Polarity of TRH receptors in transfected MDCK cells is independent of endocytosis signals and G protein coupling

Functional role of the NPxxY motif in internalization of the type 2 vasopressin receptor in LLC-PK1 cells

Interaction of the type 2 vasopressin receptor (V2R) with hormone causes desensitization and internalization. To study the role of the V2R NPxxY motif (which is involved in the clathrin-mediated endocytosis of several other receptors) in this process, we expressed FLAG-tagged wild-type V2R and a Y325F mutant V2R in LLC-PK1a epithelial cells that have low levels of endogenous V2R. Both proteins had a similar apical (35%) and basolateral (65%) membrane distribution. Substitution of Tyr325 with Phe325 prevented ligand-induced internalization of V2R determined by [3H]AVP binding and immunofluorescence but did not prevent ligand binding or signal transduction via adenylyl cyclase. Desensitization and resensitization of the V2R-Y325F mutation occurred independently of internalization. The involvement of clathrin in V2R downregulation was also shown by immunogold electron microscopy. We conclude that the NPxxY motif of the V2R is critically involved in receptor downregulation via clathrin-mediated internalization. However, this motif is not essential for the apical/basolateral sorting and polarized distribution of the V2R in LLC-PK1a cells or for adenylyl cyclase-mediated signal transduction.

Characterization of a di-leucine-based signal in the cytoplasmic tail of the nucleotide-pyrophosphatase NPP1 that mediates basolateral targeting but not endocytosis

Enzymes of the nucleotide pyrophosphatase/phosphodiesterase (NPPase) family are expressed at opposite surfaces in polarized epithelial cells. We investigated the targeting signal of NPP1, which is exclusively expressed at the basolateral surface. Full-length NPP1 and different constructs and mutants were transfected into the polarized MDCK cell line. Expression of the proteins was analyzed by confocal microscopy and surface biotinylation. The basolateral signal of NPP1 was identified as a di-leucine motif located in the cytoplasmic tail. Mutation of either or both leucines largely redirected NPP1 to the apical surface. Furthermore, addition of the conserved sequence AAASLLAP redirected the apical nucleotide pyrophosphatase/phosphodiesterase NPP3 to the basolateral surface. Full-length NPP1 was not significantly internalized. However, when the cytoplasmic tail was deleted upstream the di-leucine motif or when the six upstream flanking amino acids were deleted, the protein was mainly found intracellularly. Endocytosis experiments indicated that these mutants were endocytosed from the basolateral surface. These results identify the basolateral signal of NPP1 as a short sequence including a di-leucine motif that is dominant over apical determinants and point to the importance of surrounding amino acids in determining whether the signal will function as a basolateral signal only or as an endocytotic signal as well.

Identification of a basolateral sorting signal for the M3 muscarinic acetylcholine receptor in Madin-Darby canine kidney cells

Muscarinic acetylcholine receptors (mAChRs) can be differentially localized in polarized cells. To identify potential sorting signals that mediate mAChR targeting, we examined the sorting of mAChRs in Madin-Darby canine kidney cells, a widely used model system. Expression of FLAG-tagged mAChRs in polarized Madin-Darby canine kidney cells demonstrated that the M(2) subtype is sorted apically, whereas M(3) is targeted basolaterally. Expression of M(2)/M(3) receptor chimeras revealed that a 21-residue sequence, Ser(271)-Ser(291), from the M(3) third intracellular loop contains a basolateral sorting signal. Substitution of sequences containing the M(3) sorting signal into the homologous regions of M(2) was sufficient to confer basolateral localization to this apical receptor. Sequences containing the M(3) sorting signal also conferred basolateral targeting to M(2) when added to either the third intracellular loop or the C-terminal cytoplasmic tail. Furthermore, addition of a sequence containing the M(3) basolateral sorting signal to the cytoplasmic tail of the interleukin-2 receptor alpha-chain caused significant basolateral targeting of this heterologous apical protein. The results indicate that the M(3) basolateral sorting signal is dominant over apical signals in M(2) and acts in a position-independent manner. The M(3) sorting signal represents a novel basolateral targeting motif for G protein-coupled receptors.

The rat liver Na(+)/bile acid cotransporter. Importance of the cytoplasmic tail to function and plasma membrane targeting

To understand the potential functions of the cytoplasmic tail of Na(+)/taurocholate cotransporter (Ntcp) and to determine the basolateral sorting mechanisms for this transporter, green fluorescent protein-fused wild type and mutant rat Ntcps were constructed and the transport properties and cellular localization were assessed in transfected COS 7 and Madin-Darby canine kidney (MDCK) cells. Truncation of the 56-amino acid cytoplasmic tail demonstrates that the cytoplasmic tail of rat Ntcp is involved membrane delivery of this protein in nonpolarized and polarized cells and removal of the tail does not affect the bile acid transport function of Ntcp. Using site-directed mutagenesis, two tyrosine residues, Tyr-321 and Tyr-307, in the cytoplasmic tail of Ntcp have been identified as important for the basolateral sorting of rat Ntcp in transfected MDCK cells. Tyr-321 appears to be the major basolateral-sorting determinant, and Tyr-307 acts as a supporting determinant to ensure delivery of the transporter to the basolateral surface, especially at high levels of protein expression. When the two Tyr-based basolateral sorting motifs have been removed, the N-linked carbohydrate groups direct the tyrosine to alanine mutants to the apical surface of transfected MDCK cells. The major basolateral sorting determinant Tyr-321 is within a novel beta-turn unfavorable tetrapeptide Y(321)KAA, which has not been found in any naturally occurring basolateral sorting motifs. Two-dimensional NMR spectroscopy of a 24-mer peptide corresponding to the sequence from Tyr-307 to Thr-330 on the cytoplasmic tail of Ntcp confirms that both the Tyr-321 and Tyr-307 regions do not adopt any turn structure. Since the major motif YKAA contains a beta-turn unfavorable structure, the Ntcp basolateral sorting may not be related to the clathrin-adaptor complex pathway, as is the case for many basolateral proteins.

Studying heterotrimeric G-protein-linked signal transduction using replication-deficient adenoviruses

Plasma membrane-spanning G-protein-linked receptors transduce approximately 60% of all extracellular stimuli in higher animals. Many G-protein-linked receptor pathways are yet to be elucidated, with the receptor, G-protein or effector system as yet unidentified. In addition, many fundamental issues pertaining to G-protein signalling remain unresolved, such as the factors governing the specificity of G-protein receptor coupling and the control of signal amplitude in response to G-protein activation. In order to address some of these issues, the use of replication-deficient adenoviruses as gene transfer vectors for investigations of G-protein signalling has been developed, facilitating dissection of G-protein-linked signal transduction pathways in an extensive range of cultured cells, as well as in vivo. The present review focuses on the versatility and utility of adenoviruses for the investigation of signalling by heterotrimeric G-proteins and explores some of the recent advances in adenoviral technology as they relate to the study of signal transduction.

New perspectives on mechanisms involved in generating epithelial cell polarity

Polarized epithelial cells form barriers that separate biological compartments and regulate homeostasis by controlling ion and solute transport between those compartments. Receptors, ion transporters and channels, signal transduction proteins, and cytoskeletal proteins are organized into functionally and structurally distinct domains of the cell surface, termed apical and basolateral, that face these different compartments. This review is about mechanisms involved in the establishment and maintenance of cell polarity. Previous reports and reviews have adopted a Golgi-centric view of how epithelial cell polarity is established, in which the sorting of apical and basolateral membrane proteins in the Golgi complex is a specialized process in polarized cells, and the generation of cell surface polarity is a direct consequence of this process. Here, we argue that events at the cell surface are fundamental to the generation of cell polarity. We propose that the establishment of structural asymmetry in the plasma membrane is the first, critical event, and subsequently, this asymmetry is reinforced and maintained by delivery of proteins that were constitutively sorted in the Golgi. We propose a hierarchy of stages for establishing cell polarity.

Human calcitonin receptor is directly targeted to and retained in the basolateral surface of MDCK cells

The human calcitonin receptor (hCTR) is expressed in polarized cells of the kidney, bone, and nervous system. In the kidney, hCTRs are found in cells of the distal nephron to which blood-borne calcitonin has access only at the basolateral surface. We expressed hCTR subtypes 1 and 2 in Madin-Darby canine kidney (MDCK) cells to establish a cell model useful for delineating the molecular mechanisms underlying hCTR polarity. Selective cell surface incubation demonstrated functional polarity of hCTRs by equilibrium binding or cross-linking of radioiodinated salmon calcitonin (125I-sCT) and cAMP accumulation stimulated by sCT. We estimated that at the steady state there are 40-fold more hCTRs on the basolateral than on the apical side. Domain-selective cell surface biotinylation followed by immunoblotting of streptavidin-agarose-fractionated biotinylated glycoproteins independently confirmed the polarized distribution of FLAG epitope-tagged hCTR-2 in the basolateral domain. Confocal microscopy of immunostained receptors revealed that hCTRs are concentrated on a lateral subdomain of the basolateral membrane. Cell surface arrival assay of newly formed receptors demonstrated that direct delivery to the basolateral domain is the mechanism by which hCTRs become polarized. Measurement of receptor turnover on the basolateral surface showed that retention contributes to hCTR distribution at the steady state.

Targeting of G protein-coupled receptors to the basolateral surface of polarized renal epithelial cells involves multiple, non-contiguous structural signals

Truncations and chimeras of the alpha2A-adrenergic receptor (alpha2AAR) were evaluated to identify membrane domains responsible for its direct basolateral targeting in Madin-Darby canine kidney cells. An alpha2AAR truncation, encoding transmembrane (TM) regions 1-5, was first delivered basolaterally, but within minutes appeared apically, and at steady-state was primarily lateral in its immunocytochemical localization. A TM 1-5 truncation with the third intracellular loop revealed more intense lateral localization than for the TM 1-5 structure, consistent with the role of the third intracellular loop in alpha2AAR stabilization. Addition of TM 6-7 of A1 adenosine receptor (A1AdoR) to alpha2AARTM1-5 creates a chimera, alpha2AARTM1-5/A1AdoRTM6-7, which was first delivered apically, resulting either from loss of alpha2AAR sorting information in TM 6-7 or acquisition of apical trafficking signals within A1AdoRTM6-7. Evidence that alpha2AARTM6-7 imparts basolateral targeting information is revealed by the significant basolateral localization of the A1AdoRTM1-5/alpha2AARTM6-7 and A1AdoRTM1-5/alpha2AARTM6-7+i3 chimeras, in contrast to the dominant apical localization of A1AdoR. These results reveal that sequences within TM 1-5 and within TM 6-7 of the alpha2AAR confer basolateral targeting, providing the first evidence that alpha2AAR basolateral localization is not conferred by a single region but by non-contiguous membrane-embedded or proximal sequences.

Polarized signaling: basolateral receptor localization in epithelial cells by PDZ-containing proteins

Extracellular signals are normally presented to one surface of epithelial cells and to one end of neurons, and so neuronal and epithelial cell signaling is inherently polarized. Another aspect of signaling polarity is that receptors are often asymmetrically distributed on the surfaces of polarized cells. Recent evidence from studies of Caenorhabditis elegans shows that signaling polarity plays an important role in development. The underlying mesoderm induces the overlying ectoderm to form the vulva, and asymmetric distribution of the signal receptor on the basolateral surface of the epithelium is crucial for this signaling. In neurons, the localization of neurotransmitter receptors and ion channels at synapses allows neurons to be exquisitely sensitive to synaptic inputs. Exciting recent reports suggest that receptor localization to neuronal synapses and the basolateral membrane domains of epithelia may involve a common molecular mechanism involving localization by PDZ-containing proteins.

The O-glycosylated stalk domain is required for apical sorting of neurotrophin receptors in polarized MDCK cells

Delivery of newly synthesized membrane-spanning proteins to the apical plasma membrane domain of polarized MDCK epithelial cells is dependent on yet unidentified sorting signals present in the luminal domains of these proteins. In this report we show that structural information for apical sorting of transmembrane neurotrophin receptors (p75(NTR)) is localized to a juxtamembrane region of the extracellular domain that is rich in O-glycosylated serine/threonine residues. An internal deletion of 50 amino acids that removes this stalk domain from p75(NTR) causes the protein to be sorted exclusively of the basolateral plasma membrane. Basolateral sorting stalk-minus p75(NTR) does not occur by default, but requires sequences present in the cytoplasmic domain. The stalk domain is also required for apical secretion of a soluble form of p75(NTR), providing the first demonstration that the same domain can mediate apical sorting of both a membrane-anchored as well as secreted protein. However, the single N-glycan present on p75(NTR) is not required for apical sorting of either transmembrane or secreted forms.

Constitutive activity of native thyrotropin-releasing hormone receptors revealed using a protein kinase C-responsive reporter gene

The native TRH receptor (TRH-R), which is a G protein-coupled receptor that signals via the phosphoinositide transduction pathway, has been assumed to be inactive in the absence of agonist. In contrast, a mutant mouse TRH-R (C335Stop TRH-R) was shown previously to exhibit constitutive (or agonist-independent) signaling activity. In this report, we measured signaling activity of TRH-Rs using a protein kinase C-responsive reporter gene instead of formation of inositol phosphate second messenger molecules. Using this more sensitive system, we show that native mouse TRH-Rs exhibit agonist-independent signaling activity that is directly proportional to the number of receptors expressed in COS-1 cells and is inhibited by negative antagonist benzodiazepine drugs. As expected, the basal signaling activity of native TRH-Rs is lower than C335Stop TRH-Rs. Constitutive activity of native TRH-Rs is not peculiar to COS-1 cells in which receptor density is markedly elevated, because it can also be demonstrated in Madin Darby canine kidney cells stably expressing mouse TRH-Rs and GH4C1 cells endogenously expressing rat TRH-Rs. These findings support the thesis that native TRH-Rs oscillate between active and inactive states. We suggest that demonstration of constitutive activity of native receptors may depend on the sensitivity of the signaling assay employed.