Carbonic anhydrases IV and IX: subcellular localization and functional role in mouse skeletal muscle

Histological Methods to Assess Skeletal Muscle Degeneration and Regeneration in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a progressive disease caused by the loss of function of the protein dystrophin. This protein contributes to the stabilisation of striated cells during contraction, as it anchors the cytoskeleton with components of the extracellular matrix through the dystrophin-associated protein complex (DAPC). Moreover, absence of the functional protein affects the expression and function of proteins within the DAPC, leading to molecular events responsible for myofibre damage, muscle weakening, disability and, eventually, premature death. Presently, there is no cure for DMD, but different treatments help manage some of the symptoms. Advances in genetic and exon-skipping therapies are the most promising intervention, the safety and efficiency of which are tested in animal models. In addition to in vivo functional tests, ex vivo molecular evaluation aids assess to what extent the therapy has contributed to the regenerative process. In this regard, the later advances in microscopy and image acquisition systems and the current expansion of antibodies for immunohistological evaluation together with the development of different spectrum fluorescent dyes have made histology a crucial tool. Nevertheless, the complexity of the molecular events that take place in dystrophic muscles, together with the rise of a multitude of markers for each of the phases of the process, makes the histological assessment a challenging task. Therefore, here, we summarise and explain the rationale behind different histological techniques used in the literature to assess degeneration and regeneration in the field of dystrophinopathies, focusing especially on those related to DMD.

Roles of Carbonic Anhydrases and Carbonic Anhydrase Related Proteins in Zebrafish

During recent decades, zebrafish (Danio rerio) have become one of the most important model organisms in which to study different physiological and biological phenomena. The research field of carbonic anhydrases (CAs) and carbonic anhydrase related proteins (CARPs) is not an exception to this. The best-known function of CAs is the regulation of acid–base balance. However, studies performed with zebrafish, among others, have revealed important roles for these proteins in many other physiological processes, some of which had not yet been predicted in the light of previous studies and suggestions. Examples include roles in zebrafish pigmentation as well as motor coordination. Disruption of the function of these proteins may generate lethal outcomes. In this review, we summarize the current knowledge of CA-related studies performed in zebrafish from 1993–2021 that was obtained from PubMed search.

Carbonic Anhydrases in Metazoan Model Organisms: Molecules, Mechanisms, and Physiology

During the past three decades, mice, zebrafish, fruit flies, and Caenorhabditis elegans have been the primary model organisms used for the study of various biological phenomena. These models have also been adopted and developed to investigate the physiological roles of carbonic anhydrases (CAs) and carbonic anhydrase-related proteins (CARPs). These proteins belong to eight CA families and are identified by Greek letters: α, β, γ, δ, ζ, η, θ, and ι. Studies using model organisms have focused on two CA families, α-CAs and β-CAs, which are expressed in both prokaryotic and eukaryotic organisms with species-specific distribution patterns and unique functions. This review covers the biological roles of CAs and CARPs in light of investigations performed in model organisms. Functional studies demonstrate that CAs are not only linked to the regulation of pH homeostasis, the classical role of CAs but also contribute to a plethora of previously undescribed functions.

Carbonic Anhydrase IX-Mouse versus Human

In contrast to human carbonic anhydrase IX (hCA IX) that has been extensively studied with respect to its molecular and functional properties as well as regulation and expression, the mouse ortholog has been investigated primarily in relation to tissue distribution and characterization of CA IX-deficient mice. Thus, no data describing transcriptional regulation and functional properties of the mouse CA IX (mCA IX) have been published so far, despite its evident potential as a biomarker/target in pre-clinical animal models of tumor hypoxia. Here, we investigated for the first time, the transcriptional regulation of the Car9 gene with a detailed description of its promoter. Moreover, we performed a functional analysis of the mCA IX protein focused on pH regulation, cell-cell adhesion, and migration. Finally, we revealed an absence of a soluble extracellular form of mCA IX and provided the first experimental evidence of mCA IX presence in exosomes. In conclusion, though the protein characteristics of hCA IX and mCA IX are highly similar, and the transcription of both genes is predominantly governed by hypoxia, some attributes of transcriptional regulation are specific for either human or mouse and as such, could result in different tissue expression and data interpretation.

Suppression of CHRN endocytosis by carbonic anhydrase CAR3 in the pathogenesis of myasthenia gravis

Myasthenia gravis is an autoimmune disorder of the neuromuscular junction manifested as fatigable muscle weakness, which is typically caused by pathogenic autoantibodies against postsynaptic CHRN/AChR (cholinergic receptor nicotinic) in the endplate of skeletal muscle. Our previous studies have identified CA3 (carbonic anhydrase 3) as a specific protein insufficient in skeletal muscle from myasthenia gravis patients. In this study, we investigated the underlying mechanism of how CA3 insufficiency might contribute to myasthenia gravis. Using an experimental autoimmune myasthenia gravis animal model and the skeletal muscle cell C2C12, we find that inhibition of CAR3 (the mouse homolog of CA3) promotes CHRN internalization via a lipid raft-mediated pathway, leading to accelerated degradation of postsynaptic CHRN. Activation of CAR3 reduces CHRN degradation by suppressing receptor endocytosis. CAR3 exerts this effect by suppressing chaperone-assisted selective autophagy via interaction with BAG3 (BCL2-associated athanogene 3) and by dampening endoplasmic reticulum stress. Collectively, our study illustrates that skeletal muscle cell CAR3 is critical for CHRN homeostasis in the neuromuscular junction, and its deficiency leads to accelerated degradation of CHRN and development of myasthenia gravis, potentially revealing a novel therapeutic approach for this disorder.

The Image Data Resource: A Bioimage Data Integration and Publication Platform

This Resource describes the Image Data Resource (IDR), a prototype online system for biological image data that links experimental and analytic data across multiple data sets and promotes image data sharing and reanalysis.

Access to primary research data is vital for the advancement of science. To extend the data types supported by community repositories, we built a prototype Image Data Resource (IDR). IDR links data from several imaging modalities, including high-content screening, multi-dimensional microscopy and digital pathology, with public genetic or chemical databases and cell and tissue phenotypes expressed using controlled ontologies. Using this integration, IDR facilitates the analysis of gene networks and reveals functional interactions that are inaccessible to individual studies. To enable reanalysis, we also established a computational resource based on Jupyter notebooks that allows remote access to the entire IDR. IDR is also an open-source platform for publishing imaging data. Thus IDR provides an online resource and a software infrastructure that promotes and extends publication and reanalysis of scientific image data.

Carbonic anhydrases and their interplay with acid/base-coupled membrane transporters

Carbonic anhydrases (CAs) have not only been identified as ubiquitous enzymes catalyzing the fast reversible hydration of carbon dioxide to generate or consume protons and bicarbonate, but also as intra- and extracellular proteins, which facilitate transport function of many acid/base transporting membrane proteins, coined 'transport metabolon'. Functional interaction between CAs and acid/base transporters, such as chloride/bicarbonate exchanger (AE), sodium-bicarbonate cotransporter (NBC) and sodium/hydrogen exchanger (NHE) has been shown to require both catalytic CA activity as well as direct binding of the enzyme to specific sites on the transporter. In contrast, functional interaction between different CA isoforms and lactate-proton-cotransporting monocarboxylate transporters (MCT) has been found to be isoform-specific and independent of CA catalytic activity, but seems to require an intramolecular proton shuttle within the enzyme. In this chapter, we review the various types of interactions between acid/base-coupled membrane carriers and different CA isoforms, as studied in vitro, in intact Xenopus oocytes, and in various mammalian cell types. Furthermore, we discuss recent findings that indicate the significance of these 'transport metabolons' for normal cell functions.

Membrane associated carbonic anhydrase IV (CA IV): a personal and historical perspective

Carbonic anhydrase IV is one of 12 active human isozymes and one of four expressed on the extracellular surfaces of certain endothelial and epithelial cells. It is unique in being attached to the plasma membrane by a glycosyl-phosphatiydyl-inositol (GPI) anchor rather than by a membrane-spanning domain. It is also uniquely resistant to high concentrations of sodium dodecyl sulfate (SDS), which allows purification from tissues by inhibitor affinity chromatography without contamination by other isozymes. This unique resistance to SDS and recovery following denaturation is explained by the two disulfide bonds. The 35-kDa human CA IV is a "high activity" isozyme in CO2 hydration activity, like CA II, and has higher activity than other isozymes in catalyzing the dehydration of HCO3 (-). Human CA IV is also unique in that it contains no oligosaccharide chains, where all other mammalian CA IVs are glycoproteins with one to several oligosaccharide side chains.Although CA IV has been shown to be active in mediating CO2 and HCO3 (-) transport in many important tissues like kidney and lung, and in isolated cells from brain and muscle, the gene for CA IV appears not to be essential. The CA IV knockout mouse produced by targeted mutagenesis, though slightly smaller and produced in lower than expected numbers, is viable and has no obvious mutant phenotype. Conversely, several dominant negative mutations in humans are associated with one form of reitinitis pigmentosa (RP-17), which we attribute to unfolded protein accumulation in the choreocapillaris, leading to apoptosis of cells in the overlying retina.

Expression of carbonic anhydrase IX in human fetal joints, ligaments and tendons: a potential marker of mechanical stress in fetal development?

Carbonic anhydrase type IX (CA9) is known to express in the fetal joint cartilage to maintain pH against hypoxia. Using paraffin-embedded histology of 10 human fetuses at 10-16 weeks of gestation with an aid of immunohistochemistry of the intermediate filaments, matrix components (collagen types I and II, aggrecan, versican, fibronectin, tenascin, and hyaluronan) and CA9, we observed all joints and most of the entheses in the body. At any stages examined, CA9-poisitive cells were seen in the intervertebral disk and all joint cartilages including those of the facet joint of the vertebral column, but the accumulation area was reduced in the larger specimens. Glial fibrillary acidic protein (GFAP), one of the intermediate filaments, expressed in a part of the CA9-positive cartilages. Developing elastic cartilages were positive both of CA9 and GFAP. Notably, parts of the tendon or ligament facing to the joint, such as the joint surface of the annular ligament of the radius, were also positive for CA9. A distribution of each matrix components examined was not same as CA9. The bone-tendon and bone-ligament interface expressed CA9, but the duration at a site was limited to 3-4 weeks because the positive site was changed between stages. Thus, in the fetal entheses, CA9 expression displayed highly stage-dependent and site-dependent manners. CA9 in the fetal entheses seemed to play an additional role, but it was most likely to be useful as an excellent marker of mechanical stress at the start of enthesis development.

Expression of carbonic anhydrase in the fetal eye and extra-ocular tissues

Carbonic anhydrases (CAs) plays a critical functional role in the ciliary body and retina for maintenance of microenvironment. With immunohistochemistry using orbital contents from 8 human fetuses (12-16 weeks of gestation), we examined expressions of CAs isozymes-1, 2, 3, 6, 7 9 and 12 and found strong reactivity of CA9 in extra-ocular fibrous tissues in the anterior and posterior eyes. CA9 is known to express in the fetal joint cartilage to maintain pH against hypoxia: actually, in the present specimens, the SO pulley and its tendon was strongly positive for CA9. The CA9-positive anterior fibrous tissues were positive for smooth muscle actin and connected the orbital aspect of the 4 rectus muscle with the palpebral conjunctiva, whereas the posterior tissue was negative for smooth muscle actin and corresponded to the lateral insertion tendon of the orbitalis muscle. The anterior CA9-positve tissues seemed to correspond to the primitive form of the sleeve and pulley system. Any of matrix substances (collagen types I and II, aggrecan, versican, fibronectin, tenascin and hyaluronan) displayed a distribution pattern specific for the CA9-positive fibrous tissues. Therefore, whether or not CA9 was positive in the fibrous tissue seemed not to depend on the tissue components such as the extracellular matrix and intermediate filaments but to suggest a stressful condition such as hypoxia, unsuitable base balance and/or under mechanical stress.

Caveolin 3, flotillin 1 and influenza virus hemagglutinin reside in distinct domains on the sarcolemma of skeletal myofibers

We examined the distribution of selected raft proteins on the sarcolemma of skeletal myofibers and the role of cholesterol environment in the distribution. Immunofluorescence staining showed that flotillin-1 and influenza hemagglutinin exhibited rafts that located in the domains deficient of the dystrophin glycoprotein complex, but the distribution patterns of the two proteins were different. Cholesterol depletion from the sarcolemma by means of methyl-β-cyclodextrin resulted in distorted caveolar morphology and redistribution of the caveolin 3 protein. Concomitantly, the water permeability of the sarcolemma increased significantly. However, cholesterol depletion did not reshuffle flotillin 1 or hemagglutinin. Furthermore, a hemagglutinin variant that lacked a raft-targeting signals exhibited a similar distribution pattern as the native raft protein. These findings indicate that each raft protein exhibits a strictly defined distribution in the sarcolemma. Only the distribution of caveolin 3 that binds cholesterol was exclusively dependent on cholesterol environment.

T tubules and surface membranes provide equally effective pathways of carbonic anhydrase-facilitated lactic acid transport in skeletal muscle

We have studied lactic acid transport in the fast mouse extensor digitorum longus muscles (EDL) by intracellular and cell surface pH microelectrodes. The role of membrane-bound carbonic anhydrases (CA) of EDL in lactic acid transport was investigated by measuring lactate flux in muscles from wildtype, CAIV-, CAIX- and CAXIV-single ko, CAIV-CAXIV double ko and CAIV-CAIX-CAXIV-triple ko mice. This was complemented by immunocytochemical studies of the subcellular localization of CAIV, CAIX and CAXIV in mouse EDL. We find that CAXIV and CAIX single ko EDL exhibit markedly but not maximally reduced lactate fluxes, whereas triple ko and double ko EDL show maximal or near-maximal inhibition of CA-dependent lactate flux. Interpretation of the flux measurements in the light of the immunocytochemical results leads to the following conclusions. CAXIV, which is homogeneously distributed across the surface membrane of EDL fibers, facilitates lactic acid transport across this membrane. CAIX, which is associated only with T tubular membranes, facilitates lactic acid transport across the T tubule membrane. The removal of lactic acid from the lumen of T tubuli towards the interstitial space involves a CO2-HCO3- diffusional shuttle that is maintained cooperatively by CAIX within the T tubule and, besides CAXIV, by the CAIV, which is strategically located at the opening of the T tubules. The data suggest that about half the CA-dependent muscular lactate flux occurs across the surface membrane, while the other half occurs across the membranes of the T tubuli.

Muscle channelopathies: does the predicted channel gating pore offer new treatment insights for hypokalaemic periodic paralysis?

Hypokalaemic periodic paralysis (hypoPP) is the archetypal skeletal muscle channelopathy caused by dysfunction of one of two sarcolemmal ion channels, either the sodium channel Nav1.4 or the calcium channel Cav1.1. Clinically, hypoPP is characterised by episodes of often severe flaccid muscle paralysis, in which the muscle fibre membrane becomes electrically inexcitable, and which may be precipitated by low serum potassium levels. Initial functional characterisation of hypoPP mutations failed to adequately explain the pathomechanism of the disease. Recently, as more pathogenic mutations involving loss of positive charge have been identified in the S4 segments of either channel, the hypothesis that an abnormal gating pore current may be important has emerged. Such an aberrant gating pore current has been identified in mutant Nav1.4 channels and has prompted potentially significant advances in this area. The carbonic anhydrase inhibitor acetazolamide has been used as a treatment for hypokalaemic periodic paralysis for over 40 years but its precise therapeutic mechanism of action is unclear. In this review we summarise the recent advances in the understanding of the molecular pathophysiology of hypoPP and consider how these may relate to the reported beneficial effects of acetazolamide. We also consider potential areas for future therapeutic development.

Dietary glutamine and oral antibiotics each improve indexes of gut barrier function in rat short bowel syndrome

Short bowel syndrome (SBS) is associated with gut barrier dysfunction. We examined effects of dietary glutamine (GLN) or oral antibiotics (ABX) on indexes of gut barrier function in a rat model of SBS. Adult rats underwent a 60% distal small bowel + proximal colonic resection (RX) or bowel transection (TX; control). Rats were pair fed diets with or without l-GLN for 20 days after operation. Oral ABX (neomycin, metronidazole, and polymyxin B) were given in some RX rats fed control diet. Stool secretory immunoglobulin A (sIgA) was measured serially. On day 21, mesenteric lymph nodes (MLN) were cultured for gram-negative bacteria. IgA-positive plasma cells in jejunum, stool levels of flagellin- and lipopolysaccharide (LPS)-specific sIgA, and serum total, anti-flagellin- and anti-LPS IgG levels were determined. RX caused gram-negative bacterial translocation to MLN, increased serum total and anti-LPS IgG and increased stool total sIgA. After RX, dietary GLN tended to blunt bacterial translocation to MLN (-29%, P = NS) and significantly decreased anti-LPS IgG levels in serum, increased both stool and jejunal mucosal sIgA and increased stool anti-LPS-specific IgA. Oral ABX eliminated RX-induced bacterial translocation, significantly decreased total and anti-LPS IgG levels in serum, significantly decreased stool total IgA and increased stool LPS-specific IgA. Partial small bowel-colonic resection in rats is associated with gram-negative bacterial translocation from the gut and a concomitant adaptive immune response to LPS. These indexes of gut barrier dysfunction are ameliorated or blunted by administration of dietary GLN or oral ABX, respectively. Dietary GLN upregulates small bowel sIgA in this model.

Both radioresistant and hemopoietic cells promote innate and adaptive immune responses to flagellin

The TLR5 agonist flagellin induces innate and adaptive immune responses in a MyD88-dependent manner and is under development as a vaccine adjuvant. In vitro studies indicate that, compared with other bacteria-derived adjuvants, flagellin is a very potent activator of proinflammatory gene expression and cytokine production from cells of nonhemopoietic origin. However, the role of nonhemopoietic cells in promoting flagellin-induced immune responses in vivo remains unclear. To investigate the relative contributions of the nonhemopoietic (radioresistant) and the hemopoietic (radiosensitive) compartments, we measured both innate and adaptive immune responses of flagellin-treated MyD88 radiation bone marrow chimeras. We observed that radiosensitive and radioresistant cells played distinct roles in the innate response to flagellin, with the radiosensitive cells producing the majority of the TNF-alpha, IL-12, and IL-6 cytokines and the radioresistant cells most of the KC, IP-10, and MCP-1 cytokines. Direct activation of either compartment alone by flagellin initiated dendritic cell costimulatory molecule up-regulation and induced a significant humoral immune response to the protein itself as well as to coinjected OVA. However, robust humoral responses were only observed when MyD88 was present in both cell compartments. Further studies revealed that hemopoietic and nonhemopoietic expression of the cytokines TNF-alpha and IL-6, but not IL-1, played an important role in promoting flagellin-induced Ab responses. Thus, in vivo both radioresistant and hemopoietic cells play key nonredundant roles in mediating innate and adaptive immune responses to flagellin.