Cathepsin D and G expression correlates with human fracture healing phases and specific neutrophil N1 and N2 phenotypes

Subsequently, to identify CTSD and G genes, the expression of them was assessed in human FH samples throughout different phases of the fracture healing cascade, and potential correlations with patient characteristics were explored. Finally, human neutrophils were harvested and polarized into N0, N1, or N2 phenotypes, after which their expression of CTSD and CTSG was analysed. Bioinformatics analysis revealed distinct expression patterns of CTSD and CTSG in the fracture healing cascade in one earlier rodent study.

In human, 58 FHs (0 – 19 days post-trauma) were harvested. The expression of CTSD significantly increased over fracture healing time, while the expression of CTSG remained constant throughout the early phases of fracture healing. In neutrophils from five human donors, the expression of CTSD was higher in N2 neutrophils compared to N1, while CTSG was expressed more in N1 compared to N2 neutrophils.

This study was the first to investigate the association of CTSD and CTSG in the fracture healing cascade. It was shown that the expression of CTSD enzyme was associated with early fracture healing phases, as well as with specific neutrophil phenotypes (N1 or N2). Furthermore, these expression dynamics of CTSD and CTSG support the increasing N2/N1 phenotype ratio over time during fracture healing in humans, reflecting a shift from inflammation to regeneration.