Formation and maintenance of the neuromuscular junction needs as prerequisite secretion of agrin by the nerve and expression of the receptor tyrosin kinase MuSK and Lrp4 in the muscle fiber. The interaction of agrin with MuSK/Lrp4 mediates the development of a synaptic site by aggregation of the components of the subsynaptic apparatus and of acetylcholine receptors (AChR). In addition the motor neuron releases neuregulins. Nerve-derived neuregulins are thought to maintain synaptic AChR expression by activating ErbB receptors. Neural Agrin aggregates AChRs by activating phosphorylation of MuSK and inducing the formation of a subsynaptic apparatus. As we have shown recently, Erbin is interlinking the MuSK with the ErbB pathway. The integral membrane protein Rapsyn is additionally required for the aggregation of AChRs as part of the postsynaptic machinery. Rapsyn is connected to the extracellular part of MuSK by a linker protein. We identified a number of proteins interacting with the cytosolic part of MuSK, like p.ex. Erbin and the proteinkinase CK2. The biological relevance of the identified proteins will be determined by cell culture, in muscle fibres and in genetically altered mice. Better understanding of the MuSK signalling cascade might lead to new strategies for the treatment of muscular dystrophies, mystenia gravis and similar diseases.
GCMa and GCMb belong to a recently identified small transcription factor family involved in a number of fundamental processes in mammals. According to their 3D structure, GCM proteins represent a new zinc containing class of transcription factors with a conserved novel type of DNA binding domain assembled of two subdomains both rich in beta-pleated sheets. A five-stranded beta-sheet from the large domain protrudes into the major groove perpendicularly to the DNA axis. Although the prototype of GCM proteins is mainly expressed in the nervous system of Drosophila, up to now GCMa has only been identified in placenta, kidney and thymus of mammals. First, we aim to understand the biological role of GCMa in the kidney by generation and characterization of genetically altered mice. Second, we are conducting screens for identification of GCMa binding partners and GCMa target genes. Third, we are investigating molecular details of the protein kinase A mediated GCMa-Syncytin pathway. Syncytin, a fusogenic protein expressed in placental trophoblast cells, mediates the formation of placental syncytiotrophoblasts. Recently, it has been demonstrated that GCMa regulates syncytin gene expression. Since GCMa might play a role in pathological states of the placenta, like pre-ecplampsia, further knowledge of the molecular action of mammalian GCM proteins might clarify mechanistical details of its contribution to disease.