nNOS Signaling at Neuronal Synapses
Explore and order pathway-specific siRNAs, real-time PCR assays, and expression vectors. View pathway information and literature references for your pathway.
  • Click on your proteins of interest in the pathway image or review below
  • Select your genes of interest and click "add selection"
  • When you have finished your gene selection, click "Find Products" to find assays, arrays, or create custom products
Download Image Terms of Use Download PPT
Pathway Navigator
nNOS Signaling at Neuronal Synapses
NO (Nitric Oxide) is formed endogenously by a family of enzymes known as NOS (NO Synthases). The distribution of different isoforms of NOS is largely related to their respective functions. Three distinct isoforms of NOS have been identified: nNOS (also known as NOSI and NOS-1) being the isoform first found (and predominating) in neuronal tissue, iNOS (also known as NOSII and NOS-2) which is inducible in a wide range of cells and tissues and eNOS (also known as NOSIII and NOS-3) isoform first found in vascular endothelial cells. Glutamate, the major excitatory neurotransmitter in the brain, is the most effective activator of NO biosynthesis in most brain regions (Ref.1).

NO is generated via a five-electron oxidation of a terminal guanidine nitrogen on L-Arginine. The reaction is both oxygen and NADPH-dependent and yields L-Citrulline in addition to NO, in a 1:1 stoichiometry. NO functions as an endogenous signaling molecule in numerous organs and tissues throughout the animal and plant kingdoms. The most important regulator of nNOS activity is free cytosolic Ca2+, which stimulates nNOS through interaction with Calm (Calmodulin) and Caln (Calcineurin). Arrival of action potentials activates Voltage-dependent Ca2+ Channels situated in the neurolemma, and stimulates the release of Ca2+ from intracellular stores. This elevates cytosolic Ca2+ concentrations required for Calm binding to nNOS, thereby activating the enzyme. When the concentration of Ca2+ falls, it dissociates from the Calm, which in turn dissociates from the nNOS, thus acting as a switch that turns the enzyme on and off (Ref.6). Phosphorylation, although less well analyzed, constitutes an additional mechanism for regulating nNOS activity. The catalytic activity of the enzyme is decreased following phosphorylation by cAMP (cyclic Adenosine Monophosphate)-dependent PKC (Protein Kinase-C) or Ca2+/Calm-dependent Protein Kinase-II (Ref.2). This process occurs in the majority of peripheral and in some central nitrergic neurons. However, in the CNS (Central Nervous System), NO synthesis is predominantly regulated by the influx of Ca2+ through receptor-dependent channels, in particular following postsynaptic stimulation of NMDAR (N-Methyl-D-Aspartate type-Glutamate Receptor) by Glutamate. Recent studies show that NO actions in brain and muscle also rely crucially upon the association of nNOS with specific protein complexes in neurons and muscle cells, respectively. These physical interactions with nNOS allow for integration of NO signaling into distinct transduction cascades in specific cell types. In the brain, the 160kDa nNOS-Alpha is the predominant splice variant, and contains an N-terminal PSD/Discs-large/ZO-1 homologous (PDZ)-binding domain, which anchors this complex to the postsynaptic density in the vicinity of the NMDAR. The PDZ domain of nNOS binds to a similar PDZ domain from the postsynaptic density protein, PSD95, which in turn binds to the cytosolic tail of the NMDAR. These molecular interactions explain how Ca2+ influx through NMDARs is efficiently coupled to NO synthesis and activity. Following its synthesis at postsynaptic sites NO diffuses back to the presynaptic terminal and increase cGMP levels through activation of soluble GC (Guanylate Cyclase) (Ref.3).This membrane-localized nNOS complex is further linked to cytoplasmic signal transduction pathways via the physical interaction of nNOS with DexRas1 and the adapter protein CAPON (C-Terminal PDZ Domain Ligand of Neuronal Nitric Oxide Synthase), which activates a downstream MAPK (Mitogen-Activated Protein Kinase) cascade and modulate nuclear transcription. CAPON competes with nNOS for PDZ domains, binding to the enzyme and forcing it to disassociate itself from the plasma membrane. Therefore, CAPON determines the amount of nNOS tethered to the plasma membrane and, in this way, regulates NO formation in neurons of the CNS. Furthermore, CAPON anchors nNOS to other macromolecules, such as DexRas1. Functionally, nNOS also represents a central component that regulates synaptic transmission and intercellular signaling, through negative regulation of the NMDAR by S-nitrosylation and NO-dependent activation of DexRas1. Additionally, the half-life of neuronal nNOS-Alpha protein is regulated by the Ca2+ sensitive protease Calpain (Ref.4). nNOS may also be inhibited through an interaction with PIN (Protein Inhibitor of nNOS), a highly conserved small protein that was originally thought to destabilize nNOS dimers and thus act as an endogenous inhibitor of nNOS. However, recent reports suggest that PIN is an axonal transport protein for nNOS, rather than its regulator. nNOS may also be inhibited through an interaction with Caveolin-1 and Caveolin-3 that can displace Calm from nNOS (Ref.1).

NO is a unique messenger molecule involved in the regulation of diverse physiological processes including smooth muscle contractility, platelet reactivity, central and peripheral neurotransmission, and the cytotoxic actions of immune cells. At the cellular level, NO signaling is essential for two forms of neuronal plasticity: LTP (Long-Term Potentiation) in the hippocampus and long-term depression in the cerebellum. Neuron-derived NO also plays a major role in regulation of blood flow. Functions for NO in brain remain less certain, but most probably it is associated with an increase in local blood flow, and this response is prevented by NOS inhibitors. Particularly high levels of nNOS occur in vasodilator nerves that innervate the large cerebral blood vessels. Neuron-derived NO mediates penile erection through regulation of blood flow. NO plays both detrimental and protective roles in focal ischaemia. Neuronally produced NO exacerbates the damage associated with the early stages of stroke (Ref.5). Large amounts of NO produced during periods of cerebral ischemia mediate neuronal injury in various forms of stroke. Similar NO-mediated damage may account for neurodegeneration in other conditions as well, including Parkinsons Disease, Amyotrophic Lateral Sclerosis, and Huntingtons Disease. NO signaling is also perturbed in various muscle diseases, particularly in Duchenne Muscular Dystrophy, and these derangements may contribute to the disease processes. Therefore, pharmacological regulation of NO synthesis offers an important strategy for treatment of neurodegenerative and muscle diseases.