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Free keywords:
Osr1; Odd skipped related 1; muscle; regeneration; injury; fibroadipogenic; connective tissue
Abstract:
Myogenesis in development as well as during muscle regeneration involves a coordi-
nated interplay of various cell and tissue types. Careful orchestration of immune cells,
satellite cells, connective tissue and resident cell types is required for effective tissue
repair. The muscle interstitium comprises various resident cell types such as intersti-
tial fibroblasts, pericytes or fibro/ adipogenic progenitors (FAPs). While FAPs reside
quiescent in the muscle interstitium in homeostatic tissue they are activated upon
injury and provide a pro-myogenic environment necessary for regeneration. However,
due to the lack of markers to particularly identify activated FAPs the clarification
of exact underlying cell specific mechanisms is impeded, and a clear cell discrimi-
nation challenging. FAPs are proposed as originating from mesenchymal cells, but
confirmation remains elusive.
Odd skipped related-1 ( Osr1 ) is expressed in muscle connective tissue derived
from the lateral plate mesoderm during embryonic and fetal myogenesis in the limbs.
Unpublished data demonstrate its crucial functional role for muscle patterning and
contribution to muscle interstitial cells at the time of birth. However, postnatal ex-
pression of Osr1 was not clear, and functional studies in the adult require a conditional
Osr1 allele due to the lethality of a constitutive knock-out of Osr1 at early fetal stage.
Here, the generation of two novel tools for investigating Osr1 expression in adult
mice was performed. The Osr1 promotor-driven lacZ-reporter mouse model Osr1-
MFA enables identification of Osr1+ cells by histological and immunohistochemical
methods as well as by fluorescent-activated cell sorting (FACS) technique. The con-
ditional line Osr1-flox carries a floxed allele of Osr1 thus allows for a conditional
knock-out of Osr1 .
Previous reports described a transient muscle interstitial cell population present
during the first 3 weeks after birth, comprising myogenic non-satellite cell progenitors
and juvenile FAPs. Here, expression of Osr1 in the FAP subpopulation is demonstrated
which allows for specific identification of these cells in muscle tissue. Genetic lineage
tracing demonstrated that these Osr1+ juvenile FAPs, at least in part, give rise to
quiescent FAPs in adult muscle, where Osr1 expression is downregulated. These data
contribute to the clarification of the origin of FAPs in the adult.
Muscle damage triggers Osr1 expression in FAPs after cell activation, where it
remains upregulated during the early phase of muscle regeneration. In a model of
muscle injury that promotes fat formation, limited adipocytic contribution of Osr1+
FAPs was observed after lineage tracing. These data are in contrast to previous reports
attributing a strong contribution of FACS isolated and engrafted FAPs to ectopic
fatty accumulation. This highlights the importance of the tissue environment and the
proneness of FAPs to external cues when removed from the tissue context. Moreover,
in regenerated muscle the majority of FAPs had expressed Osr1 during the early
phase of regeneration. These results provide evidence for Osr1 expression as a specific
marker for activated FAPs after muscle injury. A large fraction of TCF4+ fibroblasts in
regenerated muscle is derived from TCF4–/ Osr1+ FAPs. This observation elucidates
TCF4+ fibroblasts as progenies of FAPs.
A conditional knock-out of Osr1 after injury results in significantly reduced pro-
inflammatory macrophage abundance and abnormal accumulation of aggregates in
myofibers during an early stage of regeneration. These results suggest a functional
involvement in the plasmin system. It is hypothesized that Osr1 indirectly regulates
the conversion of plasminogen to plasmin, thus mediates extracellular matrix deposi-
tion, macrophage recruitment and myofiber degeneration. This hypothesis assigns an
essential role to Osr1 expression in FAPs for an effective muscle regeneration after
injury.
