Sensitization of spinal cord nociceptive neurons with a conjugate of substance P and cholera toxin
© Caudle et al; licensee BioMed Central Ltd. 2007
Received: 21 December 2006
Accepted: 10 May 2007
Published: 10 May 2007
Several investigators have coupled toxins to neuropeptides for the purpose of lesioning specific neurons in the central nervous system. By producing deficits in function these toxin conjugates have yielded valuable information about the role of these cells. In an effort to specifically stimulate cells rather than kill them we have conjugated the neuropeptide substance P to the catalytic subunit of cholera toxin (SP-CTA). This conjugate should be taken up selectively by neurokinin receptor expressing neurons resulting in enhanced adenylate cyclase activity and neuronal firing.
The conjugate SP-CTA stimulates adenylate cyclase in cultured cells that are transfected with either the NK1 or NK2 receptor, but not the NK3 receptor. We further demonstrate that intrathecal injection of SP-CTA in rats induces the phosphorylation of the transcription factor cyclic AMP response element binding protein (CREB) and also enhances the expression of the immediate early gene c-Fos. Behaviorally, low doses of SP-CTA (1 μg) injected intrathecally produce thermal hyperalgesia. At higher doses (10 μg) peripheral sensitivity is suppressed suggesting that descending inhibitory pathways may be activated by the SP-CTA induced sensitization of spinal cord neurons.
The finding that stimulation of adenylate cyclase in neurokinin receptor expressing neurons in the spinal cord produces thermal hyperalgesia is consistent with the known actions of these neurons. These data demonstrate that cholera toxin can be targeted to specific cell types by coupling the catalytic subunit to a peptide agonist for a g-protein coupled receptor. Furthermore, these results demonstrate that SP-CTA can be used as a tool to study sensitization of central neurons in vivo in the absence of an injury.
Several groups have developed potential therapeutics that produce highly selective lesions in vivo by exploiting specific g-protein coupled receptors (GPCRs) as transporters to deliver a toxin to an intracellular target [1–9]. When GPCRs bind a peptide agonist they are internalized by the cell; delivering the peptide, and any attached toxin, to the inside of the cell . The toxin is then able to act on its intracellular target. Some of these investigators have used lethal toxins such as saporin, diphtheria and pseudomonas exotoxin coupled to the neuropeptide substance P to target the agents to cells expressing neurokinin receptors [1, 3, 7–9, 11]. These toxins produce highly specific lesions of neurokinin receptor expressing cells while not harming cells in the region that do not express these receptors. The investigators have also demonstrated by ablating these cells that neurons expressing the NK1 receptor in the spinal cord are required for central sensitization. Thus, these targeted toxins were found to be valuable tools for evaluating the function of neurons in the central nervous system [2–4, 12]. Moreover, it has been suggested that these targeted toxins may have clinical utility for the treatment of intractable pain.
In an effort to compliment the armamentarium of targeted toxins we sought to selectively activate, rather than kill, neurokinin receptor expressing cells by coupling cholera toxin to the neuropeptide substance P. Cholera toxin, unlike previously used toxins, is not universally lethal to the cells. The toxin is useful because it ADP ribosylates the g-protein Gs, which results in the uncoupling of the protein from GPCRs and activation of the g-protein [13–16]. Cholera toxin activation of Gs stimulates adenylate cyclase activity to produce higher levels of cAMP in the cells, altered protein kinase activity and altered ion channel activity [13, 16–21]. Thus, we hypothesized that a conjugate of substance P and the catalytic subunit of cholera toxin (SP-CTA) would selectively activate neurokinin receptor expressing neurons and would provide a novel tool for evaluating cell function in vivo.
Synthesis of SP-CTA
In situ evaluation of SP-CTA
We further evaluated the functionality of the SP-CTA by examining the ability of the conjugate to stimulate cAMP production in CHO-NK1 cells. Figure 2B demonstrates the concentration response relationship for SP-CTA (0.1 ng/ml (3.4 pM) to 1,000 ng/ml (34 nM)) when the SP-CTA was applied for 1 hour and the cAMP was measured after culturing the cells for an additional 24 hours. The time course of SP-CTA's effect on cAMP production was evaluated by treating the cells for 1 hour with 100 ng/ml SP-CTA and then harvesting the cells for cAMP analysis 1, 2, 3 and 4 days following exposure to the conjugate. As demonstrated in figure 2C, SP-CTA's effects on cAMP peaked at 1 day in the CHO-NK1 cells and remained significantly elevated for 3 days.
Figure 2D demonstrates that 48 hours following a 4 hour exposure to 100 ng/ml of either substance P, CTA or SP-CTA only SP-CTA treated CHO-NK1 cells produced an increase in cAMP production. When 100 ng/ml of substance P, CTA or SP-CTA remained in the culture media for the full 48 hours the substance P treated cells and the SP-CTA treated cells had significantly elevated levels of cAMP, whereas the CTA treated cells did not differ from control cells (Figure 2E). The findings illustrated in figures 2D and 2E indicate that substance P activation of NK1 receptors can stimulate adenylate cyclase, but that the effects of SP-CTA on cAMP production outlast any stimulation of the NK1 receptors produce by the substance P portion of the conjugate when the material is washed out of the culture media. The ability of substance P to stimulate cAMP production was previously demonstrated in several different cell types including CHO-NK1 cells [23–25], which is consistent with our findings.
In vivo evaluation of SP-CTA
Previously used lethal toxins targeted to GPCRs via coupling to a peptide agonist were useful for evaluating the function of the cells that expressed the GPCRs. By using the receptor to direct the toxins to the desired cell type the agents produced highly specific lesions, even when the targeted cells were a minor constituent of a heterogeneous population of cells [1–9, 11, 12]. However, because these toxins kill the targeted cells, the cells' function must be inferred by the deficit that is produced by the lesion. Ideally, a method to selectively stimulate the cells could provide more information about the cells' function. To achieve this goal we coupled the neuropeptide substance P, which targets neurokinin receptors [2, 3, 8], to the catalytic subunit of cholera toxin (SP-CTA). Cholera toxin is not lethal when taken up by cells, but it does enhance the activity of adenylate cyclase resulting in an increase in intracellular cAMP . Thus, by targeting cholera toxin to neurokinin receptor bearing neurons with substance P we altered the function of these cells rather than killing them.
We found that SP-CTA produced equivalent effects on cAMP production in cultured cells expressing NK1 or NK2 receptors, but did not influence cAMP production in cells that expressed NK3 receptors or non-transfected cells. These data suggest that the conjugate binds to NK1 and NK2 receptors with approximately the same affinity. Takeda and colleagues previously demonstrated that substance P had a ten fold higher affinity for NK1 than NK2 receptors . It is possible that the presence of CTA on the N terminus of substance P reduces the NK1 selectivity of the peptide. Alternatively, since we used a functional assay, any affinity differences between the receptors may have been obscured by uptake mechanisms and by the action of the toxin on Gs. For example, the association rate of substance P is much higher for NK1 than NK2 receptors . Because of the higher binding rate the NK1 receptors may be internalized faster than the NK2 receptors. However, if the NK1 receptors do not rapidly recycle to the cell surface the NK2 receptors will eventually bind and internalize an equal amount of SP-CTA. Thus our functional assay would negate the differences in binding affinity of substance P for the NK1 and NK2 receptors. Additionally, because cholera toxin stimulates cAMP production and protein kinase A activity it is possible that due to excessive phosphorylation the neurokinin receptors are trapped in the internalized state making the receptors unavailable to transport more toxin. This process could also mask any binding affinity differences.
In vivo our data demonstrate that when SP-CTA was injected intrathecally in rats the animals became hypersensitive to thermal stimuli (Figure 6A and 6B). This data supports the previous lesion studies indicating that central sensitization is mediated by NK1 receptor expressing neurons in the spinal cord [1–3, 6]. We further found that with higher doses of SP-CTA peripheral sensitivity was suppressed (Figure 6A). However, the animals demonstrated behaviors that suggested centrally mediate nociception. We hypothesize that the suppression of peripheral hypersensitivity was due to stimulation of inhibitory pathways by the NK1 receptor expressing neurons that have taken up the SP-CTA. These inhibitory systems may be previously characterized descending inhibitory pathways  or local inhibitory neurons. Further work should clarify this finding. These data, however, support the idea that selectively stimulating cells provides unique information on their function that is not available when a lesioning strategy is used. What is most notable about these results is that the inhibition of peripheral nociception could not have been predicted from several previous lesioning studies [1–3, 6]. However, when specifically examining descending inhibition Suzuki and colleagues' found that lesioning of NK1 expressing neurons in the spinal cord suppressed peripheral afferent stimulated descending inhibition , which is consistent with our findings.
The effects of intrathecally administered SP-CTA also contrast with the effects produced by intrathecal wild type cholera toxin. In a previous study wild type cholera toxin inhibited the hyperalgesia and allodynia produced by a variety of peripheral injuries . The inhibitory effect of the toxin was blocked by the opioid antagonist naloxone. The mechanism that produced this endogenous opioid mediated inhibition of nociception was not investigated, but work by Shen and Crain indicates that cholera toxin can enhance the inhibitory effects of opioids on primary afferent neurons . Thus, it can be hypothesized that wild type cholera toxin acts principally on primary afferent neurons to inhibit peripherally mediated nociception; while SP-CTA affects NK1 receptor expressing spinothalamic tract neurons to generate centrally mediated nociception.
We have synthesized a novel tool for activating adenylate cyclase in neurokinin receptor expressing cells to evaluate the function of these cells in heterogeneous populations, as are typically found in vivo. Intrathecal injections of SP-CTA in rats demonstrated that the toxin conjugate produced thermal hyperalgesia as would be expected if NK1 receptor expressing spinothalamic tract neurons were sensitized by the treatment. These data indicate that GPCRs can be exploited to transport cholera toxin into a host of different cell types. Because the toxin is targeted to the GPCR by a peptide agonist it is remarkably simple to change the peptide to direct the conjugate to receptors other than neurokinin receptors. These targeted cholera toxin conjugates could find utility in a number of biomedical research endeavors.
Synthesis of SP-CTA
The A subunit of cholera toxin (CTA) was purchased from List Biological Laboratories inc. (Cambell, CA, USA). CTA has two cysteine residues in the C-terminal region [15, 31] therefore these cysteine residues were used to attach substance P to CTA. The synthesis of SP-CTA was accomplished using a modification of Pierce Biotechnology inc.'s maleimide protein cross-linking procedure. The synthesis was carried out in two stages. The first stage was to link maleimide to the N-terminus of substance P by combining a 5 fold excess of Sulfosuccinimidyl 4-N-maleimidomethyl cyclohexane-1-carboxylate (Sulfo-SMCC) with substance P in phosphate buffered saline (PBS, pH 7.4). The mixture was incubated at room temperature for 1 hour. The substance P maleimide conjugate was separated from unreacted Sulfo-SMCC using a Sephadex G-10 (30 × 1.5 cm) column eluted with PBS. For the second phase of the synthesis the substance P maleimide conjugate was linked to the two cysteines on CTA by adding a 10 fold excess of the conjugate to CTA in PBS. This mixture was then incubated at room temperature for another hour. The SP-CTA was separated from the unreacted substance P maleimide conjugate, washed with PBS three times and concentrated using Centricon Plus-20 filters. A sample of the final product was evaluated by western blots. Briefly, the sample was run on 4–20% polyacrylamide electrophoresis gels, transferred to Polyvinylidene fluoride (PVDF) membranes and then probed with antibodies to either substance P or the catalytic subunit of cholera toxin. A secondary antibody coupled to horse radish peroxidase (HRP) and enhanced chemiluminescence were used to visualize the bands. Figure 1 illustrates the synthetic pathway as well as western blots of the final product.
Chinese Hamster Ovary cells stably expressing NK1, NK2 or NK3 receptors (a generous gift from Dr. James Krause, Neurogen Corp. ) were plated on 100 mm plates for cAMP assays or 13 mm cover slips in 24 well culture plates for immunocytochemistry experiments. The cultures were grown in F12K media, 10% Fetal Bovine Serum, 1% L-glutamine, 1% penicillin-Streptomyosin, 25 mM Hepes buffer, and G418 (500 μg/ml). The cells were cultured at 37°C in a 5% CO2 atmosphere. Additionally, plasmids containing either NK1 or NK2 receptors were purchased (UMR cDNA Resource Center, Rolla, MO, USA) and transfected into CHO cells using Lipofectamine (Invitrogen, Carlsbad, CA, USA) as per the manufacturer's instructions. These cells were cultured as described for the stably transfected cells.
To assay cAMP levels in the cell cultures Sigma inc's (St. Louis, MO, USA) Direct cAMP Enzyme Immunoassay was used according to the manufacturer's instructions. Briefly, the media on the cell cultures was removed and the cells were washed once with PBS (pH 7.4). The PBS was removed and 1 ml 0.1 M HCL was added to the cells. The cells were scraped from the plates into the HCL solution, sonicated and centrifuged (600 g, 10 minutes, 5°C) and the supernatant collected. The protein in each sample was measured using Bio-Rad's (Hercules, CA, USA) protein assay. The cAMP was acetylated with the addition of 100 μl of the kit's acetic anhydride solution. A 100 μl sample was then neutralized with 50 μl of the kit's neutralizing buffer and the samples were added to the kit's 96 well plates that were pre-absorbed with antibodies to cAMP. A standard curve and controls were set up as suggested by the manufacturer. A cAMP- alkaline phosphatase conjugate (50 μl) was added to the wells and the solution was incubated for 2 hours. The plates were then washed 3 times and 200 μl of p-nitrophenyl phosphate solution (substrate) was added to each well and the plates were incubated for 1 hour. The reaction was stopped with 50 μl 0.1 M HCL and the plate was read at 405 nm. The concentration of cAMP in the samples was extrapolated from the data collected for the cAMP standards and expressed as the number of moles of cAMP per mg protein.
Male Sprague Dawley rats (200 – 300 g) were housed in pairs and supplied standard rat chow and water ad libitum in the University of Florida's vivarium, which is an AAALAC certified facility.
Intrathecal injections were performed under isoflurane anesthesia via lumbar puncture between L4 and L5. All animal procedures in this project were reviewed and approved by the University of Florida's Institutional Animal Care and Use Committee.
Immunocytochemistry and immunohistochemistry
Rats were euthanized with pentobarbital and immediately transcardially perfused with ice cold PBS and then ice cold 4% paraformaldehyde in phosphate buffered saline (PBS)(pH 7.4). Cell cultures on cover slips were washed with PBS and fixed with 4% paraformaldehyde in PBS. The spinal cords were removed and post fixed overnight in 4% paraformaldehyde in PBS. The tissue was cryoprotected in 30% sucrose, mounted and sectioned in a cryostat (-20°C)(10–20 μm) and mounted on slides. The sections or cells were then blocked with 3% normal goat serum for 60 minutes with 0.75% triton X-100. The primary antibody was added to the blocking solution (1:500) and the sections were incubated for 48 hours at 4°C. The sections or cells were then washed (8 × 5 mins) in PBS. Following the wash the sections or cells were incubated for 1 hour at room temperature in PBS with a secondary antibody that was coupled to horse radish peroxidase (HRP), Alexa Fluro 594 or Alexa Fluro 488 (Molecular Probes, Boston, MA). The sections labeled with the Alexa Fluro stains were washed (8 × 5 mins) and viewed using fluorescence microscopy. The HRP label sections were washed similarly and treated with diaminobenzidine. The cultured cells, which were treated with HRP coupled secondary antibodies, were washed similarly and labeled using rhodamine labeled tyramide as described by the manufacturer (NEN, Boston, MA, USA). Antibodies to c-Fos were obtained from Dr. Michael Iadarola (National Institute of Dental and Craniofacial Research, National Institutes of Health) , antibodies to CTA were purchased from Sigma (St. Louis, MO), antibodies to pCREB were purchased from New England Biolabs (Ispwich, MA) and antibodies to NK1 receptor were purchased from Zymed (San Francisco, CA).
Thermal nociception assay
Thermal nociception was measured using the method of Hargreaves et al. . Briefly, the rats were placed on a clear glass surface and allowed 15 minutes to accommodate to the enclosure. An infrared light was directed onto a hind paw's plantar surface approximately in the middle of the foot. The latency for the animal to remove its foot from the path of the light was used as the dependent measure for thermal sensitivity. The test was performed 4 times for each time point (2 tests for each hind foot) with each test separated by 2 minutes to prevent paw sensitization. The withdrawal latencies of the tests were averaged to obtain the final value for that time point.
Data were analyzed using a one way ANOVA followed by Dunnett's post-hoc test, one way repeated measures ANOVA or two way ANOVA as appropriate. Significance was assigned to p ≤ 0.05.
This work was supported by the National Institute on Drug Abuse, National Institutes of Health, DA016562. The authors would also like to thank Daniel Martinez and Anthony Carter for their excellent technical assistance.
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