Segmented ring-mesh model of glycosaminoglycan chains based on the 3D analysis of normal individual and Musculocontractural Ehlers-Danlos syndrome skin using scanning transmission electron microscopy.

Collagen fibrils in the dermis are bundled by glycosaminoglycan (GAG) chains of decorin, which contribute to its strength. The three-dimensional structure of collagen fibrils and GAG chains has been discussed on the basis of observations and experiments. This study uses scanning transmission electron microscope (STEM) tomography with high Z-axis resolution to analyze the three-dimensional structure of GAG chains in the dermis from a healthy individual and a patient with Musculocontractural Ehlers-Danlos syndrome caused by pathogenic variants in CHST14 (mcEDS-CHST14). This observation revealed that the dermis from a healthy individual featured multiple GAG chains that wrapped around collagen fibrils and formed incomplete ring structures. However, in the dermis from a patient with mcEDS-CHST14, GAG chains were linear and did not form rings. Based on the relationship between collagen fibrils and GAG chains, we suggest the three-dimensional structure of normal GAG chains in a new model named the "segmented ring-mesh model." The interactions between collagen fibrils and GAG chains in this model also apply to the dermis of mcEDS-CHST14 patients, in which the GAG chain composition changes to become CS-rich and more linear. This change leads to an increased inter-fibrillar space, which inhibits the dense packing of collagen fibrils. These findings suggest that this phenomenon contributes to the skin fragility observed in mcEDS-CHST14 patients. Our study suggests the "segmented ring-mesh model" of GAG chains is essential for the dense packing of collagen fibrils in normal dermis. STEM tomography is highly effective in analyzing the three-dimensional structure of collagen fibrils and GAG chains.