# ROS Activated NETosis of Bone Marrow CD55+ Intermediate Mature Neutrophils Through HIF1α‐PADI4 Pathway to Initiate Bone Aging

## TL;DR

This study shows that neutrophils in the bone marrow of aging mice cause bone aging through a process called NETosis, which is linked to inflammation and cell aging.

## Contribution

The study identifies a specific neutrophil subset and a novel HIF1α-PADI4 pathway mechanism linking NETosis to bone aging.

## Key findings

- CD55+ intermediate mature neutrophils in bone marrow show increased NETosis, leading to bone aging.
- Senescent BMSCs produce ROS, which contributes to a cycle of inflammation and aging in bone marrow.
- Clearing NETs in vivo improves bone aging in SAMP6 mice.

## Abstract

Neutrophil NETosis is markedly dysregulated in the aging body. Bone marrow serves as the powerhouse of neutrophil differentiation, while the state of neutrophil NETosis therein and its relationship with bone aging remains largely elusive. Moreover, it remains unclear how neutrophil heterogeneity and pro‐inflammatory cues within bone marrow synergistically regulate neutrophil NETosis. Here, we find neutrophil NETosis is highly activated in the bone marrow of 3‐mon male senescence‐accelerated mouse prone 6 (SAMP6), and the released NETs induce BMSCs senescence and impairs their osteogenesis. Further, we verify in vivo NETs‐clearance significantly ameliorates bone aging of 3‐mon male SAMP6 mice. Next, through scRNA‐seq we find a CD55+ intermediate mature neutrophil subset enriching in the 3‐mon male SAMP6 bone marrow, characterized by significantly upregulated NETosis. Through cell transfer, we demonstrate this subset directly induces bone aging. Mechanistically, elevated ROS within the bone marrow of SAMP6 integrates with a CD55‐primed HIF1ɑ‐PADI4 pathway to trigger NETosis, and senescent BMSCs serve as a ROS‐producer. In summary, our results demonstrate that activated NETosis in CD55+ intermediate‐mature neutrophils plays a key role in initiating bone aging. Also, we uncover the vicious cycle of inflammaging between immune dysregulation and cellular senescence in bone marrow, providing potential targets for osteoporosis treatment.

In this study, we find CD55+ neutrophils show activated NETosis within bone marrow, induce BMSC senescence and osteogenesis inhibition, finally leading to bone aging initiation. Mechanistically, ROS synergizes with the CD55‐driven HIF1α‐PADI4 pathway to promote NETosis. We also verify senescent BMSCs as a ROS source, forming a vicious cycle of inflammaging with neutrophils within bone marrow.