Neuregulin induces the expression of transcription factors and myosin heavy chains typical of muscle spindles in cultured human muscle

Abstract

Neuregulin (NRG) (also known as ARIA, GGF, and other names) is a heparin sulfate proteoglycan secreted into the neuromuscular junction by innervating motor and sensory neurons. An integral part of synapse formation, we have analyzed NRG-induced changes in gene expression over 48 h in primary human myotubes. We show that in addition to increasing the expression of acetylcholine receptors on the myotube surface, NRG treatment results in a transient increase of several members of the early growth response (Egr) family of transcription factors. Three Egrs, Egr1, -2, and -3, are induced within the first hour of NRG treatment, with Egr1 and -3 RNA levels showing the most significant increases of Ϸ9-and 16-fold, respectively. Also noted was a corresponding increase in protein levels for both of these transcription factors. Previous literature indicates that Egr3 expression is required for the formation of muscle spindle fibers, sensory organs that are distinct from skeletal muscle contractile fibers. At the molecular level, muscle spindle fibers express a unique subset of myosin heavy chains. Two isoforms of the myosin heavy chain, the slow development and neonatal, were found to be increased in our myotube cultures after 48 h of treatment with NRG. Taken together, these results indicate that not only can NRG induce the expression of a transcription factor key to spindle fiber development (Egr3), but that a portion of this developmental process can be replicated in vitro.

Tissue engineering intrafusal fibers: Dose- and time-dependent differentiation of nuclear bag fibers in a defined in vitro system using neuregulin 1-β-1

“…This finding supports in vivo and in vitro experiments indicating that Nrg 1-β-1 activates transcription factors highly expressed in muscle spindle fibers and is required for their development. Although the incomplete differentiation of all myotubes into nuclear bag or chain fibers in culture would suggest a role for other factors in this process as well [25][26][27]. Additionally, the incomplete differentiation could represent that certain myotubes are refractory to Nrg 1-β-1 treatment.…”

“…Also, much effort has been towards identifying growth factors, hormones and culture conditions that yield robust primary myocyte growth, as well as subsequent fusion into contractile myotubes in vitro [22][23][24]. For example, in vitro studies have identified neuregulin as a growth factor that induces expression of the transcription factor Egr3 which is critical for intrafusal fiber development [25][26][27]. These studies have provided some insight into the conditions required for myocyte growth and differentiation.…”

Genetic analysis of the calcineurin pathway identifies members of the EGR gene family, specifically EGR3 , as potential susceptibility candidates in schizophrenia

“…For instance, neuregulin 1 (NRG1) has been identified as a potential susceptibility gene for schizophrenia (25), and alterations in NRG1-ErbB (one of the NRG1 receptors) signaling have been shown to occur in the prefrontal cortex of schizophrenic patients (26,27). EGR1, EGR2, and EGR3 are downstream targets for NRG1 (28) and have been shown to be regulated by the epidermal growth factor ␤1 domain of NRG1 in cultured human muscle cells. This regulatory effect of the ␤1 domain is most pronounced for EGR3 (28).…”

“…EGR1, EGR2, and EGR3 are downstream targets for NRG1 (28) and have been shown to be regulated by the epidermal growth factor ␤1 domain of NRG1 in cultured human muscle cells. This regulatory effect of the ␤1 domain is most pronounced for EGR3 (28). It is intriguing that defects in NRG1-ErbB signaling could affect oligodendrocyte development and myelination (26,29), given that multiple lines of evidence converge to implicate abnormalities in oligodendroglia and myelin in schizophrenia (30).…”

Mice Lacking the Immediate Early Gene Egr3 Respond to the Anti-Aggressive Effects of Clozapine Yet are Relatively Resistant to its Sedating Effects

“…The biological positioning of Egr3 also supports its potential role in this illness as Egr3 is activated downstream of several key proteins implicated in schizophrenia risk or pathogenesis. These include neuregulin 1 (NRG1) (Sweeney et al, 2001;Hippenmeyer et al, 2002;Jacobson et al, 2004), a gene showing genetic association with schizophrenia (Moises et al, 2002;Stefansson et al, 2002); calcineurin (CN) (Mittelstadt and Ashwell, 1998), a calcium calmodulindependent phosphatase implicated in schizophrenia risk based on studies of mutant mice as well as human genetic association studies Miyakawa et al, 2003); and N-methyl D-aspartate receptors (NMDARs) (Yamagata et al, 1994), a receptor pathway implicated in schizophrenia (Javitt and Zukin, 1991;Olney et al, 1999).…”