👤 David V Glidden

Differentiation of skeletal myoblast cells to functional myotubes involves highly regulated transcriptional dynamics. The myocyte enhancer factor 2 (MEF2) transcription factors are critical to this process, synergizing with the master regulator MyoD to promote muscle specific gene transcription. MEF2 is extensively regulated by myogenic stimuli, both transcriptionally and post-translationally, but to date there has been little progress in understanding how signals upstream of MEF2 are coordinated to produce a coherent response. In this study, we define a novel interaction between the muscle A-kinase anchoring protein (mAKAP) and MEF2 in skeletal muscle. Discrete domains of MEF2 and mAKAP bind directly. Their interaction was exploited to probe the function of mAKAP-tethered MEF2 during myogenic differentiation. Dominant interference of MEF2/mAKAP binding was sufficient to block MEF2 activation during the early stages of differentiation. Furthermore, extended expression of this disrupting domain effectively blocked myogenic differentiation, halting the formation of myotubes and decreasing expression of several differentiation markers. This study expands our understanding of the regulation of MEF2 in skeletal muscle and identifies the mAKAP scaffold as a facilitator of MEF2 transcription and myogenic differentiation. Show less

Postsynaptic density (PSD)-93 and PSD-95 are the major membrane-associated guanylate kinases (MAGUKs) at excitatory synapses of the brain linking the N-methyl-d-aspartate receptor (NMDAR) with neuronal nitric oxide synthase (nNOS), which contributes to cell death after neonatal hypoxia-ischemia (HI). We investigated whether deletion of PSD-93 would dissociate the NMDAR from nNOS and be neuroprotective. Postnatal day 7 wild-type (+/+), heterozygous (+/-), and homozygous (-/-) PSD-93 knockout mice were subjected to HI by permanent ligation of the right carotid artery, followed by exposure to 8% O2/92% N2 for 1 hour. Brains were scored 5 days later for damage with cresyl violet and iron stains. Western blot and coimmunoprecipitation were used to determine the expression and association of the major PSD proteins. There was no significant difference between PSD-93 (-/-) and (+/+) mice in mortality or degree of brain injury. In the absence of PSD-93, PSD-95 still interacted with NR2B and nNOS. Under physiological conditions, PSD-95, nNOS, NR2A, and NR2B were unaltered in the (-/-) pups. However, at 24 hours after HI, protein expression of PSD-95, nNOS, and NR2A but not NR2B was markedly higher in the (-/-) than in the (+/+) pups. In (+/+) pups, HI resulted in decreased expression of NR2A but not NR2B in cortex and decreased NR2A and NR2B expression in hippocampus, but this reduction was not observed in (-/-) pups. PSD-93 is not essential for baseline synaptic function but may participate in regulation of NMDAR-associated signaling pathways after HI injury. Deletion of PSD-93 alone does not provide neuroprotection after neonatal HI, possibly a result, in part, of upregulation of PSD-95. MAGUKs may substitute for one another, allowing normal NMDAR function in the postnatal period. Show less