As Aβ plaques accumulate in the brain, microglia are increasingly called upon to clean up the mess. This relentless phagocytic feasting requires enormous amounts of energy in the form of adenosine triphosphate, but by boosting glycolysis, the microglia fail to provide enough ATP, according to a study published October 6 in Nature Metabolism. Scientists led by Jie Zhang, Xiamen University in China, reported that hexokinase 2 (HK2)—a pivotal enzyme in glucose metabolism—ramps up in microglia in the Alzheimer's disease (AD) brain. Curiously, deleting or inhibiting HK2 in mouse models of amyloidosis boosted microglial ATP production. The cells mustered the extra energy by transforming into fat-burning machines, the scientists found. They cranked up expression of lipoprotein lipase and other lipid metabolism genes. These lipid-utilizing microglia were supreme consumers of Aβ plaques and other neuronal debris, and spared AD mice from memory loss. The findings support the idea that metabolic shifts and functional states in microglia are intimately intertwined.
Russell Swerdlow of Kansas University Medical Center in Kansas City wrote that the study answers important questions while raising many others. “I certainly hope it will help to further focus the field on the relevance of energy metabolism in AD, whether it is in neurons, astrocytes, or microglia.” (See full comment below.)
Glucose metabolism wanes in the brain with age, and even more so in neurodegenerative disease. As an energy-hogging organ, the brain consumes vast amounts of glucose, both via the fast-burning path of aerobic glycolysis in the cytosol, then via the slower, more lucrative burn of oxidative phosphorylation, which churns out even more ATP within the mitochondria. Yet cells also have an energy source that is wholly independent from glucose–fatty acids. Derived from the processing and oxidation of lipids, these provide another substrate for energy production via oxidative phosphorylation, and a slew of studies have implicated lipid processing and metabolism as central in microglial function and AD pathogenesis (Aug 2019 news; Nov 2021 news; Sep 2022 conference news).
As Aβ plaques and other pathological insults build up in the brain with age, how do microglia rally their metabolisms to respond? First author Lige Leng and colleagues addressed this by first looking for expression of hexokinases, which catalyze the first rate-limiting step of glycolysis, namely converting glucose into glucose-6-phosphate. One isoform of this enzyme—HK2—was elevated in postmortem brain samples from people with AD, and in the 5xFAD mouse model of amyloidosis. The researchers pinned most of this uptick on microglia. Notably, in 5xFAD mice and in the human AD brain, microglia surrounding plaques expressed the highest levels.
Was this revved HK2 expression helpful, or harmful? To find out, the researchers conditionally deleted HK2 from microglia in wild-type and 5xFAD mice at 5 months of age, then examined them over the following two months. Wild-type animals suffered no obvious ill effects. The 5xFAD had a lower Aβ plaque load, more synaptic proteins, and better memory than 5xFAD controls. Leng found that their microglia extended processes, transforming from a hunched, amoeboid shape into a ramified one resembling microglia in wild-type mice. Two hexokinase inhibitors—Lonidamine or 3-BromoPyruvate (3-BP)—had similar effects. In 5xFAD mice, and in cultured, human embryonic stem cell-derived microglia, 3-BP improved microglial phagocytosis, ramped up lysosomal degradation of internalized Aβ peptides, and dampened microglial secretion of pro-inflammatory cytokines.
How might hobbling glycolysis improve microglial function? Surprisingly, the scientists found the 3-BP-treated cells had more ATP, not less. In contrast, neurons or astrocytes suffered an energy deficit when glycolysis was blocked. To investigate why only microglia thrived, Leng examined their metabolomes and transcriptomes. Microglia treated with 3-BP as well as Aβ42 peptides ramped up lipid metabolism, producing more fatty acids, triglycerides, and acylcarnitine, while lactate, a product of glycolysis, declined.
Similarly, transcriptomics indicated that microglia had cranked up expression of genes involved in lipid metabolism, signaling a shift in their prime source of ATP from glucose to lipids. Upregulation of one gene in particular—lipoprotein lipase (Lpl)—explained much of the enhanced lipid processing in response to the HK2 blockade. Primarily expressed by microglia in the brain, lpl processes lipids into fatty acids. Its upregulation has been identified in microglial gene expression signatures in AD as well as other neurodegenerative diseases (Loving and Bruce, 2020).
To understand how HK2 expression relates to lipid processing and microglial transcriptional states, the scientists analyzed data from a prior, single-cell RNA sequencing study of an APP knock-in mice model of amyloidosis (Sala Frigerio et al., 2019). In short, they found that microglia expressing the least HK2 expressed the most Lpl, and the strongest DAM signature. In all, the findings suggest that the switch to fat-burning mode would go hand in hand with a transition into disease associated states, which many researchers now view as beneficial (Sep 2022 conference news). As such, the authors proposed HK2 inhibition as a potential therapeutic strategy for AD.
Flipping the Fat Switch. In the AD brain, or in a mouse model of amyloidosis (left), microglia rev up expression of HK2, deriving much of their ATP from glycolysis. Blocking HK2 (right) switches microglia to lipid metabolism, which produces more ATP and enhances phagocytosis and reduces release of pro-inflammatory cytokines. [Courtesy of Choi and Mook-Jung, News and Views, Nature Metabolism, 2022.]
To Swerdlow, the findings highlight a fundamental point about metabolism: that the pathways that make or break energy are all interconnected. “If you alter one, others will change, and glucose, amino acid, and fatty acid-fueled bioenergetics are intertwined.” Swerdlow added that microglia are known to have different functional states that are determined by their bioenergetic infrastructures, including mitochondrial phenotypes. “In that regard, I would have liked to have seen data that informed the state of mitochondria in the microglia assessed,” he wrote.
In an editorial accompanying the paper, Inhee Mook-Jung and Hayoung Choi of Seoul National University College of Medicine made a related point. They noted that the health of mitochondria will be pivotal to the success of any lipid-metabolism promoting therapy, because fatty acid oxidase, the rate-limiting enzyme for metabolism of fatty acids, resides within these powerhouse organelles.
If lipids provide the best fuel for favorable microglial functions, why do microglia increasingly rev up expression of HK2 in the face of amyloidosis? While that remains unanswered, Zhang speculated that microglia, like other cells in the brain, sense waning glucose metabolism and ramp up HK2 to compensate.
Mark Mattson of Johns Hopkins University in Baltimore noted that the 5xFAD mice used in the study were fed a high-carb diet. “To begin with, they were not in a lipid-using state,” he said. In this glucose-dependent context, microglia may upregulate HK2 as a compensatory response to protect their own survival, he suggested. He wondered if HK2 would have ramped up to the same extent had the mice had been given a ketogenic diet or fed intermittently—both scenarios that promote a shift from glucose to lipid metabolism in the brain. “Under these conditions, perhaps HK2 inhibition would not have provided an added benefit,” he predicted.
The Aβ-induced uptick in HK2 expression observed by Leng dovetails with previous observations that microglia revitalize their use of glycolysis in response to Aβ accumulation (Baik et al., 2019). Other studies have found that while the brain gradually shifts from fast-burning glycolysis to the slower, more productive process of oxidative phosphorylation with age, some regions— for example, the Aβ-plaque prone default mode network—may rekindle glycolysis in response to growing stresses (Aug 2017 news; Sep 2010 news).
Leng’s findings also jibe with a recent report that microglia contribute substantially to signals on FDG-PET scans, which measure glucose uptake in the brain (Oct 2021 news).
Róisín McManus of the German Center for Neurodegenerative Diseases in Bonn noted that Leng’s findings also resonate with studies implicating activation of the NLRP3 inflammasome in instigating glycolysis in microglia (Finucane et al., 2019). Aβ oligomers and aggregates are well-known triggers of the inflammasome, suggesting that the glycolytic shift seen in Aβ-exposed microglia could be facilitated by this inflammatory pathway (Lučiūnaitė et al., 2019). "Because HK2 depletion or inhibition was so effective at reducing Aβ pathology, this would have extensive downstream consequences, reducing long-term microglial activation by removing the very trigger (i.e. Aβ) that chronically activates these cells," she wrote (comment below).—Jessica Shugart
- In the AD brain, microglia boost expression of the glycolytic enzyme hexokinase 2.
- Blocking HK2 in mice improved microglial phagocytosis and plaque clearance.
- HK2 inhibition switched on lipid metabolism, boosting ATP production.
Russell Swerdlow of Kansas University Medical Center in Kansas City wrote that the study answers important questions while raising many others. “I certainly hope it will help to further focus the field on the relevance of energy metabolism in AD, whether it is in neurons, astrocytes, or microglia.” (See full comment below.)
Glucose metabolism wanes in the brain with age, and even more so in neurodegenerative disease. As an energy-hogging organ, the brain consumes vast amounts of glucose, both via the fast-burning path of aerobic glycolysis in the cytosol, then via the slower, more lucrative burn of oxidative phosphorylation, which churns out even more ATP within the mitochondria. Yet cells also have an energy source that is wholly independent from glucose–fatty acids. Derived from the processing and oxidation of lipids, these provide another substrate for energy production via oxidative phosphorylation, and a slew of studies have implicated lipid processing and metabolism as central in microglial function and AD pathogenesis (Aug 2019 news; Nov 2021 news; Sep 2022 conference news).
As Aβ plaques and other pathological insults build up in the brain with age, how do microglia rally their metabolisms to respond? First author Lige Leng and colleagues addressed this by first looking for expression of hexokinases, which catalyze the first rate-limiting step of glycolysis, namely converting glucose into glucose-6-phosphate. One isoform of this enzyme—HK2—was elevated in postmortem brain samples from people with AD, and in the 5xFAD mouse model of amyloidosis. The researchers pinned most of this uptick on microglia. Notably, in 5xFAD mice and in the human AD brain, microglia surrounding plaques expressed the highest levels.
Was this revved HK2 expression helpful, or harmful? To find out, the researchers conditionally deleted HK2 from microglia in wild-type and 5xFAD mice at 5 months of age, then examined them over the following two months. Wild-type animals suffered no obvious ill effects. The 5xFAD had a lower Aβ plaque load, more synaptic proteins, and better memory than 5xFAD controls. Leng found that their microglia extended processes, transforming from a hunched, amoeboid shape into a ramified one resembling microglia in wild-type mice. Two hexokinase inhibitors—Lonidamine or 3-BromoPyruvate (3-BP)—had similar effects. In 5xFAD mice, and in cultured, human embryonic stem cell-derived microglia, 3-BP improved microglial phagocytosis, ramped up lysosomal degradation of internalized Aβ peptides, and dampened microglial secretion of pro-inflammatory cytokines.
How might hobbling glycolysis improve microglial function? Surprisingly, the scientists found the 3-BP-treated cells had more ATP, not less. In contrast, neurons or astrocytes suffered an energy deficit when glycolysis was blocked. To investigate why only microglia thrived, Leng examined their metabolomes and transcriptomes. Microglia treated with 3-BP as well as Aβ42 peptides ramped up lipid metabolism, producing more fatty acids, triglycerides, and acylcarnitine, while lactate, a product of glycolysis, declined.
Similarly, transcriptomics indicated that microglia had cranked up expression of genes involved in lipid metabolism, signaling a shift in their prime source of ATP from glucose to lipids. Upregulation of one gene in particular—lipoprotein lipase (Lpl)—explained much of the enhanced lipid processing in response to the HK2 blockade. Primarily expressed by microglia in the brain, lpl processes lipids into fatty acids. Its upregulation has been identified in microglial gene expression signatures in AD as well as other neurodegenerative diseases (Loving and Bruce, 2020).
To understand how HK2 expression relates to lipid processing and microglial transcriptional states, the scientists analyzed data from a prior, single-cell RNA sequencing study of an APP knock-in mice model of amyloidosis (Sala Frigerio et al., 2019). In short, they found that microglia expressing the least HK2 expressed the most Lpl, and the strongest DAM signature. In all, the findings suggest that the switch to fat-burning mode would go hand in hand with a transition into disease associated states, which many researchers now view as beneficial (Sep 2022 conference news). As such, the authors proposed HK2 inhibition as a potential therapeutic strategy for AD.
Flipping the Fat Switch. In the AD brain, or in a mouse model of amyloidosis (left), microglia rev up expression of HK2, deriving much of their ATP from glycolysis. Blocking HK2 (right) switches microglia to lipid metabolism, which produces more ATP and enhances phagocytosis and reduces release of pro-inflammatory cytokines. [Courtesy of Choi and Mook-Jung, News and Views, Nature Metabolism, 2022.]
To Swerdlow, the findings highlight a fundamental point about metabolism: that the pathways that make or break energy are all interconnected. “If you alter one, others will change, and glucose, amino acid, and fatty acid-fueled bioenergetics are intertwined.” Swerdlow added that microglia are known to have different functional states that are determined by their bioenergetic infrastructures, including mitochondrial phenotypes. “In that regard, I would have liked to have seen data that informed the state of mitochondria in the microglia assessed,” he wrote.
In an editorial accompanying the paper, Inhee Mook-Jung and Hayoung Choi of Seoul National University College of Medicine made a related point. They noted that the health of mitochondria will be pivotal to the success of any lipid-metabolism promoting therapy, because fatty acid oxidase, the rate-limiting enzyme for metabolism of fatty acids, resides within these powerhouse organelles.
If lipids provide the best fuel for favorable microglial functions, why do microglia increasingly rev up expression of HK2 in the face of amyloidosis? While that remains unanswered, Zhang speculated that microglia, like other cells in the brain, sense waning glucose metabolism and ramp up HK2 to compensate.
Mark Mattson of Johns Hopkins University in Baltimore noted that the 5xFAD mice used in the study were fed a high-carb diet. “To begin with, they were not in a lipid-using state,” he said. In this glucose-dependent context, microglia may upregulate HK2 as a compensatory response to protect their own survival, he suggested. He wondered if HK2 would have ramped up to the same extent had the mice had been given a ketogenic diet or fed intermittently—both scenarios that promote a shift from glucose to lipid metabolism in the brain. “Under these conditions, perhaps HK2 inhibition would not have provided an added benefit,” he predicted.
The Aβ-induced uptick in HK2 expression observed by Leng dovetails with previous observations that microglia revitalize their use of glycolysis in response to Aβ accumulation (Baik et al., 2019). Other studies have found that while the brain gradually shifts from fast-burning glycolysis to the slower, more productive process of oxidative phosphorylation with age, some regions— for example, the Aβ-plaque prone default mode network—may rekindle glycolysis in response to growing stresses (Aug 2017 news; Sep 2010 news).
Leng’s findings also jibe with a recent report that microglia contribute substantially to signals on FDG-PET scans, which measure glucose uptake in the brain (Oct 2021 news).
Róisín McManus of the German Center for Neurodegenerative Diseases in Bonn noted that Leng’s findings also resonate with studies implicating activation of the NLRP3 inflammasome in instigating glycolysis in microglia (Finucane et al., 2019). Aβ oligomers and aggregates are well-known triggers of the inflammasome, suggesting that the glycolytic shift seen in Aβ-exposed microglia could be facilitated by this inflammatory pathway (Lučiūnaitė et al., 2019). "Because HK2 depletion or inhibition was so effective at reducing Aβ pathology, this would have extensive downstream consequences, reducing long-term microglial activation by removing the very trigger (i.e. Aβ) that chronically activates these cells," she wrote (comment below).—Jessica Shugart
