We need a better solution for the repair and replacement of joints. Biomaterials, cells and specialised organ culture bioreactors could be the answer.
Orthopaedic Research focuses on finding solutions for regenerating parts of the musculoskeletal apparatus that often fail while ageing as they are constantly loaded. Hereby, it can be mentioned that the joints who are often involved are the knees with problems such as cartilage degeneration and/or meniscus degeneration, but also ruptures of the anterior cruciate ligament. Furthermore, the rotator-cuff joint of the shoulder and the intervertebral discs of the spine are potentially affected with a very poor self-healing capacity. These problems might arise by ageing but also due to post-trauma induced degeneration or genetic predisposition.
In terms of implants the most successful surgery for Orthopaedics remains the hip implant. Here, complication rates as low as 1-2% were reported. However, for all other joints no equally good solutions exist until now. Here, an improved understanding of the mechano-biology of joints and the tissue homeostasis is needed. Current solutions are often to replace the joint with a full implant. However, these natural joints possess six-degree-of-freedom for motion, e.g. the knee and the spine, but are also equipped with proprio-and mechano-receptors that provide important information about the actual position and movement/loading of the joint.
An increasing elderly population demands for an increased supply of joint substitutes or replacement parts. However, these are unmet clinical needs as implants do not often give back full range of motion and proprioception, which is important for certain movements. Thus, for the intervertebral discs of the spine, cartilage and meniscus, currently no good solutions exist in the clinics that provide long-time satisfaction. Here, tissue engineering is expected to unleash new solutions.
The current research
The Tissue Engineering, Orthopaedics & Mechanobiology (TOM) Group of the Department for BioMedical Research (DBMR),at the University of Bern conducts translational research at the intersection of tissue engineering, biology and applied clinical research. The group is experienced in musculoskeletal connective tissues, such as bone, cartilage, ligaments and tendons. The TOM research Group has established core competence how these tissues and/or cells can be targeted and cultured into various 3D systems from biomaterials to organoids.
The primary aims of the TOM group are on the one hand to investigate cell therapy options to regenerate the intervertebral disc (IVD) of the spine and on the other hand, to elucidate bone metabolism and signalling of the bone morphogenic proteins (BMPs) in order to improve patient’s outcome for spinal fusion. The third focus is the understanding of ruptures of the anterior cruciate ligament of the knee and its options to heal this structure. Here, the aim is to find superior solutions for the healing of the anterior cruciate ligament (ACL).
To achieve these goals, the group applies a broad spectrum of methods, such as cell targeting by sorting, 3D hydrogel culture, organ 3D culture and specialised bioreactors that maintain the joint tissue’s mechano-biological requirements. The common focus of the TOM group is to advance in vitro organ culture models, which match closely the human situation and where regenerative therapy strategies, such as novel biomaterials and cells, can be tested in a most authentic in vitro set-up.
Biomaterials might be promising materials to repair the intervertebral disc
In regenerative medicine, and especially in tissue engineering, several open questions still need to be tackled, such as which cells or which materials to take for healing and/or regeneration. It would be great if materials could be designed for the recruitment of cells on site-of-request with growth factors that would either induce cell migration or even lead to differentiation of progenitor cells into the correct cell type. In line of this research, many research teams have tried to develop novel smart biomaterials containing growth factors. Indeed, some of these approaches seem to carry potential for further optimisation.
For the IVD, growth and differentiation factor five and six (i.e., GDF5 and GDF6) are especially promising in this respect, as they were shown to be of central relevance for thriving mesenchymal stromal cells (MSCs) towards an IVD phenotype. In this project, silk scaffolds were produced by transduced Bombyx mori (silkworm) cultures with a cassette that contained the human growth factor GDF6.1.
Recent advancements in the field of engineering, such as electro-spinning and/or 3D printing might lead to new options and products. The group has been driving research in the fields of silk electro-spinning in collaboration with René Rossi and Guisepino Fortunato from the Empa, St. Gallen, Laboratory for Biomimetic Membranes and Textiles. With these collaborations new protocols were established to generate different scaffolds mimicking either the outer part of the disc, i.e. the annulus fibrosus (AF) with a parallel fibre orientation, using a rotating mandrill to collect the silk fibres in a parallel orientation.
To mimic the inner part a static collector to accumulate randomly oriented fibres, which mimics the centre of the IVD, and scaffolds were seeded with nucleopulpocytes. Here, the basic question was more on the production of fibres and their orientation and mechanical properties for IVD repair.
Genetically engineered silk
Furthermore, the group performed in vitro pre-clinical models using genetically engineered silk. This successful Gebert-Rüf-founded project involved the collaboration of a German company that produced the silk (Spintec Engineering, GmbH, Aachen, Germany). The project was a collaboration with specific know-how on producing a GMP (general molecular practise)-compliant silk containing two of very promising growth factors for IVD repair.
The feasibility was then tested using in vitro 3D cell culture experiments seeding adult stem cells isolated from the bone-marrow aspirated from spinal surgery at the University Hospital of Bern. Prof Dr Lorin Benneker, head of Spine of the Insel University Hospital of Bern, from the Department of Traumatology and Orthopedics, was involved and provided valuable clinical tissue that allowed to assess the important question whether human primary IVD cells can be expanded with these materials and whether these cells can be stimulated towards.
The feasibility of IVD repair was tested in a 3d explant organ culture model. The team of Prof Dr Gantenbein could show that adult mesenchymal stromal cells from bone marrow could be differentiated towards more intervertebral-disc-like cells producing extra cellular matrix as expected of the so-called ‘nucleopulpocytes’.
In terms of repair for the centre of the IVD, the so-called nucleus pulposus, hydrogels are very attractive. Here, it was shown that the mechanical properties of such hydrogels are often not optimised for the orthopaedic application. For IVD and cartilage repair the stiffness of these materials should be adopted to better match the one of a native nucleus pulposus of the human lumbar disc. This increased stiffness could be achieved by incorporation of linkers into the hydrogels such as genipin. This has been tested successfully in the established bioreactor using live bovine IVD explants.
‘iPSpine’ – a H2020 project
In a recently started European consortium H2020 project named ‘iPSpine’, Project number #825925, that is led by Prof Dr Marianna Tryfonidou of the University of Utrecht, the TOM group is contributing knowledge how to target a rare progenitor cell population from the centre of the disc that might be useful for regenerative purposes. The presented findings were related to their recently published work alongside supporting partners of the iPSpine consortium that compares cell sorting methods for bovine cells disc progenitors (Tissue Engineering part C Methods, June 2019). According to the TOM group’s findings, Fluorescent Associated Cell Sorting (FACS) was the best isolation method for sorting the tissue specific progenitor cells and led to the most potent and functional cells. The aim is to apply soon this process for therapeutic purposes in humans.
Members of Prof Gantenbein’s lab are working to build on these findings within the iPSpine consortium. Dr Julien Guerrero is working on a Standard Operational Protocol for the isolation of these progenitors cells from human intervertebral discs. These cells can then be further considered and employed for the purposes of the iPSpine consortium. Within this consortium it is foreseen to derive GMP-grade specialised cells (induced pluripotent stem cells; iPSCs).
These steps are all advancing the goals of iPSpine to develop an advanced cell therapy to soon ease the widespread health problems of people suffering from chronic low back pain, the most common cause of job-related disability and missed work.
Prof Dr Benjamin Gantenbein and Dr Julien Guerrero
Department for BioMedical Research (DBMR)
Tissue Engineering for Orthopaedics & Mechanobiology (TOM)
Department of Orthopaedics
Insel University Hospital
University of Bern
+41 31 632 88 15