PDE4D And HCN1 Ultrastructure In Rhesus Macaque Entorhinal Cortex: Relevance For Aging And Alzheimer's Disease
Abstract Tau pathology emerges in a distinct spatial and temporal pattern in Alzheimer’s Disease (AD). Anatomical studies in AD subjects and rhesus macaques show earliest signs of tau pathology in the stellate cell islands in entorhinal cortex (ERC) layer II. However, the molecular mechanisms that confer vulnerability to ERC layer II cells early in AD is unknown. cAMP-PKA magnification of calcium release has been seen in prefrontal cortex, associated with HCN channel opening to dynamically regulate synaptic strength. This process is regulated by phosphodiesterases (PDE), regulation that is lost with age. The current study examined whether this “signature of flexibility” could also be seen in layer II ERC, underlying vulnerability to tau pathology with aging. We used high-spatial resolution immunoEM to localize PDE4D and HCN1 in young rhesus macaque (7-10y) ERC layer II. Our results suggest that PDE4D was concentrated on the SER-spine apparatus and in postsynaptic density, and HCN1 expressed in the membrane near excitatory synapses in dendritic spines. Within dendritic shafts, PDE4D labeling was observed along microtubules and near mitochondria, whereas HCN1 was organized in discrete clusters along the plasma membrane. These data suggest that PDE4D is optimally positioned to modulate cAMP microdomains and control calcium extrusion from the SER. HCN1 channels are localized in subcompartments to facilitate dynamic physiological representation of sensory experience and visual space governed by cAMP-PKA signaling. The anatomical patterns in ERC layer II corroborate our findings in vulnerable glutamatergic circuits in prefrontal cortex, suggesting conserved molecular features in association cortices most susceptible in AD.