A moderate oestradiol level enhances neutrophil number and activity in muscle after traumatic injury but strength recovery is accelerated

Abstract

Oestrogen has been shown to protect against skeletal muscle injury and a reduced inflammatory response has been suggested as a possible protective mechanism. There are, however, dissenting reports. Our objective was to conduct an unbiased, comprehensive study of the effect of oestradiol on the inflammatory response following muscle injury. Female C57BL6/J mice were ovariectomized and supplemented with and without oestradiol. Tibialis anterior muscles were freeze injured and studied primarily at 1-4 days post-injury. Oestradiol supplementation increased injured muscle gene expression of neutrophil chemoattractants (Cxcl1 and Cxcl5) and to a lesser extent that of monocyte/macrophage chemoattractants (Ccl2 and Spp1). Oestradiol markedly increased gene expression of the neutrophil cell surface marker (Ly6g) but had less consistent effects on the monocyte/macrophage cell surface markers (Cd68, Cd163 and Cd206). These results were confirmed at the protein level by immunoblot with oestradiol increasing LY6G/C content and having no significant effect on CD163 content. These findings were confirmed with fluorescence-activated cell sorting counts of neutrophils and macrophages in injured muscles; oestradiol increased the proportion of CD45 cells that were neutrophils (LY6G ) but not the proportion that were macrophages (CD68 or CD206 ). Physiological impact of the oestradiol-enhanced neutrophil response was assessed by strength measurements. There was no significant difference in strength between oestradiol-supplemented and -unsupplemented mice until 2 weeks post-injury; strength was 13-24% greater in supplemented mice at 2-6 weeks post-injury. In conclusion, a moderate level of oestradiol supplementation enhances neutrophil infiltration in injured muscle and this is associated with a beneficial effect on strength recovery.

Oestrogen and a Goldilocks zone for post‐damage muscle inflammation and repair?

“…In an article in this issue of The Journal of Physiology , Le et al . () note that these previous studies have contradictory findings, in that oestrogen has been reported to either attenuate or augment post‐damage muscle inflammatory responses, and they suggest that differing levels of circulating oestrogen used in these studies might explain the discrepancies. Le et al .…”

“…Interestingly, Le et al . () also found a more rapid muscle force recovery in mice with ‘normal’ oestrogen levels than in the non‐supplemented mice with lower than ‘normal’ oestrogen levels. Hence, they suggested that augmented post‐damage muscle inflammation as induced by modest or normal oestrogen levels may result in enhanced stimulation of various signalling factors that upregulate the rate of muscle repair.…”

“…Le et al . () also note that oestrogen concentration generally affects inflammatory responses in various other tissues in an ‘inverted U’ manner such that the greatest post‐damage augmentation of inflammation occurs at modest or ‘normal female’ levels of circulating oestrogen (Straub, ).…”

“…To determine if this is also how ‘normal’ levels oestrogen may affect post‐damage inflammation in skeletal muscle, Le et al . () employed an ovariectomized mouse model either without oestrogen replacement (‘low oestrogen’) or with replacement that achieved ‘normal’ oestrogen levels (as found in non‐pregnant females). They reported that ‘normal’ levels of oestrogen did indeed augment various indices of post‐damage muscle inflammation, including neutrophil infiltration, relative to ovariectomized mice not supplemented with oestrogen, which expressed lower levels of post‐damage muscle inflammation indices.…”

“…To follow up on a possible relationship with oestrogen concentration, inflammation and consequent muscle repair, Le et al . () also assessed the post‐damage rate of muscle repair in these mice with ‘normal’ or ‘low’ levels of oestrogen. They did so by determining the rate of return of muscle force over several weeks of recovery.…”