Upon activation, neutrophils undergo histone citrullination by protein arginine deiminase (PAD)4, exocytosis of chromatin and enzymes as neutrophil extracellular traps (NETs), and death. In diabetes, neutrophils are primed to release NETs and die by NETosis. Although this process is a defense against infection, NETosis can damage tissue. Therefore, we examined the effect of NETosis on the healing of diabetic foot ulcers (DFUs). Using proteomics, we found that NET components were enriched in nonhealing human DFUs. In an independent validation cohort, a high concentration of neutrophil elastase in the wound was associated with infection and a subsequent worsening of the ulcer. NET components (elastase, histones, neutrophil gelatinase-associated lipocalin, and proteinase-3) were elevated in the blood of patients with DFUs. Circulating elastase and proteinase-3 were associated with infection, and serum elastase predicted delayed healing. Neutrophils isolated from the blood of DFU patients showed an increased spontaneous NETosis but an impaired inducible NETosis. In mice, skin PAD4 activity was increased by diabetes, and FACS detection of histone citrullination, together with intravital microscopy, showed that NETosis occurred in the bed of excisional wounds. PAD4 inhibition by Cl-amidine reduced NETting neutrophils and rescued wound healing in diabetic mice. Cumulatively, these data suggest that NETosis delays DFU healing.

Despite the tremendous technical advancements in 3D bioprinting, the concept of fabricating 3D structures and functional tissues directly in live animals remains a visionary challenge. We show that 3D cell-laden hydrogels can be efficiently bioprinted across tissues and within tissues of living animals.We developed photo-sensitive polymers that allow in vitro and in vivo fabrication of hydrogels into pre-existing structures, by bio-orthogonal two-photon cycloaddition and crosslinking at wavelengths longer than 850 nm, without byproducts. By this technique, that we name intravital 3D bioprinting, after injection of these polymers in vivo it is possible to fabricate complex 3D structures inside tissues of living mice, including the dermis across epidermis, the skeletal muscle across epimysium or the brain across meninges. The use of commonly available multi-photon microscopes allows accurate (XYZ) positioning and orientation of bioprinted structures into specific anatomical sites. Finally, we show that intravital 3D bioprinting of donor muscle-derived stem cells allows de novo formation of myofibers in host animals. We envision that this strategy will offer an alternative in vivo approach to conventional bioprinting technology, holding great promises to substantially change the paradigm of 3D bioprinting for pre-clinical and clinical use.

Metformin treatment reduced the concentrations of NET components independently from glucose control. This effect was reproducible in vitro and was related to the inhibitory effect exerted by metformin on the PKC-NADPH oxidase pathway.