Harnessing stem cell-based approaches for clinically meaningful discoveries in schizophrenia

The advent of cellular reprogramming in human cells has transformed biomedical research (Karagiannis, 2019). The ability to de-differentiate and reprogram human somatic cells to induced pluripotent stem cells (iPSCs) has enabled the study of live human cells and tissue differentiated from iPSCs, including those that comprise the human brain (Shi, 2012; Benito-Kwiecinski, 2020; Kathuria, 2020a). Technical advances now enable the generation and study of a wide range of patient-derived neuronal and non-neuronal cells as well as of three-dimensional brain organoids (Watmuff, 2017; Kathuria, 2020b; Breitmeyer, 2023; Kathuria, 2023a; Lizano, 2023; Shim, 2023). The iPSC-based studies have led to a greater understanding of many disorders and these approaches are now being applied increasingly to the study of psychotic disorders (Watmuff, 2016; Park, 2021; Bhat, 2022). In this Special Issue of Schizophrenia Research, we present a series of thoughtful articles from a number of research groups that are actively studying schizophrenia and related psychotic disorders using iPSC-based methods.

The guest editorial introduction for the Special Issue on “Pluripotent Stem Cell Research in Psychosis Spectrum” is organized by starting with articles that provide a high-level understanding of the current pathophysiological and pathogenic understanding of schizophrenia from a human stem cell perspective, as well as limitations related to the current work. The editorial progresses to highlight two studies in this special issue that have attempted to address some of these limitations. Overall, it is clear from this work that human stem cell-derived models can provide a promising avenue for increased understanding of schizophrenia neurobiology and for the identification of novel therapeutic strategies.

Probing the pathogenic mechanisms of schizophrenia using human stem cells

Sebastian et al. (2022) (SCHRES-D-22–00271), in their review paper, aimed to summarize some of the major findings from studies published in the last 10 years, which have used human iPSCs for enhancing our understanding of the cell type-specific effects in schizophrenia. It is clear from this summary that multiple cell types are affected throughout development, including 1) neural progenitor cell depletion, 2) deficits in neurogenesis/gliogenesis, 3) morphological alterations in GABAergic interneurons, hippocampal neurons, dopaminergic neurons, glutamatergic excitatory neurons, astrocytes, brain endothelial cells, and microglia, which have functional implications. Lastly, a common feature identified among these alterations is oxidative stress and mitochondrial dysfunction, which was observed in two-dimensional cellular cultures using various cell types as well as three-dimensional brain organoids. However, whether mitochondrial function is causal or a consequence of the disease remains to be determined.

Overview of the use of genomics and stem cells in schizophrenia

Choudhary et al. (2022) (SCHRES-D-22–00503) extended on the review by Sebastian et al. (2022) (SCHRES-D-22–00271) by reviewing the current literature from a genetic signature perspective in human iPSC studies. First, they highlight that over 100 genomic loci, identified from studies of common or rare variants in schizophrenia, have been identified and hold promise for understanding the pathophysiology of schizophrenia. Studies have also begun to detangle the transcriptomic profiles of schizophrenia with differentially expressed genes and non-coding RNAs being identified. While these findings have helped to increase our understanding genetic underpinnings of schizophrenia, the causative role of these genomic changes have been difficult to recapitulate and replicate in animal models. However, human iPSC-based models, using two-dimensional neuronal cultures and three-dimensional brain organoids, have provided a promising avenue for elucidating the mechanistic implications of these genomic findings in schizophrenia. For example, key cellular and molecular pathways have been identified involving WNT signaling dysregulation, oxidative stress and mitochondrial dysfunction. The authors note that these studies have mainly been performed in different subtypes of psychosis with varied clinical phenotypes, and further work is needed to clinically stratify patients with schizophrenia using biological or behavioral markers. The authors also suggest that further methodological advancements in human stem cell modeling is needed to recapitulate a more realistic in vivo brain environment.

Modeling common and rare genetic factors in psychiatric disorders

Muhtaseb et al. (2022) (SCHRES-D-22–00124) have written a timely review discussing the critical need to bridge the gap between genetic findings and disease biology for psychiatric disorders. In order to make causal inferences about disease mechanisms in psychiatry, the authors suggest carefully defining the research question and experimental system with an eye towards a rigorous and well-powered study. This approach will allow researchers to overcome some of the current limitations in the literature, which include a small number of stem cell lines and single variant/gene studies. Thus, a robust and reproducible model of disease is key, which can be scaled up using novel methods, such as CRISPR/Cas9 gene editing and/or single cell multi-omic methods. They also suggest accounting for the functional effects of missing heritability by modeling context-specific regulation, gene by environment interactions, and mitochondrial DNA mutations. Lastly, given the polygenic nature of psychiatric disorders, the authors suggest modeling individual risk variants/genes by perturbing networks using multiplex CRISPR/Cas9 editing in human stem cells.

Mitochondrial alteration in psychiatric disorders

Ni et al. (2022) (SCHRES-D-22–00011) delved further into the literature focusing on the pathogenic role of mitochondrial dysfunction in psychiatric disorders, such as schizophrenia, major depressive disorder, and bipolar disorder. In support of their hypothesis, the authors present a comprehensive literature review of animal and human studies using tissue, neuroimaging, genetic and stem cell studies, showing that mitochondrial abnormalities impact neuronal function, which in turn manifests as brain-behavior disruptions. Moreover, they identified a range of mitochondrial alterations across psychiatric diseases, each of which may have a different approach for treatment. For example, medications that improve mitochondrial function have been shown to result in clinical benefit in a range of psychiatric disorders, and based on animal studies, these strategies could be preventative. However, further work is needed to understand the pathogenic role of mitochondrial dysfunction in psychiatric disorders and to optimize treatment strategies.

Synaptic dysfunction and cognition in schizophrenia

Santarriaga et al. (2023) (SCHRES-D-22–0077) conducted a review examining the role of synaptic dysfunction in cognitive impairment, and provide a summary of human stem cell based studies evaluating the role of synaptic dysfunction in schizophrenia. The authors show that there is converging evidence from genetic, postmortem, neuroimaging, and iPSC studies demonstrating how synaptic dysfunction may contribute to cognitive impairment in schizophrenia. They show that transcriptomic and proteomic data from iPSC-derived neurons mirror the signatures identified in postmortem data and align with the evidence from genome wide association studies. Moreover, they acknowledge the presence of variability in synaptic phenotypes, which may be due to the specific subtype of individuals with schizophrenia that are being studied. They also note that it is important to examine the intersection of different cellular and molecular pathways that have been implicated in schizophrenia, such as immune, mitochondrial, and synaptic pathway dysfunction, using human iPSC-based models. While these findings are exciting, further work is needed to better understand the role of ex vivo synaptic dysfunction and how it impacts in vivo cognitive performance in individuals with schizophrenia.

Clinical, neuroimaging, and molecular considerations in 22q11.2 deletion carriers

Reid et al. (2022) (SCHRES-D-22–00163) provide an original contribution in their pilot study of human iPSCs generated from two 22q11.2 deletion carriers with distinct clinical phenotypes (one case with a diagnosis of autism spectrum disorder, mild intellectual disability and schizophrenia, while the other case had cardiac defects but no mental health diagnosis), and both demonstrated high levels of dopamine synthesis capacity based on ¹⁸F-DOPA PET imaging. These cell lines and those from neurotypical individuals were used to develop midbrain dopaminergic neurons from stem cells to better understand what molecular changes are associated with the dopamine neuroimaging phenotype, and whether they occur independent of clinical phenotypes. The authors found alterations in the expression of genes associated with dopamine metabolism and signaling which differed between the two 22q11.2 deletion cell lines. The authors suggest that future mechanistic studies should consider clinical, genetic, and molecular information when choosing donors to generate stem cells from.

Biological consequences of ZMYND11 in a schizophrenia using CRISPR/Cas9 editing

Tordai et al. (2024) (SCHRES-D-23–00313) present an original research contribution examining the role of a de novo mutation in schizophrenia using human stem cell models and CRISPR/Cas9 editing. The authors used a schizophrenia case-parent trio, where the individual with schizophrenia carried a potentially disease causing 1495C>T nonsense de novo mutation in the zinc finger MYND domain-containing protein 11 (ZMYND11). ZMYND11 is implicated in biological processes related to schizophrenia, and in neurons, it is a repressor of neuronal differentiation. Hippocampal neuron progenitor cells and dentate gyrus granule cells were generated from the trio. The authors found that hippocampal progenitor cell morphology was preserved, but transcriptomic changes associated with upregulation of neuronal differentiation genes and downregulation of cell adhesion genes were observed. The authors also identified functional alterations as evidenced by lower reactivity to glutamate in calcium imaging experiments. This proof-of-concept study shows that de novo mutations in schizophrenia can be mechanistically examined using human stem cell models.

Schizophrenia and related psychotic disorders are extensively complex with regard to phenomenology, pleiotropy, etiopathogenesis, genetics and gene-environment interactions (van Os, 1998; Owen, 2016). Genome wide association studies (GWAS) of schizophrenia indicate the presence of hundreds of common variants that are associated with the disease diagnosis, with very small effect sizes (Trubetskoy, 2022). Studies of rare copy number variants have led to the identification of a number of genes that have strong effect sizes but these genetic differences are rare and do not necessarily recapitulate the underlying disease process in the vast majority of patients (Singh, 2022). The iPSC-based cellular models, in conjunction with cutting-edge methods in gene editing and multi-omic studies, provide a wide array of opportunities for innovative experimental studies (Karmacharya, 2016; Stachowiak, 2017; Kozlova, 2022; Kathuria, 2023b). However, the subtle and nuanced differences in the human brain that result in psychosis do not lend themselves to simplistic reductionist models (Frangou, 2020). For the recent advances in stem cell biology to translate to meaningful advances in the clinic will require a multi-pronged collaborative approach that include dynamic crosstalk and collaborations between the bench and the clinic (Karmacharya, 2020). A deeper recognition of clinical complexity, combined with parallel in vivo and ex vivo studies of well-defined patients, can lead to a more nuanced understanding of the biology underlying the various clinical domains in psychotic disorders (Hathy, 2020; Li, 2021; Pak, 2021; Page, 2022; Sawada, 2024; Stern, 2024).