The S1PR2‐CCL2‐BDNF‐TrkB pathway mediates neuroinflammation and motor incoordination in hyperammonaemia

The neurotrophin brain-derived neurotrophic factor (BDNF) and its receptor tropomyosin-related kinase B (TrkB) have emerged as key mediators in the pathophysiology of several mood disorders, including anxiety and depression. However, therapeutic compounds that interact with TrkB receptors have been difficult to develop. Using a combination of structure-based in silico screening and high-capacity functional assays in recombinant and neuronal cells, we identified a low-molecular weight TrkB ligand (ANA-12) that prevented activation of the receptor by BDNF with a high potency. ANA-12 showed direct and selective binding to TrkB and inhibited processes downstream of TrkB without altering TrkA and TrkC functions. KIRA-ELISA analysis demonstrated that systemic administration of ANA-12 to adult mice decreased TrkB activity in the brain without affecting neuronal survival. Mice administered ANA-12 demonstrated reduced anxiety- and depression-related behaviors on a variety of tests predictive of anxiolytic and antidepressant properties in humans. This study demonstrates that structure-based virtual screening strategy can be an efficient method for discovering potent TrkB-selective ligands that are active in vivo. We further propose that ANA-12 may be a valuable tool for studying BDNF/TrkB signaling and may constitute a lead compound for developing the next generation of therapeutic agents for the treatment of mood disorders.

Liver failure affects brain function, leading to neurological and psychiatric alterations; such alterations are referred to as hepatic encephalopathy (HE). Early diagnosis of minimal HE reveals an unexpectedly high incidence of mild cognitive impairment and psychomotor slowing in patients with liver cirrhosis - conditions that have serious health, social and economic consequences. The mechanisms responsible for the neurological alterations in HE are beginning to emerge. New therapeutic strategies acting on specific targets in the brain (phosphodiesterase 5, type A GABA receptors, cyclooxygenase and mitogen-activated protein kinase p38) have been shown to restore cognitive and motor function in animal models of chronic HE, and NMDA receptor antagonists have been shown to increase survival in acute liver failure. This article reviews the latest studies aimed at understanding how liver failure affects brain function and potential ways to ameliorate these effects.

SUMMARY Amyloid plaques and tau tangles are common pathological hallmarks for Alzheimer’s disease (AD), however reducing Aβ production failed to relieve the symptoms of AD patients. Here we report a high GABA (γ-aminobutyric acid) content in reactive astrocytes in the dentate gyrus (DG) of a mouse model for AD (5xFAD) that results in increased tonic inhibition and memory deficit. We also confirm in human AD patient brains that dentate astrocytes have a high GABA content, suggesting that high astrocytic GABA level may be a novel biomarker and a potential diagnostic tool for AD. The excessive GABA in 5xFAD astrocytes is released through an astrocyte-specific GABA transporter GAT3/4, and significantly enhanced tonic GABA inhibition in dentate granule cells. Importantly, reducing tonic inhibition in 5xFAD mice rescues the impairment of long-term potentiation (LTP) and memory deficit. Thus, reducing tonic GABA inhibition in the DG may lead to a novel therapy for Alzheimer’s disease.