Electrophoretic mobility and glycosylation characteristics of heterogeneously expressed calcitonin receptors

CGRP and the Calcitonin Receptor are Co-Expressed in Mouse, Rat and Human Trigeminal Ganglia Neurons

The neuropeptide calcitonin gene-related peptide (CGRP) is expressed in the trigeminal ganglia, a key site in craniofacial pain and migraine. CGRP potently activates two receptors: the CGRP receptor and the AMY₁ receptor. These receptors are heterodimers consisting of receptor activity-modifying protein 1 (RAMP1) with either the calcitonin receptor-like receptor (CLR) to form the CGRP receptor or the calcitonin receptor (CTR) to form the AMY₁ receptor. The expression of the CGRP receptor in trigeminal ganglia has been described in several studies; however, there is comparatively limited data available describing AMY₁ receptor expression and in which cellular subtypes it is found. This research aimed to determine the relative distributions of the AMY₁ receptor subunit, CTR, and CGRP in neurons or glia in rat, mouse and human trigeminal ganglia. Antibodies against CTR, CGRP and neuronal/glial cell markers were applied to trigeminal ganglia sections to investigate their distribution. CTR-like and CGRP-like immunoreactivity were observed in both discrete and overlapping populations of neurons. In rats and mice, 30–40% of trigeminal ganglia neurons displayed CTR-like immunoreactivity in their cell bodies, with approximately 78–80% of these also containing CGRP-like immunoreactivity. Although human cases were more variable, a similar overall pattern of CTR-like immunoreactivity to rodents was observed in the human trigeminal ganglia. CTR and CGRP appeared to be primarily colocalized in small to medium sized neurons, suggesting that colocalization of CTR and CGRP may occur in C-fiber neurons. CGRP-like or CTR-like immunoreactivity were not typically observed in glial cells. Western blotting confirmed that CTR was expressed in the trigeminal ganglia of all three species. These results confirm that CTR is expressed in trigeminal ganglia neurons. The identification of populations of neurons that express both CGRP and CTR suggests that CGRP could act in an autocrine manner through a CTR-based receptor, such as the AMY₁ receptor. Overall, this suggests that a trigeminal ganglia CTR-based receptor may be activated during migraine and could therefore represent a potential target to develop treatments for craniofacial pain and migraine.

Calcitonin receptor antibody validation and expression in the rodent brain

BACKGROUND AND AIM

Therapeutics that reduce calcitonin gene-related peptide activity are effective migraine treatments. However, gaps remain in our understanding of the molecular mechanisms that link calcitonin gene-related peptide to migraine. The amylin 1 receptor responds potently to calcitonin gene-related peptide, and to the related peptide amylin, but its role in relation to either peptide or to migraine is unclear. We sought to better understand the expression of the amylin 1 receptor protein subunit, the calcitonin receptor, in the rodent brain.

METHODS

We profiled three antibodies for immunodetection of calcitonin receptor, using immunocytochemistry, western blotting, and calcitonin receptor conditional knockout mouse tissue. Selected migraine-relevant rat brain regions were then examined for calcitonin receptor-like immunoreactivity.

RESULTS

All three antibodies detected calcitonin receptor protein but only one (188/10) produced robust immunostaining in rodent brain, under the conditions used. Calcitonin receptor-like immunoreactivity was apparent in the rat brainstem and midbrain including the locus coeruleus, periaqueductal grey and spinal trigeminal nucleus.

CONCLUSIONS

Anti-calcitonin receptor antibodies require comprehensive profiling to ensure confidence in the detection of calcitonin receptor. Using a validated antibody, calcitonin receptor-like immunoreactivity was detected in several brain regions relevant to migraine. Further research is needed to understand the functional consequences of calcitonin receptor expression for calcitonin gene-related peptide or amylin physiology and pathophysiology.

Identification and sub-cellular localization of a NAD transporter in Leishmania braziliensis (LbNDT1)

Nicotinamide adenine dinucleotide (NAD) is one of the central molecules involved in energy homeostasis, cellular signaling and antioxidative defense systems. Consequently, its biosynthetic pathways and transport systems are of vital importance. The nicotinamide/nicotinate mononucleotide adenylyltransferase (NMNAT), a key enzyme in the biosynthesis of NAD, is distributed in all domains of life and exhibits various isoforms in free-living organisms in contrast with intracellular parasites, which displays a single enzyme. In Leishmania braziliensis a unique cytosolic NMNAT has been reported to date and the mechanisms through which adequate levels of NAD are maintained among the different sub-cellular compartments of this parasite are unknown. Experimental evidences have related the transport of NAD to the Nucleotide Transporters (NTTs) family, whose members are located in the cytoplasmic membrane of parasitic life organisms. Additionally, the Mitochondrial Carrier Family (MCF), a group of proteins located in the membrane of internal organelles such as the mitochondria of free life organisms, has been implicated in NAD transport. Applying bioinformatics tools, the main characteristics of the MCF were found in a transporter candidate that we have designated as Nicotinamide Adenine Dinucleotide Transporter 1 of L. braziliensis (LbNDT1). The expression of LbNDT1 was tested both in axenic amastigotes and promastigotes of L. braziliensis, through immunodetection using polyclonal avian antibodies produced in this study. N-glycosylation of LbNDT1 was observed in both stages. Additionally, a possible partial mitochondrial distribution for LbNDT1 in amastigotes and a possible glycosomal location in promastigotes are proposed. Finally, the capability of LbNDT1 to transport NAD was confirmed by complementation assays in Saccharomyces cerevisiae. Our results demonstrate the existence of LbNDT1 in L. braziliensis becoming the first NAD transporter identified in protozoan parasites to date.

LRRK2 Kinase Inhibition Rescues Deficits in Lysosome Function Due to Heterozygous GBA1 Expression in Human iPSC-Derived Neurons

A growing number of genes associated with Parkinson’s disease are implicated in the regulation of lysosome function, including LRRK2, whose missense mutations are perhaps the most common monogenic cause of this neurodegenerative disease. These mutations are collectively thought to introduce a pathologic increase in LRRK2 kinase activity, which is currently a major target for therapeutic intervention. Heterozygous carriers of many missense mutations in the GBA1 gene have dramatically increased risk of Parkinson’s disease. A critical question has recently emerged regarding the potential interplay between the proteins encoded by these two disease-linked genes. Our group has recently demonstrated that knockin mutation of a Parkinson’s-linked GBA1 variant induces severe lysosomal and cytokine abnormalities in murine astrocytes and that these deficits were normalized via inhibition of wild-type LRRK2 kinase activity in these cells. Another group independently found that LRRK2 inhibition increases glucocerebrosidase activity in wild-type human iPSC-derived neurons, as well as those whose activity is disrupted by GBA1 or LRRK2 mutation. Fundamental questions remain in terms of the lysosomal abnormalities and the effects of LRRK2 kinase inhibition in human neurons deficient in glucocerebrosidase activity. Here, we further elucidate the physiological crosstalk between LRRK2 signaling and glucocerebrosidase activity in human iPSC-derived neurons. Our studies show that the allelic loss of GBA1 manifests broad defects in lysosomal morphology and function. Furthermore, our data show an increase in both the accumulation and secretion of oligomeric α-synuclein protein in these GBA1-heterozygous-null neurons, compared to isogenic controls. Consistent with recent findings in murine astrocytes, we observed that multiple indices of lysosomal dysfunction in GBA1-deficient human neurons were normalized by LRRK2 kinase inhibition, while some defects were preserved. Our findings demonstrate a selective but functional intersection between glucocerebrosidase dysfunction and LRRK2 signaling in the cell and may have implications in the pathogenesis and treatment of Parkinson’s disease.

Calcitonin Receptor N-Glycosylation Enhances Peptide Hormone Affinity by Controlling Receptor Dynamics

The class B G protein-coupled receptor (GPCR) calcitonin receptor (CTR) is a drug target for osteoporosis and diabetes. N-glycosylation of asparagine 130 in its extracellular domain (ECD) enhances calcitonin hormone affinity with the proximal GlcNAc residue mediating this effect through an unknown mechanism. Here, we present two crystal structures of salmon calcitonin-bound, GlcNAc-bearing CTR ECD at 1.78 and 2.85 Å resolutions and analyze the mechanism of the glycan effect. The N130 GlcNAc does not contact the hormone. Surprisingly, the structures are nearly identical to a structure of hormone-bound, N-glycan-free ECD, which suggested that the GlcNAc might affect CTR dynamics not observed in the static crystallographic snapshots. Hydrogen-deuterium exchange mass spectrometry and molecular dynamics simulations revealed that glycosylation stabilized a β-sheet adjacent to the N130 GlcNAc and the N-terminal α-helix near the peptide-binding site while increasing flexibility of the peptide-binding site turret loop. These changes due to N-glycosylation increased the ligand on-rate and decreased its off-rate. The glycan effect extended to RAMP-CTR amylin receptor complexes and was also conserved in the related CGRP receptor. These results reveal that N-glycosylation can modulate GPCR function by altering receptor dynamics.

N-Glycosylation of Asparagine 130 in the Extracellular Domain of the Human Calcitonin Receptor Significantly Increases Peptide Hormone Affinity

The calcitonin receptor (CTR) is a class B G protein-coupled receptor that is activated by the peptide hormones calcitonin and amylin. Calcitonin regulates bone remodeling through CTR, whereas amylin regulates blood glucose and food intake by activating CTR in complex with receptor activity-modifying proteins (RAMPs). These receptors are targeted clinically for the treatment of osteoporosis and diabetes. Here, we define the role of CTR N-glycosylation in hormone binding using purified calcitonin and amylin receptor extracellular domain (ECD) glycoforms and fluorescence polarization/anisotropy and isothermal titration calorimetry peptide-binding assays. N-Glycan-free CTR ECD produced in Escherichia coli exhibited ∼10-fold lower peptide affinity than CTR ECD produced in HEK293T cells, which yield complex N-glycans, or in HEK293S GnTI^- cells, which yield core N-glycans (Man₅GlcNAc₂). PNGase F-catalyzed removal of N-glycans at N73, N125, and N130 in the CTR ECD decreased peptide affinity ∼10-fold, whereas Endo H-catalyzed trimming of the N-glycans to single GlcNAc residues had no effect on peptide binding. Similar results were observed for an amylin receptor RAMP2-CTR ECD complex. Characterization of peptide-binding affinities of purified N → Q CTR ECD glycan site mutants combined with PNGase F and Endo H treatment strategies and mass spectrometry to define the glycan species indicated that a single GlcNAc residue at CTR N130 was responsible for the peptide affinity enhancement. Molecular modeling suggested that this GlcNAc functions through an allosteric mechanism rather than by directly contacting the peptide. These results reveal an important role for N-linked glycosylation in the peptide hormone binding of a clinically relevant class B GPCR.

Human testis-expressed sequence 101 is limitedly distributed in germinal epithelium of testis and disappears in seminoma

Background

Testis-expressed sequence 101 (TEX101) was found to be highly expressed in testis and involved in acrosome reaction in previous studies. Recently, the metastasis suppressor function of TEX101 in cancer was disclosed, but the comprehensive investigation of its expression has rarely been reported. In this study, the expression features of TEX101 in normal human organs and seminoma were systematically analyzed.

Results

Immunohistochemistry demonstrated intense staining of TEX101 in human testis tissues; however, its expression in 27 other types of normal human organs, including the ovary, was negligible. Higher expression of TEX101 was observed in the spermatocytes and spermatids of the testis, but relatively lower staining was detected in spermatogonia. Western blotting showed a single TEX101 band of 38 kDa in human testis, but it did not correspond to the predicted molecular weight of its mature form at 21 KDa. Furthermore, we examined seminoma tissues by immunohistochemistry and found that none of the 36 samples expressed TEX101.

Conclusions

Our data confirmed TEX101 to be a testis protein that could be related to the maturation process of male germ cells. The lack of TEX101 in seminoma indicated its potential role in tumor progression. This characteristic expression of TEX101 could provide a valuable reference for understanding its biological functions.

Spatial proximity between a photolabile residue in position 19 of salmon calcitonin and the amino terminus of the human calcitonin receptor

Calcitonins are 32-amino acid peptide hormones with both peripheral and central actions mediated via specific cell surface receptors, which belong to the class II subfamily of G protein-coupled receptors. Understanding receptor function, particularly in terms of ligand recognition by calcitonin receptors, may aid in the rational design of calcitonin analogs with increased potency and improved selectivity. To directly identify sites of proximity between calcitonin and its receptor, we carried out photoaffinity labeling studies followed by protein digestion and mapping of the radiolabeled photoconjugated receptor. A fully active salmon calcitonin analog [Arg(11,18),Bpa19]sCT, incorporating a photolabile p-benzoyl-L-phenylalanine into position 19 of the ligand, has been used to demonstrate spatial proximity between residue 19 of the peptide and the amino-terminal extracellular domain of the receptor. Cyanogen bromide cleavage together with endoproteinase Asp-N digestion indicated that binding was predominantly to the region delimited by receptor residues Cys134 and Met187. Binding to this fragment was supported further by cyanogen bromide-digestion of receptors that were mutated to remove the predicted cleavage site at Met133 (M133A, M133L). Binding within the 54-amino acid fragment was refined further by digestion with endoproteinase Lys-C to the 8-amino acid region corresponding to Cys134-Lys141. These results provide the first direct demonstration of a contact domain between salmon calcitonin and its receptor and will contribute toward modeling of the calcitonin-receptor interface.

Importance of the amino terminus in secretin family G protein-coupled receptors. Intrinsic photoaffinity labeling establishes initial docking constraints for the calcitonin receptor

The calcitonin receptor is a member of the class B family of G protein-coupled receptors, closely related to secretin and parathyroid hormone receptors. Although mechanisms of ligand binding have been directly explored for those receptors, current knowledge of the molecular basis of calcitonin binding to its receptor is based only on receptor mutagenesis. In this work we have utilized the more direct approach of photoaffinity labeling to explore spatial approximations between distinct residues within calcitonin and its receptor. For this we have developed two human calcitonin analogues incorporating a photolabile p-benzoyl-l-phenylalanine residue in the mid-region and carboxyl-terminal half of the peptide in positions 16 and 26, respectively. Both probes specifically bound to the human calcitonin receptor with high affinity and were potent stimulants of cAMP accumulation in calcitonin receptor-bearing human embryonic kidney 293 cells. They covalently labeled the calcitonin receptor in a saturable and specific manner. Further purification, deglycosylation, specific chemical and enzymatic cleavage, and sequencing of labeled wild type and mutant calcitonin receptors identified the sites of labeling for the position 16 and 26 probes as receptor residues Phe137 and Thr30, respectively. Both were within the extracellular amino terminus of the calcitonin receptor, with the former adjacent to the first transmembrane segment and the latter within the distal amino-terminal tail of the receptor. These data are consistent with affinity labeling of other members of the class B G protein-coupled receptors using analogous probes and may suggest a common ligand binding mechanism for this family.

Calcitonin receptor isoforms expressed in the developing rat kidney

BACKGROUND

Development in the metanephric-kidney transition period involves the precise expression of paracrine and autocrine events in an ordered spatio-temporal manner. Expression of these molecular events is tightly controlled and includes positive and negative growth factors and cognate receptors within close proximity in developing structures in the expanding renal cortex and medulla. The expression of calcitonin receptor (CTR) isoforms C1a and C1b in this context has not previously been described. Our current study also explored the relationship between the expression of CTR isoforms and amylin binding sites.

METHODS

Techniques included immunohistochemistry with novel antibodies that detect CTR isoforms, real time PCR for the quantification of CTR isoforms, Western blot and in vitro autoradiography, on tissues from embryo day 18 to postnatal day 30.

RESULTS

The CTR C1a isoform is expressed in the ureteric ducts of the metanephros and both isoforms are expressed in the developing distal convoluted tubules, ascending limbs of the loop of Henle and collecting ducts in the postnatal rat kidney. There was a 60-fold excess of C1a versus C1b isoforms. An apparent molecular weight of 63 kD was found. In vitro autoradiography demonstrated that while amylin binding sites were predominantly in the cortex, CTR expression was largely localized in the medulla in an earlier event, followed by cortical expression.

CONCLUSIONS

CTR C1a protein expression has been identified in the ureteric ducts in the metanephros and both isoforms expressed in the distal portions of the developing nephrons and collecting ducts. Since amylin binding sites have been localized on the proximal tubules of the cortex, it is unlikely that amylin receptors can be represented by modification of CTR affinity with receptor activity modifying proteins in the kidney.

Phospholipase D- and protein kinase C isoenzyme-dependent signal transduction pathways activated by the calcitonin receptor

The calcitonin receptor expressed by the porcine LLC-PK1 renal tubule cells is a seven-transmembrane domain, G protein-coupled receptor activating adenylyl-cyclase and phospholipase C. Salmon calcitonin stimulated dose- and time-dependent release of the phospholipase D-dependent phosphatidylcholine product [3H] choline with an EC50 = 2.5 +/-0.3 x 10(-8) M, similar to that determined for phosphoinositide metabolism (EC50 = 4.5 +/-1.0 x 10(-8)M). The hormone failed to induce release of [3H]phosphocholine and [3H]glycerophosphocholine, ruling out activation of phosphatydilcholine-specific phospholipase C and phospholipase A. Calcitonin stimulated phosphatidic acid, a product of phospholipase D-dependent phosphatydilcholine hydrolysis. Activation of phospholipase D was confirmed by release of [3H]phosphatydilethanol, a specific and stable product in the presence of a primary alcohol. Activation of calcitonin receptor induced diacylglycerol formation, with a rapid peak followed by a prolonged increase, due to activation of phospholipase C and of phospholipase D. Consequently, the protein kinase-C alpha, but not the delta isoenzyme, was cytosol-to-membrane translocated by approximately 50% after 20 min exposure to calcitonin, whereas protein kinase-C zeta, which was approximately 40% membrane-linked in unstimulated cells, translocated by approximately 19%. The human calcitonin receptor expressed by BIN-67 ovary tumor cells, although displaying higher affinity for calcitonin, failed to activate phospholipase D and protein kinase-C in response to the hormone. This receptor lacks the G protein binding consensus site due to the presence of a 48-bp cassette encoding for a 16-amino acid insert in the predicted first intracellular loop. This modification is likely to prevent the calcitonin receptor from associating to phospholipase-coupled signaling.

Characterization of amylin and calcitonin receptor binding in the mouse alpha-thyroid-stimulating hormone thyrotroph cell line

Recently, a high affinity amylin binding site was identified in the mouse alpha-TSH thyrotroph cell line. In this study, we have characterized binding sites for 125I-salmon calcitonin (125I-sCT), 125I-rat alpha-calcitonin gene-related peptide (125I-CGRP), and 125I-rat amylin in alpha-TSH cells. Using 125I-CGRP or 125I-rat amylin, equilibrium was rapidly reached, and binding was fully reversible. Competition binding revealed the relative potency of peptides was sCT>amylin, CGRP>>rCT, which is similar to the specificity profile of amylin receptors characterized in rat brain. Furthermore, specific binding of 125I-rat amylin and 125I-CGRP to membrane preparations was reduced by 52% and 39%, respectively, in the presence of 20 microM GTP-gamma-s, indicating a requirement of G protein coupling for high affinity binding. In contrast, 125I-sCT binding reached equilibrium more slowly, was essentially irreversible, and was unaltered by GTP-gamma-s. Competition binding studies using 125I-sCT as radioligand demonstrated only weak interaction by CGRP or amylin, consistent with other described CT receptors. Assessment of ligand-induced cAMP accumulation and intracellular calcium signaling revealed a relative specificity profile of sCT>rCT with little or no second messenger signaling stimulated by amylin or CGRP, consistent with a C1-CT receptor phenotype. RT-PCR amplification of messenger RNA indicated that the predominant isoform was the C1a CT receptor. In cross-linking studies, 125I-rat amylin and 125I-CGRP specifically labeled a major band of relative molecular mass (Mr) approximately 80K, being approximately 10 kDa higher than the major 125I-sCT binding protein. Full deglycosylation of N-linked carbohydrates with endoglycosidase F reduced the Mr of each of the labeled proteins to approximately 50K. Cross-linked amylin or CT receptors were immunoprecipitated with C-terminally directed antimouse or antirat CT receptor antibodies but were not immunoprecipitated with nonimmune sera or antihuman CT receptor antibodies. The current data demonstrate expression of two biochemically distinct receptor phenotypes in mouse alpha-TSH cells, a CT receptor phenotype and an amylin receptor phenotype that have highly similar protein backbones.