They are a type of cell found in the upper part of the oxyntic glands in the stomach.
Parietal cells, or oxyntic cells, are the cells of the epithelium of the stomach that secrete gastric acid and intrinsic factor acetylcholine (M3 receptors) and gastrin (CCK2 receptors).
Parietal cells are responsible for the secretion of concentrated hydrochloric acid in the gastric lumen. To accomplish this task, it is equipped with a wide variety of functionally coupled apical and basolateral ion transport proteins.
Concerted scientific effort in recent years by a variety of researchers has provided us with the molecular identity of many of these transport mechanisms, thus contributing to the clarification of persistent controversies in the field.
Physiology of parietal cells and ion transport in particular
As an organ, the stomach fulfills a unique function, as it is the main site for the digestion and sterilization of food and water.
To produce its acidic environment and at the same time generate a protective barrier, the gastric epithelium is made up of a complex collection of specialized cells that secrete acid, bicarbonate, mucus, hormones, and digestive enzymes.
During the digestive process, the interior pH of the stomach can drop to a pH of 1–2, so the glands must generate a large amount of acid to reach this concentration.
To generate acid inside the stomach, the gastric glands must have specialized cells that can rapidly produce large amounts of hydrochloric acid (HCl) on demand.
These specialized cells are the parietal cells and are found in great abundance in the gastric gland, with typically 70-90 parietal cells per gland.
Since HCl secretion is an energy-consuming process, the parietal cell depends on the generation of large amounts of ATP.
To meet these high demands, it is particularly rich in mitochondria; in fact, it is the cell with one of the highest densities of mitochondria in the human body with fractional volumes reaching up to 40% of total cell volume compared to a mere 5% in major cells.
When the parietal cell is stimulated by secretagogues, the apical pole undergoes a morphological transformation as the acid secreting pump (H + -K + -ATPase) moves to the cell surface allowing the secretion of a proton in exchange for a potassium.
At the same time, the stomach must prevent self-digestion by secreting protective mucus from the mucus neck cells, which are located in the neck of the gastric gland closest to the interior of the stomach.
Surface cells that can also secrete mucus but have an additional capacity for HCO 3 -help secretion in this process.
This allows the pH of the stomach to remain acidic without exposing the surface cells to the caustic acid environment from the gland opening.
Other specialized cells found in the gland are the main cells that are located at the base of the gland and release pepsinogen, which is cleaved by acid in the lumen of the gland to produce pepsin, the active digestive enzyme that facilitates digestion. of food.
Furthermore, histamine-secreting enterochromaffin-like cells (ECL) and gastrin-secreting G cells, which promote acid secretion, make the stomach an endocrine organ and groups of immunocompetent cells that are exposed to early contact with antigens. the various tasks that the stomach has to perform.
The acid secretion process is tightly regulated, in part by the ECL and G cells mentioned above, but also by neuronal stimulation through the vagus nerve and the release of inhibitory substances such as somatostatin.
An imbalance in this regulation can lead to pathological conditions related to acid hypersecretion, such as gastroesophageal reflux disease (GERD) or gastric ulcer disease (GUD).
Currently, up to 20% of people in Western countries suffer from GERD, experiencing symptoms such as heartburn, regurgitation, dysphagia, or a combination of the above, at least three times a week.
Therefore, GERD not only represents a major health concern, it also has a massive economic impact on our healthcare systems.
While short episodes of gastric acid reflux into the esophagus are physiological, the basic pathophysiology of GERD can be attributed to prolonged exposure of the esophageal mucosa to acid loads. The most fatal consequence of this chronic acid exposure is cancer of the esophagus.
Current state-of-the-art treatment for GERD focuses on reducing gastric acid secretion through the administration of proton pump inhibitors (PPIs) [although this approach has been criticized recently].
PPIs specifically impair the function of H + -K + -ATPase, which can be found in the apical membrane of parietal cells during the active acid-secreting phase.
However, recent studies suggest that up to 40% of patients on PPIs develop a breakthrough acid secretion while taking these drugs, thus remaining symptomatic and susceptible to long-term complications, such as Barrett’s or esophageal carcinoma.
Despite recent insights into the physiology of the parietal cell, this high percentage of progress raises the question of whether the process of gastric acid secretion is fully understood.
Three independent apical mechanisms are required to successfully secrete HCl into the gastric lumen:
- Proton extrusion through H + -K + -ATPase.
- Cl – secretion.
- K + recycling.
Although one might intuitively consider H + -K + -ATPase to be the most important contributor, disruption of any of the mechanisms mentioned above leads to achlorhydria, which not only underscores the impressive degree of mutual regulation in this system, but also it also opens up new horizons in the system.
H + -K + -ATPASE
The parietal cell H + -K + -ATPase transports protons (H +) in exchange for K + while consuming ATP. The product of this reaction releases concentrated H + into the stomach lumen and thus contributes essentially to the formation of concentrated HCl.
Transport is achieved by conformational changes in ATPase that are driven by cyclic phosphorylation and dephosphorylation of the catalytic α subunit. H + -K + -ATPase consists of two subunits (α and β) and is a member of the alkaline cation P-type ionic ion ATPase family, which also includes Na + -K + -ATPase and Ca 2+ -ATPase .
Although transport is specific for a particular cation, all ATPases share a significant degree of homology.
Approximately 63% sequence homology is found between the α subunits of H + -K + -ATPase and Na + -K + -ATPase, while the β subunit genes share 35% of the nucleotides.
This homology is also manifested in the functional characteristics of both proteins. The catalytic cycle of Na + -K + -ATPase, already established in the 1970s, shows significant similarity to that of H + -K + -ATPase.
The α subunit of gastric H + -K + -ATPase consists of 1,035 amino acids that form 10 transmembrane segments and the β subunit of 291 amino acids with one transmembrane segment.
Both subunits are linked in the M7 / loop / M8 region (M7 describes the seventh transmembrane region) of the α subunit, with an additional binding present in the M5 / loop / M6 region of the α subunit when K + is present.
Chloride is the second component, which must be secreted in the gastric lumen, to allow the formation of HCl. The importance of intact chloride secretion for successful proton extrusion was established very early.
Pharmacological inhibition of chloride channels was shown to effectively inhibit gastric acid secretion, suggesting a close functional coupling between these channels and H + -K + -ATPase.
The cystic fibrosis transmembrane regulator (CFTR) is a potential candidate for chloride secretion in the parietal cell.
Although its degree of expression is lower than in other tissues, such as intestinal or respiratory tract epithelia, it has been shown to play a fundamental role during acid secretion in the murine stomach.
H + -K + -ATPase activity could be inhibited in wild-type mice by exposure to a specific small molecule CFTR inhibitor.
Furthermore, mice carrying the homozygous ΔF508 mutation (the most common CFTR mutation responsible for cystic fibrosis) have a significantly lower ability to secrete gastric acid.
Chloride channel type 2 (ClC-2) has been proposed as an alternative route for chloride secretion to CFTR and calcium-activated chloride channels in the intestine.
The secretion of K + in the gastric lumen is essential to maintain the sustained activity of H + -K + -ATPase.
In the activated parietal cell, the opening of the apical K + channels allows the cell to maintain constant concentrations of cytosolic K + and provides the H + -K + -ATPase with a sufficient substrate for the reciprocal extrusion of protons.
One can think of this process as K + recycling. Strong evidence supports the assumption that the K + KCNQ1 channel is responsible for this task. KCNQ1 belongs to a large family of voltage-gated K + channels and was originally identified in the heart, where, if mutated, it is responsible for long QT syndrome.
Subsequently, the channel was found to be expressed in both mouse and human gastric mucosa. The modeling of KCNQ1 (- / -) has recently provided a more detailed insight into its functional role in the parietal cell.
Acid production in affected mice was reduced by up to 90% compared to wild-type animals.
Mice lacking the KCNE2 regulatory subunit showed a similar degree of achlorhydria and developed hypergastrinemia and massive glandular hyperplasia due to non-parietal cell proliferation.
KCNE2 can dramatically change the conductance properties of KCNQ1 by decreasing its sensitivity to changes in voltage and by altering its biophysical response to low extracellular pH conditions.
While KCNQ1 is only inhibited by low extracellular pH, K + conductance increases in an acidic environment when the KCNQ1 / KCNE2 complex is formed.
Naturally, this is of particular importance in the stomach. This interesting KCNE2-mediated change in KCNQ1 activation and conductance during external acidification may explain the observed lack of acid secretion in KCNE2 (- / -) mice.
The purpose of basolateral ion transport is to compensate for apically secreted ions and to maintain homeostasis of intracellular pH and cell volume during active acid secretion.
Consequently, the parietal cell is equipped with basolateral capacities of K + and Cl – uptake in the form of the Na + -2Cl – K + cotransporter (NKCC), Cl – / HCO 3 – exchangers, and the Na + -K + -ATPase.
The regulation of pH occurs mainly through Cl – / HCO 3 -Sodium-hydrogen exchangers and exchangers (NHE).
Therefore, basolateral ion transport provides the basis for the formation of functional HCl in the stomach and thus plays an integral role in the acid secretion process.
The SLC4 family of transporters comprises three different types of exchangers, one of which is the electroneutral and Na + -independent Cl – / HCO 3 – exchanger subgroup that includes SLC4A1 (AE1), SLC4A2 (AE2), and SLC4A3 (AE3).
As a consequence of multiple observations, the basolateral exchange of Cl – / HCO 3 – of parietal cells has been mainly assigned to the activity of AE2.
AE2 expression was shown to be found at higher levels in parietal cells than in any other cell type, and the localization of three different AE2 variants (AE2a, b, c) has been confirmed on the basolateral membrane.
Furthermore, parietal cells exhibit an inhibitable Cl 4,4-diisothiocyanostilbene-2,2-disulfonic acid (DIDS) – and HCO 3 – transport pattern that is in agreement with that of AE2.
The importance of AE2 as a primary participant in gastric acid secretion is also highlighted in studies of AE2 knockouts.
AE2 (- / -) mice exhibited achlorhydria and mild chronic degeneration of the gastric mucosa, while subsequent experiments in AE2a, b (- / -) demonstrated normal basal acid secretion but a 70% reduced response in production. from acid to stimulants such as histamine or carbachol.
All three AE2 polypeptide variants were shown to be inhibited by protons with slightly different but still overlapping ranges in extracellular and intracellular pH. AE2 activation can occur through hypertonicity, NH 4 +, and calmidazolium.
In summary, the AE2 family appears to be the major player in the parietal cell Cl – / HCO 3 – exchange and, as demonstrated by knock-out experiments, can be seen as a necessary prerequisite for functional acid secretion.
Almost all prokaryotic and eukaryotic cells show NHE activity, which provides for the electroneutral exchange of hydrogen and sodium. NHE NHE1–4 isoforms have been shown to be expressed in gastric parietal cells and contribute to multiple cellular functions.
NHE1 was recently shown to play a key role in maintaining parietal cell volume homeostasis after stimulated acid secretion and subsequent secretagogue-induced cell contraction.
However, the expression of this transporter in parietal cells is relatively low. NHE1 clearance studies suggest that this isoform does not appear to be directly involved in gastric acid secretion.
NHE2, like NHE1, does not appear to be essential for HCl production, but it appears to play an important role in parietal cell viability and long-term survival, as NHE1 and NHE2 knockouts show atrophy of the gastric mucosa.
The localization and functional activity of NHE3 in the process of proton extrusion of parietal cells has recently been examined by Kirchhoff et al. The authors were able to locate the protein in the apical membrane of parietal cells and demonstrate its putative function.
NHE3 was shown to be downregulated by histamine in the active phase of H + -K + -ATPase stimulation and to be active under resting conditions, providing a potential helper mechanism for proton secretion in the unstimulated parietal cell. Despite the apical location, NHE3 was discussed in this section for simplicity.
NHE4 exhibits very high expression in the basolateral membrane of the stomach with particularly high levels in the parietal and major cells.
Targeted disruption of NHE4 leads to reduced parietal cell numbers, poor differentiation and maturation, and defective development of the secretory membrane.
It is the most abundant NHE isoform in parietal cells and, in light of current evidence, clarifies its role as an important protein responsible for basolateral Na + / H + exchange activity.
NHE4 is functionally coupled to Cl mediated by AE2 – / HCO 3 – exchange. Thus, NHE4 relative to AE2 provides a mechanism for NaCl entry into the parietal cell, providing a significant electrochemical driving force for HCl secretion.
On + -K + -ATPasa
The ubiquitous Na + -K + -ATPase, originally described in 1957, is, like the gastric H + -K + -ATPase, a member of the P-type ATPase family.
Its crystalline structure has recently been revealed by Morth et al. The sodium pump is responsible for the generation of the electrochemical gradient across the cell membrane, which in turn fuels nutrient absorption, pH regulation, and ion transport in the parietal cell.
Removal of Na + from the serous bath solution or inhibition of Na + -K + -ATPase by ouabain leads to decreased HCl secretion.
Gastric acid secretion is stimulated by a variety of endocrine, paracrine, and neuronal signals in vivo.
Two intracellular signaling pathways have been postulated to play a key role in the action of H + -K + -ATPase recruitment:
The PKA cAMP-mediated activation pathway, typically induced by histaminergic stimulation of the H 2 receptor, and the Ca i 2+ pathway, typically induced by cholinergic stimulation of the muscarinic M 3 receptor or gastrin binding to the CCK B receptor.
However, it is believed that there is some overlap between the two pathways and that it may even be necessary to effectively trigger acid secretion.
Although cholinergic stimulation by the vagus nerve plays an important role in stimulating acid secretion, which is reflected in the fact that surgical vagotomy was a widely used treatment for GUD, it is believed that histamine released from ECL cells stimulated by the surrounding gastrin is essential.
The importance of histamine for intact acid secretion is underlined by the inability of H 2 (- / -) mice to secrete acid in response to gastrin and histamine.
Pharmacological H 2 receptor blockade was therefore the most advanced therapy for acid hypersecretion-related diseases before PPIs developed.
Although its effectiveness in eliminating acid production is lower compared to PPIs, H 2 -blockers are still in clinical use for patients experiencing breakthrough under PPI therapy and for patients on a clopidogrel regimen. platelet aggregation inhibitor.
The latter is the result of an adverse pharmacodynamic interaction between PPIs and clopidogrel.
Although this interaction has been shown to exist, it does not appear to impose an increased risk in terms of a worse clinical outcome, which is why current clinical guidelines on co-administration of PPIs and clopidogrel are under review.
H + -K + -ATPasa traffic
In the non-secretory parietal cell, the H + -K + -ATPase is stored in intracellular tubulovesicles, which, by stimulating acid secretion, fuse with the apical canalicular membrane, to allow the transport of protons to the lumen of the stomach .
A complex interaction between the vesicle-bound proteins and the outer membrane is necessary for the membrane fusion process to take place.
The group of proteins involved in the vesicle docking and fusion process is called the soluble N-ethylmaleimide-sensitive factor-binding receptor (SNARE) receptor complex and basically consists of three proteins: SNAP-25, the membrane protein. vesicle-associated (VAMP) alternately called synaptobrevin, and syntaxin.
Syntaxin 3 and VAMP2 were shown to both be specifically associated with tubulovesicles containing H + -K + -ATPase.
Syntaxin 3 was also identified by Karavar et al. to transfer from the cytosolic compartment to the apical membrane, where it co-localizes with H + -K + -ATPase, syntaxin 1 and F-actin in stimulated parietal cells.
By fluorescent labeling of vesicle-bound VAMP2 and apical membrane-bound SNAP-25, the same group demonstrated the fusion of both fluorescent signals at the apical pole of the parietal cell after histamine stimulation, consistent with the formation of the SNARE complex. in the process of vesicular fusion.
Furthermore, SNAP-25 defective mutants showed significantly lower acid secretion capacity.
Recent proteomic investigation of human parietal cell tubulovesicles has established a large number of additional proteins involved in the H + -K + -ATPase trafficking process, including Rab10, VAMP8, Syntaxin 7, and Syntaxin 12/13, all of which were shown to colocalize with H + -K + -ATPase.
Considering the large number of interactions between receptors, messengers, transport proteins, channels, and ion pumps that are a prerequisite for acid secretion and for maintaining secretion, one cannot help but admire the ingenuity of the highly specialized parietal cellular machinery.
As we have seen, the disruption of a single component in this complex cascade can render the parietal cell functionally useless in terms of its function to secrete. Scientific effort in recent years has provided us with the potential identities of previously anonymous ion channels and transporters.
Thus, for example, it has been possible to identify KCNQ1 as the main channel responsible for K + Recycle and establish the contribution of various actors such as CFTR or SLC26A9 to the chloride secretion process.
With the advent of various knockout mouse models, new antibodies, and functional studies, our understanding of the physiological regulation of acid secretion has markedly improved.
These models have contributed significantly to our understanding of the regulation of apical membrane acid secretion and have helped to discover potential novel therapeutic targets for the treatment of acid-related disorders.
In light of the high rates of progression that occur with conventional PPI therapy, an alternative or complementary drug approach is highly desirable and would bring relief to the growing population of patients suffering from diseases such as GERD.
There is no doubt about the high degree of efficacy of the PPI; however, it is also obvious that there is still room for improvement and that this potential needs to be exploited with some of the new transport proteins and receptors that have been described in this review.