Impaired brain glymphatic flow in experimental hepatic encephalopathy

Abstract

Background & aims: Neuronal function is exquisitely sensitive to alterations in the extracellular environment. In patients with hepatic encephalopathy (HE), accumulation of metabolic waste products and noxious substances in the interstitial fluid of the brain is thought to result from liver disease and may contribute to neuronal dysfunction and cognitive impairment. This study was designed to test the hypothesis that the accumulation of these substances, such as bile acids, may result from reduced clearance from the brain.

Methods: In a rat model of chronic liver disease with minimal HE (the bile duct ligation [BDL] model), we used emerging dynamic contrast-enhanced MRI and mass-spectroscopy techniques to assess the efficacy of the glymphatic system, which facilitates clearance of solutes from the brain. Immunofluorescence of aquaporin-4 (AQP4) and behavioural experiments were also performed.

Results: We identified discrete brain regions (olfactory bulb, prefrontal cortex and hippocampus) of altered glymphatic clearance in BDL rats, which aligned with cognitive/behavioural deficits. Reduced AQP4 expression was observed in the olfactory bulb and prefrontal cortex in HE, which could contribute to the pathophysiological mechanisms underlying the impairment in glymphatic function in BDL rats.

Conclusions: This study provides the first experimental evidence of impaired glymphatic flow in HE, potentially mediated by decreased AQP4 expression in the affected regions.

Lay summary: The 'glymphatic system' is a newly discovered brain-wide pathway that facilitates clearance of various substances that accumulate in the brain due to its activity. This study evaluated whether the function of this system is altered in a model of brain dysfunction that occurs in cirrhosis. For the first time, we identified that the clearance of substances from the brain in cirrhosis is reduced because this clearance system is defective. This study proposes a new mechanism of brain dysfunction in patients with cirrhosis and provides new targets for therapy.

Keywords: Cirrhosis; Glymphatic system; Hepatic encephalopathy; MRI; Mass spectrometry.

Graphic abstract

Fig. 1. Schematic of proposed alterations in glymphatic function in hepatic encephalopathy.

In health, glymphatic inflow of CSF occurs parallel to arterial flow along the periarterial space (between the basement membrane of smooth muscle cells and pia mater), where the water component of CSF crosses the astrocytic AQP4 channels polarised to astrocyte end-feet and enters the brain parenchyma. Here, CSF exchanges with ISF, allowing interstitial solutes to be cleared out of the parenchyma via astroglial transporters or channels, or pass through the astrocytic end-feet clefts to the perivenous space. Effluxed waste is then cleared out of the CSF pool via absorption by the meningeal lymphatic system. In hepatic encephalopathy, our data suggests that this brain-wide clearance system becomes dysfunctional, possibly because of reduced vessel coverage and polarisation of AQP4, leading to toxic accumulation of interstitial solutes in the parenchyma. Arrows indicate direction of flow. AQP4, aquaporin-4; CSF, cerebrospinal fluid; ISF, interstitial fluid.

Fig. 2. Impaired contrast agent penetration in the brain of animals with HE.

(A) Representative images of dynamic contrast-enhanced MRI of sham-operated (n = 5) and BDL (n = 5) animals. Pseudocolour scaling illustrates distribution of gadolinium throughout the brain over 144 min of recording, with the BDL brain showing reduced contrast agent and therefore glymphatic inflow in rostral areas. (B) Summary data illustrating resting ICP in sham-operated and BDL animals. Summary data showing MR contrast intensity changes in the (C) olfactory bulb (D) prefrontal cortex and, (E) hippocampus of sham-operated and BDL rats. Inset: Schematic drawing illustrating brain regions of interest. Shading indicates period of contrast agent infusion. Statistical comparisons were performed using 2-way ANOVA followed by Bonferroni post hoc test. p values indicate the level of significant differences between the groups. BDL, bile duct ligation; HE, hepatic encephalopathy.

Fig. 3. Unchanged contrast agent penetration in the striatum, caudal cortex, midbrain, thalamus, and hypothalamus of animals with HE.

Intensity (% change from baseline) vs. time plots of contrast agent penetration showing no difference between sham-operated (n = 5) and BDL (n = 5) rats in the (A) striatum, (B) caudal cortex, (C) midbrain, (D) thalamus, (E) temporalis muscle, acting as negative control with lack of contrast agent in the temporalis muscle representing a sealed system and no CSF leakage due to the cannula implantation, and (F) hypothalamus. Grey shading indicates period of contrast agent infusion. Statistical comparisons were performed using 2-way ANOVA followed by Bonferroni post hoc test. p values indicate the level of differences between the sham-operated and BDL groups. BDL, bile duct ligation; HE, hepatic encephalopathy.

Fig. 4. Contrast agent inflow in CSF filled compartments (aqueduct, lateral ventricles and third ventricle) and volumes of select brain regions are not altered in HE.

Intensity (% change from baseline) vs. time plots of contrast agent penetration indicating no difference between sham-operated (n = 5) and BDL (n = 5) rats in the (A) aqueduct, (B) lateral ventricles and (C) 3rd ventricle. Grey shading indicates period of contrast agent infusion. (D) Summary data illustrating volume (mm) of the brain regions of interest obtained from 3D ROI measurements of contrast-enhanced MR images from sham-operated and BDL animals, with no significant differences reported. Statistical comparisons were performed using 2-way ANOVA followed by Bonferroni post hoc test. p values indicate the level of differences between the sham-operated and BDL groups. BDL, bile duct ligation; CSF, cerebrospinal fluid; HE, hepatic encephalopathy; ROI, region of interest.

Fig. 5. Impaired Gd-DTPA clearance in the prefrontal cortex, evaluated by SF-ICP-MS in plasma, CSF and brain tissue of animals with HE.

Summary data illustrating Gd concentration in the (A) plasma, (B) CSF and (C) frontal, middle and hind brain tissue of sham-operated (n = 6) and BDL (n = 6) animals. Statistical comparisons for CSF and plasma samples were performed using a Student’s t test while statistical comparisons of brain data were performed using 2-way ANOVA followed by Bonferroni post hoc test. p values indicate the level of differences between the sham-operated and BDL groups. BDL, bile duct ligation; CSF, cerebrospinal fluid; HE, hepatic encephalopathy.

Fig. 6. Altered AQP4 vs. Isolectin expression, in the olfactory bulb and prefrontal cortex of animals with HE.

(A-D) Plots of fluorescence signal intensity vs. distance across blood vessels for AQP4 and Isolectin expression in the olfactory bulb and prefrontal cortex of sham-operated (n = 4) and BDL rats (n = 4). (E) Summary data indicating area under the curve of the peaks in the intensity graphs, showing significantly lower AQP4 expression compared to Isolectin in both the olfactory bulb and prefrontal cortex of BDL compared to sham-operated animals. Statistical comparisons between immunofluorescence line profile data, and area under the curve were performed using 2-way ANOVA followed by Bonferroni post hoc test. For within group comparison *p <0.05, **p <0.01, ***p <0.001. p values indicate the level of differences between Isolectin and AQP4 within each animal group. AQP4, aquaporin-4; BDL, bile duct ligation.

Fig. 7. AQP4 polarisation and vessel coverage is not significantly lower in the olfactory bulb and prefrontal cortex of animals with HE.

(A) Summary data illustrating AQP4 polarisation in the olfactory bulb and prefrontal cortex of sham-operated and BDL animals, with no significant differences reported. (B) Summary data illustrating AQP4 vessel coverage in the olfactory bulb and prefrontal cortex of sham-operated and BDL animals, with no significant differences reported. Statistical comparison was performed using 2-way ANOVA followed by Bonferroni post hoc test. p values indicate the level of differences between the sham-operated and BDL groups. AQP4, aquaporin-4; BDL, bile duct ligation; HE, hepatic encephalopathy.

Fig. 8. Cognitive/behavioural deficits in HE.

(A) Summary data illustrating time latency required to reach the escape box in the working memory task in sham-operated (n = 5) and BDL (n = 5) rats. Summary data illustrating (B) retention latency in the working memory task, (C) latency, in the spatial reference memory task, (D) speed of animal during the reference memory task, (E) distance travelled during the reference memory task and (F) path efficiency from the starting point to the escape box during the reference memory task comparing sham-operated and BDL rats. Statistical comparisons of data between sham-operated and BDL animals were performed via 2-way ANOVA with Bonferroni post hoc test. For comparison of data within animal groups along training days, 2-way ANOVA was applied followed by Tukey post hoc test. p values indicate the level of significant differences between the sham-operated and BDL groups. For within group comparison *p <0.05, ***p <0.001. BDL, bile duct ligation.