NF-κB Functions in Stromal Fibroblasts to Regulate Early Postnatal Muscle Development

Abstract

Classical NF-B activity functions as an inhibitor of the skeletal muscle myogenic program. Recent findings reveal that even in newborn RelA/p65 ؊/؊ mice, myofiber numbers are increased over that of wild type mice, suggesting that NF-B may be a contributing factor in early postnatal skeletal muscle development. Here we show that in addition to p65 deficiency, repression of NF-B with the IB␣-SR transdominant inhibitor or with muscle-specific deletion of IKK␤ resulted in similar increases in total fiber numbers as well as an up-regulation of myogenic gene products. Upon further characterization of early postnatal muscle, we observed that NF-B activity progressively declines within the first few weeks of development. At birth, the majority of this activity is compartmentalized to muscle fibers, but by neonatal day 8 NF-B activity from the myofibers diminishes, and instead, stromal fibroblasts become the main cellular compartment within the muscle that contains active NF-B. We find that NF-B functions in these fibroblasts to regulate inducible nitric-oxide synthase expression, which we show is important for myoblast fusion during the growth and maturation process of skeletal muscle. Together, these data broaden our understanding of NF-B during development by showing that in addition to its role as a negative regulator of myogenesis, NF-B also regulates nitric-oxide synthase expression within stromal fibroblasts to stimulate myoblast fusion and muscle hypertrophy.

NF-κB Signaling: A Tale of Two Pathways in Skeletal Myogenesis

“…The reason for these functional redundancies is most likely to ensure that myoblasts are maintained in an undifferentiated state prior to receiving their extracellular cues to engage in their conversion to contractile myotubes. The timing dur ing skeletal muscle development at which this regulation of NF-kB manifests itself is not known, but recent findings from our laboratory indicate that this may be restricted to postnatal muscle development, as the p65 subunit ap pears dispensable in primary ntyofiber formation during embryogenesis (Bakkar and Gull ridge, unpublished ob servations) but required for maturation of neonatal mus cles (36). Findings that adult muscles lacking IKK/3 un dergo enhanced regeneration in response to acute (103) or chronic injury (1) seem to support this claim.…”

“…4). Although this regulatory switch between the classical and alternative was first described in a myogenic culture system, new evidence indicates that this same switch functions in vivo during early postnatal skeletal muscle development (36). However, unlike the classical pathway that functions to regulate the timing of myotube formation, induction of the alternative pathway appears to be dispensable for this process, and instead functions to promote mitochondrial biogenesis, presumably to ensure that sufficient ATP quantities are available to sustain myofiber contractile activity, as well as provide more efficient energy use in conditions of starvation stress.…”

Connective tissue fibroblasts and Tcf4 regulate myogenesis

“…Recently, the molecular nature of some of these postnatal fibroblast signals has begun to be revealed. Neonatal connective tissue fibroblasts express nitric oxide synthase, and nitric oxide is a known stimulator of myoblast fusion (Dahlman et al, 2010). Potentially, during development fibroblasts could be a source of nitric oxide, which promotes myoblast fusion.…”

Glycosaminoglycan mimetics trigger IP3-dependent intracellular calcium release in myoblasts

“…NF-κB is also a Ca 2+ -dependent factor activated in depolarized skeletal muscle cells (Valdes et al, 2007). Its activation in myoblasts triggers their fusion possibly through iNos synthesis (Lee et al, 1997;Dahlman et al, 2010). Hence, whether these transcription factors are activated in GAG-treated myoblasts and contribute to the stimulation of their differentiation (see supplementary data) needs further study.…”