Single molecule effects of osteogenesis imperfecta mutations in tropocollagen protein domains

Abstract

Osteogenesis imperfecta (OI) is a genetic disease characterized by fragile bones, skeletal deformities and, in severe cases, prenatal death that affects more than 1 in 10,000 individuals. Here we show by full atomistic simulation in explicit solvent that OI mutations have a significant influence on the mechanical properties of single tropocollagen molecules, and that the severity of different forms of OI is directly correlated with the reduction of the mechanical stiffness of individual tropocollagen molecules. The reduction of molecular stiffness provides insight into the molecular-scale mechanisms of the disease. The analysis of the molecular mechanisms reveals that physical parameters of side-chain volume and hydropathy index of the mutated residue control the loss of mechanical stiffness of individual tropocollagen molecules. We propose a model that enables us to predict the loss of stiffness based on these physical characteristics of mutations. This finding provides an atomistic-level mechanistic understanding of the role of OI mutations in defining the properties of the basic protein constituents, which could eventually lead to new strategies for diagnosis and treatment the disease. The focus on material properties and their role in genetic diseases is an important, yet so far only little explored, aspect in studying the mechanisms that lead to pathological conditions. The consideration of how material properties change in diseases could lead to a new paradigm that may expand beyond the focus on biochemical readings alone and include a characterization of material properties in diagnosis and treatment, an effort referred to as materiomics.

Figure 1

Tropocollagen molecular model and loading conditions. Panel (a): Molecular geometry of the tropocollagen molecule, indicating the residues that are replaced in the mutation. All tropocollagen-like peptides considered in this study share the same structure, consisting of three identical chains made of gly-pro-hyp triplets: [(GPO)₅-(XPO)-(GPO)₄]₃. The X position of each chain (highlighted in red in the upper part) is one of seven replacing residues related to OI (lower part). Panel (b) depicts the loading condition, subjecting the tropocollagen molecule to tensile deformation. The N-terminus of the molecule is kept fix with a strong position restrain, while the C-terminus is linked to a moving spring. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 2

Young's modulus of the different peptides as a function of the glycine replacement [panel (a)] and as a function of OI severity [panel (b)]. In panel (a), the mechanical properties of each different tropocollagen molecule are depicted as a function of the replacing amino acid residues, which are ordered based on the resulting disease severity, that is, from the physiological glycine (left) to the most severe OI mutation, aspartic acid (right). Panel (b) shows the Young's modulus relative to that of the reference (glycine) as a function of the mutation severity. The severity parameter, introduced for the first time in this work, is meant to quantify the degree of the OI severity due to specific mutations. It is generally accepted that some mutations leads to more severe phenotype. The severity parameter quantify this trend, using all available data on OI mutations, that is more than 200 occurrences catalogued in the Database of Human Collagen Mutations (http://www.le.ac.uk/genetics/collagen). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 3

Measured and predicted value of Young's modulus. The plot shows Young's modulus as measured from atomistic simulation for each tropocollagen molecule (blue) is compared with the mechanical properties calculated using Eq. (1) (purple). The results from the first six peptides (Gly-Val) are use to fit the parameters of Eq. (1), which then predicted the Young's modulus of the remaining two peptides (Glu and Asp). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 4

Influence of the hydropathy index and the side-chain volume of replacing residues on tropocollagen Young's modulus. Most severe cases of OI fall into the dark blue domain, with a hydropathy index of −3.75 and a side-chain volume of ≈100 ų. The most severe mutations feature the smallest distance from the center of the dark blue domain (distances of individual cases to the center of the dark blue domain are indicated by white lines). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 5

Ramachandran plot relative to the mutation positions and to the two adjacent amino acids in each direction. In the reference case [panel (a)] the configuration is close to that of the polyproline II chain [Psi = 150°; Phi = −75°; red circle in panel (a)]. The presence of the mutations alters the coiling of the molecule, shown by the scattering in the Phi and Psi values [panel (b)]. In the case of valine [panel (c)] this scattering is reduced, leading to a coiling similar to the reference case. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]