Basic Science and Pathogenesis

Abstract

Background: Effective disease-modifying regimens for Alzheimer's Disease (AD) remain lacking due to insufficient understanding of its pathogenic drivers. It was shown previously that upregulation of the calcium-sensing receptor (CaSR), an excitatory family C GPCR, induces neurodegeneration by interfering with the inhibitory γ-aminobutyric acid (GABA) signaling following acute brain injuries (Ann_Clin_Transl_Neurol, 1:851-66). Herein, we determined whether CaSR overexpression is causally associated with the AD.

Method: (1) Proteomic profiles of hippocampal CA1 neurons in postmortem brains of subjects at progressive AD Braak stages (0, 1, 2, 4 and 6) were compared using a NanoString GeoMx profiler. (2) Proximity ligation assays (PLA) were performed to assess interactions of neuronal CaSR with type B GABA receptors (GABA_B1R and GABA_B2R). (3) Y-maze and new object recognition (NOR) tests were performed to assess the impact of neuronal ablation of the Casr gene by Camk2a-Cre-mediated gene recombination (^Camk2aCaSR^{Δflox/Δflox}) or pharmacological inhibition of neuronal CaSR activities by daily subcutaneous injections of a brain-permeable calcilytic (NPS-2143, 20 µmole/kg) on cognitive functions of aging C57/B6 mice and two mouse models of early-onset AD, 5XFAD (MMRRC_034840-JAX) and hAPP^NL-G-F (Nature_Neuroscience, 5:661) mice.

Result: The abundance of CaSR protein was significantly (p<0.0001) increased from the Braak stage 2 to 4/6 (Fig. 1A, C) and positively correlated (r² = 0.2448 - 0.4655, p<0.0001) with the abundance of APP, Aβ_1-42, BACE1, PSEN1, and phospho-tau proteins (Fig. 1E-I) in the somas of CA1 neurons (Fig. 1B) of the human brains. PLA showed increases in heterodimerization of CaSR with the GABA_B1R in human brain from the Braak 0 to 5 (Fig. 2A) and in cultured hippocampal neurons overexpressing the CaSR by lentiviruses along with reduced GABA_B1R/GABA_B2R heterodimerization (Fig. 2B). While the control mice showed significant (p<0.001) cognitive declines from 12 to 18/20 months of age (MOA), these defects were prevented in the age-matched ^Camk2aCaSR^{Δflox/Δflox} or NPS-2143-injected mice (Fig. 3A, B). Similarly, the early cognitive declines in the 5XFAD (Fig. 3C-F) and hAPP^NL-G-F (Fig. 3G, H) mice at 6 MOA were completely alleviated when the mice were bred into the ^Camk2aCaSR^{Δflox/Δflox} background or injected with NPS-2143.

Conclusion: The neuronal CaSR is a critical driver of dementia by interfering with GABA_BR signaling and a druggable target to prevent or treat AD.