The molecular process regulating the localization of acetylcholine receptors (AChRs) at the neuromuscular junction (NMJ) requires agrin, a factor secreted by motor neurons. This process involves various other molecules, but the signaling process in muscles initiated by agrin is poorly understood. We are examining this process in zebrafish which are vertebrates amenable to genetic analysis for the identification of genes important for a biological process and analysis of the in vivo function of these genes. We generated the zebrafish ennui mutation in which AChRs are mislocalized and identified the ennui gene as one encoding for LRP4 that in mammals is required for proper clustering of AChRs. We propose to use the ennui mutants to better understand the in vivo role of LRP4 for the formation of the vertebrate NMJ. We isolated the viable ennui mutation that showed a decreased electrophysiological response at the NMJ. The reduced response was due to a dramatic decrease in synaptic AChRs and high levels of AChRs mislocalized to the ends of muscles. The mutant phenotype is cell autonomous, and exogenous agrin induced AChR clusters in wildtype muscles but not in ennui muscles. These results suggested that the ennui gene encoded for a muscle factor required for agrin-induced localization of AChRs to the NMJ. The ennui gene was identified as lrp4 by a combination of genetic mapping of the mutation and genomic analysis. LRP4 is a member of the low-density lipoprotein receptor family and is expressed by early stage muscles. Although lrp4 was recently found to be critical for proper localization of AChRs in mice, there is little known about how LRP4 may mediate agrin signaling and how it might interact with other well studied components of the agrin-initiated signaling pathway. We propose to explore these issues with experiments that utilize the advantages of zebrafish for examining in vivo gene function.
Aim 1 : We will examine how LRP4 is distributed in muscle by generating antibodies and/or expression of fluorescently labeled LRP4 and see if LRP4 co-localizes with other known NMJ components.
Aim 2 : We will analyze how LRP4 regulates aneural AChR clusters that form prior to innervation by a combination of antisense knockdowns and expression of specific forms of LRP4.
Aim 3 : We will establish whether LRP4 is an aggregation factor and see how LRP4 and MuSK, a component of the agrin receptor complex, are functionally related.
Aim 4 : We will assay how the interaction of LRP4 and MuSK affects the in vivo development of the NMJ.
Zebrafish mutants could serve as animal models for human neuromuscular disorders such as myasthenia gravis and congenital myasthenic syndromes. The phenotype of ennui mutants is reminiscent of the muscle weakness and AChR deficiencies seen in patients afflicted with these diseases. This makes lrp4 a candidate gene for neuromuscular junction disorders in patients of unknown etiology. The fact that ennui mutants are viable makes them more analogous to human disorders and useful for assaying therapeutic agents.
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