Test tube knees

Every step, every turn, and every bend: each movement we make translates into a strain on a joint and the millimeter-thin layer of cartilage within. It’s no surprise when this wear and tear leads to injury and joint disease. Normally cartilage isn’t very good at repairing itself, but what if an injection could teach achy joints how to grow cartilage as good as new? 

Scientists working toward this goal must overcome a major obstacle: cartilage will only form in the lab when it can establish the same 3D contacts that would exist in a real joint. To make the situation even more complicated, living cartilage constantly responds to mechanical stress by altering its physical and biochemical characteristics. Now, scientists are closer than ever before to creating a “test tube knee” that effectively models our joint and is helping to accelerate the discovery of new treatments for cartilage damage.  

The project started at the Genomics Institute of the Novartis Research Foundation (GNF) in La Jolla, California. Kristen Johnson, a senior investigator at GNF, had been following evidence that a type of stem cell in the joint can be persuaded to produce new cartilage cells, or chondrocytes, if given the right stimulus.

“The cells are almost there, they have the machinery to repair, but they’re just not getting the last step right,” says Johnson. “Our vision is to direct how cells develop – to literally just inject a therapy into a joint to make mature cartilage.”  

To find such triggers, Johnson wanted to test 2.5 million molecules on the stem cells, but first she had to figure out how to set up the massive experiment. For years, researchers had been growing cartilage in the lab using a 3D pellet culture where the cells could grow in any direction, but these conditions were challenging to mimic in the two-dimensional, plate-based format needed for an automated screen of millions of molecules. As described in Johnson’s 2012 Science publication, the trick was to position the cells just right in the plate, providing them with the contacts required to grow. The resulting screen led to the discovery of cartilage-inducing molecules like kartogenin.  

The next step was to explore the inner workings of the newly-formed cartilage, but Johnson’s models were limited in how well they represented a real-life scenario. This cartilage wasn’t truly 3D and it never experienced any of the mechanical stresses that would influence its development and potential to repair. The team needed a healthier, more authentic model, and they also needed to test whether the lab-grown tissue would actually behave like real cartilage mechanically.  

Matteo Centola builds models that mimic the structure and properties of cartilage in knee joints. Photo by Marta Sanchez-Oro

Matteo Centola, an engineer in the Musculoskeletal Diseases group at the Novartis Institutes for BioMedical Research in Basel, Switzerland, provided the solution with a spongy, biodegradable support structure that can be seeded with stem cells. The tissue is then grown in a bioreactor, which in combination with the porous support material, allows the nutrients and potential treatments to easily flow throughout the developing cartilage, keeping it healthy and uniform.  

“These models are truly 3D and much closer to what would exist in a real joint,” says Johnson.  

While scientists at GNF are studying the molecular pathways that underlie the cartilage formation, Centola’s models can be used to measure the elasticity and toughness of the tissue. His models will also help the team uncover pathways that only become activated when the cartilage experiences mechanical stress.  

“Our ultimate goal is to mimic walking and running in culture,” says Centola. “We want to see how this lab-grown tissue responds to real-world stress.”

Main image: Scientists track the amount of healthy cartilage in a joint to measure repair before and after a treatment. Image: Kristen Johnson/GNF