The Role Of Cell Proliferation In Hepatic Encephalopathy

Abstract

Chronic alcohol misuse or alcohol use disorder is a common problem worldwide. Excessive alcohol consumption can affect almost every organ in the body but neurological complications can occur due to acute intoxicating effects as well as longer term damage known as alcohol-related brain damage. The cognitive impairments associated with chronic alcohol misuse can be compounded by associated liver damage leading to an increase in circulating neurotoxic substances such as ammonia giving rise to a condition known as hepatic encephalopathy. The pathogenesis of hepatic encephalopathy is currently unknown however animal models of alcohol misuse suggest that aberrant cell proliferation attributed to neurogenesis may play a role.

Neurogenesis occurs in the adult mammalian brain in two neurogenic niches the subventricular zone lining the lateral ventricles and the subgranular zone of the hippocampus and involves the proliferation of a neural stem cell and eventual integration of an immature neuron into the existing circuitry. Although this process has been widely proven in animals its existence in humans remains controversial. So, prior to addressing a role for neurogenesis in disease its existence needs to be proven in normal individuals. Here, cell proliferation and neurogenesis were simultaneously examined in the subventricular zone and subgranular zone of 23 individuals aged 0.2-59 years, using immunohistochemistry and immunofluorescence in combination with unbiased stereology. This demonstrated a marked decline in proliferating cells in both the subventricular zone and subgranular zone in early infancy such that the levels of proliferation were similar to the adjacent parenchyma by four and one years, respectively. Furthermore, the phenotype of proliferating cells changed with age such that in the adult subventricular zone and subgranular zone, and adjacent parenchyma, all proliferating cells co-localised with the microglial marker, Iba1. Taken together this suggests that adult neurogenesis is a residual process and that any potential disease-related alterations in proliferation in the adult brain are likely associated with microglia.

Indeed, widespread proliferating cells that co-localised with the microglial marker Iba1 were found in a subset of chronic alcoholics with a pathological diagnosis of HE. In contrast cases without microglial proliferation displayed microglial dystrophy and associated neuronal loss in the prefrontal cortex. There were no obvious differences between these subsets from the clinical and pathological data available.

To determine the cause and pathogenic significance of this microglial proliferation, a pilot study was conducted to develop an animal model of chronic hepatic encephalopathy using the hepatotoxin, thioacetamide and combinations of known risk factors for hepatic encephalopathy; chronic alcohol use and a high-fat diet. Animals receiving thioacetamide had macroscopic evidence of liver injury, elevations of transaminases and associated anxiety-like behaviour measured in an open-field test. There were however no associated microglial or astrocytic changes in these animals and combinations of alcohol and high-fat diet had no additional effects.

In conclusion, this work has shown that the majority of the rare proliferative events in the adult human brain are microglia. Chronic alcoholism with a pathological diagnosis of hepatic encephalopathy results in shifts in microglial phenotype with one subset of patients demonstrating proliferation and another dystrophy. Future work is required to develop an animal model of chronic hepatic encephalopathy, where the role of microglial dysfunction in hepatic encephalopathy pathogenesis can be further elucidated.