3D Reconstruction of the Neurovascular Unit Reveals Differential Loss of Cholinergic Innervation in the Cortex and Hippocampus of the Adult Mouse Brain.

Shereen Nizari,Ignacio A Romero,Roxana O Carare,Cheryl A Hawkes

doi:10.3389/fnagi.2019.00172

Shereen Nizari, Ignacio A Romero + Show 2 more

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https://doi.org/10.3389/fnagi.2019.00172

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Abstract

Increasing evidence supports a role for cerebrovasculature dysfunction in the etiology of Alzheimer’s disease (AD). Blood vessels in the brain are composed of a collection of cells and acellular material that comprise the neurovascular unit (NVU). The NVU in the hippocampus and cortex receives innervation from cholinergic neurons that originate in the basal forebrain. Death of these neurons and their nerve fibers is an early feature of AD. However, the effect of the loss of cholinergic innervation on the NVU is not well characterized. The purpose of this study was to evaluate the effect of the loss of cholinergic innervation of components of the NVU at capillaries, arteries and veins in the hippocampus and cortex. Adult male C57BL/6 mice received an intracerebroventricular injection of the immunotoxin p75NTR mu-saporin to induce the loss of cholinergic neurons. Quadruple labeling immunohistochemistry and 3D reconstruction were carried out to characterize specific points of contact between cholinergic fibers and collagen IV, smooth muscle cells and astrocyte endfeet. Innate differences were observed between vessels of the hippocampus and cortex of control mice, including a greater amount of cholinergic contact with perivascular astrocytes in hippocampal capillaries and a thicker basement membrane in hippocampal veins. Saporin treatment induced a loss of cholinergic innervation at the arterial basement membrane and smooth muscle cells of both the hippocampus and the cortex. In the cortex, there was an additional loss of innervation at the astrocytic endfeet. The current results suggest that cortical arteries are more strongly affected by cholinergic denervation than arteries in the hippocampus. This regional variation may have implications for the etiology of the vascular pathology that develops in AD.

Highlights

The loss of cholinergic neurons in the basal forebrain and the areas innervated by their fiber projections is a hallmark of Alzheimer’s disease (AD; Whitehouse et al, 1982; Francis et al, 1999)
Recent genetically-driven technologies such as optogenetics and designer receptor exclusively activated by designer drug (DREDD) have led to more targeted approaches to silence specific cholinergic populations (Hangya et al, 2015; Zhang et al, 2017), it is not clear if these techniques replicate the loss of cholinergic innervation that is seen in AD
Cholinergic fiber projections were observed in the hippocampus (Figure 1B) and cortex (Figure 1C)

Summary

Introduction

The loss of cholinergic neurons in the basal forebrain and the areas innervated by their fiber projections is a hallmark of Alzheimer’s disease (AD; Whitehouse et al, 1982; Francis et al, 1999). Numerous experimental models have been used to mimic the loss of basal forebrain cholinergic neurons and their fiber projections These include injection of ibotenic acid into the substantia innominata (Vaucher and Hamel, 1995), lesioning of the fimbria fornix (van der Staay et al, 1989) and electric pulse ablation of the medial septum (Scheiderer et al, 2006; Nelson et al, 2014). These models can result in widespread degeneration that may not target cholinergic cell populations. Recent genetically-driven technologies such as optogenetics and designer receptor exclusively activated by designer drug (DREDD) have led to more targeted approaches to silence specific cholinergic populations (Hangya et al, 2015; Zhang et al, 2017), it is not clear if these techniques replicate the loss of cholinergic innervation that is seen in AD

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