Clocking the Pace of Organoid Research

rganoids are 3-dimensional structures that reflect Osome of the complex features of a native tissue, making them “organ like.” Organoids have seen a rapid growth in popularity and are becoming a mainstay in gastrointestinal research, owing in part to the ability of organoids to recapitulate some physiological functions of the tissue from which they are derived. For example, intestinal organoids possess the full complement of epithelial cell types and, unlike most other in vitro systems, provide the cellular heterogeneity of the native intestine. Because organoids represent only part of the native organ (ie, the epithelium) and lack other components such as the immune system, vasculature, and nervous tissue normally present, they are a reductionist system that allows researchers to address targeted questions free of the complex interactions making up the organ. Despite their utility, organoids are notwithout limitations. For example, because they are grown as 3-dimensional structures in a droplet of extracellular matrix, their culture can be cumbersome, and accessing the apical (luminal) surface of the epithelium is difficult and requires time-consuming and labor-intensive approaches, such as microinjection. Three-dimensional culture methods also limit use of organoids in established pipelines, such as high-throughput screening and other platforms designed for cells grown as 2-dimensional monolayers. Two new articles published in this issue of Cellular and Molecular Gastroenterology and Hepatology involve the use of organoids as models of normal physiology and for the development of strategies that overcome limitations inherent to 3-dimensional culture methods. In the first study, Stokes et al investigate a role for the circadian clock during intestinal regeneration. It is well appreciated that the circadian clock plays an important role regulating normal physiology and behavior by controlling hormonal fluctuations, sleep patterns, feeding, etc. In this study, the authors demonstrate that the intestinal epithelium displays a BMAL1-dependent circadian rhythm during normal homeostasis and during regeneration. Interestingly, although the epithelium displays a rhythm at steady state and following injury, it is only during injury that mitosis is also synchronized with the epithelial clock. In this study, organoids were used to examine if these observations were intrinsic to the epithelium, or if they were being influenced by extrinsic factors such as the nervous system. Based on these experiments, the authors were able to conclude that the BMAL1-dependent clock was intrinsic to the epithelium, a finding that would have been difficult to delineate in vivo. Mechanistically, the study demonstrated that during injury or inflammation BMAL1 regulates the timing of cell proliferation by regulating tumor necrosis factor expression, which was shown to have a direct effect on proliferation. Additionally, BMAL1 mutant mice did not respond properly to tumor