Axonal trafficking of heparanase 1

Abstract

Heparanase 1 (HPA1) is the mammalian β-D-endoglycosidase that degrades heparan sulfate (HS). It is synthesized as a latent form (proheparanase) that lacks enzymatic activity. Proheparanase will be secreted to the extracellular environment before it is reuptake by the cells via heparan sulfate proteoglycan (HSPG). The internalized proheparanase will then be processed to the mature form that displays enzymatic activity in lysosome. Neurons, unlike other mammalian cells, have highly arborized morphology. However, it is still unknown how neuronal heparanase may be transported and processed. In cultured hippocampal neuros, we found axonal localization of heparanase 1. Axonal heparanase distributes along the neurofilament as shown by image from structure illumination microscopy, suggesting its active axonal trafficking, which was further confirmed by the presynaptic rather than postsynaptic localization of heparanase. Similar to other mammalian cells, neuronal heparanase, predominantly the latent form, can also be released. The exocytosed heparanase however, show low affinity for heparin. In addition, we found, unexpectedly, that even the purified recombinant heparanase, when exogenously added, fail to bind to heparan sulfate at the cell surface significantly, indicating that heparanase may not be extensively internalized by the neurons. This is confirmed by sparse co-localization between heparanase and early endosome. Additionally, we also identified the activity-regulated expression of HPA1. Specifically, long term suppression or increase of neuronal activity will result in decrease of increase of HPA1 expression respectively. Collectively, our data suggests a novel trafficking pathway of HPA1 and suggest that HPA1 may be involved in the process of homeostatic plasticity.