Elastic microenvironment of skin cells: drift and repair View more
The loss of elasticity is a hallmark of systemic aging and is amplified in skin photoaging or in genetic syndromes (e.g. cutis laxa) with consequences on tissue functions and social interactions. Soft tissue elasticity is mainly provided by large elastic fibers composed of supramolecular complexes of elastin and microfibrils. In the skin, the mature elastic fibers are located in the dermal compartment, they are tangential to the surface in the reticular dermis to allow the stretching and recoiling of the skin. Fine elastic fibers, poor in elastin, are observed perpendicular to the surface in the papillary dermis and participate in the structuring of the epidermal ridges and the dermal papillae. The synthesis of elastic fibers (elastogenesis) occurs during the last third of fetal life with a peak in the perinatal period and then slowly decreases until the end of growth. In adulthood, elastogenesis is very weak or leads to disorganized and non-functional fibers, while the initial fibers gradually degrade according to a rate depending on lifestyle.
The lack of functional elastic fibers has an impact on cell behavior and morphology. Dermal fibroblasts from aged individuals, or senescent fibroblasts, show specific changes in their nuclear morphology associated to a decrease of circularity and a reorganization of lamins A/C and B. The same modulations can be observed when culturing healthy cells on too soft substrates, demonstrating that the cells adapt their mechanical properties to the elastic modulus of their environment across a physical continuum from extracellular matrix to chromatin. In keratinocytes, a stiff environment promotes cell proliferation, while a soft substrate induces cell differentiation below a specific threshold.
To date, no treatment exits to restore or repair deficient or degraded elastic fibers. A few pharmacological compounds have been proposed, but their efficacy/side effects balance remains very unfavorable. We developed a synthetic elastin protein (SEP) inspired from the human tropoelastin sequence to provide an elastic molecular prosthesis as a substitution material to replace or strengthen endogenous elastic fibers.
The SEP is easily produced in E. coli and purified by inversed transition cycling. The resulting 55 kDa protein recapitulates the main physicochemical properties of the tropoelastin as thermal responsiveness, secondary structures, and spherical self-assembly. The SEP, or its fragments obtained by enzymatic digestion, do not induce pro-inflammatory cytokines (IL-8, 1b and TNFa) from a monocyte cell line, nor neutrophil recruitment after injection in a relevant in vivo zebrafish model. Considering post-confluent dermal fibroblasts treated or not with the SEP, the SEP colocalized with fibrillar deposits of elastin, while no colocalization is observed in the presence of an inhibitor of lysyl oxidases (crosslinking enzyme). This indicates that the SEP is actively integrated into pre-existing elastic material by the cellular machinery. Reconstructed dermal sheets treated with the SEP show larger amount of fibrillar structures positive for elastin and SEP compared to untreated samples. Moreover, treated sheets show a 4-fold increase of the elastic modulus (E’). Altogether, these results designate the SEP as a good candidate for improving tissue elasticity.