**1. Introduction**

In the first part of this chapter we will summarize the main clinical aspects of cystic fibrosis as well as the pathophysiology of lung and liver diseases, with particular reference to the role of airway and biliary duct epithelia, where the cystic fibrosis gene is expressed. In the second part we will describe the main features of placenta-derived stem cells and their potential use for the treatment of lung and liver diseases in cystic fibrosis.

#### **1.1. Cystic fibrosis**

Cystic fibrosis (CF) is an autosomal recessive disease of epithelia in the lung, liver, pancreas, small intestine, reproductive organs, sweat glands and other fluid-transporting tissues [1, 2]. In Caucasians the disease affects about 1 in 2500 live births and is the most common eventually lethal genetic disease [3]. The cause of CF is different mutations in the *CFTR* (cystic fibrosis transmembrane conductance regulator) gene, the product of which is a protein expressed in the apical membrane of most epithelia. This membrane protein is a cyclic AMP (cAMP) regulated chloride (Cl- )-channel involved in different regulatory processes of the cell, *e.g.* both transcellular and paracellular ion and water transport [1, 4].

Chronic progressive obstructive lung disease and pancreatic insufficiency are the main clinical symptoms of CF, where pulmonary disease is the major cause (95%) of morbidity and mortality [5]. However, liver disease is also increasing as the life span of these individuals becomes longer.

© 2013 Carbone et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The succession of events leading from the defective CFTR to the clinical symptoms is not completely understood. However, it is obvious that the abnormal ion transport with hyper‐ absorption of Na+ and impaired Cl and HCO3 secretion in airway epithelial cells and cholangiocytes leads to a disturbance of the fluid lining the airways and the bile ducts [6-10].

#### *1.1.1. The CFTR gene*

The *CFTR* gene was identified in 1989 and this has sharply accelerated the research on CF. The gene, which is situated on the long arm of human chromosome 7 (7q31.2), spans approximately 250 kilobases (kb) of nucleotide sequences together with its promoter and regulatory regions. The 27 exons form a 6.5 kb long coding sequence, which is capable of encoding a protein of 1480 amino acids [11].

The *CFTR* gene product is not limited to the cells of epithelial origin. In fact*, CFTR* mRNA transcripts and/or CFTR protein have been demonstrated in lung fibroblasts, blood cells, hematopoietic stem/progenitor stem cells (HSPC), alveolar macrophages, and smooth muscle cells [12-14]. In addition to its typical plasma membrane location, CFTR was also found in membranous organelles such as lysosomes of alveolar macrophages [15] and in both apical and basolateral membrane of the sweat duct [16].

Although over 1,900 different mutations in the *CFTR* gene are known (Cystic Fibrosis Mutation Database, http://www.genet.sickkids.on.ca/cftr/Home.html), approximately 66% of the patients worldwide carry the F508del mutation (a deletion of three nucleotides that results in a loss of phenylalanine at position 508 of the CFTR protein) with somewhat higher prevalence in Western Europe and USA [17]. This type of mutation causes an incorrectly assembled CFTR protein resulting in endoplasmatic reticulum (ER) retention and degradation of the protein [18] as well as defective regulation [19]. Patients homozygous for F508del usually have more pronounced clinical manifestations compared to heterozygotes and genotypes without F508del [20-22] although these differences are highly variable [23].

#### *1.1.2. The CFTR protein*

Based on the amino-acid sequence and its structure, CFTR is identified as a member of the superfamily of ATP-binding cassette (ABC) transporters. However, among the thousands of ABC family members, only CFTR is an ion channel [24, 25]. ABC transporters are ubiquitous in the entire animal kingdom due to their role in coupling transport to ATP hydrolysis. They also are involved in many genetic diseases [26]. Like other ABC transporters CFTR contains two membrane-spanning domains (MSDs), two hydrophilic nucleotide-binding domains (NBDs) located at the cytoplasmic site of the protein, and, as a unique feature among ABC transporters, a regulatory domain (R domain) located between NBD1 and MSD2. The R domain contains several consensus phosphorylation sites for protein kinases A (PKA) and C (PKC) [27]. The opening and closing of the CFTR Cl channel is tightly controlled by the balance of kinase and phosphatase activity within the cell and by cellular ATP levels [28]. Activation of PKA causes the phosphorylation of multiple serine residues within the R domain leading to conformational changes in this domain [29] relieving its inhibitory functions on CFTR channel gating [30]. Once the R domain is phosphorylated, channel opening requires binding of cytosolic ATP. NBD1-NBD2 dimerization induces channel opening, whereas ATP hydrol‐ ysis at the NBD2 induces dimer disruption and channel closure [24, 31, 32]. Finally, channel activity is terminated by protein phosphatases that dephosphorylate the R domain and return CFTR to its quiescent state [28].

Besides its cAMP-induced chloride channel function, CFTR is reported to have important regulatory functions on other ion channels and transporters. Below some of these interactions are presented: HCO3 is conducted from the cell into the lumen [33] through reciprocal regulatory interactions between CFTR and the SLC26 chloride/bicarbonate exchanger [34] and loss of this mechanism contributes to both airway and pancreatic-duct disease in CF [33, 35]. CFTR enhances ATP release by a separate channel [36], not yet identified [37]. This CFTR mediated release, although debated, is thought to be stimulated by hypotonic challenge to strengthen autocrine control of cell volume regulation through a purinergic receptor-depend‐ ent signalling mechanism [36, 37]. Furthermore, transport of glutathione is directly mediated by CFTR, which is essential for control of oxidative stress [38]. The interaction between CFTR and epithelial sodium channel (ENaC) is of crucial importance for lung disease development (see below). CFTR downregulates calcium-activated chloride channels (CaCC) [39], and stimulates outwardly rectifying chloride channels [40]. Other channels regulated are the volume-regulated anion channel [41] and ATP-sensitive KATP channels such as inwardly rectifying outer medullary potassium channels [42].

Regulatory sites on NBD1 interact with several of the above processes. For example, NBD1 contains a CFTR-specific regulatory site that downregulates ENaC. This regulatory site is also needed for CFTR-mediated interactions with other transporting membrane proteins [1, 43]. Several studies also have identified a short stretch of amino acids (-DTRL-) at the COOH terminal end, forming a PDZ binding domain [1, 44]. This PDZ binding domain interacts with different PDZ-domain-containing proteins, anchors CFTR to the cytoskeleton and stimulates the channel activities through downstream signaling elements [44, 45].
