2002/03/01 by Juan V Esplugues, Juan V. Esplugues · 31 citations
Medicine · Biochemistry, Genetics and Molecular Biology · Neuroscience · #Nitric Oxide and Endothelin Effects #Receptor Mechanisms and Signaling #Neuropeptides and Animal Physiology
paper · pdf · doi:10.1038/sj.bjp.0704569
The discovery that nitric oxide (NO) functions as a signalling molecule in the nervous system has radically changed the concept of neural communication. Indeed, the adoption of the term nitrergic for nerves whose transmitter function depends on the release of NO or for transmission mechanisms brought about by NO (Moncada et al., 1997) emphasizes the specific characteristics of this mediator. The physical properties of NO prevent its storage in lipid-lined vesicles and metabolism by hydrolytic degradatory enzymes. Therefore, unlike established neurotransmitters, NO is synthesized on demand and is neither stored in synaptic vesicles nor released by exocytosis, but simply diffuses from nerve terminals. The distance of this NO diffusion (40 – 300 μm in diameter) implies that structures in the vicinity of the producing cell, both neuronal and non-neuronal, are influenced following its release. This suggests that, as well as acting as a neurotransmitter, NO has a neuromodulatory role (Garthwaite & Boulton, 1995). In addition, it diffuses into rather than binds with protein receptors on adjacent cells, and most of its known actions are the consequence of interplay with intracellular targets that would usually be regarded as secondary messengers. The activity of conventional neurotransmitters is terminated either by re-uptake mechanisms or enzymatic degradation while inactivation of NO follows reaction with a substrate. There are multiple points at which biological control can be exerted over the production and activity of conventional neurotransmitters. However, control of the synthesis of NO is the key to regulating its activity. Endothelial NOS (eNOS) and inducible NOS (iNOS) are present in the nervous system and will be duly addressed here. However, neuronal NOS (nNOS) is the principal isoform present in said system and will be the main focus of this review. All nNOS positive neurones exhibit α-nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase activity, which has become the histochemical marker of nitrergic neurones. However, early results demonstrating this may have been limited by inappropriate fixation procedures and should be viewed with caution (Wolf, 1997). The original cloning of full-length nNOS produced what is now designated as nNOSα, and which accounts for the majority of nNOS activity in nervous tissue (Bredt et al., 1991). In addition, four splice variants have recently been identified (nNOSβ, nNOSγ, nNOSμ and nNOS-2) and these appear to exhibit distinct cellular and tissue locations (Gibson, 2001; Nakane et al., 1993; Silvagno et al., 1996; Alderton et al., 2001). In particular, there is growing evidence that nNOS biosynthesis in excitable tissues is not restricted to neurones while substantial amounts of this enzyme have been identified in skeletal muscle, where it is involved in the regulation of metabolism and muscle contractility (Stamler & Meissner, 2001). The magnitude of literature dealing with the role of NO in the nervous system is so great that it would be impossible to include in this review the entirety of the research carried out. For logistical reasons, only groundbreaking references have been quoted, but when necessary, recent reviews dealing with specific areas within the field have been included and are intended to act as a guideline for further reading. The most important regulator of nNOS activity seems to be free cytosolic Ca2+, which stimulates nNOS through interaction with calmodulin. 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 above the 400 nM required for calmodulin to bind to nNOS, thereby activating the enzyme. When the concentration of Ca2+ falls, it dissociates from the calmodulin, which in turn dissociates from the nNOS, thus acting as a switch that turns the enzyme on and off (Knowles et al., 1989; Sheng et al., 1992). Phosphorylation, although less well analysed, constitutes an additional mechanism for regulating nNOS activity. The catalytic activity of the enzyme is decreased following phosphorylation by cyclic adenosin monophosphate (cyclic AMP)-dependent protein kinase (Bredt et al., 1992; Brüne & Lapetina, 1991), protein kinase C (Bredt et al., 1992; Nakane et al., 1991) or Ca2+/calmodulin-dependent protein kinase II (Bredt et al., 1992; Hayashi et al., 1999; Komeima et al., 2000; Nakane et al., 1991; Schmidt et al., 1992b). This process occurs in the majority of peripheral and in some central nitrergic neurones (Figures 1 and 2). However, in the CNS, NO synthesis seems predominantly regulated by the influx of Ca2+ through receptor-dependent channels, in particular following postsynaptic stimulation of NMDA receptors by the excitatory neurotransmitter, glutamate (Bredt & Snyder, 1989; Garthwaite et al., 1989). The amino acid terminal of nNOS possesses a PDZ domain which is not present in the β and γ splice variants (Alderton et al., 2001; Brenman et al., 1996). The aforementioned domains are modular structures of approximately 100 amino acids, which occur in a number of proteins, anchoring them to cytoskeletal elements such as synaptic densities and related membrane-associated guanylate kinases (Tomita et al., 2001). In the case of nNOS, its PDZ domain interacts with the postsynaptic density protein PSD-95, whereas the N-Methyl-D-Aspartate (NMDA) receptor contains a Ser/Thr-X-Val motif (tSXV) that also binds with PSD95. By facilitating the proximity of NMDA receptors to the enzyme, the scaffolding protein, PSD95 directly exposes nNOS to the flux of Ca2+ entering the ion channel of activated NMDA receptors (Kornau et al., 1995; Tomita et al., 2001). Transient Ca2+ fluxes following the activation of other receptors would be too diluted to have a similar effect by the time they reach the vicinity of nNOS. It is possible that NO bioactivity feeds back to control the activity of the channel as S-nitrosylation of critical cysteines seems to down-regulate the NMDA receptor (Choi et al., 2000). There are many other potential regulators of the NMDA receptor/nNOS coupling and downstream signalling pathways. The protein carboxy-terminal PDZ ligand of nNOS (CAPON) is thought to be selectively associated with nNOS and to exhibit a similar regional distribution. CAPON competes with nNOS for PDZ domains, binding to the enzyme and forcing it to disassociate itself from the plasma membrane (Jaffrey et al., 1998). Therefore, CAPON determines the amount of nNOS tethered to the plasma membrane and, in this way, regulates NO formation in neurones of the CNS. Furthermore, CAPON anchors nNOS to other macromolecules, such as the small G-protein Dexras-1 (Fang et al., 2000), although the importance of this relationship remains to be determined. At this point, it must be said that recent proteomic analysis has not identified CAPON in the vicinity of the NMDA receptor, which raises some doubts about the above hypotheses (Husi et al., 2000). Activation of nNOS in the CNS. Release of glutamate activates NMDA receptors (NMDAr), and the consequent flux of Ca2+ entering the ion channel activates nNOS, which is linked to the receptor via the postsynaptic density protein PSD-95. It is possible that NO bioactivity feeds back to control the presynaptic neuron and the activity of the channel. The protein CAPON is thought to be selectively associated with nNOS and regulates NO formation in neurones. nNOS in mysenteric neurones is regulated by the flux of Ca2+ through voltage-dependent calcium channels (VDCC). NO relaxes the adjacent smooth muscle following activation of sGC. Various other receptors and domains contain the tSXV motif and are also potentially associated with central nNOS and regulated by this multifunctional protein – protein interaction (Tomita et al., 2001). nNOS may also be inhibited through an interaction with protein inhibitor of nNOS (PIN), a highly conserved small protein that was originally thought to destabilize nNOS dimers and thus act as an endogenous inhibitor of nNOS (Jaffrey & Snyder, 1996). However, recent reports suggest that PIN is an axonal transport protein for nNOS, rather than its regulator (Hemmens et al., 1998; Rodriguez-Crespo et al., 1998). nNOS may also be inhibited through an interaction with caveolin-1 and caveolin-3 that, in a way similar to the effect of caveolin-1 on eNOS, could displace calmodulin from nNOS (García-Cerdeña et al., 1997; Venema et al., 1997). Furthermore, these members of the caveolin family interact with other signalling molecules such as c-src, Ha-ras and GSα, which suggests a potential role for nNOS in some signalling complexes (Couet et al., 1997). Finally, a role for heat shock protein NOS-hsp90/heterocomplexes in the modulation of the haem's interaction with nNOS has recently been suggested (Bender et al., 1999). In skeletal muscle, nNOS activity is related to muscle ACh receptors and membrane depolarization (Figure 3). Again, nNOS is targeted to membrane structures due to the association of its PDZ domain with α1-syntropin, a dystrophin-associated protein that shares homology with postsynaptic density proteins PDS95 y PDS93. Interactions with PIN, highly expressed in skeletal muscle, and with caveolin-3 are also possible (Brenman et al., 1995; Chao et al., 1996; Stamler & Meissner, 2001; Venema et al., 1997). Activation of nNOS in the skeletal muscle follows the influx of Ca2+ through voltage-dependent calcium channels (VDCC) induced by activation of ACh receptors (AChr) and membrane depolarization. The release of Ca2+ from the sarcoplasmic reticulum (SR) is also implicated. nNOS targets the membrane due to its association with α1-syntropin, a component of the dystrophin complex (DC). Although considered to be constitutive, levels of nNOS activity and expression appear to be subject to dynamicup- or down-regulation induced by a large variety of stimuli, including nerve (Steel et al., 1994; Verge et al., 1992) and brain injury (Kitchener et al., 1993; Regidor et al., 1993), aging (Carrier et al., 1997; Mollace et al., 1995), pharmacological treatment (Bagetta et al., 1993), lactation (Ceccatelli & Eriksson, 1993), hypoxia (Guo et al., 1997), stress (Cazal et al., 1993), gonadectomy (Ceccatelli et al., 1993), light exposure (Schaad et al., 1994) and exercise (Tidball et al., 1998). The existence of presynaptic automodulation in nitrergic neurones has also been proposed. Although not yet fully characterized, this action may result from the combination of NO with the haem group of NOS, which inhibits the enzyme (Klatt et al., 1992; Rogers & Ignarro, 1992). Finally, there is evidence that cells maintain low levels of cyclic guanosin monophosphate (cyclic GMP) while producing NO. This occurs because increases in Ca2+ levels, similar to those needed to stimulate nNOS, also activate a Ca2+/calmodulin-dependent cyclic GMP phosphodiesterase that facilitates the degradation of cyclic GMP (Mayer et al., 1992). The actions of NO are a consequence of its influence on a variety of protein functions which it exerts through its reaction with cysteine thiol, S-nitrosylation, and transition metal centres (Drapier & Bouton, 1996; Jaffrey et al., 2001; Lane et al., 2001). The enzyme soluble guanylyl cyclase (sGC) has long been considered to be the major physiological target for neuronal NO, and there is ample evidence that increases in cyclic GMP levels mediate a large number of the physiological actions of NO. Thus, immunohistochemical techniques have found that the distribution of sGC and cyclic GMP is complementary to that of nNOS (Schmidt et al., 1992a; Southam & Garthwaite, 1993; Young et al., 1993). Functionally, either nitrergic nerve stimulation or administration of NO-donors increases intracellular cyclic GMP concentrations (Bredt & Snyder, 1989; Torphy et al., 1986). In both cases, these responses are mimicked by analogues of cyclic GMP (Gibson & Mirzazadeh, 1989), whereas inhibition of the destruction of this intracellular mediator potentiates the results of nitrergic stimulation (Barbier & Lefebvre, 1995; Bayguinov & Sanders, 1993). The mechanisms linking the rise in cyclic GMP content to the various effects of NO in the CNS and peripheral smooth muscle are not fully understood, although in both cases the final step seems to be a reduction of ([Ca2+]i). Alternative targets of cyclic GMP may involve direct channel gating with the opening of inward Ca2+ and Na2+ channels, activation of cyclic GMP-dependent kinases, actions related to cyclic adenosin diphosphate (ADP) ribose, and interactions with cyclic AMP resulting from regulation of cyclic GMP-dependent phosphodiesterases (Hunter, 2000; Jaffrey et al., 2001; Lincoln et al., 2001). NO modulates oxygen consumption in the mitochondria. In particular, nanomolar concentrations of NO inhibit cytochrome oxidase, the terminal haem-containing enzyme in the mitochondrial respiratory chain. Recent evidence demonstrates that this effect is reversible and competitive with oxygen, and suggests that NO is a crucial regulator in the generation of energy and the mediation of cell death by mitochondria (Beltrán et al., 2000). The consequences of such an activity are still to be evaluated but, obvious physiological implications aside, they could clarify the mechanisms by which NO is involved in cell or tissue damage. Furthermore, and in conjunction with differences in glycolytic capacity, this activity may explain why neurones and glia show variations in sensitivity to NO-induced damage (Brown, 2000; Almeida et al., 2001). NO has been linked to the release (Meffert et al., 1996) of other neurotransmitters and the effects which they produce, in particular acetylcholine (Gustafsson et al., 1990; Li & Rand, 1989b), noradrenaline (Boeckxstaens et al., 1993; Li & Rand, 1989a) dopamine (Hanbauer et al., 1992), glutamate (Montague et al., 1994; Sorkin, 1993), γ-aminobutyric acid (GABA) (Beltran et al., 1999; Kuriyama & Ohkuma, 1995), serotonin (Bogers et al., 1991; Reiser, 1990b), adenosin triphosphate (ATP) (Boeckxstaens et al., 1991a), bombesin (Beltran et al., 1999), carbon monoxide (Xue et al., 2000), opioids (Barnette et al., 1990) and endothelin (Reiser, 1990a). The mechanisms responsible for these interactions are still not fully understood, but direct S-nitrosylation of receptors, activation of cyclic GMP-dependent protein phosphorylation cascades, regulation of neuronal energy dynamics and a modulating effect on transporters are potentially involved (Choi et al., 2000; Kiss & Vizi, 2001; Pieper et al., 2000). In addition, a presynaptic modulation of NO release through the activation of α2-adrenoceptors, nicotinic receptors, purinergic receptors etc. has also been proposed (Boeckxstaens et al., 1993). Finally, it has been suggested that NO modulates gene transcription and translation in neurones and glia (Hess et al., 1993; Peunova & Enikolopov, 1993; 1995). However, these effects would seem to be indirect since there is little evidence of the existence of DNA elements within the promotor regions of eukaryotic cells that respond directly to NO (Morris, 1995). NO was first characterized in the CNS as the intercellular messenger mediating the increase in cyclic GMP levels that follows activation of glutamate receptors (Garthwaite et al., 1988). The majority of the information available deals with nNOS, of which the brain contains the highest activity found in any tissue, and which, although present in some cerebral vessels and in glial cells, is predominantly found in neurones (Bredt et al., 1990; Salter et al., 1991). nNOS-containing neurones are present in many areas of the CNS (Figure 4), with the highest densities occurring in the accessory olfactory bulb and granule cells of the cerebellum. Although nNOS neurones represent only roughly 1% of cell bodies in the cerebral cortex, virtually every neurone in the cortex is exposed to nNOS nerve terminals. From a morphological point of view nNOS neurones display a great heterogeneity in their localization within the CNS, constituting a small population of varying interneurones. Furthermore, the number and chemical characteristics of nNOS neurones vary considerably depending on the area of the brain while the enzyme itself does not co-localize with any single neurotransmitter (Braissant et al., 1999; Iwase et al., 1998; Vincent, 1995; Wolf, 1997). nNOS can be located either pre- or post-synaptically and is particularly implicated in neural signalling, neurotoxicity, synaptic plasticity and modulation of behavioural pathways such as learning or expression of pain. Representative microphotograph of basal nNOS immunoreactivity (monoclonal antibody) in the dorsal vagal complex (DVC) of the brainstem. Scale bar=100 μm. eNOS is mainly involved in the regulation of vascular function and, although also present in some populations of neurones (Dinerman et al., 1994) and glia (Wiencken & Casagrande, 1999), is predominantly located in the endothelial cells of cerebral vessels. Finally, induction of iNOS in glial cells is implicated in the unspecific immune response of the brain and is usually associated with pathological conditions (Murphy, 2000). NO has been proposed as the retrograde messenger which co-ordinates the enhancement of both pre- and post-synaptic mechanisms involved in two forms of synaptic plasticity; namely long-term potentiation (LTP) and long-term depression (LTD). LTP is a property of many central excitatory synapses characterized by a prolonged enhancement of synaptic transmission, or an activity-dependent increase in synaptic strength, lasting from hours to weeks or even longer. The process by which LTP is induced is not completely clear, but it involves glutamate acting on amino-3-hydroxy-5-methylisooxazole-4-propionic acid (AMPA) or NMDA-receptors. This activates a series of events in which Ca2+/calmodulin-dependent protein kinase II, NOS and protein tyrosine kinases are implicated. LTP is thought to be a synaptic correlate of learning and memory, and is most pronounced in higher brain centres involved in cognitive functions, particularly in the cerebral cortex and hippocampus. The basic evidence for said involvement in LTP stems from in vitro studies in which inhibition of NOS prevented the development of LTP (Bohme et al., 1991; O'dell et al., 1991; Schuman & Madison, 1991). Gene targeting suggests that both nNOS and neuronally located eNOS are implicated in LTP. Thus, while LTP is only slightly reduced in nNOS or eNOS null mice (O'dell et al., 1994), animals deficient in both NOS isozymes exhibit a substantially decreased LTP (Son et al., 1996). Guanylate cyclase seems to be the main effector of NO in the induction of LTP (Bohme et al., 1991; Haley et al., 1992), however, ADP-ribosylation (Brüne & Lapetina, 1989) and activation of calmodulin-dependent kinases (Soderling, 2000; Tomita et al., 2001) have also been implicated. LTD is characterized by a long lasting depression of parallel fibre synapses, which follows repeated excitation of the climbing of The reduction in synaptic to result from a of postsynaptic receptors which is by activation of protein kinases C and and of the GMP signalling et al., 1993; & 1991). LTD can be in higher regions of the although it has been particularly well in the where it has been proposed as a for the learning of Finally, the role of NO in both forms of synaptic plasticity involves interaction with and related membrane-associated guanylate kinases, by the that both LTP and LTD are in mice with targeted of et al., 1998). NO has complex on brain formation and through regulation of synaptic of NO synthesis & 1992), learning and olfactory (Bohme et al., 1993; et al., 1997), behavioural and activity in et al., 1995). NO has also been implicated in neuronal targeting and brain development 2000; & 2000; et al., & 1999), learning et al., and et al., 1992; & 1992), et al., 1995), and 1999; & et al., 1995), et al., 1994) and respiratory generation et al., 1992). behavioural responses by and pathways are thought to involve NO generation or stimulation of central nitrergic neurones et al., 1994; 1995). The of NO in behavioural mechanisms seems in nNOS animals show behavioural when but exhibit and In nNOS in mice et al., 1994; et al., 1995). There is an of peripheral functions in which a role for central NO has been although the physiological of these further to an there is evidence central NO in the regulation of et al., 1992), et al., 1992), nerve activity et al., 1992), acid and et al., 1996; et al., 2000; et al., 2001) and associated with et al., 1995). NO in the CNS to be involved in to a to the and the modulation of various including the production of and 1994; et al., 1993; et al., 1993; & NO has been implicated at various levels of the neural both neurones and dorsal and and structures of the 1999). Functionally, most involve the interaction of NO and NMDA receptors, and it is established that synthesis of NO of the to the cortex and in behavioural responses (Mayer et al., 1999). However, the role of NO to the of NO has effects when stems from peripheral nerve and in of or et al., 1992; & 1993; et al., 1991; 1993), while administration of by a to a et al., 1995). In of NO synthesis in of et al., 1993). The of nNOS mice has not such for these animals display a to some of damage by NOS et al., 1995). The of NO in the effects of is also For of the cyclase system the actions of while the effects of & 1995; & 2001). Furthermore, there is evidence which suggests that the splice variants of nNOS, modulates but not et al., 1997). The neuronal damage that cerebral involves an release of glutamate and a activation of NMDA receptors that, for a of a influx of Ca2+ into the postsynaptic neurone which, in the activation of nNOS and of NO. In NO produced by activation of eNOS et al., 1996; et al., 1997), and even NMDA receptors & 1997; & 1997), a role in brain by regional cerebral The first that NO could mediate effects with the discovery that inhibition of NOS glutamate in neuronal from the cerebral cortex et al., 1991) and in of et al., 1991). studies by which that inhibition of NO synthesis NMDA both in et al., 1992; et al., and in vitro et al., 1993), and which concentrations of NO in various et al., 1993; et al., 1993). due to the of in vitro and in reports in which was not & 1999; et al., 1997). However, such from variations in conditions and of NOS Thus, inhibition of nNOS with concentrations of NOS that not eNOS activity whereas the of nNOS is in of et al., 1994; et al., 1996). of injury seems to occur through inhibition of eNOS with of NOS which results in of cerebral and a increase in of the role of nNOS and eNOS in neuronal damage has been nNOS mice substantially less brain damage following than those of the et al., 1994), while neuronal from such animals are to damage by glutamate and hypoxia et al., 1996). The reduction of in nNOS null mice does not occur when NOS are at concentrations that inhibit of vessels. the other eNOS mice exhibit damage following an effect associated with a reduction of in the and administration of NOS to these animals injury et al., 1996). The interactions and signalling mechanisms involved in these effects are is linked to cyclic mechanisms et al., 1998). In addition, of by NO has been implicated in the neuronal while NO is thought to oxygen and induced damage et al., 1993). NO could be directly by with a specific of the resulting in a decreased binding of glutamate or a of Ca2+ through the channel activation (Choi et al., 2000; et al., 1992). of seems to be the mediator in damage et al., 1998; et al., 1993; et al., 1997). The production of has been in et al., 1998; et al., 1997). with of and nNOS exhibit decreased following vascular et al., 1994), by a production et al., 1998). to DNA by NO and to be an important This is due to the activation of the enzyme which is of energy resulting in cellular death the DNA damage is et al., 1999; et al., 1993; et al., of nNOS while inhibition of with a of exhibit to cerebral than nNOS mice or those with NOS or NMDA et al., 1997). In addition, inhibition of mitochondrial respiratory the of neuronal energy (Beltrán et al., 2000; et al., 1994; 1995; et al., 1998; & concentrations of NO may also cellular by including C et al., and & Snyder, 1992), or by with the present in haem or complexes associated with such as cytochrome or (Drapier & Bouton, 1996). conditions and the expression of iNOS in various populations of cerebral However, iNOS expression occurs than that of nNOS and eNOS, and the cellular of this expression is on the of injury et al., iNOS