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Description
Psoriasis is an inflammatory skin disease affecting around 2-4 % of the western European population. Patients suffer from red, itchy and scaly plaques and an impaired quality of life. Several treatment options are available, but with higher efficacy often toxicity and/or therapy costs are rising.
The Yersinia outer protein M (YopM) was the first bacterial effector protein discovered to be a cell-penetrating peptide (CPP), meaning it is able to autonomously enter eukaryotic cells independent of the T3 secretion system. It has been demonstrated that recombinant YopM (rYopM) down-regulated the expression of pro-inflammatory cytokines, like TNF-alpha or IL-1beta, after penetration of the host cell. Current theories about the mechanism of action focus on the inhibition of caspase-1 activation by either binding directly to pro-caspase-1 or to the scaffolding protein IQGAP1 and the downstream inhibition of the formation of the NLRP3 inflammasome. Hence, rYopM might be a promising molecule for further investigation in the context of local therapies for inflammatory skin diseases like psoriasis.
Subcutaneous injection of full-length rYopM already showed promising results in ongoing Imiquimod (IMQ)-induced psoriasis-like skin inflammation in mice. To narrow down the functional groups of the protein a truncated version of the protein was generated where only the two N-terminal α-helices (essential for translocation) and the first three leucine-rich repeats (LRRs) are present, called rYopMLRR1-3. As a negative control a construct wherein only the first three LRRs are missing was used, termed rYopMΔNLS1. To elicit psoriasis-like inflammation in mice, the animals were creamed daily with 62.5 mg of Aldara, containing 5 % IMQ on the shaved lower back for 8 consecutive days. Following disease onset the mice were additionally injected with 50 µg rYopM, rYopMLRR1-3, rYopM ΔNLS1 or PBS as control on a daily basis. Throughout the experiment the clinical score of the mice was determined and on the final day of the experiment skin samples were taken for histological examination, RNA analysis and protein quantification. The results clearly showed a reduced clinical score and scratching behavior of the rYopM and rYopMLRR1-3 treated animals, which was reflected in the reduced epidermal thickness as assessed by hematoxylin & eosin staining. Furthermore, keratinocyte proliferation was examined by the means of Ki-67 stainings and was markedly decreased in these groups resulting in lower levels of acanthosis. The quantification of inflammatory protein levels in the skin revealed reduced concentrations of pro-inflammatory and psoriasis-associated cytokines, like IL-17A, TNF-alpha or IL-1beta, as well as neutrophil-attracting chemokines, like CXCL1 and CXCL5. This finding was also confirmed by RNAseq analysis, suggesting a similar mode of action of rYopM and rYopMLRR1-3. Consequently, rYopM and rYopMLRR1-3 treatment significantly impaired neutrophil immigration into affected skin areas. Therefore, the anti-inflammatory capacity of rYopM treatment in psoriasis-like skin inflammation has clearly been shown. In addition, the essential part of rYopM for mediating this effect was narrowed down to the first three leucine-rich-repeats.
To sum this up, YopM is a promising target for further analysis in the context of local treatment of inflammatory skin diseases.
