Elsevier

Nitric Oxide

Volume 20, Issue 4, 1 June 2009, Pages 223-230
Nitric Oxide

Review
Neuronal nitric oxide synthase: Structure, subcellular localization, regulation, and clinical implications

https://doi.org/10.1016/j.niox.2009.03.001Get rights and content

Abstract

Nitric oxide (NO), a free gaseous signaling molecule, is involved in the regulation of the cardiovascular, nervous and immune system. The neurotransmitter function of nitric oxide is dependent on dynamic regulation of its biosynthetic enzyme, nitric oxide synthase (NOS). There are three types of NOS, neuronal nitric oxide synthase (nNOS), endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS). Of the three NOS, we focus on nNOS in the present review. Brain nNOS exists in particulate and soluble forms and the differential subcellular localization of nNOS may contribute to its diverse functions. Proteins bearing PDZ domains can interact directly with the PDZ domain of nNOS, influencing the subcellular distribution and/or activity of the enzyme. During the past several years, an increasing number of reports have demonstrated the importance of nNOS in a variety of synaptic signaling events. nNOS has been implicated in modulating physiological functions such as learning, memory, and neurogenesis, as well as being involved in a number of human diseases. In this review we concentrate on recent findings regarding the structural features, subcellular localization and factors regulating nNOS function. In particular, we conclude with a section discussing the role of nNOS in a wide range of physiological and pathological conditions.

Introduction

Since awarding the Nobel Prize to R. Furchgott, L. Ignarro and F. Murad for their discovery of nitric oxide (NO) as a biological mediator, a rapidly expanding body of data have indicated the importance of nitric oxide in the physiology of the central nervous system [1], [2], [3]. There are three genetically different isoforms of nitric oxide synthase (NOS) which account for NO production. They include neuronal nitric oxide synthase (also known as nNOS, Type I, NOS-I and NOS-1) being the isoform found in neuronal tissues, inducible nitric oxide synthase (also known as iNOS, Type II, NOS-II and NOS-2) being the isoform which can be synthesized following induction by pro-inflammatory cytokines or endotoxin and endothelial nitric oxide synthase (also known as eNOS, Type III, NOS-III and NOS-3) being the isoform expressed in endothelial cells [4]. nNOS and eNOS are constitutively expressed and their activities are calcium-dependent, whereas the activity of iNOS is fully activated at basal intracellular calcium concentration, so its activity is calcium-independent [5]. Of the three NOS isoforms, nNOS constitutes the predominant source of NO in neurons and localizes to synaptic spines. Additionally, nNOS is also present in skeletal muscle, cardiac muscle and smooth muscle [6], [7], [8], [9], where NO controls blood flow and muscle contractility [10], [11], [12].

This review is intended to display the advances about nNOS over the last several years. It will address the structure, subcellular localization, regulation and concludes with a section discussing the role of nNOS in human physiologies and pathologies.

Section snippets

Structure

The neuronal NOS consists of 1434 amino acids with a predicted molecular weight of 160.8 kDa [13]. Monomer of nNOS is inactive, dimer is its active form and the dimerization requires tetrahydrobiopterin (BH4), heme and l-arginine binding [14]. nNOS monomer exhibits a bidomain structure containing an oxygenase domain (N-terminal) and a reductase domain (C-terminal) which can be separated by a calmodulin binding motif. The oxygenase domain which binds the substrate l-arginine contains a

Subcellular localization

nNOS is expressed in both immature and mature neurons [19], [20]. Besides, nNOS has also been found in rat astrocytes, the adventitia of rat brain blood vessels, rat cardiac myocytes, etc [21], [22]. Because NO can not be stored in the cells, it depends on new synthesis to exert its functional properties. Thus, to some extent, nNOS must be bond to the plasma membrane directly or be anchored to the plasma membrane by adapter proteins. Fractionation studies have demonstrated that brain nNOS

nNOS dimerization

Active nNOS is in a dimeric form, with an extensive interface being formed between the two oxygenase domains creating high-affinity binding sites for BH4 and l-arginine [28], [29]. nNOS monomer has two cysteine residues which can form a disulphide bridge or ligate a zinc ion between the monomers covalently linking the two oxygenase domains. Moreover, there is an ‘N-terminal hook’ domain which can also stabilize the dimer. Additionally, interactions across the dimmer between the reductase

Involvement of nNOS in physiologies and pathologies

Although nNOS-derived NO is a critical molecule in mediating synaptic plasticity and neuronal signaling, it changes from a physiological neuromodulator to a neurotoxic factor when excessive amount of NO is produced. So nNOS may play an important role in a wide range of physiological and pathological conditions.

Conclusions and perspectives

Collectively, nNOS is implicated in a wide range of functions and pathologies with pleiotropic effects. In view of its ubiquitous expression in the CNS, there are extensive and unique chances for nNOS to interact with other neuronal elements, thus exerting appropriate functional properties. Given increased nNOS activity and expression in many diseases, inhibiting nNOS might have putative therapeutic effects. Unfortunately, inhibiting nNOS directly may disturb physiological functions and produce

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