Formation of postsynaptic-like membranes during differentiation of embryonic stem cells in vitro

To analyze the formation of neuromuscular junctions, mouse pluripotent embryonic stem (ES) cells were differentiated via embryoid bodies into skeletal muscle and neuronal cells. The developmentally controlled expression of skeletal muscle-specific genes coding for myf5, myogenin, myoD and myf6, α₁ subunit of the L-type calcium channel, cell adhesion molecule M- cadherin, and neuron-specific genes encoding the 68-, 160-, and 200-kDa neurofilament proteins, synaptic vesicle protein synaptophysin, brain- specific proteoglycan neurocan, and microtubule-associated protein tau was demonstrated by RTPCR analysis. In addition, genes specifically expressed at neuromuscular junctions, the γ- and ε-subunits of the nicotinic acetylcholine receptor (AChR) and the extracellular matrix protein S- laminin, were found. At the terminal differentiation stage characterized by the formation of multinucleated spontaneously contracting myotubes, the myogenic regulatory gene myf6 and the AChR ε-subunit gene, both specifically expressed in mature adult skeletal muscle, were found to be coexpressed. Only the terminally differentiated myotubes showed a clustering of nicotinic acetylcholine receptors (AChR) and a colocalization with agrin and synaptophysin. The formation of AChRs was also demonstrated on a functional level by using the patch clamp technique. Taken together, our results showed that during ES cell differentiation in vitro neuron- and muscle-specific genes are expressed in a developmentally controlled manner, resulting in the formation of postsynaptic-like membranes. Thus, the embryonic stem cell differentiation model will be helpful for studying cellular interactions at neuromuscular junctions by 'loss of function' analysis in vitro.