Mini Brains & More: Stem Cells In Neuroscience

Cellular Time Travel: induced Pluripotent Stem Cells

Everyone has once wondered: what if I were able to go back in time? Fueled either by the Back to the Future franchise or an embarrassing mistake, almost everyone has played around with the idea of returning to an earlier state. Imagine being able to go back to your childhood, before decisions that have brought you to where you are now. Now bring that idea to a much smaller scale. Imagine applying that idea to the cells in our body. Early in development, progenitor (stem) cells have the capability to differentiate into different types (neurons, cardiomyocytes, etc). During development, cells become more specialized over time and as an organism matures, they become more limited in the types of cells they can differentiate into [1]. Scientists are now able to perform cellular “time travel” through induced pluripotent stem cells (iPSCs), taking mature cells back to a stage of pluripotency, when they are able to specialize into almost any cell type (with a few exceptions). 

Our bodies are made of millions of cells, each of them specialized for a specific function. All cells develop their specialization in a process called differentiation, occurring early in fetal development [2]. Differentiation causes changes in a cell’s shape and features, to help it adapt to its environment and be considered a mature cell [1]. These changes are driven by transcription factors, proteins involved with determining which genes are expressed by a cell. Comparing this process to music, transcription factors represent pianists deciding what notes (genes) to play, in order to produce a harmony (in this case, a specialized cell). Interestingly enough, expression of four transcription factors, named Yamanaka factors after the person who discovered them, will push a cell back to a state of pluripotency [3]. Because they are induced to re-enter this stage from a mature state, these cells are termed induced pluripotent stem cells (iPSCs). iPSCs represent a new frontier in medical research: they play a major role in developing more accurate models and treatments, specifically through cerebral organoids. These cells offer the opportunity to understand the causes behind neurodegeneration and aid in the development of new therapies. 

Neurodegeneration and iPSCs

Neurodegenerative conditions are defined by the progressive loss of neurons, the functional units of the brain [4]. Conversations about neurodegeneration sometimes focus on Alzheimer’s Disease (AD), one of the most common neurodegenerative conditions, with 6.2 million Americans currently diagnosed [5]. If you looked into the brain of an Alzheimer’s patient, you might find aggregates of amyloid-beta, plaques of misfolded proteins outside of neurons, which are thought to trigger AD progression [6]. You might also find neurofibrillary tangles, clumps of tau protein within neurons, which are specifically associated with neuron loss & cognitive decline [6]. AD can be classified as either sporadic (SAD), which are the majority of AD cases, or familial (FAD), which represents about 5% of AD cases. FAD is linked to three known genes (PSEN1, PSEN2, and APP) while SAD is more complex [7]. As AD currently has no cure, nor does it have an established disease-modifying therapy (which would help reduce symptoms), researchers are working towards developing better models to search for novel treatments for Alzheimer’s patients [8]. 

This is where induced pluripotent stem cells (iPSCs) shine, as they present a new frontier for disease modeling and drug screening. First described in 2006, induced pluripotent stem cells have quickly made their way into labs around the world [3]. One of the key benefits to the pluripotency of iPSCs is that they can be selectively pushed into maturing into specific cell types [3, 9]. This has tremendous potential in terms of disease modeling, especially for neurodegenerative conditions like Alzheimer’s. 

Existing models of AD primarily include immortalized cell lines derived from humans or animals and live animal models, which include rodents and zebrafish [9]. However, these models have their limitations. Although cells in culture provide information on the impact of amyloid-beta on actual cellular function, they do not account for the impact of age-related neurodegenerative changes that occur in human patients [9]. On the other hand, mice are predominantly used for Alzheimer’s research because they can model AD in living organisms at a relatively low cost, and there are many established techniques to measure cognitive function and performance in mice [9]. One limitation to mice is that they are not similar enough to humans. A genomic data project published in 2018 found that many proteins encoded by risk genes for SAD were only 50% similar between humans and mice [9, 10]. That is not to say that mice are altogether unrepresentative: the same project found that 90% of FAD proteins were similar between mice and humans. This difference indicates that mice are good models for certain human conditions, though limited in their representativeness of neurodegenerative conditions in humans. Everything considered, despite the considerable progress in the field achieved through the use of cell and animal models, there is a clear gap in translation to clinical applications - only 10% of drugs for neurodegenerative conditions progress from clinical trials to FDA approval [11].

Cerebral Organoids

This gap is exactly where cerebral organoids offer hope. One use of iPSCs are cerebral organoids, ‘mini brains’ which are used to model neurological disorders and develop more effective therapies. Despite the moniker ‘mini brains,’ these models do not look exactly like the ridged organ enclosed in all of our skulls. Rather, they are small and smooth pellet-looking models of the human brain, derived from human tissue, and grown in petri dishes. Once a fantasy from the world of science fiction, cerebral organoids are now a well developed research tool, and just one exciting application of induced pluripotent stem cells (iPSCs) in neuroscience. Researchers can reprogram skin cells collected from a patient back to pluripotency by manipulating expression of certain transcription factors [12]. From there, these cells can be selectively pushed into differentiating into a specific cell type, like neurons [9]. In the context of Alzheimer’s Disease, researchers have successfully created an Alzheimer’s cerebral organoid model using iPSCs, by exposing the cells to the misfolded amyloid-beta [7, 13]. This was an important development, because these organoids allowed for amyloid-beta and tau in one consolidated three-dimensional model system. Cerebral organoids offer a more representative model of the overall environment of the brain. Our brains are not just made of a single-layer of one cell type, as represented by “2D” cultured cells in petri dishes, so 3D organoids seemingly present as a more accurate representation of the brain [7, 14]. 

Also, since organoids are composed of human cells, any discoveries using these models may be more translatable to clinical applications compared to those made with animal models. This feature is being capitalized on in drug screening experiments; cerebral organoids are already being used to assess drug efficacy for Alzheimer’s Disease [15]. Despite all the benefits of cerebral organoids, it is important to recognize their limitations. Firstly, organoids generally are not very representative models of aging, because they are far more similar to developing brains [7]. Additionally, most organoid models lack blood vessels, and therefore are limited in both how much they can grow as well as their ability to represent the effects of the blood-brain barrier [16]. Though imperfect, cerebral organoids definitely bring us one step closer to improving the translatability between disease models and real patients. 

Ethical Considerations

As scientific discoveries have expanded research possibilities, researchers have had to grapple with more questions. For example: just because we can conduct an experiment, is it ethical to do so? 

Since iPSCs are relatively new scientific developments, there is much we do not know about them. Cerebral organoids are a stride towards the goal of replicating brain function in a model. But, since our brains are responsible for consciousness, a representative model seemingly calls for a deliberate attempt to recreate consciousness in a petri dish. A 2020 study investigating organoids got shut down for ethical concerns [17]. The reason? These organoids were discovered to be producing electrical activity, similar to that of premature babies [18]. This was concerning because it raised questions about the potential consciousness of organoids, because coordinated electrical activity is a property of conscious brains [17].

Another study, conducted back in 2017, found similar self-organizing activity with cerebral organoids. These researchers found that human stem cells, provided with nutrients over a long period of time, spontaneously organized into organoids consisting of different cell types. Even more shocking was that these organoids contained light-sensitive cells (similar to those found in the eye), which fired when exposed to light [17, 19]. However, it is important to note that the study’s investigator, Dr. Paola Arlotta at Harvard University, cautioned that this firing did not necessarily indicate that the organoids were able to see and process the world -- just that they could represent these visual circuits accurately [17]. 

In general, scientists are divided as to what all this means. Some are of the opinion that cerebral organoids are too premature in their development and therefore cannot claim consciousness [17]. Others believe that cerebral organoids are on the fast-track to Frankenstein’s monster. However, as advocates point out, animal models are conscious. Reputable researchers, including Dr. Alysson Muotri, suggest that there may not be an ethical difference between animals and lab-grown conscious organoids [17]. Moreover, the apparently unmatched ability of cerebral organoids to accurately model the human brain seems to swing the discussion in favor of using them over animal models. Yet, without a universal agreement on what qualifies an organism as conscious, it is hard to analyze the ethics of this development. As scientists proceed with constructing more accurate models of the brain, it appears that this discussion between ethicists and scientists will need to speed up to keep up with the quickening pace of scientific research. 

iPSCs offer a glimmer of hope for those suffering from neurodegenerative conditions. Currently these disorders lack cures, much less adequate disease-altering therapies for symptom relief [14]. Developing cerebral organoids from iPSCs allows for a comprehensive three-dimensional representation of the human brain: a better model on which to test novel treatments. This is important because millions of Americans are affected by neurologic disorders, which include devastating neurodegenerative conditions such as Alzheimer’s Disease and Parkinson’s Disease [5]. iPSCs were once a far-fetched idea. Who would have thought cellular time travel would eventually be possible, even commonplace in labs worldwide? In the same vein, though personalized treatments derived from mini-brains almost seem too good to be true, they may someday become a reality. In light of the promise that iPSCs hold for research, it is exciting to think that someday iPSC-derived mini brains may help patients live longer, happier lives.

References:

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