Subcellular localization of PDE4D and HCN1 in rhesus macaque entorhinal cortex layer II: Signature of vulnerability in 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 the disease course is unknown. Our previous research in monkeys showed early calcium dysregulation in layer II ERC, where phosphorylated tau accumulated on the calcium-storing smooth endoplasmic reticulum (SER) under glutamatergic synapses, and PKA-phosphorylated ryanodine receptors on the SER showed evidence of calcium leak. 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 advancing age. We used high-spatial resolution immunoEM to localize PDE4D and HCN1 in young rhesus macaque (7-10y) ERC layer II. PDE4D and HCN1 were primarily observed in postsynaptic compartments in macaque ERC layer II. In dendritic spines, PDE4D was concentrated on the SER spine apparatus and in postsynaptic density, and HCN1 expressed in the membrane near excitatory synapses. Within dendritic shafts, PDE4D labeling was observed along microtubules and near mitochondria, whereas HCN1 was organized in discrete clusters along the plasma membrane. PDE4D immunolabeling was also observed in astroglial leaflets ensheathing excitatory asymmetric synapses. 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. These data suggest that PDE4D and HCN1 are positioned to provide a signature of flexibility in postsynaptic compartments in ERC layer II stellate cells, which becomes a signature of vulnerability when abrogated by advancing age.