CFTR in the Plasma Membrane
A highly prevalent mutation observed in the genetic, autosomal recessive disease of cystic fibrosis (CF) is the misfolding of the delta F508-CFTR mutant protein. This defect is due to a deletion of the amino acid phenylalanine at position 508 in the first nucleotide-binding domain (NBD1) of the CF transmembrane conductance regulator (CFTR) (Cormet-Boyaka et al., 2009). The CFTR is essentially an ion channel known to be a transporter, belonging to the ATP binding cassette family, a family of proteins that couples ATP binding activity and hydrolysis at the NBDs of a transporter like CFTR to, ultimately, allow for domain rearrangements that permit solute movement (Mendoza & Thomas, 2007). The ΔF508 CFTR misfolding mutation causes impairment in the trafficking of CFTR to the plasma membrane (PM), and thus, disrupts CFTR's function as a transporter (Mendoza & Thomas, 2007).
In the normal patient, however, there is no such misfolding. CFTR biogenesis starts with the folding of the CFTR protein in the endoplasmic reticulum (ER). To ensure adequate intramolecular interaction between the two membrane-spanning domains of CFTR, it is necessary for the cytoplasmic membranes to be properly folded. Proper structure conformation results in the formation of an ion channel between CFTR's NBD and regulatory region (Turnbull, Rosser, and Cyr, 2007). ATP binding and hydrolysis are required for the functioning of the ion channel. The immature B- form of CFTR then leaves the ER to be transported to the Golgi apparatus, via coat protein complex II (COPII)-coated vesicles. In the Golgi, B-form CFTR is converted, via glycosylation, to the mature C-form CFTR. The wild-type CFTR (WT-CFTR) is then carried off to the PM, where it assumes its function as a transporter, or a chloride ion channel. At the surface of the PM, or the apical membrane, CFTR levels are regulated by the recycling of CFTR molecules to the PM, or by lysosomal degradation (Turnbull, Rosser, and Cyr, 2007). Once at the cell surface, CFTR pursues its normal and proper function with the vital aid of regulatory molecules, such as kinases, phosphates, and the cytoskeleton (Guggino and Banks-Schlegel, 2004). Non-CF patients have the normally-functioning CFTR protein mainly localized to the apical membrane of ciliate cells that line the gut, which includes organs like the pancreas, and lungs. Chloride ion movement across such epithelia is regulated, so as to not cause CF (Turnbull, Rosser, and Cyr, 2007).
Contrastingly, ΔF508 CFTR is a misfolded protein, which is recognized by the ER and translocated to the cytosol, where it is degraded (Guggino and Banks-Schlegel, 2004; Cormet-Boyaka et al., 2009 ). The ER quality control system recognizes and retains the misfolded proteins. The retained molecules in the ER are in the immature, B-form; they are targeted for degradation in ER-associated degradation (ERAD) pathways. There, they are ubiquinated and retrotranslocated to the cytosol, where they ungergo ubiquitin-proteasome-mediated degradation- a dominant mechanism for the disposal of misfolded proteins in mammalian cells (Cormet-Boyaka et al., 2009). CFTR mutations, which inhibit trans-epithelial ion transport, also lead to the onset of CF-related illnesses, such as lung disease or pancreatic failure (Turnbull, Rosser, and Cyr, 2007). Perturbed ion transport, in addition, leads to a reduced volume of airway surface liquid (ASL), mucus dehydration, decreased mucus transport, and mucus plugging of the airways (Guggino and Banks-Schlegel, 2004). Furthermore, the absence of CFTR from the PM is said to be main cause of CF (Rennolds et al., 2008). However, when the mutant CFTR does reach the PM and forms functional chloride ion channels, it is known to have a shorter half-life in the PM than WT-CFTR (Guggino and Banks-Schlegel, 2004). Furthermore, it exhibits a defect in phosphorylation, which could explain the observed reduction in ΔF508 CFTR functional activity, in comparison to WT-CFTR (Cormet-Boyaka et al., 2009).
Anti-inflammatory agents, pancreatic enzyme supplements, and antibiotics- all of which are characterized as more traditional therapies, attempt to ameliorate the effect of the impaired function of CFTR, evident in CF patients (Guggino and Banks-Schlegel, 2004). Recently, however, drug design has been targeted at correcting the function of defective CFTR. For instance, Cormet-Boyaka et al conducted a recent study that investigated the effects of a truncated (i.e. exogenous) CFTR protein on the function of the chloride ion channel in ΔF508 CFTR mice (Cormet-Boyaka et al., 2009). Results from the study showed that coexpression of small CFTR fragments can specifically rescue the processing of ΔF508 CFTR from the Golgi to the PM and, accordingly, restore the normal function of the chloride transport. Therefore, these findings established that in vivo transcomplementation by a fragment of the exogenous protein can correct the defective function of the same endogenous protein (Cormet-Boyaka et al., 2009). Another study investigated the effects of base treatments on the functioning of ΔF508 CFTR (Namkung, Kim, and Lee, 2005). Since the retrieval of misfolded protein from the Golgi intermediate compartment is a critical step in ER retention and ERAD of the ΔF508 protein, inhibition of retrograde Golgi-to-ER traffic by alkalisation, by NaHCO3, of the intracellular Golgi lumen was tested to see if it permits functional ΔF508 CFTR to reach the apical membrane, or the cell surface, in mice. The study concluded that base treatments do indeed correct the misfolded CFTR loss-of-function type of conformational mutations, like those pertaining to CF (Namkung, Kim, and Lee, 2005). Furthermore, drugs like Miglustat and small molecules like dynamin-inhibiting Dynasore, have also been found to correct a specific function of ΔF508 CFTR's. For instance, Miglustat was examined by Decchecci el al and it has been found to correct ΔF508 CFTR function in human nasal epithelial cells, as well as, CF bronchial cells (Dechecchi et al, 2008). Similarly, Dynasore was found to correct ΔF508 CFTR by inhibiting the internalization, or endocytosis, of both wild-type and mutant CFTR, in order to increase their levels and functioning at the cell surface (Young et al, 2009).
Cormet-Boyaka,E., Hong, J.S., Berdiev, B.K., Fortenberry, J.A., Rennolds, J., Clancy, J.P., Benos, D.J., Boyaka, P.N., Sorscher, E.J. (2009). A truncated CFTR protein rescues endogenous ΔF508 CFTR and corrects chloride transport in mice. The FASEB Journal, 23, 3743-3751
* An exogenous protein fragment of CFTR can correct the function of the chloride ion channel misfolded ΔF508 CFTR. This takes the therapeutic path of correcting, rather than ameliorating the protein function
Dechecchi, M.C.a,⁎, Nicolis, E. a,Norez, C. ,Bezzerri, V., Borgatti, M., Mancini, I., Rizzotti, P., Ribeiro, C.M.P., Gambari, R., Becq, F., Cabrini, G. (2008). Anti-inflammatory effect of miglustat in bronchial epithelial cells. Journal of Cystic Fibrosis, 7, 555-565
* Miglustat is a kind of drug, which again corrects the function of ΔF508 CFTR, by focusing on specific mechanisms and conformational changes leading to the functioning of ΔF508 CFTR at the surface of the PM
Guggino, W.B., Banks-Schlegel, S.P. (2004). Macromolecular interactions and ion transport in cystic fibrosis [Review]. American Journal of Respiratory and Critical Care Medicine, 170, 815-820
* This review article described the many mechanisms in which CFTR is involved, whether it's normal or defective. It also discussed clinical features and effects of the ΔF508 CFTR.
Mendoza, J.L., Thomas, P.J. (2007). Building an understanding of cystic fibrosis on the foundation of ABC transporter structures. J Bioenerg Biomembr, 39, 499-505
* This article perceived CFTR as a transporter, which essentially is the description for its function as an ion channel. This article helped me in understanding the various pathways taken by CFTR in the normal and mutated model of CF.
Namkung, W., Kim, K.H., Lee, M.G. (2005). Base treatment corrects defects due to misfolding of mutant cystic fibrosis transmembrane conductance regulator. Gastroenterology, 129, 1979-1990
* This article focused on base treatment of intracellular compartments as a method of correcting the function of ΔF508 CFTR. This was considered a therapeutic method to enhance CFTR function at the cell surface.
Rennolds, J., Tower, C., Musgrove, L., Fan, L., Maloney, K., Clancy, J., Kirk, K., Sztul, E., Cormet-Boyaka, E. (2008). Cystic fibrosis transmembrane conductance regulator trafficking is mediated by the COPI coat in epithelial cells. Journal of Biological Chemistry, 283, 833-839
* This article although focused on COP1 processes and interaction with CFTR, was used to understand the antero- and retrograde movement of particles including CFTR inside the cell
Turnbull, E.L., Rosser, M.F.N., Cyr, D.M. (2007). The role of UPS in cystic fibrosis. BMC Biochemistry, 8(Suppl 1), 1-10
* This article was used to understand the role of ubiquitin in the degradation process of CFTR.
Young,A., Gentzch, M., Abban, C.Y., Jia, Y., Meneses, P.I., Bridges, R.J., Bradbury, N.A. (2009). Dynasore inhibits removal of wild-type and _F508 cystic fibrosis transmembrane conductance regulator (CFTR) from the plasma membrane. Biochem J., 421, 377-385
Problem Summary Personal Reflection
This assignment was definitely not easy. The process of looking for relevant articles was tedious and, for me, it took almost as much time as writing the paper. Also, just reading the articles and piecing together the relevant and important facts was very difficult, as I had to keep track of which article had which piece of information. On the other hand, the positive thing about this assignment is that I discovered that I learn and memorize better the ideas learned from class when I see them applied in real cases, where all mechanisms follow a logical flow and lead up to a certain outcomes, such as symptoms pertaining to a certain disease. The one thing that really frustrated me though was the annotated references. Personally, I like to focus on one thing for an assignment and that would be the writing part. I think that it's enough that I had to go through the trouble of finding the articles, but then to take the time and further unnecessary pressure to provide a mini-summary or just even a comment, was just redundant and I didn't find it beneficial in any way. I already applied what I learned and synthesized from the articles into the paper- that should be enough. Overall, I think I definitely benefitted from the assignment, in that it taught me more about my way of studying than anything else. Of course, knowing more about cystic fibrosis was interesting as well. I acquired researching and information-synthesis skills. Also, looking at facts from different perspective, for example from the molecular and cellular levels, I thought helped me open my mind to new insights.