PDE4D and neuroblastoma: Next, complementing the findings in the mouse neuroblastoma N2a cells, we sought to identify the role of PDE4D in regulating neuronal morphology in the mouse hippocampal HT22 cell line to better approximate the in vivo context in which we found changes in hippocampal PDE4D-mediated cAMP-PKA-CREB signaling (Figs. 1 and 2).