real-time-analysis-gut-brain-neural-communication-cortex-wide-calcium

Real-time Analysis of Gut-brain Neural Communication: Cortex wide Calcium Dynamics in Response to Intestinal Glucose Stimulation

Communication between the gastrointestinal tract and the brain after nutrient absorption plays an essential role in food preference, metabolism, and ...

Ochanomizu University

The glymphatic system is a brain-wide clearance pathway; its impairment contributes to the accumulation of amyloid-β. Influx of cerebrospinal fluid (CSF) depends upon the expression and perivascular localization of the astroglial water channel aquaporin-4 (AQP4). Prompted by a recent failure to find an effect of  knock-out (KO) on CSF and interstitial fluid (ISF) tracer transport, five groups re-examined the importance of AQP4 in glymphatic transport. We concur that CSF influx is higher in wild-type mice than in four different  KO lines and in one line that lacks perivascular AQP4 ( KO). Meta-analysis of all studies demonstrated a significant decrease in tracer transport in KO mice and rats compared to controls. Meta-regression indicated that anesthesia, age, and tracer delivery explain the opposing results. We also report that intrastriatal injections suppress glymphatic function. This validates the role of AQP4 and shows that glymphatic studies must avoid the use of invasive procedures.

Aquaporin-4-dependent glymphatic solute transport in the rodent brain.

Spontaneous waves of cortical spreading depolarization (CSD) are induced in the setting of acute focal ischemia. CSD is linked to a sharp increase of extracellular K that induces a long-lasting suppression of neural activity. Furthermore, CSD induces secondary irreversible damage in the ischemic brain, suggesting that K homeostasis might constitute a therapeutic strategy in ischemic stroke. Here we report that adrenergic receptor (AdR) antagonism accelerates normalization of extracellular K, resulting in faster recovery of neural activity after photothrombotic stroke. Remarkably, systemic adrenergic blockade before or after stroke facilitated functional motor recovery and reduced infarct volume, paralleling the preservation of the water channel aquaporin-4 in astrocytes. Our observations suggest that AdR blockers promote cerebrospinal fluid exchange and rapid extracellular K clearance, representing a potent potential intervention for acute stroke.

Adrenergic receptor antagonism induces neuroprotection and facilitates recovery from acute ischemic stroke.

Transcranial direct current stimulation (tDCS) has been reported for its beneficial effects on memory formation and various brain disorders. While the electrophysiological readout of tDCS effects is subtle, astrocytes have been demonstrated to elicit Ca elevations during tDCS in a rodent model. This study aimed to elucidate the effects of tDCS on another major glial cell type, microglia, by histology and  imaging. tDCS performed in awake conditions induced a significant change in the pixel intensity distribution of Iba-1 immunohistochemistry, and microglial somata were enlarged when examined 3 h after tDCS. These effects were blocked by adrenergic receptor antagonists or in IPR2 (inositol trisphosphate receptor type 2)-deficient mice, which lack large cytosolic Ca elevations in astrocytes. No obvious changes were observed in isoflurane-anesthetized mice. Furthermore,  two-photon imaging of microglia showed a reduction of motility that was blocked by a β-adrenergic receptor antagonist. Our observations add support for the influence of noradrenaline in tDCS and suggest possible interactions between microglia and astrocytes to express functional changes associated with tDCS.

Transcranial Direct Current Stimulation (tDCS) Induces Adrenergic Receptor-Dependent Microglial Morphological Changes in Mice.

Cortical spreading depolarization (CSD) is a propagating wave of tissue depolarization characterized by a large increase of extracellular potassium concentration and prolonged subsequent electrical silencing of neurons. Waves of CSD arise spontaneously in various acute neurological settings, including migraine aura and ischemic stroke. Recently, we have reported that pan-inhibition of adrenergic receptors (AdRs) facilitates the normalization of extracellular potassium after acute photothrombotic stroke in mice. Here, we have extended that mechanistic study to ask whether AdR antagonists also modify the dynamics of KCl-induced CSD and post-CSD recovery in vivo. Spontaneous neural activity and KCl-induced CSD were visualized by cortex-wide transcranial Ca imaging in G-CaMP7 transgenic mice. AdR antagonism decreased the recurrence of CSD waves and accelerated the post-CSD recovery of neural activity. Two-photon imaging revealed that astrocytes exhibited aberrant Ca signaling after passage of the CSD wave. This astrocytic Ca activity was diminished by the AdR antagonists. Furthermore, AdR pan-antagonism facilitated the normalization of the extracellular potassium level after CSD, which paralleled the recovery of neural activity. These observations add support to the proposal that neuroprotective effects of AdR pan-antagonism arise from accelerated normalization of extracellular K levels in the setting of acute brain injury.

Adrenergic inhibition facilitates normalization of extracellular potassium after cortical spreading depolarization.

A report of a family of Darier's disease with mood disorders drew attention when the causative gene was identified as ATP2A2 (or SERCA2), which encodes a Ca2+ pump on the endoplasmic reticulum (ER) membrane and is important for intracellular Ca2+ signaling. Recently, it was found that loss-of-function mutations of ATP2A2 confer a risk of neuropsychiatric disorders including depression, bipolar disorder and schizophrenia. In addition, a genome-wide association study found an association between ATP2A2 and schizophrenia. However, the mechanism of how ATP2A2 contributes to vulnerability to these mental disorders is unknown. Here, we analyzed Atp2a2 heterozygous brain-specific conditional knockout (hetero cKO) mice. The ER membranes prepared from the hetero cKO mouse brain showed decreased Ca2+ uptake activity. In Atp2a2 heterozygous neurons, decays of cytosolic Ca2+ level were slower than control neurons after depolarization. The hetero cKO mice showed altered behavioral responses to novel environments and impairments in fear memory, suggestive of enhanced dopamine signaling. In vivo dialysis demonstrated that extracellular dopamine levels in the NAc were indeed higher in the hetero cKO mice. These results altogether indicate that the haploinsufficiency of Atp2a2 in the brain causes prolonged cytosolic Ca2+ transients, which possibly results in enhanced dopamine signaling, a common feature of mood disorders and schizophrenia. These findings elucidate how ATP2A2 mutations causing a dermatological disease may exert their pleiotropic effects on the brain and confer a risk for mental disorders.

Brain-specific heterozygous loss-of-function of ATP2A2, endoplasmic reticulum Ca2+ pump responsible for Darier's disease, causes behavioral abnormalities and a hyper-dopaminergic state.

Author Correction: Adrenergic inhibition facilitates normalization of extracellular potassium after cortical spreading depolarization.

Visualization and tracking of the information flow in the broader brain area are essential because nerve cells make a vast network in the brain. Fluorescence Ca imaging is a simultaneous visualization of brain cell activities in a wide area. Instead of classical chemical indicators, developing various types of transgenic animals that express Ca-sensitive fluorescent proteins enables us to observe brain activities in living animals at a larger scale for a long time. Multiple kinds of literature have reported that transcranial imaging of such transgenic animals is practical for monitoring the wide-field information flow across the broad brain regions, although it has a lower spatial resolution. Notably, this technique is helpful for the initial evaluation of cortical function in disease models. This review will introduce fully intact transcranial macroscopic imaging and cortex-wide Ca imaging as practical applications.

Transcranial cortex-wide Ca imaging for the functional mapping of cortical dynamics.

Optical errors, including spherical aberrations, hinder high-resolution imaging of biological samples due to biochemical components and physical properties. We developed the Deep-C microscope system to achieve aberration-free images, employing a motorized correction collar and contrast-based calculations. However, current contrast-maximization techniques, such as the Brenner gradient method, inadequately assess specific frequency bands. The Peak-C method addresses this issue, but its arbitrary neighbor selection and susceptibility to the noise limit its effectiveness. In this paper, we emphasize the importance of a broad spatial frequency range for accurate spherical aberration correction and propose Peak-F. This spatial frequency-based system utilizes a fast Fourier transform (FFT) as a band-pass filter. This approach overcomes Peak-C's limitations and comprehensively covers the low-frequency domain of image spatial frequencies.

Article	Year
Aquaporin-4-dependent glymphatic solute transport in the rodent brain. eLife\| PubMed ID: 30561329	2018
Adrenergic receptor antagonism induces neuroprotection and facilitates recovery from acute ischemic stroke. Proceedings of the National Academy of Sciences of the United States of America\| PubMed ID: 31097598	2019
Transcranial Direct Current Stimulation (tDCS) Induces Adrenergic Receptor-Dependent Microglial Morphological Changes in Mice. eNeuro\| PubMed ID: 31444225	2019
Adrenergic inhibition facilitates normalization of extracellular potassium after cortical spreading depolarization. Scientific reports\| PubMed ID: 33854148	2021
Brain-specific heterozygous loss-of-function of ATP2A2, endoplasmic reticulum Ca2+ pump responsible for Darier's disease, causes behavioral abnormalities and a hyper-dopaminergic state. Human molecular genetics\| PubMed ID: 34104969	2021
Author Correction: Adrenergic inhibition facilitates normalization of extracellular potassium after cortical spreading depolarization. Scientific reports\| PubMed ID: 35304562	2022
Transcranial cortex-wide Ca imaging for the functional mapping of cortical dynamics. Frontiers in neuroscience\| PubMed ID: 36875638	2023
Spatial frequency-based correction of the spherical aberration in living brain imaging. Microscopy (Oxford, England)\| PubMed ID: 37315186	2023

Hiromu Monai

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