Detection of truncated isoforms of human neuroserpin lacking the reactive center loop: Implications in noninhibitory role

Neuroserpin is a serine protease inhibitor expressed mainly in the brain and at low levels in other tissues like the kidney, testis, heart, and spinal cord. It is involved in the inhibition of tissue plasminogen activator (tPA), plasmin, and to a lesser extent, urokinase-type plasminogen (uPA). Neuroserpin has also been shown to plays noninhibitory roles in the regulation of N-cadherin-mediated cell adhesion. It is involved in neuroprotection from seizure and stroke through tPA-mediated inhibition and also through its other protease targets. Mutations in critical domains of neuroserpin lead to its polymerization and neuronal death. In this study, a novel truncated isoform of human neuroserpin was identified in the brain and liver, which was confirmed by reverse transcriptase-PCR and DNA sequencing using exon-specific primers. Structural characterization of novel isoform using MD simulations studies indicated that it lacks the reactive center loop (RCL) but largely maintains its secondary structure fold. The novel truncated variant was cloned, expressed, and purified. A comparative intrinsic fluorescence and 4,4 '-bis-1-anilino naphthalene 8-sulfonate studies revealed a decrease in fluorescence emission intensity and a more exposed hydrophobic surface as compared to the reported isoform. However, the novel isoform has lost its ability for tPA inhibition and complex formation. The absence of RCL indicates a noninhibitory role for the truncated isoform, prompting a detailed search and identification of two smaller isoforms in the human brain. With indications of the noninhibitory role of neuroserpin, identifying novel isoforms that appear to be without the tPA recognition domain is significant.