Abstract
BackgroundComplex changes in the brain microenvironment following traumatic brain injury (TBI) can cause neurological impairments for which there are few efficacious therapeutic interventions. The reactivity of astrocytes is one of the keys to microenvironmental changes, such as neuroinflammation, but its role and the molecular mechanisms that underpin it remain unclear.MethodsMale C57BL/6J mice were subjected to the controlled cortical impact (CCI) to develop a TBI model. The specific ligand of AXL receptor tyrosine kinase (AXL), recombinant mouse growth arrest-specific 6 (rmGas6) was intracerebroventricularly administered, and selective AXL antagonist R428 was intraperitoneally applied at 30 min post-modeling separately. Post-TBI assessments included neurobehavioral assessments, transmission electron microscopy, immunohistochemistry, and western blotting. Real-time polymerase chain reaction (RT-PCR), siRNA transfection, and flow cytometry were performed for mechanism assessments in primary cultured astrocytes.ResultsAXL is upregulated mainly in astrocytes after TBI and promotes astrocytes switching to a phenotype that exhibits the capability of ingesting degenerated neurons or debris. As a result, this astrocytic transformation promotes the limitation of neuroinflammation and recovery of neurological dysfunction. Pharmacological inhibition of AXL in astrocytes significantly decreased astrocytic phagocytosis both in vivo and in primary astrocyte cultures, in contrast to the effect of treatment with the rmGas6. AXL activates the signal transducer and activator of the transcription 1 (STAT1) pathway thereby further upregulating ATP-binding cassette transporter 1 (ABCA1). Moreover, the supernatant from GAS6-depleted BV2 cells induced limited enhancement of astrocytic phagocytosis in vitro.ConclusionOur work establishes the role of AXL in the transformation of astrocytes to a phagocytic phenotype via the AXL/STAT1/ABCA1 pathway which contributes to the separation of healthy brain tissue from injury-induced cell debris, further ameliorating neuroinflammation and neurological impairments after TBI. Collectively, our findings provide a potential therapeutic target for TBI.
Highlights
Complex changes in the brain microenvironment following traumatic brain injury (TBI) can cause neurological impairments for which there are few efficacious therapeutic interventions
The results showed that TBI mice spent more time immobile in the TST compared to the sham group, and recombinant mouse growth arrest-specific 6 (rmGas6)-treated mice spent less time immobile compared to the TBI vehicle group
The Growth arrest specific 6 (GAS6)-deficient supernatant significantly abrogated the effect of promoting astrocytic phagocytosis compared with the supernatant from scramble siRNA-transfected BV2 cells while the scramble siRNA-treated BV2-derived supernatant has a similar effect with the untreated BV2 supernatant (Fig. 7G). These results suggested that microglia can regulate the astrocytic transformation and phagocytosis which is mediated by AXL receptor tyrosine kinase (AXL) signaling transduction via the secretion of GAS6
Summary
Complex changes in the brain microenvironment following traumatic brain injury (TBI) can cause neurological impairments for which there are few efficacious therapeutic interventions. The initiation of mechanical damage and subsequent injurious biochemical cascades induce cerebral microenvironment changes that underlie many neurological and psychiatric function disorders observed in TBI patients. Microglia are resident immune cells involved in brain homeostasis surveillance [3] that have a sensitive response and rapid activation within 1 hour after TBI. This is followed by reactive astrocytes, which are resident brain cells that can dramatically transform their phenotype as a result of brain environment changes [4, 5]. Regarding glial activation under pathological conditions, it remains unclear how these two types of glial cells interact
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
