Functional characterization of melanocortin 2 receptor (Mc2r) from a lobe-finned fish (Protopterus annectens) and insights into the molecular evolution of melanocortin receptors.

Recent studies from our group on melanocortin 2 receptors (Mc2r) from basal families of actinopterygians have served to resolve that Mrap1 dependence and ACTH selectivity are features of even the most basal ray-finned fishes. However, there have been no studies on Mc2r function of the basal sarcopterygians, the lobe-finned fishes, represented by the extant members coelacanths and lungfishes. Here, we offer the first molecular and functional characterization of an Mc2r from a lobe-finned fish, the West African lungfish (Protopterus annectens). Plasmids containing cDNA constructs of lungfish (lf) Mc2r and Mrap1 were expressed in mammalian and zebrafish cell lines. Cells were then stimulated by human ACTH(1-24) and melanocyte stimulating hormone (α-MSH), as well as alanine-substituted analogs of hACTH(1-24) targeting residues within the H6F7R8W9 and K15K16R17R18P19 motifs. Activation of lfMc2r was assessed using a cAMP-responsive luciferase reporter gene assay. In these assays, lfMc2r required co-expression with lfMrap1, was selective for ACTH over α-MSH at physiological concentrations of the ligands, and was completely inhibited by multiple-alanine substitutions of the HFRW (A6-9) and KKRRP (A15-19) motifs. Single- and partial-alanine substitutions of the HFRW and KKRRP motifs varied in their impacts on receptor-ligand affinity from having no effect to completely inhibiting lfMc2r activation. This characterization of the Mc2r of a lobe-finned fish fulfills the last major extant vertebrate group for which Mc2r function had yet to be characterized. In doing so, we resolve that all basal bony vertebrate groups exhibit Mc2r function that substantially differs from that of the cartilaginous fishes, indicating that rapid and dramatic shift in Mc2r function occurred between the radiation of cartilaginous fishes and the emergence of bony fishes. We support this interpretation with a molecular clock analysis of the melanocortin receptors, which demonstrates the uniquely high rate of sequence divergence in Mc2r. Much remains to be understood regarding the molecular evolution of Mc2r during the early radiation of vertebrates that resulted in the derived functional characteristics of Mrap1 dependence and exclusive selectivity for ACTH.

Molecular and pharmacological analysis of the melanocortin-2 receptor and its accessory proteins Mrap1 and Mrap2 in a Squalomorph shark, the Pacific spiny dogfish.

The hypothalamus-pituitary-adrenal/interrenal (HPA/I) axis is a conserved vertebrate neuroendocrine mechanism regulating the stress response. The penultimate step of the HPA/I axis is the exclusive activation of the melanocortin-2 receptor (Mc2r) by adrenocorticotropic hormone (ACTH), requiring an accessory protein, Mrap1 or Mrap2. Limited data for only three cartilaginous fishes support the hypothesis that Mc2r/Mrap1 function in bony vertebrates is a derived trait. Further, Mc2r/Mrap1 functional properties appear to contrast among cartilaginous fishes (i.e., the holocephalans and elasmobranchs). This study sought to determine whether functional properties of Mc2r/Mrap1 are conserved across elasmobranchs and in contrast to holocephalans. The deduced amino acid sequences of Pacific spiny dogfish (Squalus suckleyi; pd) pdMc2r, pdMrap1, and pdMrap2 were obtained from a de novo transcriptome of the interrenal gland and validated against the S. suckleyi genome. pdMc2r showed high primary sequence similarity with elasmobranch and holocephalan Mc2r except at extracellular domains 1 and 2, and transmembrane domain 5. pdMraps showed similarly high sequence similarity with holocephalan and other elasmobranch Mraps, with all cartilaginous fish Mrap1 orthologs lacking an activation motif. cAMP reporter gene assays demonstrated that pdMc2r requires an Mrap for activation, and can be activated by stingray (sr) ACTH(1-24), srACTH(1-13)NH2 (i.e., α-MSH), and γ-melanocyte-stimulating hormone at physiological concentrations. However, pdMc2r was three orders of magnitude more sensitive to srACTH(1-24) than srACTH(1-13)NH2. Further, pdMc2r was two orders of magnitude more sensitive to srACTH(1-24) when expressed with pdMrap1 than with pdMrap2. These data suggest that functional properties of pdMc2r/pdMrap1 reflect other elasmobranchs and contrast what is seen in holocephalans.

Hypothalamus-pituitary-interrenal (HPI) axis signaling in Atlantic sturgeon (Acipenser oxyrinchus) and sterlet (Acipenser ruthenus).

In vertebrates, the hypothalamic-pituitaryadrenal/interrenal (HPA/HPI) axis is a highly conserved endocrine axis that regulates glucocorticoid production via signaling by corticotropin releasing hormone (CRH) and adrenocorticotropic hormone (ACTH). Once activated by ACTH, Gs protein-coupled melanocortin 2 receptors (Mc2r) present in corticosteroidogenic cells stimulate expression of steroidogenic acute regulatory protein (Star), which initiates steroid biosynthesis. In the present study, we examined the tissue distribution of genes involved in HPI axis signaling and steroidogenesis in the Atlantic sturgeon (Acipenser oxyrinchus) and provided the first functional characterization of Mc2r in sturgeon. Mc2r of A. oxyrinchus and the sterlet sturgeon (Acipenser ruthenus) are co-dependent on interaction with the melanocortin receptor accessory protein 1 (Mrap1) and highly selective for human (h) ACTH over other melanocortin ligands. A. oxyrinchus expresses key genes involved in HPI axis signaling in a tissue-specific manner that is indicative of the presence of a complete HPI axis in sturgeon. Importantly, we co-localized mc2r, mrap1, and star mRNA expression to the head kidney, indicating that this is possibly a site of ACTH-mediated corticosteroidogenesis in sturgeon. Our results are discussed in the context of other studies on the HPI axis of basal bony vertebrates, which, when taken together, demonstrate a need to better resolve the evolution of HPI axis signaling in vertebrates.

Trends in the evolution of the elasmobranch melanocortin-2 receptor: Insights from structure/function studies on the activation of whale shark Mc2r.

To understand the mechanism for activation of the melanocortin-2 receptor (Mc2r) of the elasmobranch, Rhincodon typus (whale shark; ws), wsmc2r was co-expressed with wsmrap1 in CHO cells, and the transfected cells were stimulated with alanine-substituted analogs of ACTH(1-24) at the "message" motif (H6F7R8W9) and the "address" motif (K15K16R17R18P19). Complete alanine substitution of the H6F7R8W9 motif blocked activation, whereas single alanine substitution at this motif indicated the following hierarchy of position importance for activation: W9 > R8, and substitution at F7 and H6 had no effect on activation. The same analysis was done on a representative bony vertebrate Mc2r ortholog (Amia calva; bowfin; bf) and the order of position importance for activation was W9 > R8 = F7, (alanine substitution at H6 was negligible). Complete alanine substitution at the K15K16R17R18P19 motif resulted in distinct outcomes for wsMc2r and bfMc2r. For bfMc2r, this analog blocked activation-an outcome typical for bony vertebrate Mc2r orthologs. For wsMc2r, this analog resulted in a shift in sensitivity to stimulation of the analog as compared to ACTH(1-24) by two orders of magnitude, but the dose response curve did reach saturation. To evaluate whether the EC2 domain of wsMc2r plays a role in activation, a chimeric wsMc2r was made in which the EC2 domain was replaced with the EC2 domain from a melanocortin receptor that does not interact with Mrap1 (i.e., Xenopus tropicalis Mc1r). This substitution did not negatively impact the activation of the chimeric receptor. In addition, alanine substitution at a putative activation motif in the N-terminal of wsMrap1 did not affect the sensitivity of wsMc2r to stimulation by ACTH(1-24). Collectively, these observations suggest that wsMc2r may only have a HFRW binding site for melanocortin-related ligand which would explain how wsMc2r could be activated by either ACTH or MSH-sized ligands.

Receptor-mediated activation of CFTR via prostaglandin signaling pathways in the airway.

Cystic fibrosis (CF) is a genetic disease caused by mutations of the gene encoding a cAMP-activated Cl- channel, the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR modulator therapies consist of small-molecule drugs that rescue mutant CFTR. Regimens of single or combinations of CFTR modulators still rely on endogenous levels of cAMP to regulate CFTR activity. We investigated CFTR activation by the natural mediator prostaglandin E2 (PGE2) and lubiprostone (a Food and Drug Administration-approved drug known to target prostaglandin receptors) and tested the hypothesis that receptor-mediated CFTR activators can be used in combination with currently available CFTR modulators to increase function of mutant CFTR. Primary-cultured airway epithelia were assayed in Ussing chambers. Experimental CFTR activators and established CFTR modulators were applied for 24 h and/or acutely and analyzed for their effect on CFTR activity as measured by changes in short-circuit current (ISC). In non-CF airway epithelia, acute application of lubiprostone and PGE2 activated CFTR to the levels comparable to forskolin (Fsk). Pretreatment (24 h) with antagonists to prostaglandin receptors EP2 and EP4 abolished the ability of lubiprostone to acutely activate CFTR. In F508del homozygous airway epithelia pretreated with the triple combination of elexacaftor, tezacaftor, and ivacaftor (ELEXA/TEZ/IVA; i.e., Trikafta), acute application of lubiprostone was able to maximally activate CFTR. Prolonged (24 h) cotreatment of F508del homozygous epithelia with ELEXA/TEZ/IVA and lubiprostone increased acute CFTR activation by ∼60% compared with the treatment with ELEXA/TEZ/IVA alone. This work establishes the feasibility of targeting prostaglandin receptors to activate CFTR on the airway epithelia and demonstrates that cotreatment with lubiprostone can further restore modulator-rescued CFTR.

A basal actinopterygian melanocortin receptor: Molecular and functional characterization of an Mc2r ortholog from the Senegal bichir (Polypterus senegalus).

In bony vertebrates, melanocortin 2 receptor (Mc2r) specifically binds adrenocorticotropic hormone (ACTH) and is responsible for mediating anterior pituitary signaling that stimulates corticosteroid production in the adrenal gland/interrenal cells. In bony fishes Mc2r requires the chaperoning of an accessory protein (Mrap1) to traffic to the membrane surface and bind ACTH. Here, we evaluated the structure and pharmacological properties of Mc2r from the Senegal bichir (Polypterus senegalus), which represents the most basal bony fish from which an Mc2r has been pharmacologically studied to date. In our experiments, cDNA constructs of the Mc2r from the Senegal bichir (sbMc2r) and various vertebrate Mrap1s were heterologously co-expressed in Chinese hamster ovary (CHO) cells, stimulated by ACTH or melanocyte-stimulating hormone (α-MSH) ligands, and assessed using a luciferase reporter gene assay. When expressed without an Mrap1, sbMc2r was not activated by ACTH. When co-expressed with Mrap1 from either chicken (Gallus gallus) or bowfin (Amia calva), sbMc2r could be activated in a dose-dependent manner by ACTH, but not α-MSH. Co-expression of sbMrap2 with sbMc2r resulted in no detectable activation of the receptor. Collectively, these results demonstrate that sbMc2r has pharmacological properties similar to those of Mc2rs of later-evolved bony fishes, such as Mrap1 dependence and ACTH selectivity, indicating that these qualities of Mc2r function are ancestral to all bony fish Mc2rs.

The isosmotic point as critical salinity limit for growth and osmoregulation, but not survival, in the wolf eel Anarrhichthys ocellatus.

Members of wolf fish family Anarhichadidae have emerged as potential cold-water marine aquaculture species. This study examined growth performance and osmoregulation in juvenile wolf eel (Anarrhichthys ocellatus) held in a series of dilute salinities (30, 14, 9, and 6 ‰) over an 8-week trial. At the conclusion of the growth study, fish were sampled for analysis of gill and intestine enzyme activity, plasma ion content, and muscle moisture. Growth rate remained positive in all salinities throughout the 8-week trial. Specific growth rate was maintained above 3.0% mass day-1 at salinities of 30 and 14 ‰, but was significantly reduced at 9 (2.9% mass day-1) and 6 ‰ (2.0% mass day-1). Muscle water content increased with increasing salinity dilution (77.9% water in 30 ‰; 79.8% water in 6 ‰), and plasma osmolality (~ 320 mOsm kg-1) was maintained in salinities as dilute as 9 ‰ but was significantly lower (~ 280 mOsm kg-1) in the most dilute salinity of 6 ‰. Segmental linear regression analyses revealed that the calculated isosmotic point for wolf eel of ~ 10.6 ‰ was a critical limit for maintaining growth performance and osmoregulatory homeostasis. It is an important finding that fish considered to be a typical marine stenohaline organism could maintain ion and water balance as low as the isosmotic point, and exhibit survival and positive growth rates in salinities as dilute as 6 ‰. This work delivers a fundamental step in the empirical examination of this emerging aquaculture species and provides a model for evaluating osmoregulatory performance of marine stenohaline fishes in low-salinity aquaculture.

11-Deoxycortisol is a stress responsive and gluconeogenic hormone in a jawless vertebrate, the sea lamprey (Petromyzon marinus).

Although corticosteroid-mediated hepatic gluconeogenic activity in response to stress has been extensively studied in fishes and other vertebrates, there is little information on the stress response in basal vertebrates. In sea lamprey (Petromyzon marinus), a representative member of the most basal extant vertebrate group Agnatha, 11-deoxycortisol and deoxycorticosterone are the major circulating corticosteroids. The present study examined changes in circulating glucose and 11-deoxycortisol concentrations in response to a physical stressor. Furthermore, the gluconeogenic actions of 11-deoxycortisol and deoxycorticosterone were examined. Within 6 h of exposure of larval and juvenile sea lamprey to an acute handling stress, plasma 11-deoxycortisol levels increased 15- and 6-fold, respectively, and plasma glucose increased 3- and 4-fold, respectively. Radiometric receptor binding studies revealed that a corticosteroid receptor (CR) is present in the liver at lower abundance than in other tissues (gill and anterior intestine) and that the binding affinity of the liver CR was similar for 11-deoxycortisol and deoxycorticosterone. Transcriptional tissue profiles indicate a wide distribution of cr transcription, kidney-specific transcription of steroidogenic acute regulatory protein (star) and liver-specific transcription of phosphoenolpyruvate carboxykinase (pepck). Ex vivo incubation of liver tissue with 11-deoxycortisol resulted in dose-dependent increases in pepck mRNA levels. Finally, intraperitoneal administration of 11-deoxycortisol and deoxycorticosterone demonstrated that only 11-deoxycortisol resulted in an increase in plasma glucose. Together, these results provide the first direct evidence for the gluconeogenic activity of 11-deoxycortisol in an agnathan, indicating that corticosteroid regulation of plasma glucose is a basal trait among vertebrates.

Corticosteroid control of Na+/K+-ATPase in the intestine of the sea lamprey (Petromyzon marinus).

Anadromous sea lamprey (Petromyzon marinus) larvae undergo a months-long true metamorphosis during which they develop seawater (SW) tolerance prior to downstream migration and SW entry. We have previously shown that intestinal Na+/K+-ATPase (NKA) activity increases during metamorphosis and is critical to the osmoregulatory function of the intestine in SW. The present study investigated the role of 11-deoxycortisol (S) in controlling NKA in the anterior (AI) and posterior (PI) intestine during sea lamprey metamorphosis. In a tissue profile, nka mRNA and protein were most abundant in the gill, kidney, and AI. During metamorphosis, AI nka mRNA increased 10-fold, whereas PI nka mRNA did not change. Specific corticosteroid receptors were found in the AI, which had a higher binding affinity for S compared to 11-deoxycorticosterone (DOC). In vivo administration of S in mid-metamorphic lamprey upregulated NKA activity 3-fold in the AI and PI, whereas administration of DOC did not affect intestinal NKA activity. During a 24 h SW challenge test, dehydration of white muscle moisture was rescued by prior treatment with S, which was associated with increased intestinal nka mRNA and NKA activity. These results indicate that intestinal osmoregulation in sea lamprey is a target for control by S during metamorphosis and the development of SW tolerance.

Functional characterization and osmoregulatory role of the Na+-K+-2Cl- cotransporter in the gill of sea lamprey (Petromyzon marinus), a basal vertebrate.

The present study provides molecular and functional characterization of Na+-K+-2Cl- cotransporter (NKCC1/Slc12a2) in the gills of sea lamprey (Petromyzon marinus), the most basal extant vertebrate with an osmoregulatory strategy. We report the full-length peptide sequence for the lamprey Na-K-Cl cotransporter 1 (NKCC1), which we show groups strongly with and occupies a basal position among other vertebrate NKCC1 sequences. In postmetamorphic juvenile lamprey, nkcc1 mRNA was present in many tissues but was fivefold higher in the gill than any other examined tissue, and NKCC1 protein was only detected in the gill. Gill mRNA and protein abundances of NKCC1 and Na+-K+-ATPase (NKA/Atp1a1) were significantly upregulated (20- to 200-fold) during late metamorphosis in fresh water, coinciding with the development of salinity tolerance, and were upregulated an additional twofold after acclimation to seawater (SW). Immunohistochemistry revealed that NKCC1 in the gill is found in filamental ionocytes coexpressing NKA, which develop during metamorphosis in preparation for SW entry. Lamprey treated with bumetanide, a widely used pharmacological inhibitor of NKCC1, exhibited higher plasma Cl- and osmolality as well as reduced muscle water content after 24 h in SW; there were no effects of bumetanide in freshwater-acclimated lamprey. This work provides the first functional characterization of NKCC1 as a mechanism for branchial salt secretion in lampreys, providing evidence that this mode of Cl- secretion has been present among vertebrates for ~550 million years.

Reduced thermal tolerance during salinity acclimation in brook trout (Salvelinus fontinalis) can be rescued by prior treatment with cortisol.

The aims of this study were to assess whether thermal tolerance of brook trout (Salvelinus fontinalis) is affected during seawater (SW) acclimation and to investigate the role of cortisol in osmoregulation and thermal tolerance during SW acclimation. Freshwater (FW)-acclimated brook trout at 18°C (Tacc) were exposed to SW for 16 days, whilst maintaining a FW control. Fish were examined for critical thermal maximum (CTmax) 0 (before), 2, 5 and 16 days after SW exposure, and sampled at Tacc and CTmax for analysis of plasma cortisol, glucose and Cl-, gill Na+/K+-ATPase (NKA) activity and heat shock protein 70 (HSP70) abundance, and white muscle water content. At 2 days in SW, CTmax was significantly reduced (from 31 to 26°C), and then recovered by 16 days. This transient decrease in thermal tolerance coincided with a transient increase in plasma Cl- and decrease in muscle moisture content. Salinity itself had no effect on gill HSP70 abundance compared with the large and immediate effects of high temperature exposure during CTmax testing. To examine the role of cortisol in osmoregulation, brook trout were administered a cortisol implant (5 and 25 µg g-1 CORT) prior to SW exposure. Both CORT doses significantly increased their capacity to maintain plasma Cl- during SW acclimation. Treatment with the 25 µg g-1 CORT dose was shown to significantly improve CTmax after 2 days in SW, and CTmax was associated with plasma Cl- and muscle moisture content. These findings indicate that brook trout are sensitive to temperature during SW acclimation and that thermal tolerance is associated with ion and water balance during SW acclimation.

Interaction of osmoregulatory and acid-base compensation in white sturgeon (Acipenser transmontanus) during exposure to aquatic hypercarbia and elevated salinity.

Migratory fishes encounter a variety of environmental conditions, including changes in salinity, temperature, and dissolved gases, and it is important to understand how these fishes are able to acclimate to multiple environmental stressors. The gill is the primary site of both acid-base balance and ion regulation in fishes. Many ion transport mechanisms involved with acid-base compensation are also required for the regulation of plasma Na+ and Cl+, the predominant extracellular ions, potentially resulting in a strong interaction between iono- and acid-base regulation. The present study examined the physiological interaction of elevated dissolved CO2 (an acid-base disturbance) on osmoregulation during seawater acclimation (an ionoregulatory disturbance) in juvenile white sturgeon (Acipenser transmontanus). Blood pH (pHe), plasma [HCO3-], [Na+], [Cl-], and osmolality, white muscle water content, and gill Na+/K+-ATPase (NKA) and Na+/K+/2Cl- cotransporter (NKCC) abundance were examined over a 10-day seawater (SW) acclimation period under normocarbia (NCSW) or during prior and continued exposure to hypercarbia (HCSW), and compared to a normocarbic freshwater (NCFW) control. Hypercarbia induced a severe extracellular acidosis (from pH 7.65 to pH 7.2) in HCSW sturgeon, and these fish had a 2-fold greater rise in plasma osmolarity over NCSW by day 2 of SW exposure. Interestingly, pHe recovery in HCSW was associated more prominently with an elevation in plasma Na+ prior to osmotic recovery and more prominently with a reduction in plasma Cl- following osmotic recovery, indicating a biphasic response as the requirements of osmoregulation transitioned from ion-uptake to ion-excretion throughout SW acclimation. These results imply a prioritization of osmoregulatory recovery over acid-base recovery in this period of combined exposure to acid-base and ionoregulatory disturbances.

Interaction of osmoregulatory and acid-base compensation in white sturgeon (Acipenser transmontanus) during exposure to aquatic hypercarbia and elevated salinity.

Migratory fishes encounter a variety of environmental conditions, including changes in salinity, temperature and dissolved gases, and it is important to understand how these fishes are able to acclimate to multiple environmental stressors. The gill is the primary site of both acid-base balance and ion regulation in fishes. Many ion transport mechanisms involved with acid-base compensation are also required for the regulation of plasma Na(+) and Cl(+), the predominant extracellular ions, potentially resulting in a strong interaction between ionoregulation and acid-base regulation. The present study examined the physiological interaction of elevated dissolved CO2 (an acid-base disturbance) on osmoregulation during seawater acclimation (an ionoregulatory disturbance) in juvenile white sturgeon (Acipenser transmontanus). Blood pH (pHe), plasma [HCO3 (-)], [Na(+)], [Cl(-)] and osmolality, white muscle water content, and gill Na(+)/K(+)-ATPase (NKA) and Na(+)/K(+)/2Cl(-) co-transporter (NKCC) abundance were examined over a 10 day seawater (SW) acclimation period under normocarbia (NCSW) or during prior and continued exposure to hypercarbia (HCSW), and compared with a normocarbic freshwater (NCFW) control. Hypercarbia induced a severe extracellular acidosis (from pH 7.65 to pH 7.2) in HCSW sturgeon, and these fish had a 2-fold greater rise in plasma osmolarity over NCSW by day 2 of SW exposure. Interestingly, pHe recovery in HCSW was associated more prominently with an elevation in plasma Na(+) prior to osmotic recovery and more prominently with a reduction in plasma Cl(-) following osmotic recovery, indicating a biphasic response as the requirements of osmoregulation transitioned from ion-uptake to ion-excretion throughout SW acclimation. These results imply a prioritization of osmoregulatory recovery over acid-base recovery in this period of combined exposure to acid-base and ionoregulatory disturbances.

Endocrine control of gill ionocyte function in euryhaline fishes.

The endocrine system is an essential regulator of the osmoregulatory organs that enable euryhaline fishes to maintain hydromineral balance in a broad range of environmental salinities. Because branchial ionocytes are the primary site for the active exchange of Na+, Cl-, and Ca2+ with the external environment, their functional regulation is inextricably linked with adaptive responses to changes in salinity. Here, we review the molecular-level processes that connect osmoregulatory hormones with branchial ion transport. We focus on how factors such as prolactin, growth hormone, cortisol, and insulin-like growth-factors operate through their cognate receptors to direct the expression of specific ion transporters/channels, Na+/K+-ATPases, tight-junction proteins, and aquaporins in ion-absorptive (freshwater-type) and ion-secretory (seawater-type) ionocytes. While these connections have historically been deduced in teleost models, more recently, increased attention has been given to understanding the nature of these connections in basal lineages. We conclude our review by proposing areas for future investigation that aim to fill gaps in the collective understanding of how hormonal signaling underlies ionocyte-based processes.

Net benefit of ivacaftor during prolonged tezacaftor/elexacaftor exposure in vitro.

BACKGROUND: A decrease in the lumacaftor-mediated increase in F508del-CFTR function and expression upon prolonged exposure to ivacaftor (VX-770) has previously been described. However, the efficacy observed with ivacaftor-containing CFTR modulator therapies in vivo is in conflict with these reports. We hypothesized that a portion of the apparent decrease in CFTR function observed after prolonged ivacaftor exposure in vitro was due to an increase in constitutive CFTR-mediated ion transport. METHODS: Human nasal epithelial (HNE) cells were obtained by brushings from three CF individuals homozygous for the F508del CFTR mutation. Differentiated epithelia were pre-treated with prolonged (24 h) exposure to either lumacaftor (VX-809; 3 µM), tezacaftor (VX-661; 3 µM), elexacaftor (VX-445; 3 µM), and/or ivacaftor (0.1-6.4 µM) or DMSO (vehicle control), and CFTR function was assayed by Ussing chamber electrophysiology. RESULTS: In cells treated with lumacaftor, constitutive CFTR activity was not increased at any concentration of co-treatment with ivacaftor. Constitutive CFTR activity was also unchanged in cells treated with the combination of tezacaftor and elexacaftor. An increase in constitutive CFTR activity above the DMSO controls was only observed in cells treated with the combination of tezacaftor and elexacaftor and co-treated with at least 0.1 µM ivacaftor. CONCLUSIONS: These results demonstrate that ivacaftor is a critical component in the triple combination therapy along with tezacaftor and elexacaftor to increase constitutive CFTR function. This work further elucidates the mechanism of action of the effective triple combination therapeutic that is now the primary clinical tool in treating CF.

Human Melanocortin-2 Receptor: Identifying a Role for Residues in the TM4, EC2, and TM5 Domains in Activation and Trafficking as a Result of Co-Expression with the Accessory Protein, Mrap1 in Chinese Hamster Ovary Cells.

Human melanocortin-2 receptor (hMC2R) co-expressed with the accessory protein mouse (m)MRAP1 in Chinese Hamster Ovary (CHO) cells has been used as a model system to investigate the activation and trafficking of hMC2R. A previous study had shown that the N-terminal domain of mMRAP1 makes contact with one of the extracellular domains of hMC2R to facilitate activation of hMC2R. A chimeric receptor paradigm was used in which the extracellular domains of hMC2R were replaced with the corresponding domains from Xenopus tropicalis MC1R, a receptor that does not interact with MRAP1, to reveal that EC2 (Extracellular domain 2) is the most likely contact site for hMC2R and mMRAP1 to facilitate activation of the receptor following an ACTH binding event. Prior to activation, mMRAP1 facilitates the trafficking of hMC2R from the ER to the plasma membrane. This process is dependent on the transmembrane domain (TM) of mMRAP1 making contact with one or more TMs of hMC2R. A single alanine substitution paradigm was used to identify residues in TM4 (i.e., I163, M165), EC2 (F167), and TM5 (F178) that play a role in the trafficking of hMC2R to the plasma membrane. These results provide further clarification of the activation mechanism for hMC2R.

Structure/Function Studies on the Activation Motif of Two Non-Mammalian Mrap1 Orthologs, and Observations on the Phylogeny of Mrap1, Including a Novel Characterization of an Mrap1 from the Chondrostean Fish, Polyodon spathula.

In derived bony vertebrates, activation of the melanocortin-2 receptor (Mc2r) by its ACTH ligand requires chaperoning by the Mc2r accessory protein (Mrap1). The N-terminal domain of the non-mammalian tetrapod MRAP1 from chicken (c; Gallus gallus) has the putative activation motif, W18D19Y20I21, and the N-terminal domain in the neopterygian ray-finned fish Mrap1 from bowfin (bf; Amia calva) has the putative activation motif, Y18D19Y20I21. The current study used an alanine-substitution paradigm to test the hypothesis that only the Y20 position in the Mrap1 ortholog of these non-mammalian vertebrates is required for activation of the respective Mc2r ortholog. Instead, we found that for cMRAP1, single alanine-substitution resulted in a gradient of inhibition of activation (Y20 >> D19 = W18 > I21). For bfMrap1, single alanine-substitution also resulted in a gradient of inhibition of activation (Y20 >> D19 > I21 > Y18). This study also included an analysis of Mc2r activation in an older lineage of ray-finned fish, the paddlefish (p), Polyodon spathula (subclass Chondronstei). Currently no mrap1 gene has been found in the paddlefish genome. When pmc2r was expressed alone in our CHO cell/cAMP reporter gene assay, no activation was observed following stimulation with ACTH. However, when pmc2r was co-expressed with either cmrap1 or bfmrap1 robust dose response curves were generated. These results indicate that the formation of an Mc2r/Mrap1 heterodimer emerged early in the radiation of the bony vertebrates.

IL-13-programmed airway tuft cells produce PGE2, which promotes CFTR-dependent mucociliary function.

Chronic type 2 (T2) inflammatory diseases of the respiratory tract are characterized by mucus overproduction and disordered mucociliary function, which are largely attributed to the effects of IL-13 on common epithelial cell types (mucus secretory and ciliated cells). The role of rare cells in airway T2 inflammation is less clear, though tuft cells have been shown to be critical in the initiation of T2 immunity in the intestine. Using bulk and single-cell RNA sequencing of airway epithelium and mouse modeling, we found that IL-13 expanded and programmed airway tuft cells toward eicosanoid metabolism and that tuft cell deficiency led to a reduction in airway prostaglandin E2 (PGE2) concentration. Allergic airway epithelia bore a signature of PGE2 activation, and PGE2 activation led to cystic fibrosis transmembrane receptor-dependent ion and fluid secretion and accelerated mucociliary transport. These data reveal a role for tuft cells in regulating epithelial mucociliary function in the allergic airway.

Molecular mechanisms of Cl- transport in fishes: New insights and their evolutionary context.

With remarkably few exceptions, aquatic vertebrates maintain internal Cl- homeostasis despite strong and sometimes fluctuating Cl- concentration gradients between extracellular fluids and external environments. In this "Perspective," we discuss recent advances in the understanding of epithelial Cl- transport at the molecular level within key osmoregulatory organs in fishes. New insights into mechanisms for epithelial Cl- transport in basal lineages are highlighted to provide an evolutionary context. We describe Cl- transport processes that employ: cystic fibrosis transmembrane conductance regulator, cation-chloride cotransporters, voltage-gated chloride channels, and chloride-anion exchangers. As the collective understanding of Cl- transport processes continues to expand, investigators are equipped to more precisely characterize how endocrine factors promote hydromineral balance. We, therefore, conclude our discussion by paying special attention to recently defined roles for prolactin and corticosteroids in the regulation of Cl- transport in basal and derived clades.

Elexacaftor is a CFTR potentiator and acts synergistically with ivacaftor during acute and chronic treatment.

Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), which lead to early death due to progressive lung disease. The development of small-molecule modulators that directly interact with CFTR to aid in protein folding ("correctors") and/or increase channel function ("potentiators") have proven to be highly effective in the therapeutic treatment of CF. Notably, incorporation of the next-generation CFTR corrector, elexacaftor, into a triple combination therapeutic (marketed as Trikafta) has shown tremendous clinical promise in treating CF caused by F508del-CFTR. Here, we report on a newly-described role of elexacaftor as a CFTR potentiator. We explore the acute and chronic actions, pharmacology, and efficacy of elexacaftor as a CFTR potentiator in restoring function to multiple classes of CFTR mutations. We demonstrate that the potentiating action of elexacaftor exhibits multiplicative synergy with the established CFTR potentiator ivacaftor in rescuing multiple CFTR class defects, indicating that a new combination therapeutic of ivacaftor and elexacaftor could have broad impact on CF therapies.