2 2 0
chapter 12
Gastrointestinal Digestion and Absorption
abnormality in CF and the determination of the sweat chlo-
ride concentration is used as the standard diagnostic test.
The cause of elevated Cl- concentration in the sweat is
due to failure of reabsorption in the reabsorptive portion
of the sweat gland. Normally sweat is a hypotonic fluid as
it emerges at the surface of the skin. This occurs because
the secretory and absorptive activities of the sweat gland
are located in two different regions.
The CF gene encodes a protein designated as the cys-
tic fibrosis transmembrane conductance regulator (CFTR)
and is on chromosome 7q31.2. CFTR is a glycosylated
protein containing 1480 amino acid residues. It is a 170-
to 180-kDa protein and the variations in molecular weight
are due to differences in glycosylation. CFTR belongs to
a family of channel proteins known as ATP-binding cas-
sette (ABC) transporters which are essential to virtually
all cells. Abnormalities in channel proteins have both in-
herited and noninherited causes, and associated disorders
have been called
channelopathies.
Other ABC transport
defects are multidrug resistance (mdr) transporter (known
as P-glycoprotein) and sulfonylurea receptors (SUR1 and
SUR2). P-Glycoprotein gene is up-reregulated in its ex-
pression in response to certain chemotherapeutic drugs
(e.g., vinca alkaloids). This causes the cells to become
multidrug resistant, because the drugs are exported out
of the target cells in an ATP-dependent process. The nor-
mal physiological role of P-glycoprotein may reside in its
participation in the transport of phosphatidylcholine and
other phospholipids (Chapter 19). It is also thought that
the P-glycoprotein functions in the detoxification process
of pumping toxins that are xenobiotic out of cells. The
sulfonylurea-receptor transporter is an ATP-sensitive K+
channel found in the
fi
cells of the pancreatic islets of
Langerhans and in cardiac and skeletal muscle. Regulation
of insulin secretion from
ft
cells is governed by sulfony-
lurea receptor proteins (Chapter 22). TAP 1 and TAP2, sub-
units of the major histocompatibility complex, are ABC
transporters involved in peptide transport into the endo-
plasmic reticulum of antigen-presenting cells. Recurrent
respiratory infections and bronchiectasis occur in patients
with TAP2 defects. ABC transporters are also involved
in many other biochemical processes. These include
peroxisome biogenesis (
X-linked adrenoleukodystrophy,
Zelweger’s syndrome,
Chapter 18) cholesterol efflux,
Tangier disease,
Chapter 20), and
all-trans
retinaldehyde
efflux from the inside of the disk to the cytoplasm in retinal
rod cells (
early-onset macular degeneration,
Chapter 28).
The CFTR protein consists of five domains: two
membrane spanning domains (MSDs), two nucleotide-
binding domains
(NBDs),
and a regulatory domain
(Figure 12-14). Both the amino terminus and carboxy ter-
minus are located in the cytoplasm and each of the two
membrane spanning domains contains six transmembrane
F I G U R E 1 2 -1 4
Diagrammatic representation of cystic fibrosis transmembrane regulatory
protein (CFTR). CFTR consists of two transmembrane (TM) domains, two
nucleotide binding domains (NBD) and one regulatory R-domain. The
opening of the chloride channel requires ATP binding to at least one of the
NBDs and phosphorylation of R-domain by cAMP-dependant protein
kinase. There are more than 700 mutations and polymorphisms of CFTR.
NBDs are hot spots for mutation, and three severe CF causing mutations
AF508, G551D, and G1349D are shown. The AF508 mutation is the most
common mutation, occurring in about 70% of CF patients.
segments. The two nucleotide binding domains interact
with ATP. The unique regulatory domain contains several
consensus phosphorylation sites. The phosphorylation of
the R-protein occurs both by cAMP dependent protein ki-
nase and protein kinase C. CFTR protein is located in the
apical cell membrane of most epithelial cells; however, it
is also found in the basolateral cell membranes and pre-
sumably participates in Cl- reabsorption. The basolateral
cell membrane sites are located in sweat duct epithelial
cells and the proximal convoluted tubule and the thick
ascending limbs of Henle’s loop in the kidney.
The understanding of CFTR function in chloride
transport has been aided by insertion of CFTR into ar-
tificial lipid membranes and by recording currents flow-
ing through single membrane channels using patch-clamp
technology. Activation of the channel for Cl- secre-
tion requires phosphorylation of sites located in the
R-domain and binding and/or hydrolysis of ATP at the
nucleotide binding domain. Phosphorylation by protein
kinases and dephosphorylation by phosphatases at the
R-subunit controls Cl- secretion. Thus, if CFTR is defec-
tive, Cl- secretion does not occur. If CFTR is kept contin-
ually active by increasing intracellular cAMP levels, the
Cl- transport is enormously increased with accompanying
fluid loss (Figure 12-15).
More than 700 mutations located throughout the gene
have been identified in the CFTR gene in CF patients. The
mutations include missense, nonsense, and frameshift
mutations. In terms of functional defects, CFTR mutations
have been grouped into four categories:
1. Protein production,
2. Protein processing,
