Excess filaggrin in keratinocytes is removed by extracellular vesicles to prevent premature death and this mechanism can be hijacked by Staphylococcus aureus in a TLR2-dependent fashion

Abstract

Filaggrin (FLG) protein is indispensable for multiple aspects of the epidermal barrier function but its accumulation in a monomeric filaggrin form may initiate premature keratinocytes death; it is unclear how filaggrin levels are controlled before the formation of storing keratohyalin granules. Here we show that keratinocyte-secreted small extracellular vesicles (sEVs) may contain filaggrin-related cargo providing a route of eliminating excess filaggrin from keratinocytes. Filaggrin-containing sEVs are found in plasma in both healthy individuals and atopic dermatitis patients. Staphylococcus aureus (S. aureus) enhances packaging and secretion of filaggrin-relevant products within the sEVs for enhanced export via a TLR2-mediated mechanism which is also linked to the ubiquitination process. This filaggrin removal system, preventing premature keratinocyte death and epidermal barrier dysfunction, is exploited by S. aureus which promotes filaggrin elimination from the skin that could help safeguard bacterial growth.

In silico analysis of the profilaggrin sequence indicates alterations in the stability, degradation route, and intracellular protein fate in filaggrin null mutation carriers

“…It is possible that in this scenario, the remaining routes (proteasomal degradation, vesicular export) would be enhanced to control the profilaggrin/filaggrin levels in the cytosol. Indeed, we have previously made a very unexpected observation of the increased amount of the profilaggrin/filaggrin protein cargo in the sEVs fractions isolated from the blood of AD patients compared to the healthy controls ( Gutowska-Owsiak, 2022 ), and this would perfectly align with such a scenario. Thus, it is conceivable that keratinocytes could expel the excess profilaggrin/filaggrin-related products to prevent the toxic consequences in the event of a diminished capacity to generate KHGs ( Sybert et al, 1985 ).…”

“…We have previously described two separate mechanisms that control intracellular filaggrin levels during keratinocyte differentiation; the actin-based Akt-1/HspB1-dependent mechanism governing profilaggrin sequestration in KHGs ( Gutowska-Owsiak et al, 2018 ) as well as the small extracellular vesicle (sEV)-mediated removal of excess free-floating profilaggrin/filaggrin from the cytosol ( Gutowska-Owsiak, 2022 ). As for the latter, we also determined that Staphylococcus aureus , a skin pathogen with high prevalence and significant contribution to the pathology in AD patients, enhances filaggrin loading into the sEV cargo, facilitating its removal from the skin.…”

“…We believe that this discrepancy results from the use of different models. Specifically, we used cells growing in monolayers, where many cells present with low, but detectable profilaggrin expression in relatively undifferentiated cells ( Gutowska-Owsiak, 2022 ). This is where the greatest control over free profilaggrin must be exerted and where it could undergo the UPS-mediated turnover.…”

Filaggrin insufficiency renders keratinocyte-derived small extracellular vesicles capable of affecting CD1a-mediated T cell responses and promoting allergic inflammation

“…Small extracellular vesicles (sEVs), enriched in exosomes, are secreted organelles falling within the 30-150nm size range, released by all nucleated cells, including keratinocytes [18][19][20][21][22][23] . Due to the unique biogenesis pathway, exosome-enriched sEVs acquire distinct characteristics enabling them to penetrate between cells and enter the systemic circulation without damage.…”