The Role Of Dysregulated Protein Expression In Abnormal Behaviors Of SAPAP4 Knockout Mice

SAPAP(SAP90 / PSD95 Associated Proteins,SAPAP)is a very important family of scaffold proteins in the postsynaptic density.As one of the SAPAP protein family members,SAPAP4 is highly expressed in the prefrontal cortex,hippocampus,striatum and other brain areas related to higher cognitive functions.It is involved in the regulation of dendritic spine development and synaptic structure maintenance by interacting with a variety of proteins and plays a crucial role in the normal function of synapses.In our previous study,we found that SAPAP4 knockout mice displayed manic-like behaviors,such as hyperactivity,antidepressant,memory deficits,etc,but the molecular mechanism that underlies the abnormal behaviors of SAPAP4 knockout mice is still unclear.Given that manic behavior is closely related to the dysfunction of prefrontal cortex,hippocampus and striatum,in the present study,Western blot was used to determine whether loss of SAPAP4 impacts the protein expression of multiple postsynaptic scaffolds and receptors in these three brain regions.Our results showed that the expression level of Shank3 protein was significantly increased in prefrontal cortex in SAPAP4 knockout mice,expression of NMDA receptor subunits(NR2A and NR2B)reduced in both hippocampus and striatum,and expression of mGluR5 receptor was found decreased in hippocampus.To further investigate whether altered expression of Shank3 or NMDA receptor contributes to the manic-like behaviors of SAPAP4 knockout mice,Shank3 was decreased in SAPAP4 knockout mice by crossing with Shank3 knockout mice.We found that abnormal behaviors of SAPAP4 knockout mice were not rescued by such strategy.In addition,combining behavioral and pharmacological methods,we found that short-term administration of D-cycloserine,a partial NMDA receptor agonist,could enhance contextual memory in WT mice,but could not rescue memory deficit in SAPAP4 knockout mice.Previous studies have shown that monoamine system abnormalities played important roles in mood disorders such as mania.Therefore,we used Western blot to examine protein expression levels of monoamine transporters and receptors in the three brain regions mentioned above.We found that the protein level of 5HT2 C and NET were significantly decreased in prefrontal cortex and hippocampus,respectively.Further study with high performance liquid chromatography showed that the concentrations of 5-hydroxytryptamine,dopamine and other monoamine transmitters were not significantly changed in the hippocampus of SAPAP4 knockout mice.In summary,with the application of technologies including biochemistry,behavior study and pharmacology,we investigated the molecular mechanism underlying the behavioral abnormalities in SAPAP4 knockout mice.Here,we demonstrated that loss of SAPAP4 function resulted in abnormal expression of several synaptic proteins and monoamine system related proteins,which is likely to be related to the abnormal behaviors of SAPAP4 knockout mice.Our findings may facilitate future studies on the potential role of the SAPAP4 in manic-like behaviors.