Altération Des Fibres élastiques Dermiques au Cours Des Dermatoses Calcifiantes : étude Structurelle en Microscopie Multiphoton

The variety of pathologies accompanied by mineral deposits in tissues is large (cancers, infectious processes, environmental diseases) (1). In dermatology, several pathologies are associated with calcium mineral deposits, such as calciphylaxis and pseudoxanthoma elasticum (PXE). To date, few studies have been carried out on the chemical characteristics of cutaneous mineral deposits, their physiopathological consequences, the occurrence of associated pathologies and their repercussions on human organs.

This has motivated the PhD project of Hester COLBOC, M.D., on the characterization of calcium deposits in various calcifying dermatosis, including calciphylaxis (2) and sarcoidosis (3). This fundamental research project is carried out in the framework of a collaboration between the LCP (Institute of Physico-Chemistry, CNRS, Paris-Saclay University) and AP-HP, in particular in the fields of uro-nephrology (urinary lithiasis) and oncology (4) (calcifications in breast and thyroid cancer). We have shown that skin calcifications consist of carbapatite in calciphylaxis and calcite in sarcoidosis. We have also shown that these deposits are mainly located in the dermal or vessel wall elastic fibres, suggesting a molecular nucleation pathway within these fibers to be explored.

This structural and chemical description is innovative, but does not answer all the questions that arise around these calcium deposits: why do some patients, who present a normal phosphocalcium balance, have massive skin calcifications? Why, on the contrary, do other patients, such as some dialysis patients, present extremely disturbed phosphocalcic balance sheets, but no skin calcification? This paradox raises the possibility of the existence of "nucleation foci" within the elastic dermal and vascular fibers, favouring the precipitation of phosphocalcic deposits in some patients. Some in vitro models have explored these nucleation foci within elastic fibres. (ref Gourgas et al) Quite remarkably, most calcifying dermatosis are preceded by a cutaneous inflammatory state. These skin inflammatory phenomena could also induce the precipitation of calcium deposits. Some authors thus speculate that elastin is digested by metalloproteinase (MMP) in the extracellular matrix, thus creating nucleation foci favouring phosphocalcic deposits. Munavalli et al. showed an increase level of MMP in urinary sample of a patient with calciphylaxis in a context of rapid weight loss (5). They hypothesize that elevated serum MMP levels in this patient could lead to an alteration of the external elastic limit and vascular calcifications. This hypothesis is supported on the other hand by other models of elastic fiber alteration, such as during skin aging (dermatoporosis), in which there is no inflammation and no calcification.

The background hypothesis of our project is that skin inflammation leads to a specific alteration of the elastic fibres, which is conducive to their calcification.

In order to explore this hypothesis, particularly during calciphylaxis and PXE, we propose to use multiphoton microscopy, an imaging technique available at the Laboratoire d'Optique et Biosciences (LOB) at the Ecole Polytechnique. Multiphoton microscopy has been developed in the early 1990s as an alternative to confocal microscopy, to improve in-depth imaging of biological tissues with sub-micrometric resolution. Most importantly, multiphoton microscopy can combine several lodes of contrast in parallel and without any labeling to identify the various elements of a tissue. Second harmonic generation (SHG) signals allow specific imaging of unstained fibrillar collagen with unequalled sensitivity, while two-photon excited fluorescence (2PEF) allows cellular imaging thanks to various intrinsic cellular chromophores as well as elastin imaging in the dermis or in other tissues (see Figure 1) (6).