An autoimmune theory of endogenous vasoactive neuropeptides in the aetiology of Chronic Fatigue Syndrome

Introduction

Endogenous vasoactive neuropeptides (VNs) of the pituitary adenylate cyclase-activating polypeptide (PACAP)/ vasoactive intestinal peptide (VIP) family may be implicated in the aetiology of chronic fatigue syndrome (CFS) [1]. These VNs are ubiquitous substances strongly preserved in evolutionary terms indicating their crucial roles for survival. They have extraordinary commonality between species. Substantial amino acid sequence homology exists between them, suggesting evolution from a common ancestral gene, and they demonstrate some degree of overlap of structure and function as well as potential for immunological cross-reactivity. They exert their effects at high level in controlling brain and hypothalamic-pituitary-adrenal axis functions as well as at important peripheral sites such as heart, gut, blood, lung, pancreas and reproductive tracts [2].

VN roles and functions

These VNs belong to the secretin-glucagon super-family [3], exerting significant control over carbohydrate and lipid metabolism. They have important roles in neurotrophic function, neuroregulation and neurotransmission, and immunological and hormonal modulation [4]. They are susceptible to catalysis and antibody action [5]. Compromise of their function is therefore likely to have serious consequences for homeostasis. Pain and fatigue in a number of vasoactive neuropeptide associated disorders are potentially explicable. Endogenous opioid activity is functionally related to cytokine and vasoactive neuropeptide activity. Thus pain mediation and perception will be altered in conditions where endogenous opioid function is impaired through these mechanisms [6]. Nitric oxide metabolism is implicated in immuno-modulation as well as mediating chemical sensitivity in these conditions through co-location of nitric oxide synthases [7]. These VNs influence receptor accumulation and activity for acetylcholine [8] and other neurotransmitters. Hence co-transmitter functions with acetylcholine may be the linkage to fatigue mediation in some syndromes. Vasoactive neuropeptides have complex roles and structures and are mediated by G protein-coupled receptors (GPCR). The secondary transmitter cyclic adenosine monophosphate (cAMP) is generated from ATP via adenylate cyclase which is known to exist in multiple isoforms and exhibits a range of processing functions [9].

Endogenous VNs exert a wide spectrum of immunological functions and have a critical role in homeostasis of the immune system through different receptors expressed in various immunocompetent cells [10]. They operate via multiple signaling processes [11]. Disturbances in their function are recognised as potential causes of autoimmune disease and they appear to have a role in protecting bystander lysis, a process in the pathogenesis of several autoimmune and inflammatory diseases [12].The development of autoantibodies to PACAP and VIP in mammalian tissues [13] is known but not extensively documented. However autoantibody responses to this group of neuropeptides indicate immunological tolerance may be readily broken. Hence it has been postulated that depletion of VIP by specific antibodies in autoimmune disease may interfere with VIP regulation of T cells and inflammatory cells and result in further amplification of autoreactive immunological responses [14].

Implications for neuroregulation

PACAP and VIP exert an extraordinary array of functions in the brain and peripheral tissues. VIP has been identified in all regions of the brain including cerebral vessels [15].  PACAP is co-located with vesicular acetylcholine transporter in nerve terminals in all mouse adrenomedullary cholinergic synapses [16]. PACAP has a potentiating additive effect with adrenal and noradrenaline on cAMP production in rat cerebral cortex indicating a crucial role in neuroregulation [17]. Human PACAP and VIP have been shown to exert circadian oscillations in mice [18] and these substances have specific roles in setting rhythms at certain times of the circadian cycle [19]. Disruption to these processes would therefore be expected to have significant impacts on physiological functions and homeostasis.

The role of VNs in protection from brain cell apoptosis is well documented. Cerebellar granule neurons cultured in the presence of KCl undergo spontaneous apoptosis which is reduced by exposure to PACAP [20]. Interestingly apoptosis is known to be higher in sudden infant death syndrome (SIDS) cases than controls with hippocampus and brainstem, including dorsal nuclei being affected [21] and apoptotic neurodegeneration is postulated as the specific pathophysiological mechanism [22]. Mechanisms associated with apoptosis would therefore indicate a suitable area for investigation, particularly those mechanisms usually protective against apoptosis.  These roles are fulfilled by VNs, for example hippocampal ischaemia induced apoptosis in the rat is protected by PACAP through inhibition of the JNK/SAPK and p38 signalling pathways [23]. PACAP is known to have a neuroprotective effect against a range of insults. Ethanol-induced apoptosis occurs via caspase pathways resulting in DNA fragmentation, mitochondrial permeability and cell death. PACAP, acting via its receptor PAC1, protects against ethanol-induced cell death and may have a therapeutic role in conditions such as fetal alcohol syndrome [24].

Other regulatory roles of VNs

PACAP is a powerful respiratory stimulant in dogs [25]. A potentially important finding is that late-gestation blockade of VIP activity in pregnant mice produced distinct morphological abnormalities in the somatosensory cortex of offspring. Their response to hypoxia was subsequently impaired. A significant arousal deficit was seen in anti-VIP mice, which was not associated with deranged peripheral or brainstem-mediated responses to hypoxia during sleep [26].  This finding may have significant implications for respiratory control mechanisms.

Cardiovascular function is in part regulated by VNs. Some variation in the cardiac roles of PACAP exists and is dose dependent, for example PACAP may promote both tachycardia and bradycardia [27]. PACAP activates intracardiac postganglionic parasympathetic nerves by shortening the effective refractory period, and has a greater profibrillatory effect than vagal stimulation [28]. Conceivably PACAP opposition, for example through autoimmune effects on the PAC1 receptor might result in decreased cardiac responsiveness. Adipose tissue metabolism and thermal stress have been linked to VN function. High levels of uncoupling protein are produced to regulate heat production. Neonatal adipose tissue is a primary site of cytokine and cytokine receptor action [29]. While the precise mechanism of adipose tissue metabolism dysfunction is unclear, a combination of factors including metabolic stress, infection, necrosis and vascular hypoperfusion has been suggested [30].

Nitric oxide and chemical sensitivity

Chemical sensitivity to environmental substances is associated with CFS. This may be modulated via nitric oxide and its metabolites.  PACAP and VIP coexist with nitric oxide synthases (NOSs) and other neuropeptides within the nervous system and peripheral tissues and have a role in NO modulation. PACAP and VIP also demonstrate neuroprotective effects against ischaemia and glutamate-induced toxicity [31]. Hence symptoms attributable to nitric oxide and glutamate might therefore occur in association with VN disturbance. Peroxynitrites are associated with toxicity related to brain injury and infection. Loss of neurones is significantly higher in peroxynitrite exposed brains of rats [32,33] and neurotropic virus infection produces inflammatory responses dependent on the activity of peroxynitrite [34]. NO may mediate both neuroprotective and neurodegenerative actions during ischaemic/reperfusion injuries as well as mediating neurotoxicity [35]. Experimental autoimmune disease is known to cause an increase in the production of nitric oxide metabolites correlating with indices of autoimmune expression, morphological impairment and levels of cyclic nucleotides[36]. Interestingly, inhibition of nitric oxide synthesis in rats causes rapid tachyphylaxis to the haemodynamic effects of PACAP-27 [37]. This may prove to be an important modulating mechanism for diminishing vasoactive neuropeptide responsiveness in an NO generating environment with suppression of NO synthase expression.

Immunoregulatory dysfunction

Autoimmune dysfunction of VNs or their receptors may potentially arise via a number of pathways. Cytosine-guanosine dinucleotide DNA (CpG) elements in promoter regions of VN receptors [38,39] may be vulnerable to assault mechanisms such as hypomethylation, resulting in dysregulation of transcriptional/translational capacity. CpG elements are also known to be potent stimulators of immune responses [40] and antibody responses to these VN receptors might hypothetically occur, giving rise to IgM or IgG antibody types, resulting in long term autoimmunity to these vital substances. Such mechanisms might also result from mimicry with bacterial residues, giving rise to mistaken recognition of self VN-CpG for bacterial CpG and perverse VN autoimmunity. Heat shock proteins may also play a role. They are important chaperone molecules for proper intracellular functioning of VNs and are known to have a key role in immunoregulation [41,42,43,44].

Conclusion and future directions

The autoimmune hypothesis of VNs suggests that relatively minor infection or inflammation results in predictable pro-inflammatory cytokine and other responses. Other pro-inflammatory effects such as nitric oxide release and possible chemical sensitivities may also occur. Modulation and termination of these inflammatory responses is required by VNs. Autoimmune effects eg on PACAP/VIP or the PAC1/VPAC1/VPAC2 receptors will have a negating effect on neuropeptide function and also subsequent effects on intracellular mechanisms. While some inflammatory or infectious events may be trivial, compromise of the functions of PACAP/VIP is not. These vital functions in the brain include thermoregulation, olfaction, circadian rhythm, cardio-respiratory activation and sleep-wake cycles. Cardiac and other organs known to exhibit similar PACAP/VIP receptor function would also be expected to demonstrate dysfunction somewhat simultaneously. Further understanding of the roles of the autoimmune dysfunction of these VNs and their receptors may elucidate the mechanisms of CFS and open the way for routine laboratory investigations and prevention options.

1. Staines DR. Is Chronic Fatigue Syndrome an autoimmune disorder of endogenous neuropeptides, exogenous infection and molecular mimicry?  Med Hypoth. 2004. 62:646-652
2. Arimura A. Perspectives on pituitary adenylate cyclase-activating polypeptide (PACAP) in the neuroendocrine, endocrine and nervous systems. Jpn J Physiol 1998;48(5):301-31
3. Nussdorfer GG, Bahcelioglu M, Neri G, Malendowicz LK. Secretin, glucagon, gastric inhibitory polypeptide, parathyroid hormone, and related peptides in the regulation of the hypothalamus-pituitary adrenal axis. Peptides. 2000 Feb; 21(2):309-24
4. Delgado M, Abad C, Martinez C, et al. Vasoactive intestinal peptide in the immune system: potential therapeutic role in inflammatory and autoimmune diseases. J Mol Med. 2002 Jan;80(1):16-24
5. Ikezaki H, Paul S, Alkan-Onyuksel H, et al. Vasodilation elicited by liposomal VIP is unimpeded by anti-VIP antibody in hamster cheek pouch. Am J Physiol.1998 Jul;275(1 Pt 2):R56-62
6. Peterson PK, Molitor TW, Chao CC. The opioid-cytokine connection. J Neuroimmunol. 1998 Mar 15; 83(1-2):63-9
7. Pall ML. Elevated nitric oxide/peroxynitrite theory of multiple chemical sensitivity: central role of N-methyl-D-aspartate receptors in the sensitivity mechanism. Environ Health Perspect. 2003 Sep; 111(12):1461-4
8. Buffelli M, Pasino E, Cangiano A. In vivo acetylcholine receptor expression induced by calcitonin gene-related peptide in rat soleus muscle. Neuroscience. 2001; 104(2):561-7
9. Vaudry D, Gonzalez BJ, Basille M et al. Pituitary adenylate cyclase-activating polypeptide and its receptors: from structure to functions. Pharmacol Rev 2000;52(2):269-324
10. Gomarez RP, Martinez C, Abad D, et al. Immunology of VIP: a review and therapeutical perspectives. Curr Pharm Des 2001 Jan; 7(2):89-111
11. Zhou CJ, Shioda S, Yada T, et al. PACAP and its receptors exert pleiotropic effects in the nervous system by activating multiple signaling pathways. Curr Protein Pept Sci. 2002 Aug; 3 (4): 423-39.
12. Delgado M, Abad C, Martinez C, et al. PACAP in immunity and inflammation. Ann N Y Acad Sci. 2003 May;992:141-57
13. Paul S. Catalytic activity of anti-ground state antibodies, antibody subunits, and human autoantibodies. Appl Biochem Biotechnol. 1994 May-Jun;47(2-3):241-53
14. Bangale Y, Cavill D, Gordon T, et al. Vasoactive intestinal peptide binding autoantibodies in autoimmune humans and mice. Peptides. 2002 Dec;23(12):2251-7
15. Suzuki N, Shimizu T, Takao M, et al. Neurokinin-1 receptors in the cerebrovascular vasoactive intestinal polypeptide-containing nerves of the rat. Auton Neurosci. 2002 Jan 10;95(1-2):103-11
16. Hamelink C, Tjurmina O, Damadzic R, et al. Pituitary adenylate cyclase-activating polypeptide is a sympathoadrenal neurotransmitter involved in catecholamine regulation and glucohomeostasis. Proc Natl Acad Sci USA. 2002 Jan 8;99 (1): 461-6.
17. Nowak JZ, Kuba K. Pituitary adenylate cyclase-activating polypeptide and vasoactive intestinal peptide stimulated cyclic AMP synthesis in rat cerebral cortex slices: interaction with noradrenaline, adrenaline, and forskolin. J Mol Neurosci. 2002 Feb-Apr;18(1-2):47-52
18. Shen S, Spratt C, Sheward WJ, et al. Overexpression of the human VPAC2 receptor in the suprachiasmatic nucleus altering the circadian phenotype of mice. Proc natl Acad Sci USA. 2000 Oct10;97(21):11575-80
19. Gillette MU, Mitchell JW. Signaling in the suprachiasmatic nucleus: selectively responsive and integrative. Cell Tissue Res. 2002 Jul;309(1):99-107. Epub 2002 Jun 06
20. Bhave SV, Hoffman PL. Phosphatidylinositol 3’-OH kinase and protein kinase A pathways mediate the anti-apoptotic effect of pituitary adenylyl cyclase-activating polypeptide in cultured cerebellar granule neurons: modulation by ethanol. J Neurochem. 2004 Jan;88(2):359-69
21. Waters KA, Meehan B, Huang JQ, et al. Neuronal apoptosis in sudden infant death syndrome. Pediatr Res. 1999 Feb;45(2):166-72
22. Sparks DL, Hunsaker JC3rd. Neuropathology of sudden infant death (syndrome): literature review and evidence of a probable apoptotic degenerative cause. Childs Nerv Syst. 2002 Nov;18(11):568-92. Epub 2002 Sep04
23. Shioda S, Ozawa H, Dohi K, et al. PACAP protects hippocampal neurons against apoptosis: involvement of JNK/SAPK signalling pathway. Ann NY Acad  Sci. 1998 Dec 11;865:100-10
24. Vaudry D, Rouselle C, Basille M, et al. Pituitary adenylate cyclase-activating polypeptide protects rat cerebellar granule neurons against ethanol-induced apoptotic cell death. Proc Natl Acad Sci USA. 2002 Apr30;99(9):6398-403. Epub 2002 Apr23
25. Runcie MJ, Ulman LG, Potter EK. Effects of adenylate cyclase-activating polypeptide on cardiovascular and respiratory responses in anaesthetised dogs. Regul Pept. 1995 dec 14;60(2-3):193-200
26. Cohen G, Gressens P, Gallego J, Gaulter C. Depression of hypoxic arousal response in adolescent mice following antenatal vasoactive intestinal polypeptide blockade. J Physiol. 2002 Apr 15;540(Pt2):691-9
27. Ishizuka Y, Kashimoto K, Mochizuki T, et al. Cardiovascular and respiratory actions of adenylate cyclase-activating polypeptides. Regul Pept. 1992 Jul 2;40(1):29-39
28. Hirose M, Leatmanoratn Z, Laurita KR, Carlson MD. Mechanism for pituitary adenylate cyclase-activating polypeptide-induced atrial fibrillation. J Cardiovasc Electrophysiol. 2001 Dec;12(12):1381-6
29. Stephenson T, Budge H, Mostyn A, et al. Fetal and neonatal adipose maturation: a primary site of cytokine and cytokine-receptor action. Biochem Soc Trans. 2001 May;29(Pt2):80-5
30. Stephenson TJ, Variend S. Visceral brown fat necrosis in postperinatal mortality. J Clin Pathol. 1987 Aug;40(8):896-900
31. Onoue S, Endo K, Yajima T, et al. Pituitary adenylate cyclase-activating polypeptide and vasoactive intestinal peptide attenuate glutamate-induced nNOS activation and cytotoxicity. Regula Pept. 2002 Jul 15;107(1-3):43-7
32. Pall ML. NMDA sensitisation and stimulation by peroxynitrite, nitric oxide and organic solvents as the mechanism of chemical sensitivity in multiple chemical sensitivity. FASEB J.  2002 Sep;16(11):1407-17
33. Bao F, Liu D. Peroxynitrite generated in the rat spinal cord induces neuron death and neurological deficits. Neuroscience.  2002;115(3):839-49
34. Hooper DC, Kean RB, Scott GS, et al. The central nervous system inflammatory response to neurotropic virus infections is peroxynitrite dependent. J Immunol. 2001 sep 15;167(6):3470-7
35. He Y, Imam SZ, Dong Z, et al. Role of nitric oxide in rotenone-induced nigro-striatal injury. J Neurochem.  2003 Sep;86(6):1338-45
36. Zviagina TV, Siniachenko OV, Grin’ VK, et al. The blood level of nitric oxide metabolites in experimental autoimmune disease during immunosuppressive therapy. Patol Fitziol Eksp Ter.  2003 Jan-Mar;(1):32-3
37. Whalen EJ, Travis MD, Johnson AK, Lewis SJ. Rapid tachyphylaxis to hemodynamic effects of PACAP –27 after inhibition of nitric oxide synthesis. Am J Physiol Heart Citc Physiol. 1999. 276(6):H2117-H2126.
38.  Cauli B, Tong XK, Rancillac A, et al. Cortical GABA interneurons in neurovascular coupling: relays for subcortical vasoactive pathways. J Neurosci. 2004 Oct 13;24(41):8940-9
39. Broad PM, Symes AJ, Thakker RV, Craig RK. Structure and methylation of the human calcitonin/alpha-CGRP gene. Nucleic Acids Res. 1989 Sep 12;17(17):6999-7011
40. Krieg AM. CpG motifs: the active ingredient in bacterial extracts. Nat Med 2003;9(7):831-5
41. Bandholtz L, Guo Y, Palmberg C et al. Heat shock protein binds CpG oligonucleotides directly: implications for hsp90 as a missing link in CpG signalling and recognition. Cell Mol Life Sci. 2003 Feb;60(2):422-9
42. van Eden W, Koets A, van KootenP, Prakken B, van der Zee R. Immunopotentiating heat shock proteins: negotiators between innate danger and control of autoimmunity. Vaccine. 2003 Feb 14;21(9-10):897-901
43. Pockley AG. Heat shock proteins as regulators of the immune response. Lancet. 2003. 362(9382):469-76
44. Srivistava P. Interaction of heat shock proteins with peptides and antigen presenting cells: chaperoning of the innate and adaptive immune responses. Annu Rev Immunol. 2002;20:395-425.

Latest News | Research | Information | Advocacy | Conference | Guidelines