Nitric oxide signaling via the second messenger molecule cGMP is essential for normal physiological brain function. NO itself is produced by the NO synthase (NOS) family of enzymes, and NO bioactivity is also exerted by metabolites of NO and by cGMP-independent pathways. In many brain regions, activation of NOS and NO/cGMP signaling is a consequence of activation of glutamatergic excitatory amino acid receptors and cholinergic muscarinic receptor subtypes. The NO/sGC/cGMP signal transduction system is considered to be important for modulating synaptic transmission and plasticity in brain regions such as the hippocampus and cerebral cortex, which are critical for learning and memory. We have long argued that NO plays a decisive role in signal transduction cascades that are compromised in AD, and therefore that drugs delivering NO bioactivity represent targets for AD therapy. Acceptance of this argument has been impeded by a popular view that NO is neurotoxic and causative in diseases such as AD.

NO, before realization of its essential role in human physiology, was...  Read more

Nitric oxide signaling via the second messenger molecule cGMP is essential for normal physiological brain function. NO itself is produced by the NO synthase (NOS) family of enzymes, and NO bioactivity is also exerted by metabolites of NO and by cGMP-independent pathways. In many brain regions, activation of NOS and NO/cGMP signaling is a consequence of activation of glutamatergic excitatory amino acid receptors and cholinergic muscarinic receptor subtypes. The NO/sGC/cGMP signal transduction system is considered to be important for modulating synaptic transmission and plasticity in brain regions such as the hippocampus and cerebral cortex, which are critical for learning and memory. We have long argued that NO plays a decisive role in signal transduction cascades that are compromised in AD, and therefore that drugs delivering NO bioactivity represent targets for AD therapy. Acceptance of this argument has been impeded by a popular view that NO is neurotoxic and causative in diseases such as AD.

NO, before realization of its essential role in human physiology, was best known as a toxic atmospheric pollutant, and it is easy to demonstrate NO toxicity toward brain cells in vitro. Excitotoxic neurodegeneration as occurs in ischemic stroke has been linked with increased NO levels. It has been proposed that disease states where chronic inflammation is a causative factor would benefit from the use of inhibitors of iNOS, which is induced under such conditions and can produce high levels of cellular NO. Some researchers have espoused a simplistic paradigm of “bad” iNOS and “good” nNOS and eNOS. NO has been accepted as a causative factor in brain diseases despite many reports demonstrating anti-neurodegenerative properties and even neuroprotection in response to insults such as amyloid-β (Aβ) neurotoxicity. There is also 130 years of drug therapy with nitrates, drug sources of NO bioactivity, to support the safety of NO and NO-based medications.

Recently, 1) nitrates have been reported to reverse cognition deficits induced by cholinergic neurodegeneration and reduce amyloid load in transgenic AD mouse models, and 2) direct links have been shown from amyloid protein to neuronal dysfunction mediated by damage to NO signaling networks.

The present work of Colton and coworkers serves to emphasize the fallacy in the presumption that NO and/or iNOS causes neurodegeneration. The work draws further links between loss of brain NO bioactivity and both buildup of amyloid and hyperphosphorylated tau deposits, the key pathological brain markers of AD. The Colton transgenic mouse model superimposes an iNOS knockout on a standard AD transgenic model leading to a pathophysiology that mimics aspects of human AD:

• formation of hyperphosphorylated tau in hippocampal neurons
• intracellular tangle-like aggregates
• increased amyloid load and plaques
• widespread cortical neuronal damage

Interpretation of data from NOS knockout transgenics is especially difficult because knockout of one isozyme leads to compensatory changes in the remaining two isoforms. In this transgenic, the researchers took care to note that eNOS protein levels increased and nNOS levels fell, which may have also contributed to the observed pathology. The observation of apoptotic cell death and raised caspase activity may provide a pathway for formation of neurofibrillary tangles of hyperphosphorylated tau initiated by loss of NO bioactivity. There remain many questions to answer, in particular the mechanism of loss of iNOS activity in the early stages of AD, but the work provides another strong supporting plank for the argument that drugs that reinforce NO bioactivity represent a valid and urgent approach to AD.

View all comments by Greg Thatcher

Colton and colleagues reported that crossing the APPsw transgenic line with the NOS2-/- mouse led to the novel Tg2576/NOS2-/- bigenic mouse which recapitulates the key pathological features of Alzheimer disease (AD), that is, β amyloid deposition, accumulation of hyperphosphorylated tau, and neuronal loss.

The significance of these results is twofold: they indicate that nitric oxide (NO) plays a role in the development of AD pathological hallmarks and they highlight the neuroprotective properties of NO. This view is supported by other findings obtained using NO-releasing derivatives of anti-inflammatory and antioxidant compounds (reviewed in Gasparini et al., 2004; 2005). In particular, HCT 1026 and NCX 2216, two NO-releasing derivatives of the nonsteroidal anti-inflammatory drug flurbiprofen, have been investigated in neuroinflammation and AD transgenic models. Besides showing improved anti-inflammatory activity (Prosperi et al., 2001; 2004), these compounds have additional properties of potential benefit for AD. For example, it has been shown that chronic administration of...  Read more

Colton and colleagues reported that crossing the APPsw transgenic line with the NOS2-/- mouse led to the novel Tg2576/NOS2-/- bigenic mouse which recapitulates the key pathological features of Alzheimer disease (AD), that is, β amyloid deposition, accumulation of hyperphosphorylated tau, and neuronal loss.

The significance of these results is twofold: they indicate that nitric oxide (NO) plays a role in the development of AD pathological hallmarks and they highlight the neuroprotective properties of NO. This view is supported by other findings obtained using NO-releasing derivatives of anti-inflammatory and antioxidant compounds (reviewed in Gasparini et al., 2004; 2005). In particular, HCT 1026 and NCX 2216, two NO-releasing derivatives of the nonsteroidal anti-inflammatory drug flurbiprofen, have been investigated in neuroinflammation and AD transgenic models. Besides showing improved anti-inflammatory activity (Prosperi et al., 2001; 2004), these compounds have additional properties of potential benefit for AD. For example, it has been shown that chronic administration of both HCT 1026 and NCX 2216 reduce β amyloid load in APPsw/presenilin 1 mutant double transgenic mice (Jantzen 2002; Van Groen and Kadish, 2005). Besides this, they also have peculiar activities that are mediated by NO. Specifically, these compounds act as PPARγ agonists on cultured rat microglia (Bernardo et al., 2005; 2006), inhibit Nf-kB activation (Fratelli et al., 2003), attenuate the loss of cholinergic cells following LPS-induced neuroinflammation, and reduce LPS-induced caspase-3, -8 and -9 activity in rat brain (Wenk et al., 2000).

It is worth noting that the kinetics of NO release and its concentration at the tissue level are critical to achieve such effects. The above-mentioned compounds release small amounts of NO with slow kinetics. Fast NO donors do not share the same effects (Gasparini, unpublished observations), indicating that the amount of NO and the timing are important to determine the neuroprotective effects of NO.

Altogether, these results show that approaches focusing on NO bioavailability and biogenesis could represent a valuable therapeutic strategy for AD treatment.

References:
Bernardo A, Ajmone-Cat MA, Gasparini L, Ongini E, Minghetti L. Nuclear receptor peroxisome proliferator-activated receptor-γ is activated in rat microglial cells by the anti-inflammatory drug HCT1026, a derivative of flurbiprofen. J Neurochem. 2005 Feb;92(4):895-903. Abstract

Bernardo A, Gasparini L, Ongini E, Minghetti L. Dynamic regulation of microglial functions by the non-steroidal anti-inflammatory drug NCX 2216: implications for chronic treatments of neurodegenerative diseases. Neurobiol Dis. 2006 Apr;22(1):25-32. Epub 2005 Nov 22. Abstract

Fratelli M, Minto M, Crespi A, Erba E, Vandenabeele P, Del Soldato P, Ghezzi P. Inhibition of nuclear factor-kappaB by a nitro-derivative of flurbiprofen: a possible mechanism for antiinflammatory and antiproliferative effect. Antioxid Redox Signal. 2003 Apr;5(2):229-35. Abstract

Gasparini L, Ongini E, Wenk G. Non-steroidal anti-inflammatory drugs (NSAIDs) in Alzheimer's disease: old and new mechanisms of action. J Neurochem. 2004 Nov;91(3):521-36. Review. Abstract

Gasparini L, Ongini E, Wilcock D, Morgan D. Activity of flurbiprofen and chemically related anti-inflammatory drugs in models of Alzheimer's disease. Brain Res Brain Res Rev. 2005 Apr;48(2):400-8. Epub 2005 Jan 28. Review. Abstract

Jantzen PT, Connor KE, DiCarlo G, Wenk GL, Wallace JL, Rojiani AM, Coppola D, Morgan D, Gordon MN. Microglial activation and β -amyloid deposit reduction caused by a nitric oxide-releasing nonsteroidal anti-inflammatory drug in amyloid precursor protein plus presenilin-1 transgenic mice. J Neurosci. 2002 Mar 15;22(6):2246-54. Abstract

Prosperi C, Scali C, Barba M, Bellucci A, Giovannini MG, Pepeu G, Casamenti F. Comparison between flurbiprofen and its nitric oxide-releasing derivatives HCT-1026 and NCX-2216 on Aβ(1-42)-induced brain inflammation and neuronal damage in the rat. Int J Immunopathol Pharmacol. 2004 Sep-Dec;17(3):317-30. Abstract

Prosperi C, Scali C, Pepeu G, Casamenti F. NO-flurbiprofen attenuates excitotoxin-induced brain inflammation, and releases nitric oxide in the brain. Jpn J Pharmacol. 2001 Jun;86(2):230-5. Abstract

van Groen T, Kadish I. Transgenic AD model mice, effects of potential anti-AD treatments on inflammation and pathology. Brain Res Brain Res Rev. 2005 Apr;48(2):370-8. Abstract

Wenk GL, McGann K, Mencarelli A, Hauss-Wegrzyniak B, Del Soldato P, Fiorucci S. Mechanisms to prevent the toxicity of chronic neuroinflammation on forebrain cholinergic neurons. Eur J Pharmacol. 2000 Aug 18;402(1-2):77-85. Abstract

View all comments by Laura Gasparini

A practical problem with transgenic animals is that most focus on single aspects of the biology of AD such as amyloid. This paper broadens the scope of inquiry to a second variable, in this case, nitric oxide. It is of interest that aluminum, which epidemiology identifies as a risk factor in AD, has significant effects on NOS levels. Bondy et al. (1998) found that aluminum treatment induces NOS in the rat brain over 3 days and 3 weeks. Rodella et al. (2006) reported that aluminum exposure impaired the glutamate-nitric oxide-cGNP pathway and reduced numbers of nitroxidergic neurons in the rat somatosensory cortex with the largest effects seen after 3 months. Aluminum appeared to downregulate NOS in the 1-3-month period, and then decreased the NPY system at 6 and 12 months that is colocalized with NOS in cortical neurons. Kim (2003) found a decrease in nNOS immunoreactive neurons in rat pups after perinatal exposure.

References:
Bondy SC, Liu D, Guo-Ross S. Aluminum treatment induces nitric oxide synthase in the rat brain. Neurochem Int. 1998 Jul;33(1):51-4. Abstract

Rodella LF, Ricci F, Borsani E, Rezzani R, Stacchiotti A, Mariani C, Bianchi R. Exposure to aluminium changes the NADPH-diaphorase/NPY pattern in the rat cerebral cortex. Arch Histol Cytol. 2006 Mar;69(1):13-21. Abstract

Kim K. Perinatal exposure to aluminum alters neuronal nitric oxide synthase expression in the frontal cortex of rat offspring. Brain Res Bull. 2003 Aug 30;61(4):437-41. Abstract

View all comments by Erik Jansson

I found this study to be most interesting. Of particular interest from other studies is the suggestion that ionic mercury may play a role in Alzheimer's disease. Recent studies have shown that mercury in submicromolar concentrations inhibits glutamate transporters, triggering excitotoxicity.
The finding that NO protects neurons from tau pathology is of interest. While NO is produced during excitotoxicity, the mechanism of toxiciity is assumed to be the production of peroxynitrite in the face of elevated superoxide production. By suppressing mitochondrial energy production, excitotoxicity is greatly increased. This study provides more evidence for mercury's involvement in this process, since it has been shown that mercury reduces NOS activity. This would block NO's protective effects, without blocking excitotoxicity.

View all comments by Russell Blaylock

Antioxidant Defenses—Nitric Oxide, Amyloid-β and Tau Phosphorylation: A Zero Sum Game
This paper is exceptionally well executed and may lead to a paradigm shift. Dogma indicates that nitric oxide, amyloid-β, and tau phosphorylation are all bad. By deleting nitric oxide synthase 2 (NOS2) in APP transgenic lines, the authors find increased amyloid-β and tau phosphorylation. In our opinion, one can reconcile these data only by viewing nitric oxide, amyloid-β, and tau phosphorylation as protective antioxidants (Smith et al., 2002; Lee et al., 2005; Castellani et al., 2006; Lee et al., 2006). Mutations in APP cause oxidative stress in vitro (Marques et al., 2003), in animal models (Pappolla et al., 1998; Smith et al., 1998), and in humans (Nunomura et al., 2004). Such oxidative stress leads to cellular adaptations, including increases in amyloid-β, tau phosphorylation, and nitric oxide. In the Colton study, by deleting NOS2, remaining cellular adaptations (amyloid-β and tau phosphorylation) are increased, though in this case not sufficiently to prevent neurodegeneration.

References:
Castellani RJ, Lee HG, Perry G, Smith MA (2006) Antioxidant protection and neurodegenerative disease: the role of amyloid-beta and tau. Am J Alzheimers Dis Other Demen 21, 126-130. Abstract

Lee HG, Perry G, Moreira PI, Garrett MR, Liu Q, Zhu X, Takeda A, Nunomura A, Smith MA (2005) Tau phosphorylation in Alzheimer's disease: pathogen or protector? Trends Mol Med 11, 164-169. Abstract

Lee HG, Zhu X, Nunomura A, Perry G, Smith MA (2006) Amyloid beta: the alternate hypothesis. Curr Alzheimer Res 3, 75-80. Abstract

Marques CA, Keil U, Bonert A, Steiner B, Haass C, Muller WE, Eckert A (2003) Neurotoxic mechanisms caused by the Alzheimer's disease-linked Swedish amyloid precursor protein mutation: oxidative stress, caspases, and the JNK pathway. J Biol Chem 278, 28294-28302. Abstract

Nunomura A, Chiba S, Lippa CF, Cras P, Kalaria RN, Takeda A, Honda K, Smith MA, Perry G (2004) Neuronal RNA oxidation is a prominent feature of familial Alzheimer's disease. Neurobiol Dis 17, 108-113. Abstract

Pappolla MA, Chyan YJ, Omar RA, Hsiao K, Perry G, Smith MA, Bozner P (1998) Evidence of oxidative stress and in vivo neurotoxicity of beta-amyloid in a transgenic mouse model of Alzheimer's disease: a chronic oxidative paradigm for testing antioxidant therapies in vivo. Am J Pathol 152, 871-877. Abstract

Smith MA, Casadesus G, Joseph JA, Perry G (2002) Amyloid-beta and tau serve antioxidant functions in the aging and Alzheimer brain. Free Radic Biol Med 33, 1194-1199. Abstract

Smith MA, Hirai K, Hsiao K, Pappolla MA, Harris PL, Siedlak SL, Tabaton M, Perry G (1998) Amyloid-beta deposition in Alzheimer transgenic mice is associated with oxidative stress. J Neurochem 70, 2212-2215. Abstract

View all comments by Hyoung-gon Lee
View all comments by George Perry
View all comments by Mark A. Smith
View all comments by Xiongwei Zhu

Reply to Lee, Perry, Smith, and Zhu
My colleagues and I are delighted to see the enthusiastic interest in our mouse model for Alzheimer disease. The APPsw NOS2-/- mouse represents a novel approach to understanding the relationship between amyloid and tau pathology. The concept that NO serves as an antioxidant and promotes cell survival has been well developed in a number of physiological systems, such as the cardiovascular system. Thus, it is timely and appropriate that the role of NO as a neuroprotective agent, rather than as an “unrelenting killer” be more thoroughly explored in chronic neurodegenerative diseases. In many ways, the physiological adaptations to a long-lasting disease state are central to this study. As stated by Lee, Perry, Smith, and Zhu, multiple mechanisms including the formation of Aβ may serve to maintain a normal brain redox balance during chronic disease. Through its ability to bind reactive copper, Aβ peptide can serve as a Fenton-type oxidant in the presence of ascorbate (Dikalov et al., 2004) or as an “antioxidant” through modification of...  Read more

Reply to Lee, Perry, Smith, and Zhu
My colleagues and I are delighted to see the enthusiastic interest in our mouse model for Alzheimer disease. The APPsw NOS2-/- mouse represents a novel approach to understanding the relationship between amyloid and tau pathology. The concept that NO serves as an antioxidant and promotes cell survival has been well developed in a number of physiological systems, such as the cardiovascular system. Thus, it is timely and appropriate that the role of NO as a neuroprotective agent, rather than as an “unrelenting killer” be more thoroughly explored in chronic neurodegenerative diseases. In many ways, the physiological adaptations to a long-lasting disease state are central to this study. As stated by Lee, Perry, Smith, and Zhu, multiple mechanisms including the formation of Aβ may serve to maintain a normal brain redox balance during chronic disease. Through its ability to bind reactive copper, Aβ peptide can serve as a Fenton-type oxidant in the presence of ascorbate (Dikalov et al., 2004) or as an “antioxidant” through modification of its histidine groups (Schoneich, 2004). Thus, what has appeared to be a destructive redox molecule may in fact reduce toxic levels of reactive copper under certain conditions. In our mouse, the ability of nitric oxide to counteract H2O2 and lipid peroxidation is lost, and thus the redox balance is likely to be tipped in favor of oxidation over time, regardless of the source of oxidation. NO’s ability to regulate cGMP, caspases, and kinases, however, greatly extends the level at which NO can alter cell function.

When and why NO falls and/or NOS fails is of great interest. Both Blaylock and Jansson suggest different, but potentially interesting mechanisms for NOS destruction in the brain. We favor the view that NO is siphoned away initially by reactive scavengers, such as the iron associated with heme oxygenase 1 activity or other reactive NO scavengers. The essential question to neurodegenerative disease, however, is “can NO be successfully replaced and tissue destruction consequently slowed?” The use of NO NSAIDs as described by Gasparini is one such category of agents that have been used. However, in the case of flurbiprofen it may be difficult to separate NO’s contributions from the ferulic acid group. Other therapeutic candidates are clearly being developed by Thatcher and his group. Although we would caution that the ubiquitous nature of NO makes targeting to select tissue locations a critical issue, there is the possibility that useful new therapeutics can be developed from this concept. It would be interesting to know if the incidence and severity of AD are altered in patient populations who have used long-term nitroglycerin as a therapy for cardiovascular disease.

References:
Dikalov SI, Vitek MP, Mason RP. Cupric-amyloid-beta peptide complex stimulates oxidation of ascorbate and generation of hydroxyl radical. Free Radic Biol Med. 2004 Feb 1;36(3):340-7. Abstract

Schoneich C. Selective Cu2+/ascorbate-dependent oxidation of alzheimer's disease beta-amyloid peptides. Ann N Y Acad Sci. 2004 Mar;1012:164-70. Abstract

View all comments by Carol Colton

This paper is highly interesting in demonstrating a role for NO in the pathogenesis of Alzheimer-like neuropathology in transgenic mice. However, the seemingly new aspect of NO's protective role in neuropathology is not so new. Already in 1999, Willenborg and coworkers reviewed the controversial role of NO in the pathogenesis of a rodent model of multiple sclerosis—experimental autoimmune encephalomyelitis (EAE). We and others have shown that NO may ameliorate the EAE disease course by exerting immunomodulatory functions (Kahl et al., 2003 and 2004). However, the effects were very dependent on the type of EAE model, with NO mediating CNS damage in other models (reviewed by Willenborg et al., 1999).

Another aspect is that findings from rodents regarding NOS type 2 cannot easily be transferred to the human situation, as—like Colton and coworkers also mention in their discussion—the expression of NOS-2 appears to be under much tighter control than in rodents.

Finally, one must not forget that NO has a large range of physiological functions, such as vasodilation and...  Read more

This paper is highly interesting in demonstrating a role for NO in the pathogenesis of Alzheimer-like neuropathology in transgenic mice. However, the seemingly new aspect of NO's protective role in neuropathology is not so new. Already in 1999, Willenborg and coworkers reviewed the controversial role of NO in the pathogenesis of a rodent model of multiple sclerosis—experimental autoimmune encephalomyelitis (EAE). We and others have shown that NO may ameliorate the EAE disease course by exerting immunomodulatory functions (Kahl et al., 2003 and 2004). However, the effects were very dependent on the type of EAE model, with NO mediating CNS damage in other models (reviewed by Willenborg et al., 1999).

Another aspect is that findings from rodents regarding NOS type 2 cannot easily be transferred to the human situation, as—like Colton and coworkers also mention in their discussion—the expression of NOS-2 appears to be under much tighter control than in rodents.

Finally, one must not forget that NO has a large range of physiological functions, such as vasodilation and neurotransmission. Thus, modulating the NO system at any point may lead to compensatory processes or side effects.

Taken together, the role of NO in the pathogenesis of amyloid plaque and tau aggregate formation warrants further studies, especially in vitro investigations in human cell systems and comparative analyses in different rodent models of Alzheimer neuropathology.

References:
Kahl KG, Schmidt HHHW, Jung S, Sherman P, Toyka KV and Zielasek J. Experimental autoimmune encephalomyelitis in mice with a targeted deletion of the inducible nitric oxide synthase gene: increased T-helper 1 response. Neurosci Lett 2004; 358: 58-62. Abstract

Kahl KG, Zielasek J, Uttenthal LO, Rodrigo J, Toyka KV, Schmidt HHHW. Protective role of the cytokine-inducible isoform of nitric oxide synthase induction and nitro-sative stress in experimental autoimmune encephalomyelitis of the DA rat. J Neurosci Res 2003; 73:198-205. Abstract

Willenborg DO,Staykova MA, Cowden WB. Our shifting understanding of the role of nitric oxide in autoimmune encephalomyelitis: a review. J Neuroimmunol 1999; 100: 21-35. Abstract

View all comments by Jürgen Zielasek