Mechanistic Study On RAB39B Gene Mutation Caused X-linked Intellectual Disability

RAB39B is located on the X chromosome(Xq28)and encodes the RAB39B protein that belongs to the RAB family.Similar to other small GTPases,the activity of RAB proteins is regulated by GTP-bound(active)and GDP-bound(inactive)states.As an evolutionarily conserved vesicle transport regulator,RAB proteins can determine the specificity and directionality of membrane transport by localizing to specific membrane structures.RAB39B gene mutations have been found in multiple diseases,including X-linked intellectual disability,Parkinson's disease,and autism spectrum disorders.Although RAB39B mutations are not common in these diseases,the direct correlation between RAB39B mutations and diseases provides a new revenue for elucidating the molecular mechanism underlying these diseases and identifying new therapeutic targets.Resently,the pathological and physiological functions of RAB39B remain largely unknown,and there is an urgent need to scrunitize this protein.In this study,we first analyzed the expression pattern of RAB39B protein in wild type(WT)mice and identified RAB39B as a neuron-specific protein.We constructed Rab39b knockout mice and found that they had overall normal birth rate and morphologies as WT mice.However,in various behavioral tests on 2-month-old male mice,we found that Rab39b knockout mice had abnormal behaviors compared to WT control mice.Using open field test and high-elevated plus maze test,we found that Rab39b knockout mice had less anxiety behavior than WT mice.Using novel object recognition test,T maze test and Morris water maze test,we found that Rab39b knockout mice had impairment in learning and memory.Using three-chamber social interaction test,we found that deletion of Rab39b altered mouse social behavior.Using walking analysis,hanging test and rotarod test,we found that RAB39B deficiency impaired mouse motor abilities.By analyzing neuronal morphology,we found that Rab39b knockout mouse had reduced dendritic spine numbers in both cortex and hippocampus,accompanied by a reduction of NMDA receptors in the post-synaptic density(PSD).Using electrophysiological analysis,we found that Rab39b knockout mouse had impaired excitatory synaptic transmission capacity and synaptic plasticity.Finally,we found that downregulation of Rab39b affected the mTOR signaling and interfered with rapamycin-induced autophagy.Treatment with rapamycin promoted autophagy and patially rescued impaired memory and synaptic plastisity in Rab39b knockout mice,without affecting the PSD distribution of NMDA receptors.In summary,our study indicates that RAB39B is a neuron-specific protein and plays a crucial role in regulating mouse behaviors.Deletion of Rab39b leads to synaptic dysfunction and autophagy disturbance.Rapamycin can partially rescue imparied memory and synaptic plasticity in Rab39b knockout mice.These results shed light on our understanding about the mechanism underlying Rab39b gene mutation-caused diseases.