The main hypothesis of this proposal is that degeneration of the locus coeruleus (LC) noradrenergic projection system promotes cognitive impairment in Alzheimer?s disease (AD) by driving forebrain cerebrovascular dysfunction. We recently demonstrated that LC neuron loss occurs in amnestic mild cognitive impairment (aMCI) and correlates with poorer cognitive function. Hence, understanding how LC degeneration impairs cognition may open up new therapeutic avenues. To date, most studies have focused on the role of the LC in regulating AD neuropathology, yet virtually no attention has been paid to the role of the LC in regulating cerebrovascular permeability and perfusion in AD. To gauge the scientific premise for pursuing this question, we administered a dopamine-?-hydroxylase IgG-saporin (DBH-SAP) immunotoxin into the prefrontal cortex (PFC) of Tg344-19 AD rats, which mimicked LC neuron and fiber loss and resulted in memory impairments. Strikingly, postmortem analysis revealed evidence of widespread blood-brain barrier leakage and increased cerebral amyloid angiopathy in DBH-SAP-lesioned AD rats compared to control IgG-saporin (CTL-SAP) rats. Moreover, pressure myography studies showed blunted vasoreactivity of PFC parenchymal arterioles following DBH-SAP lesions. To begin to understand the mechanisms for these LC-mediated pathologies, RNA sequencing was performed on laser-captured PFC vessels from Rush Religious Orders Study (RROS) subjects. These data revealed a dysregulation of genes mediating vessel permeability and calcium signaling in aMCI and AD compared to controls. Many of these genes were also dysregulated in vessels harvested from DBH-SAP-lesioned AD rats, suggesting specific pathomechanic pathways linking LC degeneration with forebrain vascular dysfunction. Therefore, we designed our Specific Aims to test the extent to which LC degeneration drives cerebrovascular pathology in target fields (Aim 1) and the potential mechanisms underlying this phenomenon (Aim 2).
In Aim 1, Tg344-19 AD rats will be administered DBH-SAP or CTL-SAP in the PFC in the presence or absence of the norepinephrine pro-drug L-DOPS or the reuptake inhibitor atomoxetine. Rats will be assessed for memory function and for cortical perfusion by MRI. Pressure myography studies will analyze vessel physiology and postmortem analysis will quantify vascular and AD-like pathology. Spontaneously Hypertensive Stroke Prone rats will also be included to test whether LC cell loss exacerbates vascular risk factors.
In Aim 2, PFC parenchymal vessels from RROS subjects will be analyzed by RNA-Seq. Target genes that are also dysregulated in PFC vessels from DBH-SAP rats will be validated and tested for their role in vascular dysfunction using in vitro mechanistic assays. If successful, this proposal will show that 1) LC degeneration is a nexus lesion that impacts both vascular and neuropathology during the earliest stages of AD, and that 2) targeting noradrenergic mechanistic pathways in small vessels may allow for more comprehensive disease modification in AD by reducing vascular contributions to cognitive impairment.
The contribution of cerebrovascular disease and pathology to the onset and progression of Alzheimer?s disease (AD) is a significant question that has received renewed interest. This proposal expounds upon compelling in vivo and human preliminary data to test that degeneration of the noradrenergic locus coeruleus (LC) projection system during the AD prodrome contributes to cognitive impairment through its damaging effects on cerebrovascular integrity in target fields. If successful, this proposal will advance the clinical rationale for targeting LC signaling pathways as part of a comprehensive disease modifying therapeutic regimen.