Tracing Early Neurodevelopment in Schizophrenia with Induced Pluripotent Stem Cells

Abstract

Schizophrenia (SCZ) is a devastating mental disorder that is characterized by distortions in thinking, perception, emotion, language, sense of self, and behavior. Epidemiological evidence suggests that subtle perturbations in early neurodevelopment increase later susceptibility for disease, which typically manifests in adolescence to early adulthood. Early perturbations are thought to be significantly mediated through incompletely understood genetic risk factors. The advent of induced pluripotent stem cell (iPSC) technology allows for the in vitro analysis of disease-relevant neuronal cell types from the early stages of human brain development. Since iPSCs capture each donor's genotype, comparison between neuronal cells derived from healthy and diseased individuals can provide important insights into the molecular and cellular basis of SCZ. In this review, we discuss results from an increasing number of iPSC-based SCZ/control studies that highlight alterations in neuronal differentiation, maturation, and neurotransmission in addition to perturbed mitochondrial function and micro-RNA expression. In light of this remarkable progress, we consider also ongoing challenges from the field of iPSC-based disease modeling that call for further improvements on the generation and design of patient-specific iPSC studies to ultimately progress from basic studies on SCZ to tailored treatments.

Keywords: differentiation; early neurodevelopment; micro-RNA; mitochondria; neuroleptics; neurotransmission; patient-specific iPSCs; schizophrenia.

Figure 1

Tracing early neurodevelopment. Key morphogens in anterior–posterior patterning are WNT (wingless), FGF (fibroblast growth factors), and RA (retinoid acid). Gradients of these morphogens regulate differentiation into specific types of neural progenitor cells of forebrain (purple), midbrain (orange), hindbrain (yellow), and anterior spinal cord (green). Key morphogens in dorsal–ventral patterning are gradients of BMP (bone morphogenetic proteins), WNT, SHH (sonic hedgehog), and Nodal. BMP and WNT determine the dorsal fates of neural progenitor cells, whereas SHH and Nodal determines ventral fates. Different types of neural cells for tracing early neurodevelopment in schizophrenia (SCZ) are shown: Ventral midbrain progenitors give rise to tyrosine hydroxylase-positive dopaminergic neurons. Neural progenitor cells (green) from the ventricular zone (VZ) of the dorsal telencephalon generate all excitatory glutamatergic pyramidal neurons. Dorsal progenitors use radial glia cells (red) as scaffold to migrate radially to the cortical plate (CP) and from there to destined cortical layers. Interneurons (green) are derived from the ventral neurogenic zone termed medial ganglion eminence (MGE) and migrate tangentially to the pallium. There, interneurons can use radial glia cells to ascend to the cortical plate as is the case for inhibitory GABA-ergic interneurons or to descend to the ventricular zone. Differential interactions between subsets of interneurons and the radial glial scaffold are indicated by shades of green and red, respectively. On top of this, local guidance cues that are indicated by a gradient of pink can influence interneuron positioning as shown by blue arrows. Part of Figure 1 is reprinted from K. Sue O’Shea and Melvin G. McInnis, Neurodevelopmental origins of bipolar disorder: iPSC models; published by Molecular and Cellular Neuroscience, volume 73, pp. 63–83, 2016, with permission from Elsevier.

Figure 2

Temporal course of selected neurodevelopmental processes. The x axis depicts time from fertilization to young adulthood and the y axis shows the relative percentage of the maximum. The average age of schizophrenia (SCZ) onset in males and females in late adolescence and early adulthood, respectively, is indicated. Neurogenesis and subsequent migration of neurons to the cortex begin within a few weeks of gestation in humans and is completed around birth. Dendritic arborization and myelination continue postnatally towards adolescence and beyond. The progressive reduction of grey-mater volume detected with longitudinal neuroimaging (top) is thought to result from the combined effect of pruning of the neural arbor and myelin deposition. The formation of prefrontal excitatory synapses reaches a maximum in childhood at five years and declines thereafter until adolescence. By contrast, the formation of prefrontal inhibitory synapses strongly increases from 15 years onward through adolescence and levels off in early adulthood. Deregulation of the cortical excitatory-inhibitory balance is hypothesized to contribute to the development and manifestation of SCZ. Adapted from Thomas R. Insel, Rethinking Schizophrenia, published by Nature, 2010.

Figure 3

Structure and function of the mitochondrion. Outer and inner double-layered membranes enclose the intermembrane space, the cristae space formed by infoldings of the inner membrane, and the matrix as the space within the inner membrane. The matrix hosts the circular mitochondrial DNA (mtDNA) and a large number of enzymes catalyzing various biochemical reactions. Blood-borne glucose is metabolized intracellularly to pyruvate and then to acetyl-CoA, the only substrate fueling the tricarboxylic acid (TCA) cycle. Oxidation of pyruvate produces reduced cofactors that transfer free electrons to the electron transport chain (ETC) hosted at the inner mitochondrial membrane. The ETC consists of complex I to V that build an electrochemical gradient (∆φ_m) across the inner mitochondrial membrane driving the synthesis of adenosine triphosphate (ATP). Electron leakage mainly at complex I and III (0.1 to 2%) reduces prematurely oxygen to superoxide anion (O₂⁻) that is converted by mitochondrial superoxide dismutase MnSOD and CuZnSOD to hydrogen peroxide (H₂O₂) in the matrix or intermembrane space, respectively. Thereafter, H₂O₂ is oxidized to highly reactive hydroxyl (OH⁻) free radicals that cause mtDNA damage, reduced transcription, and oxidation of ETC proteins and membrane lipids. Radical production is counteracted by enzymatic defense mechanisms, including glutathione peroxidase (GPx) and catalase (CAT) that detoxify H₂O₂ into water. Ub = ubiquinone, also known as Coenzyme Q10; Cyt c = cytochrome c.