Question: 

Skeletal muscle regeneration after soft tissue trauma is regulated by complex interactions between myeloid cells and skeletal muscle cells. The removal of tissue debris and the release of growth factors by invading macrophages are required for adequate muscle repair. Hypoxia inducible factor-1 (HIF-1) is essential for myeloid cell function like aggregation, invasion, motility and phagocytosis. In order to investigate the meaning of myeloid HIF-1a for skeletal muscle regeneration, we examined mice with a conditional HIF-1a knock-out in myeloid cells in a model of soft tissue trauma of the mouse hind limb.

Methods: 

At 3h, 24h, 48h, 7d and 10d after trauma, injured and control muscles were collected for histological and RT-PCR analysis. Serum concentration of creatine kinase served as a measure for the severity of trauma.

Results: 

In acute muscle injury, 3h and 24h after trauma, no differences between wild-type and knock-out mice were detected with respect to creatine kinase levels and the extension of the traumatic lesion. However, immunohistochemical staining for F4/80-positive macrophages revealed that the invasion of macrophages to the site of injury was significantly delayed in knock-out compared to control mice. In control mice the number of macrophages peaked at 48h after trauma and already decreased 168h post injury. In contrast, the highest value of macrophages in knock-out mice was not observed until 168h following muscle injury. Moreover 7d and 10d after muscle trauma control mice showed advanced satellite cell differentiation and angiogenesis whereas the injury site of knock-out mice was characterized by larger areas of necrotic cell debris, small myoblasts and a significantly lower number of endothelial cells. RT-PCR analysis of whole muscle lysates also reflected the delay of muscle regeneration in knock-out mice as the increase of muscle regeneration markers (MyoD and myogenin) occurred significantly later in these mice.

Conclusion: 

Our findings collectively show that myeloid HIF-1a does not affect the acute extent of the traumatic lesion, but plays a significant role in skeletal muscle regeneration and angiogenesis after soft tissue trauma.