Aging-associated modifications of collagen affect its degradation by matrix metalloproteinases

Highlights

• Aging-associated modifications of collagen such as mineralization and AGE-modification reduce susceptibility toward MMPs.

• Removal of collagen-associated glycosaminoglycans only affect collagenolysis by cathepsin K but not by MMPs.

• Only cathepsin K but not non-collagenolytic cathepsins can finalize MMP-initiated collagen degradation.

• Mechanical stability of aged collagens is reduced after MMP exposure.

• Aging-associated modifications of collagen likely contribute to the pathology of diseases and general aging processes.

Abstract

The natural aging process and various pathologies correlate with alterations in the composition and the structural and mechanical integrity of the connective tissue. Collagens represent the most abundant matrix proteins and provide for the overall stiffness and resilience of tissues. The structural changes of collagens and their susceptibility to degradation are associated with skin wrinkling, bone and cartilage deterioration, as well as cardiovascular and respiratory malfunctions. Here, matrix metalloproteinases (MMPs) are major contributors to tissue remodeling and collagen degradation. During aging, collagens are modified by mineralization, accumulation of advanced glycation end-products (AGEs), and the depletion of glycosaminoglycans (GAGs), which affect fiber stability and their susceptibility to MMP-mediated degradation. We found a reduced collagenolysis in mineralized and AGE-modified collagen fibers when compared to native fibrillar collagen. GAGs had no effect on MMP-mediated degradation of collagen. In general, MMP digestion led to a reduction in the mechanical strength of native and modified collagen fibers. Successive fiber degradation with MMPs and the cysteine-dependent collagenase, cathepsin K (CatK), resulted in their complete degradation. In contrast, MMP-generated fragments were not or only poorly cleaved by non-collagenolytic cathepsins such as cathepsin V (CatV). In conclusion, our data indicate that aging and disease-associated collagen modifications reduce tissue remodeling by MMPs and decrease the structural and mechanic integrity of collagen fibers, which both may exacerbate extracellular matrix pathology.

Introduction

The main constituents of the extracellular matrix (ECM) are fibrillar collagens, elastin, and glycosaminoglycans (GAGs), which form the structural and mechanical support in connective tissues. Other than cathepsins, matrix metalloproteinases (MMPs) are responsible for the homeostatic tissue repair and degradation of collagens [1]. Previous studies have described the mechanism of collagen degradation by collagenolytic MMPs, which cleave interstitial collagens at a distinct site within their triple-helical region, resulting in 3/4- and 1/4-length fragments [2], [3], [4]. These fragments are thought to readily denature and to be further degraded by the gelatinases, MMP-2, MMP-9, and other proteases [5], [6].

Collagen turnover is crucial during normal physiological processes including wound healing, organogenesis, cell migration, and various inflammatory processes [7], [8]. Overexpression of collagen-degrading MMPs results in an excessive ECM degradation that can lead to a variety of pathologies such as vascular disease, rheumatoid arthritis, osteoarthritis, cancer, and fibrosis [9], [10], [11]. The most abundant type of collagen is type I collagen that is found in bone, tendons, skin, artery walls, cornea, fibrocartilage, and teeth [12]. Proteolytic degradation of type I collagen by proteases including MMPs plays a crucial role in osteoporosis, bone tumors, wound healing, and the destabilization of atherosclerotic plaques. Moreover, aging-associated modifications have a direct impact on matrix homeostasis by affecting collagen degradation, the control of endogenous inhibitors, as well as the activity of embedded tissue growth factors [13], [14]. Aging-associated modifications of collagens include mineralization, cross-linking via advanced glycation end-products (AGEs), and the removal or addition of GAGs. Accumulation of AGEs in ECM and their cross-linking abilities interfere with proteolytic degradation, the mechanical stability of matrix proteins, and contribute to numerous clinical complications [15], [16], [17]. Age-related mineralization of tissues may result in tissue stiffness and impaired remodeling. Tissue calcification and AGE accumulation can cause matrix destruction and functional failure of the connective tissue [18], [19]. The collagen-associated GAG content alters during aging and with the degradation of the collagen-associated proteoglycans [20], [21], [22], [23]. Therefore, it is crucial to understand how these aging processes affect the enzymatic activities of matrix degrading proteases. Recently, we have shown that collagen degradation by cathepsin K (CatK) is reduced by mineralization and AGE-modification and is completely blocked when GAGs are removed [24]. In the present study, we investigated the effects of these aging modifications on the degradability of type I collagen fibrils and fibers by the Zn-dependent collagenases MMP-1, MMP-8, and MMP-13 and their cooperative actions with collagenolytic and non-collagenolytic cathepsins.