Microglial,metabolic,reconfiguration,provides,a,new,strategy,for,the,treatment,of,Alzheimer’s,disease

Lige Leng, Jie Zhang

Alzheimer’s disease (AD) shares multiple characteristics of metabolic diseases:AD is an age-associated neurodegenerative disease characterized by progressive loss of memory and cognitive functions, which is classically manifested by the deposition of β-amyloid (Aβ) plaques, the neurofibrillary tangles, and neuronal loss. Mounting evidence also suggests that AD shares multiple characteristics of metabolic diseases. The impairment in cerebral glucose metabolism and insulin resistance are considered typical features of AD and its occurrence precedes cognitive dysfunction for decades in patients.

Introduction of the function and metabolic states of microglia:Microglia are critical nervous systemspecific immune cells serving as tissue-resident macrophages influencing brain development, maintenance of the neural environment, response to injury and repair. Microglia in the central nervous system are usually maintained in a quiescent state. When activated, they can perform many diverse functions which may be either beneficial or harmful depending on the situation. As influenced by their environment, microglia assume a diversity of phenotypes and retain the capability to shift functions to maintain tissue homeostasis. In comparison with peripheral macrophages, microglia demonstrate similar and unique features with regard to phenotype polarization, allowing for innate immunological functions. Under these conditions, energy demands would be associated with functional activities and cell survival and thus, may serve to influence the contribution of microglia activation to various neurodegenerative conditions (Colonna and Butovsky, 2017; Ghosh et al., 2018).

As the main Aβ clearance cells, microglia are targets, with great potential, for pharmaceutical intervention for AD. In AD, glucose hypometabolism is a typical feature, while energy/adenosine triphosphate (ATP) consumption is vital for the key functions of microglia including phagocytosis and migration. It may be some kinds of compensatory mechanism to upregulate the level or activity of enzymes in the glucose metabolism pathway. On the other hand, many neurons are inevitably injured with axons breaking in AD. In the process of phagocytosis of axon fragments, microglia uptake and clearance of a large amount of sphingolipids seem to provide a material basis and stimulation for metabolic reconfiguration.

Due to the high dependence of neurons on glucose, previous studies mainly focused on the imbalance of glucose metabolism in AD, while ignoring the potential shift of other energy substances. Microglia express transporters for the three main energy substrates (glucose, fatty acids, and glutamine), indicating a flexible use of available energy resources. The survival and activation of microglia depend on a sufficient energy supply. Microglia are sensitive toin vitrochanges in ambient glucose levels which impact their function and express the glucose transporters GLUT1, GLUT2, and GLUT5 (Payne et al., 1997). Cell-type-specific RNA-seq analysis reveals that microglia express the full set of genes required for both glycolysis and OXPHOS, but their bioenergetic phenotype is activation-statedependent (Aldana, 2019). Resting microglia depend mainly on OXPHOS for ATP production, whereas activated microglia favor glycolysis as manifested by increased lactate production and decreased mitochondrial oxygen consumption (Aldana, 2019). Additionally, transcriptome data suggest that microglia express key enzymes for fatty acids mobilization and β-oxidation, which may alternatively meet their elevated energy demand upon activation. Microglia express CD36 and loss of this fatty acid transporter impacts microglial functionin vitroandin vivoin response to neurological insult (Aldana, 2019). This suggests that the ability to take up and utilize fatty acids as an energy source is critical for normal glial function in response to an insult, and is likely related to concomitant immune-metabolic responses (Baik et al., 2019).

Lipoprotein lipase (LPL) and its function in microglia:LPL, a key enzyme necessary for the breakdown of triglycerides and uptake of fatty acids, is found in microglia (Baik et al., 2019; Wu et al., 2021). It has been shown that LPL is colocalized with senile plaques in AD brains, and its mutations are associated with the severity of AD pathophysiological features (D Bruce et al., 2020). LPL was also reduced in AD cerebrospinal fluid samples relative to those in controls. It is found higher expression in microglia far more than in neurons and astrocytes, which may indicate that intact fatty acid uptake by microglia is a key event in normal microglial functions. One study showed that microglia associated with plaques which play a protective role in AD present a high LPL expression (Baik et al., 2019). These LPL high-expressed microglia represent stronger phagocytosis ability. More importantly, LPL may facilitate lipid uptake in microglia in AD during neuron death and axon demyelination and it is a novel feature of a microglial phenotype that supports neuron protection and myelination repair through the phagocytosis and clearance of lipid debris. This indicates that intact fatty acid uptake by microglia is a key event in normal microglial function. The H+allele of the LPL HindIII intronic polymorphism also showed a risk for sporadic lateonset Alzheimer’s disease (Scacchi et al., 2004).

Microglial metabolic reconfiguration to lipid metabolism by inhibition of hexokinase 2 provides a new strategy for the treatment of AD:In glucose metabolism, the first key ratelimiting enzymes are hexokinases, which convert glucose to glucose-6-phosphate and initiate the downstream pathways of glucose utilization. There are four members of the HK family, of which HK1-3 is expressed in the central nervous system and HK4 is mainly expressed in the peripheral tissues (Wilson, 2003). At present, little is known about the functions of HK isoforms in the brain.

Writing inNature Metabolism(Leng et al., 2022), the researchers report that the expression of hexokinase 2 (HK2) was specifically elevated in microglia from AD patients and an AD mouse model. The researchers generated microglial HK2-specific knockout mice and crossed them with 5xFAD mice, and found that genetic depletion of HK2 in microglia significantly promoted microglia phagocytosis, lowed the amyloid plaque burden, and attenuated cognitive impairments in AD mice. Pharmaceutically, hexokinases inhibitors (Lonidamine and 3-BP) are also effective to promote Aβ clearance by promoting microglia phagocytosis. Furthermore, the researchers show that two downstream metabolites of HK2, glucose-6-phosphate and fructose-6-phosphate, reverse HK2-deficiency-induced upregulation of LPL, ATP production, and microglial phagocytosis (Figure 1A).

Activated microglia express several glycolytic enzymes resulting in enhanced ATP production, highlighting the necessity of enhanced cellular metabolism to regulate their adaptability. The expression of many genes involved in glycolysis increases in AD, which may be due to the positive feedback for glucose hypometabolism. The researchers here identified a glycolysis enzyme hexokinase (HK) isoform called HK2 that is specifically expressed in reactive microglia and increased by Aβ stimulation. Among all the hexokinases, HK2 specifically increased in amyloidassociated microglia of AD patients and an AD mouse model.

The ATP level was found to increase dramatically in microglia but not in neurons or astrocytes upon HK2 inhibitions. ATP generation should be decreased if glycolysis is inhibited, but the researchers found the opposite effects in microglia but not in other brain cells. Contrary to expectations, they find higher levels of ATP in HK2 deficient or inhibited microgliain vitro, which they interpret as the consequence of a compensatory increase in lipid metabolism via LPL upregulation. There exists a metabolic reconfiguration upon inhibition of HK2 regarding fatty acid oxidation as the major source of ATP upon HK2 inhibition (Figure 1B). The researchers also show that two key downstream metabolites of HK2, namely glucose-6-phosphate and fructose-6-phosphate, abolish the effect of HK2-deficiency on LPL elevation, ATP upregulation, and Aβ phagocytosis.To our knowledge, it is the first time to found that the hypometabolism of glucose can increase ATP generation in microglia (but not other types of brain cells such as neurons and astrocytes). Many previous studies have detailed described the metabolic reprogramming of glycolysis and oxidative phosphorylation in microglia in AD. The study (Leng et al., 2022) broadened the metabolic reconfiguration of microglia to between three main energy substrates. This finding is novel and exciting and shall impact the microglia research field.

Figure 1｜Inhibition or deletion of HK2 triggers metabolic reconfiguration to lipid metabolism, ensuring sufficient ATP production, and resulting in microglia with high phagocytic activity to ameliorate the Aβ plaque burden and improve cognitive function.

The deficiency of HK2 in microglia instead increases ATP production through LPL-mediated fatty acid metabolism. The researchers also found that genetic ablation or pharmacological inhibitions of HK2 increases phagocytosis of Aβ in microgliain vitroandin vivo, resulting in decreased amyloid burden and improved cognition. These results connect glucose metabolism to critical microglial responses, including phagocytosis, with possible relevance for AD pathogenesis, as glucose hypometabolism is a critical manifestation of prodromal AD.

HK2 inhibitions (Lonidamine and 3-BP) can efficiently promote microglia β-amyloid phagocytosis and attenuate cognitive impairments in AD mice. By inferior vena cava injections of both inhibitors for 48 hours, the researchers found that amyloid plaque deposition in 5XFAD mice was greatly reduced. Further, intraperitoneal injection of 3-bromopyruvic acid is also capable to lower the amyloid plaque burden and attenuate cognitive impairments.

Based on the results, it is found that changes in the levels of key enzymes in glucose metabolism may lead to dysfunction of microglial functions, thereby exacerbating the process of AD. Therapeutic strategies targeting these key metabolic enzymes can improve the energy balance of microglia, improve microglial functions, and then reverse the pathological process of AD. To realize this process, it needs to find out the interrelationship of metabolic networks and the key nodes of the transformation of metabolic patterns. Every research progress is to add bricks and tiles to the blueprint and constantly supplement the unknown parts. Microglial HK2 pharmaceutical inhibition is a potential AD therapeutic strategy. Given the crucial role of microglia in neurodegenerative diseases and demyelinating diseases, the same treatment strategy may also be extended to broader kinds of diseases.

These findings may raise other compelling questions. LPL is a well-known disease-associated microglia (DAM) marker and its increase is known to increase more in the late stage. The DAM, express high levels of genes involved in phagocytic and lipid metabolism. Interestingly, almost all DAM express high levels of LPL and contain high contents of intracellular phagocytic particles of Aβ. HK2 is the rate-limiting enzyme in the first step of glycolysis. In the present study, we found that HK2 inhibition or deficiency stimulates LPL expression in microglia, and the inhibition of LPL decreases the beneficial effects of HK2 inhibition in microglia.

According to the single-cell analysis data, HK2 expression correlates with the expression of many DAM stage 2 genes includinglpl. Therefore, it is possible that the imbalance of glucose metabolism causes the changes in the metabolism-related gene HK2, which may initiate progressive defects in microglia. The results also inferred that HK2 might be a new DAM stage 2 marker gene. The relationship between HK2 and other DAM stage 1 and stage 2 genes also needs to be further explored. HK2 might be only the tip of the iceberg in an ever-expanding scenario of metabolic enzymes in DAM function and AD pathology progression.

In summary, excessive HK2 activity causes insufficient ATP production in microglia in AD mice, while inhibition of HK2 induces a metabolic shift towards lipid metabolism instead of glucose metabolism to ensure sufficient ATP, resulting in microglia with high phagocytic activity to ameliorate the Aβ plaque burden and improve cognitive function. The study also sheds light on the cross-talk between glucose and lipid metabolism, the two of three energy substrates, which ensure efficient energy usage by microglia in the brains of individuals with AD.

This work was supported by the National Nature Science Foundation of China (Nos. 91849205,81925010, and U1905207 to JZ; Nos. 81801337 and 82071520 to LL; No. 92149303 to JZ);

The National Key Research and Development

Program of China (No. 2021YFA1101402 to JZ); The Fundamental Research Funds for the Central Universities (Nos. 20720190118 and 20720180049 to JZ; No. 20720190075 to LL); Nature Science Foundation of Fujian Province (No. 2019J05006 to LL); Xiamen Youth Innovation Fund (No. 3502Z20206031 to LL).

*Correspondence to:Jie Zhang, PhD, jiezhang@xmu.edu.cn; Lige Leng, PhD, lenglige@xmu.edu.cn.

https://orcid.org/0000-0003-1040-5848 (Jie Zhang)

https://orcid.org/0000-0002-4288-4743 (Lige Leng)

Date of submission:November 4, 2022

Date of decision:November 30, 2022

Date of acceptance:December 11, 2022

Date of web publication:January 30, 2023

https://doi.org/10.4103/1673-5374.367842

How to cite this article:Leng L, Zhang J (2023) Microglial metabolic reconfiguration provides a new strategy for the treatment of Alzheimer’s disease. Neural Regen Res 18(9):1946-1947.

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