# Myeloid cell-specific β2-adrenergic receptor deletion improves early cardiac injury resolution via de-repression of Anxa1

## TL;DR

Deleting the β2-adrenergic receptor in myeloid cells improves heart recovery after injury by boosting the clearance of damaged cells.

## Contribution

Myeloid cell-specific β2AR deletion enhances efferocytosis via Anxa1 de-repression, improving cardiac recovery.

## Key findings

- LB2 mice showed better heart function and less fibrosis after myocardial infarction.
- β2AR deletion increased Anxa1 expression and efferocytosis in myeloid cells.
- miR-374b-5p repression of Anxa1 is mediated by β2AR signaling.

## Abstract

Rationale: Myeloid cells, including neutrophils (Nu), monocytes (Mo) and macrophages (Mac), rapidly accumulate after ischemic cardiac injury where they play integral roles in inflammation and repair. β2-adrenergic receptor (β2AR) signaling regulates numerous facets of immune cell behavior, but its impact on myeloid cell-specific responses to acute cardiac injury is unclear.

Methods: Myeloid cell-specific β2AR knockout (LB2) and control mice were subjected to myocardial infarction (MI) with or without prior transplantation with shRNA-modified bone marrow. The impact of myeloid cell-specific β2AR deletion, and mechanistic basis for the effect, were assessed via echocardiography, immunohistochemistry, flow cytometry, gene expression and efferocytosis analyses.

Results: LB2 mice displayed better cardiac function and less fibrotic remodeling post-MI. Despite a similar initial influx of myeloid cell subtypes, by 4 days post-MI LB2 mice had significantly reduced Nu, concurrent with increased Nu-containing Mac, indicating an enhanced efferocytosis capacity. Indeed, the pro-efferocytotic protein annexin A1 (AnxA1) was elevated in several β2AR-deficient myeloid cell types following MI. Mechanistically, we found the expression of several miRs known to repress Anxa1 expression were elevated in response to β2AR stimulation, an effect absent with β2AR deletion, and miR-374b-5p mimic in particular was sufficient to decrease Anxa1 expression. Finally, lentivirus-encoded shRNA was used to induce knockdown of Anxa1 expression in the bone marrow of LB2 mice prior to MI, which reduced Nu efferocytosis in vitro and prevented the ameliorative effects on cardiac fibrosis and function observed with LB2 mice following MI in vivo.

Conclusion: Myeloid cell-specific β2AR deletion leads to loss of miR-374b-5p-mediated repression of AnxA1, which allows for enhanced efferocytosis-mediated Nu clearance, thereby limiting infarct expansion and improving post-injury cardiac function and fibrotic remodeling.