate K(m) of 7.5 μm and V(max) of 1800 nmol min(-1) mg(-1) of protein. This same recombinant protein also has GC activity in vitro that is dependent on the presence of either Mg(2+) or Mn(2+). Overexpression of the full-length AtPSKR1 receptor in Arabidopsis leaf protoplasts raised the endogenous basal cGMP levels over 20-fold, indicating that the receptor has GC activity in vivo. In addition, PSK-α itself, but not the non-sulfated backbone, induces rapid increases in cGMP levels in protoplasts. Together these results indicate that the PSKR1 contains dual GC and kinase catalytic activities that operate in vivo and that this receptor constitutes a novel class of enzymes with overlapping catalytic domains.

Plant natriuretic peptides are apoplastic and paracrine stress response molecules

Higher plants contain biologically active proteins that are recognized by antibodies against human atrial natriuretic peptide (ANP). We identified and isolated two Arabidopsis thaliana immunoreactive plant natriuretic peptide (PNP)-encoding genes, AtPNP-A and AtPNP-B, which are distantly related members of the expansin superfamily and have a role in the regulation of homeostasis in abiotic and biotic stresses, and have shown that AtPNP-A modulates the effects of ABA on stomata. Arabidopsis PNP (PNP-A) is mainly expressed in leaf mesophyll cells, and in protoplast assays we demonstrate that it is secreted using AtPNP-A:green fluorescent protein (GFP) reporter constructs and flow cytometry. Transient reporter assays provide evidence that AtPNP-A expression is enhanced by heat, osmotica and salt, and that AtPNP-A itself can enhance its own expression, thereby generating a response signature diagnostic for paracrine action and potentially also autocrine effects. Expression of native AtPNP-A is enhanced by osmotica and transiently by salt. Although AtPNP-A expression is induced by salt and osmotica, ABA does not significantly modulate AtPNP-A levels nor does recombinant AtPNP-A affect reporter expression of the ABA-responsive RD29A gene. Together, these results provide experimental evidence that AtPNP-A is stress responsive, secreted into the apoplastic space and can enhance its own expression. Furthermore, our findings support the idea that AtPNP-A, together with ABA, is an important component in complex plant stress responses and that, much like in animals, peptide signaling molecules can create diverse and modular signals essential for growth, development and defense under rapidly changing environmental conditions.

Heterogeneity amongst 5-HT₃ receptor subunits: is this significant?

The serotonin 3 (5-HT₃) receptor is a ligand gated ion channel unlike the other 5-HT receptors which are G protein coupled receptors. The functional 5-HT₃ receptor forms a pentamer of five symmetrically arranged subunits surrounding a central pore. The 5-HT(3A) subunit was first identified at a molecular level and can form functional homomers or heteromers with the 5-HT(3B) subunit. Recently, three new 5-HT₃ subunits have been discovered and these can also form functional heteromers with the 5-HT(3A) subunit. In addition, splice variants of the 5-HT₃ subunits have also been reported. These findings have markedly increased the complexity of the 5-HT₃ receptor and may form part of the explanation of unresolved differences between studies investigating 5-HT₃ receptor function in cell lines compared with native tissues. In this review we discuss the properties of the different subunits and their distribution to determine if they contribute to functional changes in the 5-HT₃ receptor. Several recent pharmacogenomic studies have revealed single nucleotide polymorphisms (SNPs) and other variations in the different 5-HT₃ receptor subunits that are associated with various clinical conditions. We discuss the implications of these findings with respect to drug design and tailored pharmacogenomic therapies.

Developing a model of plant hormone interactions

Plant growth and development is influenced by mutual interactions among plant hormones. The five classical plant hormones are auxins, cytokinins, gibberellins, abscisic acid and ethylene. They are small diffusible molecules that easily penetrate between cells. In addition, newer classes of plant hormones have been identified such as brassinosteroids, jasmonic acid, salicylic acid and various small proteins or peptides. These hormones also play important roles in the regulation of plant growth and development. This review begins with a brief summary of the current findings on plant hormones. Based on this knowledge, a conceptual model about interactions among plant hormones is built so as to link and develop an understanding of the diverse functions of different plant hormones as a whole in plants.

Synthesis and preliminary screening of novel tryptamines as 5-HT4 receptor ligands

For the development of novel 5-HT(4) receptor ligands we have designed and synthesized two series of 5-methoxytryptamine derivatives varying the substitution on the primary amine. Their biological activities were evaluated in a receptor binding assay where a subset of compounds showed comparable potency to the agonists serotonin and 5-methoxytryptamine. Structure-activity analyses have highlighted promising avenues for further synthetic work and binding modes were proposed by docking these compounds into a homology model of the 5-HT(4) receptor.

Tissue dependent differences in G-protein coupled receptor kinases associated with 5-HT4 receptor desensitization in the rat gastro-intestinal tract

Desensitization of 5-HT(4) receptors is regulated by G-protein coupled receptor kinases (GRKs). However, the specific GRK(s) that regulates the desensitization of 5-HT(4) receptors in the in vivo setting is unknown. We investigated the in situ expression of 5-HT(4) receptors and the GRKs in the rat gastrointestinal tract using immunohistochemistry and their interaction using coimmunoprecipitation. 5-HT(4) receptors were expressed in the tunica muscularis mucosae of the oesophagus, longitudinal muscle, myenteric plexus, circular muscle, submucosal plexus and muscularis mucosae of both the proximal and distal colon. GRK2 was expressed in longitudinal muscle and occasionally in myenteric plexus whilst GRK5 showed limited expression in the nerve endings of the myenteric plexus and submucosal plexus of the colon. GRK3 was expressed in the tunica muscularis mucosae of the oesophagus, circular muscle, submucosal plexus and muscularis mucosae of the colon. GRK6 was expressed in the tunica muscularis mucosae of the oesophagus, longitudinal muscle, circular muscle, and muscularis mucosae of the colon. Stimulation of tunica muscularis mucosae of the oesophagus and distal colon using the 5-HT(4) receptor agonist, tegaserod, followed by analysis of the 5-HT(4) receptor antibody immunoprecipitate, revealed the coimmunoprecipitation of GRK6 with 5-HT(4) receptors in the tunica muscularis mucosae of oesophagus while GRK2 and GRK6 were coimmunoprecipitated with 5-HT(4) receptors in the distal colon. This study indicates that GRK6 may be involved in the regulation of the desensitization of 5-HT(4) receptors in the rat oesophagus whilst GRK2 and GRK6 may be involved in regulation of the desensitization of 5-HT(4) receptors in the distal colon.

Human 5-HT(4) and 5-HT(7) receptor splice variants: are they important?

G-protein-coupled receptors (GPCRs), which are encoded by >300 genes in the human genome, are by far the largest class of targets for modern drugs. These macromolecules display inherent adaptability of function, which is partly due to the production of different forms of the receptor protein. These are commonly called 'isoforms' or 'splice variants' denoting the molecular process of their production/assembly. Not all GPCRs are expressed as splice variants, but certain subclasses of 5-HT receptors are for example, the 5-HT(4) and 5-HT(7) receptors. There are at least 11 human 5-HT(4) and three h5-HT(7) receptor splice variants. This review describestheir discoveries, nomenclature and structures. The discovery that particular splice variants are tissue specific (or prominent) has highlighted their potential as future drug targets. In particular, this review examines the functional relevance of different 5-HT(4) and 5-HT(7) receptor splice variants. Examples are given to illustrate that splice variants have differential modulatory influences on signalling processes. Differences in agonist potency and efficacies and also differences in desensitisation rates to 5-HT occur with both 5-HT(4) and 5-HT(7) receptor splice variants. The known and candidate signalling systems that allow for splice variant specific responses include GPCR interacting proteins (GIPs) and GPCR receptor kinases (GRKs) which are examined.Finally, the relevance of 5-HT receptor splice variants to clinical medicine and to the pharmaceutical industry is discussed.

Induction of apoptosis by plant natriuretic peptides in rat cardiomyoblasts

Atrial natriuretic peptide (ANP) has an important role in maintaining the homeostasis of body fluids and blood pressure and also in preventing cardiac hypertrophy and initiating the process of apoptosis. An immunoreactive analog of ANP was discovered in plants over a decade ago and termed plant natriuretic peptide (PNP). PNP is a small protein that contains sequence and structural similarity to ANP within a predicted protruding psi (psi) loop. Since application of ANP or PNP stimulates similar functional effects in plants, it is conceivable that PNP may have effects on mammalian cells. In this report, we show that purified recombinant PNP induces apoptosis in a dose dependent and cell type specific manner. Rat cardiac myoblasts (H9c2 cells) were more susceptible to the apoptotic promoting effects of PNP and ANP than HEK-293T cells where PNP had a protective effect at lower concentrations. Similarly rat thoracic myoblasts (A-10) were less responsive to both PNP and ANP than the H9c2 cells. Since PNP is mimicking the effect of ANP in this instance, PNP may prove to be a useful lead molecule for developing novel therapeutic natriuretic peptides.

Investigations into the binding affinities of different human 5-HT4 receptor splice variants

This study examined whether the drug-receptor-binding sites of 5 selected human 5-HT(4) receptor splice variants [h5-HT4(a), h5-HT4(b), h5-HT4(c), h5-HT4(d) and h5-HT4(g)] display preferential affinities towards agonists. The agonists selected on the basis of chemical diversity and clinical relevance were: 5-HT4 benzamides, renzapride, zacopride and prucalopride; the benzimidazolones, DAU 6236 and BIMU 1; the aromatic ketone, RS67333, and the indole carbazimidamide tegaserod. The rank order of affinities ranging across the splice variants was: tegaserod (pKi: 7.38-7.91) > or = Y-36912 (pKi: 7.03-7.85) = BIMU 1 (pKi: 6.92-7.78) > or = DAU 6236 (pKi: 6.79-7.99) > or = 5-HT (pKi: 5.82-7.29) > or = 5-MeOT (pKi: 5.64-6.83) > or = renzapride (pKi: 4.85-5.56). We obtained affinity values for the 5-HT4(b), (d) and (g) variants for RS67333 (pKi: 7:48-8.29), prucalopride (pKi: 6.86-7.37) and zacopride (pKi: 5.88-7.0). These results indicate that the ligands interact with the same conserved site in each splice variant. Some splice variants have a higher affinity for certain agonists and the direction of selectivity followed a common trend of lowest affinity at the (d) variant. However, this trend was not evident in functional experiments. Our findings suggest that it may be possible to design splice variant selective ligands, which may be of relevance for experimental drugs but may be difficult to develop clinically.

Distribution of serotonin receptors and interacting proteins in the human sigmoid colon

This study aimed to examine the distribution of 5-HT receptors in the human colon. 5-HT induces desensitization of the circular muscle and as this is facilitated by G-protein coupled receptor kinases (GRKs) and other proteins, we also examined their distribution. Human sigmoid colon samples were dissected into three separate layers (mucosa, taeniae coli and intertaenial strips) and RNA was amplified by RT-PCR. The 5-HT(2B) receptor and all 5-HT(7) receptor splice variants were expressed in all tissues. 5-HT(4) a,b,c and n splice variants were also expressed in all tissues and 5-HT(4d), 5-HT(4g) and 5-HT(4i) were only detected in some samples. The 5-HT(2A) receptor was seen predominantly in the intertaenial strips of the colon. Only one transcript of the serotonin transporter (SERT) was detected in the muscle layers. Variation was seen in GRK expression with GRK2 and 3 predominantly expressed in the mucosa, while GRK5 and 6 were found more commonly in the taeniae coli. PDZ (named after postsynaptic density protein, Drosophila disc large tumour suppressor and tight junction protein ZO-1) domain containing proteins, which may be involved in 5-HT receptor trafficking, were also detected throughout the sigmoid colon. The 5-HT(3A) subunit was expressed in all tissues, whereas the 5-HT(3E) subunit was mainly found in the mucosa layer while the 5-HT(3B) subunit was more common in the muscle layers. Receptor interacting chaperone (RIC-3), which is involved in transporting 5-HT(3) receptor subunits, is expressed less in mucosa compared to muscle layers. In conclusion, these results show that there is variation in distribution of 5-HT receptors and interacting proteins within the sigmoid colon that may contribute to colonic function.

Synthesis of Quinoline Derivatives as 5-HT4 Receptor Ligands

A general and convenient synthesis of 6-methoxyquinoline-3-carboxamides commencing with a cyclization step that involves ρ-anisidine and diethyl (ethoxymethylene)malonate is described. An additional tetrahydroquinoline scaffold 19 is prepared from 6-methoxyquinoline-3-carboxamide and this represents a novel serotinergic lead structure. These compounds show reasonable affinity at 1 × 10–6 M, and docking experiments suggest that they may bind in a similar manner to serotonin.

Plant natriuretic peptide active site determination and effects on cGMP and cell volume regulation

Natriuretic peptides (NP) were first identified in animals where they play a role in the regulation of salt and water balance. This regulation is partly mediated by intracellular changes in cyclic GMP (cGMP). NP immunoanalogues occur in many plants and have been isolated, with two NP encoding genes characterised in Arabidopsis thaliana L. (AtPNP-A and AtPNP-B). Part of AtPNP-A contains the region with homology to human atrial (A)NP. We report here on the effects of recombinant AtPNP-A and smaller synthetic peptides within the ANP-homologous region with a view to identifying the biologically active domain of the molecule. Furthermore, we investigated interactions between AtPNP-A and the hormone, abscisic acid (ABA). ABA does not significantly affect Arabidopsis mesophyll protoplast volume regulation, whereas AtPNP-A and synthetic peptides promote water uptake into the protoplasts causing swelling. This effect is promoted by the membrane permeable cGMP analogue, 8-Br-cGMP, and inhibited by guanylate cyclase inhibitors indicating that increases in cGMP are an essential component of the plant natriuretic peptides (PNP) signalling cascade. ABA does not induce cGMP transients and does not affect AtPNP-A dependent cGMP increases, hence the two regulators differ in their second messenger signatures. Interestingly, AtPNP-A significantly delays and reduces the extent of ABA stimulated stomatal closure that is also based on cell volume regulation. We conclude that a complex interplay between observed PNP effects (stomatal opening and protoplast swelling) and ABA is likely to be cell type specific.

Comparison of 5-HT4 and 5-HT7 receptor expression and function in the circular muscle of the human colon

Serotonin receptors are potential targets for treating functional bowel disorders. This study investigated the functional roles and expression of the 5-HT4 and the 5-HT7 receptor, which coexist in human colon circular smooth muscle. 5-HT3 receptor expression was also investigated. Part of the relaxant response to 5-HT was due to activation of 5-HT4 receptors as the apparent pKB value of the selective 5-HT4 antagonist, GR 113808, was 9.36. 5-HT4 mRNA levels were low in five tissues and undetectable in four others, but all responded to 5-HT with an EC50 value of 102.54+/-19.32 nM. The contribution of 5-HT7 receptors to the response was not readily demonstrated using the selective 5-HT7 antagonist, SB-269970, as its apparent pKB value of 7.19 (5-HT4 block with 1 microM GR 113808) was lower than the value obtained using the 5-HT7 guinea pig ileum assay (8.62). Nevertheless, the 5-HT7 receptor was expressed more consistently than the 5-HT4, but at similar levels. The 5-HT(3Ashort) and 5-HT(3B) subunits were co-expressed at similar levels, but the 5-HT(3Along) subunit was detected in only five of the nine samples tested. The findings show that 5-HT4-induced relaxation occurs at low to undetectable levels of tissue mRNA, as measured by qPCR. Although 5-HT7 receptor mRNA is detected at low, but consistent levels, the functional activity of this receptor is not readily identified given the currently available drugs.

Activation of 5-HT3 receptors in the rat and mouse intestinal tract: a comparative study

This study provides a comprehensive evaluation of 5-HT(3) receptor functional distribution in both the rat and mouse intestinal tract. 5-HT(3A-S) receptor splice variant mRNA was expressed throughout the intestine of the rat and mouse; the 5-HT(3A-L) variant being more common in the rat.5-HT, m-CPB, 1-PBG and 2-methyl-5-hydroxytryptamine (2m5-HT) induced contraction in the jejunum, ileum, proximal colon and distal colon of the rat (pEC(50) range: 2m5-HT, 5.86+/-0.40 to m-CPB, 7.47+/-0.27) and mouse (pEC(50) range: 1-PBG, 5.34+/-0.06 to m-CPB, 6.49+/-0.14) in the presence of nontarget 5-HT receptor antagonists, methysergide (1 muM) and GR125487 (0.1 microM). The rank orders of potency in the four regions of the rat and mouse intestine were concordant with the accepted order and the responses to 5-HT were inhibited by ondansetron (0.1 microM).5-HT(3)-induced contractions to 5-HT were reduced by tetrodotoxin (1 microM). Pargyline (10 muM) and fluoxetine (1 microM) potentiated responses in the rat jejunum. Atropine (0.1 microM) potentiated 5-HT(3)-induced responses in the rat jejunum (E(max) 49-65%), but attenuated responses in most other regions of the rat and mouse (e.g. mouse ileum: E(max) 57-26%). In the rat jejunum, L-NAME (100 microM) mimicked the effect of atropine, hexamethonium (100 microM) suppressed 5-HT(3)-induced responses, but tachykinin receptor antagonists were without effect. It is concluded that functional 5-HT(3) receptors are present in nerves along the length of the rat and mouse intestinal tract. The mouse proximal colon was found to discriminate 5-HT(3) receptor agonist profiles better than any other region in the rat or mouse. The rat jejunum shows evidence of 5-HT uptake and inactivation processes as well as inhibitory nitrergic and nontachykinin excitatory pathways associated with the 5-HT(3)-induced response.