14 Jul 2024

Microglia respond to proteinopathies by changing their shapes and expression profiles in different ways. Here’s one that has been somewhat overlooked: rod-shaped microglia. These long, straight cells have been observed in several neurodegenerative diseases, including Alzheimer’s, Parkinson’s, and Huntington’s, but few studies have characterized what they do. In a July 2 preprint on bioRxiv, researchers led by Long-Jun Wu at the University of Texas Health Science Center in Houston shed some light on this, and also reported the first evidence of rod-shaped microglia in the brains of people who died from amyotrophic lateral sclerosis, a TDP-43 proteinopathy.

In a mouse model, the cells popped up in the motor cortex soon after turning on a TDP-43 transgene. They snuggled up to pyramidal cell dendrites and engulfed excitatory synapses, dampening network activity. Preventing these microglia from forming, on the other hand, amped up neuronal firing, worsened motor abilities, and hastened death. “Our results suggest that rod-shaped microglia play a neuroprotective role by attenuating cortical hyperexcitability,” the authors concluded.

Adam Bachstetter at the University of Kentucky, Lexington, called the functional findings a major discovery. He also appreciated the study’s identification of potential markers for these cells, including galectin-3, which supports cell-cell signaling among other roles. “Utilizing new, more specific tools to study rod-shaped microglia could significantly advance our understanding of how microglia promote healthy aging and combat disease,” he wrote to Alzforum (comment below).

Ironically, Wu and colleagues did not set out to study rod-shaped microglia. They were interested in how microglia sculpt neuronal circuits through interactions with synapses (Umpierre and Wu, 2021; Zhao et al., 2024). In particular, they wondered if microglia help counteract the hyperexcitability that occurs in the motor cortex early in the course of ALS.

To investigate, they used rNLS8 mice, whose inducible transgene overexpresses human TDP-43 without a nuclear localization signal. When the transgene is switched on via the mice’s diet, the truncated protein accumulates in inclusions in the cytoplasm, mimicking the proteinopathy of ALS. First author Manling Xie turned on this transgene in 2-month-old adult mice while monitoring neuronal activity via probes implanted in the motor cortex. Almost immediately, excitatory pyramidal cells revved up, firing 50 percent more often. This hyperactivity peaked after two to three weeks, dropping back to baseline or even below.

What did hyperactivity do to brain cells? The authors sacrificed mice at 3 weeks and analyzed gene expression in several regions using spatial and single-cell RNA-Seq. Most expression changes occurred in microglia in the cortex. These cells turned on genes characteristic of disease-associated microglia (DAM), including TREM2, APOE, and galectin-3 (Jun 2017 news).

When the authors immunostained cortical brain sections for activated microglia, they were struck by how many of them had straightened into a rod shape. Before transgene activation, rod-shaped cells were sparse, making up five percent of all microglia. After activation, however, their number swelled, peaking at about one-third of all microglia three weeks later, around the time neuronal hyperactivity peaked. The rod microglia seemed to be a subtype of DAM, expressing genes involved in phagocytosis, cell adhesion, and cell remodeling. Almost two-thirds of them expressed galectin-3, hinting at a potential marker gene.

What did the rod microglia do? Immunostaining showed these long cells aligning themselves along the apical dendrites of pyramidal cells. In some cases, their processes embraced the dendrite; in others, the whole cell body made contact, or even surrounded the dendrite (see image above). Tellingly, these microglia often contained pre- and post-synaptic material engulfed from excitatory synapses (see below). To the authors, the data suggested rod-shaped microglia were pruning synapses to restore homeostasis in the motor cortex.

Xie and colleagues tested this idea by crossing rNLS8 mice with TREM2 knockout mice. TREM2 regulates the transition to DAM (Jul 2018 conference news). As expected, in its absence the rod-shaped microglia did not form. As a result, pyramidal cells fired more wildly. The mice were less coordinated, falling off a rotating rod more quickly. Most dramatically, survival time shrank. Three-fourths of rNLS8 TREM2 knockouts died within a month of TDP-43 transgene activation, compared with about 10 percent of rNLS8 mice that had TREM2. Rod-shaped microglia help protect the brain, the authors concluded.

The authors extended the findings to ALS brain, analyzing postmortem motor cortices from 31 patients and 25 age-matched controls. As in the rNLS8 mice, rod-shaped microglia were abundant in the ALS samples, making up about 15 percent of all microglia, but were nearly nonexistent in control brain (see image to left). In AD, PD, HD, and dementia with Lewy bodies, they make up about 10 percent of all microglia, as well (McGeer et al., 1988; Sapp et al., 2001; Bachstetter et al., 2015). They have also been seen in the APP^NL-F mouse model of amyloidosis, and ramp up in APP^NL-F and P301S crosses, suggesting tau pathology might energize them further (Dec 2016 conference news).

“Given Long-Jun Wu’s lab expertise in imaging of microglia in vivo, I am incredibly excited to see where this project goes next,” wrote Bachstetter.  One question that needs more work is whether the cells in question are truly microglia or might possibly be infiltrating macrophages, wrote Neta Rosenzweig, Brigham and Women’s Hospital, Boston (comment below).—Madolyn Bowman Rogers

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References

News Citations

1. Hot DAM: Specific Microglia Engulf Plaques 9 Jun 2017
2. TREM2: Diehard Microglial Supporter, Consequences Be DAMed 13 Jul 2018
3. Next-Generation Mouse Models: Tau Knock-ins and Human Chimeras 8 Dec 2016

Paper Citations

1. Umpierre AD, Wu LJ. How microglia sense and regulate neuronal activity. Glia. 2021 Jul;69(7):1637-1653. Epub 2020 Dec 28 PubMed.
2. Zhao S, Umpierre AD, Wu LJ. Tuning neural circuits and behaviors by microglia in the adult brain. Trends Neurosci. 2024 Mar;47(3):181-194. Epub 2024 Jan 19 PubMed.
3. McGeer PL, Itagaki S, Boyes BE, McGeer EG. Reactive microglia are positive for HLA-DR in the substantia nigra of Parkinson's and Alzheimer's disease brains. Neurology. 1988 Aug;38(8):1285-91. PubMed.
4. Sapp E, Kegel KB, Aronin N, Hashikawa T, Uchiyama Y, Tohyama K, Bhide PG, Vonsattel JP, Difiglia M. Early and progressive accumulation of reactive microglia in the Huntington disease brain. J Neuropathol Exp Neurol. 2001 Feb;60(2):161-72. PubMed.
5. Bachstetter AD, Van Eldik LJ, Schmitt FA, Neltner JH, Ighodaro ET, Webster SJ, Patel E, Abner EL, Kryscio RJ, Nelson PT. Disease-related microglia heterogeneity in the hippocampus of Alzheimer's disease, dementia with Lewy bodies, and hippocampal sclerosis of aging. Acta Neuropathol Commun. 2015 May 23;3:32. PubMed.

Further Reading

News

1. ApoE and Trem2 Flip a Microglial Switch in Neurodegenerative Disease 24 Sep 2017
2. In TDP-43 Mouse, Hesitant Microglia Eventually Swoop to the Rescue 6 Mar 2018