The APOE gene exacerbates pathology not only of the amyloid variety, but tau, as well. Previous research has tied ApoE’s effect on tau to microglial activation, but how does ApoE trigger microglia? In the June 10 Neuron online, scientists led by David Holtzman at Washington University, St. Louis, implicate the low-density lipoprotein receptor (LDLR), one of two ApoE receptors. In a tauopathy mouse model, overexpressing LDLR lowered ApoE and calmed microglia. This, in turn, slowed tau pathology and neurodegeneration. Curiously, excess LDLR was also a good thing for other glia. It dampened astrocyte reactivity and boosted the number of oligodendrocyte precursor cells (OPCs), bolstering myelination.
- In tauopathy mice, increased LDLR dramatically lowered ApoE in the brain.
- This shifted microglia from a DAM into protein clearance/neuron support mode ...
- ... which, in turn, hit the brakes on tangles and neurodegeneration.
Holtzman noted that LDLR has been highly studied for cardiovascular diseases, but less so in the brain. The new findings imply that this receptor can protect neurons and might make a promising therapeutic target in AD and other tauopathies, he told Alzforum.
Others said the findings fit with existing literature on lipid dysregulation in neurodegenerative disease. “This is a great paper that highlights the important role of apolipoproteins and lipids in mediating the pathophysiology of AD,” noted Shane Liddelow at New York University.
LDLR: Guardian Angel? Synapses (red) are dense in wild-type mouse hippocampus (left), lost in P301S mouse hippocampus (center), and partially preserved by LDLR overexpression (right). [Courtesy of Shi et al., Neuron.]
The Holtzman group previously found that ApoE, especially the AD risk allele E4, drove P301S tauopathy mice to accumulate phosphorylated tau. This dialed up neuroinflammation, accelerating brain atrophy (Sep 2017 news). Later studies by this and other groups implicated microglia (Oct 2019 news; Mancuso et al., 2019).
To further dissect ApoE biology, Holtzman turned to its LDLR. He had previously found that amping up LDLR levels in amyloidosis mouse models not only squelched Aβ and amyloid plaques, but also slashed the amount of soluble ApoE by 90 percent (Dec 2009 news). This is because when ApoE binds LDLR, the complex is internalized and degraded (Heeren et al., 2006).
What would boosting LDLR do in a tauopathy model? To answer this, first author Yang Shi crossed P301S mice with LDLR over-expressers. By 9 months of age, P301S mice normally have extensive loss of synapses and gray matter in the hippocampus. LDLR overexpression roughly halved synapse loss and atrophy (see image above). It also cut the amount of soluble p-tau in half. Knocking out APOE similarly protects these mice, strengthening the idea that LDLR acts via its effect on ApoE.
Most cell types in the brain express LDLR. To pin down its effects on microglia, the authors isolated them from P301S/LDLR mice. Microglia from 3-month-old mice made about half as much ApoE as did wild-types. In 20-month-old P301S/LDLR mice, the difference was more dramatic, with ApoE slashed by about 90 percent. Normally, ApoE expression increases with age, helping trigger the disease-associated microglial (DAM) activation state (Jun 2017 news; Sep 2017 news). LDLR overexpression seemed to prevent this age-associated microglial increase in ApoE. In keeping with this, microglia from 20-month-old mice expressed few DAM genes, suggesting this activation state was suppressed.
State Shifting. In wild-type mice (left), homeostatic microglia (red and blue) predominate, while in P301S mice (middle), the majority are DAM (green). When LDLR expression rises (right), a subtype (russet) that supports neuronal function replaces DAM. Lysosomal specialist microglia (yellow) also crop up in the presence of tau tangles. [Courtesy of Shi et al., Neuron.]
Single-nucleus RNA-Seq of hippocampal cells from 9-month-old P301S/LDLR mice and controls bolstered this idea. In P301S mice, nearly two-thirds of microglia had the DAM phenotype; in LDLR over-expressers, almost none did (see image above). Instead, P301S/LDLR hippocampi contained a microglial subtype not found in wild-type mice. This subtype, which made up about 10 percent of the microglia in their brains, ramped up lysosomal degradation while turning down cellular metabolism and protein synthesis. It seems to represent an alternate activation state specialized to clear proteins and lipids. The same subtype occurred in APOE knockouts.
Steve Barger at the University of Arkansas for Medical Sciences in Little Rock was intrigued by this lysosomal phenotype. His work has shown that ApoE binds to a DNA sequence that serves as a master regulator of lysosomal genes, suppressing their expression (Parcon et al., 2018). Thus, depleting ApoE may boost lysosomal gene expression and improve protein clearance. “A generalized effect of ApoE4 on autophagy may explain why it worsens conditions involving aggregation of Tau, α-synuclein, or TDP43, irrespective of Aꞵ,” Barger noted (full comment below).
This “lysosome specialist” microglia was not the only new subtype in P301S/LDLR mice. Almost half their microglia assumed a unique phenotype not seen in wild-type or P301S mice. These microglia suppressed DAM and antigen-presenting genes, while boosting ion channels, neurotransmitter receptors such as NMDAR, and genes that regulate synaptic plasticity. Possibly, these microglia support neuronal functions. Curiously, this subtype was absent in APOE knockouts.
“It would be very interesting to study this sub-population in more detail and evaluate if it is also abundant in other neurodegenerative diseases or affected by Aβ pathology,” Susanne Krasemann at University Medical Center in Hamburg, Germany, wrote to Alzforum (full comment below).
LDLR overexpression also soothed astrocytes. Almost two-thirds of astrocytes in P301S mice are reactive, but in P301S/LDLR mice, a fifth were, barely more than in wild-type. Reactive astrocytes turn up production of lipids and extracellular matrix proteins, as well as proinflammatory cytokines (Aug 2019 news). Other work by the Holtzman group has reported benefits from lowering astrocytic ApoE in tauopathy models (Apr 2021 news).
Perhaps most surprisingly, LDLR overexpression nearly doubled the number of OPCs. And it showed: P301S/LDLR mice had more intact myelin than did P301S controls, hinting that the larger progenitor crew repaired damage to the brain’s insulation. Knocking out APOE had similar effects. Wild-type mice accrue myelin damage with age and, intriguingly, LDLR overexpression or APOE knockout both ameliorated this. The data suggest that targeting this system could improve the health of the aging brain, Holtzman said.
Would increasing LDLR be a good strategy in Alzheimer’s, where both plaques and tangles plague the brain? This is unclear, as some evidence suggests activated microglia, with high ApoE and TREM2, help contain amyloid plaques (Jul 2019 conference news; Mar 2020 news). However, this may depend on disease stage, noted Julia TCW at Icahn School of Medicine in New York (full comment below). TREM2 expression helps early in disease, when plaques are forming, but can become harmful later, when tau pathology predominates (Oct 2017 news; Apr 2020 news). Holtzman plans to probe the issue in mouse models that develop both pathologies.
So, how about targeting LDLR in people? Because of the receptor’s central importance in cardiovascular disease, numerous medications have been developed to boost it. For example, statins increase LDLR in the liver. The challenge is to get these medications into the brain, Holtzman said.
Joachim Herz at the University of Texas Southwestern Medical Center in Dallas agreed. “This paper raises the question of whether it is practical and clinically safe … to design lipophilic statins that preferentially partition into the brain,” he wrote to Alzforum (full comment below). Meanwhile, Holtzman is exploring small molecule or gene-based approaches to boost LDLR in the brain.—Madolyn Bowman Rogers
Research Models Citations
- ApoE4 Makes All Things Tau Worse, From Beginning to End
- In Tauopathy, ApoE Destroys Neurons Via Microglia
- Mind Over Heart—LDL Receptors Crimp ApoE, Aβ Accumulation
- Hot DAM: Specific Microglia Engulf Plaques
- ApoE and Trem2 Flip a Microglial Switch in Neurodegenerative Disease
- ApoE4 Glia Bungle Lipid Processing, Mess with the Matrisome
- Squelching ApoE in Astrocytes of Tau-Ravaged Mice Dampens Degeneration
- TREM2, Microglia Dampen Dangerous Liaisons Between Aβ and Tau
- Paper Alert: Mouse TREM2 Antibody Boosts Microglial Plaque Clean-Up
- Changing With the Times: Disease Stage Alters TREM2 Effect on Tau
- With TREM2, Timing Is Everything
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