Transcriptomic and proteomic characterization of the locus coeruleus in a rat model of Alzheimer’s disease

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AbstractBackgroundThe noradrenergic locus coeruleus (LC) is the first region to accumulate hyperphosphorylated tau pathology (p‐tau) in Alzheimer’s disease (AD). Using TgF344‐AD transgenic rats, which develop endogenous p‐tau in the LC prior to pathology in forebrain regions, we have recently demonstrated age‐dependent effects of p‐tau on LC firing rates. Specifically, LC neurons exhibit early hyperactivity followed by late hypoactivity. These changes in LC firing rates are coincident with the emergence of behavioral phenotypes that characterize different stages of AD (neuropsychiatric symptoms followed by cognitive deficits). Here, we sought to identify molecular mechanisms that could contribute to these changes using RNAscope and proteomics in TgF344‐AD rats.Methods32 male and female TgF344‐AD rats and wild‐type (WT) littermates aged 6 and 15 months were used for this study. Brains were flash‐frozen in isopentane on dry ice and stored at ‐80°C until being sectioned. At the level of the LC, the left hemisphere LC was collected with a 1 mm tissue punch. Punches of the cerebellum were taken as a control to verify LC enrichment in the brainstem punches. The right LC hemisphere was sectioned at 16 µm for processing with fluorescent in situ hybridization using the RNAscope Multiplex Fluorescent V2 Assay according to the manufacturer’s protocol. Probes included noradrenergic markers (tyrosine hydroxylase, norepinephrine transporter) and those that influence LC activity (α2‐adrenergic receptors, GRIA2, GRIN, ADCYAP1R1). Separately, tissue punches were submitted to the Emory Integrated Proteomics Core for label‐free quantification of proteins.ResultsRNAscope revealed no differences in transcript levels of noradrenergic markers at either age. Similarly, there were no differences in transcript levels of α2‐adrenergic receptors, GRIA, or ADCYAP1R1 at either age, or GRIN in 6‐month animals. We observed an increase of GRIN mRNA in 15‐month TgF344‐AD animals, suggesting an upregulation of excitatory neurotransmission.ConclusionsNoradrenergic neuronal identity remains in‐tact up to 15 months of age, a timepoint when there is no LC cell death in these rats. We have also uncovered a potential compensatory mechanism to maintain LC activity in late stages of disease. Proteomics and RNAscope analysis of other markers that may affect LC activity are ongoing.