Analysis Of The Role And Molecular Mechanism Of Tmem132e In Bipolar Disorder Using Mouse Models

Bipolar disorder(BD)is a common major psychiatric disorder characterized by high prevalence,high morbidity,and high mortality,with a prevalence of BD of 1%-1.5%in China.The etiology of bipolar disorder is complex,it is generally believed that genetic factors constitute the etiological factors and tendencies,and its pathogenesis is ambiguous at present,generally involving a variety of neurotransmitters and pathways.The single transmembrane protein TMEM132 family includes five members of TMEM132A-E.The N-terminal extracellular domain of this family contains three Ig domains,which belong to the IgSF and may play an important role in regulating neuronal cell morphology,motility,and migration.Among them,TMEM132E is a deafness gene previously identified by our group.At present,there are few studies on the function of the TMEM132E gene,and some studies have found that the TMEM132E gene is closely related to various psychiatric diseases such as bipolar disorder,panic disorder,and anxiety disorder through genome-wide association study(GWAS).To clarify the role of Tmem132e in bipolar disorder and explore its possible molecular mechanism,a number of behavioral and immunohistochemical experimental analyses were performed on the constructed Tmem132e knockout mice to obtain the following findings:First,this paper performed a behavioral analysis of the constructed Tmem132eflox/flox;Sox2Cre+/-(T132e ScKO)mice.Compared with control(Ctrl)mice,OFT trials showed that T132e ScKO mice had reduced residence time to enter the central region but unchanged total movement distance;EPM trials showed that T132e ScKO mice had reduced number and time to enter the open arm;TST and FST trials showed that T132e ScKO mice had significantly increased immobility time;and SPT trials showed that sucrose preference was reduced in T132e ScKO mice.The results indicated that T132e ScKO mice showed anxiety and depression-like behaviors.Given that the knockout caused by Sox2-Cre+/-mice approximated the full knockout effect in various tissue,we used Emxl-Cre+/-mice to construct Tmem132eflox/flox;Emxl-Cre+/-(T132e EcKO)mice with specific knockout of Tmeml32e in forebrain neurons and glia,and behavioral experimental analysis indicated that T132e EcKO mice also showed anxiety-and depressionlike behaviors.Both mouse models illustrate that Tmem132e knockout can lead to mice showing anxiety/depression-like emotions similar to human bipolar patients,suggesting that Tmem132e is involved in the development of bipolar disorder.Secondly,to investigate the possible mechanism by which Tmem132e is involved in the development of bipolar disorder,Nissl staining and H&E staining were used to visualize the brain tissue morphology of knockout mice.The results showed that knockout of Tmeml32e did not affect the overall morphogenesis of the brain under physiological conditions;immunofluorescence staining and Western blot revealed that knockout of Tmem132e resulted in astrogliosis without affecting the number of neurons;Golgi staining results showed that knockout of Tmem132e resulted in a decrease in dendritic spine density;Western blot results showed that synaptic protein expression was decreased,suggesting that Tmem132e knockout could lead to impaired synaptic plasticity.Again,given that the BDNF-TrkB signaling pathway plays a vital role in BD and other psychiatric diseases,the expression of TrkB receptors in the cortex and hippocampal brain regions of T132e ScKO mice was examined in this paper,and Western blot results showed that TrkB was significantly decreased in T132e ScKO mice.All three signaling pathways downstream of TrkB receptor are inhibited to varying degrees.This suggests that anxietydepression-like behavior resulting from Tmem132e knockout is associated with decreased TrkB expression inhibiting its downstream pathway.When we examined TrkB receptor expression in cortical hippocampal brain regions of T132E-OE mice,we found that TrkB was significantly elevated in the hippocampal brain regions of T132E-OE mice.This result suggested that Tmem132e correlates with TrkB expression.Finally,to investigate the mechanism by which TMEM132E affects TrkB,PLA,Co-IP and immunofluorescence staining methods were used to analyze it from in vitro and in vivo,and the results showed that TMEM132E protein co-localizes with TrkB in neural cells and interacts with both full-length TrkB protein(TrkB-FL)and truncated TrkB protein(TrkB-T1).In summary,the present study confirmed that Tmem132e knockout caused mice to exhibit anxiety-depression-like behavior similar to bipolar patients,suggesting that Tmem132e plays a vital role in the development of bipolar disorder;preliminarily explored that Tmem132e may affect the BDNF-TrkB signaling pathway by synergizing with TrkB or by regulating TrkB expression,resulting in impaired synaptic plasticity and ultimately leading to the generation of anxiety-depression-like behavior.Current studies have shown that antidepressants can directly bind and allosterically act on TrkB,and TMEM132E may become a therapeutic target for mental stabilizers.Further clarifying the mechanism of action of TMEM132E and TrkB can provide a theoretical basis for elucidating the biological mechanism of anxiety and depression comorbidity in bipolar disorder and improving treatment options.