Characterization of RHAU conditional Knockout mice and the role of RHAU in maintaining post-synaptic stability at neuromuscular junction

The RNA helicase RHAU conditional knockout mice generated in Yoshikuni Nagamine laboratory show neuromuscular-paralytic disorder. However, the mechanisms underlying this neuromuscular-paralytic phenotype caused by RHAU loss of function remains unknown. In this thesis, the goal is to characterize the RHAU conditional knockout mice and to understand the mechanisms of how conditional knockout of RHAU in mice leads to neuromuscular –paralytic disorder.
By inducing RHAU knockout at different ages, we found that mice with RHAU knockout at early stage (3-week-old) die faster with much lower bodyweight than Wildtype mice, while the mice with RHAU knockout at late stage (5-month-old) develop age-dependent progressive neuromuscular paralytic disorder with muscle shrinkage. To understand the synaptic basis of this neuromuscular disorder, the motor axons were genetically labeled with membrane targeted GFP (mGFP). We could not find the denervation of motor axons at NMJs in paralyzed RHAU knockout mice, indicating that the paralytic phenotype caused by loss of RHAU is independent of axon degeneration. To further understand the role of motoneuron in contributing to paralysis observed in RHAU conditional knockout mice, RHAU was specifically deleted in motoneurons by breeding with HB9-cre mice. Consistently, the motoneuron-specific knockout mice show normal motor behavior as WT mice with normal NMJ morphology.
We then knockout RHAU specifically in muscle by expressing Cre recombinase in muscle using Adeno associated virus (AAV). The post-synaptic AChR clusters are destabilized and fragmented into individual boutons upon muscle-specific RHAU knockout. Thus, our results demonstrate that RNA helicase RHAU plays an essential role in maintaining post-synaptic AChR clusters stability at NMJ.