Target Discovery and Validation Reviews and Protocols

214 Vijayaraj, Söhl, and Magin
ulcerative colitis and Crohn’s disease (51,52). Crohn’s disease and ulcerative
colitis, together known as inflammatory bowel disease, are chronic illnesses of
unknown origin. The inflammation within the intestinal tract (within the colon
in ulcerative colitis and anywhere from the mouth to the anus in Crohn’s disease)
leads to some or all of the following clinical symptoms: diarrhea (with or without
blood), abdominal pain, fever, and fatigue.
1.1.5. Phenotypes of Keratin Knockout and Transgenic Mice
1.1.5.1. SIMPLE AND EMBRYONIC EPITHELIA
Ablation of keratins expressed in simple epithelia and during embryogenesis
only partially helped to identify their functional significance. Side effects
because of the strain background apparently influence the phenotypical outcome
of generated knockouts. Keratin K8-deficient mice, initially thought to
die from hepatocyte fragility leading to fetal liver bleeding at ED12.5 in the
C57BL/6 strain (53,54), have been later demonstrated to collapse from placental
malfunctions (55). However, keratin K8 deficiency crossed into FVB/N
strain results in viable offspring featuring colonic hyperplasia, diarrhea, and
colitis (53,54). Mistargeting of apical H + /K + -ATPase-β, anion exchanger AE1/2
and the Na + transporter might lead to the observed phenotype, possibly because
of the net Na + absorption associated with the Cl – secretion in jejunal epithelia
(33,56). The colonic hyperplasia and colitis were later determined as chronic
spontaneous helper T-cell 2 colitis that probably arises from colonic bacterial
infection (57). Either distinct modifier genes or the second type II keratin, K7,
expressed early during development, might compensate for the loss of K8 in
FVB/N K8(–/–) mice (Tables 1 and 2).
Hepatocytes from K8(–/–) mice are more susceptible to mechanical stress and
hepatotoxic drugs and secrete less bile acids and ecto-ATPase, a phenomenon that
is mechanistically still unexplored (58). Concanavalin A (ConA)-induced liver
damage left K8(–/–) [and also K18(–/–)] epithelial cells approx 100 times more
sensitive to TNF-induced cell death and increased both downstream targeted
JNK- and nuclear factor-κB-protein levels. ConA-induced immunoresponse
through activated T-cells was therefore thought to release TNFα, acting on the
TNF receptor (TNFR)2 (59). Moreover, glutathione S-transferase-fusion protein
binding assays suggested an interaction of the N-terminal parts of K8 and K18
with the TNFR2 (59). Primary hepatocytes from K8(–/–) mice being more sensitive
to Fas-mediated apoptosis fall into the same category (60). Although transported
by microtubules, Fas receptor abundance at the cell surface was increased
in K8(–/–) mice, thus implying K8 to contribute to Fas receptor distribution (60).
Concerning other keratins, a direct interaction between the TNFR1-associated
death domain protein (TRADD) and the coil 1a domain of K18 and K14 was