PDCD4 and depressive disorder: To further validate the causal relationship between the expression of Pdcd4 and depressive disorder, a recombinant AAV9 virus carrying mouse Pdcd4 gene and GFP (OE-Pdcd4) was bilaterally injected into the hippocampus of mice, the control group received an AAV9 virus only expressing GFP (Fig. 1g).