Skeletal muscle PGC-1α1 promotes motor neuron recruitment and neuromuscular junction formation

R. Mills,¹ L. Agudelo,² I. Allodi,³ V. Martinez-Redondo,² V. Lundin,¹ J. Correia,² L. Comley,³ E. Hedlund,³ J. Ruas² and A. Teixeira,¹ ¹Department of Cell and Molecular Biology, Karolinska Institutet, 171 77, Stockholm, Sweden, ²Molecular and Cellular Exercise Physiology, Department of Physiology and Pharmacology, Karolinska Institutet, Stockholm, Sweden and ³Department of Neuroscience, Karolinska Institutet, Stockholm, Sweden.

Peroxisome-proliferator-activated receptor γ coactivator 1α (PGC-1α) is a key regulator of mitochondrial biogenesis and energy metabolism; having an important role in the biology of skeletal muscle. Expression of PGC-1α in skeletal muscle is induced by exercise, regulating many of the effects of endurance training (Lin et al., 2002). PGC-1α gene has been recently found to encode for different splice variants (α1, α2, α3, α4) (Ruas et al., 2012). A form of PGC-1α, termed PGC-1α4, was identified that does not regulate known PGC-1α targets (now termed PGC-1α1). PGC-1α4 instead induces IGF-1 expression and repression of myostatin, resulting in muscle hypertrophy and increased strength. Taken together, PGC-1α4 integrates resistance training with a gene program of muscle hypertrophy whilst PGC-1α1 regulates skeletal muscles adaptations to endurance exercise.

PGC-1α has also been identified as a regulator of neuromuscular junction (NMJ) structure and activity. It was initially noted that PGC-1α expression reduces muscle-mass loss and maintains muscle fibre volume during denervation in comparison to normal mice (Handschin et al., 2007). To investigate how these two PGC-1α isoforms, representing the different exercise modalities, affect the structure and function of the NMJ we developed an in vitro NMJ model. We used a commercially available microfluidic device to generate a functional NMJ model, which allows the specific manipulation of each cell type and control of the cellular microenvironment. Using this model we over-expressed PGC-1α1 and PGC-1α4 within mouse myotubes and investigated NMJ morphology and functionality. Whilst we found both isoforms caused pre-synaptic and post-synaptic NMJ changes; PGC-1α1 overexpression promoted motor neuron recruitment and formation of functional NMJs. To determine the factor(s) responsible for the pre-synaptic and post-synaptic NMJ modifications, we investigated the potential role of myokines, muscle-derived cytokines, on mediating the changes observed after PGC-1α1 myotube overexpression. Using a bioinformatics approach, combining gene array data and in silico prediction servers, we identified the myokines responsible for the observed motor neuron recruitment and NMJ morphological changes.

Handschin C, Kobayashi YM, Chin S, Seale P, Campbell KP, Spiegelman BM. (2007). PGC-1α regulates the neuromuscular junction program and ameliorates Duchenne muscular dystrophy. Genes & Development, 21: 770-783.

Lin J, Wu H, Tarr PT, Zhang CY, Wu Z, Boss O, Michael LF, Puigserver P, Isotani E, Olson EN, Lowell BB, Bassel-Duby R, Spiegelman BM. (2002). Transcriptional co-activator PGC-1α drives the formation of slow-twitch muscle fibres. Nature, 418: 797-801.

Ruas JL, White JP, Rao RR, Kleiner S, Brannan KT, Harrison BC, Greene NP, Wu J, Estall JL, Irving BA, Lanza IR, Rasbach KA, Okutsu M, Nair KS, Yan Z, Leinwand LA, Spiegelman BM. (2012) A PGC-1α isoform induced by resistance training regulates skeletal muscle hypertrophy. Cell, 151: 1319-1331.