Complex mechanisms are required to form neuromuscular synapses, direct their subsequent maturation, and maintain the synapse throughout life. Transcriptional and post-translational pathways play impor Show more
Complex mechanisms are required to form neuromuscular synapses, direct their subsequent maturation, and maintain the synapse throughout life. Transcriptional and post-translational pathways play important roles in synaptic differentiation and direct the accumulation of the neurotransmitter receptors, acetylcholine receptors (AChRs), to the postsynaptic membrane, ensuring for reliable synaptic transmission. Rapsyn, an intracellular peripheral membrane protein that binds AChRs, is essential for synaptic differentiation, but how Rapsyn acts is poorly understood. We screened for proteins that coisolate with AChRs in a Rapsyn-dependent manner and show that microtubule actin cross linking factor 1 (MACF1), a scaffolding protein with binding sites for microtubules (MT) and actin, is concentrated at neuromuscular synapses, where it binds Rapsyn and serves as a synaptic organizer for MT-associated proteins, EB1 and MAP1b, and the actin-associated protein, Vinculin. MACF1 plays an important role in maintaining synaptic differentiation and efficient synaptic transmission in mice, and variants in Show less
Congenital myasthenic syndromes (CMS) with underlying RAPSN mutations turned out to be of high clinical relevance due to their worldwide frequency. To date, all reported patients with CMS with sequenc Show more
Congenital myasthenic syndromes (CMS) with underlying RAPSN mutations turned out to be of high clinical relevance due to their worldwide frequency. To date, all reported patients with CMS with sequence variations in the translated region of RAPSN carry the mutation N88K on at least one allele. The authors report two patients lacking the common N88K allele but harboring differing novel mutations of the RAPSN gene on both alleles: one patient is homozygous for a missense mutation (R164C); the second patient is compound heterozygous for a splice (IVS1-15C>A) and another missense mutation (L283P). The authors analyzed the RAPSN gene for sequence variations and carried out in vitro studies in order to delineate the potential pathogenicity of the three novel RAPSN mutations. For the putative splice mutation (IVS1-15C>A), the authors constructed wild-type and mutated RAPSN minigenes for transfection and subsequent RNA analysis. The mutation generates a novel acceptor splice site leading to retention of 13 nucleotides of intron 1 in the mature mRNA and subsequently to a frameshift transcript. Cotransfection of wild-type AChR subunits with RAPSN-constructs carrying R164C and L283P indicate that both mutations diminish coclustering of AChR with rapsyn. Screening for the common mutation RAPSN N88K facilitates targeted genetic analysis in congenital myasthenic syndromes. However, absence of a N88K allele does not exclude underlying RAPSN mutations as cause of the congenital myasthenic syndromes. Sequencing of the entire gene may be considered in patients with joint contractures and respiratory problems even in the absence of the mutation N88K. Show less