Neuronal Ddit4 overexpression in the medial prefrontal cortex reduces synaptic density and impairs cognitive function in mice

Chronic stress exposure causes neurobiological and behavioral changes that resemble those reported in psychiatric conditions such as major depressive disorder (MDD). Preclinical stress models and studies using postmortem tissue from MDD patients have shown that DNA Damage-Inducible Transcript 4 ( Ddit4 ) is increased in the prefrontal cortex (PFC). This is important because DDIT4 negatively regulates the mammalian target of rapamycin (mTOR) pathway, which may lead to behavioral deficits through diminished neuroplasticity and PFC function. Our prior studies indicate that coordinated neuron-microglia interactions contribute to synaptic remodeling in the PFC. The present studies aimed to test the hypothesis that increased neuronal Ddit4 expression is sufficient to drive structural remodeling of PFC neurons, in part by provoking microglia activation, and this leads to behavioral and cognitive deficits. To this end, we bilaterally infused AAV5-hSyn1-Ddit4-tdTomato or a control vector into the PFC of male Thy1-GFP and C57BL/6 mice and examined molecular, cellular, and behavioral endpoints. Mice with Ddit4 overexpression (Ddit4-OV) showed no change in passive stress coping yet exhibited a deficit in temporal order memory. Immunohistology analyses showed a decrease in dendritic spine density of Ddit4-OV mice. However, we found no changes in microglia count, microglia size, or nearest neighbor distance. Bulk RNA sequencing of Ddit4-OV PFC revealed increases in transcripts involved with dendrite and synapse function and decreases in transcripts involved with mitochondrial function, implicating mTOR dysregulation. Altogether, these results indicate that Ddit4 overexpression recapitulates some of the broad molecular, cellular, and behavioral adaptations observed following chronic stress exposure through a cell-autonomous mechanism. Significance Statement This work provides more context for the neurobiological effects of neuronal DNA Damage-Inducible Transcript 4 ( Ddit4 ). Ddit4 , an inhibitor of the mammalian target of rapamycin (mTOR) pathway, exhibits increased expression in the prefrontal cortex (PFC) of both rats exposed to pre-clinical chronic stress models and humans diagnosed with major depressive disorder (MDD). Our findings demonstrate that Ddit4 overexpression specifically in neurons is sufficient to reduce spine density in the PFC, impair temporal order memory, and induce transcriptional changes associated with stress and depression. These results indicate that neuronal Ddit4 can disrupt PFC function and cognitive performance in a cell-autonomous manner.