Accumulation of Kv7.2 channels in putative ectopic transduction zones of mice nerve-end neuromas
© Roza et al; licensee BioMed Central Ltd. 2011
Received: 1 March 2011
Accepted: 14 August 2011
Published: 14 August 2011
Modulation of M-type currents has been proposed as a new strategy for the treatment of neuropathic pain due to their role in regulating neuronal excitability. Using electrophysiological techniques we showed previously that the opening of Kv7 channels with retigabine, blocked ectopic discharges from axotomized fibers but did not alter transduction at intact skin afferents. We hypothesized that after nerve damage, accumulation of Kv7 channels in afferent fibers may increase M-type currents which then acquired a more important role at regulating fiber excitability.
In this study, we used an immunohistochemical approach to examine patterns of expression of Kv7.2 channels in afferent fibers after axotomy and compared them to patterns of expression of voltage gated Na+ channels (Nav) which are key electrogenic elements in peripheral axons known to accumulate in experimental and human neuromas.
Axotomy induced an enlargement and narrowing of the nodes of Ranvier at the proximal end of the neuroma together with a dramatic demyelination and loss of structure at its distal end in which naked accumulations of Nav were present. In addition, axotomy also induced accumulations of Kv7.2 that co-localized with those of Nav channels.
Whilst Nav channels are mandatory for initiation of action potentials, (i.e. responsible for the generation/propagation of ectopic discharges) an increased accumulation of Kv7.2 channels after axotomy may represent a homeostatic compensation to over excitability in axotomized fibers, opening a window for a peripheral action of M-current modulators under conditions of neuropathy.
Recent data from our laboratory showed that the specific Kv7 channel opener, retigabine, blocked ectopic discharges recorded from single axotomized fibers in response to stimulation of the neuroma . This action was not modality specific but rather dependent upon hyperpolarization. However, modulation of M-currents did not change responses to stimulation of intact skin receptors. On the basis of these experimental observations we proposed that the prominent role of M-type currents newly acquired after axotomy emerged as a consequence of the accumulation of Kv7 channels in the vicinity of newly formed transduction zones known to occur at neuromatose endings.
Nerve damage has been shown to produce local demyelination and disruption of the molecular organization of nodes  characterized by large accumulations of voltage gated Na+ channels (Nav) which are essential for the initiation of action potentials [3–6]. M-type currents have an important role stabilizing membrane potential at rest  and preliminary data suggest that ion channel rearrangement at ectopic areas could include an altered expression of Kv7 subunits .
Several lines of evidence suggest the presence of Kv7 channels/M-type currents at specialized receptor endings [9, 10] and at nodes of Ranvier co-localizing with Nav channels [11, 12]. Although there is no data on the distribution of Kv7 channels along unmyelinated axons, they are present in small diameter dorsal root ganglion neurons [13, 14]. Kv7.2 is the subunit predominantly expressed in nodes of Ranvier [11, 12, 15] and may form homomeric channels to yield particular M-type currents .
Here we have examined Kv7.2 channel reorganization at aberrant transduction zones produced by saphenous nerve sectioning to further understand the enhanced role of M-type currents after axotomy. To this end, we have used Nav channel labeling as a marker of nodes and putative transduction zones formed after axotomy.
Findings and discussion
Antibodies, dilutions and sources
Manufacturer, species antibody was raised in, mono- vs. Polyclonal
1500-1518 aa of the α-subunit of Voltage Gated Sodium Channels
The epitope for sc-49303 maps within amino acids 50-100 in the internal region of Myelin P2 of human origin (accession #P02689)
Santa Cruz, Goat,
Fusion protein 1308-1381 (cytoplasmic domain) of rat Caspr (accession number #P97846)
NeuroMab (UC Davis, Davis CA), mouse monoclonal, clone K65/35
Fusion protein 1066-1174 (intracellular domain common to NF -155 and NF-186) of rat NF-155 (accession number AAL27854)
NeuroMab (UC Davis, Davis CA), mouse monoclonal, clone L11A/41
aa 578-593 at the C-terminal domain
Alomone, rabbit, polyclonal
aa 578-593 at the C-terminal domain
Chemicon, rabbit, polyclonal
aa 578-593 at the C-terminal domain
Sigma, rabbit, polyclonal
C-18; The epitope for sc-7792 maps within the last 50 amino acids at the C-terminus of KCNQ2 of human origin (accesion #O43526)
Santa Cruz, goat,
N-19; The epitope for sc-7793 maps within the first 50 amino acids at the N-terminus of KCNQ2 of human origin (accession #O43526)
Santa Cruz, goat,
Measurements on nodes of Ranvier
Node Caliber (μm)
1.9 ± 0.06
1.1 ± 0.05*
1.5 ± 0.05 #
0.8 ± 0.02*
Node Length (μm)
1.7 ± 0.05
3.3 ± 0.2 *
1.9 ± 0.05 #
3.4 ± 0.13*
The data presented here confirms previous observations on the disruption of axonal organization following axotomy [2, 6]. The original contribution of the present work is the confirmation of an abundant expression of Kv7.2 channels in newly formed aberrant loci, which also express abundant Nav channels and presumably represent newly transduction zones [4, 18, 19].
At the distal end of the neuroma, we observed severe loss of myelin and nerve organization where the development of naked accumulations of Nav and Kv7.2 becomes a salient feature. In the proximal end of the neuroma, we observed a significant two-fold widening of the nodes together with a decreased node caliber which still expressed Nav or Kv7.2 channels in a continuous form. Large increases of nodal length have been reported for tissue samples obtained from multiple sclerosis patients . Bostock and Sears  reported continuous conduction from chemically demyelinited rat spinal roots, demonstrating that bared internodal axons can become excitable. This feature is likely due to the presence of Nav channels at the site of injury and also in newly formed nodes of Ranvier during remyelination . Furthermore, Nav channel accumulation has been also described in fibers from human painful neuromas  and painful human dental pulp . A recent study demonstrated that 1/3 of the patients that underwent surgical repair after peripheral nerve section and still suffer from chronic pain, exhibited a significant slowing in the conduction velocities of their repaired nerves as compared to pain-free patients, probably due to less successful nerve regeneration . These series of events reinforce the connection of Nav channel clusters with ectopic loci and pain.
Schwarz  reported that Kv7.2 was present in nodes of fibers from all diameters, and, although no co-staining with Nav was performed, the staining pattern of both Kv7.2 and Nav were similar. Our present results confirm these previous observations and, in addition, show that following axotomy, the occurrence of co-localization of Nav and Kv7.2 channels increase considerably (1.75-fold increase) in proximal areas of the neuroma where nerve structure and myelin are relatively conserved. Nav and Kv7.2 channels share an ankyrin-G domain for membrane retention , and elevated levels of ankiryn-G have been reported in samples from human painful neuromas . This is supposed to facilitate Nav channel insertion into the axon membrane  and may facilitate the insertion of Kv7.2 channels as well.
The present immunohistochenical data, indicate the presence of Kv7.2 channels in a proportion of nodes of the intact nerve as reported by others [11, 12, 15], although M-type currents seem to have only a minor role at coding acute nociceptive stimuli of cutaneous origin . After axotomy, there is an accumulation of Kv7.2 channels which parallels that of Nav channels in de-myelinated areas thought to constitute ectopic loci [3, 19]. A concentration of Kv7.2 channels is likely to result in larger M-like currents which then acquire a more preponderant role at controlling axonal excitability. These results open up the possibility of interacting with peripheral Kv7.2 channels to attenuate neuropathic pain symptoms.
Adult outbreed CD1 mice of both sexes were used. European Union and State legislation for the regulation of animal experiments were followed and the local Animal Care Facility approved the experimental protocols. Nerve-end neuromas were produced by total section of both saphenous nerves in mice following the methods previously described . Under deep anesthesia with isofluorane (~3.5-4% in pure O2) and with sterile precautions, the saphenous nerve was exposed at the level of the mid-thigh, dissected free and tightly ligated with 8-0 silk. The nerve was cut distal to the ligature and the cut end inserted into a 3 mm long silicone tube (0.45 mm internal diameter) to prevent lateral innervation of surrounding tissue. The tube was tied in place with the same piece of silk and a ~2 mm piece of the distal nerve stump was excised to prevent reinnervation. The incision was closed. The animals were housed in groups of two to four and inspected periodically for infections or abnormal behavior. The mice had access to water and food ad libitum. Saphenous nerves from naive mice were used as controls.
Animals were perfused with cold PBS and either intact nerves or ~4 week old nerve end-neuromas were extracted, embebed in OCT (Tissue-Tek) and submerged in acetone previously cooled in dry ice for a minimum of 30 minutes. Preparations were placed in cold PBS solution and fibres were teased out from samples, transferred to Superfrost poly-L-lysine coated slides (SuperFrost-Plus, Menzel-Glaser) and allowed to dry. In each slide, teased samples from normal nerves and nerve-end neuromas were present to minimize potential inter-staining variability. Slides were stored at -20°C until used for staining with antibodies.
Slides were permeabilized by immersion in -20°C acetone for 10 minutes. After washing with PBS, non-specific binding sites were blocked for 1 hour at room temperature (TBS containing 5% gelatin fish, 0.5% Triton X-100). Slides were incubated overnight at 4°C in a humidified chamber with primary antibodies (see Table 1) in blocking buffer (0.2% of Triton X-100). Slides were then washed and incubated for 2 hours at room temperature with the appropriate secondary antibodies (cyanine-2-conjugated and cyanine-5-conjugated diluted 1:800 and 1:200, respectively, Jackson Immunoresearch Labs) in blocking buffer. After washing, slides were cover-slipped using Fluor-Save antifade reagent. These methods have been adapted from previous relevant publications in the literature [11, 12].
Tissue specimens were evaluated with an Olympus Fluorescence Microscope. Olympus software Cell-R was used for acquisition of images. Final image processing for illustration purposes (i.e. scales, improve in brightness and framing) was done with Adobe Photoshop.
This work was supported by the Spanish Ministry of Education and Science (SAF 2009-07876). The authors would like to thank Ivan Rivera-Arconada for helpful comments on the manuscript and David Vega-Avelaira for his help with the figures.
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