Clever Stem Cells Self-Assemble Into Mini-Organoids
Explained | Biology | Sarah Moir
A decade ago, Lancaster discovered the first ‘organoid’ upon inspection of her neural stem cells. Now, cell scientists are exploring the diverse possibilities and ethics of this technology, from models of disease and development to their creation and use.
In 2011, postdoc Madeline Lancaster discovered she accidentally cultivated spherical clumps of neuronal cells on petri dishes. These clumps were resemblant of an embryonic brain, featuring with it the cells of developing retina [1]. Predictably, over the past decade stem cell scientists have been excited by the development of these lab grown ‘mini-organs’, or organoids. Almost anything from intestine to kidneys, gut, liver, lungs, and even a brain, are able to (rudimentarily) be generated in miniature from stem cells, informed upon her intended research to generate neural rosettes (a signature of neural progenitors). Evidently, we have learned our cells are pretty talented at directing their own assembly into three-dimensional diverse and hierarchical cellular networks, and surprisingly at the minimal manipulation of human hands [1].
This is not so surprising, however. We all know that our sophisticated body plan develops from a single cell, the fertilised egg. Through this process, cells self-direct their organisation and development using chemical signals to ‘communicate’ with surrounding cells. Remarkable, considering we have a guesstimate of a few trillion cells with around 200 different cell types. However, observation and testing beyond the 14-day stage of embryo development leaves much of organ development understudied. What’s useful about organoids is that scientists can study the development of organs in vitro to avoid the ethical and accessibility caveats associated with embryonic developmental research. I.e. We might study the full or near development of the ‘embryonic brain’ in a lab, although these methods present their own logistical and ethical complications to be discussed later.
Mini Brains as a Model for Neurodevelopment and Disease
Nonetheless, Trujillo (2019) and their lab partners have already established brain organoids that are in resemblance to premature baby brains. This work measured the EEG patterns of organoid brains and showed patterns of synchronised activity in short bursts similar to that of 25-39-week-old infants’ post-conception. While exciting, there is emphasis on the leaps required before these organoids could be considered a ‘real human brain’ [2]. Missing cell types, key brain structures, and connectivity within the structure remain to be developed. Additionally, neurophysiologist Sampsa Vanhalto (who developed an infant EEG database) highlights that EEG resemblance does not necessarily equate to functional resemblance [3]. Therefore, the suitability for neural organoids as a model for neural dynamics is yet to be clearly defined; although, this group suggest the robustness of their small-scale cortical organoid model could be used to study not only neurodevelopment but also neuropsychiatric pathologies like epilepsy [2].
Mini Brains as a Model for SARS-CoV-2 Infection
Organoid technology is not limited to organogenesis and developmental research. In fact, creativity within the stem cell field yields many possibilities including personalised patient drug testing, transplant therapies, a model for disease, and biotechnologies. One relevant example is how we are using organoids to learn about the SARS-CoV-2 virus. You may have heard of the neurological effects associated with SARS-CoV-2: dizziness, confusion, or stroke. However, limited accessibility to the brains of living patients limits what we can learn about the virus and how — or if — the virus affects neuronal cells. In a pre-print, Muotri et al. (2020) used human brain organoids to determine that SARS-CoV-2 can infect and kill neural cells and cortical neurons, while also impairing synapse function [4]. Development of an infected brain model also meant the group could test drugs on the model instead of infected patients. They found an FDA approved anti-hep C drug (Sofosbuvir) RNA polymerase blocker — that could also treat coronaviruses — yielded improved neural cell survival in the model [4]. Another group led by Akiko Iwasaki also used brain organoids to determine evidence for infection but by observing their metabolic changes using single-cell RNA sequencing [5]. Using monoclonal antibodies to block the cell surface ACE2 receptor, they detected low viral concentrations in cells [5]. Imagine SARS-CoV-2 is the key that unlocks the door (ACE2), for cell entry. Now imagine the antibody as a large barricade blocking access between the key and the door. By blocking this mechanism and observing reduced infection, we can validate the hypothesis that ACE2 is important for viral entry. Ultimately, mini-brain organoids are a promising and developing model for investigating viral infection in living cells, which was previously largely impossible and only accessible post-mortem.
A Recipe for Organoids
To create an organoid with a variety of cell types organised into a 3D structure, we need to begin with stem cells; cells capable of a variety of fates that become differentiated into their respective roles and location within the organ [6]. But not all stem cells are equal. The ‘steminess’ of a stem cell dictates its capacity to differentiate into a variety of cell types. I.e. blood progenitor cells are limited to producing cell types associated with the blood cell lineage. Embryonic Stem Cells (ESC) are capable of differentiating into any cell-type, thus, deemed ‘pluripotent’ [6]. We need pluripotent cells to produce organoids, but these can be sourced from more ethical sources than ESC. Induced pluripotent stem cells (iPSC) are a popular option, used in aforementioned research by Muotri and Iwasaki [4-5]. This method takes human somatic cells and reprogrammes them back to their undifferentiated state using Yamanaka or OKSM transcription factors [7]. Note that if cells can be sampled from a patient to generate organoids, we can create personalised cell therapies using models identical to the patient’s genetic profile. iPSCs are placed within the appropriate cell medium and scaffold to guide desired differentiation pathways as the cells self-organise. Currently, cell scientists are overcoming a few major hurdles that limit organoid potential. These include the lack of homogeneity between organoid models which is inherent of their self-assembly [8]. Additionally, lack of vessel networks prevents nutrient supply and waste removal that reduces organoid lifespan and subsequent accessibility [8]. Thus, organoids lack the complete functional repertoire of their respective organ. They lack in the morphological and cell-type complexity that is a feature of multi-organ systems, rather than in isolation [8].
Just Because We Can... Should We?
Talking about growing mini brains in a dish inevitably stirs interesting and slightly unsettling debate around the ethics of such pursuits. What if this ‘brain’ has intelligence? Is it conscious? Does it feel emotion? We need to consider if ethics should differ between brain, heart, or liver organoids, and what it all means within the context of ‘personhood’, rights, and ownership of the organ [9]. Bioethicist Sarah Chan highlights brain organoids are far behind this level of cognition, emphasising this isn’t something we need to worry about but rather issues of consent, ownership, and cell sources are more pressing [10]. Chan suggests the multitude of organoid research benefits far outweigh the risks, at least in terms of where the technology currently resides, but as technology develops risks should be re-assessed.
The Future of Organoids
Lancaster’s surprising observations a decade ago sparked a frenzy in the stem cell world to explore the possibilities of organoid technology [1]. From the study of neurodevelopment and neurodegenerative disease to testing the consequences of viral infection or therapeutics, organoids grant access to the previously difficult-to-study organs of the human body. Exciting yet somewhat unsettling, we are all captivated to see where this technology leads us.
Sarah Moir - BSc, Biological Sciences
Sarah has just finished her third year of study at UoA as a Biology major. She is staying with Scientific as the head editor this year before she returns for post-grad, and is currently underway on her summer research project investigating umbilical stem cells.