A2 Astrocytes Counteract the Blood-Brain Barrier Dysfunction Via α7nAChR/JAK2/STAT3 Signaling in HIV-Associated Neurocognitive Disorders

HIV-associated neurocognitive disorders (HAND), containing wide-spectrum manifestations, lead to cognitive declines among HIV-1 infected patients. Blood-brain barrier (BBB) disruption putatively underlies the progression of HAND, while molecular mechanisms involving BBB breakdown remain elusive. Recent studies highlight the crucial role of astrocytes in HAND, as astrocytes, the most abundant glial cells supporting intact BBB functions, undergo a pronounced phenotypic conversion upon disease context. Neurotoxic reactive astrocytes, termed A1 astrocytes, are destructive to BBB via upregulation of classical complement cascade genes, while A2 astrocytes are postulated to be protective. Although extensive astrocyte activation is present in the CNS of patients suffering from HAND, little is known about the contribution of reactive astrocytes to HAND neuropathology.

Considering the dual role of reactive astrocytes, mechanisms facilitating A2 astrocytes formation could regulate the integrity of BBB. Robust evidence demonstrates that α7 nicotinic acetylcholine receptors (α7nAChR) with various cellular functions participate in the morphological changes of astrocytes. Here, we took advantage of primary astrocytes, HIV-1 gp120 transgenic mice (gp120tg mice), gp120tg mice and aimed to interrogate whether α7nAChR-mediated A2 astrocyte formation contributes to maintain the BBB functions. Our results in primary astrocytes showed that blockage of α7nAChR promoted a plethora of A2-specific gene activation, including S100a10, Slc10a6, Tm4sf1, etc. These genes encode proteins that are related to BBB development and functioning. Then we sought to characterize what cytokines associated with the maintenance of BBB altered in gp120-induced astrocytes followed A2 astrocyte formation. Strikingly, the increase of A2-specific genes was consonant with the dramatic production of neurotrophic factors, including nerve growth factor, brain-derived neurotrophic factor, and glial cell line-derived neurotrophic factor. To pinpoint the role of α7nAChR-mediated A2 astrocytes in BBB development, we straightforwardly monitored the A2 astrocyte responses in α7^-/-gp120tg mice. In line with the in vitro findings, α7nAChR knockout was sufficient to trigger the activation of the predominant gene pattern characterized by A2 astrocytes in the hippocampus of α7^-/-gp120tg mice as compared to gp120tg mice, resulting in the limited levels of serum Il-1α, Il-1β, Il-6, and Tnf-α in α7^-/-gp120tg mice. In particular, the increase of A2-specific genes was consonant with the dramatic production of neurotrophic factors, including nerve growth factor, brain-derived neurotrophic factor, and glial cell line-derived neurotrophic factor. Notably, the α7^-/-gp120tg mice with A2-specific transcriptomic profiling exhibited a normal morphology in the cortex and hippocampus, where gp120tg mice displayed visible neuronal loss and aberrations. Furthermore, the α7^-/-gp120tg mice with robust A2 astrocyte responses showed a clear preference for the target quadrant during the Water Maze test, suggesting that A2 astrocyte activation mediated by α7nAChR deficiency ameliorated spatial learning and memory impairment in gp120tg mice.

Studies showed that a shift towards increased KYNA synthesis is associated with the increased KYNA levels when the primary route for tryptophan catabolism was inhibited in HD flies and Alzheimer's disease mouse model. To examine the elevated levels of KYNA in CNS of gp120tg mice, the endogenous synthesis strategies were utilized to convert dietary tryptophan to KYNA, as KYNA cannot penetrate across the blood-brain barrier due to its polar structure. Our results demonstrated that tryptophan administration dramatically promotes A2-astrocyte formation in gp120tg mice, suggesting the protective mechanism of tryptophan and its metabolites in neuronal damage and cognitive decline. Collectively, we found that the protective impacts of A2 astrocyte generation mediated by the α7nAChR/JAK2/STAT3 signaling pathway are associated with robust elevated neurotrophic factor levels and limited expression of proinflammatory cytokines, laying a consolidation for the therapeutic potential targeting A2 astrocyte formation. (Acknowledgements: Corresponding author: Hong Cao, gzhcao@smu.edu.cn; National Natural Science Foundation of China, No. 82172259 to H.C.)

No relevant conflicts of interest to declare.