†These authors contributed equally.
Academic Editor: Graham Pawelec
The transport of chloride and bicarbonate across epithelia controls the pH and
volume of the intracellular and luminal fluids, as well as the systemic pH and
vascular volume. The anion exchanger pendrin (SLC26A4) and the cystic fibrosis
transmembrane conductance regulator (CFTR) channel are expressed in the apical
membrane of epithelial cells of various organs and tissues, including the
airways, kidney, thyroid, and inner ear. While pendrin drives chloride
reabsorption and bicarbonate, thiocyanate or iodide secretion within the apical
compartment, CFTR represents a pathway for the apical efflux of chloride,
bicarbonate, and possibly iodide. In the airways, pendrin and CFTR seems to be
involved in alkalinization of the apical fluid via bicarbonate secretion,
especially during inflammation, while CFTR also controls the volume of the apical
fluid via a cAMP-dependent chloride secretion, which is stimulated by pendrin. In
the kidney, pendrin is expressed in the cortical collecting duct and connecting
tubule and co-localizes with CFTR in the apical membrane of
Transcellular Cl
Sequence alterations in the SLC26A4 gene, encoding pathogenic pendrin protein variants, cause Pendred syndrome [14] and non-syndromic sensorineural deafness DFNB4 [15]. Pendred syndrome (OMIM #274600) is an autosomal recessive disease that was first described, in terms of clinical manifestations, in 1896 by Vaughan Pendred [16]. This syndrome has an estimated prevalence ranging between 7.5 and 10 per 100,000 individuals [17, 18]. Pendred syndrome is characterized by hearing loss and thyroid dysfunction possibly due to abnormal iodide organification. About 4–10% of the described inherited deafness cases are ascribed to Pendred syndrome [19]. DFNB4 (OMIM #600791) is the second most common cause of hearing loss worldwide [20]. In both cases, patients present with a malformation of the temporal bone called enlarged vestibular aqueduct (EVA), accompanied by a malformation of the membranous labyrinth, which displays an enlargement of the endolymphatic sac and duct. A cochlear incomplete partition type II (Mondini malformation) may occasionally be found (reviewed in [21]).
Up to now, 840 sequence alterations of the SLC26A4 gene have been reported (https://www.ncbi.nlm.nih.gov/clinvar, accessed on the 15.12.2021) and only in part characterized concerning their clinical significance. Two hundred and eighty of them are missense mutations generating protein variants of which 125 have been classified as pathogenic/likely pathogenic and may be retained in the endoplasmic reticulum. Pathogenic protein variants deriving from nonsense mutations, altered splicing, insertions, deletions, and partial gene duplications have been also described [22]. Abnormal and high expression of pendrin has been correlated with lung disorders [23, 24]. Conversely, specific inhibition of renal pendrin has been suggested to face hypertension and the condition of diuretic-resistant water overload [25]. Based on the crystal structure of the transmembrane domain of Deinococcus geothermalis fumarate transporter (SLC26Dg), it has been proposed that pendrin holds fourteen transmembrane alpha-helices with cytosolic N-terminal and C-terminal domains [26]. The C-terminal region contains a Sulfate Transporter and Anti-Sigma Factor Antagonist (STAS) domain and a putative consensus protein kinase A (PKA) phosphorylation motif (714)RKDT(717). Deletion mutants lacking the putative PKA consensus site displayed a lower basal functionality and a reduced insertion in the plasma membrane [27]. Interestingly, additional putative phosphorylation sites for other kinases including PKC can be predicted by bioinformatics (https://services.healthtech.dtu.dk/service.php?NetPhos-3.1; NCBI Reference Sequence: NP_000432.1). In this respect, additional studies would be needed to clarify the possible role of phosphorylation on pendrin function and subcellular distribution.
In this review, we focus on the functional interaction between the anion exchanger pendrin and the CFTR channel in orchestrating the anion transport as well as the control of the pH and volume of the luminal fluids in epithelia of various organs. We summarize the evidence of a clear interplay between these two molecular entities in the airways and kidney, and information supporting a similar scenario in the inner ear, thyroid, and liver. Tissue-specific pendrin expression and function is also reviewed; for information concerning the structure, expression, and function of CFTR in different organs as well as clinical aspects of cystic fibrosis (CF; OMIM #219700) we refer to recent excellent reviews [28, 29, 30] and other publications of this same Special Issue.
In the last few years, the involvement of pendrin in different respiratory disorders has been widely described [31, 32, 33]. In the airways, pendrin is expressed mainly in secretory non-ciliated cells, while ciliated cells lack significant pendrin expression [34, 35]. In a mouse model of acute lung injury (ALI) induced by lipopolysaccharide (LPS), pendrin expression increased [36]. Interestingly, treatment with YS-01, a selective pendrin inhibitor, mitigated the typical ALI inflammatory responses by downregulating the expression of proinflammatory mediators and Nuclear factor kappa B (NF-kB) activation [37]. Also, pendrin-deficient mice showed a reduced allergen-induced airway hyperreactivity with respect to control animals [23].
Pendrin is highly expressed in animal models of asthma and chronic obstructive
pulmonary disease (COPD) [32] and in patients with chronic rhinosinusitis [38].
Asthma and COPD are chronic inflammatory diseases affecting the airways and
causing airflow impairment possibly due to bronchoconstriction and mucus
overproduction. Several interleukins (ILs), including IL4, IL13, and IL17 play
key roles in the pathogenesis of asthma and COPD [4, 32, 35]. Indeed, IL13 and
IL4 are involved in mucus overproduction [39]. In the respiratory system, the
electroneutral antiporter pendrin mediates the transport of Cl
In human bronchiolar cells, IL4/IL13 modulate the transport of different ions.
In particular, these cytokines downregulate Na
Ion transport in the airway epithelial cells. On the
apical membrane, airway epithelial cells express the vacuolar H
In the airways, chloride and bicarbonate homeostasis is crucial for pH
regulation and the dynamic maintenance of ASL. ASL is generated by secretion from
submucosal glands and transepithelial osmotic water transport. Physiologically,
ASL pH is slightly acidic compared to the interstitium. The apical secretion of
protons is mainly mediated by the H
Altered mechanisms regulating ASL pH cause several respiratory disturbances
[46]. CFTR controls bicarbonate efflux by transporting HCO
Secretory and primary cells deriving from differentiated human airway epithelia
express both pendrin and CFTR [50]. Studies in polarized Calu-3 cells that
co-express CFTR and pendrin revealed that stimulation with forskolin potentiates
HCO
Recent data revealed that stimulation of pendrin with IL4 or IL13 facilitates
Cl
Combined treatment with TNF
Other studies, however, revealed that pendrin can play a role in controlling ASL
volume and not in regulating ASL pH in IL13-stimulated cells. In IL13-treated
airway epithelial cells isolated from healthy subjects or patients with cystic
fibrosis, selective inhibition of pendrin significantly increased ASL volume,
which might result from an impaired pendrin-dependent Cl
The distal tract of the nephron plays a key role in the regulation of the
systemic pH owing to the unique property of secreting HCO
Functional interplay of pendrin and CFTR in the distal nephron.
In the kidney, pendrin is expressed in the cortical collecting duct and
connecting tubule and co-localizes with CFTR in the apical membrane of
Ectopic expression in HEK-293 cells showed that pendrin can function in the
electroneutral Cl
Alterations in the kidney function of pendrin knockout mice become more obvious
when these mice are exposed to aldosterone analogues, such as deoxycorticosterone
pivalate (DOCP), or challenged by salt restriction. Pendrin knockout mice are
resistant to DOCP-induced hypertension but develop metabolic alkalosis, denoting
that (i) pendrin is upregulated by aldosterone in wild type mice and (ii)
counteracts metabolic alkalosis in this setting [68]. Moderate NaCl restriction
increased urinary volume and Cl
The activity of pendrin in the kidney is under control of angiotensin II/aldosterone, which changes pendrin cellular abundance and distribution in part through a direct effect on the mineralocorticoid receptor of intercalated cells [68, 73, 74].
Interestingly, the activity of pendrin in the kidney is tightly connected to that of other ion-transporting systems. For example, in various mouse models, negative or positive changes in pendrin abundance are mirrored by changes in abundance and/or activity of the epithelial sodium channel ENaC in principal cells and the sodium-driven chloride/bicarbonate exchanger (Ndcbe) [74, 75, 76, 77]. In contrast, gene ablation of the sodium-chloride cotransporter NCC in the distal convoluted tubule provoked a compensatory upregulation of pendrin [78], probably masking the effect of NCC loss of function on salt reabsorption; consequently, double knockout of pendrin and NCC caused severe salt wasting, volume depletion and metabolic alkalosis under baseline conditions [79].
CFTR mRNA and/or protein have been found in all nephron segments of the human
and rat kidney, and are particularly abundant in the renal cortex and outer
medulla. In the mouse kidney, CFTR was found in endosomes of the cells of the
terminal part of the proximal tubule [80]. In humans, protein immunoreactivity
was detected at the apical region of both the proximal and the distal renal
tubules [81]; the expression starts during early embryogenesis [82]. In addition
to wild-type CFTR, a truncated form (TNR-CFTR) containing only the first
transmembrane domain, the first nucleotide-binding domain, and the regulatory
domain is expressed in the kidney and demonstrated cAMP-dependent, PKA-stimulated
Cl
The first hint of a possible functional interplay between CFTR and pendrin in
the kidney arose from experiments where the two proteins were expressed in
HEK-293 cells. In these cells, co-expression with CFTR activated the
Cl
The importance of studying the interplay between pendrin and CFTR in the kidney
is linked to the fact that CF patients are prone to develop life threatening
metabolic alkalosis [88]. In this context, Soleimani and his research group first
showed that the kidneys of CF mice have an impaired ability to excrete excess
bicarbonate. Following a three-day oral bicarbonate load, CF mice showed a
significantly more acidic urine as well as elevated bicarbonate and arterial
blood pH compared to wild type animals, all indicative of the development of
metabolic alkalosis. These effects were linked to transposition of pendrin
towards the subcellular compartment of
Very recently, Berg et al. [91] dissected the molecular derangements
leading to metabolic alkalosis in CF. By utilizing a global and a kidney
tubule-specific CFTR knockout mouse as well as a pendrin knockout mouse, these
authors found that bicarbonate excretion into the urine following i.p. injection
of secretin or an acute oral load of bicarbonate is prevented in these mice [91].
Secretin, which is the first hormone that was discovered, is a peptide initially
described as produced by the S cells of the duodenum following a drop of the
duodenal content pH below a value of four. In addition to inhibiting the
production of gastric acid from the parietal cells of the stomach, secretin also
stimulates the secretion of bicarbonate from the pancreas. More recently, a
direct effect of secretin on renal water reabsorption was found [92, 93]. Berg
et al. [91] found that secretin receptors are expressed on the
basolateral membrane of
The role of CFTR in governing the pendrin-mediated bicarbonate excretion into
the urine was recently confirmed also in a mouse model of chronic base load (oral
supplementation of NaHCO
If CFTR directly activates pendrin or simply provides a pathway for the apical recycling of chloride, thus enabling pendrin function, is currently unknown. One can imagine that a direct molecular interaction between pendrin and CFTR is necessary for pendrin activation and/or stability within the plasma membrane; it is tempting to speculate that this interaction might involve the STAS domain of pendrin and the R domain of CFTR, as it is reported for other members of the SLC family [49, 94]. How CFTR can regulate pendrin protein abundance and whether this involves a possible transcriptional regulation is more difficult to envision and remains to be elucidated. Also, CFTR is capable of bicarbonate secretion in the pancreas and various other organs [95]; whether and to what extent CFTR itself participates in the renal bicarbonate excretion in the CCD independently of pendrin remains to be explored.
The thyroid gland contains two different endocrine units, the follicular and
parafollicular cells. Follicular cells are involved in thyroid hormone
generation. Thyroid hormone production is tightly controlled by the
hypothalamic-pituitary-thyroid axis. Specifically, the thyrotropin-releasing
hormone (TRH) promotes the pituitary thyroid stimulating hormone (TSH) release,
which in turn stimulates thyroid follicles for protein synthesis and thyroxine
(T4) and triiodothyronine (T3) secretion (Fig. 3). TSH is a pivotal modulator of
thyroid cell functions. Thyroid hormones are involved in several metabolic
functions including body temperature control, cardiovascular functioning, growth,
and normal development. Therefore, the reduced activity of the thyroid gland
observed in hypothyroidism can cause numerous dysfunctions such as bradycardia,
cold intolerance, and fat accumulation. In the thyroid follicular cells, iodide
uptake occurs via the sodium-iodide symporter (NIS), which is expressed at the
basolateral membrane. This process requires the activity of the
Na
Ion transport in the follicular cells of the thyroid. In the
follicular thyroid cells, The TSH receptor (TSHR) is localized at the basolateral
membrane, where it transduces signals resulting in an increased protein synthesis
and thyroid hormones (T3, T4) production. Moreover, at the basolateral side, the
cotransporter NIS is involved in sodium and iodide entry accompanied by
Na
Pendrin has been proposed to mediate the apical efflux of iodide [1]. Consistently, patients with Pendred syndrome can be affected by a partial defect in iodide organification that may lead to goiter development. Nevertheless, thyroid dysfunction is very variable even within the same family, with some patients having large goiters and others with minimal size increase of the thyroid. Importantly, in addition to pendrin, the calcium-dependent chloride channel anoctamin 1 (ANO1/TMEM16A) and CFTR [98, 99] can also mediate the apical iodide efflux (Fig. 3).
These findings may explain why several patients with Pendred syndrome are euthyroid, although it cannot be excluded that these patients would develop thyroid dysfunction under conditions of dietary iodide deficiency. Also, the high variability in the clinical manifestations of thyroid disorders may be due to different dietary iodine intake between patients [22]. Iodide transport to the luminal follicular side is strictly dependent on the iodide uptake at the basolateral side from the blood into the cytosol of thyrocytes (Fig. 3). The basolateral uptake is indeed useful to create a positive gradient for the apical iodide exit. In line, in Chinese hamster ovary cells (CHO) transfected either with NIS or with NIS and pendrin, iodide transport resulted significantly higher in pendrin positive cells [100]. Moreover, in COS-7 cells, the pendrin-dependent iodide efflux was potentiated by the increase of the extracellular chloride concentration, underlining the important role of the extracellular chloride levels in sustaining the iodide efflux [101]. As a matter of fact, co-expression of pendrin and NIS in polarized cells grown in a bicameral system promoted the transcellular flux of iodide into the apical compartment [102].
Physiologically, thyroid cells reabsorb sodium and release chloride in a cAMP-dependent manner. Indeed, in a porcine thyroid culture model, CFTR controlled the cAMP-dependent chloride secretion, raising the hypothesis that chloride may represent the counter-ion for pendrin-dependent iodide transport [8, 103, 104, 105]. On the other hand, it has been proposed that CFTR may modulate the activity of pendrin by binding the STAS domain [49, 106]. Interestingly, some patients with cystic fibrosis can develop mild hypothyroidism [107]. However, considering that CFTR can mediate either the apical efflux of iodide, similarly to pendrin, or the chloride secretion in the thyroid lumen, whether CFTR dysfunction contributes to hypothyroidism directly or following impairment of pendrin activity is still unclear.
In the inner ear pendrin is found in the apical membrane of epithelial cells of
the cochlea, vestibular system, and endolymphatic sac and duct. Within the murine
cochlea, pendrin is expressed in epithelial cells of the spiral prominence, outer
sulcus cells, root cells, and spindle-shaped cells of the stria
vascularis [108, 109]. The stria vascularis is the structure devoted to
the generation of the endocochlear potential, which is essential to the hearing
function. In this location, pendrin-driven chloride reabsorption and bicarbonate
secretion control the volume and pH of the endolymph, respectively; indeed, loss
of pendrin function in knockout mice causes (i) an abnormal enlargement of the
luminal volume and (ii) acidification of the endolymph, which are two events that
lead to a complex scenario of pathophysiological derangements, including impaired
cell-to-cell communication, retarded development of the stria vascularis
and organ of Corti, free radical stress, alteration of expression and function of
other ion transporters, such as loss of expression of the Kcnj10 in the
intermediate cells of the stria vascularis and BK K+ channels in inner
hair cells, as well as a greatly increased Ca
In the murine endolymphatic sac, pendrin is abundantly expressed in mitochondria-rich cells and in a subset of ribosome-rich cells, although at a much lower levels. In the endolymphatic sac, pendrin drives apical reabsorption of chloride followed by vectorial absorption of fluid [114]. Following pendrin gene ablation in mice, the main derangements seen in the cochlea, i.e. luminal enlargement and endolymph acidification, are also seen in the endolymphatic sac. Transgenic mice with pendrin expression restricted to the mitochondria-rich cells of the endolymphatic sac, but lacking pendrin expression in the cochlea or vestibular labyrinth, did not develop a luminal enlargement. Most importantly, hearing and balance were preserved in these mice [115]. These findings point to a specific function of pendrin in mitochondria-rich cells of the endolymphatic sac in controlling the volume of the endolymph.
In the cochlea, CFTR transcript and protein were found in the inner (IHCs) and outer (OHCs) hair cells of the organ of Corti of adult mice. In the lateral membrane of OHCs, CFTR establishes a direct molecular interaction with the motor protein Slc26a5/prestin and stimulates its activity [116]. An interplay between CFTR and pendrin in these sensory cells is unlikely, as pendrin expression appears to be restricted to epithelial cells. In addition to the organ of Corti, CFTR expression was also documented in the stria vascularis, spiral ligament, basilar membrane, and cochlear ganglion in guinea pigs and its expression was increased in a model of endolymphatic hydrops [117]. These findings are consistent with a role of CFTR in handling of ion composition and volume of the endolymph. A hypothesis of a functional interplay between CFTR and pendrin in the stria vascularis is tempting and would require the precise identification of the cell types expressing CFTR. Various degrees of hearing loss and alteration of the inner ear structures, including atrophy of the stria vascularis, have been described in CF patients [118]. Although these derangements have been linked to the use of ototoxic antibiotics, it has been suggested that exposure to aminoglycosides is not the only causal factor for hearing loss in CF [119]. It is plausible that loss of CFTR might affect proper pendrin function, thus contributing to hearing loss and/or increased sensitivity to ototoxic insults in CF.
In the intermediate portion of the rat endolymphatic sac, CFTR was found expressed on the apical membrane of epithelial cells and co-localized with ENaC, although the specific cell type was not determined in this study [120]. Recently, a newly discovered cell type, called the ionocyte, has been described as the main source of CFTR transcript and activity in mouse and human airways [121, 122]. These cells share morphological and functional similarities with the intercalated cells of the kidney and mitochondria-rich cells of the inner ear [123]. Indeed, CFTR and pendrin transcripts are co-expressed in mitochondria-rich cells of the mouse endolymphatic sac [114]. CFTR is also expressed in the semicircular canal duct epithelium of rat, and seems to drive a cAMP-stimulated apical secretion of chloride; this may apply to other structures of the inner ear as well. These considerations lead to speculate that CFTR and pendrin might establish a functional interplay in the apical membrane of mitochondria rich cells of the endolymphatic sac in handling the ion composition, pH and volume of the endolymph.
The parotid duct is a salivary duct emerging from the parotid gland, that is the
major salivary gland. CFTR is expressed on the apical membrane of the ductal
system of salivary glands, and its dysfunction contributes to the abnormal
salivary secretion in CF [124]. In the parotid duct, pendrin and Slc26a6 are
expressed in the luminal membrane and mainly mediate I
The presence of pendrin transcript and protein in the mouse liver was first reported in 2011. The expression levels changes of pendrin in the liver during metabolic alkalosis and acidosis appeared to be opposite to those observed in the kidney, as pendrin was downregulated in metabolic alkalosis and upregulated in metabolic acidosis [8]. As the liver participates in the systemic acid-base balance by disposing of bicarbonate via the hepatic ureogenesis, it is conceivable that the acid-base status might regulate bicarbonate transporters in this organ; however, the function of pendrin in the liver needs to be further explored.
In the liver, CFTR expression is restricted to the apical membrane of the
epithelial cells – the cholangiocytes - lining the biliary ducts and is not
found in the hepatocytes [125]. In the liver, CFTR is essential to the transport
of chloride, bicarbonate, and osmotically coupled water into the bile and to the
biliary HCO
The precise cellular localization of pendrin in the liver is unknown; in
addition, the main bicarbonate secretion pathway in cholangiocytes appears to be
the AE2/SLC4A2-mediated Cl
According to a number of cell-based and animal studies as well as clinical observations, the functional interplay between pendrin and CFTR seems to play an important role in the airways and in the kidney in physiological and pathological conditions including, and probably not limited to, CF and inflammation.
In the airways, pendrin and CFTR expression is driven by selective interleukins that are involved in the inflammatory responses and ASL physiology. In particular, the combined function of CFTR and pendrin may be important to control bicarbonate efflux that increases ASL pH and water secretion. However, the precise role of pendrin in terms of ion transport selectivity in CF and in other inflammatory diseases is still missing.
In the kidney, the pendrin-driven bicarbonate excretion relies on the functional
and molecular integrity of CFTR and serves to counteract metabolic alkalosis.
Whether and to what extent bicarbonate secretion also occurs via CFTR remains to
be determined. Also, whether CFTR stimulates pendrin via a direct molecular
interaction, favors its expression, folding and/or stabilization in the plasma
membrane, or simply provides a pathway for Cl
Several observations indicate that pendrin and CFTR might cooperate to control the apical anion transport as well as the pH and volume of the luminal fluid in other organs, including the inner ear and thyroid. Additional studies are needed to explore this intriguing hypothesis.
GT and SD conceived the study, performed the bibliographic research, prepared the figures, wrote the manuscript, contributed to editorial changes in the manuscript, read and approved the final manuscript.
Not applicable.
The authors acknowledge the expert secretarial assistance of Elisabeth Mooslechner.
This work was supported in part by the Paracelsus Medical University Research Fund, grant number PMU-FFF R-20/04/136-DOS to SD and Progetto PRIN (2017R5ZE2C_002) to GT.
The authors declare no conflict of interest.