Accumulation of non‐erythroid αII‐spectrin and calpain‐cleaved αII‐spectrin breakdown products in cerebrospinal fluid after traumatic brain injury in rats

Although a number of increased CSF proteins have been correlated with brain damage and outcome after traumatic brain injury (TBI), a major limitation of currently tested biomarkers is a lack of specificity for defining neuropathological cascades. Identification of surrogate biomarkers that are elevated in CSF in response to brain injury and that offer insight into one or more pathological neurochemical events will provide critical information for appropriate administration of therapeutic compounds for treatment of TBI patients. Non‐erythroid αII‐spectrin is a cytoskeletal protein that is a substrate of both calpain and caspase‐3 cysteine proteases. As we have previously demonstrated, cleavage of αII‐spectrin by calpain and caspase‐3 results in accumulation of protease‐specific spectrin breakdown products (SBDPs) that can be used to monitor the magnitude and temporal duration of protease activation. However, accumulation of αII‐spectrin and αII‐SBDPs in CSF after TBI has never been examined. Following a moderate level (2.0 mm) of controlled cortical impact TBI in rodents, native αII‐spectrin protein was decreased in brain tissue and increased in CSF from 24 h to 72 h after injury. In addition, calpain‐specific SBDPs were observed to increase in both brain and CSF after injury. Increases in the calpain‐specific 145 kDa SBDP in CSF were 244%, 530% and 665% of sham‐injured control animals at 24 h, 48 h and 72 h after TBI, respectively. The caspase‐3‐specific SBDP was observed to increase in CSF in some animals but to a lesser degree. Importantly, levels of these proteins were undetectable in CSF of uninjured control rats. These results indicate that detection of αII‐spectrin and αII‐SBDPs is a powerful discriminator of outcome and protease activation after TBI. In accord with our previous studies, results also indicate that calpain may be a more important effector of cell death after moderate TBI than caspase‐3.