Studying Heterotypic Cell⁻Cell Interactions in the Human Brain Using Pluripotent Stem Cell Models for Neurodegeneration

Abstract

Human cerebral organoids derived from induced pluripotent stem cells (iPSCs) provide novel tools for recapitulating the cytoarchitecture of the human brain and for studying biological mechanisms of neurological disorders. However, the heterotypic interactions of neurovascular units, composed of neurons, pericytes (i.e., the tissue resident mesenchymal stromal cells), astrocytes, and brain microvascular endothelial cells, in brain-like tissues are less investigated. In addition, most cortical organoids lack a microglia component, the resident immune cells in the brain. Impairment of the blood-brain barrier caused by improper crosstalk between neural cells and vascular cells is associated with many neurodegenerative disorders. Mesenchymal stem cells (MSCs), with a phenotype overlapping with pericytes, have promotion effects on neurogenesis and angiogenesis, which are mainly attributed to secreted growth factors and extracellular matrices. As the innate macrophages of the central nervous system, microglia regulate neuronal activities and promote neuronal differentiation by secreting neurotrophic factors and pro-/anti-inflammatory molecules. Neuronal-microglia interactions mediated by chemokines signaling can be modulated in vitro for recapitulating microglial activities during neurodegenerative disease progression. In this review, we discussed the cellular interactions and the physiological roles of neural cells with other cell types including endothelial cells and microglia based on iPSC models. The therapeutic roles of MSCs in treating neural degeneration and pathological roles of microglia in neurodegenerative disease progression were also discussed.

Keywords: endothelial; heterotypic; mesenchymal stem cells; microglia; neural-vascular interactions; pluripotent stem cells.

Figure 1

Cellular complexity in the central nervous system (CNS). The blood vessels in human brain form the blood–brain barrier (BBB) with endothelial cells, pericytes, astrocytes, and neurons. The endothelial cells also interact with microglia for immune response. Microglia have different activation pathways. Surveying microglia can be classically activated (M1) induced by lipopolysaccharide (LPS) or IFN-γ to release pro-inflammatory molecules, such as reactive oxygen species (ROS), TNF-α, IL-6, IL-1β, MMP and glutamate or alternatively activated (M2) by IL-4 or IL-13 to phagocytize pathogens and cell debris to induce an anti-inflammatory response with upregulation of IL-10 and arginase 1. Mesenchymal stem cells (MSCs), close to pericytes, secrete neurotrophic factors and angiogenesis factors.

Figure 2

A working model of the dynamic interactions between neural progenitor cells (NPCs) and brain microvascular endothelial cells. VEGF: vascular endothelial growth factor; BDNF: brain-derived neurotrophic factor; NO: Nitric oxide; eNOS: endothelial nitric oxide synthase 3; VEGFR2: vascular endothelial growth factor receptor 2; TrkB: Tropomyosin receptor kinase B.

Figure 3

The blood–brain barrier (BBB) in vivo and the role in neural degeneration. The blood vessels in human brain form BBB with endothelial cells, pericytes, astrocytes, and neurons. BBB breakdown due to endothelial and pericyte degeneration leads to neural degeneration, associated with inflammatory response, loss of neurons, and synaptic dysfunction. LRP1: low density lipoprotein receptor-related protein 1; RAGE: receptor for advanced glycation end-products; PGP1: phosphatidylglycerolphosphate synthase 1.

Figure 4

Schematic of the proposed signaling in spheroid human mesenchymal stem cells (MSCs). Interaction between VEGF and Notch signaling in neurovascular coupling. BBB: Blood-brain barrier; NPCs: neural progenitor cells; ECs: endothelial cells; MMP: matrix metalloproteinase; ECM: extracellular matrix; NICD: the intracellular domain of the notch protein; VEGF: vascular endothelial growth factor: VEGFR2: vascular endothelial growth factor receptor 2; TrkB: Tropomyosin receptor kinase B.

Figure 5

The physiological role of microglia in CNS development. Microglia originate from primitive hematopoietic stem cell at the extra-embryonic yolk sac (YS). Microglia migrate from the yolk sac into the central nervous system as the resident immune cells during brain development. Microglia regulate the neuronal activities and promote neuronal differentiation by secreting neurotrophic factors and anti-inflammatory molecules. In the developing brain, microglia can promote synaptic pruning and phagocytose neural progenitor cells.

Figure 6

Vicious cycle of neuro-inflammation [116]. Aβ plaques activate the primed microglia into neurotoxic microglia phenotype via the toll-like receptor (TLR) and released a variety of pro-inflammatory molecules, including IL-6, IL-1β, and TNF-α, which induce astrocyte and neuronal damage with increased level of apoptosis. In turn, the activation of purinergic P2X7 receptors in microglia amplified alternative activation. Neuroprotective microglia are beneficial via the secretion of Aβ-degrading enzymes or by phagocytotic clearance of toxic Aβ plaques. From Jacobs et al, 2012.

Figure 7

Neural-microglia interactions in hiPSC-based organoid models. Co-culturing the isogenic microglia with hiPSC-derived dorsal and ventral spheroids showed response to pro-inflammatory stimuli, Aβ42 oligomers. Dorsal-microglia group were less pro-inflammatory and showed higher anti-inflammatory cytokine secretion, while ventral-microglia group showed higher TNF-α expression under Aβ42 stimulation. All co-cultured spheroids stimulated cell proliferation and reduced reactive oxygen species (ROS) production, better resembling the tissue-specific microenvironment and the homeostasis.