Carlos Morra: The use of human-induced pluripotent stem cells (iPSC) in schizophrenia. Will this technology generate a leap forward in the disease´s modeling?*
The development of induced pluripotent stem cells (iPSC) may constitute a significant advance in medical research.
The differentiation process is a natural mechanism in which embryonic stem cells produce all the body's different mature cells. The belief that this process was a one-way road changed when the Nobel laureate Shinya Yamanaka in 2006 identified four factors, Oct3/4, Sox2, c-Myc and Klf4, from a list of 24 genetic factors he found in the literature associated with pluripotency. When mature cells obtained from living organisms, like fibroblasts or adult peripheral blood, T cells were exposed to them by being infected with their retroviral vectors and regressed into iPSC (Yamanaka 2008).
There are many significant advantages in using cultures derived from iPSC.
- They are pluripotent, meaning that they can differentiate in any human cell.
- They can self-renew, replicating themselves continuously.
- Finally, they can be generated from the same individuals (Alvarez-Buylla, Seri and Doetsch 2002; Kim and de Vellis 2009; Chen, Lin, Foxe et al. 2013; Avior, Sagi and Benvenisty 2016; Gao, Yourick and Sprando 2017; Szabó, Juhász, Hathy et al. 2020; Larijani, Parhizkar RP, Hadavandkhani et al. 2021).
This essay will
- introduce schizophrenia and some current disease models for schizophrenia
- present some of the findings obtained utilizing iPSC in schizophrenia
- describe some of the research with iPSCs that might provide newer models for understanding and treating
Schizophrenia is a complex nosological entity empirically derived by Kraepelin (1893), with consideration of the course and outcome, of cross-sectional psychopathology. It affects approximately 1% of the general population. It produces a devastating impact on patients and their families, with a substantial economic burden worldwide (Collins, Patel, Joestl et al. 2011).
Authors, like Stephan Heckers, questioned the validity of the concept, mainly based on the poorly defined molecular abnormalities and focal pathology - with different onsets, clinical manifestations and outcomes. Its phenotypical heterogeneity occasioned many inconsistencies when adopting models to fully explain its etiology or biological manifestations (Ban 2007; Heckers 2008; Moncrieff and Middleton 2015).
There are several disease models of schizophrenia that are currently based on findings from other methodologies, such as post-mortem brain studies, morphological and functional brain images, and genetic and tissue cultures.
The morphologic abnormalities described in schizophrenic brains (Shepherd, Laurens, Matheson et al. 2012) include macroscopic and microscopic manifestations like the reduction of total brain volume, abnormal cell size, reduced spine density and variations of neural distribution in the prefrontal cortex and hippocampus (Harrison 2000; Brennand, Simone, Jou et al. 2011).
This methodology contributed to the development or validation of many current conceptualizations, such as the dopaminergic, glutamatergic, serotonergic, noradrenergic, cannabinoid, GABAergic, cholinergic and kappa opioid theories (Gierke, Zhao C, Bernstein et al. 2008; Jones, Watson and Fone 2011; Steeds, Carhart-Harris and Stone 2015; Falk, Heine, Harwood et al. 2016; Coyle, Ruzicka and Balu 2020; Christian, Song and Ming 2020; Larijani, Parhizkar RP, Hadavandkhani et al. 2021).
Most of the existing theories failed to address the disease's dynamic totality and sometimes wrongly assume that concurrent factors are causes or consequences, making it difficult to reach a valid conclusion (Reynolds and Harte 2007; Coyle, Ruzicka and Balu 2020).
The recently developed iPSC generation technique introduced a novel approach for identifying and understanding the “etiopathogenic” abnormalities of schizophrenia.
The possibility of differentiating stem cells in neurons and glial cells, like astrocytes, oligodendrocytes and ependymal cells, can replicate the disease's heterogeneous abnormalities in patient's cells cultures (Brennand, Simone, Jou et al. 2011; Liu, Osipovitch, Benraiss et al. 2019).
A paper published by Brennand, Savas, Kim et al. (2015) reported that even neural progenitor cells (NPCs) of schizophrenic patients showed abnormal gene expression and protein levels that may result in changes in migration and responses to oxidative stress.
Other authors described in cells derived from iPSC several abnormalities such as transcriptome alterations, levels of neuronal differentiation, changes in axonal, dendritic, and synaptic morphology and function, mitochondrial damage, increased expression of mRNA-9, phosphoinositide 3-kinase/glycogen synthase kinase 3 (PI3K/GSK3) signaling alterations, accelerated neural differentiation, GABAergic dysfunctions, changes in WNT signaling, dysregulation of potassium channel-encoding genes in SCZ glial cells, diminished Ca2+ response to glutamatergic signals and altered reactivity to environmental risk factors or challenges (Hashimoto-Torii, Torii, Fujimoto et al. 2014; Liszewska and Jaworski 2018; Ishii, Ishikawa, Fujimori et al. 2019; Wen, Christian, Song and Ming 2016; Hathy, Szabó, Varga et al. 2020; Stertz, Di Re, Pei et al. 2021).
These findings assisted in the elaboration of many current disease models. For example, the alterations in WNT signaling initially allegedly responsible for the abnormal neuronal migration patterns in schizophrenia forebrains.
However, for further consideration, a valid disease model still will be necessary to determine whether these are causal factors leading to altered neural positioning and development or merely the secondary consequences of changes in oxidative stress (Panaccione, Napoletano, Forte et al. 2013; Topol, Zhu, Tran et al. 2015; Hoseth, Krull, Dieset et al. 2018).
Some other findings described in schizophrenic cell cultures, like the GABAergic interneurons or the glial calcium anomalies, provided new evidence to support other pre-existent theories, like glutamatergic (Boissart, Poulet A, Georges et al. 2013; Hathy, Szabó, Varga et al. 2020; Coyle, Ruzicka and Balu 2020).
Neuronal cultures allowed to record high spatial and temporal-resolution in vivo images, obtain sequential real-time information of the disease's pathogeny and maintain all subjects' genetic information and reduce inter-species variability.
Moreover, manipulating them using invasive techniques, pharmacologic compounds or genome editing techniques provided newer reliable ways to produce and test schizophrenia models and identify future treatment targets (Avior, Sagi and Benvenisty 2016; Quadrato, Brown and Arlotta 2016; Wen, Christian, Song and Ming 2016).
By recent improvements in the generation process, homogenous cultures of proliferative NPCs were directly generated from mature cells before reaching a pluripotent state (iPSC); this suggested the existence of alternative shortcuts, with a substantial cost and time reduction, but no other significant differences with the cultures derived from iPSC (Szabó, Juhász, Hathy et al. 2020).
In conclusion, well-defined, enhanced CNS cell cultures derived from iPSC can represent realistic schizophrenia molecular pathophysiology (Larijani, Parhizkar RP, Hadavandkhani et al. 2021).
The evidence obtained of receptor abnormalities using this technology would allow scientists to identify new treatment targets and propose new disease models focusing on aspects not explained at least entirely, using post-mortem, neuroimaging, animal or genetic studies.
In the future, developing three-dimensional cultures and organoids might improve studying the disease and reducing, even more, the number of studies employing non-human subjects (Quadrato and Arlotta 2017; Christian, Song and Ming 2020; Larijani, Parhizkar, Hadavandkhani et al. 2021).
Reaching universal conclusions from the findings of schizophrenics' iPSC cultures has been almost impossible because of the relatively small sample sizes presented in every study; however, using worldwide databases would simplify identifying homogenous phenotypes within the patients.
Finally, the combination of iPSC with modern gene-editing techniques, like transcription activator-like effector nucleases (TALENs), clustered regularly interspaced short palindromic repeats (CRISPR) provides the ability to reproduce the abnormal findings in genetic “knockout” animals and test the functional consequences of the molecular or genetic abnormalities and validate future models; moreover, finding molecular targets will improve therapeutical applications with all their heuristic implications (Musunuru 2013; Quadrato, Brown and Arlotta 2016; St. Clair and Johnstone 2018).
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*This essay is based on an assignment Carlos Morra completed for a Master’s course in Science in Applied in Neuroscience at King´s College London and submitted in February 2021.
June 24, 2021