IMMUNOHISTOCHEMICAL STUDY OF THE BRAIN AQUAPORIN-4 IN THE RAT ACUTE LIVER FAILURE MODEL

Abstract

Acute liver failure as well as acute-on-chronic liver failure result in the development of acute hepatic encephalopathy (HE) characterized by the major pathophysiologic event in form of the brain edema formation. Among brain cells, astrocytes are considered to be a central cellular population most sensitive to ammonia exposure in HE being the primarily cellular source of glutamine synthetase for ammonia metabolism. Astrocyte swelling is considered to be a principal sign of acute HE, while the exact molecular mechanisms of this event are still not fully understood. According to current concepts on HE, alteration in AQP4 regulation can play one of the central roles in the brain edema development and progression in hyperammonemia states. Considering high region- and context-specific heterogeneity of astroglial populations in the CNS, AQP4 involvement in the links of HE can also sustain mentioned conventional diversity.
 The aim of the study: to determine the immunohistochemical level of the brain aquaporin 4 (AQP4) expression in the experimental acute liver failure in rats.
 Materials and methods. The study was conducted in Wistar rats: 5 sham (control) animals and 10 rats with acetaminophen induced liver failure model (AILF). The immunohistochemical study of AQP4 expression was carried out in the sensorimotor cortex, white matter, hippocampus, thalamus and caudate nucleus/putamen regions between 12 and 24 h after acetaminophen treatment.
 Results. Starting from the 6th hour after acetaminophen treatment all AILF-animals showed the progressive impairment of clinical signs of acute liver failure, evidenced histologically by spread liver centrilobular necrosis and finished in 6 rats by comatose state up to 24 h (constituted subgroup AILF-B, “non-survived”). 4 animals survived until the 24 h - subgroup AILF-A, “survived”. In the AILF-B group, starting from 16 to 24 hours after acetaminophen treatment, a significant (relative to control) regionally-specific dynamic increase AQP4 levels was observed in the brain: in the cortex – by 405.17 %, hippocampus – by 387.38%, caudate nucleus/putamen – by 314.11%; from 12th hour: in the thalamus – by 342.66% and subcortical white matter – by 297.77%; with the highest elevation of AQP4 expression in the cortex among other studied regions: by 5.05 times.
 Conclusions. AILF in rats induces dynamic increase in AQP4 levels in the cortex, hippocampus and caudate nucleus/putamen by 12th hours and in the white matter and thalamus – by 16th h after the acetaminophen overdosing with the highest elevation in the cortical region. The heterogeneity in the degree of AQP4 elevation among different brain regions potentially may indicate brain territories more susceptible for systemic toxic exposure and damage in acute liver failure. Furthermore, the earliest reliable increase of AQP4 levels in the cortex, hippocampus and caudate/putamen might propose the faster reactivity of the local astroglial populations in response to the hyperammonemia among other regions. Consequently, the later and lower rates of AQP4 elevation in the white matter might indicate local astroglia as less reactive and/or more protected from the harmful exposure at a certain time period of the experiment. The higher cortical levels of AQP4 in the non-survived animals compared to survived ones reflect the significance of AQP4-involving mechanisms in the aggravation of acute HE, as well as the role of AQP4 alterations in thanatogenesis in the conditions of acute liver failure.