Nano and Micro Morphology of Type I Collagen as a Function of Disease and Drug Treatment.

Abstract

Type I collagen, the most abundant protein in the human body, is the primary organic component of bone and forms fibrils with a periodic spacing (D-spacing) along the fibril axis. These collagen fibrils are organized into larger hierarchical structures. Using atomic force microscopy (AFM), individual microfibrils were quantitatively characterized and a minimum of 90 collagen molecules were required before D-spacing was observed. The arrangement of type I collagen fibrils into microstructures in bone tissue altered upon estrogen depletion, a model for postmenopausal osteoporosis, as well as drug treatments for osteoporosis.

Micro-scale structural changes were measured in a rabbit animal model from the following treatment groups: Sham, ovariectomized (OVX), OVX + alendronate (ALN), OVX + cathepsin K inhibitor (CatKI), and OVX + estrogen replacement therapy (ERT). AFM images were collected for 84 animals across all five treatment groups. A fibril-by-fibril analysis was conducted by hand-coding fibrils into Parallel (collagen bundles or collagen sheets) or Oblique microstructures. In cortical bone, OVX increased the proportion of fibrils coded as Oblique and decreased the proportion of fibrils coded as Parallel with statistical significance (p < 0.05). Treatment with ALN or ERT partially prevented this change from occurring and treatment with CatKI completely prevented this change from occurring within error.

An automated imaged level analysis was conducted using an autocorrelation technique and calculating a fibril alignment parameter (FAP) to describe the degree of local collagen fibril alignment in an image. At this level of hierarchical structure, OVX altered trabecular bone and showed no changes to cortical bone. Treatment with CatKI prevented the trabecular microstructural change from occurring but also introduced a change to the cortical bone. Treatment with ALN altered both the cortical and trabecular microstructure.

Sites of collagen interactions with the collagen chaperone PEDF, pigment epithelium derived factor, were imaged in bone tissue by phase imaging during tapping mode AFM. Gold nanoparticles were used as a secondary tag on PEDF to allow detection of PEDF binding in tissue. PEDF bound heterogeneously and was detected between collagen fibrils with a low amount of alignment.