# Loss of Brd4 alleviates pathological bone loss via Slc9b2 suppression in osteoclastogenesis

## TL;DR

Deleting Brd4 in bone cells prevents bone loss by blocking osteoclast activity, offering a new treatment for osteoporosis.

## Contribution

This study identifies Brd4 as a key regulator of osteoclast metabolism and reveals Slc9b2 as a novel downstream target.

## Key findings

- Brd4 deletion increases bone mass and prevents bone loss by suppressing osteoclastogenesis.
- Slc9b2 is a downstream effector of Brd4 in osteoclast differentiation.
- Brd4-targeting PROTACs inhibit osteoclastogenesis and reduce pathological bone loss.

## Abstract

Epigenetic regulation plays a crucial role in skeletal degenerative diseases, including osteoporosis. As an epigenetic reader, bromodomain protein 4 (Brd4) is known as a key driver of gene activation; however, its role in maintaining skeletal homeostasis remains largely unknown.

We examined Brd4 expression in bone specimens from osteoporotic patients and mouse models, and generated two types of Brd4 conditional knockout mice using Lyz2‐Cre and Ctsk‐Cre systems. Bone mass, osteoclast differentiation, and metabolic activity were assessed under physiological and pathological conditions, including ovariectomy and lipopolysaccharide (LPS) challenge. Mechanistic analyses were performed using transcriptomic screening, gene overexpression, and pharmacological interventions.

Brd4 expression was markedly elevated in bones from osteoporotic patients and mice compared with normal controls. Deletion of Brd4 increased basal bone mass and prevented bone loss induced by ovariectomy or LPS, primarily by suppressing osteoclastogenesis through inhibition of glycolysis. Unbiased screening identified solute carrier family 9 member B2 (Slc9b2) as a downstream effector of Brd4. Overexpression of Slc9b2 partially rescued the impaired osteoclastogenesis caused by Brd4 depletion. Moreover, phosphatidylserine‐containing nanoliposomes loaded with Brd4‐targeting PROTACs (e.g., dBET6) effectively suppressed osteoclastogenesis and alleviated pathological bone loss.

Brd4 serves as a crucial regulator of osteoclast metabolism and differentiation. Targeting Brd4 represents a promising therapeutic strategy for the prevention and treatment of osteoporosis and pathological bone loss.

Brd4 is highly expressed in osteoporotic patients and animals.Brd4 is crucial for glycolysis‐mediated OC differentiation.The loss of Brd4 in bone marrow monocytes or osteoclasts increases basal bone mass and prevents pathological bone loss.Slc9b2 is a novel target of Brd4 in mediating osteoclastogenesis.Targeting Brd4 by dBET6@PSLs could alleviate osteoporosis progression.

Brd4 is highly expressed in osteoporotic patients and animals.

Brd4 is crucial for glycolysis‐mediated OC differentiation.

The loss of Brd4 in bone marrow monocytes or osteoclasts increases basal bone mass and prevents pathological bone loss.

Slc9b2 is a novel target of Brd4 in mediating osteoclastogenesis.

Targeting Brd4 by dBET6@PSLs could alleviate osteoporosis progression.

Brd4 regulates bone metabolism through Slc9b2 suppression: a targeted therapeutic approach for osteoporosis. Elevated Brd4 expression is strongly correlated with osteoporosis, primarily by promoting osteoclastogenesis. Mechanistically, Brd4 is crucial role in regulating glycolysis, a prerequisite for osteoclastogenesis (left). In contrast, the loss of Brd4 has been shown to increase basal bone mass and prevent pathological bone loss induced by OVX or LPS, through the suppression of OC markers, particularly Slc9b2. Targeting Brd4 with PROTACs loaded on PSLs (dBET6@PSLs) significantly inhibited osteoclastogenesis and alleviated pathological bone loss. These findings suggest that Brd4 inhibition could be a promising therapeutic strategy for preventing pathological bone loss, including osteoporosis.