Quantification of axonal damage in traumatic brain injury: Affinity purification and characterization of cerebrospinal fluid tau proteins

Diffuse axonal injury is a primary feature of head trauma and is one of the most frequent causes of mortality and morbidity, Diffuse axonal injury is microscopic in nature and difficult or impossible to detect with imaging te chniques. The objective of the present study was to determine whether axona l injury in head trauma patients could be quantified by measuring levels of CSF tau proteins. Tau proteins are structural microtubule binding proteins primarily localized in the axonal compartment of neurons. Monoclonal antib odies recognizing the form of tau found in the CSF of head trauma patients were developed by differential CSF hybridoma screening using CSF from head trauma and control patients. Clones positive for head trauma CSF tau protei ns were used to characterize this form of tau and for ELISA development. Us ing the developed ELISA, CSF tau levels were elevated >1,000-fold in head t rauma patients (mean, 1,519 ng/ml of CSF) when compared with patients with multiple sclerosis (mean, 0.014 ng/ml of CSF; p < 0.001), normal pressure h ydrocephalus (nondetectable CSF tau), neurologic controls (mean, 0.031 ng/m l of CSF; p < 0.001), or nonneurologic controls (nondetectable CSF tau; p < 0.001), in head trauma, a relationship between clinical improvement and de creased CSF tau levels was observed. These data suggest that CSF tau levels may prove a clinically useful assay for quantifying the axonal injury asso ciated with head trauma and monitoring efficacy of neuroprotective agents. Affinity purification of CSF tau from head trauma patients indicated a unif orm cleavage of similar to 18 kDa from all six tau isoforms, reducing their apparent molecular sizes to 30-50 kDa, These cleaved forms of CSF tau cons isted of the interior portion of the tau sequence, including the microtubul e binding domain, as judged by cyanogen bromide digestion. Consistent with these data, CSF cleaved tau bound taxolpolymerized microtubules, indicating a functionally intact microtubule binding domain. Furthermore, epitope map ping studies suggested that CSF cleaved tau proteins consist of the interio r portion of the tau sequence with cleavage at both N and C terminals.