Cathepsin X cleaves the C-terminal dipeptide of alpha- and gamma-enolase and impairs survival and neuritogenesis of neuronal cells.

The cysteine carboxypeptidase cathepsin X has been recognized as an important player in degenerative processes during normal aging and in pathological conditions. In this study we identify isozymes alpha- and gamma-enolases as targets for cathepsin X. Cathepsin X sequentially cleaves C-terminal amino acids of both isozymes, abolishing their neurotrophic activity. Neuronal cell survival and neuritogenesis are, in this way, regulated, as shown on pheochromocytoma cell line PC12. Inhibition of cathepsin X activity increases generation of plasmin, essential for neuronal differentiation and changes the length distribution of neurites, especially in the early phase of neurite outgrowth. Moreover, cathepsin X inhibition increases neuronal survival and reduces serum deprivation induced apoptosis, particularly in the absence of nerve growth factor. On the other hand, the proliferation of cells is decreased, indicating induction of differentiation. Our study reveals enolase isozymes as crucial neurotrophic factors that are regulated by the proteolytic activity of cathepsin X.