The dynamic impairment of synaptic transmission in the PCx-IL engram circuit contributes to early olfactory memory decline in Alzheimer's disease.

Olfactory dysfunction has emerged as a promising target for the early diagnosis and treatment of Alzheimer's disease (AD). However, the mechanisms underlying neural circuit disruption associated with olfactory dysfunction in AD remain poorly understood. We conducted single-cell RNA sequencing (RNA-seq) and ex vivo electrophysiological studies to determine the link between olfactory memory in AD and dynamic synaptic transmission disorders in PCx-IL engram cell circuits. Clinical functional magnetic resonance imaging (fMRI) data revealed that connectivity between the piriform cortex (PCx) and the infralimbic cortex (IL) was impaired during the early mild cognitive impairment (MCI) stage of AD. Optogenetic stimulation of IL-projecting PCx engram neurons successfully improved olfactory memory retrieval in 5xFAD mice. In addition, single-cell RNA sequencing was employed to investigate the mechanisms of damage in IL engram cells, which revealed increased glutamate expression and impaired synaptic function as key alterations. Guided by single-cell sequencing data, we analyzed glutamatergic synaptic transmission in the PCx-IL engram cell circuit in 5xFAD mice. These results indicated dynamic impairments in AMPA receptor-associated synaptic transmission within this circuit. Optical long-term potentiation (LTP) of synaptic transmission restored directional engram synaptic transmission and prevented olfactory memory decline. Therefore, dynamic impairment of synaptic transmission in the PCx-IL engram cell circuit underlies the early decline in olfactory memory in AD. Impairment of PCx-IL functional connectivity may represent a new target for the diagnosis and treatment of early-stage AD.