Acute systemic LPS-exposure impairs perivascular CSF distribution in mice.

BACKGROUND: The exchange of cerebrospinal (CSF) and interstitial fluid is believed to be vital for waste clearance in the brain. The sleep-dependent glymphatic system, which is comprised of perivascular flow of CSF and is largely dependent on arterial pulsatility and astrocytic aquaporin-4 (AQP4) expression, facilitates much of this brain clearance. During the last decade, several observations have indicated that impaired glymphatic function goes hand in hand with neurodegenerative diseases. Since pathologies of the brain carry inflammatory components, we wanted to know how acute inflammation, e.g., with lipopolysaccharide (LPS) injections, would affect the glymphatic system. In this study, we aim to measure the effect of LPS on perivascular CSF distribution as a measure of glymphatic function. METHODS: Three hours after injection of LPS (1 mg/kg i.p.), C57bl/6 mice were (1) imaged for two CSF tracers, injected into cisterna magna, (2) transcardially perfused with buffer, or (3) used for physiological readouts. Tracer flow was imaged using a low magnification microscope on fixed brains, as well as using vibratome-cut slices for measuring tracer penetration in the brain. Cytokines, glial, and BBB-permeability markers were measured with ELISAs, Western blots, and immunohistochemistry. Cerebral blood flow was approximated using laser Doppler flowmetry, respiration and heart rate with a surgical monitor, and AQP4-polarization was quantified using confocal microscopy of immunolabeled brain sections. RESULTS: LPS-injections significantly lowered perivascular CSF tracer flow and penetration into the parenchyma. No differences in AQP4 polarization, cytokines, astroglial and BBB markers, cerebral blood flow, or respiration were detected in LPS-injected mice, although LPS did elevate cortical Iba1+ area and heart rate. CONCLUSIONS: This study reports another physiological response after acute exposure to the bacterial endotoxin LPS, namely the statistically significant decrease in perivascular distribution of CSF. These observations may benefit our understanding of the role of systemic inflammation in brain clearance.

The neuritic plaque in Alzheimer's disease: perivascular degeneration of neuronal processes.

Cerebrovascular pathology is common in aging and Alzheimer's disease (AD). The microvasculature is particularly vulnerable, with capillary-level microhemorrhages coinciding with amyloid beta deposits in senile plaques. In the current analysis, we assessed the relationship between cerebral microvessels and the neuritic component of the plaque in cortical and hippocampal 50- to 200-µm sections from 11 AD, 3 Down syndrome, and 7 nondemented cases in neuritic disease stages 0-VI. We report that 77%-97% of neuritic plaques are perivascular, independently of disease stage or dementia diagnosis. Within neuritic plaques, dystrophic hyperphosphorylated tau-positive neurites appear as clusters of punctate, bulbous, and thread-like structures focused around capillaries and colocalize with iron deposits characteristic of microhemorrhage. Microvessels within the neuritic plaque are narrowed by 1.0 ± 1.0 µm-4.4 ± 2.0 µm, a difference of 16%-65% compared to blood vessel segments with diameters 7.9 ± 2.0-6.4 ± 0.8 µm (p < 0.01) outside the plaque domain. The reduced capacity of microvessels within plaques, frequently below patency, likely compromises normal microlocal cerebrovascular perfusion. These data link the neuritic and amyloid beta components of the plaque directly to microvascular degeneration. Strategies focused on cerebrovascular antecedents to neuritic dystrophy in AD have immediate potential for prevention, detection, and therapeutic intervention.

Intranasal Losartan Decreases Perivascular Beta Amyloid, Inflammation, and the Decline of Neurogenesis in Hypertensive Rats.

The contribution of the local angiotensin receptor system to neuroinflammation, impaired neurogenesis, and amyloid beta (Aß) accumulation in Alzheimer's disease (AD) and in hypertension is consistent with the remarkable neuroprotection provided by angiotensin receptor blockers (ARBs) independent of their blood pressure-lowering effect. Considering the causal relationship between hypertension and AD and that targeting cerebrovascular pathology with ARBs does not necessarily require their systemic effects, we tested intranasal losartan in the rat model of chronic hypertension (spontaneously hypertensive stroke-prone rats, SHRSP). Intranasal losartan at a subdepressor dose decreased mortality, neuroinflammation, and perivascular content of Aß by enhancing key players in its metabolism and clearance, including insulin-degrading enzyme, neprilysin, and transthyretin. Furthermore, this treatment improved neurologic deficits and increased brain IL-10 concentration, hippocampal cell survival, neurogenesis, and choroid plexus cell proliferation in SHRSP. Losartan (1 µM) also reduced LDH release from cultured astroglial cells in response to toxic glutamate concentrations. This effect was completely blunted by IL-10 antibodies. These findings suggest that intranasal ARB treatment is a neuroprotective, neurogenesis-inducing, and Aß-decreasing strategy for the treatment of hypertensive stroke and cerebral amyloid angiopathy acting at least partly through the IL-10 pathway.

Quantitative and qualitative assessment of glymphatic flux using Evans blue albumin.

BACKGROUND: The glymphatic system is a proposed pathway for clearance of proteins and macromolecules from brain, and disrupted glymphatic flux is implicated in neurological disease. We capitalized on colorimetric, fluorescent, and protein-binding properties of Evans blue to evaluate glymphatic flux. NEW METHOD: Twenty-five µL of 1% Evans blue-labeled albumin (EBA) in artificial cerebrospinal fluid (aCSF) was injected into the intracisternal space of anesthetized postnatal day 17 rats. Serum was collected at various time points after injection (n = 37) and EBA was measured spectrophotometrically. In separate rats (n = 3), a cranial window was placed over the parietal cortex and EBA transit was evaluated using in vivo multiphoton microscopy. Separate rats (n = 6) were processed for immunohistochemistry to examine localization of EBA. In some rats, intracranial pressure (ICP) was increased via intracisternal injection of aCSF. RESULTS: EBA was detected in serum as early as 30 min, was maximal at 4 h, and was undetectable at 72 h after intracisternal injection. Using intra-vital microscopy and immunohistochemistry EBA could be tracked from CSF to perivascular locations. Consistent with removal via glymphatic flux, increasing ICP to 40 mmHg accelerated transit of EBA from CSF to blood. COMPARISON WITH EXISTING METHODS: Transit of EBA from CSF to serum could be quantified spectrophotometrically without radioactive labeling. Glymphatic flux could also be qualitatively evaluated using EBA fluorescence. CONCLUSION: We present a novel technique for simultaneous quantitative and qualitative evaluation of glymphatic flux in rats.

Loss of perivascular aquaporin-4 in idiopathic normal pressure hydrocephalus.

Idiopathic normal pressure hydrocephalus (iNPH) is a subtype of dementia that may be successfully treated with cerebrospinal fluid (CSF) diversion. Recently, magnetic resonance imaging (MRI) using a MRI contrast agent as a CSF tracer revealed impaired clearance of the CSF tracer from various brain regions such as the entorhinal cortex of iNPH patients. Hampered clearance of waste solutes, for example, soluble amyloid-ß, may underlie neurodegeneration and dementia in iNPH. The goal of the present study was to explore whether iNPH is associated with altered subcellular distribution of aquaporin-4 (AQP4) water channels, which is reported to facilitate CSF circulation and paravascular glymphatic drainage of metabolites from the brain parenchyma. Cortical brain biopsies of 30 iNPH patients and 12 reference individuals were subjected to AQP4 immunogold cytochemistry. Electron microscopy revealed significantly reduced density of AQP4 water channels in astrocytic endfoot membranes along cortical microvessels in patients with iNPH versus reference subjects. There was a significant positive correlation between density of AQP4 toward endothelial cells (perivascular) and toward parenchyma, but the reduced density of AQP4 toward parenchyma was not significant in iNPH. We conclude that perivascular AQP4 expression is attenuated in iNPH, potentially contributing to impaired glymphatic circulation, and waste clearance, and subsequent neurodegeneration. Hence, restoring normal perivascular AQP4 distribution may emerge as a novel treatment strategy for iNPH.