Function of human Rh based on structure of RhCG at 2.1 A

How sensor Amt-like proteins integrate ammonium signals

Unlike aquaporins or potassium channels, ammonium transporters (Amts) uniquely discriminate ammonium from potassium and water. This feature has certainly contributed to their repurposing as ammonium receptors during evolution. Here, we describe the ammonium receptor Sd-Amt1, where an Amt module connects to a cytoplasmic diguanylate cyclase transducer module via an HAMP domain. Structures of the protein with and without bound ammonium were determined to 1.7- and 1.9-Ångstrom resolution, depicting the ON and OFF states of the receptor and confirming the presence of a binding site for two ammonium cations that is pivotal for signal perception and receptor activation. The transducer domain was disordered in the crystals, and an AlphaFold2 prediction suggests that the helices linking both domains are flexible. While the sensor domain retains the trimeric fold formed by all Amt family members, the HAMP domains interact as pairs and serve to dimerize the transducer domain upon activation.

Molecular dynamics of the human RhD and RhAG blood group proteins

Introduction: Blood group antigens of the RH system (formerly known as "Rhesus") play an important role in transfusion medicine because of the severe haemolytic consequences of antibodies to these antigens. No crystal structure is available for RhD proteins with its partner RhAG, and the precise stoichiometry of the trimer complex remains unknown. Methods: To analyse their structural properties, the trimers formed by RhD and/or RhAG subunits were generated by protein modelling and molecular dynamics simulations were performed. Results: No major differences in structural behaviour were found between trimers of different compositions. The conformation of the subunits is relatively constant during molecular dynamics simulations, except for three large disordered loops. Discussion: This work makes it possible to propose a reasonable stoichiometry and demonstrates the potential of studying the structural behaviour of these proteins to investigate the hundreds of genetic variants relevant to transfusion medicine.

Branchial CO2 and ammonia excretion in crustaceans: Involvement of an apical Rhesus-like glycoprotein

AIM

To determine whether the crustacean Rh1 protein functions as a dual CO2 /ammonia transporter and investigate its role in branchial ammonia excretion and acid-base regulation.

METHODS

Sequence analysis of decapod Rh1 proteins was used to determine the conservation of amino acid residues putatively involved in ammonia transport and CO2 binding in human and bacterial Rh proteins. Using the Carcinus maenas Rh1 protein (CmRh1) as a representative of decapod Rh1 proteins, we test the ammonia and CO2 transport capabilities of CmRh1 through heterologous expression in yeast and Xenopus oocytes coupled with site-directed mutagenesis. Quantitative PCR was used to assess the distribution of CmRh1 mRNA in various tissues. Western blotting was used to assess CmRh1 protein expression changes in response to high environmental ammonia and CO2 . Further, immunohistochemistry was used to assess sub-cellular localization of CmRh1 and a membrane-bound carbonic anhydrase (CmCAg).

RESULTS

Sequence analysis of decapod Rh proteins revealed high conservation of several amino acid residues putatively involved in conducting ammonia transport and CO2 binding. Expression of CmRh1 in Xenopus oocytes enhanced both ammonia and CO2 transport which was nullified in CmRh1 D180N mutant oocytes. Transport of the ammonia analog methylamine by CmRh1 is dependent on both ionized and un-ionized ammonia/methylamine species. CmRh1 was co-localized with CmCAg to the apical membrane of the crustacean gill and only experienced decreased protein expression in the anterior gills when exposed to high environmental ammonia.

CONCLUSION

CmRh1 is the first identified apical transporter-mediated route for ammonia and CO2 excretion in the crustacean gill. Our findings shed further light on the potential universality of dual ammonia and CO2 transport capacity of Rhesus glycoproteins in both vertebrates and invertebrates.

Biological ammonium transporters from the Amt/Mep/Rh superfamily: mechanism, energetics, and technical limitations

The exchange of ammonium across cellular membranes is a fundamental process in all domains of life and is facilitated by the ubiquitous Amt/Mep/Rh transporter superfamily. Remarkably, despite a high structural conservation in all domains of life, these proteins have gained various biological functions during evolution. It is tempting to hypothesise that the physiological functions gained by these proteins may be explained at least in part by differences in the energetics of their translocation mechanisms. Therefore, in this review, we will explore our current knowledge of energetics of the Amt/Mep/Rh family, discuss variations in observations between different organisms, and highlight some technical drawbacks which have hampered effects at mechanistic characterisation. Through the review, we aim to provide a comprehensive overview of current understanding of the mechanism of transport of this unique and extraordinary Amt/Mep/Rh superfamily of ammonium transporters.

Biological ammonium transporters: evolution and diversification

Although ammonium is the preferred nitrogen source for microbes and plants, in animal cells it is a toxic product of nitrogen metabolism that needs to be excreted. Thus, ammonium movement across biological membranes, whether for uptake or excretion, is a fundamental and ubiquitous biological process catalysed by the superfamily of the Amt/Mep/Rh transporters. A remarkable feature of the Amt/Mep/Rh family is that they are ubiquitous and, despite sharing low amino acid sequence identity, are highly structurally conserved. Despite sharing a common structure, these proteins have become involved in a diverse range of physiological process spanning all domains of life, with reports describing their involvement in diverse biological processes being published regularly. In this context, we exhaustively present their range of biological roles across the domains of life and after explore current hypotheses concerning their evolution to help to understand how and why the conserved structure fulfils diverse physiological functions.

Use of computational biology to compare the theoretical tertiary structures of the most common forms of RhCE and RhD

BACKGROUND AND OBJECTIVES

Computational biology analyses the theoretical tertiary structure of proteins and identifies the 'topological' differences between RhD and RhCE. Our aim was to identify the theoretical structural differences between the four isoforms of RhCE and RhD using computational biological tools.

MATERIALS AND METHODS

Physicochemical profile was determined by hydrophobicity and electrostatic potential analysis. Secondary and tertiary structures were generated using computational biology tools. The structures were evaluated and validated using Ramachandran algorithm, which calculates the single score, p-value and root mean square deviation (RMSD). Structures were overlaid on local refinement of 'RhAG-RhCE-ANK' (PBDID 7uzq) and RhAG to compare their spatial distribution within the membrane.

RESULTS

All proteins differed in surface area and electrostatic distance due to variations in hydrophobicity and electrostatic potential. The RMSD between RhD and RhCE was 0.46 ± 0.04 Å, and the comparison within RhCE was 0.57 ± 0.08 Å. The percentage of amino acids in the hydrophobic thickness was 50.24% for RhD while for RhCE it ranged between 73.08% and 76.68%. The RHAG hydrophobic thickness was 34.2 Å, and RhCE's hydrophobic thickness was 33.83 Å. We suggest that the C/c antigens differ exofacially at loops L1 and L2. For the E/e antigens, the difference lies in L6. By contrast, L4 is the same for all proteins except Rhce.

CONCLUSION

The physicochemical properties of Rh proteins made them different, although their genes are homologous. Using computational biology, we model structures with sufficient precision, similar to those obtained experimentally. An amino acid variation alters the folding of the tertiary structure and the interactions with other proteins, modifying the electrostatic environment, the spatial conformations and therefore the antigenic recognition.

Prokaryotic ammonium transporters: what has three decades of research revealed?

The exchange of ammonium across cellular membranes is a fundamental process in all domains of life. In plants, bacteria and fungi, ammonium represents a vital source of nitrogen, which is scavenged from the external environment. In contrast, in animal cells ammonium is a cytotoxic metabolic waste product and must be excreted to prevent cell death. Transport of ammonium is facilitated by the ubiquitous Amt/Mep/Rh transporter superfamily. In addition to their function as transporters, Amt/Mep/Rh proteins play roles in a diverse array of biological processes and human physiopathology. Despite this clear physiological importance and medical relevance, the molecular mechanism of Amt/Mep/Rh proteins has remained elusive. Crystal structures of bacterial Amt/Rh proteins suggest electroneutral transport, whilst functional evidence supports an electrogenic mechanism. Here, focusing on bacterial members of the family, we summarize the structure of Amt/Rh proteins and what three decades of research tells us concerning the general mechanisms of ammonium translocation, in particular the possibility that the transport mechanism might differ in various members of the Amt/Mep/Rh superfamily.

Purification and structural characterization of the Na+-translocating ferredoxin: NAD+ reductase (Rnf) complex of Clostridium tetanomorphum

Various microbial metabolisms use H⁺/Na⁺-translocating ferredoxin:NAD⁺ reductase (Rnf) either to exergonically oxidize reduced ferredoxin by NAD⁺ for generating a transmembrane electrochemical potential or reversely to exploit the latter for producing reduced ferredoxin. For cryo-EM structural analysis, we elaborated a quick four-step purification protocol for the Rnf complex from Clostridium tetanomorphum and integrated the homogeneous and active enzyme into a nanodisc. The obtained 4.27 Å density map largely allows chain tracing and redox cofactor identification complemented by biochemical data from entire Rnf and single subunits RnfB, RnfC and RnfG. On this basis, we postulated an electron transfer route between ferredoxin and NAD via eight [4Fe-4S] clusters, one Fe ion and four flavins crossing the cell membrane twice related to the pathway of NADH:ubiquinone reductase. Redox-coupled Na⁺ translocation is provided by orchestrating Na⁺ uptake/release, electrostatic effects of the assumed membrane-integrated FMN semiquinone anion and accompanied polypeptide rearrangements mediated by different redox steps.

Never-homozygous genetic variants in healthy populations are potential recessive disease candidates

The rapid pace with which genetic variants are now being determined means there is a pressing need to understand how they affect biological systems. Variants from healthy individuals have previously been used to study blood groups or HLA diversity and to identify genes that can apparently be nonfunctional in healthy people. These studies and others have observed a lower than expected frequency of homozygous individuals for potentially deleterious alleles, which would suggest that several of these alleles can lead to recessive disorders. Here we exploited this principle to hunt for potential disease variants in genomes from healthy people. We identified at least 108 exclusively heterozygous variants with evidence for an impact on biological function. We discuss several examples of candidate variants/genes including CCDC8, PANK3, RHD and NLRP12. Overall, the results suggest there are many, comparatively frequent, potentially lethal or disease-causing variants lurking in healthy human populations.

The protein organization of a red blood cell

Red blood cells (RBCs) (erythrocytes) are the simplest primary human cells, lacking nuclei and major organelles and instead employing about a thousand proteins to dynamically control cellular function and morphology in response to physiological cues. In this study, we define a canonical RBC proteome and interactome using quantitative mass spectrometry and machine learning. Our data reveal an RBC interactome dominated by protein homeostasis, redox biology, cytoskeletal dynamics, and carbon metabolism. We validate protein complexes through electron microscopy and chemical crosslinking and, with these data, build 3D structural models of the ankyrin/Band 3/Band 4.2 complex that bridges the spectrin cytoskeleton to the RBC membrane. The model suggests spring-like compression of ankyrin may contribute to the characteristic RBC cell shape and flexibility. Taken together, our study provides an in-depth view of the global protein organization of human RBCs and serves as a comprehensive resource for future research.

Architecture of the human erythrocyte ankyrin-1 complex

The stability and shape of the erythrocyte membrane is provided by the ankyrin-1 complex, but how it tethers the spectrin-actin cytoskeleton to the lipid bilayer and the nature of its association with the band 3 anion exchanger and the Rhesus glycoproteins remains unknown. Here we present structures of ankyrin-1 complexes purified from human erythrocytes. We reveal the architecture of a core complex of ankyrin-1, the Rhesus proteins RhAG and RhCE, the band 3 anion exchanger, protein 4.2, glycophorin A and glycophorin B. The distinct T-shaped conformation of membrane-bound ankyrin-1 facilitates recognition of RhCE and, unexpectedly, the water channel aquaporin-1. Together, our results uncover the molecular details of ankyrin-1 association with the erythrocyte membrane, and illustrate the mechanism of ankyrin-mediated membrane protein clustering.

A Rhesus channel in the coral symbiosome membrane suggests a novel mechanism to regulate NH3 and CO2 delivery to algal symbionts

Reef-building corals maintain an intracellular photosymbiotic association with dinoflagellate algae. As the algae are hosted inside the symbiosome, all metabolic exchanges must take place across the symbiosome membrane. Using functional studies in Xenopus oocytes, immunolocalization, and confocal Airyscan microscopy, we established that Acropora yongei Rh (ayRhp1) facilitates transmembrane NH₃ and CO₂ diffusion and that it is present in the symbiosome membrane. Furthermore, ayRhp1 abundance in the symbiosome membrane was highest around midday and lowest around midnight. We conclude that ayRhp1 mediates a symbiosomal NH₄⁺-trapping mechanism that promotes nitrogen delivery to algae during the day-necessary to sustain photosynthesis-and restricts nitrogen delivery at night-to keep algae under nitrogen limitation. The role of ayRhp1-facilitated CO₂ diffusion is less clear, but it may have implications for metabolic dysregulation between symbiotic partners and bleaching. This previously unknown mechanism expands our understanding of symbioses at the immediate animal-microbe interface, the symbiosome.

Coexistence of Ammonium Transporter and Channel Mechanisms in Amt-Mep-Rh Twin-His Variants Impairs the Filamentation Signaling Capacity of Fungal Mep2 Transceptors

Ammonium translocation through biological membranes, by the ubiquitous Amt-Mep-Rh family of transporters, plays a key role in all domains of life. Two highly conserved histidine residues protrude into the lumen of the pore of these transporters, forming the family’s characteristic Twin-His motif. It has been hypothesized that the motif is essential to confer the selectivity of the transport mechanism. Here, using a combination of in vitro electrophysiology on Escherichia coli AmtB, in silico molecular dynamics simulations, and in vivo yeast functional complementation assays, we demonstrate that variations in the Twin-His motif trigger a mechanistic switch between a specific transporter, depending on ammonium deprotonation, to an unspecific ion channel activity. We therefore propose that there is no selective filter that governs specificity in Amt-Mep-Rh transporters, but the inherent mechanism of translocation, dependent on the fragmentation of the substrate, ensures the high specificity of the translocation. We show that coexistence of both mechanisms in single Twin-His variants of yeast Mep2 transceptors disrupts the signaling function and so impairs fungal filamentation. These data support a signaling process driven by the transport mechanism of the fungal Mep2 transceptors.

Posttranslational regulation of transporters important for symbiotic interactions

Coordinated sharing of nutritional resources is a central feature of symbiotic interactions, and, despite the importance of this topic, many questions remain concerning the identification, activity, and regulation of transporter proteins involved. Recent progress in obtaining genome and transcriptome sequences for symbiotic organisms provides a wealth of information on plant, fungal, and bacterial transporters that can be applied to these questions. In this update, we focus on legume-rhizobia and mycorrhizal symbioses and how transporters at the symbiotic interfaces can be regulated at the protein level. We point out areas where more research is needed and ways that an understanding of transporter mechanism and energetics can focus hypotheses. Protein phosphorylation is a predominant mechanism of posttranslational regulation of transporters in general and at the symbiotic interface specifically. Other mechanisms of transporter regulation, such as protein-protein interaction, including transporter multimerization, polar localization, and regulation by pH and membrane potential are also important at the symbiotic interface. Most of the transporters that function in the symbiotic interface are members of transporter families; we bring in relevant information on posttranslational regulation within transporter families to help generate hypotheses for transporter regulation at the symbiotic interface.

Treatment of Hyperammonemia by Transplanting a Symbiotic Pair of Intestinal Microbes

Hyperammonemia is a deleterious and inevitable consequence of liver failure. However, no adequate therapeutic agent is available for hyperammonemia. Although recent studies showed that the pharmabiotic approach could be a therapeutic option for hyperammonemia, its development is clogged with poor identification of etiological microbes and low transplantation efficiency of candidate microbes. In this study, we developed a pharmabiotic treatment for hyperammonemia that employs a symbiotic pair of intestinal microbes that are both able to remove ammonia from the surrounding environment. By a radioactive tracing experiment in mice, we elucidated how the removal of ammonia by probiotics in the intestinal lumen leads to lower blood ammonia levels. After determination of the therapeutic mechanism, ammonia-removing probiotic strains were identified by high-throughput screening of gut microbes. The symbiotic partners of ammonia-removing probiotic strains were identified by screening intestinal microbes of a human gut, and the pairs were administrated to hyperammonemic mice to evaluate therapeutic efficacy. Blood ammonia was in a chemical equilibrium relationship with intestinal ammonia. Lactobacillus reuteri JBD400 removed intestinal ammonia to shift the chemical equilibrium to lower the blood ammonia level. L. reuteri JBD400 was successfully transplanted with a symbiotic partner, Streptococcus rubneri JBD420, improving transplantation efficiency 2.3×10³ times more compared to the sole transplantation while lowering blood ammonia levels significantly. This work provides new pharmabiotics for the treatment of hyperammonemia as well as explains its therapeutic mechanism. Also, this approach provides a concept of symbiotic pairs approach in the emerging field of pharmabiotics.

The Significance of RHD Genotyping and Characteristic Analysis in Chinese RhD Variant Individuals

Background

RhD is the most important and complex blood group system because of its highly polymorphic and immunogenic nature. RhD variants can induce immune response by allogeneic transfusion, organ transplantation, and fetal immunity. The transfusion strategies are different for RhD variants formed by various alleles. Therefore, extensive investigation of the molecular mechanism underlying RhD variants is critical for preventing immune-related blood transfusion reactions and fetal immunity.

Methods

RhD variants were collected from donors and patients in Zhejiang Province, China. The phenotypes were classified using the serologic method. The full coding regions of RHD gene were analyzed using the PCR-SBT method. The multiplex ligation-dependent probe amplification (MLPA) assay was used to analyze the genotype and gene copy number. SWISS-MODLE and PyMOL software were used to analyze 3D structures of RhD caused by the variant alleles. The effect of non-synonymous substitutions was predicted using Polymorphism Phenotyping algorithm (PolyPhen-2), Sorting Intolerant From Tolerant (SIFT), and Protein Variation Effect Analyzer (PROVEAN) software.

Results

In the collected RhD variants, 28 distinct RHD variant alleles were identified, including three novel variant alleles. RH-MLPA assay is advantageous for determining the copy number of RHD gene. 3D homology modeling predicted that protein conformation was disrupted and may explain RhD epitope differential expression. A total of 14 non-synonymous mutations were determined to be detrimental to the protein structure.

Discussion

We revealed the diversity of RHD alleles present in eastern Chinese RhD variants. The bioinformatics of these variant alleles extended our knowledge of RhD variants, which was crucial for evaluating their impact to guide transfusion support and avoid immune-related blood transfusion reactions.

COVID-19 coagulopathies: Human blood proteins mimic SARS-CoV-2 virus, vaccine proteins and bacterial co-infections inducing autoimmunity: Combinations of bacteria and SARS-CoV-2 synergize to induce autoantibodies targeting cardiolipin, cardiolipin-binding proteins, platelet factor 4, prothrombin, and coagulation factors

Severe COVID-19 is often accompanied by coagulopathies such as thrombocytopenia and abnormal clotting. Rarely, such complications follow SARS-CoV-2 vaccination. The cause of these coagulopathies is unknown. It is hypothesized that coagulopathies accompanying SARS-CoV-2 infections and vaccinations result from bacterial co-infections that synergize with virus-induced autoimmunity due to antigenic mimicry of blood proteins by both bacterial and viral antigens. Coagulopathies occur mainly in severe COVID-19 characterized by bacterial co-infections with Streptococci, Staphylococci, Klebsiella, Escherichia coli, and Acinetobacter baumannii. These bacteria express unusually large numbers of antigens mimicking human blood antigens, as do both SARS-CoV-2 and adenoviruses. Bacteria mimic cardiolipin, prothrombin, albumin, and platelet factor 4 (PF4). SARS-CoV-2 mimics complement factors, Rh antigens, platelet phosphodiesterases, Factors IX and X, von Willebrand Factor (VWF), and VWF protease ADAMTS13. Adenoviruses mimic prothrombin and platelet factor 4. Bacterial prophylaxis, avoidance of vaccinating bacterially infected individuals, and antigen deletion for vaccines may reduce coagulopathy risk. Also see the video abstract here: https://youtu.be/zWDOsghrPg8.

Optimisation of Neuraminidase Expression for Use in Drug Discovery by Using HEK293-6E Cells

Influenza virus is a highly contagious virus that causes significant human mortality and morbidity annually. The most effective drugs for treating influenza are the neuraminidase inhibitors, but resistance to these inhibitors has emerged, and additional drug discovery research on neuraminidase and other targets is needed. Traditional methods of neuraminidase production from embryonated eggs are cumbersome, while insect cell derived protein is less reflective of neuraminidase produced during human infection. Herein we describe a method for producing neuraminidase from a human cell line, HEK293-6E, and demonstrate the method by producing the neuraminidase from the 1918 H1N1 pandemic influenza strain. This method produced high levels of soluble neuraminidase expression (>3000 EU/mL), was enhanced by including a secretion signal from a viral chemokine binding protein, and does not require co-expression of additional proteins. The neuraminidase produced was of sufficient quantity and purity to support high resolution crystal structure determination. The structure solved using this protein conformed to the previously reported structure. Notably the glycosylation at three asparagine residues was superior in quality to that from insect cell derived neuraminidase. This method of production of neuraminidase should prove useful in further studies, such as the characterisation of inhibitor binding.

A guide to plasma membrane solute carrier proteins

This review aims to serve as an introduction to the solute carrier proteins (SLC) superfamily of transporter proteins and their roles in human cells. The SLC superfamily currently includes 458 transport proteins in 65 families that carry a wide variety of substances across cellular membranes. While members of this superfamily are found throughout cellular organelles, this review focuses on transporters expressed at the plasma membrane. At the cell surface, SLC proteins may be viewed as gatekeepers of the cellular milieu, dynamically responding to different metabolic states. With altered metabolism being one of the hallmarks of cancer, we also briefly review the roles that surface SLC proteins play in the development and progression of cancer through their influence on regulating metabolism and environmental conditions.

Exploration of novel candidate genes involved in epidermal keratinocyte differentiation and skin barrier repair in man

A proper skin barrier function requires constant formation of stratum corneum, i.e. the outermost layer of epidermis composed of terminally differentiated keratinocytes. The complex process of converting proliferative basal keratinocytes into corneocytes relies on programmed changes in the activity of many well-established genes. Much remains however to be investigated about this process, e.g. in conjunction with epidermal barrier defects due to genetic errors as in ichthyosis. To this end, we re-analyzed two sets of microarray-data comparing altered gene expression in differentiated vs. proliferating keratinocytes and in the skin of patients with autosomal recessive congenital ichthyosis (ARCI) vs. healthy controls, respectively. We thus identified 24 genes to be upregulated in both sets of array and not previously associated with keratinocyte differentiation. For 10 of these genes (AKR1B10, BLNK, ENDOU, GCNT4, GLTP, RHCG, SLC15A1, TMEM45B, TMEM86A and VSNL1), qPCR analysis confirmed the array results and subsequent immunostainings of normal epidermis showed superficial expression of several of the proteins. Furthermore, induction of keratinocyte differentiation using phorbol esters (PMA) resulted in increased expression of eight of the genes, whereas siRNA silencing of PPARδ, a transcription factor supporting differentiation, had the opposite effect. In summary, our results identify ten new candidate genes seemingly involved in human epidermal keratinocyte differentiation and possibly important for epidermal repair in a genetic skin disease characterized by barrier failure.

Carbon dioxide transport across membranes

Carbon dioxide (CO2) movement across cellular membranes is passive and governed by Fick's law of diffusion. Until recently, we believed that gases cross biological membranes exclusively by dissolving in and then diffusing through membrane lipid. However, the observation that some membranes are CO2 impermeable led to the discovery of a gas molecule moving through a channel; namely, CO2 diffusion through aquaporin-1 (AQP1). Later work demonstrated CO2 diffusion through rhesus (Rh) proteins and NH3 diffusion through both AQPs and Rh proteins. The tetrameric AQPs exhibit differential selectivity for CO2 versus NH3 versus H2O, reflecting physico-chemical differences among the small molecules as well as among the hydrophilic monomeric pores and hydrophobic central pores of various AQPs. Preliminary work suggests that NH3 moves through the monomeric pores of AQP1, whereas CO2 moves through both monomeric and central pores. Initial work on AQP5 indicates that it is possible to create a metal-binding site on the central pore's extracellular face, thereby blocking CO2 movement. The trimeric Rh proteins have monomers with hydrophilic pores surrounding a hydrophobic central pore. Preliminary work on the bacterial Rh homologue AmtB suggests that gas can diffuse through the central pore and three sets of interfacial clefts between monomers. Finally, initial work indicates that CO2 diffuses through the electrogenic Na/HCO3 cotransporter NBCe1. At least in some cells, CO2-permeable proteins could provide important pathways for transmembrane CO2 movements. Such pathways could be amenable to cellular regulation and could become valuable drug targets.

Insights into anti-D formation in carriers of RhD variants through studies of 3D intraprotein interactions

BACKGROUND

Many RhD variants associated with anti-D formation (partial D) in carriers exposed to the conventional D antigen carry mutations affecting extracellular loop residues. Surprisingly, some carry mutations affecting transmembrane or intracellular domains, positions not thought likely to have a major impact on D epitopes.

STUDY DESIGN AND METHODS

A wild-type Rh trimer (RhD₁ RhAG₂ ) was modeled by comparative modeling with the human RhCG structure. Taking trimer conformation, residue accessibility, and position relative to the lipid bilayer into account, we redefine the domains of the RhD protein. We generated models for RhD variants carrying one or two amino acid substitutions associated with anti-D formation in published articles (25 variants) or abstracts (12 variants) and for RHD*weak D type 38. We determined the extracellular substitutions and compared the interactions of the variants with those of the standard RhD.

RESULTS

The findings of the three-dimensional (3D) analysis were correlated with anti-D formation for 76% of RhD variants: 15 substitutions associated with anti-D formation concerned extracellular residues, and structural differences in intraprotein interactions relative to standard RhD were observed in the others. We discuss the mechanisms by which D epitopes may be modified in variants in which the extracellular residues are identical to those of standard RhD and provide arguments for the benignity of p.T379M (RHD*DAU0) and p.G278D (RHD*weak D type 38) in transfusion medicine.

CONCLUSION

The study of RhD intraprotein interactions and the precise redefinition of residue accessibility provide insight into the mechanisms through which RhD point mutations may lead to anti-D formation in carriers.

Highlighting membrane protein structure and function: A celebration of the Protein Data Bank

Biological membranes define the boundaries of cells and compartmentalize the chemical and physical processes required for life. Many biological processes are carried out by proteins embedded in or associated with such membranes. Determination of membrane protein (MP) structures at atomic or near-atomic resolution plays a vital role in elucidating their structural and functional impact in biology. This endeavor has determined 1198 unique MP structures as of early 2021. The value of these structures is expanded greatly by deposition of their three-dimensional (3D) coordinates into the Protein Data Bank (PDB) after the first atomic MP structure was elucidated in 1985. Since then, free access to MP structures facilitates broader and deeper understanding of MPs, which provides crucial new insights into their biological functions. Here we highlight the structural and functional biology of representative MPs and landmarks in the evolution of new technologies, with insights into key developments influenced by the PDB in magnifying their impact.

Single molecule distribution of RhD binding epitopes on ultraflat erythrocyte ghosts

The Rh blood group system plays a key role in transfusion and organ transplant medicine. The complex transmembrane Rh polypeptides RhD and RhCE carry numerous antigens, including the extremely immunogenic D antigen. The Rh polypeptides form multimolecular Rh complexes with certain transmembrane and skeletal proteins, with so far only incompletely understood physiological functions. Determination of the energy landscape of individual Rh binding epitopes towards their specific interaction partners as well as their localization across the red blood cell (RBC) membrane requires single molecule approaches including large area high resolution recognition imaging. Atomic force microscopy based molecular recognition force spectroscopy in combination with single molecule recognition imaging fulfills these requirements. For unbiased single molecule results, nano-mechanical influences due to cell elasticity have to be eliminated. This is realized by generation of ultra flat erythrocyte ghosts on a solid support. We developed a protocol for the preparation of complete ultraflat erythrocyte ghosts and determined the molecular binding behaviour of different anti-D antibodies towards their binding epitopes on RhD positive and negative erythrocytes. Performing optimized topography and recognition imaging at 16 Mpixel resolution allowed localisation of individual RhD molecules at the single molecule level across an entire RBC. A map of Rh antigens across integer ultraflat RBC ghosts was generated with nanometer resolution. Here we show a homogeneous distribution on rim and dimple regions with comparable receptor densities. Furthermore, differences in the energy landscape between specific monoclonal antibodies were determined at the single molecule level.

Toward a Systematic Structural and Functional Annotation of Solute Carriers Transporters-Example of the SLC6 and SLC7 Families

SLC transporters are emerging key drug targets. One important step for drug development is the profound understanding of the structural determinants defining the substrate selectivity of each transporter. Recently, the improvement of computational power and experimental methods such as X-ray and cryo-EM crystallography permitted to conduct structure-based studies on specific transporters having important pharmacological impact. However, a lot remains to be discovered regarding their dynamics, transport modulation and ligand recognition. A detailed functional characterization of transporters would provide opportunities to develop new compounds targeting these key drug targets. Here, we are giving an overview of two major human LeuT-fold families, SLC6 and SLC7, with an emphasis on the most relevant members of each family for drug development. We gather the most recent understanding on the structural determinants of selectivity within and across the two families. We then use this information to discuss the benefits of a more generalized structural and functional annotation of the LeuT fold and the implications of such mapping for drug discovery.

A two-lane mechanism for selective biological ammonium transport

The transport of charged molecules across biological membranes faces the dual problem of accommodating charges in a highly hydrophobic environment while maintaining selective substrate translocation. This has been the subject of a particular controversy for the exchange of ammonium across cellular membranes, an essential process in all domains of life. Ammonium transport is mediated by the ubiquitous Amt/Mep/Rh transporters that includes the human Rhesus factors. Here, using a combination of electrophysiology, yeast functional complementation and extended molecular dynamics simulations, we reveal a unique two-lane pathway for electrogenic NH₄⁺ transport in two archetypal members of the family, the transporters AmtB from Escherichia coli and Rh50 from Nitrosomonas europaea. The pathway underpins a mechanism by which charged H⁺ and neutral NH₃ are carried separately across the membrane after NH₄⁺ deprotonation. This mechanism defines a new principle of achieving transport selectivity against competing ions in a biological transport process.

Rh type C-glycoprotein functions as a novel tumor suppressor gene by inhibiting tumorigenicity and metastasis in head and neck squamous cell carcinoma

Head and neck squamous cell carcinoma (HNSCC), a major histologic subtype of head and neck cancer, presents great mortality and morbidity worldwide. The aim of this study is to discover new potential biomarkers closely correlated with HNSCC progression. In this study, weighted gene co-expression network analysis was applied to construct a co-expression network, and the brown module was identified as the most correlated with HNSCC progression. Hub gene identification combined with survival analyses determined RHCG as a candidate biomarker for cancer progression and prognosis prediction. Further experimental results proved that RHCG was aberrantly downregulated in HNSCC tissues and cell lines. Moreover, decreased RHCG expression was shown to be associated with advanced stage and dismal prognosis in HNSCC patients. Functional assays revealed that RHCG could inhibit cell viability, clonogenicity, cell migration in vitro and suppress tumor formation in vivo. Further bioinformatics study demonstrated that DNA promoter hypermethylation of RHCG could lead to its downregulation and serve as potential prognostic maker in HNSCC. Our study reveals that RHCG acts as a tumor suppressor gene that plays a crucial role in inhibiting tumorigenicity and metastasis in HNSCC, which will shed light on the potential diagnostic and therapeutic strategies for HNSCC.

Yeast filamentation signaling is connected to a specific substrate translocation mechanism of the Mep2 transceptor

The dimorphic transition from the yeast to the filamentous form of growth allows cells to explore their environment for more suitable niches and is often crucial for the virulence of pathogenic fungi. In contrast to their Mep1/3 paralogues, fungal Mep2-type ammonium transport proteins of the conserved Mep-Amt-Rh family have been assigned an additional receptor role required to trigger the filamentation signal in response to ammonium scarcity. Here, genetic, kinetic and structure-function analyses were used to shed light on the poorly characterized signaling role of Saccharomyces cerevisiae Mep2. We show that Mep2 variants lacking the C-terminal tail conserve the ability to induce filamentation, revealing that signaling can proceed in the absence of exclusive binding of a putative partner to the largest cytosolic domain of the protein. Our data support that filamentation signaling requires the conformational changes accompanying substrate translocation through the pore crossing the hydrophobic core of Mep2. pHluorin reporter assays show that the transport activity of Mep2 and of non-signaling Mep1 differently affect yeast cytosolic pH in vivo, and that the unique pore variant Mep2^H194E, with apparent uncoupling of transport and signaling functions, acquires increased ability of acidification. Functional characterization in Xenopus oocytes reveals that Mep2 mediates electroneutral substrate translocation while Mep1 performs electrogenic transport. Our findings highlight that the Mep2-dependent filamentation induction is connected to its specific transport mechanism, suggesting a role of pH in signal mediation. Finally, we show that the signaling process is conserved for the Mep2 protein from the human pathogen Candida albicans.

Fungal Mep2-type ammonium transport proteins of the conserved Mep-Amt-Rh family that includes human Rhesus factors are specifically required to allow filamentation in response to ammonium limitation. These proteins were therefore assigned a receptor role while the underlying mechanism of signal transduction remains poorly understood. The “transceptor” property has subsequently been proposed to concern transporters of all kind of micro- and macro- nutrients in eukaryotes, from fungi to human. However, bringing the firm demonstration of their existence remains challenging as variants with full uncoupling of transport and receptor functions are difficult to obtain. Our data question the involvement of the C-terminal extremity of Saccharomyces cerevisiae Mep2 in the signal mediation leading to filamentation. If signaling partners exist, they should also bind to cytosolic loops and/or membrane-embedded domains. The capacity of Mep2 to enable filamentation is closely intertwined to the mechanism of substrate translocation through the pore of the hydrophobic core of the protein. In Xenopus oocytes, the transport activity of non-signaling Mep1 is electrogenic while it is electroneutral for Mep2, the latter likely translocating the weak base NH₃, but not the proton released after NH₄⁺ recognition and depronotation. We propose that given consequences of a Mep2-specific transport process, such as an intracellular pH modification, could be the underlying cause of the filamentation signal ensured by Mep2-type proteins.

Development of Aedes aegypti (Diptera: Culicidae) mosquito larvae in high ammonia sewage in septic tanks causes alterations in ammonia excretion, ammonia transporter expression, and osmoregulation

Larvae of the disease vector mosquito, Aedes aegypti (L.) readily develop in ammonia rich sewage in the British Virgin Islands. To understand how the larvae survive in ammonia levels that are lethal to most animals, an examination of ammonia excretory physiology in larvae collected from septic-water and freshwater was carried out. A. aegypti larvae were found to be remarkably plastic in dealing with high external ammonia through the modulation of NH₄⁺ excretion at the anal papillae, measured using the scanning ion-selective electrode technique (SIET), and NH₄⁺ secretion in the primary urine by the Malpighian tubules when developing in septicwater. Ammonia transporters, Amt and Rh proteins, are expressed in ionoregulatory and excretory organs, with increases in Rh protein, Na⁺-K⁺-ATPase, and V-type-H⁺-ATPase expression observed in the Malpighian tubules, hindgut, and anal papillae in septic-water larvae. A comparative approach using laboratory A. aegypti larvae reared in high ammonia septic-water revealed similar responses to collected A. aegypti with regard to altered ammonia secretion and hemolymph ion composition. Results suggest that the observed alterations in excretory physiology of larvae developing in septic-water is a consequence of the high ammonia levels and that A. aegypti larvae may rely on ammonia transporting proteins coupled to active transport to survive in septic-water.

Functional Attributes of Antibodies, Effector Cells, and Target Cells Affecting NK Cell-Mediated Antibody-Dependent Cellular Cytotoxicity

Ab-dependent cellular cytotoxicity (ADCC) is one of the most important effector mechanisms of tumor-targeting Abs in current immunotherapies. In ADCC and other Ab-dependent activation of myeloid effector cells, close cell-cell contact (between effector and target cell) and formation of immunological synapses are required. However, we still lack basic knowledge on the principal factors influencing ADCC potential by therapeutic Abs. In this study we investigated the combined roles of five factors affecting human NK cell-mediated ADCC, namely: 1) Ag density, 2) target cell membrane composition, 3) IgG FcγR polymorphism, 4) FcγR-blocking cytophilic Abs, and 5) Ab fucosylation. We demonstrate that the magnitude of NK cell-mediated ADCC responses is predominantly influenced by Ag density and Ab fucosylation. Afucosylation consistently induced efficient ADCC, even at very low Ag density, where fucosylated target Abs did not elicit ADCC. On the side of the effector cell, the FcγRIIIa-Val/Phe158 polymorphism influenced ADCC potency, with NK cells expressing the Val158 variant showing more potent ADCC. In addition, we identified the sialic acid content of the target cell membrane as an important inhibitory factor for ADCC. Furthermore, we found that the presence and glycosylation status of aspecific endogenous Abs bound to NK cell FcγRIIIa (cytophilic Abs) determine the blocking effect on ADCC. These five parameters affect the potency of Abs in vitro and should be further tested as predictors of in vivo capacity.

Detergent-free isolation of native red blood cell membrane complexes

Over the past few decades, studies on the red blood cell (RBC) membrane gave rise to increasingly sophisticated although divergent models of its structural organization, since investigations were often performed in denaturing conditions using detergents. To access soluble isolated RBC membrane complexes with the preservation of their interactions and conformations, we decided to apply the recent SMALP (Styrene Maleic Acid Lipid Particles) technology to RBC ghosts. Depending on the ionic strength of buffers in which ghost membranes were resuspended, the isolated proteins within SMALPs could differ on Coomassie-stained gels, but with few changes when compared to ghost membrane SDS lysates. We subsequently produced SMALPs derived from ghosts from two different blood group phenotypes, RhD-positive and RhD-negative, both types of RBC expressing the RhCE proteins but only RhD-positive cells being able to express the RhD proteins. This allowed the isolation, by size exclusion chromatography (SEC), of soluble fractions containing the Rh complex, including the RhD protein or not, within SMALPs. The use a conformation-dependent anti-RhD antibody in immunoprecipitation studies performed on SEC fractions of SMALPs containing Rh proteins clearly demonstrated that the RhD protein, which was only present in SMALPs prepared from RhD-positive RBC ghosts, has preserved at least one important conformational RhD epitope. This approach opens new perspectives in the field of the erythroid membrane study, such as visualization of RBC membrane complexes in native conditions by cryo-electron microscopy (CryoEM) or immuno-tests with conformation-dependent antibodies against blood group antigens on separated and characterized SMALPs containing RBC membrane proteins.

Nutrient and Stress Sensing in Pathogenic Yeasts

More than 1.5 million fungal species are estimated to live in vastly different environmental niches. Despite each unique host environment, fungal cells sense certain fundamentally conserved elements, such as nutrients, pheromones and stress, for adaptation to their niches. Sensing these extracellular signals is critical for pathogens to adapt to the hostile host environment and cause disease. Hence, dissecting the complex extracellular signal-sensing mechanisms that aid in this is pivotal and may facilitate the development of new therapeutic approaches to control fungal infections. In this review, we summarize the current knowledge on how two important pathogenic yeasts, Candida albicans and Cryptococcus neoformans, sense nutrient availability, such as carbon sources, amino acids, and ammonium, and different stress signals to regulate their morphogenesis and pathogenicity in comparison with the non-pathogenic model yeast Saccharomyces cerevisiae. The molecular interactions between extracellular signals and their respective sensory systems are described in detail. The potential implication of analyzing nutrient and stress-sensing systems in antifungal drug development is also discussed.

Biocompatible coupling of therapeutic fusion proteins to human erythrocytes

Carriage of drugs by red blood cells (RBCs) modulates pharmacokinetics, pharmacodynamics, and immunogenicity. However, optimal targets for attaching therapeutics to human RBCs and adverse effects have not been studied. We engineered nonhuman-primate single-chain antibody fragments (scFvs) directed to human RBCs and fused scFvs with human thrombomodulin (hTM) as a representative biotherapeutic cargo (hTM-scFv). Binding fusions to RBCs on band 3/glycophorin A (GPA; Wright b [Wr^b] epitope) and RhCE (Rh17/Hr0 epitope) similarly endowed RBCs with hTM activity, but differed in their effects on RBC physiology. scFv and hTM-scFv targeted to band 3/GPA increased membrane rigidity and sensitized RBCs to hemolysis induced by mechanical stress, while reducing sensitivity to hypo-osmotic hemolysis. Similar properties were seen for other ligands bound to GPA and band 3 on human and murine RBCs. In contrast, binding of scFv or hTM-scFv to RhCE did not alter deformability or sensitivity to mechanical and osmotic stress at similar copy numbers bound per RBCs. Contrasting responses were also seen for immunoglobulin G antibodies against band 3, GPA, and RhCE. RBC-bound hTM-scFv generated activated protein C (APC) in the presence of thrombin, but RhCE-targeted hTM-scFv demonstrated greater APC generation per bound copy. Both Wr^b- and RhCE-targeted fusion proteins inhibited fibrin deposition induced by tumor necrosis factor-α in an endothelialized microfluidic model using human whole blood. RhCE-bound hTM-scFv more effectively reduced platelet and leukocyte adhesion, whereas anti-Wr^b scFv appeared to promote platelet adhesion. These data provide a translational framework for the development of engineered affinity ligands to safely couple therapeutics to human RBCs.

Structural biology and structure–function relationships of membrane proteins

The study of structure–function relationships of membrane proteins (MPs) has been one of the major goals in the field of structural biology. Many Noble Prizes regarding remarkable accomplishments in MP structure determination and biochemistry have been awarded over the last few decades. Mutations or improper folding of these proteins are associated with numerous serious illnesses. Therefore, as important drug targets, the study of their primary sequence and three-dimensional fold, combined with cell-based assays, provides vital information about their structure–function relationships. Today, this information is vital to drug discovery and medicine. In the last two decades, many have been the technical advances and breakthroughs in the field of MP structural biology that have contributed to an exponential growth in the number of unique MP structures in the Protein Data Bank. Nevertheless, given the medical importance and many unanswered questions, it will never be an excess of MP structures, regardless of the method used. Owing to the extension of the field, in this brief review, we will only focus on structure–function relationships of the three most significant pharmaceutical classes: G protein-coupled receptors, ion channels and transporters.

Optimising the transient expression of GABA(A) receptors in adherent HEK293 cells

Owing to their therapeutic relevance, considerable efforts are devoted to the structural characterisation of membrane proteins. Such studies are limited by the availability of high quality protein due to the difficulty of overexpression in recombinant mammalian systems. We sought to systematically optimise multiple aspects in the process of transiently transfecting HEK293 cells, to allow the rapid expression of membrane proteins, without the lengthy process of stable clone formation. We assessed the impact of medium formulation, cell line, and harvest time on the expression of GABA_A receptors, as determined by [³H]muscimol binding in cell membranes. Furthermore, transfection with the use of calcium phosphate/polyethyleneimine multishell nanoparticles was optimised, and a dual vector system utilising viral enhancing elements was designed and implemented. These efforts resulted in a 40-fold improvement in GABA_A α₁β₃ receptor expression, providing final yields of 22 fmol/cm². The findings from this work provide a guide to the optimisation of transient expression of proteins in mammalian cells and should assist in the structural characterisation of membrane proteins.

MpAMT1;2 from Marchantia polymorpha is a High-Affinity, Plasma Membrane Ammonium Transporter

Plant ammonium transporters in the AMT/MEP/Rh (ammonium transporter/methylammonium and ammonium permease/Rhesus factor) superfamily have only been previously characterized in flowering plants (angiosperms). Plant AMT1s are electrogenic, while plant AMT2s are electroneutral, and MEP and Rh transporters in other organisms are electroneutral. We analyzed the transport function of MpAMT1;2 from the basal land plant Marchantia polymorpha, a liverwort. MpAMT1;2 was shown to localize to the plasma membrane in Marchantia gametophyte thallus by stable transformation using a C-terminal citrine fusion. MpAMT1;2 expression was studied using quantitative real-time PCR and shown to be higher when plants were N deficient and lower when plants were grown on media containing ammonium, nitrate or the amino acid glutamine. Expression in Xenopus oocytes and analysis by electrophysiology revealed that MpAMT1;2 is an electrogenic ammonium transporter with a very high affinity for ammonium (7 µM at pH 5.6 and a membrane potential of -137 mV). A conserved inhibitory phosphorylation site identified in angiosperm AMT1s is also present in all AMT1s in Marchantia. Here we show that a phosphomimetic mutation T475D in MpAMT1;2 completely inhibits ammonium transport activity. The results indicate that MpAMT1;2 may be important for ammonium uptake into cells in the Marchantia thallus.

Structural biology of solute carrier (SLC) membrane transport proteins

The human solute carriers (SLCs) comprise over 400 different transporters, organized into 65 families ( http://slc.bioparadigms.org/ ) based on their sequence homology and transport function. SLCs are responsible for transporting extraordinarily diverse solutes across biological membranes, including inorganic ions, amino acids, lipids, sugars, neurotransmitters and drugs. Most of these membrane proteins function as coupled symporters (co-transporters) utilizing downhill ion (H⁺ or Na⁺) gradients as the driving force for the transport of substrate against its concentration gradient into cells. Other members work as antiporters (exchangers) that typically contain a single substrate-binding site with an alternating access mode of transport, while a few members exhibit channel-like properties. Dysfunction of SLCs is correlated with numerous human diseases and therefore they are potential therapeutic drug targets. In this review, we identified all of the SLC crystal structures that have been determined, most of which are from prokaryotic species. We further sorted all the SLC structures into four main groups with different protein folds and further discuss the well-characterized MFS (major facilitator superfamily) and LeuT (leucine transporter) folds. This review provides a systematic analysis of the structure, molecular basis of substrate recognition and mechanism of action in different SLC family members.

Ammonia Excretion in an Osmoregulatory Syncytium Is Facilitated by AeAmt2, a Novel Ammonia Transporter in Aedes aegypti Larvae

The larvae of the mosquito Aedes aegypti inhabit ammonia rich septic tanks in tropical regions of the world that make extensive use of these systems, explaining the prevalence of disease during dry seasons. Since ammonia (NH₃/NH4+) is toxic to animals, an understanding of the physiological mechanisms of ammonia excretion permitting the survival of A. aegypti larvae in high ammonia environments is important. We have characterized a novel ammonia transporter, AeAmt2, belonging to the Amt/MEP/Rh family of ammonia transporters. Based on the amino acid sequence, the predicted topology of AeAmt2 consists of 11 transmembrane helices with an extracellular N-terminus and a cytoplasmic C-terminus region. Alignment of the predicted AeAmt2 amino acid sequence with other Amt/MEP proteins from plants, bacteria, and yeast highlights the presence of conserved residues characteristic of ammonia conducting channels in this protein. AeAmt2 is expressed in the ionoregulatory anal papillae of A. aegypti larvae where it is localized to the apical membrane of the epithelium. dsRNA-mediated knockdown of AeAmt2 results in a significant decrease in NH4+ efflux from the anal papillae, suggesting a key role in facilitating ammonia excretion. The effect of high environmental ammonia (HEA) on expression of AeAmt2, along with previously characterized AeAmt1, AeRh50-1, and AeRh50-2 in the anal papillae was investigated. We show that changes in expression of ammonia transporters occur in response to acute and chronic exposure to HEA, which reflects the importance of these transporters in the physiology of life in high ammonia habitats.

A RHAG point mutation selectively disrupts Rh antigen expression

OBJECTIVES

The aim of this study was to characterise a novel mutation in the gene encoding RhAG in order to elucidate a molecular mechanism for Rh antigen expression and spherocytosis.

BACKGROUND

Rhesus-associated glycoprotein (RhAG) is critical for maintaining the structure and stability of erythrocytes. Single missense mutations in the gene encoding RhAG are sufficient to induce spherocytosis and deficiencies in Rh complex formation. We report a novel missense mutation that incompletely disrupts Rh antigen expression and selectively knocks out RhD antigen expression.

METHODS

Blood samples were taken from a 38-year-old male, his brother, his wife and his daughter in Xi'an, China. To detect the proband's RhAG and D antigen expression, the RBC were stained with anti-D and anti-RhAG and analysed by flow cytometry. Red blood cell morphology was detected with atomic force microscopy (AFM). Genomic DNA was isolated from whole blood samples, and the RHD, RHCE and RHAG alleles were sequenced and analysed. The mutation was mapped onto a predicted crystal structure of RhAG by the I-TASSER server and visualised using PyMOL.

RESULTS

Morphological testing by AFM found clear evidence of spherocytosis in the proband's erythrocytes. RHAG gene sequencing identified the mutation at sequence 236G > A, resulting in a serine to asparagine substitution at residue 79 (S79N). Family survey indicated that inheriting this allele is necessary and sufficient to cause the condition. Mapping the mutation onto a predicted crystal structure of RhAG revealed the proximity of the mutation to the critical structural elements of the protein.

CONCLUSIONS

A novel RHAG mutation significantly lowers RhAG antigen expression and antigen-mediated agglutination intensity.

The ammonia transporter RhCG modulates urinary acidification by interacting with the vacuolar proton-ATPases in renal intercalated cells

Ammonium, stemming from renal ammoniagenesis, is a major urinary proton buffer and is excreted along the collecting duct. This process depends on the concomitant secretion of ammonia by the ammonia channel RhCG and of protons by the vacuolar-type proton-ATPase pump. Thus, urinary ammonium content and urinary acidification are tightly linked. However, mice lacking Rhcg excrete more alkaline urine despite lower urinary ammonium, suggesting an unexpected role of Rhcg in urinary acidification. RhCG and the B1 and B2 proton-ATPase subunits could be co-immunoprecipitated from kidney. In ex vivo microperfused cortical collecting ducts (CCD) proton-ATPase activity was drastically reduced in the absence of Rhcg. Conversely, overexpression of RhCG in HEK293 cells resulted in higher proton secretion rates and increased B1 proton-ATPase mRNA expression. However, in kidneys from Rhcg-/- mice the expression of only B1 and B2 subunits was altered. Immunolocalization of proton-ATPase subunits together with immuno-gold detection of the A proton-ATPase subunit showed similar localization and density of staining in kidneys from Rhcg+/+ and Rhcg-/-mice. In order to test for a reciprocal effect of intercalated cell proton-ATPases on Rhcg activity, we assessed Rhcg and proton-ATPase activities in microperfused CCD from Atp6v1b1-/- mice and showed reduced proton-ATPase activity without altering Rhcg activity. Thus, RhCG and proton-ATPase are located within the same cellular protein complex. RhCG may modulate proton-ATPase function and urinary acidification, whereas proton-ATPase activity does not affect RhCG function. This mechanism may help to coordinate ammonia and proton secretion beyond physicochemical driving forces.

Ammonia excretion in aquatic invertebrates: new insights and questions

Invertebrates employ a variety of ammonia excretion strategies to facilitate their survival in diverse aquatic environments, including freshwater, seawater and the water film surrounding soil particles. Various environmental properties set innate challenges for an organism's ammonia excretory capacity. These include the availability of NaCl and the respective ion-permeability of the organism's transport epithelia, and the buffering capacity of their immediate surrounding medium. To this end, some transporters seem to be conserved in the excretory process. This includes the Na+/K+(NH4+)-ATPase (NKA), the NH3/CO2 dual gas-channel Rhesus (Rh)-proteins and novel ammonia transporters (AMTs), which have been identified in several invertebrates but appear to be absent from vertebrates. In addition, recent evidence strongly suggests that the hyperpolarization-activated cyclic nucleotide-gated K+ channel (HCN) plays a significant role in ammonia excretion and is highly conserved throughout the animal kingdom. Furthermore, microtubule-dependent vesicular excretion pathways have been found in marine and soil-dwelling species, where, unlike freshwater systems, acid-trapping of excreted ammonia is difficult or absent owing to the high environmental buffering capacity of the surroundings. Finally, although ammonia is known to be a toxic nitrogenous waste product, certain marine species readily maintain potentially toxic hemolymph ammonia as a sort of ammonia homeostasis, which suggests that ammonia is involved in physiological processes and does not exist simply for excretion. Such findings are discussed within this Commentary and are hypothesized to be involved in acid–base regulation. We also describe excretory organs and processes that are dependent on environmental constraints and indicate gaps in the current knowledge in these topics.

RHCG Suppresses Tumorigenicity and Metastasis in Esophageal Squamous Cell Carcinoma via Inhibiting NF-κB Signaling and MMP1 Expression

Background and Aims: Esophageal squamous cell carcinoma (ESCC), a major histologic subtype of esophageal cancer, is increasing in incidence, but the genetic underpinnings of this disease remain unexplored. The aim of this study is to identify the recurrent genetic changes, elucidate their roles and discover new biomarkers for improving clinical management of ESCC. Methods: Western blotting and immunohistochemistry were performed to detect the expression level of RHCG. Bisulfite genomic sequencing (BGS) and methylation-specific PCR (MSP) were used to study the methylation status in the promoter region of RHCG. The tumor-suppressive effect of RHCG was determined by both in-vitro and in-vivo assays. Affymetrix cDNA microarray was used to identify the underlying molecular mechanism. Results:RHCG was frequently downregulated in ESCCs, which was significantly correlated with poor differentiation (P = 0.001), invasion (P = 0.003), lymph node metastasis (P = 0.038) and poorer prognosis (P < 0.001). Demethylation treatment and bisulfite genomic sequencing analyses revealed that the downregulation of RHCG in both ESCC cell lines and clinical samples was associated with its promoter hypermethylation. Functional assays demonstrated that RHCG could inhibit clonogenicity, cell motility, tumor formation and metastasis in mice. Further study revealed that RHCG could stabilize IκB by decreasing its phosphorylation, and subsequently inhibit NF-κB/p65 activation by blocking the nuclear translocation of p65, where it acted as a transcription regulator for the upregulation of MMP1 expression. Conclusions: Our results support the notion that RHCG is a novel tumor suppressor gene that plays an important role in the development and progression of ESCC.

Identification of the role of Rh protein in ammonia excretion of swimming crab Portunus trituberculatus

In Portunus trituberculatus, a full-length cDNA of Rhesus-like glycoprotein (Rh protein), the whole 478 amino acids, has been identified in gills, which plays an essential role in ammonia (NH3 /NH4+) excretion. Phylogenetic analysis of the Rh-like proteins from crabs was clustered, showing high conservation of the ammonium transporter domain and transmembrane segments essential to the function of Rh protein. Rh protein of P. trituberculatus (PtRh) was detected in all tested tissues, and showed the highest expression in gills. To further characterize the role of PtRh in ammonia metabolism and excretion, a double-stranded RNA-mediated RNA interference of PtRh was employed. The knockdown of PtRh up-regulated mRNA expression of ammonia excretion related genes aquaporin (AQP), K+-channel, vesicle associated membrane protein (VAMP), increased activities of Na+ /K+ -ATPase (NKA) and V-type H+-ATPase (V-ATPase), whereas the Na+/H+-exchanger (NHE) expression reduced firstly and then elevated. dsRNA-mediated reductions in PtRh significantly reduced ammonia excretion rate and increased ammonia and glutamine (Gln) levels in hemolymph, together with increase of glutamate dehydrogenase (GDH) and glutamine synthetase (GS) activites, indicating a central role for PtRh in ammonia excretion and detoxification mechanisms. Taken together, we conclude that the Rh protein is a primary contributor to ammonia excretion of P. trituberculatus, which may be the basis of their ability to inhabit benthic water with high ammonia levels.