Age-related changes in dermal collagen physical properties in human skin.

Collagen is the major structural protein in the skin. Fragmentation and disorganization of the collagen fibrils are the hallmarks of the aged human skin dermis. These age-related alterations of collagen fibrils impair skin structural integrity and make the tissue microenvironment more prone to skin disorders. As the biological function of collagen lies predominantly in its physical properties, we applied atomic force microscopy (AFM) and nanoindentation to evaluate the physical properties (surface roughness, stiffness, and hardness) of dermal collagen in young (25±5 years, N = 6) and aged (75±6 years, N = 6) healthy sun-protected hip skin. We observed that in the aged dermis, the surface of collagen fibrils was rougher, and fiber bundles were stiffer and harder, compared to young dermal collagen. Mechanistically, the age-related elevation of matrix metalloproteinase-1 (MMP-1) and advanced glycation end products (AGEs) are responsible for rougher and stiffer/harder dermal collagen, respectively. Analyzing the physical properties of dermal collagen as a function of age revealed that alterations of the physical properties of collagen fibrils changed with age (22-89 years, N = 18). We also observed that the reticular dermis is rougher and mechanically stiffer and harder compared to the papillary dermis in human skin. These data extend the current understanding of collagen beyond biological entities to include biophysical properties.

Reduced expression of Collagen 17A1 in naturally aged, photoaged, and UV-irradiated human skin in vivo: Potential links to epidermal aging.

Collagen 17A1 (COL17A1) is a transmembrane structural component of the hemidesmosome that mediate adhesion of keratinocytes to the underlying membrane. Recent work in mouse showed that COL17A1 deficiency leads to premature skin aging. Although the role COL17A1 in skin aging is becoming recognized in mouse models, its connection to human skin natural aging/photoaging/ultraviolet (UV)-irradiated human skin has received little attention. To determine COL17A1 expression in naturally aged and photoaged as well as acutely UV irradiated human skin, skin samples were obtained from: (1) young (N = 10, 26.7±1.3 years) and aged (N = 10, 84.0 ± 1.7 years) sun-protected buttock skin; (2) photoaged extensor forearm and subject matched sun-protected underarm skin (N = 6, 56.0 ± 3.4 years); (3) solar-simulated UV-irradiated buttock skin (N = 6, 51.2 ± 3.6 years). COL17A1 levels were determined by immunohistology and RT-PCR, and the potential role of COL17A1 in epidermal aging was investigated by immunostaining of the marker for interfollicular epidermal stem cells and keratinocytes proliferation. We found that COL17A1 is specifically expressed in interfollicular epidermal stem cell niches, and that significantly reduced in naturally aged, photoaged, and acute UV-irradiated human skin in vivo. COL17A1 is identified as keratinocyte-specific collagen, and UV irradiation significantly downregulates COL17A1 expression in keratinocytes. Reduced expression of COL17A1 is positively correlated with impaired regeneration of keratinocytes and reduced dermal-epidermal junction as well as thin epidermis in aged human skin (epidermal aging). We also confirmed that keratinocyte-specific integrin ß4 (ITGB4), which interacts with COL17A1, is reduced in aged human skin. Mechanistically, we found that matrix metalloproteinases (MMPs) are responsible for UV-mediated COL17A1 degradation in both in vitro keratinocytes and in vivo mouse skin. These data suggest the possible links between reduced expression of COL17A1 and epidermal aging in human skin.