Signal transduction pathways during myogenesis and at the neuromuscular junction in health and disease
Our lab is interested in signal transduction pathways regulating different aspects of myogenesis and the formation and maintenance of neuromuscular junctions (NMJs). During development the release of a heparan sulfate proteoglycan (agrin) by motoneurons activates the muscle-specific receptor tyrosine kinase (MuSK) as the main organizer of subsynaptic specializations and formation of NMJs. MuSK downstream signaling is largely undefined.
One of these signaling pathways we study involves LAP adaptor proteins (leucine rich repeats and PDZ domain). We identified LAP protein members Erbin, Lano, and Scribble, but not Densin-180, in muscle cells, where they are important in regulating the clustering of nicotinic acetylcholine receptors (AChRs). Erbin-null skeletal muscles did not reveal any conspicuous impairment of the muscle fiber itself, but quantitative 3D morphometry and electrophysiology showed that their NMJs were impaired. We speculate that Erbin, Lano, and Scribble act at the postsynaptic membrane of NMJs in a concerted fashion to regulate AChR cluster morphology and neural transmission.
In order to understand more about MuSK-downstream signaling, we searched and identified protein kinase CK2 to interact with MuSK. We observed CK2-mediated phosphorylation of serine residues of MuSK and other protein components of the postsynaptic apparatus, like Dok-7, Rac and 14-4-4-gamma. Muscle-specific CK2-beta knockout mice developed a myasthenic phenotype due to impaired NMJ structure and function and muscle fiber homeostasis due to affected mitochondrial physiology. We found out that CK2 also phosphorylates TOMM22, the main receptor for mitochondrial protein import in skeletal muscle fibers. Moreover, we showed that skeletal muscles from skeletal muscle CK2-beta knockout mice contain dysfunctional mitochondria and are positive for mitophagy markers involved in the removal of impaired mitochondria.
More recently, our lab started to elucidate the role of canonical Wnt and Hippo signaling members in muscles using a reporter mouse paradigm. We detected active canonical Wnt signaling (1) in myotubes, (2) in specific adult muscle fiber types, (3) at NMJs, and (4) during regeneration of skeletal muscle after injury. Interestingly, Hippo signaling members mediated signaling accompanied canonical Wnt signaling in adult muscle fibers.
Altogether, these projects contribute to better understand of individual and concerted roles of intracellular signaling pathways regarding neuromuscular biology in health and disease.
The role of GCM proteins in mammals
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.