The recent paper of Frédéric Darios and Bazbek Davletov adds up to recent studies trying to decipher the molecular mechanism of action of neuroactive lipids such as omega-3 (n3) polyunsaturated fatty acids (PUFAs). Many cellular effectors of n3PUFAs have been previously identified (including neuroprotectin [1-2], cell signaling kinase Akt and PI3K [3-5], retinoid X receptors [6-7], etc.), but here the authors bring compelling in vitro evidence for a causal relationship linking PUFAs and syntaxin 3 to neurite growth. A key observation is the capacity of long-chain PUFAs (docosahexaenoic acid and arachidonic acid) to alter the 3D-structure of syntaxin 3 to make it complex with another synaptic protein, synaptosomal-associated protein-25. Among the series of fatty acids tested, only those altering syntaxin 3 conformation also promoted neurite growth. However, no distinction between n3 and n6 PUFAs was reported, contrasting with most in vivo studies, which favor n3PUFAs as neuroprotective agents. The next step is to determine if this scenario observed in cultured PC12 and hippocampal...  Read more

The recent paper of Frédéric Darios and Bazbek Davletov adds up to recent studies trying to decipher the molecular mechanism of action of neuroactive lipids such as omega-3 (n3) polyunsaturated fatty acids (PUFAs). Many cellular effectors of n3PUFAs have been previously identified (including neuroprotectin [1-2], cell signaling kinase Akt and PI3K [3-5], retinoid X receptors [6-7], etc.), but here the authors bring compelling in vitro evidence for a causal relationship linking PUFAs and syntaxin 3 to neurite growth. A key observation is the capacity of long-chain PUFAs (docosahexaenoic acid and arachidonic acid) to alter the 3D-structure of syntaxin 3 to make it complex with another synaptic protein, synaptosomal-associated protein-25. Among the series of fatty acids tested, only those altering syntaxin 3 conformation also promoted neurite growth. However, no distinction between n3 and n6 PUFAs was reported, contrasting with most in vivo studies, which favor n3PUFAs as neuroprotective agents. The next step is to determine if this scenario observed in cultured PC12 and hippocampal cells also occurs in vivo. If it does, it is possible that n3PUFAs exert their benefit on cognitive function through their action on syntaxin 3. This hypothesis will need to be tested in animal models of cognitive deficit. Syntaxin 3 could evolve as a potential drug target for Alzheimer disease.

References:
1. Bazan NG. Neuroprotectin D1 (NPD1): a DHA-derived mediator that protects brain and retina against cell injury-induced oxidative stress. Brain Pathol. 2005 Apr;15(2):159-66. Review. Abstract

2. Lukiw WJ, Cui JG, Marcheselli VL, Bodker M, Botkjaer A, Gotlinger K, Serhan CN, Bazan NG. A role for docosahexaenoic acid-derived neuroprotectin D1 in neural cell survival and Alzheimer disease. J Clin Invest. 2005 Oct;115(10):2774-83. Abstract

3. Akbar M, Calderon F, Wen Z, Kim HY. Docosahexaenoic acid: a positive modulator of Akt signaling in neuronal survival. Proc Natl Acad Sci U S A. 2005 Aug 2;102(31):10858-63. Epub 2005 Jul 22. Abstract

4. Akbar M, Kim HY. Protective effects of docosahexaenoic acid in staurosporine-induced apoptosis: involvement of phosphatidylinositol-3 kinase pathway. J Neurochem. 2002 Aug;82(3):655-65. Abstract

5. Calon F, Lim GP, Yang F, Morihara T, Teter B, Ubeda O, Rostaing P, Triller A, Salem N Jr, Ashe KH, Frautschy SA, Cole GM. Docosahexaenoic acid protects from dendritic pathology in an Alzheimer's disease mouse model. Neuron. 2004 Sep 2;43(5):633-45. Abstract

6. de Urquiza AM, Liu S, Sjoberg M, Zetterstrom RH, Griffiths W, Sjovall J, Perlmann T. Docosahexaenoic acid, a ligand for the retinoid X receptor in mouse brain. Science. 2000 Dec 15;290(5499):2140-4. Abstract

7. Samadi P, Gregoire L, Rouillard C, Bedard PJ, Di Paolo T, Levesque D. Docosahexaenoic acid reduces levodopa-induced dyskinesias in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine monkeys. Ann Neurol. 2006 Feb;59(2):282-8. Abstract

View all comments by Frederic Calon

We read this interesting paper, which offers a novel mechanism of action for PUFA-mediated neurite outgrowth. The mechanism involves fatty acid modulation of formation of syntaxin 3 complexes with SNARE proteins. The latter are required for regulating fusion of incoming vesicles with the expanding growth cone. Both the omega-6 and omega-3 fatty acids are similarly competent to promote syntaxin 3 protein complex formation and neurite outgrowth. Although high—100 micromolar half-maximal—concentrations of fatty acid are required for efficacy, the authors argue that these are achievable at the plasma membrane bilayer and they provide data for how the fatty acids impact physiological neurite outgrowth in living cells. Since Morris and collaborators have reported that low essential PUFA intake increases AD risk, these data suggest that one reason for this increased risk may be compromised neurite outgrowth and related loss of synapses. Increased oxidation of omega-3 and omega-6 fatty acids and their focal loss in AD is implied by measured...  Read more

We read this interesting paper, which offers a novel mechanism of action for PUFA-mediated neurite outgrowth. The mechanism involves fatty acid modulation of formation of syntaxin 3 complexes with SNARE proteins. The latter are required for regulating fusion of incoming vesicles with the expanding growth cone. Both the omega-6 and omega-3 fatty acids are similarly competent to promote syntaxin 3 protein complex formation and neurite outgrowth. Although high—100 micromolar half-maximal—concentrations of fatty acid are required for efficacy, the authors argue that these are achievable at the plasma membrane bilayer and they provide data for how the fatty acids impact physiological neurite outgrowth in living cells. Since Morris and collaborators have reported that low essential PUFA intake increases AD risk, these data suggest that one reason for this increased risk may be compromised neurite outgrowth and related loss of synapses. Increased oxidation of omega-3 and omega-6 fatty acids and their focal loss in AD is implied by measured increases in both F2 and F4 isoprostanes measured by multiple groups (Halliwell, Montine, Morrow, Pratico). However, these results showing similar impact from both omega-6 (AA) and omega-3 (DHA) cannot explain a selective benefit of omega-3 fatty acids or fish consumption as suggested by both epidemiology and AD animal model research, including work from our own lab, showing selective protection with DHA and adverse effects of high omega-6 (linoleic acid). Thus, more selective protection with DHA likely involves other mechanisms that may include reductions in amyloid accumulation, as seen in our data, as well as regulation of Akt translocation and activation as championed by Akbar et al., and generation of neuroprotectins like NPD1 as described by Nick Bazan's group. To our mind, it seems more and more probable that multiple protective mechanisms can be ascribed to these beneficial fats in your head.

View all comments by Gregory Cole
View all comments by Sally A. Frautschy

The paper by Darios and Davletov provides significant information on the triggers of cell membrane expansion underlying neurite outgrowth. The importance of omega-3 and omega-6 polyunsaturated fatty acids in enabling syntaxin 3 to complex with SNAP25—eventually generating the SNARE ternary complex that results in membrane fusion and expansion—is a new and important finding. These results raise intriguing questions regarding the molecular events involved in AD pathogenesis. Recent research has shown that memory impairment in AD results from synaptic dysfunction with loss of LTP, in turn resulting from the toxic effects of Aβ oligomers at the cell membrane (Walsh et al., 2002; Cleary et al., 2005; Lesné et al., 2006). A recent paper by the group led by William Klein has proposed that such an effect is mediated by the specific interaction of Aβ oligomers with dendritic arbors at discrete puncta containing synaptic markers such as PSD-95, and...  Read more

The paper by Darios and Davletov provides significant information on the triggers of cell membrane expansion underlying neurite outgrowth. The importance of omega-3 and omega-6 polyunsaturated fatty acids in enabling syntaxin 3 to complex with SNAP25—eventually generating the SNARE ternary complex that results in membrane fusion and expansion—is a new and important finding. These results raise intriguing questions regarding the molecular events involved in AD pathogenesis. Recent research has shown that memory impairment in AD results from synaptic dysfunction with loss of LTP, in turn resulting from the toxic effects of Aβ oligomers at the cell membrane (Walsh et al., 2002; Cleary et al., 2005; Lesné et al., 2006). A recent paper by the group led by William Klein has proposed that such an effect is mediated by the specific interaction of Aβ oligomers with dendritic arbors at discrete puncta containing synaptic markers such as PSD-95, and possibly others including glutamate receptors, suggesting a clue to explain neuronal selectivity of damage (Lacor et al., 2004). Is it possible that Aβ aggregates may disrupt synaptic memory by interfering with synaptic expansion by the mechanisms outlined by Darios and Davletov? The possibility that Aβ oligomers interact with SNARE ternary complexes, or their precursors, and so compromise synaptogenesis is worth further investigation.

Another consideration concerns the effect of arachidonic acid and possibly other PUFAs in stabilizing a folding variant of syntaxin 3, causing it to interact with SNAP25. This is another example of a protein folding variant stabilized by a fatty acid. The HAMLET story has shown that a molecule of oleic acid interacts with α-lactalbumin, stabilizing a molten, globule-like folding variant of the latter, which is endowed with cytotoxic activity specific to tumor cells. A full understanding of how fatty acids, arising from specific metabolic modifications of membrane lipids, participate in membrane processing and/or membrane interactions with specific or nonspecific ligands is of fundamental importance to our understanding of the molecular basis of cell function and dysfunction.

View all comments by Massimo Stefani