The role of Densin-180 on mouse behavior and synaptic protein distribution

Abstract

Lrrc7 (Leucine Rich Repeat Containing 7) encodes Densin-180, a scaffold protein located in the postsynaptic density of excitatory synapses. A transgenic mouse line with disruption of Lrrc7 by the transgene displayed severe abnormalities, including intensive fighting with littermates, anxious and attack during handling, and death at weaning due to dehydration. Homozygous females did not nurse their babies. As the transgene was not expressed and there was almost a complete loss of expression of Densin-180 in the mutant brain, the behavioral defects might be due to molecular abnormalities in excitatory synapses induced by the deficiency of Densin-180. Also, to reduce injury due to fighting caused by mutant mice, a colored transparent plastic house and paper nesting strips were used to enhance the nursing condition in the home cage. Surprisingly, fighting was no longer observed. Therefore, the first objective of this study was to analyze the behavioral and biochemical defects in Densin-180 mutants, and then to examine the effect of environmental enrichment (EE) on these defects in the mutants. The ultimate hope is to rescue the abnormalities displayed in Densin-180 mutants.

The behavioral assessments confirmed that Lrrc7 mutants were anxious as illustrated in three independent tests for the level of anxiety: Open field, elevated plus maze and light-dark box. Also, the sucrose preference test suggested anhedonia-like behavior in Lrrc7 mutants. Interestingly, the Lrrc7 mutants displayed a normal social interaction in three-chamber test, but reduced aggressiveness in one-chamber test. At the cellular level, the reduced levels of Calcium/calmodulin-dependent kinase II (CamKII) α and NMDA receptor subunit 2B (Nr2B) in the synaptosomal fraction of mutant cortex was consistent with the scaffold role of Densin-180. Furthermore, the reduction of neurite length and breadth in primary hippocampal neurons isolated from mutant embryos suggested that loss of Densin-180 affected early neurite development. Taken together, the data not only consolidated the scaffold role of Densin-180 in the synapse, but also revealed an important function of Densin-180 in neurite development.

With the above-mentioned EE protocol, the difference in the levels of both CamKIIα and Nr2B were no longer observed in the mutant from western analysis. Concerning their behavior, enrichment was capable to rescue the anhedonia-like behavior, but it did not reduce the anxiety level of the mutant mice. Exploration and locomotion were increased in both wild-type (WT) and mutant mice in the open field and light-dark box. Even though EE did not exert any effect on the anxiety-like behavior, the beneficial influence of EE on the general well-being of Lrrc7 mouse mutants and their littermates was remarkable. To conclude, Lrrc7 mutants exhibited excessive fighting with littermates before weaning. Behavioral tests of adult mice showed that homozygous mutant mice had increased anxiety but might have lost the aggressiveness. Loss of Densin-180 impaired neurite development, as well as the proper localization of CamKIIα and NR2B to synapses. The synaptic and behavioral abnormalities exhibited in Lrrc7 mutants could be ameliorated by EE.