3D Bioprinted Muscle and Tendon Tissues for Drug Development.

Abstract

In our aging societies, there is a huge medical need for treatments of degenerative muscle and tendon diseases, for which there are currently no approved pharmaceutical therapies. Furthermore, also devastating muscle diseases that affect children and younger patients such as Duchenne muscular dystrophy or amyothrophic lateral sclerosis (ALS) lack curative drug treatments. A major hurdle in new drug discovery and development is the nonexistence of normal functional human tissues and diseased tissues for compound screening and testing. Currently, most high-throughput drug screening campaigns are performedwith target-centeredbiochemicalor simplehumancellbased assays if the drug target is known, or with two-dimensional (2D) cell culture phenotypic screens, if the target is unknown. Identified hits and further optimized compounds (leads) are then usually analyzed in low-throughput rodent ex vivo and/or in vivo preclinical animal models for efficacy, potency, specificity and safety. Besides the obvious slowness of this step to assess the pharmacodynamic and physiological effects of new drug candidates the jump from human cell-based systems to animal preclinical models and to human clinical trials is very often too large and not reliable enough to master. Fortunately, recent years have seen an incredible progress in new approaches generating functional 3D human tissues from normal and disease donors. Three-dimensional (3D) human organotypic tissue cultures are generally more predictive for in vivo effects, because they model much more in vivo tissue physiology than conventional 2D cell cultures. Thus, 3D tissue culture has the potential to revolutionize drug discovery and development. The paradigm shift from 2D to 3D cell culture is already showing great benefit in basic research of tissues differentiation and homeostasis as well as in tissue engineering efforts for regenerative medicine applications.[1] Skeletal muscle and tendon tissues basically consist each of one single functional cell type: the multinucleated myotube and the tenocyte. Myotubes are the basic building block of skeletal muscle and are also called myofibers. They are formed by cell fusion of myoblasts. Myofibers are bundled into fascicles and an individual skeletal muscle can be made up of many fascicles surrounded by a connective tissue sheet. At each end, skeletal muscles are seamlessly and tightly connected to tendon through an interface called the myotendinous junction (MTJ). In contrast to skeletal muscle, which is mainly composed of cells with very little extracellularmatrix (ECM), tendon tissuemainly consists of collagen ECM with few embedded threads of aligned cells. Over the last years, different groups have generated 3D skeletal muscle and tendon tissues for basic research and screening purposes.[2–5] Tissues were generated between different kinds of attachment posts allowing mechanical tension and/or stimulation and functional readouts. However, although these systems allowed better in vitro investigations of skeletal muscle and tendon tissue generation and physiology as well as compound screenings they all suffer from a lack of reproducibility, robustness and cost effectiveness for high-throughput analyses and routine compound screenings.