Mice Kash Protein Syne-1, Syne-2 And Sun Protein Sun1, Sun2 Function In The Process Of Muscle Cell Nuclear Anchoring And Migration Of Nerve Cells In The Study

Nuclear positioning is essential for many biological processes, defects of which can cause severe human diseases. Both microtubule and actin cytoskeleton have been shown to be involved in proper nuclear positioning. Recent pivotal studies in C.elegans and Drosophila have demonstrated that KASH proteins and SUN proteins, two kinds of nuclear envelope (NE) proteins, play critical roles in nuclear migration and anchorage through interaction with different sets of cytoskeletons. Although several kinds of functions of mammalian KASH and SUN proteins have been reported using in vitro tissue culture cells, their essential in vivo functions remain largely unknown. Using knockout mice against Syne-1, Syne-2 and SUN1, SUN2, we have studied their functions during myonuclear anchorage and neuronal migration in this research.Each skeletal muscle cell of mammals contains hundreds of nuclei which are evenly spaced beneath the cell membrane, except for 3～8 synaptic nuclei clustering under the neuromuscular junction (NMJ). The arrangement of myonuclei has been thought to be essential for intracellular transport and NMJ structure maintenance for many years. However, the underlying mechanism of nuclear positioning remains unclear. We found that all four proteins were localized on the NE of myonuclei. The deletion of Syne-1 KASH domain completely disrupted the anchorage of myonuclei and the clustering of synaptic nuclei, which in turn affected the innervation of motor nerves. The overexpression of Syne-2 KASH domain caused a similar nuclear anchorage defect through disturbing the function of endogenous Syne-1. Although the deletion of either SUN 1 or SUN2 did not affect myonuclear positioning, the double knockout of SUN proteins disrupted the anchorage of mynuclei and the branching process of motor nerves through abolishing the NE localization of Syne-1. These results demonstrated that Syne-1, together with SUN1 and SUN2 are essential for myonuclear positioning.Neuronal migration failure is an important cause for many kinds of severe human diseases, such as epilepsy, mental retardation and schizophrenia. Nucleokinesis is a key process of neuronal migration. However, it has long been a question mark how the pulling force is transfered from microtubule and dynein to the "huge" nucleus. In this research, we found the Syne DKO and SUN DKO mice were succefully delivered but died soon after birth from breath failure. The brains of Syne DKO or SUN DKO mice were significantly smaller than their littermate controls. Histological analysis showed that the cerebral cortex of Syne DKO and SUN DKO mice displayed severe laminary defects. Birth-dating experiments demonstrated that both Syne DKO and SUN DKO disrupted the radial neuronal migration in the cerebral cortex. Furthermore, we showed that Syne-1 and Syne-2 colocalized with Lis1/dynein complex both in vivo and in vitro. Based on these observations, we propose that Syne-1 and Syne-2 are the potential nuclear envelope binding sites of Lis1/dynein complex, with which Syne and SUN proteins mediate the coupling between microtubule and the nucleus during neuronal migration.Our studies on Syne-1, Syne-2 and SUN1, SUN2 discovered their crucial functions during myonuclear anchorage and neuronal migration. These results could advance our knowledge on the molecular mechanism of nuclear positioning, and provide new insights into the pathology of human muscular dystrophy and mental diseases.