Inflammation is a fickle friend indeed. Some pathways have benefits, while others do great harm. Two new papers add data to the latter category. Scientists led by Li Gan, University of California, San Francisco, report that inflammatory signaling in microglia, brought on when progranulin goes missing, causes mice to groom themselves incessantly. Appearing in the April 24 Early Edition from the Proceedings of the National Academy of Sciences USA, the mouse data show similar compulsive behavior as seen in people carrying progranulin mutations that cause frontotemporal dementia (FTD). The results suggest that hyperactivation via the microglial NFκB-tumor necrosis factor alpha axis may mediate some of the behavioral problems in FTD. A second study in the April 24 Cell Reports links excess immune signaling to premature neurodegeneration and early death in flies. Petros Ligoxygakis, University of Oxford, U.K., reports that knocking back the NFκB signaling pathway extends lifespan, while revving it up shortens it. In both studies, scientists find that dialing back similar inflammatory signals rescues behavior and early death.
People with mutations in the progranulin (PGRN) gene develop FTD, which is marked by compulsive behaviors, social withdrawal, and loss of empathy. PGRN knockout mice recapitulate some of those behaviors, including a tendency to compulsively scratch at their mouths and backs to clean themselves (see Lui et al., 2016). While wild-type mice prefer the company of other mice, PGRN knockouts are just as likely to hang out with inanimate objects (Yin et al., 2010).
These progranulin-deficient mice reportedly express more proinflammatory cytokines, such as TNFα, and have a larger number of active microglia (Yin et al., 2010). Gan and colleagues wondered whether the excess inflammation in these mice might explain their behavior.
To find out, co-first authors Grietje Krabbe, Sakura Minami, and Jon Etchegaray compared PGRN knockout mice with those that also lacked one or both copies of TNFα. While the PGRN knockouts cleaned themselves incessantly, the double transgenics did so about a third less, comparable to wild-type mice. They still spent the same amount of time with another mouse as with an inanimate object, however, suggesting TNFα signaling did not account for the antisocial behaviors.
Compulsive behavior has been traced back to hyperactive medium spiny neurons in the nucleus accumbens (Ahmari et al., 2013). In the present study, whole cell patch-clamp recording confirmed that knocking out PGRN in mice caused these medium spiny neurons to fire faster than normal. However, taking out one or both copies of TNFα slowed that firing back down to wild-type levels.
Microglia seemed especially sensitive to these genetic alterations. In progranulin knockouts, microglia extended fewer finger-like projections into the environment to probe for signs of trouble than controls (see image above). They also reacted more sluggishly to a laser-induced injury (see video below). In fact, knocking out progranulin in microglia alone was enough to elicit the compulsive self-grooming behavior in mice. What’s more, deleting NF-κB—a master regulator of TNFα (see image below)—in the microglia of mice with no progranulin restored this behavior to normal, along with other behaviors that had gone haywire in these mice, including social deficits and excessive marble burying. This suggests NF-κB signaling has a broader influence on behavior.
Together, the results support microglial NF-κB and TNFα signaling as an operative pathway in PGRN-deficient FTD, wrote the authors. “We now have direct proof that microglia abnormalities contribute to FTD-like behavior,” said Gan. “That had not been shown before.” Her group saw similar compulsive behaviors in people with PGRN mutations, such as obsessive hygiene and collecting objects. “They carry out these repetitive behaviors because their circuit drives them to do it—they just can’t stop,” Gan went on. Previous studies confirmed high levels of TNFα in the plasma of these patients (Miller et al., 2013). Gan’s research now suggests these behaviors could be calmed by modulating the inflammatory pathways in microglia. “It provides a new angle for intervention in FTD,” Gan said. She next plans to work out how TNFα changes the intrinsic excitability of medial spiny neurons, and whether it can be reversed with therapy.
“Taken together with other published literature, it is clear that genetic ablation of progranulin leads to profound neuroinflammation and alterations in microglia activities,” wrote Malú Tansey and Thomas Kukar, Emory University School of Medicine, Atlanta, to Alzforum (see full comment below). They cautioned that TNF-deficient mice also lack many other cytokines and chemokines, so drugs that target TNFα would help single out its role.
In the paper by Ligoxygakis and colleagues, first authors Ilias Kounatidis and Stanislava Chtarbanova note that as people age, immune signaling ramps up and inflammation becomes chronic. To find out if that leads to neurodegeneration, the researchers created multiple fly models with mutated negative regulators of the NF-κB immune pathway. By removing the brakes, the authors escalated this signaling. In these fly mutants, neurodegeneration accelerated greatly, as measured by trouble climbing up inside the vials in which they were kept. Whereas wild-type flies lived about 65 days, these only made it to about 25.
Conversely, if the authors genetically knocked down NF-κB signaling, the flies lived an average of 105 days. They scrambled up their vials late into old age, meaning their “healthspan” was also extended. Curiously, while age-related neurodegeneration progressed as usual in these flies, they released twice as much of the adipokinetic hormone (Akh). This caused energy stores to break down into usable sugars and triglycerides, and may explain why the flies were more active, said Ligoxygakis. A similar change in hormone signaling and lifespan extension was reported in mice with reduced NF-κB signaling (Zhang et al., 2013).
“The paper shows for the first time that both increase and decrease of NF-κB activity, and therefore of immune activity, can determine lifespan,” said Ligoxygakis. “It’s all centered on one molecule.” NF-κB could become a therapeutic target for neurodegenerative diseases, or even prolong healthspan, he said, but the challenge will be to modulate this signaling locally in the brain.
“This is a fascinating report using fly models to examine the intersection of aging, neurodegeneration, and metabolism,” wrote Terrence Town, University of Southern California, Los Angeles to Alzforum (see full comment below). “This result hints at an evolutionary basis for control of a glial phenotype that maintains homeostasis and repair.”
“This work is impressive, as it clearly demonstrates that neural NF-κB is a crucial determinant of aging and lifespan in drosophila,” wrote Dongsheng Cai, Albert Einstein College of Medicine, New York, to Alzforum (see full comment below). Together with the previous research in mice, “the case is growing stronger that the hypothalamic NF-κB pathway has a programmatic role in aging and longevity, even though the responsible cell types in the brain, especially in the neuroendocrine region, are still unclear,” he added.—Gwyneth Dickey Zakaib
Feel the burn? Microglia rush to the site of a laser-induced injury in normal mice with intact progranulin (first clip). However, when progranulin is missing, the microglia are more sluggish in their approach (second clip). [Image courtesy of Krabbe, PNAS.]
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