Protein kinase complex influences the outer layer of skin to control the eventual shape of the full-grown skeleton and skull

Continuing studies about an important regulatory protein kinase complex called I-kappa-B kinase (or IKK) have now shown that a subunit of IKK – IKK alpha – influences the outer layer of skin in developing mice to control the eventual shape of the full-grown skeleton and skull.

University of California, San Diego (UCSD) School of Medicine researcher Michael Karin, Ph.D., first discovered the IKK complex and its three sub-units, alpha, beta and gamma in 1996. Since then, his group has published extensively on the numerous activities of IKK sub-units, including roles in immune response activation, in formation of the skin’s outer layer, and as an essential protein that prevents a rare disease called Incontinentia Pigmenti.*

Published in the April 8, 2004 issue of the journal Nature**, the new study illustrates how IKK alpha, acting within the ectoderm, or outer layer of developing skin, relays developmental information to the mesoderm, the middle layer of the developing embryo, which gives rise to the skull, skeleton and musculature.

Scientists already know that mesoderm cells, within the first few hours of development, migrate to the location of skeleton and head where they form a mesh into which bone can be laid. However, this is the first study to graphically illustrate the role of the ectoderm in this process.

The researchers found that one mechanism by which IKK alpha exerts a strong effect on development of cranial and skeletal bone is through regulation of a family of growth factors called fibroblast growth factors (FGF).

“By understanding the processes that control development, we have a new path to pursue for potential treatment of mutations affecting skeletal and craniofacial development in humans,” said Karin, who is a UCSD professor of pharmacology and an American Cancer Society Research Professor.

“It may also turn out that controlling FGF over-expression by IKK alpha could be relevant for certain types of skin cancer,” Karin added. “Although cancer usually appears as we grow older, some of the aberrations that lead to cancer development may have occurred early in life during embryogenesis. In this case, overproduction of FGFs, proteins that stimulate the proliferation of skin cells, may make an important contribution to the eventual development of certain types of skin cancer.”

For their experiments, the investigators utilized IKK alpha-deficient mice who are born with abnormalities that included taut and shiny skin, rudimentary limbs, absent or severely truncated tail, and a short and rounded head due to shorter jaw and nasal bones.

When embryos of IKK alpha-deficient mice were given a version of IKK alpha that deposits itself exclusively in the outer layer of the skin, the newborn mice were normal, with wrinkled and loose skin, well-developed limbs and tails, and a fully normal head.

While these results indicated that IKK alpha in the ectoderm was required for mesodermally derived skeletal development, the researchers noted that it raised the question of how the outer layer of skin exerts its control.

Hypothesizing that a possible mechanism may include increased or decreased production of certain proteins known to be involved in shaping the skeleton and skull, the team studied two specific proteins – FGFs and bone morphogenic proteins (BMPs). Using a method called polymerase chain reaction, they looked at tissue samples from normal and IKK alpha-deficient mice. The most striking alterations were highly elevated levels of FGF proteins in the IKK alpha-deficient mice. When the IKK alpha-deficient embryonic mice were given ectodermal IKK alpha, however, the FGF over-expression was reversed.

Karin noted that IKK alpha appears to regulate FGF through its ability to induce the differentiation (change to a more specialized form) of keratinocytes, which are skin cells that make the tough, insoluble protein substance called keratin that is the chief constituent of hair, nails, horns and hooves.

The study was supported by grants from the National Institutes of Health, and Superfund Basic Research Program and CERIES research awards.

In addition to Karin, the study investigators were first author Alok K. Sil, Ph.D., Laboratory of Gene Regulation and Signal Transduction, Department of Pharmacology, UCSD; and Shin Madda, M.D., Ph.D., and Yuji Sano, Ph.D., also members of the Laboratory of Gene Regulation and Signal Transduction, Department of Pharmacology, UCSD; and Dennis R. Roop, Ph.D., Departments of Molecular and Cellular Biology and Dermatology, Baylor College of Medicine, Houston, Texas.