Bioelectronic Approaches to Control Neuroimmune Interactions in Acute Kidney Injury

Abstract

Recent studies have shown renal protective effects of bioelectric approaches, including ultrasound treatment, electrical vagus nerve stimulation, and optogenetic brainstem C1 neuron stimulation. The renal protection acquired by all three modalities was lost in splenectomized mice and/or α7 subunit of the nicotinic acetylcholine receptor-deficient mice. C1 neuron-mediated renal protection was blocked by β2-adrenergic receptor antagonist. These findings indicate that all three methods commonly, at least partially, activate the cholinergic anti-inflammatory pathway, a well-studied neuroimmune pathway. In this article, we summarize the current understanding of neuroimmune axis-mediated kidney protection in preclinical models of acute kidney injury by these three modalities. Examination of the differences among these three modalities might lead to a further elucidation of the neuroimmune axis involved in renal protection and is of interest for developing new therapeutic approaches.

Figure 1.

Ultrasound (US) treatment in mice. US treatment is applied to the mice as shown in the left panel. After mice were anesthetized, fur was shaved and removed using a depilatory. Mice were then placed on a modified microscope stage, which was positioned under a US transducer held in place with a ring clamp. Prewarmed US gel was then placed on the depilated skin for US application. Mouse body temperature was monitored via rectal probe and maintained at 36.5°C with a heating pad and heat lamp. Top right panel shows the protocol for US treatment and renal ischemia-reperfusion injury (IRI). One day prior to IRI, US was applied to the mice, then IRI was performed 1 day after IRI. Bottom right panel shows the image where US was applied. This is an example of the left side of kidney. The enclosed area in red is the actual targeted region including kidney and spleen. US pulses (1 sec every 6 sec for 2 min) were applied to both kidneys including spleen. Control animals underwent the same preparation procedures but were not exposed to US pulses.

Figure 2.

Selective stimulation of vagus nerve with electricity. The left cervical vagus nerve was isolated via a midline cervical incision and placed on a bipolar silver wire electrode for stimulation. To perform intact vagus nerve stimulation (VNS), the nerve was left intact. To activate vagal afferents, the vagus nerve was cut and the central end was stimulated selectively. The peripheral end of the cut nerve was stimulated to activate vagal efferents.

Figure 3.

Optogenetic neuron activation. channel rhodopsin, or halorhodopsin are introduced into specific neurons and used for optogenetic stimulation. When blue light is applied to neurons expressing channelrhodopsin (ChR2), the opsin functions as a nonselective cation channel, resulting in activation of the neurons. When yellow light is applied to halorhodopsin-expressing neurons, the chloride pump is activated, leading to inhibition of the neurons.

Figure 4.

Optogenetic C1 neuron stimulation. Initially, virus carrying ChR2 (AAV2-ChR2-mCherry) was injected into a specific region (left rostral ventrolateral medulla, location of C1 neurons) of the mouse (dopamine β-hydroxylase-cre mice; targeting adrenergic C1 neurons) and at the same time an optical fiber was implanted close to the target region. Five to 6 weeks later, the virus-injected target region (C1 neurons) expressed ChR2. After the implanted optical fiber and the laser were connected, light pulses (470 nm, 10 msec duration, 5 Hz) were delivered for 10 min.

Figure 5.

Current understanding of the cholinergic anti-inflammatory pathway (CAP) in acute kidney injury. The activated cholinergic anti-inflammatory pathway links the nervous system and immune system, then protects the kidney from injury. The activity of afferent vagus nerve fibers is stimulated by cytokines and pathogen-associated molecular patterns (PAMPs). The signal activates efferent vagus nerve fibers through the nucleus tractus solitarius (NTS) and dorsal motor nucleus of the vagus (DMV) in the brain. The efferent vagus nerve (cholinergic) stimulates CD4 T cells in spleen via the splenic sympathetic (adrenergic) nerve. Release of norepinephrine (NE) binds to β2-adrenergic receptors (β2ARs) on CD4 T cells, which then elicits release of acetylcholine (ACh). ACh binding to α7 nicotinic acetylcholine receptors (α7nAChRs) on macrophages produces an anti-inflammatory response, such as tumor necrosis factor (TNF)-α suppression. Ultrasound and C1 neuron stimulation also activate the pathway, at least partially, not directly through vagus nerve and produce organ protective effects (Abe et al. 2017).