Patient-Derived In Vitro Models of Microglial Function and Synaptic Engulfment in Schizophrenia

Abstract

Multiple lines of evidence implicate dysregulated microglia-mediated synaptic pruning in the pathophysiology of schizophrenia. In vitro human cellular studies represent a promising means of pursuing this hypothesis, complementing efforts with animal models and postmortem human data while addressing their limitations. The challenges in culturing homogeneous populations of cells derived from postmortem or surgical biopsy brain material from patients, and their limited availability, has led to a focus on differentiation of induced pluripotent stem cells. These methods too have limitations, in that they disrupt the epigenome and can demonstrate line-to-line variability due in part to extended time in culture, partial reprogramming, and/or residual epigenetic memory from the cell source, yielding large technical artifacts. Yet another strategy uses direct transdifferentiation of peripheral mononuclear blood cells, or umbilical cord blood cells, to microglia-like cells. Any of these approaches can be paired with patient-derived synaptosomes from differentiated neurons as a simpler alternative to co-culture. Patient-derived microglia models may facilitate identification of novel modulators of synaptic pruning and identification of biomarkers that may allow more targeted early interventions.

Keywords: Microglia; PBMCs; Schizophrenia; Synaptic pruning; Therapeutics; iPSCs.

Figure 1.

Patient-derived in vitro models of microglial function and synaptic engulfment in schizophrenia. (A) Overview of patient-derived microglial and neuronal in vitro cellular models to understand disease etiology and for therapeutic discovery. Dotted lines represent common experimental pathways. (B) Biobanks of patient cells offer a trove of information to compare by i) clinical diagnosis or by ii) specific genetic signatures such as risk variant(s). The ability to reprogram these cells is important to observe cell type–specific phenotypes and responses to perturbations. (C) Cell manipulations can be applied during different reprogramming stages and may be optimal in a cell type–specific manner. i) Targeted gene knockout, ii) gene regulation, iii) silencing, and iv) transgene expression can be used to manipulate individual genes implicated in schizophrenia. In addition, the effects of v) small molecules and vi) biologics can be screened for phenotypic reversal for therapeutic discovery and development. (D) Functional in vitro assays monitor cell-specific behavior, such as i) phagocytosis (e.g., synaptosomes), ii) alterations in morphology, iii) motility/chemotaxis, and iv) activation state. Furthermore, transcriptomic data, either in v) bulk or vi) single-cell resolution, allow for gene-and pathway-level information that can lead to novel targets for further study of potential therapeutic value. CRISPR, clustered regularly interspaced short palindromic repeats; i/a, inhibiting/activating; iPSCs, induced pluripotent stem cells; KO, knockout; PBMCs, peripheral blood mononuclear cells; siRNA, small interfering RNA.