01 Sep 2017

A paper in the September 1 Nature claims a cadre of rogue microglia are all it takes to orchestrate neurodegeneration. Researchers led by Frederic Geissmann and Omar Abdel-Wahab of Memorial Sloan Kettering Cancer Center in New York, and Marco Prinz of the University of Freiburg in Germany, induced a somatic mutation in about 10 percent of microglia that switched on ERK kinase signaling. The mice later developed a severe neurodegenerative disease that paralyzed them. The researchers determined that damaging inflammation caused by the mutated microglia was likely to blame. The findings raise the possibility that similar somatic mutations in people are responsible for a rare neurodegenerative disease that occurs inchildren.

This is a great paper for many reasons, commented Bart De Strooper of the Dementia Research Institute in the U.K. I am particularly excited about the concept of acquired genetic mosaicism as a cause of neurodegenerative disorder. The paper also shows that microglia mutations can be directly causative inneurodegeneration.

Most famous for their role in causing cancer, somatic mutations can spontaneously arise in any cell, sometimes giving it a proliferative edge. Mutations in the RAS-MEK-ERK signaling pathway, for example, can cause diseases called histiocytoses if they arise in the myeloid cell lineage, which gives rise to blood and immune cells, including macrophages and microglia. Histiocytoses manifest in different ways, including leukemias, other tumors, and malfunctions in multiple organs. Mysteriously, a small fraction of carriers also get a neurodegenerative disease that manifests between childhood and middle age, with symptoms such as cerebellar ataxia and tremor (Lachenal et al., 2006; Wnorowski et al., 2008). The reason for the neurodegeneration has been amystery.

Geissmann and colleagues speculated it could be caused by microglia descended from erythro-myeloid progenitor cells (EMPs) harboring the same RAS-MEK-ERK somatic mutations. EMPs arise in the embryonic yolk sac early in development, and give rise to microglia in the brain and macrophages in other tissues (Perdiguero et al., 2014; Feb 2015 conference news).In contrast, circulating monocytes are continually replenished by hemotopoietic stem cells (HSCs) in the bonemarrow.

Doomed During Development? Histiocytoses arise from somatic mutations in hematopoietic stem cells (HSCs, left) or in erythro-myeloid progenitor (EMP) cells (right), which give rise to macrophages and microglia. The mutant microglia may cause inflammation, leading to neurodegeneration. [Courtesy of Tarnawsky and Yoder, Nature, News & Views,2017.]

To find out if somatic mutations in EMPs could beget microglia that trigger neurodegeneration, first author Elvira Mass and colleagues induced a somatic mutation that causes histiocytoses into mice. They chose the V600E variant of the BRAF gene, a substitution that switches on ERK signaling. The researchers generated transgenic mice carrying an inducible copy of the mutated BRAF gene, which could only be switched on via tamoxifen-induced Cre recombination in EMPs. This also turned on yellow fluorescent protein so the researchers could identify the cells. At embryonic day 8.5, they injected pregnant mice with a teeny dose of the drug to ensure that only a fraction of the embryos EMPs would express the mutation. About 10 percent of tissue resident macrophages, including microglia, in the resulting offspring expressed V600E BRAF at one month ofage.

The mutant microglia took up their positions in the brain, but were different from their normal counterparts from the get-go. Those carrying the V600E BRAF expressed elevated markers of proliferation, ERK signaling, and inflammation. In one-month-old mice, these feisty microglia had yet to cause trouble, but by four months of age, the researchers noticed neurological symptoms in the mice, including loss of hind limb reflexes and shortened stride. At seven months, 90 percent of the animals were affected and by nine months 60 percent of the mice had full hind limb paralysis. These symptoms, similar to cerebellar ataxia, are common in people with cerebral histiocytoses. Feeding the mice a BRAF inhibitor starting at one month of age drastically delayed onset and slowedprogression.

Compared to wild-type mice (left), animals with induced BRAF mutations in their EMPs had an expansion of mutant microglia expressing YFP in their spinal cord (middle). Microglia also expressed the activation marker CD68 (top) and phosphorylated ERK (bottom). [Courtesy of Mass et al., Nature2017.]

The researchers next searched for pathological changes that could have triggered the disorder. In month-old mice, the researchers found signs of elevated microglial and astrocyte activation, but not neuronal death. Oddly, by immunohistochemistry using the 22C11 antibody, the researchers noticed deposits of amyloid precursor protein (APP) in the inflamed areas, a phenomenon that Geissmann attributed to release of the membrane protein from newly damaged axons. In six-month-old animals, large clusters of activated, phagocytic microglia carrying the BRAF mutation crowded in the thalamus, brain stem, cerebellum, and spinal cord. These same regions were rife with synaptic and neuronal loss, demyelination, and astrogliosis. The mutant microglia had a small proliferative advantage compared with their wild-type counterparts, but Geissmann attributed the bulk of the neuronal damage to the activation of the cells, rather than their expansion. Treatment with a BRAF inhibitor mitigated theseresponses.

Gene expression analysis of mutant microglia taken from paralyzed mice revealed the differential expression of around 8,000 genes, 80 percent of which were upregulated compared to microglia from control mice. These genes included a bevy of pro-inflammatory mediators, including cytokines, phagocytosis boosters, matrix proteins, and growthfactors.

For some reason, the thalamus, brain stem, cerebellum, and spinal cord were uniquely vulnerable to the presence of the V600E BRAF mutant cells. Tissue macrophages carrying the mutation also expanded in the liver, spleen, kidney, and lung, even more so than in the brain, but did not cause damage in those regions. Geissmann speculated that differences in the tissue microenvironment could play a role in this selective vulnerability. For example, normal liver macrophages are in a near constant state of activation, Geissmann said, so the organ is equipped to deal with them. Perhaps the posterior part of the brain is unaccustomed to constant microglial activation, he said. Indeed, chronic microglial activation occurs during AD as well, and appears to ultimately inflict damage, rather than helpfulresponses.

Finally, the researchers investigated whether patients with histiocytoses also had abnormal microglia. They analyzed postmortem brain tissue from three patients with Erdheim-Chester disease (ECD), and conducted gene expression analysis on brain biopsies from one person with Langerhans cell histiocytosis (LCH), and another with juvenile xanthogranuloma (JXG). All of these patients had neurodegenerative disease associated with their histiocytoses, which were all caused by BRAF V600E mutations. In the ECD samples, the researchers spotted abundant activated microglia gathered at sites of neuronal loss, astrogliosis, and demyelination. Compared with data from five control samples, gene expression analysis on the JXG and LCH samples revealed an upregulation of genes in the MAPK pathway, including multiple pro-inflammatorycytokines.

The findings support the idea that activated microglia wreak havoc in the brain and cause neurodegeneration in people withhistiocytoses.

For a somatic mutation to have an effect, affected cells must propagate sufficiently. EMPs proliferate during early development, making it a prime time for mutant clones to multiply, Geissmann said. Perhaps the number of mutant clones born during the EMP stage would suffice to harm neurons, he said. However, if microglia are also bestowed with a proliferative edge, this would likely exacerbate the damage, he added. Either way, Geissmann proposed that inhibitors of ERK signaling might thwart neurodegeneration when mutant microglia areinvolved.

In an accompanying editorial, Stefan Tarnawsky and Mervin Yoder at Indiana University in Indianapolis noted opportunities for better diagnosis in this scenario. When somatic mutations occur in EMPs during early development, macrophages in many regions of the body will likely carry the mutations, not just microglia in the brain. This suggests that it might be possible to collect macrophage samples from more easily accessible, non-CNS tissues to look for biomarkers when diagnosing microglia-related disease, theywrote.

What about somatic mutations that might arise later in life, when tissue resident macrophages or microglia are already nestled into their permanent residences? Though recent studies reported that microglia are relatively long-lived cells, they proliferate in response to threats (Aug 2017 news),perhaps setting the stage for expansion of mutant cells, Geissmann speculated. That said, beyond people with histiocytoses, the contribution of somatic mutations in microglia to neurodegenerative disease is unclear. De Strooper and others have reported that genetic mosaicism in neurons could cause neurodegeneration (Jul 2015 news). A major impediment to studying this phenomenon is that somatic mutations that arise in the brain go undetected in standard genomic sequencing.JessicaShugart

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Somatic SNAFUCan a Few Mutant Microglia Cause Neurodegenerative Disease? - Alzforum