For individuals suffering from brittle bone disease, also known as osteogenesis imperfecta (OI), life is fraught with complications. The slightest misstep, a seemingly harmless fall, or even one false move can result in a broken bone. This is because they were born with an inherited genetic defect that makes their bones extremely brittle and often leads to physical deformity.

The root cause of brittle bones in most cases is a mutation in the gene responsible for producing type I collagen, the crucial protein for establishing a hard bone matrix. This mutation prevents the collagen protein from folding correctly, resulting in an unstable bone matrix and brittle bones.

Historically, scientists have had only a rudimentary understanding of how these mutations disrupt bone matrix formation and how to effectively treat these malformations. However, researchers at ETH Zurich’s Institute for Biomechanics have taken a significant step towards answering these questions. Led by Professor Xiao-Hua Qin, in collaboration with fellow ETH Professor Ralph Müller, the team has developed a 3D in vitro model to investigate bone formation in greater detail.

This new bone model utilizes a porous matrix made of a synthetic polymer. In this matrix, soft hydrogel serves as a scaffold where bone-forming cells, or osteoblasts, can settle, multiply, and connect to form a three-dimensional network. The researchers determined that the ideal pore size for this matrix is between 5 and 20 micrometers, allowing cells to thrive while preventing them from escaping.

Drawing inspiration from in vitro models for nerve cells, the researchers equipped their hydrogel with a peptide crosslinker that can be broken down by a matrix metalloproteinase (MMP) enzyme. This addition enables the cells to produce more mature collagen fibers, essential for bone formation.

To ensure proper bone cell growth and networking, the researchers placed the hydrogel with embedded cells onto a chip and channeled a liquid through the pores. This liquid subjected the cells to shearing forces, simulating the mechanical stimulation necessary for bone development.

Their bone model, featuring the biodegradable hydrogel matrix and mechanical stimulation, successfully emulates normal bone development. The osteoblasts reproduce and, in some cases, develop into immature osteocytes, which account for 90% of cells in healthy bones. They also secrete collagen and can mineralize the matrix, closely resembling normal bone development.

Having patented their model, the researchers plan to make it available to potential industry partners. Compared to previous bone formation models, this new in vitro model offers numerous advantages, including appropriate pore sizes for cell maneuverability and the ability to study collagen production directly.

Historically, research on OI has relied heavily on animal models, which are costly and come with numerous constraints. This new in vitro model provides a promising alternative. Zauchner, a doctoral student in Qin’s group, is poised to start the first experiments using cells from a young OI patient at the Children’s Hospital Zurich.

This project is part of the Swiss National Research Programme “Advancing 3R,” which aims to explore how to bring the Replace, Reduce, and Refine (3R) approach to animal experiments. Qin’s group seeks a better understanding of bone formation, development, and degradation processes. Beyond the new OI model, the team is also developing a model for degenerative bone diseases such as osteoporosis, focusing on terminally differentiated bone cells known as osteocytes.

“In the model we’ve just unveiled, we found immature osteocytes as well as osteoblasts,” says Qin. This achievement marks a significant step forward in building an in vitro model for bone development, potentially paving the way for new treatments and better understanding of brittle bone disease and other bone-related conditions.

By Impact Lab