Cholinergic Receptor Muscarinic 1 Co-Localized with Mitochondria in Cultured Dorsal Root Ganglion Neurons, and Its Deletion Disrupted Mitochondrial Ultrastructure in Peripheral Neurons: Implications in Alzheimer’s Disease

Abstract

Background: Loss of Cholinergic Receptor Muscarinic 1 (CHRM1) has been linked to the pathogenesis of Alzheimer’s disease (AD). Our recent study found significantly lower CHRM1 protein levels in AD patient cortices, linked to reduced survival. Furthermore, using knockout mice (Chrm1−/−) we demonstrated that deletion of Chrm1 alters cortical mitochondrial structure and function, directly establishing a connection between its loss and mitochondrial dysfunction in the context of AD. While CHRM1’s role in the brain has been extensively investigated, its impact on peripheral neurons in AD remains a crucial area of research, especially considering reported declines in peripheral nerve conduction among AD patients. Objective: The objective was to characterize Chrm1 localization and mitochondrial deficits in Chrm1−/− dorsal root ganglion (DRG) neurons. Methods: Recombinant proteins tagged with Green or Red Fluorescent Protein (GFP/RFP) were transiently expressed to investigate the localization of Chrm1 and mitochondria, as well as mitochondrial movement in the neurites of cultured primary mouse DRG neurons, using confocal time-lapse live cell imaging. Transmission electron microscopy was performed to examine the ultrastructure of mitochondria in both wild-type and Chrm1−/− DRGs. Results: Fluorescence imaging revealed colocalization and comigration of N-terminal GFP-tagged Chrm1 and mitochondrial localization signal peptide-tagged RFP-labelled mitochondria in the DRGs neurons. A spectrum of mitochondrial structural abnormalities, including disruption and loss of cristae was observed in 87% neurons in Chrm1−/− DRGs. Conclusions: This study suggests that Chrm1 may be localized in the neuronal mitochondria and loss of Chrm1 in peripheral neurons causes sever mitochondrial structural aberrations resembling AD pathology.

Editorial: Proteins and protein-complexes underlying mitochondrial structure-function and metabolism: implications in diseases

“…This contrasting finding was further corroborated by the observations that Chrm1 loss in cortex versus hippocampus differentially alters respiratory protein complexes assembly, correlating with the post-translational modifications of the key proteins involved in oxidative phosphorylation. Furthermore, a recent study by Sabbir published in the Journal of Alzheimer’s Disease ( Sabbir, 2024 ) reported the colocalization and comigration of N-terminal green fluorescence protein-tagged Chrm1 with red fluorescence protein-labelled mitochondria in cultured adult primary rat dorsal root ganglion neurons, suggesting a potential localization of Chrm1 protein with mitochondria. The study observed the presence of abnormally swollen mitochondria with a loss of cristae in the Chrm1-deleted mouse peripheral sensory neurons, indicating a role for Chrm1 in the regulation of peripheral neurons’ mitochondrial structure ( Sabbir, 2024 ).…”

“…Furthermore, a recent study by Sabbir published in the Journal of Alzheimer’s Disease ( Sabbir, 2024 ) reported the colocalization and comigration of N-terminal green fluorescence protein-tagged Chrm1 with red fluorescence protein-labelled mitochondria in cultured adult primary rat dorsal root ganglion neurons, suggesting a potential localization of Chrm1 protein with mitochondria. The study observed the presence of abnormally swollen mitochondria with a loss of cristae in the Chrm1-deleted mouse peripheral sensory neurons, indicating a role for Chrm1 in the regulation of peripheral neurons’ mitochondrial structure ( Sabbir, 2024 ). The reported structural, physiological, and molecular phenotypes under brain tissue-specific loss of Chrm1 protein represent a novel discovery that will shape the direction of Alzheimer’s research.…”