ApoE4 Makes Microglia "Greasy"
Professor Cai Lihui and his team from the MIT in the United States has been committed to studying the effects of apoE4 on various types of cells in the brain.In 2018 year,they found that apoE4 causes neurons to produce a large number of β Amyloid polypeptide 42 leads to hyperactivity of neurons.
Beside,Microglia and astrocytes will also be affected, resulting in cholesterol accumulation and inflammation, which cannot clear the deposited β Amyloid.In 2021 year, They continued to advance the research results, proving that apoE4 can seriously damage the lipid metabolism ability of astrocytes.
Time has come to 2022. This time, it's the turn of microglia.
Professor Cai and his team found that apoE4 also destroys the lipid metabolism ability of microglia, which leads to the accumulation of lipid molecules, especially cholesterol. They will combine with specific types of potassium ion channels in the neuronal cell membrane, thereby inhibiting neuronal discharge and decreasing neuronal excitability, which is a phenomenon that will occur in the later stage of AD. In addition, the accumulation of lipids can also lead to inflammation, and neuroinflammation is also one of the factors known to promote the progression of AD.This finding may also help researchers to build research models that simulate the later stage of AD in the future. The results were published in the journal <<Cell · stem cell>>.
The research relies on the crispr-cas9 genome editing of induced pluripotent stem cells (iPSCs) previously used by the research team, which can derive allelic cell lines with the same genes except APOE alleles, including neurons, astrocytes and microglia.Microglia in the brain highly express purinergic receptor p2ry12. Purinergic signal is an important regulatory signal for microglia to chemotaxis, phagocytosis and produce proinflammatory factors. P2ry12 can sense extracellular energy molecules (ADP and ATP). Many soluble neuronal factors are also involved in the monitoring of neuronal activity by microglia, such as the cx3cl1-cx3cr1 signaling axis.In addition, microglia also express a large number of neurotransmitters and receptors, which mediate neuron microglia communication. Microglia like cells (imgls) derived from iPSCs by the research team also express these important receptors, which are highly consistent with microglia.
In order to determine the effect of apoE4 on neuron microglia communication function, researchers constructed apoE3 and apoE4 imgls at the same time. ApoE3 and apoE4 are only one amino acid apart, but their effect on ad is neutral.
Through repeated experiments using different types of selective inhibitors, the researchers found that purinergic signals were impaired in apoE4 imgls. Since the decrease of purinergic receptor p2ry12 was related to the activation status of microglia, the researchers deduced that the monitoring status of apoE4 imgls had changed, weakening the monitoring of neuronal activity.
Transcriptional profiling further confirmed their inference. ApoE4 imgls have stronger transcriptional responses, and are significantly enriched in HIF-1 and JAK-STAT signals, as well as cytokine cytokine receptor interactions, suggesting that they have strong proinflammatory responses. The induction of intracellular calcium signaling pathway decreased and calcium transiently decreased. The expression level of p2ry12 was significantly decreased, but p2ry6, which is related to the "high alertness" state of microglia, was not significantly increased. In addition, the expression of many other steady-state genes was also significantly reduced, including CX3CR1 and csf1r.
We know that pro-inflammatory stimulation can induce the metabolic conversion of microglia from oxidative phosphorylation (OXPHOS) to glycolysis. Researchers have indeed found that the level of glucose transporter GLUT3 in apoE4 imgls has a slight but significant increase. In addition, the significantly enriched HIF-1 signal just mentioned also happens to be the main transcriptional regulator of glycolysis.
Energy consumption and fatty acid oxidative damage are related to lipid accumulation in activated microglia. While the expression of genes related to oxidative phosphorylation is reduced, researchers have indeed observed the reduction of differentially expressed genes related to lipid catabolism and membrane fatty acid transporter CD36 in apoE4 imgls.
After staining the lipid droplets, it was found that the content of lipid droplets in apoE4 imgls significantly increased, and the activation of the downstream calcium signaling pathway by G protein can promote lipid decomposition by activating protein kinase A and C (PKA and PKC). Consistent with this important mechanism, researchers observed that the expression of genes encoding the catalytic and regulatory domains of PKA and PKC in apoE4 imgls decreased.
ApoE, as the main cholesterol transporter in the brain, mediates the transmission of cholesterol and other lipids between neurons and glial cells. The supernatant of apoE4 imgls culture is rich in ApoE and cholesterol. Previous studies believe that the increase of cholesterol is related to the decrease of lipid transport of apoE4 glial cells, but the researchers infer that the accumulation of extracellular lipids may also be the "product" of the relatively more serious defect of lipid intracellular flow.
To verify this inference, they co cultured apoE3 and apoE4 imgls with low-density lipoprotein cholesterol (LDL) isolated from human plasma at the same time. Compared with apoE3, the cellular uptake of LDL by apoE4 imgls was greatly reduced.
For neurons, the addition of exogenous cholesterol hyperpolarizes the resting membrane potential, changes the current voltage curve, and enhances the inward rectifier potassium ion (KIR) current, which is the reason for the hyperpolarization of the resting membrane potential and reduces the excitability of neurons.
Among Kir channels, kir3.3 channel (girk3) encoded by KCNJ9 gene is abundantly expressed. Like other Kir channels, it can regulate neuronal excitability, reduce neuronal activity when it functions, and increase neuronal activity when it loses its function. Moreover, GIRK channel is just a lipid-gated channel. In neurons, cholesterol will enhance its activity, thereby reducing neuronal excitability. GIRK channels are located in the cholesterol rich microdomain, i.e., lipid rafts, on the neuronal cell membrane, whereas lipid rafts are increased in apoE4 imgls.
Researchers consulted the snRNA sequencing data of AD patients that have been published and found that the levels of GIRK2 and girk3 in excitatory neurons of AD patients were significantly increased compared with the control group without ad.
That is to say, apoE4 impairs the lipid metabolism ability of microglia, which leads to the "floating out" of lipid molecules. The accumulated lipid molecules enhance the KIR current in the neuronal cell membrane, hyperpolarize the resting membrane potential, and reduce the excitability of neurons.
Researchers tried to treat apoE4 microglia with Triadimefon C, which can inhibit the formation of lipid droplets, successfully reduced lipid accumulation and restored neuronal activity to normal. However, Triadimefon C is cytotoxic and is not suitable for the treatment of AD, but this study points out a new direction for us, that is, to restore lipid homeostasis to combat the effects of apoE4 on glial cells and neurons.
Dr. Matheus B. Victor, the first author of the study, said that it is essential to restore the lipid homeostasis of different types of cells in the brain, but it is not an easy thing. Previously, they found that choline may have this potential. In the future, more exploration is needed.
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