Stem-cell-derived human microglia transplanted in mouse brain to study human disease | Renzo Mancuso et al. | ResearchPod

Transcript: Stem-cell-derived human microglia transplanted in mouse brain to study human disease

[[RP_SECTION:microglia-niche-influence|Microglia Niche Influence]]

Sam: [steady, matter-of-fact] Human microglia grown in a dish carry an activated transcriptional profile. Transplanted into a mouse brain, they shift back toward a homeostatic state. That is the central observation from Renzo Mancuso and colleagues at the Flanders Institute for Biotechnology, and it suggests the brain niche is doing much of the work of holding a native microglial phenotype in place.

Alex: [leaning in, curious] So the culture dish is effectively a false environment? If the cells are that sensitive to their surroundings, how much of the in vitro microglia literature is describing the dish rather than the cell? [[RP_SECTION:xenotransplantation-experimental-design|Xenotransplantation Experimental Design]]

Sam: [measured, teaching mode] That is a fair implication, and the authors' data point that way. The in vitro cells consistently show an activated profile that bears little resemblance to primary microglia. To test what the niche does, they used xenotransplantation. Human embryonic stem cell-derived microglia were engrafted into Rag2 Il2rγ double-knockout mice, pre-conditioned with the CSF1R inhibitor BLZ945. <break time="0.6s" /> The inhibitor empties the host microglial niche, so the human cells can colonise it. Once integrated, they adopt a homeostatic signature that is close to human primary microglia.

Alex: [slower, processing] So it's like re-potting a plant that was struggling in a greenhouse into natural soil. But if the goal is Alzheimer's, aren't we still limited by the fact that mouse biology isn't human? [[RP_SECTION:human-and-mouse-divergence|Human and Mouse Divergence]]

Sam: [grounded, precise] That is the motivation for the design. The paper notes that 41% of human genes lack clear one-to-one mouse orthologs. Several Alzheimer's risk genes, TREM2 and the MS4A cluster among them, are divergent or absent in rodents. Putting human cells into the brain lets you examine those genes in vivo. When the authors challenged the transplanted cells with oligomeric amyloid-beta, the cytokine response was one that mouse microglia did not replicate.

Alex: [probing, analytical] So the vessel is mouse but the immune response is human. Is the nature of the inflammatory cascade different from what standard mouse models show?

Sam: [thoughtful, building the point] It appears so. The human microglia settled into a primed state between homeostasis and full activation, which the authors suggest is a distinctly human response to amyloid-beta. The authors read this as a sign that mouse models may have hidden human-specific inflammatory dynamics. The limitation is that the system is chimeric. Human cells are still signalling inside a mouse brain, which may constrain the pathways you observe.

Stem-cell-derived human microglia transplanted in mouse brain to study human disease

Renzo Mancuso, Johanna Van Den Daele, Nicola Fattorelli, Bart De Strooper

Oct 28, 2019·6:00·user_upload·Original Paper

Human MicrogliaDisease ModelingGenetic OrthologySingle-cell TranscriptomicsAlzheimer’s DiseaseTranscriptional Profiling

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6 min

Briefing

Research Question

Microglia are central to Alzheimer's disease (AD) pathology, yet modeling them in mice is challenging because many human AD-associated genes lack direct mouse orthologs or show functional divergence. This study investigates whether human embryonic stem cell (ESC)-derived microglia can be transplanted into the mouse brain to create a more accurate, humanized model for studying microglial function in disease.

Approach

The researchers differentiated human H9 ESCs into microglia-like cells and transplanted them into the brains of immunodeficient (Rag2-/- Il2rγ-/-) mice. To facilitate engraftment, the host mice were pre-treated with a CSF1R inhibitor to deplete endogenous mouse microglia. The researchers then used single-cell RNA sequencing to compare the transcriptomic profiles of these transplanted human cells against primary human microglia and host mouse microglia, both under homeostatic conditions and following an acute challenge with oligomeric amyloid-beta.

Main Findings

The transplanted human microglia successfully integrated into the mouse brain, displaying a ramified morphology and expressing homeostatic markers (TMEM119, P2RY12) similar to primary human microglia. Transcriptomic analysis confirmed that the mouse central nervous system environment effectively drives these cells from an artificial in vitro state toward a natural, homeostatic phenotype. When challenged with oligomeric amyloid-beta, the human microglia exhibited a distinct 'cytokine response' (CRM) state. Crucially, this response differed from that of the host mouse microglia, with many human-specific genes—including several AD risk genes—showing divergent expression patterns. This demonstrates that the humanized model captures aspects of the human microglial response to AD-related pathology that are missed in traditional mouse models.

Why It Matters

This study provides a robust proof-of-concept for using humanized chimeric models to study microglial biology. By overcoming the limitations of mouse-human genetic divergence, this approach allows researchers to investigate the function of human-specific AD risk genes in an in vivo environment. This model serves as a powerful tool for dissecting the cellular mechanisms of neurological diseases and could significantly improve the translational relevance of future preclinical studies.

Key Terms and Definitions

Microglia — The resident immune cells of the central nervous system that maintain brain homeostasis and respond to injury or pathology.
Orthologs — Genes in different species that evolved from a common ancestral gene and typically retain similar functions.
Engraftment — The process of successfully transplanting and integrating donor cells into a host tissue.
Transcriptome — The complete set of RNA transcripts produced by the genome at any given time, used here to define the functional state of the microglia.
Oligomeric amyloid-beta — A toxic, soluble form of the amyloid-beta peptide associated with cognitive impairment and the early stages of Alzheimer's disease.

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