The Toll of Hyperammonemia on the Brain

Acute liver failure (ALF) is a devastating disease characterized by inflammation, immune activation, and hyperammonemia.¹ These processes predispose patients to the development of cerebral edema, which can have fatal consequences.² Therefore, defining the mechanisms by which hyperammonemia results in cerebral edema in ALF is important in order to discover newer therapeutic targets. The article by Vijay et al³ in this issue of Cellular and Molecular Gastroenterology and Hepatology elucidates the role of Toll-like receptor 9 (TLR9) as a mediator of cerebral edema in a model of hyperammonemia.

In recent years, the contribution of inflammatory processes to central nervous system (CNS) dysfunction has become increasingly apparent. While explicitly autoimmune CNS disorders such as multiple sclerosis have long been associated with inflammatory changes,⁴ the pathophysiology of other CNS disorders, including neuropsychiatric disorders such as schizophrenia and autism⁵ and mood disorders such as major depression,⁶ have also implicated immune system dysfunction. These connections between the CNS and immune system reveal the vulnerability of the CNS to inflammatory changes associated with dysfunction of other organ systems. TLR3 and TLR4 ligands, for example, have been used in animals to model the gestational insults that contribute to neuropsychiatric disorders7, 8, 9 and the sickness behavior produced by TLR3 ligands¹⁰ is an explicit example of the effect of peripheral immune function on CNS performance. Given the striking inflammatory effects¹¹ and devastating CNS sequelae¹² associated with ALF, there remains an urgent need to investigate mechanisms by which immune changes associated with liver disease contribute to CNS-relevant phenotypes such as those resulting in brain edema, intracranial hypertension, and HE. Within these disorders, TLRs, given their crucial immune function¹³ and implication in both CNS¹⁴ and hepatic disease,¹⁵ are an appealing target for investigation.

Although it had previously been established that TLR9 signaling is necessary for the progression of acetaminophen-induced liver injury¹⁶ and therefore subsequent development of brain edema, the importance of the receptor for the earlier stages of this process had prevented understanding of its role for later stage disease. Here, the authors use a novel combination of ammonium acetate and Tlr9^–/– mice to directly induce hyperammonemia while maintaining liver function, allowing direct evaluation of receptor knockout’s effect on the subsequent development of brain edema. Further nuance is achieved by use of Tlr9^fl/fl mice crossed with mice expressing Cre recombinase under the control of the lysozyme promoter, generating macrophage and neutrophil conditional knockouts of TLR9. With these tools, the authors are able to establish that absence of TLR9 within these cell populations is capable of preventing ammonium acetate-induced increases in brain water, proinflammatory cytokine production, and hepatocyte swelling. The TLR9 antagonist ODN2088 is similarly able to prevent these phenotypes. Interestingly, increases in plasma DNA following ammonium acetate administration are observed regardless of TLR9 status, suggesting that this alteration is upstream of the receptor. Despite the undoubtedly interesting findings in this article, further studies are needed using models of brain edema with concomitant liver failure, which are closer to the human disease process.¹⁷

These findings suggest TLR9 as a key mediator of the progression from hyperammonemia to brain edema and associated intracranial hypertension. This knowledge of TLR9’s role in later stages of this process allows for further investigation into the therapeutic window for TLR9 antagonism and suggests a potential benefit even in cases where liver failure has progressed to hyperammonemia. This investigation of neuroimmune regulation of brain edema could set the basis for new therapeutic options for the prevention and treatment of this feared complication of acute liver failure.