Engineering Stem Cell Organoids

Abstract

Organoid systems leverage the self-organizing properties of stem cells to create diverse multi-cellular tissue proxies. Most organoid models only represent single or partial components of a tissue, and it is often difficult to control the cell type, organization, and cell-cell/cell-matrix interactions within these systems. Herein, we discuss basic approaches to generate stem cell-based organoids, their advantages and limitations, and how bioengineering strategies can be used to steer the cell composition and their 3D organization within organoids to further enhance their utility in research and therapies.

Figure 1. Model Systems in the Life Sciences

Organisms comprise a hierarchy of systems from the subcellular level to the whole body. In the life sciences, many models have been developed across this organismal hierarchy, to address specific questions across biology and medicine. Each model system possess unique attributes; in general, with increasing scale comes increasing system complexity and challenges in cell culture and the reduced availability of biochemical and quantitative tools, which can limit study insights. Organoid models provide a unique opportunity to incorporate moderate system complexity while still affording many tools for probing structure and function. When compared to tissue explants, organoid systems can mimic similar cell-cell and cell-matrix interactions while maintaining the ability for long-term cultures thanks to maintained signaling cues important for survival.

Figure 2. Organoid Development

The process of organoid formation is similar to organism development originating from a zygote and giving rise to a mature adult organism. This includes precisely controlled differentiation, proliferation, and apoptosis paired with multi-cellular self-organization and patterning, which leads to diverse mature tissues. Organoid systems are derived from ESCs (isolated from a blastocyte), iPSCs (reprogrammed from adult tissues), or ASCs (isolated from mature tissues). The driver stem cell population undergoes a similar process of culture-controlled differentiation and self-organization to give rise to tissue-specific organoids. In the case of uncontrolled differentiation (and especially following transplantation), PSCs will produce teratomas, self-organized multi-tissue tumors. Bioengineering strategies can be used to further control differentiation and organization of organoid systems to be further developed into models more representative of in vivo tissues.

Figure 3. Intestinal Organoids

(A) The intestinal epithelium encompasses a dynamic environment in which multiple cues drive rapid and sustained tissue turnover, which is emulated in part in organoid culture. The spatial arrangement of signaling cues and neighboring cells in vivo facilitates sustained epithelial regeneration from Lgr5+ stem cells. (B) Lgr5+ stem cells are directly responsible for the generation of terminally differentiated epithelial cells including the Paneth cell, which directly provides cues for sustaining the stem cell niche. (C) In organoid cultures, growth factors provided by the stem cell microenvironment and surrounding mesenchyme are supplemented with exogenous factors to sustain Lgr5+ stem cells and the organoid. (D) By applying multiple engineering strategies, it is possible to further emulate the in vivo intestinal epithelium to build improved systems for disease modeling, drug discovery, and screening. Additionally, it is possible to use directed gene editing or small molecule treatment to drive organoids down particular paths of enrichment, thereby enabling the potential for organoid transplantation therapy.

Figure 4. Brain Organoids

Brain organoids follow a modified path of in vivo development, where ESCs develop into the brain structure following specific spatio-temporal cues, ultimately leading to the mature brain structure. In vitro this process begins with either ESCs or iPSCs, and following structural and biochemical cues, multiple organoid lineages can be produced. This includes the self-organized multi-regional organoids or the patterned induction of organoids mimicking specific regions of the brain. A promising set of tools including customized and responsive biomaterials, patterned signaling, and microfluidic networks can be applied to further refine the spatial development of brain organoids.

Figure 5. Bioengineering Approaches to Advance Organoid-Based Research and Therapy

With the right combination and sequence of input signals that the niche relays to the cell, it is possible to obtain the desired output (i.e., the in vitro disease model or tissue-specific organoid) and there are multiple bioengineering tools that can be harnessed to modify these signals and monitor relevant responses. Based on the same principle, following elucidation of organoid biology, there is potential to harness new knowledge to create synthetic niches. The niche can be engineered by combining multiple bioengineering techniques that mimic specific niche components (e.g., biomimetic scaffolds, tunable stiffness, appropriate topography, and spatio-temporally controlled signaling cues). The stem cells used to seed the organoid culture can also be engineered. In addition to exogenous signaling mechanisms, cell activity can be controlled though genome editing and surface modifications including drug delivery nano/microparticles. Using these methodologies, we can gain better control over the organoid to maximize functionality and sustainability in culture and ideally more closely mimic in vivo biology.