ApoE4 delivers a blow to neurons in mouse models of tauopathy but, according to a paper published October 10 in the Journal of Experimental Medicine, it only does so if mice have their microglia. Researchers led by David Holtzman at Washington University in St. Louis reported that ridding the brain of these immune cells pre-empted this neurodegeneration. ApoE knockouts were protected from tau toxicity and neurodegeneration, as well, regardless of whether they had microglia, the authors found. The work suggests that a toxic alliance between ApoE4 and microglia accelerates tau pathology.
- In mice, ApoE4 drives tau pathology and neurodegeneration.
- Depleting microglia prevented shrinkage of the brain.
- It also halted the spread of tau pathology throughout the brain.
“Overall, the findings place ApoE and microglia as dominant contributors to neurodegeneration,” said Oleg Butovsky of Brigham and Women’s Hospital in Boston.
Holtzman and colleagues previously reported that all three isoforms of ApoE, but ApoE4 in particular, fueled neuroinflammation, tau pathology, and neurodegeneration in P301S mice. Deleting their endogenous ApoE, or the microglial receptor TREM2, for that matter, largely spared neurons (Apr 2017 conference news; Sep 2017 news; Oct 2017 news on Leyns et al., 2017). Around the same time, TREM2 and ApoE were placed at the heart of microglia’s so-called neurodegenerative phenotype (Sep 2017 news). This raised the question of whether ApoE’s effect on tau and neurons depends upon microglia.
To investigate, first author Yang Shi and colleagues wiped out microglia entirely using PLX3397, an inhibitor of the colony-stimulating factor 1 receptor. Microglia need CSF1R signaling to survive. Shi treated four different mouse lines with PLX3397: P301S-tau mice; P301S expressing human ApoE4; P301S mice with ApoE knocked out; and wild-type mice with ApoE knocked out. They started lacing the mice’s chow at 6 months of age, when tau pathology in tau/ApoE4 mice is still at an early stage and neurons are not degenerating yet, and settled on a protocol that eliminated all microglia in male mice within 21 days. Because they were unable to completely wipe out microglia in females, the researchers used males throughout the study.
When the mice were 9 months old, the researchers observed a striking result: microglial depletion completely protected the tau/ApoE4 mice from neurodegeneration. Tau/ApoE4 mice on control chow had up to 50 percent smaller hippocampi and entorhinal/piriform cortices, respectively, and a massively enlarged brain ventricle, compared with their counterparts on PLX3397 (see image above). Tau/ApoE KO mice were likewise protected. Overall, the findings suggest that ApoE drives neurodegeneration in the tauopathy mice primarily via microglia.
In fact, removing microglia completely blocked the typical march of tau pathology through the brains of the tau/ApoE4 mice. Previously, the scientists used the AT8 antibody to describe four stages of worsening neurodegeneration. Stage 1 was marked by intense p-tau staining in hippocampal mossy fiber axons, with but diffuse staining in a smattering of neuronal cell bodies in the granule cell layer and CA1 regions. As disease progressed, p-tau accumulated further in these cell bodies, spread to other hippocampal neurons, and clumped up into tangles. The entire hippocampus was covered with p-tau by stage 4, which correlated with the most brain atrophy (Shi et al., 2017). Applying this scheme, the researchers found that 9-month-old tau/ApoE4 were predominantly stage 3 and 4, while both tau/ApoE knockouts and microglial knockouts were stage 1.
Might microglia harm neurons by raising expression of ApoE in other cells? Indeed, the authors found that depleting microglia caused astrocytes and, to a lesser extent, a subset of neurons in the hippocampus, to pump out more ApoE; however, since these mice were protected from neurodegeneration, the finding suggested that in the absence of microglia, ApoE was rendered toothless.
And how was ApoE affecting microglia? Shi found that it did not change their number, but did increase their expression of CD68, a lysosomal marker of microglial activation. In contrast, neither astrocyte number nor activation were affected by expression of ApoE or by the presence/absence of microglia. These findings suggested that ApoE4 likely kills neurons in the tauopathy mice by altering microglial function.
While Holtzman’s paper was in press, another study, led by Renzo Mancuso of the University of Southampton, U.K., reported similar data. These researchers used another CSF1R inhibitor, JNJ-527, to partially deplete microglia and prevent their proliferation (see Mancuso et al., 2019). In the spinal cords of P301S mice, JNJ-527 docked microglial numbers by more than half in the spinal cord. This assuaged neuroinflammation there, reduced phospho-tau, and spared motor neurons.
Kim Green of the University of California, Irvine, called the effects of removing microglia “stunning.” He said that the finding builds on the emerging theme that microglia play a critical role in neurodegeneration. Green, who collaborated with Plexxikon in Berkeley, California, to develop PLX3397, previously reported that microglial depletion prevented the development of Aβ plaques in the 5xFAD model of amyloidosis (Sep 2019 news), while depleting microglia after plaques had already formed slowed synapse loss around dystrophic neurites (Spangenburg et al., 2016). The Glabe lab at UCI reported similar results (Mar 2018 news). Together with the present work, these studies suggest microglia can exacerbate different types of proteinopathy. Exactly how ApoE mediates the damage microglia inflict in response to different types of pathology and at different stages of disease is now a critical line of research, Green said.
Holtzman agreed, pointing out that he has not nailed down the source of the ApoE that promotes microglia-mediated neurodegeneration. Butovsky believes it is microglial. Conditional microglial ApoE knockouts could confirm that. Butovsky added that non-pharmacological approaches, such as conditional knockout of CSF1R, are needed to deplete microglia without confounding effects on other cell populations. Indeed, Holtzman and colleagues found that PLX3397 altered blood cell counts, including the number of red blood cells, monocytes, and T cells. While the inhibitor has also been reported to bind CSF1R on neurons, the researchers do not believe this explains the protection in tau/ApoE4 mice, since PLX3397 had a detrimental effect on hippocampal neurons in culture, preventing neurite growth.
Susanne Krasemann of University Medical Center in Hamburg, Germany, said that the findings dovetail with previous work implicating ApoE and TREM2 as flippers of a microglial neurodegenerative switch (Krasemann et al., 2017). Because microglial depletion boosts astrocytic ApoE expression without harming neurons, preventing aberrant microglial responses, rather than lowering ApoE levels per se, could be a therapeutic strategy for neurodegenerative disease, she said.
Holtzman and Butovsky agree it will be important to consider disease stage when targeting microglia. For example, they might exercise important protective functions early on but deliver a toxic blow to neurons in later stages (May 2016 news; Jan 2019 news).
Another unknown is whether tau pathology harms neurons directly, or if activated microglia are responsible for all the damage, Holtzman said. It is clear from previous studies that accumulation of phospho-tau incites harmful microglial responses, including induction of the complement cascade (Jul 2018 conference news; Aug 2019 news). Holtzman speculated that low amounts of phospho-tau, as seen in early stages of disease, trigger microglia, and the inflammation they subsequently unleash drives the lion’s share of the ensuing neurodegeneration. Of course, activated microglia also drove p-tau accumulation itself. Holtzman proposed that this progression could reflect a secondary response to the neuronal damage inflicted by the activated microglia.—Jessica Shugart
Research Models Citations
- ApoE and Tau: Unholy Alliance Spawns Neurodegeneration
- ApoE4 Makes All Things Tau Worse, From Beginning to End
- Changing With the Times: Disease Stage Alters TREM2 Effect on Tau
- ApoE and Trem2 Flip a Microglial Switch in Neurodegenerative Disease
- Are Microglia Plaque Factories?
- Wiping Out Microglia Prevents Neuritic Plaques
- Barrier Function: TREM2 Helps Microglia to Compact Amyloid Plaques
- Without TREM2, Plaques Grow Fast in Mice, Have Less ApoE
- Synaptic Tau Clangs the Dinner Bell for Hungry Microglia
- Nixing Complement Protein Protects Neurons in Tauopathy Model
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