Mutant (delta F508) cystic fibrosis transmembrane conductance regulator Cl- channel is functional when retained in endoplasmic reticulum of mammalian cells

Single-channel kinetics, inactivation, and spatial distribution of inositol trisphosphate (IP3) receptors in Xenopus oocyte nucleus

Single-channel properties of the Xenopus inositol trisphosphate receptor (IP3R) ion channel were examined by patch clamp electrophysiology of the outer nuclear membrane of isolated oocyte nuclei. With 140 mM K+ as the charge carrier (cytoplasmic [IP3] = 10 microM, free [Ca2+] = 200 nM), the IP3R exhibited four and possibly five conductance states. The conductance of the most-frequently observed state M was 113 pS around 0 mV and approximately 300 pS at 60 mV. The channel was frequently observed with high open probability (mean P(o) = 0.4 at 20 mV). Dwell time distribution analysis revealed at least two kinetic states of M with time constants tau < 5 ms and approximately 20 ms; and at least three closed states with tau approximately 1 ms, approximately 10 ms, and >1 s. Higher cytoplasmic potential increased the relative frequency and tau of the longest closed state. A novel "flicker" kinetic mode was observed, in which the channel alternated rapidly between two new conductance states: F1 and F2. The relative occupation probability of the flicker states exhibited voltage dependence described by a Boltzmann distribution corresponding to 1.33 electron charges moving across the entire electric field during F1 to F2 transitions. Channel run-down or inactivation (tau approximately 30 s) was consistently observed in the continuous presence of IP3 and the absence of change in [Ca2+]. Some (approximately 10%) channel disappearances could be reversed by an increase in voltage before irreversible inactivation. A model for voltage-dependent channel gating is proposed in which one mechanism controls channel opening in both the normal and flicker modes, whereas a separate independent mechanism generates flicker activity and voltage-reversible inactivation. Mapping of functional channels indicates that the IP3R tends to aggregate into microscopic (<1 microm) as well as macroscopic (approximately 10 microm) clusters. Ca2+-independent inactivation of IP3R and channel clustering may contribute to complex [Ca2+] signals in cells.

Cystic fibrosis transmembrane conductance regulator-associated ATP and adenosine 3'-phosphate 5'-phosphosulfate channels in endoplasmic reticulum and plasma membranes

Cystic fibrosis (CF) is characterized by abnormal regulation of epithelial ion and fluid transport due to mutations in the CF transmembrane conductance regulator (CFTR), an apical membrane-localized Cl- channel, that usually prevent it from exiting the endoplasmic reticulum. Defective or absent CFTR in the epithelium is believed to disrupt fluid balance in human airways and thereby contribute to chronic respiratory inflammation. Patch-clamp of the plasma membrane and outer membrane of the nuclear envelope of nuclei isolated from CFTR-expressing Chinese hamster ovary cells revealed that CFTR is associated with a regulated ATP channel in both membrane compartments. CFTR expression was also shown to be associated with permeability to another adenine nucleotide, adenosine 3'-phosphate 5'-phosphosulfate, the universal sulfate donor in cells. These results may provide a link between the ion channel function of CFTR and abnormal glycoprotein processing observed in CF.

Expression of bacteriorhodopsin in Sf9 and COS-1 cells

We report studies on the expression of the archaebacterial membrane protein bacteriorhodopsin in Sf9 insect cells and in COS-1 mammalian cells. In both cell systems, the apoprotein bacterio-opsin was expressed at levels of approximately 1 microgram/10(6) cells. Immunofluorescence studies showed that the expressed protein was accumulated in the endoplasmic reticulum. However, upon addition of all-trans retinal to membranes isolated from either Sf9 or COS-1 cells expressing bacterio-opsin, the characteristic bacteriorhodopsin chromophore (lambda max at approximately 560 nm) was rapidly generated. This is in contrast to bacterio-opsin expressed in E. coli, which cannot be functionally reconstituted with retinal unless it is first denatured, and then renatured in vitro. These studies demonstrate that the bacterio-opsin expressed is correctly folded and show that localization of a heterologously expressed membrane protein in the endoplasmic reticulum does not necessarily imply that it is misfolded.

Intracellular protein transport to the thyrocyte plasma membrane: potential implications for thyroid physiology

We present a snapshot of developments in epithelial biology that may prove helpful in understanding cellular aspects of the machinery designed for the synthesis of thyroid hormones on the thyroglobulin precursor. The functional unit of the thyroid gland is the follicle, delimited by a monolayer of thyrocytes. Like the cells of most simple epithelia, thyrocytes exhibit specialization of the cell surface that confronts two different extracellular environments-apical and basolateral, which are separated by tight junctions. Specifically, the basolateral domain faces the interstitium/bloodstream, while the apical domain is in contact with the lumen that is the primary target for newly synthesized thyroglobulin secretion and also serves as a storage depot for previously secreted protein. Thyrocytes use their polarity in several important ways, such as for maintaining basolaterally located iodide uptake and T4 deiodination, as well apically located iodide efflux and iodination machinery. The mechanisms by which this organization is established, fall in large part under the more general cell biological problem of intracellular sorting and trafficking of different proteins en route to the cell surface. Nearly all exportable proteins begin their biological life after synthesis in an intracellular compartment known as the endoplasmic reticulum (ER), upon which different degrees of difficulty may be encountered during nascent polypeptide folding and initial export to the Golgi complex. In these initial stages, ER molecular chaperones can assist in monitoring protein folding and export while themselves remaining as resident proteins of the thyroid ER. After export from the ER, most subsequent sorting for protein delivery to apical or basolateral surfaces of thyrocytes occurs within another specialized intracellular compartment known as the trans-Golgi network. Targeting information encoded in secretory proteins and plasma membrane proteins can be exposed or buried at different stages along the export pathway, which is likely to account for sorting and specific delivery of different newly-synthesized proteins. Defects in either burying or exposing these structural signals, and consequent abnormalities in protein transport, may contribute to different thyroid pathologies.

The biogenesis, traffic, and function of the cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cyclic AMP-activated chloride channel that is encoded by the gene that is defective in cystic fibrosis. This ion channel resides at the luminal surfaces and in endosomes of epithelial cells that line the airways, intestine, and a variety of exocrine glands. In this article we discuss current hypotheses regarding how CFTR functions as a regulated ion channel and how CF mutations lead to disease. We also evaluate the emerging notion that CFTR is a multifunctional protein that is capable of regulating epithelial physiology at several levels, including the modulation of other ion channels and the regulation of intracellular membrane traffic. Elucidating the various functions of CFTR should contribute to our understanding of the pathology in cystic fibrosis, the most common lethal genetic disorder among Caucasians.

A delta F508 mutation in mouse cystic fibrosis transmembrane conductance regulator results in a temperature-sensitive processing defect in vivo

The most prevalent mutation (delta F508) in cystic fibrosis patients inhibits maturation and transfer to the plasma membrane of the mutant cystic fibrosis transmembrane conductance regulator (CFTR). We have analyzed the properties of a delta F508 CFTR mouse model, which we described recently. We show that the mRNA levels of mutant CFTR are normal in all tissues examined. Therefore the reduced mRNA levels reported in two similar models may be related to their intronic transcription units. Maturation of mutant CFTR was greatly reduced in freshly excised oviduct, compared with normal. Accumulation of mutant CFTR antigen in the apical region of jejunum crypt enterocytes was not observed, in contrast to normal mice. In cultured gallbladder epithelial cells from delta F508 mice, CFTR chloride channel activity could be detected at only two percent of the normal frequency. However, in mutant cells that were grown at reduced temperature the channel frequency increased to over sixteen percent of the normal level at that temperature. The biophysical characteristics of the mutant channel were not significantly different from normal. In homozygous delta F508 mice we did not observe a significant effect of genetic background on the level of residual chloride channel activity, as determined by the size of the forskolin response in Ussing chamber experiments. Our data show that like its human homologue, mouse delta F508-CFTR is a temperature sensitive processing mutant. The delta F508 mouse is therefore a valid in vivo model of human delta F508-CFTR. It may help us to elucidate the processing pathways of complex membrane proteins. Moreover, it may facilitate the discovery of new approaches towards therapy of cystic fibrosis.

Turnover of the cystic fibrosis transmembrane conductance regulator (CFTR): slow degradation of wild-type and delta F508 CFTR in surface membrane preparations of immortalized airway epithelial cells

The protein product of the cystic fibrosis (CF) gene, termed the cystic fibrosis transmembrane conductance regulator (CFTR), is known to function as an apical chloride channel at the surface of airway epithelial cells. It has been proposed that CFTR has additional intracellular functions and that there is altered processing of mutant forms. In examining these functions we found a stable form of CFTR with slow turnover in surface membrane preparations from CF and non-CF immortalized airway epithelial cell lines. The methods used to study the turnover of CFTR were pulse/chase experiments utilizing saturation labeling of [35S] Met with chase periods of 5-24 h in the presence of 8 mM Met and cell fractionation techniques. Preparations of morphologically identifiable surface membranes were compared to total cell membrane preparations containing intracellular membranes. Surface membrane CFTR had lower turnover defined by pulse/chase ratios than that of the total cell membrane preparations. Moreover, mutant CFTR was stable in the surface membrane fraction with little degradation even after a 24 h chase, whereas wild-type CFTR had a higher pulse/chase ratio at 24 h. In the presence of 50 microM castanospermine, which is an inhibitor of processing alpha-glucosidases, a more rapid turnover of mutant CFTR was found in the total cell membrane preparation, whereas wild-type CFTR had a lower response. The results are compatible with a pool of CFTR in or near the surface membranes which has an altered turnover in CF and a glycosylation-dependent alteration in the processing of mutant CFTR.

Understanding how cystic fibrosis mutations cause a loss of Cl- channel function

Defective epithelial Cl- secretion is the hallmark of the lethal genetic disease cystic fibrosis (CF). This abnormality is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), a regulated Cl- channel. Since the identification of the single gene encoding CFTR, several hundred disease-causing mutations, associated with a wide variety of clinical phenotypes, have been reported. To understand the relationship between genotype and clinical phenotype, researchers have investigated how mutations in CFTR disrupt its function. Here, we review the recent progress in understanding how CF-associated mutations in CFTR produce defective Cl- channels, and discuss the implications of this knowledge for the development of therapy for CF.

Evidence against defective trans-Golgi acidification in cystic fibrosis

Defective organelle acidification has been proposed as a unifying hypothesis to explain the pleiotropic cellular abnormalities associated with cystic fibrosis. To test whether cystic fibrosis transmembrane conductance regulator (CFTR) participates in trans-Golgi pH regulation, intraluminal trans-Golgi pH was measured in stably transfected Swiss 3T3 fibroblasts (expressing CFTR or DeltaF508-CFTR) and CFTR-expressing and nonexpressing epithelial cells. trans-Golgi pH was measured by ratio-imaging confocal microscopy using a liposome injection procedure to label the lumen of trans-Golgi with fluid phase fluorescein and rhodamine chromophores (Seksek, O., Biwersi, J., and Verkman, A. S.(1995) J. Biol. Chem. 270, 4967-4970). Selective labeling of trans-Golgi was confirmed by colocalization of the delivered fluid phase fluorophores with N-(6-[(7-nitrobenzo-2-oxa-1, 3-diazol-4-yl)amino]caproyl)-sphingosine. In unstimulated fibroblasts in HCO3--free buffer, trans- Golgi pH was 6.25 +/- 0.04 (mean +/- S.E.; n = 80, vector control), 6.30 +/- 0.03 (n = 74, CFTR) and 6.23 +/- 0.06 (n = 60, DeltaF508) (not significant). After stimulation of plasma membrane Cl- conductance by 8-(4-chlorophenylthio)-cAMP (CPT-cAMP), trans-Golgi pH was 6.42 +/- 0.07 (n = 22, control), 6.47 +/- 0.07 (n = 20, CFTR), and 6.35 +/- 0. 07 (n = 22, DeltaF508) (not significant). Similarly, significant pH differences were not found for control versus CFTR-expressing cells in 25 mM HCO3- buffer. In epithelial cells, which do not express CFTR, trans-Golgi pH was (in 25 mM HCO3-) 6.36 +/- 0.04 (n = 33) and 6.34 +/- 0.08 (n = 23, CPT-cAMP) in MDCK cells and 6.25 +/- 0.04 (n = 18) and 6.24 +/- 0.06 (n = 15, CPT-cAMP) in SK-MES-1 cells. In Calu-3 cells, which natively express CFTR, trans-Golgi pH was (in 25 mM HCO3-) 6.19 +/- 0.05 (n = 25) and 6.17 +/- 0.08 (n = 23, CPT-cAMP). To test whether CFTR expression affects pH in the endosomal compartment in HCO3- buffer, pH was measured by ratio imaging in individual endosomes labeled with fluorescein-rhodamine dextrans. Comparing control and CFTR-expressing fibroblasts, average endosome pH (range, 5.40-5.53 after 10 min; 4.79-4.89, 30 min) differed by <0.13 unit, both before and after cAMP stimulation. These results indicate that CFTR expression and activation do not influence pH in the trans-Golgi and endosomal compartments, providing direct evidence against the defective acidification hypothesis.

An endoplasmic reticulum storage disease causing congenital goiter with hypothyroidism

In humans, deficient thyroglobulin (Tg, the thyroid prohormone) is an important cause of congenital hypothyroid goiter; further, homozygous mice expressing two cog/cog alleles (linked to the Tg locus) exhibit the same phenotype. Tg mutations might affect multiple different steps in thyroid hormone synthesis; however, the microscopic and biochemical phenotype tends to involve enlargement of the thyroid ER and accumulation of protein bands of M(r) < 100. To explore further the cell biology of this autosomal recessive illness, we have examined the folding and intracellular transport of newly synthesized Tg in cog/cog thyroid tissue. We find that mutant mice synthesize a full-length Tg, which appears to undergo normal N-linked glycosylation and glucose trimming. Nevertheless, in the mutant, Tg is deficient in the folding that leads to homodimerization, and there is a deficiency in the quantity of intracellular Tg transported to the distal portion of the secretory pathway. Indeed, we find that the underlying disorder in cog/cog mice is a thyroid ER storage disease, in which a temperature-sensitive Tg folding defect, in conjunction with normal ER quality control mechanisms, leads to defective Tg export. In relation to quality control, we find that the physiological response in this illness includes the specific induction of five molecular chaperones in the thyroid ER. Based on the pattern of chaperone binding, different potential roles for individual chaperones are suggested in glycoprotein folding, retention, and degradation in this ER storage disease.

Nuclear ion channel activity is regulated by actin filaments

Actin filaments are novel second messengers involved in ion channel regulation. Because cytoskeletal components interact with the nuclear envelope, the actin cytoskeleton may also control nuclear membrane function. In this report, the patch-clamp technique was applied to isolated nuclei from amphibian A6 epithelial cells to assess the role of actin filaments on nuclear ion channel activity under nucleus-attached or -excised conditions. The most prevalent spontaneous nuclear ion channel species, 76% (n = 46), was cation selective and had a maximal single-channel conductance of approximately 420 pS. Nuclear ion channels also displayed multiple subconductance states, including channel activity of 26 pS that was frequently observed. Nuclear ion channel activity on otherwise quiescent patches was induced by either addition of the actin cytoskeleton disrupter cytochalasin D (CD; 5 micrograms/ml, 60%, 3 of 5 patches) or actin (10-1,000 micrograms/ml) to the bathing solution of nucleus-attached patches (59%, 13 of 22 patches). Actin also induced ion channel activity in quiescent excised inside-out patches from the nuclear envelope (80%, 4 of 5 patches). In contrast, addition of bovine serum albumin (10-1,000 micrograms/ml) to the bathing solution of nucleus-attached patches was without effect on nuclear ion channel activity (5 of 5 patches). The monoclonal antibody MAb414, specific for nuclear pore complex proteins, completely prevented either spontaneous or cytosolic actin-induced nuclear ion channels under nucleus-attached conditions (4 of 4 patches) but not intranuclear actin-induced nuclear ion channels under excised inside-out conditions (3 of 3 patches). In nucleus-attached patches, channel activity was readily activated by addition of the G-actin-binding protein deoxyribonuclease I to nucleus-attached patches (56%, 5 of 9 patches) or further addition of the actin-cross-linker filamin in the presence of actin (57%, 4 of 7 patches). The data indicate that dynamic changes in actin filament organization may represent a novel mechanism to control nuclear function.

Intracellular loop between transmembrane segments IV and V of cystic fibrosis transmembrane conductance regulator is involved in regulation of chloride channel conductance state

The cystic fibrosis transmembrane conductance regulator (CFTR) contains two membrane-spanning domains; each consists of six transmembrane segments joined by three extracellular and two intracellular loops of different length. To examine the role of intracellular loops in CFTR channel function, we studied a deletion mutant of CFTR (delta 19 CFTR) in which 19 amino acids were removed from the intracellular loop joining transmembrane segments IV and V. This mutant protein was expressed in a human embryonic kidney cell line (293 HEK). Fully mature glycosylated CFTR (approximately 170 kDa) was immunoprecipitated from cells transfected with wild-type CFTR cDNA, while cells transfected with the mutant gene expressed only a core-glycosylated form (approximately 140 kDa). The chloride efflux rate (measured by 6-methoxyl-N-(3-sulfopropyl) quinolinium SPQ fluorescence) from cells expressing wild-type CFTR increased 600% in response to forskolin. In contrast, delta 19 CFTR-expressing cells had no significant response to forskolin. Western blotting performed on subcellular membrane fractions showed that delta 19 CFTR was located in the same fractions as delta F508 CFTR, a processing mutant of CFTR. These results suggest that delta 19 CFTR is located in the intracellular membranes, without reaching the cell surface. Upon reconstitution into lipid bilayer membranes, delta 19 CFTR formed a functional Cl- channel with gating properties nearly identical to those of the wild-type CFTR channel. However, delta 19 CFTR channels exhibited frequent transitions to a 6-picosiemens subconductance state, whereas wild-type CFTR channels rarely exist in this subconductance state. These data suggest that the intracellular loop is involved in stabilizing the full conductance state of the CFTR Cl- channel.