Regulation of Keratin Expression During 
Differentiation and Development in Human Skin 
Elaine Fuchs, Ph.D. — Investigator 
Dr. Fuchs is also Professor in the Departments of Molecular Genetics and Cell Biology and of Biochemistry 
and Molecular Biology at the University of Chicago. She received her B.S. degree in chemistry from the 
University of Illinois and her Ph.D. degree in biochemistry from Princeton University, where she studied 
with Charles Gilvarg. Her postdoctoral research was done with Howard Green at the Massachusetts 
Institute of Technology. Dr. Fuchs counts among her honors the R.R. Bensely Award from the American 
Association of Anatomists. 
THE long-range objective of our research is to 
understand the biochemical mechanisms that 
operate and regulate the expression of human 
genes during development and differentiation in 
skin. Present knowledge of the biochemistry of 
human skin and its diseases is limited. Although 
dermatologists have always directed their interest 
toward human skin biology and skin diseases, the 
field of molecular biology has only recently ap- 
proached a level of understanding that permits 
the complex biochemistry of human skin to be 
explored. A major factor facilitating such studies 
is the ability to grow human skin cells in tissue 
culture, including epidermal cells, dermal fibro- 
blasts, melanocytes, and dermal papillae cells. 
The recent development of technology for target- 
ing foreign genes to transgenic animals' skin has 
provided a valuable in vivo model system for the 
study of genetic skin diseases and skin cancers. 
Much of our research on human skin has fo- 
cused on the epidermis, which comprises about 
20 cell layers whose outermost is the skin sur- 
face. Only the inner or basal layer of the epider- 
mis is truly living and undergoes DNA synthesis 
and cell division. Under an influence as yet un- 
identified, a basal cell ceases to divide and makes 
a commitment to differentiate terminally. As the 
cell moves outward to the skin surface, it under- 
goes a variety of morphological and biochemical 
changes. The most pronounced of these is the pro- 
duction of a dense network of keratin filaments, 
which are tough, resilient protein fibers. Many 
skin diseases of the epidermis, including psoria- 
sis and basal and squamous cell carcinomas, in- 
volve a malfunctioning of the differentiative pro- 
cess that is frequently associated with some 
abnormality in the production or organization of 
these filaments. Our investigation is focused on 
the regulation of the expression of keratin pro- 
teins and their genes in human epidermis and in 
epidermal cells differentiating in tissue culture. 
The keratins are a group of 1 0-20 related pro- 
teins (40-70 kDa) that form the 10-nm keratin 
filaments in the cytoplasm of epidermal cells. 
Only a subset (typically 2-6) of keratins are ever 
expressed at one time. As a normal epidermal cell 
differentiates, it changes the subset of keratins it 
makes. In addition, the cell increases its keratin 
synthesis, leaving the fully differentiated epider- 
mal cell with 85 percent of its total protein as 
keratins. In diseases of the skin involving epider- 
mal hyperproliferation, including psoriasis and 
squamous cell carcinomas, a new subset of kera- 
tins not normally made in the epidermis is pro- 
duced, and can be diagnostic. 
A coordinated genetic and biochemical ap- 
proach is necessary to determine the regulation 
of the multiple keratins and to decipher their 
structural and functional roles in the differentiat- 
ing epidermal cell. A number of years ago, we 
showed that expression of different subsets of 
epidermal keratins is due to changes in the syn- 
thesis of different mRNAs. We used DNA recombi- 
nant technology to show that these mRNAs are 
encoded by about 20 different genes of two dis- 
tinct types. Type I encodes small keratins (40-53 
kDa); type II, larger keratins (53-67 kDa). 
Keratins are expressed as specific pairs of type 
I and II proteins. The basic subunit of keratin fila- 
ments is a heterodimer, composed of the two 
types. Approximately 20,000 heterodimers form 
a single 10-nm filament; the assembly process is 
energy independent and does not appear to re- 
quire auxiliary proteins or factors. Using DNA 
sequencing, we determined the amino acid 
sequences for several keratin pairs. The cytoskele- 
tal architecture of keratin filaments may be specif- 
ically tailored to suit the particular structural 
needs of each epidermal cell at various stages of 
differentiation and development. 
To determine the details of the filament assem- 
bly process and to investigate the interactions of 
keratin filaments with other proteins and organ- 
elles, we used deletion and site-directed muta- 
genesis to alter the coding sequences of K5 and 
Kl4, the pair expressed in the living cells of the 
epidermis. We generated substantial quantities of 
keratins for filament assembly studies, using ge- 
netic engineering to overexpress wild type and 
mutant human keratins in bacteria, which do not 
have keratin. We purified these keratins and iso- 
lated and examined the consequences of muta- 
tions and deletions on keratin filament assembly 
in vitro. 
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