Blood-brain barrier penetration of non-replicating SARS-CoV-2 and S1 variants of concern induce neuroinflammation which is accentuated in a mouse model of Alzheimer's disease

COVID-19 and especially Long COVID are associated with severe CNS symptoms and may place persons at risk to develop long-term cognitive impairments. Here, we show that two non-infective models of SARS-CoV-2 can cross the blood-brain barrier (BBB) and induce neuroinflammation, a major mechanism underpinning CNS and cognitive impairments, even in the absence of productive infection. The viral models cross the BBB by the mechanism of adsorptive transcytosis with the sugar N-acetylglucosamine being key. The delta and omicron variants cross the BB B faster than the other variants of concern, with peripheral tissue uptake rates also differing for the variants. Neuroinflammation induced by icv injection of S1 protein was greatly enhanced in young and especially in aged SAMP8 mice, a model of Alzheimer's disease, whereas sex and obesity had little effect.

Transport of the Proinflammatory Chemokines C-C Motif Chemokine Ligand 2 (MCP-1) and C-C Motif Chemokine Ligand 5 (RANTES) across the Intact Mouse Blood-Brain Barrier Is Inhibited by Heparin and Eprodisate and Increased with Systemic Inflammation

One important function of the vascular blood-brain barrier (BBB) is to facilitate neuroimmune communication. The BBB fulfills this function, in part, through its ability to transport cytokines and chemokines. C-C motif chemokine receptor 2 (CCL2) (MCP-1) and C-C motif chemokine receptor 5 (CCL5) (RANTES) are proinflammatory chemokines that mediate neuroimmune responses to acute insults and aspects of brain injury and neurodegenerative diseases; however, a blood-to-brain transport system has not been evaluated for either chemokine in vivo. Therefore, we determined whether CCL2 and CCL5 in blood can cross the intact BBB and enter the brain. Using CD-1 mice, we found that ¹²⁵I-labeled CCL2 and CCL5 crossed the BBB and entered the brain parenchyma. We next aimed to identify the mechanisms of ¹²⁵I-CCL2 and ¹²⁵I-CCL5 transport in an in situ brain perfusion model. We found that both heparin and eprodisate inhibited brain uptake of ¹²⁵I-CCL2 and ¹²⁵I-CCL5 in situ, whereas antagonists of their receptors, CCR2 or CCR5, respectively, did not, suggesting that heparan sulfates at the endothelial surface mediate BBB transport. Finally, we showed that CCL2 and CCL5 transport across the BBB increased following a single injection of 0.3 mg/kg lipopolysaccharide. These data demonstrate that CCL2 and CCL5 in the brain can derive, in part, from the circulation, especially during systemic inflammation. Further, binding to the BBB-associated heparan sulfate is a mechanism by which both chemokines can cross the intact BBB, highlighting a novel therapeutic target for treating neuroinflammation. SIGNIFICANCE STATEMENT: Our work demonstrates that C-C motif chemokine ligand 2 (CCL2) and C-C motif chemokine ligand 5 (CCL5) can cross the intact blood-brain barrier and that transport is robustly increased during inflammation. These data suggest that circulating CCL2 and CCL5 can contribute to brain levels of each chemokine. We further show that the transport of both chemokines is inhibited by heparin and eprodisate, suggesting that CCL2/CCL5-heparan sulfate interactions could be therapeutically targeted to limit accumulation of these chemokines in the brain.

Measurement of Blood-Brain Barrier Disruption in Mice Following Ozone Exposure Using Highly Sensitive Radiotracer Assays

Ozone is a widespread air toxicant. Although its primary target organ is the lungs, emerging evidence suggests that ozone also has harmful effects on the brain. The vascular blood-brain barrier (BBB), an endothelial interface that regulates passage of substances between the brain and peripheral tissues, is a likely mediator of ozone's adverse effects on the brain. Ozone can cause BBB disruption, a pathological state in which the BBB becomes leaky, resulting in the unregulated entry of circulating substances into the brain. BBB disruption can be detected using many methods, which each have their strengths and limitations. Recent data suggest that BBB disruption can occur in mice following ozone exposures, albeit at a low level. Therefore, robust and highly sensitive assays for BBB disruption are needed. Assays commonly used to detect BBB disruption, however, can be time consuming, lack sensitivity, and can be vulnerable to artifacts that are typically not addressed in the experimental design. Radiochemical assays are among the most sensitive and specific for detecting subtle disruptions of the BBB and require minimal sample processing for detection. Radiochemical assays can also be multiplexed to include radiotracer conjugates of large and small molecular weights, and the uptake of each of them can provide information about the severity and mechanism of BBB disruption. Here, we describe a protocol to use two of these radiotracer conjugates, ¹⁴ C-sucrose and ^99m Tc- albumin, to measure BBB disruption following an acute exposure to ozone in mice. We provide the steps to expose mice acutely to ozone, to label albumin with ^99m Tc-pertechnetate, and to measure BBB disruption by evaluating permeability to ^99m Tc-albumin and ¹⁴ C-sucrose after ozone exposure. These methods can be adapted to different ozone exposure paradigms and to different rodent species/strains, allowing for the sensitive and rapid assessment of BBB disruption that is detectable in whole brains or in brain regions. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Ozone exposures in mice Basic Protocol 2: Measurement of blood-brain barrier disruption by evaluating permeability to ¹⁴ C-sucrose and ^99m Tc-albumin Support Protocol: Labeling of bovine serum albumin with ^99m Tc.

Effects of Anesthesia on Ozone-Induced Lung and Systemic Inflammation

PURPOSE

Anesthetics are required for procedures that deliver drugs/biologics, infectious/inflammatory agents, and toxicants directly to the lungs. However, the possible confounding effects of anesthesia on lung inflammation and injury are underreported. Here, we evaluated the effects of two commonly used anesthetic regimens on lung inflammatory responses to ozone in mice.

METHODS

We tested the effects of brief isoflurane (Iso) or ketamine/xylazine/atipamezole (K/X/A) anesthesia prior to ozone exposure (4 h, 3 ppm) on lung inflammatory responses in mice. Anesthesia regimens modeled those used for non-surgical intratracheal instillations and were administered 1-2 h or 24 h prior to initiating ozone exposure.

RESULTS

We found that Iso given 1-2 h prior to ozone inhibited inflammatory responses in the lung, and this effect was absent when Iso was given 23-24 h prior to ozone. In contrast, K/X/A given 1-2 h prior to ozone increased lung and systemic inflammation.

CONCLUSION

Our results highlight the need to comprehensively evaluate anesthesia as an experimental variable in the assessment of lung inflammation in response to ozone and other inflammatory stimuli.

The S1 protein of SARS-CoV-2 crosses the blood-brain barrier in mice

It is unclear whether severe acute respiratory syndrome coronavirus 2, which causes coronavirus disease 2019, can enter the brain. Severe acute respiratory syndrome coronavirus 2 binds to cells via the S1 subunit of its spike protein. We show that intravenously injected radioiodinated S1 (I-S1) readily crossed the blood-brain barrier in male mice, was taken up by brain regions and entered the parenchymal brain space. I-S1 was also taken up by the lung, spleen, kidney and liver. Intranasally administered I-S1 also entered the brain, although at levels roughly ten times lower than after intravenous administration. APOE genotype and sex did not affect whole-brain I-S1 uptake but had variable effects on uptake by the olfactory bulb, liver, spleen and kidney. I-S1 uptake in the hippocampus and olfactory bulb was reduced by lipopolysaccharide-induced inflammation. Mechanistic studies indicated that I-S1 crosses the blood-brain barrier by adsorptive transcytosis and that murine angiotensin-converting enzyme 2 is involved in brain and lung uptake, but not in kidney, liver or spleen uptake.