Protein traffic in polarized epithelial cells: the polymeric immunoglobulin receptor as a model system

Splice-site A choice targets plasma-membrane Ca2+-ATPase isoform 2 to hair bundles

Localization of mechanotransduction in sensory hair cells to hair bundles requires selective targeting of essential proteins to specific locations. Isoform 2 of the plasma-membrane Ca2+-ATPase (PMCA2), required for hearing and balance, is found exclusively in hair bundles. We determined the contribution of splicing at the two major splicing sites (A and C) to hair-cell targeting of PMCA2. When PMCA2 isoforms were immunoprecipitated from purified hair bundles of rat utricle, 2w was the only site A variant detected; moreover, immunocytochemistry for 2w in rat vestibular and cochlear tissues indicated that this splice form was located solely in bundles. To demonstrate the necessity of the 2w sequence, we transfected hair cells with PMCA2 containing different variants at splice sites A and C. Although native hair bundles exclusively use the 2a form at splice-site C, epitope-tagged PMCA2w/a and PMCA2w/b were both concentrated in bundles, indicating that site C is not involved in bundle targeting. In contrast, PMCA2z/a was excluded from bundles and was instead targeted to the basolateral plasma membrane. Bundle-specific targeting of PMCA2w/a tagged with green fluorescent protein (GFP) was diminished, suggesting that GFP interfered with splice-site A. Together, these data demonstrate that PMCA2w/a is the hair-bundle isoform of PMCA in rat hair cells and that 2w targets PMCA2 to bundles. The 2w sequence is thus the first targeting signal identified for a hair-bundle membrane protein; moreover, the striking distribution of inner-ear PMCA isoforms dictated by selective targeting suggests a critical functional role for segregated pathways of Ca2+ transport.

Albumin transcytosis across the epithelium of the lactating mouse mammary gland

Murine milk contains 18 mg ml(-1) serum albumin, a concentration equal to that in the serum of the lactating mouse. We examined cellular transport using in vivo methods in the mouse. At steady state the specific activity of (125)I-albumin injected into the blood stream was equal in plasma and whey, confirming that milk albumin is extra-mammary in origin. Fluorescent albumin crossed the gland from basolateral surface to lumen via cytoplasmic vesicles, but was not transported in the apical to basal direction. Albumin was segregated from transferrin at the basal surface of the epithelial cells and did not colocalize with either caveolin-1 or -2. Vesicular transport was not disrupted by filipin providing additional evidence that, unlike the vascular endothelium, caveoli are not involved. Cytoplasmic albumin was localized to vesicles containing IgA and transport was disrupted by agents that interfere with clathrin-mediated endocytosis. Together, these findings provide evidence that albumin is transported across the mammary epithelium by the same pathway as immunoglobulin. The possibility that the massive transfer of albumin into mouse milk is mediated by fluid phase transport is considered.

Single chain Fv: a ligand in receptor-mediated gene delivery

We have used an anti-human polymeric immunoglobulin receptor (pIgR) single chain Fv (scFv) to deliver reporter genes to epithelial cells in vitro. The scFv was constructed from a monoclonal antibody directed against pIgR and a cysteine residue was added at the carboxyl end to facilitate its conjugation to polylysine (polyK) via the heterobifunctional cross-linker SPDP. ScFv-cys was expressed in Drosophila S2 cells and purified to homogeneity using conventional column chromatography. ScFv-polyK, and polyK as control, were condensed with a DNA expression plasmid containing the luciferase reporter gene driven by the CMV promoter into unimolecular (with respect to DNA) complexes under high salt conditions. Target cells were MDCK cells transfected with human pIgR and repeatedly sorted for high-level receptor expression, with untransfected MDCK cells as control. Receptor-bearing MDCK cells were readily transfected by scFv-cys containing, pIgR directed complexes, and expression could be blocked by addition of excess human secretory component (SC), the extracellular portion of pIgR. In contrast, MDCK cells that did not express pIgR were not transfected. Nontargeted complexes were not effective in transfecting MDCK cells with or without pIgR. Targeted complexes also transfected human tracheal epithelial cells in primary culture, corroborating the pIgR-mediated gene delivery. These data indicate that a scFv directed against human pIgR can direct foreign genes specifically into receptor-bearing cells in vitro. We have expressed and purified a ligand that is efficient and specific in pIgR-mediated gene delivery.

Fc receptor signaling and trafficking: a connection for antigen processing

Most hematopoietic cells express a wide variety of receptors for the Fc portion of immunoglobulins (FcR) belonging to the immunoreceptor family. FcRs are multichain complexes composed of ligand-binding alpha chains, which determine Ig binding, and signal tranduction subunits, bearing a conserved immunoreceptor tyrosine-based activation motif (ITAM). Besides signaling, most Fc gamma Rs also efficiently internalize antigen-antibodies complexes and thus induce efficient processing of antigens into peptides presented by major histocompatibility complex (MHC) class II molecules. Importantly, ITAMs and cytosolic effectors of cell signaling also determine Fc gamma R's endocytic transport and antigen presentation capacities.

In vitro transport of active alpha(1)-antitrypsin to the apical surface of epithelia by targeting the polymeric immunoglobulin receptor

In cystic fibrosis (CF), the intense host inflammatory response to chronic infection largely accounts for the progressive pulmonary disease, and ultimately death. Neutrophils are the prominent inflammatory cells in the lungs of patients with CF, and large amounts of neutrophil elastase (NE) are released during phagocytosis. Besides having direct effects on structural elastin, NE stimulates the release of proinflammatory mediators from the respiratory epithelium and is a potent secretogogue. Therapeutic use of elastase inhibitors in CF has been complicated by difficulties in delivery to the critical site in the airway-the surface of the epithelium. We describe a unique strategy to protect the respiratory epithelial cell surface directly by capitalizing on the nondegradative transcytotic pathway of the polymeric immunoglobulin receptor (pIgR). A recombinant fusion protein was constructed consisting of an antihuman pIgR single-chain Fv (scFv) antibody linked to human alpha(1)-antitrypsin (A1AT), an inhibitor of NE. The recombinant scFv-A1AT fusion protein bound specifically to the pIgR on the basolateral surface of an epithelial cell monolayer, and was transported and released into the apical medium where the A1AT domain was capable of forming an inactivation complex with NE. Thus, A1AT linked to an antihuman pIgR scFv was delivered in receptor-specific fashion from the basolateral to apical surface and was released as an active antiprotease, indicating that it is feasible to deliver therapeutic proteins to the apical surface of epithelia by targeting the pIgR.

Recent advances in intestinal macromolecular drug delivery via receptor-mediated transport pathways

Receptor-mediated transport mechanisms provide a pathway for the trafficking of extracellular macromolecules into (endocytosis) and across (transcytosis) the cell. This comprises the binding of a ligand to a specific cell-surface receptor, clustering of the ligand-receptor complexes in endocytotic vesicles and vesicular sorting. This review focuses on recent advances in cellular and molecular biology pertaining to receptor-mediated endocytosis. A concise overview is presented of current and potential future applications of targeting to RME mechanisms to improve oral macromolecular drug delivery.

Controlling receptor/ligand trafficking: effects of cellular and molecular properties on endosomal sorting

Receptor-mediated endocytosis is the process by which cells internalize ligands that have specifically interacted with cell surface receptors. Within intracellular endosomal compartments, receptor/ligand complexes can be targeted to lysosomes for degradation, recycled back to the plasma membrane, or sorted separately to these destinations. We have developed a mechanistic mathematical model that can account for the spectrum of experimentally observed endosomal sorting outcomes. The central hypothesis of this model is that receptors may be selectively retained by putative endosomal retention components and that this process may be modulated by receptor occupancy. This hypothesis is supported by the recent discovery of an endosomal retention component involved in targeting epidermal growth factor receptors to lysosomes. In this paper, we use the model to predict how changes in key cellular and molecular parameters alter sorting outcomes. This analysis provides guidance for rationally modulating the sorting process in a variety of biomedical applications, either by the manipulation of cellular parameters or the design of ligand properties.

Basolateral localization and transcytosis of gonadotropin and thyrotropin receptors expressed in Madin-Darby canine kidney cells

The thyrotropin (TSH) and follicle-stimulating hormone (FSH) receptors are present mainly on the basolateral cell surface in the thyroid gland and in Sertoli cells, whereas in ovarian and in testicular cells, the luteinizing hormone (LH) receptors are distributed throughout the cell surface. When expressed in Madin-Darby canine kidney (MDCK) cells, all three receptors accumulated at the basolateral cell surface showing that they carry the corresponding targeting signals. The receptors were directly delivered to the basolateral surface of the MDCK cells. A minor fraction of the gonadotropin receptors but not of TSH receptors was secondarily targeted to the apical surface through transcytosis. The mechanisms of basolateral targeting and transcytosis were analyzed using the FSH receptor as a model. Both were insensitive to brefeldin A and pertussis toxin. Gs activation by AlF4- and cholera toxin provoked a marked enhancement of FSH receptor transcytosis. The population of Gs proteins involved in this mechanism was different from that involved in signal transduction since neither FSH nor forskolin mimicked the effects of AlF4- and cholera toxin. Gs activation provoked a similar effect on LH receptor distribution in MDCK cells, whereas it did not modify the compartmentalization of the TSH receptor. Hormone-specific transcytosis was observed in MDCK cells expressing the gonadotropin (FSH and LH) receptors and was increased after cholera toxin administration.

Intracellular routing of wild-type and mutated polymeric immunoglobulin receptor in hippocampal neurons in culture

Certain epithelial cells synthesize the polymeric immunoglobulin receptor (pIgR) to transport immunoglobulins (Igs) A and M into external secretions. In polarized epithelia, newly synthesized receptor is first delivered to the basolateral plasma membrane and is then, after binding the Ig, transcytosed to the apical plasma membrane, where the receptor-ligand complex is released by proteolytic cleavage. In a previous work (Ikonen et al., 1993), we implied the existence of a dendro-axonal transcytotic pathway for the rabbit pIgR expressed in hippocampal neurons via the Semliki Forest Virus (SFV) expression system. By labeling surface-exposed pIgR in live neuronal cells, we now show (a) internalization of the receptor from the dendritic plasma membrane to the dendritic early endosomes, (b) redistribution of the receptor from the dendritic to the axonal domain, (c) inhibition of this movement by brefeldin A (BFA) and (d) stimulation by the activation of protein kinase C (PKC) via phorbol myristate acetate (PMA). In addition, we show that a mutant form of the receptor lacking the epithelial basolateral sorting signal is directly delivered to the axonal domain of hippocampal neurons. Although this mutant is internalized into early endosomes, no transcytosis to the dendrites could be observed. In epithelial Madin-Darby Canine Kidney (MDCK) cells, the mutant receptor could also be internalized into basolaterally derived early endosomes. These results suggest the existence of a dendro-axonal transcytotic pathway in neuronal cells which shares similarities with the basolateral to apical transcytosis in epithelial cells and constitute the basis for the future analysis of its physiological role.

Molecular aspects of tetanus and botulinum neurotoxin poisoning

Clostridial neurotoxins, tetanus and the botulinum toxins A-G, are high molecular weight proteins consisting of a heavy chain which is responsible for the internalisation and a light chain possessing a zinc-dependent proteolytic activity. They exclusively proteolyse either the vesicle membrane protein, synaptobrevin or two integral plasma membrane proteins, SNAP 25 and syntaxin. Together with cytosolic proteins these proteins form the SNARE complex involved in vesicle exocytosis, and their cleavage blocks the latter process. Clostridial neurotoxins have now become powerful tools to investigate the final events occurring during secretion in neuronal, endocrine, and non-neuronal cells. They are applied to dissect the specific interactions of the SNARE protein complex with cytosolic fusogens and other modulators of exocytosis. Whereas exocytosis is not essential for the survival of cells, the organism as a whole will fall victim to a few nanograms since interneuronal and neuromuscular transmission is vital to muscular control, especially in respiration. Although all clostridial neurotoxins by their light chains attack proteins of the SNARE complex, tetanus toxin and the various botulinum toxins differ dramatically in their clinical symptoms. The biological information for this difference resides on the respective heavy chains which select different transport routes carrying the light chain from the place of entrance to the final compartment of action. So far the different transport vesicles used either by the various botulinum neurotoxins or by tetanus toxin are not yet defined. Nevertheless at least one of the botulinum toxins serves as a beneficial drug in the treatment of severe neuromuscular spasms.

Endocytosis and lysosomal targeting of epidermal growth factor receptors are mediated by distinct sequences independent of the tyrosine kinase domain

Ligand-induced internalization of the epidermal growth factor receptor (EGFR) leads to accelerated receptor degradation. Two models have been proposed to explain this. In the first model, induced internalization expands the intracellular pool of receptors, leading to enhanced lysosomal targeting. The second model proposes that activation of intrinsic receptor kinase activity induces inward vesiculation of endosomes, thus interrupting receptor recycling. To test these models, we created EGFR mutants that lack the conserved tyrosine kinase domain, but retain different parts of the distal carboxyl terminus regulatory region. Mutants lacking all distal regulatory sequences underwent slow internalization (0.02 min-1) and turnover (t1/2 approximately 24 h), similar to unoccupied, holo-EGFR. Mutant receptors that lacked the kinase domain, but retained the entire distal regulatory domain, were constitutively internalized and targeted to lysosomes, even in the absence of EGF. The turnover of these receptors (t1/2 approximately 11 h) was similar to that of occupied, kinase-active holo-EGFR (t1/2 approximately 9.5 h). These results show that receptor tyrosine kinase activity is not required for the targeting of EGFR to lysosomes. Receptor mutants which expressed previously identified endocytic sequences underwent rapid internalization. Unexpectedly, enhanced turnover of EGFR mutants required additional sequences located between residues 945 and 991 in the holo-EGFR. Thus, internalization and lysosomal targeting of EGFR are separate processes mediated by distinct sequences. Our results indicate that induced internalization is necessary, but not sufficient, for enhanced EGFR degradation. Instead, down-regulation requires exposure of previously cryptic internalization and lysosomal targeting sequences. Occupied EGFR thus appear to be handled by the endocytic machinery in the same fashion as other constitutively internalized or lysosomally targeted receptors.