Microglia integration into human midbrain organoids leads to increased neuronal maturation and functionality

Abstract

The human brain is a complex, three-dimensional structure. To better recapitulate brain complexity, recent efforts have focused on the development of human specific midbrain organoids. Human iPSC-derived midbrain organoids consist of differentiated and functional neurons, which contain active synapses, as well as astrocytes and oligodendrocytes. However, the absence of microglia, with their ability to remodel neuronal networks and phagocytose apoptotic cells and debris, represents a major disadvantage for the current midbrain organoid systems. Additionally, neuro-inflammation related disease modeling is not possible in the absence of microglia. So far, no studies about the effects of human iPSC-derived microglia on midbrain organoid neural cells have been published. Here we describe an approach to derive microglia from human iPSCs and integrate them into iPSC-derived midbrain organoids. Using single nuclear RNA Sequencing, we provide a detailed characterization of microglia in midbrain organoids as well as the influence of their presence on the other cells of the organoids. Furthermore, we describe the effects that microglia have on cell death and oxidative stress-related gene expression. Finally, we show that microglia in midbrain organoids affect synaptic remodeling and increase neuronal excitability. Altogether, we show a more suitable system to further investigate brain development, as well as neurodegenerative diseases and neuro-inflammation.

Microglia-containing human brain organoids for the study of brain development and pathology

“…Whole-cell patch-clamp and multi-electrode array recordings showed that a lower threshold for action potential generation and a shorter interspike interval were observed in microglia-integrated midbrain organoids, suggesting that microglia integration promoted neural maturation. The presence of microglia in midbrain organoids also increased cytokine release (including IL6, IL10, IL1β, and TNFα), and affected expression of genes related to immune response, inflammation, phagocytosis, and synaptic remodeling and maturation [85].…”

“…Excitatory post-synaptic currents (EPSCs) were present only in neurons from microglia-containing organoids after day 107 [84]. A recent study [85] that co-cultured human midbrain organoids with hiPSCs-derived macrophage progenitors also reported that microglia integration led to increased neural maturation and functionality. Whole-cell patch-clamp and multi-electrode array recordings showed that a lower threshold for action potential generation and a shorter interspike interval were observed in microglia-integrated midbrain organoids, suggesting that microglia integration promoted neural maturation.…”

“…However, the amount of microglia and other potential mesodermal lineage cells developed in HBOs is uncontrolled after incorporation of MPCs. For example, the number of microglia developed in MC-HBOs using this strategy only accounted for 1.18% of total cells as revealed by single-cell RNA sequencing (scRNA-seq) [85]. This number is much lower than that in the healthy human brain.…”