Truncated TrkB.T1-Mediated Astrocyte Dysfunction Contributes to Impaired Motor Function and Neuropathic Pain after Spinal Cord Injury

Following spinal cord injury (SCI), astrocytes demonstrate long-lasting reactive changes, which are associated with the persistence of neuropathic pain and motor dysfunction. We previously demonstrated that upregulation of trkB. T1, a truncated isoform of the brain-derived neurotrophic factor receptor (BDNF), contributes to gliosis after SCI, but little is known about the effects of trkB. T1 on the function of astrocytes. As trkB. T1 is the sole isoform of trkB receptors expressed on astrocytes, we examined the function of trkB.T1-driven astrocytes in vitro and in vivo. Immunohistochemistry showed that trkB. T1(+) cells were significantly upregulated 7 d after injury, with sustained elevation in white matter through 8 weeks. The latter increase was predominantly found in astrocytes. TrkB.T1 was also highly expressed by neurons and microglia/macrophages at 7 d after injury and declined by 8 weeks. RNA sequencing of cultured astrocytes derived from trkB. T1(+/+) (WT) and trkB.T1(-/-) (KO) mice revealed downregulation of migration and proliferation pathways in KO astrocytes. KO astrocytes also exhibited slower migration/proliferation in vitro in response to FBS or BDNF compared with WT astrocytes. Reduced proliferation of astrocytes was also confirmed after SCI in astrocyte-specific trkB.T1 KO mice; using mechanical allodynia and pain-related measurements on the CatWalk, these animals also showed reduced hyperpathic responses, along with improved motor coordination. Together, our data indicate that trkB.T1 in astrocytes contributes to neuropathic pain and neurological dysfunction following SCI, suggesting that trkB. T1 may provide a novel therapeutic target for SCI.