"Immunity": The team of the Second Hospital of Zhejiang University found for the first time that resting microglia can directly affect the electrical excitability of neurons!

*For medical professionals only

As the main risk factor for cardiovascular disease, hypertension is easy to induce cardiovascular diseases and neurodegenerative diseases such as myocardial infarction and stroke, and the incidence rate is as high as 2/3
in > 60-year-old people.

In the current clinical treatment, antihypertensive drugs mainly achieve antihypertensive effects
by relaxing peripheral vascular resistance or reducing water and sodium retention.
However, refractory hypertension cannot be effectively alleviated even with a combination of multiple drugs; The pathogenesis of some hypertensions is still unclear, and there are no effective prevention and treatment methods
in the clinic.

Although the etiology of hypertension varies, most patients with essential hypertension have elevated sympathetic activity, and renal sympathetic ablation (RSD) can be effective in relieving symptoms of hypertension [1].

Therefore, the central homeostasis of the sympathetic nervous system is essential for the cardiovascular system, but its mechanism is not clear
.

Recently, Shi Peng, the Second Affiliated Hospital of Zhejiang University School of Medicine, collaborated with Shen Xiao and Gu Yan research teams of the School of Basic Medicine to publish research results in the journal Immunology[2].

For the first time, they found that resting microglia directly act on the hypothalamic paraventricular nucleus (PVN) presympathetic neuron receptor PDGFRα by secreting platelet-derived growth factor (PDGF) B to promote neuronal Kv4.
3 potassium channel protein expression, prevent neuronal hyperexcitation, and maintain sympathetic tone and normal
blood pressure.
If the PDGFB MG-Kv4.
3^Neuron pathway is disrupted, it induces an increase in
blood pressure.

Screenshot of the first page of the paper

Microglia act as intrinsic immune cells in the brain whose development, maturation, and aging are synchronized with the central nervous system[3].

Studies have shown that microglia monitor the local microenvironment of the central nervous system in real time through their branches[4], which not only participates in immune regulation, but also has an important regulatory role in neurodevelopment, neuronal excitability and neural circuits[5].

Microglial life cycle diagram[3]

The direct regulation of neurons by microglia is mainly through "synaptic pruning" [6-7], until 2020, when the journal Nature revealed that in the striatum, microglia can sense neuronal activation and inhibit neuronal excitability through the ATP-AMP-ADO-A1R signaling pathway, acting as a "brake buffer" [8].

However, there is no direct evidence
for microglia to directly regulate neuronal electrophysiological properties.

In the cardiovascular system, sympathetic nerve endings grow against the walls of blood vessels, and their excitation causes vasoconstriction, and improving sympathetic tone is an effective means of relieving hypertension, for example, RSD can effectively relieve refractory hypertension
in the clinic.
However, it has not been reported
whether microglia mediate central sympathetic homeostasis and thus regulate blood pressure by maintaining normal neuronal excitability.

Inspired by this, the researchers wanted to analyze the pathogenesis of hypertension from the perspective of the sympathetic nervous system
.
They combined single-cell sequencing, proteomics and epigenetics to try to reveal the regulatory mechanisms
of microglia on hypertension at the molecular level.

First, the researchers injected diphtheria toxin (DT) into the lateral ventricle of CD11b-DTR transgenic mice to remove resting microglia, resulting in sympathetic excitation in mice, which was manifested by elevated peripheral serum norepinephrine (NE) and epinephrine in mice and an increase
in basal heart rate.

Subsequently, they used patch forceps to perform an electrophysiological analysis of presympathetic neurons (i.
e.
, PVN-RVLM) projected into the ventral medulla oblongata (RVLM) of the ventral end of the head of the brainstem using patch forceps, and found that removal of microglia can lead to hyperactivation of sympathetic neurons, manifested by increased proportion and frequency of firing neurons, and inhibition
of exotic potassium current.

Removal of microglia deletion leads to sympathetic excitation and overactivation of presympathetic neurons in PVN

Potassium ion channels are a key factor in regulating neuronal excitability, and in order to explore the above mechanisms, the team conducted more in-depth research
using single-cell sequencing technology.

They found that after removal of microglia, the expression of the Kcnd3 gene encoding the Kv4.
3 potassium ion channel of PVN-RVLM neurons was significantly downregulated; ex vivo co-culture of microglia line BV2 significantly increased Kv4.
3 expression
in PC12 cells of neuron-like pheochromocytoma.

In addition, if Kv4.
3 is selectively inhibited, low-pressure-activated potassium ion currents can be significantly blocked, suggesting that resting microglia can maintain normal neuronal excitability
by increasing Kv4.
3 on neurons.

Removal of microglia reduces the expression of PVN presympathetic neurons Kv4.
3

Next, the research team tried to use transcriptomics methods to identify the underlying mechanisms by
which microglia regulate neuronal Kv4.
3 expression.

Studies have shown that PDGF can be involved in the regulation of potassium ion channels in oligodendrocytes, while Pdgfb (the gene encoding PDGFB) is highly expressed in microglia, and when microglia are removed, Pdgfb expression is significantly reduced
.

In addition, specific removal of Pdgfb from microglia or administration of PDGFR inhibitors reduces the expression of Kv4.
3 on PVN-RVLM neurons; at the same time, the former can induce a decrease in the K⁺ current of PVN presympathetic neurons and sympathetic excitation
.

The above results show that microglia regulate neurons Kv4.
3 through paracrine PDGFB
.

The removal of microglia-derived PDGFB leads to a weakening of the K⁺ current in PVN presympathetic neurons and sympathetic excitation

Studies have shown that PDGFB binds
to two receptors, PDGFRα and PDGFRβ.
Ex vivo cell experiments have found that only the use of PDGFRα antagonists can reduce microglia Kv4.
3 expression, when the specific knockout of neurons PDGFRα, can cause increased blood pressure and potassium ion current in mice, simulating the effect
of removing microglia.

Interestingly, PDGFR belongs to the tyrosine kinase family, and its inhibitors (TKIs) are widely used in anti-tumor therapy clinically, but the drug causes the adverse effect of hypertension, and this study provides a clear molecular mechanism
for this phenomenon to some extent.

Neuronal PDGFRα is a downstream target for microglia to regulate the sympathetic nerve,

Knockout of PDGFRα results in a weakening of the PVN presympathetic neuron K⁺ current

Finally, by injection of exogenous PDGFB or increased expression of neurons Kv4.
3 through lateral ventricles, there is a significant reversal of removal of microglia causing increased blood pressure and potassium current inhibition
.

Schematic of the mechanism

Overall, this study not only revealed that microglia can be directly involved in regulating neuronal membrane potentials, but also enriched the non-immune function of microglia and the mechanism by which they interact with neurons; It also reveals the central regulation mechanism of sympathetic homeostasis, and provides a new idea
for the prevention and treatment of clinical autonomic disorders (such as hypertension).

Resources:

[1].
Azizi M, Sanghvi K, Saxena M, et al.
Ultrasound renal denervation for hypertension resistant to a triple medication pill (RADIANCE-HTN TRIO): a randomised, multicentre, single-blind, sham-controlled trial.
Lancet.
2021; 397(10293):2476-2486.
doi:10.
1016/S0140-6736(21)00788-1

[2].
Bi Q, Wang C, Cheng G, et al.
Microglia-derived PDGFB promotes neuronal potassium currents to suppress basal sympathetic tonicity and limit hypertension.
Immunity.
2022; 55(8):1466-1482.
e9.
doi:10.
1016/j.
immuni.
2022.
06.
018

[3].
Spittau B.
Aging Microglia-Phenotypes, Functions and Implications for Age-Related Neurodegenerative Diseases.
Front Aging Neurosci.
2017; 9:194.
Published 2017 Jun 14.

doi:10.
3389/fnagi.
2017.
00194

[4].
Yang T, Velagapudi R, Terrando N.
Neuroinflammation after surgery: from mechanisms to therapeutic targets.
Nat Immunol.
2020; 21(11):1319-1326.
doi:10.
1038/s41590-020-0812-1

[5].
Schafer DP, Lehrman EK, Kautzman AG, et al.
Microglia sculpt postnatal neural circuits in an activity and complement-dependent manner.
Neuron.
2012; 74(4):691-705.
doi:10.
1016/j.
neuron.
2012.
03.
026

[6].
Bilimoria PM, Stevens B.
Microglia function during brain development: New insights from animal models.
Brain Res.
2015; 1617:7-17.
doi:10.
1016/j.
brainres.
2014.
11.
032

[7].
Wang C, Yue H, Hu Z, et al.
Microglia mediate forgetting via complement-dependent synaptic elimination.
Science.
2020; 367(6478):688-694.
doi:10.
1126/science.
aaz2288

[8].
Badimon A, Strasburger HJ, Ayata P, et al.
Negative feedback control of neuronal activity by microglia.
Nature.
2020; 586(7829):417-423.
doi:10.
1038/s41586-020-2777-8

Editor-in-chargeBioTalker