Target Discovery and Validation Reviews and Protocols

208 Vijayaraj, Söhl, and Magin
filament organization, which we cannot refer to because of space limitations.
For these modifications, more detailed literature is recommended (22–24).
1.1.3. Keratin Gene Expression and Regulation
Today, it is commonly accepted that in simple as well as stratified epithelia,
keratin genes are expressed in a “pairwise” manner. An acid type I keratin protein
always attaches to a basic type II keratin to assemble a dimeric structure
(8). Although redundant expression of further type I and type II keratins in different
epithelia readily occurs, expression also takes place in this already mentioned
pairwise manner. But what regulates this distinct pairwise turning-on and
-off of different type I and II keratin genes?
The human K18 keratin gene (~10 kb) (25) serves as a prime example of
what might influence the transcriptional control of a keratin gene. Regulatory
elements (Alu sequences and four putative SP1 binding sites) surrounding the
seven coding exons as well as an AP-1 consensus site in exon 6 interacting with
Ets-1 elements in intron 1 have been demonstrated to be involved in the copy
number-dependent and position-independent transgenic expression of hK18.
Additionally, two differentially methylated CpGs islands have been found in the
first intron enhancer (26–28). Recently, taking this knowledge into consideration,
Oshima and colleagues verified the coincidence of both hK18 transgene
expression and that of different reporter genes in addition to the gene coding for
the Cre-recombinase subcloned together with an internal ribosome entry side
(IRES) immediately downstream of exon 7 (29).
Results from the paradigmatic expression of the human K18 transgene imply
that putative transcription factor binding sites merely identified in the (core)promoter
might be not sufficient to explain the specific and pairwise expression
pattern of keratin genes (30–32).
In internal so-called “simple” epithelia, K8 and K18 form the pair that is predominantly
expressed from embryonic day (ED)2.5 on (32) and that is complemented
by K7, K19, K20, and K23 in most epithelia starting at ED3.5 (32).
Generally, these keratins seem to be able to replace for K8 and K18, provided
that they are expressed in the same cell type (33,34).
In stratified epithelia, the prominent keratins are K5 and K14, which are complemented
by different amounts of K15 and K17 in their basal layer (21,35,36).
The upper parts of the epidermis maintain K5 and K14 as proteins, probably
because of their long half-lives, but when keratinocytes start to differentiate they
express highly abundant K1 and K10, accompanied by K1b, K2e, and K9 present
in upper spinous and granular keratinocytes, particularly exposed to stress
(37,38). Under conditions of proliferation or differentiation such as keratinocyte
migration and wound healing, however, the so-called reinforcement keratins
K6a, K6b, K6e, and K6hf as well as K16 and K17 often replace either transiently