Sabine Werner and colleagues, at the Institute of Cell Biology, ETH Zürich, Switzerland, have determined a role for the protein SRF in the skin and found that its expression is markedly decreased in the diseased areas of skin of individuals with psoriasis.
In the study, human skin cells known as keratinocytes were found to express high levels of SRF when healthy but only low levels when psoriatic and when wounded. Mice lacking SRF in keratinocytes during embryonic development died in utero, whereas mice in which SRF was absent in keratinocytes only after birth developed psoriasis-like skin lesions.
Without sufficient SRF, skin becomes diseased
In normal, healthy skin, the cells in the upper layer of skin (epidermal keratinocytes) produce relatively large amounts of SRF. This maintains the skin’s equilibrium. However, if there is an absence of SRF for whatever reason (due to the deletion of the corresponding gene locus “srf”, for example), lab mice develop symptoms reminiscent of psoriasis in humans. Just like psoriasis patients, “srf” knockout mice developed flaky, inflamed patches on their backs, paws and tails. In these areas, the skin cells had divided excessively and were no longer able to develop normally.
The researchers were able to demonstrate that the skeleton of the skin cells is disturbed and destroyed as a direct result of the SRF deficiency. The cells lose contact with their neighbors and the matrix that surrounds them. Consequently, the outer layer of skin loses its compact layering. Fissures form, which enables water to evaporate more easily and the skin dries out more quickly, thus making the intermediate spaces more susceptible to foreign bodies and bacteria. This in turn triggers an inflammatory reaction that induces the skin cells to divide and impairs their differentiation.
Psoriasis patients lack SRF almost entirely
Interestingly, the ETH-Zurich researchers have now discovered that the affected skin in psoriasis patients almost lacks the SRF protein entirely. Together with the results of the mouse tests, this suggests that the loss of SRF is involved in the development of the common skin disease. Like the mice, psoriasis patients develop red, inflamed patches that constantly shed squames – the necrotic skin cells. The patients thus become partially disfigured and suffer from severe itching. Ointments offer some relief, restoring the protective and barrier function of the skin and, in more severe cases, anti-inflammatory substances. The renowned spa treatment in the mineral-rich water of the Dead Sea can also alleviate the effects of the disease, but not cure it.
Trigger still unknown
The researchers still do not know which factor(s) cause(s) the down-regulation of SRF. This is a key issue which Werner’s team is set to address next. The suspects include other proteins, such as cytokines or other transcription factors. Moreover, we still do not know whether and which triggers from the environment contribute to to the down-regulation of SRF.
For a long time, the school of thought was that abnormalities in the immune system trigger psoriasis. Today, however, the research is also looking for defects in the epidermis that could be genetic or even environmental. “In the case of psoriasis, it is not always the immune system that is the trigger, but often a defect in the keratinocytes,” explains Sabine Werner.
- Heidi Koegel, Lukas von Tobel, Matthias Schäfer, Siegfried Alberti, Elisabeth Kremmer, Cornelia Mauch, Daniel Hohl, Xiao-Jing Wang, Hans-Dietmar Beer, Wilhelm Bloch, Alfred Nordheim, Sabine Werner. Loss of serum response factor in keratinocytes results in hyperproliferative skin disease in mice. Journal of Clinical Investigation, 2009; DOI: 10.1172/JCI37771
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