Nerve growth factor stimulates regeneration of perivascular nerve, and induces the maturation of microvessels around the injured artery

Highlights

• Novel role of peripheral nerve on pathological angiogenesis.

• Development of new in vivo angiogenesis assay using collagen-coated tube (CCT).

• Innervation of peripheral nerves around the pre-mature vasa vasorum microvessels.

• NGF-induced maturation of microvessels around the injured vascular walls.

Abstract

An immature vasa vasorum in the adventitia of arteries has been implicated in induction of the formation of unstable atherosclerotic plaques. Normalization/maturation of the vasa vasorum may be an attractive therapeutic approach for arteriosclerotic diseases. Nerve growth factor (NGF) is a pleotropic molecule with angiogenic activity in addition to neural growth effects. However, whether NGF affects the formation of microvessels in addition to innervation during pathological angiogenesis is unclear. In the present study, we show a new role for NGF in neovessels around injured arterial walls using a novel in vivo angiogenesis assay.

The vasa vasorum around arterial walls was induced to grow using wire-mediated mouse femoral arterial injury. When collagen-coated tube (CCT) was placed beside the injured artery for 7–14 days, microvessels grew two-dimensionally in a thin layer on the CCT (CCT-membrane) in accordance with the development of the vasa vasorum. The perivascular nerve was found at not only arterioles but also capillaries in the CCT-membrane. Biodegradable hydrogels containing VEGF and NGF were applied around the injured artery/CCT. VEGF significantly increased the total length and instability of microvessels within the CCT-membrane. In contrast, NGF induced regeneration of the peripheral nerve around the microvessels and induced the maturation and stabilization of microvessels. In an ex vivo nerve-free angiogenesis assay, although NGF potentially stimulated vascular sprouting from aorta tissues, no effects of NGF on vascular maturation were observed.

These data demonstrated that NGF had potent angiogenic effects on the microvessels around the injured artery, and especially induced the maturation/stabilization of microvessels in accordance with the regeneration of perivascular nerves.

Introduction

The vasa vasorum forms a microvascular network in the adventitial layer of large arteries and supplies oxygen and nutrients to the outer layers of the vessel walls [1]. As vessel walls thickness increases in the setting of atherosclerotic diseases, the vasa vasorum within the arterial walls grows [2]. The vasa vasorum within atherosclerotic lesion has relatively thin and fragile walls, usually with fewer pericytes around the microvessels compared to normal capillaries [3]. These properties of the immature vasa vasorum contributes to the progression of unstable plaques [4], [5]. Therefore, the vasa vasorum in plaques is an attractive therapeutic target for arteriosclerotic diseases, and improving our understanding of the processes involved in the maturation and remodeling of the vasa vasorum is necessary. However, the mechanism of vasa vasorum angiogenesis in the pathophysiological setting is poorly understood because of limited appropriate methods for observing the microvessels in the arterial walls.

Perivascular nerves play an important role in the maintenance and regulation of vascular tone [6]. Regeneration of the perivascular vasomotor nerve occures during angiogenesis to form the functional vasculature. In contrast to postnatal stages, peripheral nerve play an alternative role in angiogenesis during embryonic development. During development of embryonic mouse limb skin, peripheral nerves provide a template that determines the organo-typic pattern of vessel branching and arterial differentiation [7], [8]. Neurotrophic and neural guidance factors contribute to embryonic vessel formations, forming the vascular and neural network [9], [10]. However, whether the perivascular nerve is involved in neovessels formation in postnatal physiological and pathological conditions is unclear.

Neurotrophic factors such as nerve growth factor (NGF) are well known for their roles in regulating growth and functional maintenance of peripheral and central nervous system cells [11]. NGF also has potent angiogenic activity [12], although the mechanism of the angiogenic effects of NGF is still controversial. NGF increases the density of not only capillaries but also matured vessels such as arterioles in response to hindlimb ischemia [13], [14]. The angiogenic effects of NGF are mediated through direct effects on vascular endothelial cells or indirectly by influencing the action of other endogenous growth factors such as vascular endothelial growth factor (VEGF) [12], [15]. However, the effects of NGF on vascular maturation cannot be fully explained by the previously proposed mechanisms.