In vivo evidence that truncated trkB.T1 participates in nociception
© Renn et al; licensee BioMed Central Ltd. 2009
Received: 05 August 2009
Accepted: 29 October 2009
Published: 29 October 2009
Brain-Derived Neurotrophic Factor (BDNF) is a central nervous system modulator of nociception. In animal models of chronic pain, BDNF exerts its effects on nociceptive processing by binding to the full-length receptor tropomyosin-related kinase B (trkB.FL) and transducing intracellular signaling to produce nocifensive behaviors. In addition to trkB.FL, the trkB locus also produces a widely-expressed alternatively-spliced truncated isoform, trkB.T1. TrkB.T1 binds BDNF with high affinity; however the unique 11 amino acid intracellular cytoplasmic tail lacks the kinase domain of trkB.FL. Recently, trkB.T1 was shown to be specifically up-regulated in a model of HIV-associated neuropathic pain, potentially implicating trkB.T1 as a modulator of nociception. Here, we report that trkB.T1 mRNA and protein is up-regulated in the spinal dorsal horn at times following antiretroviral drug treatment and hind paw inflammation in which nocifensive behaviors develop. While genetic depletion of trkB.T1 did not affect baseline mechanical and thermal thresholds, the absence of trkB.T1 resulted in significant attenuation of inflammation- and antiretroviral-induced nocifensive behaviors. Our results suggest that trkB.T1 up-regulation following antiretroviral treatment and tissue inflammation participates in the development and maintenance of nocifensive behavior and may represent a novel therapeutic target for pain treatment.
BDNF is a potent modulator of pain processing in the CNS [[1, 2]; for review see [3, 4]]. BDNF exerts effects on nociception by binding to trkB.FL and initiating intracellular signaling cascades that lead to transcriptional changes. Noxious stimulation increases BDNF and trkB.FL production in the spinal dorsal horn [5–9] and brainstem [10, 11], leading to hyperalgesia and the formation of nocifensive behaviors. Interfering with trkB.FL signaling via trkB.FL-specific exogenous antibody administration  or by blocking receptor activation in the chemical-genetic transgenic trkBF616A mice  prevents the development of tissue- and nerve injury-induced thermal and mechanical hypersensitivity .
In addition to trkB.FL, the Ntrk2 (trkB) locus encodes for several alternatively-spliced isoforms of the receptor , including trkB.T1, the predominant isoform expressed in the adult mammalian nervous system. The extracellular domain of trkB.T1 is identical to the full-length isoform, which enables high-affinity BDNF binding [15, 16]. However, the 11 amino acid intracellular portion of trkB.T1 lacks the kinase activation domain necessary to activate classical signal transduction pathways. Since trkB.T1 heterodimerization with trkB.FL inhibits trans-autophosphorylation of the trkB.FL kinase domain, studies support a model in which trkB.T1 functions to reduce BDNF signaling [17–22]. However, we [23, 24] and others [25–31] have demonstrated evidence that trkB.T1 may signal independently.
TrkB.T1 is up-regulated in pathological states including human Alzheimer's disease  and it is the mechanism underlying premature hippocampal cell death in a mouse model of Down Syndrome [23, 33]. Moreover, altered trkB.T1 expression can affect synaptic plasticity. Over-expression of trkB.T1 in hippocampal neurons leads to the inhibition of synaptic potentiation  and over-expressing trkB.T1 at neuromuscular synapses results in disassembly of acetylcholine receptor clustering . Recently, trkB.T1 specifically was shown to be significantly up-regulated in dorsal root ganglion neurons in a rodent model of HIV-associated neuropathic pain . Thus, it is reasonable to hypothesize that trkB.T1 up-regulation might have a specific role in pain processing.
BDNF signaling modulates nociceptive processing across a variety of pre-clinical models of pain (for review, ). Most studies have focused on trkB.FL as the receptor responsible for BDNF-mediated nociception, since antibodies specific for trkB.FL  and abrogation of trkB.FL activation using the trkBF616A transgenic mouse  attenuate thermal and mechanical hypersensitivity. Few studies, however, have examined a specific role for trkB.T1. Yajima et al.  concluded that trkB.T1 did not have a role in nociception since repeated intrathecal injections with anti-trkB.T1 did not produce changes in nerve injury-induced thermal hyperalgesia. However, we (Dorsey and Renn, unpublished) and others (Dr. Rita Balice-Gordon, University of Pennsylvania, personal communication) have shown that the trkB.T1 antibody used in the Yajima et al.  study cross-reacts with other epitopes in vivo. Moreover, as acknowledged in Wang et al. , indirect methods of targeting receptor activation, for example via exogenous antibody administration, can be inconclusive. In support of a role for trkB.T1 in nociception, a recent microarray study in a model of HIV-associated neuropathic pain found that trkB.T1, but not trkB.FL or trkB.T2 (a second truncated isoform), was up-regulated in dorsal root ganglion neurons .
Since trkB.T1 and trkB.FL are presumably regulated by a common promoter, we also undertook a careful examination of spinal dorsal horn trkB.FL mRNA and protein regulation in the antiretroviral and inflammatory models in both wildtype and trkB.T1 knockout animals. As demonstrated in Additional File 1, we detected no significant regulation of trkB.FL mRNA (panels A-D) following antiretroviral or inflammatory treatment compared with vehicle controls in either genotype. Similar results were found for protein levels (panels E-H), although trkB.FL was down-regulated approximately 25% in wildtype CFA-treated animals. Thus, we conclude that the up-regulation of trkB.T1 following noxious stimulation is specific for this isoform.
Here, we demonstrate that trkB.T1 mRNA and protein are both significantly up-regulated in the spinal dorsal horn following systemic antiretroviral drug administration and hind paw inflammation (Figure 1). We show that in vivo deletion of trkB.T1 did not affect acute nociceptive processing, since baseline mechanical and thermal thresholds were not significantly different from wildtype strain-matched controls (Figure 2). However, following antiretroviral administration or hind paw inflammation via CFA injection or capsaicin treatment, animals lacking trkB.T1 were significantly protected against the development of mechanical and thermal hypersensitivity (Figure 2A, B, D). These results suggest that trkB.T1 participates in the development and maintenance of mechanical and thermal hypersensitivity, and that upregulation specifically contributes to the degree of thermal and mechanical hypersensitivity since the null mutant demonstrates attenuated hypersensitivity to our noxious stimulation paradigms.
The expression of trkB.FL and trkB.T1 is conserved across species and throughout evolution, and trkB isoform expression is regulated during development when the dominant trkB isoform expression in the brain switches from trkB.FL to trkB.T1 . Receptor expression is cell specific and not all cells express both isoforms. For example, neurons express both isoforms  while astrocytes only express trkB.T1 [23, 40]. The prevailing view has been that trkB.T1 expression leads to dominant negative inhibition of full-length trkB signaling and a reduction in the activation of downstream signaling molecules [17–23, 33]. If trkB.T1 were functioning solely as a dominant negative inhibitor of signaling in the dorsal horn, the prediction would be that deletion of trkB.T1 would result in more, not less pain. However, in the animal models of pain tested in this study, genetic depletion of trkB.T1 results in attenuation of pain. Several studies have provided evidence to suggest that trkB.T1 can signal independently, however the precise nature and functional relevance of this signal has been less clear. The available evidence regarding trkB.T1 signaling suggests that BDNF binding to trkB.T1 causes increased intracellular calcium release . We have shown that neurons from a mouse model of neurodegeneration over-express trkB.T1, resulting in a significant increase in resting intracellular calcium; reducing trkB.T1 expression in those neurons reduced intracellular calcium to wildtype levels . In a rodent model of antiretroviral-induced neuropathy, intrathecal injection of TMB-8, a drug that buffers calcium, provided near complete abrogation of mechanical hypersensitivity . Thus, our results demonstrating attenuation in thermal and mechanical sensitivity in the absence of trkB.T1 could be because increased trkB.T1 expression induces increased intracellular calcium signaling which promotes hypersensitivity.
Our results support the idea that trkB signaling is complex, and that trkB.T1 participates in the development and maintenance of persistent pain. Genetic deletion of trkB.T1 provides significant protection from the development of thermal and mechanical hypersensitivity following noxious stimulation. Thus, we conclude that while previous studies have demonstrated a specific role for BDNF-mediated trkB.FL signaling in nociception, our results support a role for trkB.T1 in nociception that has not been previously described.
Materials and methods
Procedures have been described elsewhere [23, 33]. Primary antibodies included anti-trkB.T1 (Santa Cruz TrkB (C-13) sc-119), anti-trkB out (generous gift from David Kaplan, Hospital for Sick Kids, Toronto, Canada) and anti-actin (Sigma A2547). Secondary antibodies conjugated to peroxidase included anti-mouse IgG and anti-rabbit IgG (Amersham Pharmacia Biotech).
Methods for RNA extraction and qPCR techniques have been previously described (Dorsey et al., 2009). Primers specific for trkB.T1 included forward: 5'-TGG TGA TGT TGC TCC TGC TCA AGT-3'; reverse: 5'-CCC ATC CAG TGG GAT CTT ATG AAA C-3'. Primers specific for trkB.FL included forward: 5'-ACT TTG GCA TCA CCA ACA GTC AGC-3'; reverse: 5'-AGG TTG TAG CAC TCG GCA AGG AAA-3'. The β-actin gene was used as the reference gene with specific primers: forward: 5'-TGT GGT GCC AGA TCT TCT CCA TGT-3'; reverse: 5'-TGT GGT GCC AGA TCT TCT CCA TCT-3' (Integrated DNA Technologies).
Generation of trkB.T1 wildtype and knockout mice
Mice were generated as previously described . Wildtype and knockout experimental animals were the progeny of heterozygous trkB.T1 matings from animals at N13 generation.
All experiments were approved by the University of Maryland Baltimore Institutional Animal Care and Use Committee and were performed following the guidelines of the International Association for the Study of Pain. All experiments were performed with the tester blind to the genotype of the mice and the treatment that they received. All mice tested were adult male mice (2-6 months of age weighing approximately 30 g. Details regarding the production of the inflammatory model using Complete Freund's adjuvant (CFA) can be found in Renn et al.  and the antiretroviral model methods are detailed in Dorsey et al. . For capsaicin experiments, capsaicin (Sigma, St. Louis, MO) was dissolved in 100% dimethylsulfoxide (DMSO) followed by dilution with 0.9% saline to a concentration of 0.08 μg/μl. Using a 50 μl Hamilton syringe with a 1/2" 30-gauge needle attached, each mouse was given a 20 μl subcutaneous injection of capsaicin (1.6 μg/20 μl) into the plantar surface of the left hind paw. The mice were placed in individual Plexiglas cubicles and allowed to acclimate for approximately one hour before the injection of capsaicin. Licking induced by the capsaicin injection was used to indicate a nocifensive response. Each mouse was examined for a period of 5 minutes following the capsaicin injection by a blinded observer. The time (in seconds) to onset of licking, total accumulated time (in seconds) spent licking and the number of bouts of licking the capsaicin-injected paw was measured. Thermal Hyperalgesia: The Paw Thermal Stimulator (PTS, UARDG, Department of Anesthesiology, University of California, San Diego; attn George Ozaki) was used to assess thermal sensitivity. Mice were placed in individual Plexiglas cubicles on a glass surface maintained at a constant temperature of 30°C ± 0.1°C, and allowed to acclimate for approximately one hour prior to testing. Paw withdrawal latencies were defined as the time in seconds required from the onset of the stimulus to the point of a brisk withdrawal of the hind paw from the stimulus. An automatic 20-second cut-off was used to avoid tissue injury from prolonged exposure to the thermal stimulus. Each hind paw was tested three times and the response latency was the average of the three tests. Approximately 10 minutes elapsed between repeat test exposures of each hind paw to avoid tissue injury from the repeated applications of the thermal stimulus. Mechanical Allodynia: The nocifensive behavior of paw withdrawal from a mechanical stimulus was used to assess the development of mechanical allodynia. The mice were placed in individual Plexiglas cubicles on a wire mesh platform, and allowed to acclimate for approximately one hour. A series of von Frey filaments (Touch Test Sensory Evaluator Kit, myNeurolab.com, St. Louis, MO), with bending forces that ranged from 0.04 g to 1.40 g, was used to deliver the mechanical stimuli. Each filament was applied to the left hind paw until the filament just bent and was held in place for 5 seconds or until the mouse withdrew his paw. Each filament was tested 5 times on the left hind paw, with a period of 3-5 minutes elapsing between trials. A positive response to the stimulus was defined as a brisk withdrawal, with or without shaking or licking, of the hind paw during or immediately upon removal of the filament application. Using a procedure modified after Ren , the mechanical stimuli were applied to the plantar surface of the left hind paw, starting with the 0.4 g filament. If the 0.4 g filament elicited 3 positive responses out of 5 trials, then the mouse was tested moving downward through the series to the 0.04 g filament and the number of withdrawals was recorded for each filament. If the 0.4 g filament did not elicit 3 positive responses, then the mouse was tested moving upward through the series to the 1.4 g filament and the number of withdrawals was recorded for each filament. Threshold was defined as the filament with the lowest bending force that elicited at least 3 positive responses out of 5 trials.
This work was supported by a grant from the National Institutes of Health to S. G. D. (NR009672).
- Thompson SW, Bennett DL, Kerr BJ, Bradbury EF, McMahon SB: Brain-derived neurotrophic factor is an endogenous modulator of nociceptive responses in the spinal cord. Proc Natl Acad Sci USA 1999, 96: 7714–7718.PubMed CentralPubMedView ArticleGoogle Scholar
- Miletic G, Miletic V: Increases in the concentration of brain derived neurotrophic factor in the lumbar spinal dorsal horn are associated with pain behavior following chronic constriction injury in rats. Neurosci Lett 2002, 319: 137–140.PubMedView ArticleGoogle Scholar
- McMahon SB, Cafferty WB: Neurotrophic influences on neuropathic pain. Novartis Found Symp 2004, 261: 68–92.PubMedView ArticleGoogle Scholar
- Pezet S, McMahon SB: Neurotrophins: Mediators and modulators of pain. Annu Rev Neurosci 2006, 29: 507–538.PubMedView ArticleGoogle Scholar
- Michael GJ, Averill S, Shortland PJ, Yan Q, Priestley JV: Axotomy results in major changes in BDNF expression by dorsal root ganglion cells: BDNF expression in large trkB and trkC cells, in pericellular baskets, and in projections to deep dorsal horn and dorsal column nuclei. Eur J Neurosci 1999, 11: 3539–3551.PubMedView ArticleGoogle Scholar
- Ha SO, Kim JK, Hong HS, Kim DS, Cho HJ: Expression of brain-derived neurotrophic factor in rat dorsal root ganglia, spinal cord and gracile nuclei in experimental models of neuropathic pain. Neuroscience 2001, 107: 301–309.PubMedView ArticleGoogle Scholar
- Pezet S, Malcangio M, Lever IJ, Perkinton MS, Thompson SW, Williams RJ, McMahon SB: Noxious stimulation induces trk receptor and downstream ERK phosphorylation in spinal dorsal horn. Mol Cell Neurosci 2002, 21: 684–695.PubMedView ArticleGoogle Scholar
- Coull JA, Beggs S, Boudreau D, Boivin D, Tsuda M, Inoue K, Gravel C, Salter MW, DeKoninck Y: BDNF from microglia causes the shift in neuronal anion gradient underlying neuropathic pain. Nature 2005, 438: 1017–1021.PubMedView ArticleGoogle Scholar
- Li L, Xian CJ, Zhong JH, Zhou XF: Upregulation of brain-derived neurotrophic factor in the sensory pathway by selective motor nerve injury in adult rats. Neurotox Res 2006, 9: 269–283.PubMedView ArticleGoogle Scholar
- Guo W, Robbins MT, Wei F, Zhou S, Dubner R, Ren K: Supraspinal brain-derived neurotrophic factor signaling: A novel mechanism for descending pain faciliatation. J Neurosci 2006, 26: 126–137.PubMedView ArticleGoogle Scholar
- Renn CL, Lin L, Thomas S, Dorsey SG: Full-length tropomyosin-related kinase B expression in the brainstem in response to persistent inflammatory pain. NeuroReport 2006, 17: 1175–1179.PubMedView ArticleGoogle Scholar
- Yajima Y, Narita M, Narita M, Matsumoto N, Suzuki T: Involvement of a spinal brain-derived neurotrophic factor/full-length trkB pathway in the development of nerve injury-induced thermal hyperalgesia in mice. Brain Res 2002, 958: 338–346.PubMedView ArticleGoogle Scholar
- Chen X, Ye H, Kuruvilla R, Ramanan N, Scangos KW, Zhang C, Johnson NM, England PM, Shokat KM, Ginty DD: A chemical-genetic approach to studying neurotrophin signaling. Neuron 2005, 46: 13–21.PubMedView ArticleGoogle Scholar
- Wang X, Ratnam J, Zou B, England PM, Basbaum AI: TrkB signaling is required for both the induction and maintenance of tissue and nerve injury-induced persistent pain. J Neurosci 2009, 29: 5508–5515.PubMed CentralPubMedView ArticleGoogle Scholar
- Middlemas DS, Lindberg RA, Hunter T: TrkB, a neural receptor protein-tyrosine kinase: Evidence for a full-length and two truncated receptors. Mol Cell Biol 1991, 11: 143–153.PubMed CentralPubMedGoogle Scholar
- Biffo S, Offenhauser N, Carter BD, Barde YA: Selective binding and internalization by truncated receptors restrict the availability of BDNF during development. Development 1995, 121: 2462–2470.Google Scholar
- Eide FF, Vinig ER, Eide BL, Zang K, Wang XY, Reichardt LF: Naturally occurring truncated trkB receptors have dominant inhibitory effects on brain-derived neurotrophic factor signaling. J Neurosci 1996, 16: 3123–3129.PubMed CentralPubMedGoogle Scholar
- Gonzalez M, Ruggiero FP, Chang Q, Shi YJ, Rich MM, Kraner S, Balice-Gordon RJ: Disruption of trkB-mediated signaling induces disassembly of postsynaptic receptor clusters at neuromuscular junctions. Neuron 1999, 24: 567–583.PubMedView ArticleGoogle Scholar
- Saarelainen T, Pussinen R, Koponen E, Alhonen L, Wong G, Sirvio J, Castren E: Transgenic mice overexpressing truncated trkB neurotrophin receptors in neurons have impaired long-term spatial memory but normal hippocampal LTP. Synapse 2000, 38: 102–104.PubMedView ArticleGoogle Scholar
- Haapasalo A, Koponen E, Hoppe E, Wong G, Castren E: Truncated trkB.T1 is a dominant negative inhibitor of trkB.TK+-mediated cell survival. Biophys Res Commun 2001, 280: 1352–1358.View ArticleGoogle Scholar
- Lahteninen S, Pitkanen A, Saarelainen T, Nissinen J, Koponen E, Castren E: Decreased BDNF signaling in transgenic mice reduces epileptogenesis. Eur J Neurosci 2002, 15: 721–734.View ArticleGoogle Scholar
- Luikart BW, Nef S, Shipman T, Parada LF: In vivo role of truncated trkB receptors during sensory ganglion neurogenesis. Neuroscience 2003, 117: 847–858.PubMedView ArticleGoogle Scholar
- Dorsey SG, Renn CL, Carim-Todd L, Barrick CA, Bambrick L, Krueger BK, Ward CW, Tessarollo L: In vivo restoration of physiological levels of truncated trkB.T1 receptor rescues neuronal cell death in a trisomic mouse model. Neuron 2006, 51: 21–18.PubMedView ArticleGoogle Scholar
- Carim-Todd L, Bath KG, Fulgenzi G, Yanpallewar S, Jing D, Barrick CA, Becker J, Buckley H, Dorsey SG, Lee FS, Tessarollo L: Endogenous truncated trkB.T1 receptor regulates neuronal complexity and trkB kinase receptor function in vivo. J Neurosci 2009, 29: 678–685.PubMed CentralPubMedView ArticleGoogle Scholar
- Baxter GT, Radeke MJ, Kuo RC, Makrides V, Hinkle B, Hoang R, Medina-Selby A, Coit D, Valenzuela P, Feinstein SC: Signal transduction mediated by the truncated trkB receptor isoforms, trkB.T1 and trkB.T2. J Neurosci 1997, 17: 2683–2690.PubMedGoogle Scholar
- Haapasalo A, Saarelainen T, Moshnyakov M, Arumae U, Kiema TR, Saarma M, Wong G, Castren E: Expression of the naturally occurring truncated trkB neurotrophin receptor induces outgrowth of filopodia and processes in neuroblastoma cells. Oncogene 1999, 18: 1285–1296.PubMedView ArticleGoogle Scholar
- Yacoubian TA, Lo DC: Truncated and full-length trkB receptors regulate distinct modes of dendritic growth. Nat Neurosci 2000, 3: 342–349.PubMedView ArticleGoogle Scholar
- Rose CR, Blum R, Pichler B, Lepier A, Kafitz KW, Konnerth A: Truncated trkB-T1 mediates neurotrophin-evoked calcium signaling in glia cells. Nature 2003, 426: 74–78.PubMedView ArticleGoogle Scholar
- Hartmann M, Brigadski T, Erdmann KS, Holtmann B, Sendtner M, Narz F, Lessmann V: Truncated trkB receptor-induced outgrowth of dendritic filopodia involves the p75 neurotrophin receptor. J Cell Sci 2004, 117: 5803–5814.PubMedView ArticleGoogle Scholar
- Ohira K, Kumanogoh H, Sahara Y, Homma KJ, Hirai H, Nakamura S, Hayashi M: A truncated tropomyosin-related kinase B receptor, T1, regulates glial cell morphology via Rho GDP dissociation inhibitor 1. J Neurosci 2005, 25: 1343–1353.PubMedView ArticleGoogle Scholar
- Ohira K, Homma KJ, Hirai H, Nakamura S, Hayashi M: TrkB-T1 regulates the RhoA signaling and actin cytoskeleton in glioma cells. Biochem Biophys Res Commun 2006, 342: 867–874.PubMedView ArticleGoogle Scholar
- Ferrer I, Marin C, Rey MJ, Ribalta T, Goutan E, et al.: BDNF and full-length and truncated trkB expression in Alzheimer disease: Implications in therapeutic strategies. J Neuropathol Exp Neurol 1999, 58: 729–739.PubMedView ArticleGoogle Scholar
- Dorsey SG, Bambrick LL, Balice-Gordon RJ, Krueger BK: Failure of brain-derived neurotrophic factor-dependent neuron survival in mouse trisomy 16. J Neurosci 2002, 22: 2571–2578.PubMedGoogle Scholar
- Li YX, Xu Y, Ju D, Lester HA, Davidson N, Schuman EM: Expression of a dominant negative trkB receptor, T1, reveals a requirement for presynaptic signaling in BDNF-induced synaptic potentiation in cultured hippocampal neurons. Proc Natl Acad Sci USA 1998, 95: 10884–10889.PubMed CentralPubMedView ArticleGoogle Scholar
- Maratou K, Wallace VC, Hasnie FS, Okuse K, Hosseini R, et al.: Comparison of dorsal root ganglion gene expression in rat models of traumatic and HIV-associated neuropathic pain. Eur J Pain 2009, 13: 87–98.View ArticleGoogle Scholar
- Joseph EK, Chen X, Khasar SG, Levine JD: Novel mechanism of enhanced nociception in a model of AIDS-therapy-induced painful peripheral neuropathy in the rat. Pain 2004, 107: 147–158.PubMedView ArticleGoogle Scholar
- Dorsey SG, Leitch CC, Renn CL, Lessans S, Smith BA, Wang XM, Dionne RA: Genome-wide screen identifies drug-induced regulation of the gene giant axonal neuropathy (Gan) in a mouse model of antiretroviral-induced painful peripheral neuropathy. Biol Res Nurs 2009,11(1):7–16.PubMed CentralPubMedView ArticleGoogle Scholar
- Merighi A, Salio C, Ghirri A, Lossi L, Ferrini F, Betelli C, Bardoni R: BDNF as a pain modulator. Prog Neurobiol 2008, 85: 297–317.PubMedView ArticleGoogle Scholar
- Klein R, Smeyne RJ, Wurst W, Long LK, Auerbach BA, Joyner AL, Barbacid M: Targeted disruption of the trkB neurotrophin receptor gene results in nervous system lesions and neonatal death. Cell 1993, 75: 113–122.PubMedView ArticleGoogle Scholar
- Frisen J, Verge VM, Fried K, Risling M, Persson H, Trotter J, Hokfelt T, Lindholm D: Characterization of glial trkB receptors: Differential response to injury in the central and peripheral nervous systems. Proc Natl Acad Sci USA 1993, 90: 4971–4975.PubMed CentralPubMedView ArticleGoogle Scholar
- Ren K: An improved method for assessing mechanical allodynia in the rat. Physiol Behav 1999, 67: 711–716.PubMedView ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.