A 5-HTergic DNR vCA1 circuit and Alzheimer's disease

PROJECT SUMMARY Despite the tremendous efforts in Alzheimer’s disease (AD) research, we have not made much progress in understanding the pathophysiology of AD or inhibiting/correcting AD-related behavioral symptoms. The recent FDA-approved aducanumab demonstrated significant efficacy in reducing amyloid ß (Aß). Still, it showed a limited effect in improving AD-related memory impairments. The specific neural circuits that mediate these cognitive processes but are altered progressively in the AD brains may serve as a treatment target after the removal of Aß plaques. In my previous studies, I demonstrated that dorsal Raphe nucleus (DRN) serotonin (5- HT) neurons provide monosynaptic inputs to the hippocampal ventral CA1 (vCA1). Further, genetic ablation of 5-HT synthesis selectively in these vCA1-projecting DRN neurons impaired spatial memory in young mice. In addition, genetic deletion of the 5-HT 2C receptor (5-HT2CR) in the vCA1 led to spatial memory deficits in young mice. I also observed that lorcaserin, a selective agonist of 5-HT2CR, can ameliorate spatial memory deficits in a 6-month-old knock-in AD mouse model (APPNL-G-F), associated with restoration of synaptic plasticity in vCA1 neurons. Together, I developed a hypothesis that a 5-HTergic DRN to vCA1 circuit regulates spatial memory via 5-HT2CR, a therapeutic target for memory symptoms in Alzheimer’s disease. The K99 phase will focus on the upstream node of this circuit, the vCA1-projecting 5-HT neurons. Fiber photometry experiments will be used to monitor the real-time activity of these vCA1-projecting 5-HT neurons, as well as 5-HT release in the vCA1, corresponding to memory acquisition and retrieval behaviors. The intersectional retrograde chemogenetic approach will be used to further test whether inhibition of the vCA1-projecting 5-HT neurons would inhibit memory function and whether activating these neurons would rescue memory impairments in APPNL-G-F mice and aged mice. During the R00 phase, I will utilize the techniques and the problem-solving experience I acquire from the K99 phase to test the functional significance of the downstream 5-HT2CR-expressing vCA1 neurons. I will use fiber photometry to monitor the activity of 5-HT2CR-expressing vCA1 neurons during the memory test and will use the chemogenetic approach to assess the functional relevance of these neurons in memory function. In addition, I will also test the combination treatment of Aß-lowering (aducanumab) and 5-HT2CR agonism (lorcaserin) in APPNL-G-F mice and aged mice. The proposed studies will advance our knowledge of the circuitry mechanisms underlying memory function and evaluate the possibility of 5-HT2CR agonism as a novel therapeutic target for AD in combination with Aß-reducing medications. In addition, the K99 phase will provide an ideal training opportunity to equip me with essential techniques, knowledge, and problem-solving skills. These will prepare me for the R00 phase of research and an independent research career focusing on circuitry mechanisms of different behaviors.