In the March 29 Stem Cell Reports, researchers led by Frederick Livesey at the University of Cambridge in England described a new protocol for generating microglia-like cells from human stem cells. Compared with a handful of other recently published microglia recipes, this one yields maximal numbers of cells using a relatively simple process. The microglia expressed genes on par with primary microglia cultured directly from the brain, buddied up with neurons in coculture, and responded to inflammatory stimuli. In microglia generated from people harboring pathogenic mutations in TREM2, the immune receptor’s trafficking was profoundly altered, yet the microglia gobbled up bacteria and pumped out cytokines just fine. However, some researchers wondered if these cells were tested in a physiologically meaningful paradigm.
The pivotal role of microglia in neurodegenerative disease makes the immune cells a key subject of study, but efforts to understand them outside of their physiological environment have been fraught with challenges. For one, the highly responsive cells are shifty by nature—removing them from the brain evokes almost immediate changes in gene expression (see Jun 2017 news). “You only have to look at them funny and they’ll get aggressive,” Livesey joked. The cells are also sparse in the mouse brain, and mouse microglia can only approximate the characteristics of their human counterparts. An emerging strategy to sidestep these issues is to make human microglia from scratch. In recent years, researchers have developed a handful of protocols that generate microglia from human stem cells, including the induced pluripotent variety (iPSCs) (see Jul 2016 conference news). Reasoning that the environment of the brain is essential for microglial development, some protocols utilize complex coculture systems to coax precursors into microglia (Haenseler et al., 2017; Takata et al., 2017; Pandya et al., 2016). Others rely entirely on cocktails of specific growth factors to steer the cells toward microgliahood (Muffat et al., 2016; Abud et al., 2017; Douvaras et al., 2017).
First author Philip Brownjohn and colleagues went with the cocktail approach, first generating primitive macrophage precursors (PMPs) from iPSCs. These cells approximate the macrophage precursors that arise from erythro-myeloid precursors in the yolk sac during fetal development and that give rise to brain-resident microglia. Other researchers previously developed protocols to establish them in culture (Karlsson et al., 2008; van Wilgenburg et al., 2013). Brownjohn found that iPSCs expanded by 20- to 50-fold into PMPs over 80 days. Then, by adding in other factors, including granulocyte colony-stimulating factor (GM-CSF) and IL-34, to replicate the environment of the developing brain, the researchers steered the PMPs into microglia-like cells within roughly a week. Ninety-five percent of the cells expressed typical microglial markers, including Iba-1 and CD45, while almost 100 percent of them expressed TREM2.
Brownjohn compared gene expression profiles to see how the derived cells measured up to primary microglia and to those developed via alternate methods (Zhang et al., 2016; Gosselin et al., 2017; Abud et al., 2017). Broadly, the transcriptome of the new microglia aligned well with those of other induced microglia, and of primary microglia that had been maintained in culture for a week. The transcriptome diverged from those reported for other types of myeloid cells such as monocytes and dendritic cells. While it also differed substantially from the transcriptome of freshly extracted microglia, it matched where it counts most: in expression of microglial signature genes (Hickman et al., 2013; Butovsky et al., 2014; Bennett et al., 2016). Livesey told Alzforum that while it would be preferable to generate microglia exactly like those in the brain, it may not be possible to escape what he called the “culture artifact.” He believes the PMP-derived microglia are a useful model to study processes in the brain.
In keeping with the status of such professional phagocytes, the induced microglia efficiently engulfed bacterial particles. And, when doused with lipopolysaccharide, the cells ramped up expression of inflammatory cytokines, including IL-1β, TNF-α, and IL-6. To see if the cells would interact appropriately with neurons, the researchers added them to three-dimensional cortical organoids. The microglia migrated deeply into the organoids and set up shop, extending probing processes to monitor their new environs.
In a first stab at using these microglia to investigate processes related to neurodegeneration, the researchers next turned their sights on TREM2. They generated microglia from four iPSC sources: a patient with frontotemporal dementia who carried two copies of the T66M mutation in TREM2; two unaffected family members who carried only one copy of the mutant gene; and one person who carried two copies of the W50C mutation that causes Nasu-Hakola disease (NHD). All of the iPSC lines appeared to differentiate into microglia-like cells as well as those carrying two normal copies of TREM2. Using antibodies specific for different regions of TREM2, the researchers found that neither the W50C nor T66M variant trafficked normally to the cell surface, where the extracellular N-terminal domain is often shed following cleavage via ADAM10 protease. This cleavage leaves behind a C-terminal stub. Lysates from microglia bearing homozygous T66M or W50C mutations were devoid of this C terminal fragment. Overall, the findings indicated that the mutations disrupted TREM2 trafficking and processing, in agreement with studies reporting reduced maturation of the mutant protein in a mouse microglial cell line and in human embryonic kidney cells (Kleinberger et al., 2014).
How would this stunted maturation affect microglial responses? To address this, the researchers first provoked the T66M and W50C TREM2 microglia with lipopolysaccharide (LPS). They found no deficits in the inflammatory responses as compared with induced microglia carrying wild-type TREM2. Normal and mutant microglia also engulfed fluorescently labeled E. coli with equal gusto. A previous study reported that TREM2 facilitates the uptake of acetylated lipoproteins, along with associated Aβ, and that pathogenic TREM2 mutations or TREM2 deficiency impaired this process (Yeh et al., 2016). However, the mutant cells had no problem engulfing these lipoproteins. The results are seemingly at odds with previous studies reporting that the mutations affect phagocytic and other microglial functions (see May 2017 news).
Brownjohn and Livesey told Alzforum that the findings point to compensatory mechanisms—myriad other phagocytic receptors, for example—that could make up for TREM2’s loss of function in the mutant microglia.
In a joint comment to Alzforum, Christian Haass of the German Center for Neurodegenerative Diseases and Dominik Paquet of University Hospital, both in Munich, agreed that compensatory mechanisms were likely afoot, pointing out that they, too, had observed similar compensatory effects when assessing antibody-mediated phagocytosis of Aβ plaques in TREM2-deficient cells (Xiang et al., 2016).
Damian Crowther at AstraZeneca in Cambridge, England, offered a cautionary note. He referred to LPS as the “nuclear option,” pointing out that it maxes out pro-inflammatory responses and even downregulates TREM2 expression. This could mask differences between TREM2 mutants and wild-type cells, he said. “Of greater interest in neurodegenerative disease might be the interaction of the TREM2 pathway with weaker TLR4 agonists, such as amyloid-containing debris (reviewed in Molteni et al., 2016),” he wrote.
Likewise, Oleg Butovsky of Brigham and Women’s Hospital in Boston said he would like to see how the transcriptomes of the microglial cells change in response to disease-relevant stimuli, such as material from dying neurons. He previously reported that resting microglia express a homeostatic gene-expression signature, which is rapidly lost upon stimulation of TREM2 by neuronal detritus (see Feb 2015 conference news; Sep 2017 news). Whether the TREM2 mutant microglia developed by Brownjohn and colleagues are deficient in changing gears will be a crucial next question to address with this beautiful cellular model, he said.
Brownjohn and Livesey told Alzforum that these experiments are already in the works, assessing how the cells migrate and respond to multiple threats in coculture with neurons.
How do Brownjohn’s microglia measure up to cells developed by other protocols? The jury is still out on that, according to Haass and Paquet. “At the current—still pioneering— state of the field, independent studies confirming transferability of protocols, validity of cellular fates/transcriptomics, as well as applicability for disease research are crucial. The recent paper by the Livesey lab therefore is a welcome addition,” they wrote.
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