Ionic permeability of the frog sciatic nerve perineurium: parallel studies of potassium and lanthanum penetration using electrophysiological and electron microscopic techniques

Group IV nociceptors develop axonal chemical sensitivity during neuritis and following treatment of the sciatic nerve with vinblastine

We have previously shown that nerve inflammation (neuritis) and transient vinblastine application lead to axonal mechanical sensitivity in nociceptors innervating deep structures. We also have shown that these treatments reduce axonal transport and have proposed that this leads to functional accumulation of mechanically sensitive channels in the affected part of the axons. Though informing the etiology of mechanically induced pain, axonal mechanical sensitivity does not address the common report of ongoing radiating pain during neuritis, which could be secondary to the provocation of axonal chemical sensitivity. We proposed that neuritis and vinblastine application would induce sensitivities to noxious chemicals and that the number of chemo-sensitive channels would be increased at the affected site. In adult female rats, nerves were either untreated or treated with complete Freund's adjuvant (to induce neuritis) or vinblastine. After 3-7 days, dorsal root teased fiber recordings were taken from group IV neurons with axons within the sciatic nerve. Sciatic nerves were injected intraneurally with a combination of noxious inflammatory chemicals. Whereas no normal sciatic axons responded to this stimulus, 80% and 38% of axons responded in the neuritis and vinblastine groups, respectively. In separate experiments, sciatic nerves were partially ligated and treated with complete Freund's adjuvant or vinblastine (with controls), and after 3-5 days were immunolabeled for the histamine H₃ receptor. The results support that both neuritis and vinblastine treatment reduce transport of the histamine H₃ receptor. The finding that nociceptor axons can develop ectopic chemical sensitivity is consistent with ongoing radiating pain due to nerve inflammation.NEW & NOTEWORTHY Many patients suffer ongoing pain with no local pathology or apparent nerve injury. We show that nerve inflammation and transient application of vinblastine induce sensitivity of group IV nociceptor axons to a mixture of endogenous inflammatory chemicals. We also show that the same conditions reduce the axonal transport of the histamine H₃ receptor. The results provide a mechanism for ongoing nociception from focal nerve inflammation or pressure without overt nerve damage.

Redox-sensitive structure and function of the first extracellular loop of the cell-cell contact protein claudin-1: lessons from molecular structure to animals

The paracellular cleft within epithelia/endothelia is sealed by tight junction (TJ) proteins. Their extracellular loops (ECLs) are assumed to control paracellular permeability and are targets of pathogenes. We demonstrated that claudin-1 is crucial for paracellular tightening. Its ECL1 is essential for the sealing and contains two cysteines conserved throughout all claudins.

AIMS

We prove the hypothesis that this cysteine motif forms a redox-sensitive intramolecular disulfide bridge and, hence, the claudin-1-ECL1 constitutes a functional structure which is associated to ECLs of this and other TJ proteins.

RESULTS

The structure and function of claudin-1-ECL1 was elucidated by investigating sequences of this ECL as synthetic peptides, C1C2, and as recombinant proteins, and exhibited a β-sheet binding surface flanked by an α-helix. These sequences bound to different claudins, their ECL1, and peptides with nanomolar binding constants. C-terminally truncated C1C2 (-4aaC) opened cellular barriers and the perineurium. Recombinant ECL1 formed oligomers, and bound to claudin-1 expressing cells. Oligomerization and claudin association were abolished by reducing agents, indicating intraloop disulfide bridging and redox sensitivity.

INNOVATION

The structural and functional model based on our in vitro and in vivo investigations suggested that claudin-1-ECL1 constitutes a functional and ECL-binding β-sheet, stabilized by a shielded and redox-sensitive disulfide bond.

CONCLUSION

Since the β-sheet represents a consensus sequence of claudins and further junctional proteins, a general structural feature is implied. Therefore, our model is of general relevance for the TJ assembly in normal and pathological conditions. C1C2-4aaC is a new drug enhancer that is used to improve pharmacological treatment through tissue barriers.

Abnormal junctions and permeability of myelin in PMP22-deficient nerves

OBJECTIVE

The peripheral myelin protein-22 (PMP22) gene is associated with the most common types of inherited neuropathies, including hereditary neuropathy with liability to pressure palsies (HNPP) caused by PMP22 deficiency. However, the function of PMP22 has yet to be defined. Our previous study has shown that PMP22 deficiency causes an impaired propagation of nerve action potentials in the absence of demyelination. In the present study, we tested an alternative mechanism relating to myelin permeability.

METHODS

Utilizing Pmp22(+) (/) (-) mice as a model of HNPP, we evaluated myelin junctions and their permeability using morphological, electrophysiological, and biochemical approaches.

RESULTS

We show disruption of multiple types of cell junction complexes in peripheral nerve, resulting in increased permeability of myelin and impaired action potential propagation. We further demonstrate that PMP22 interacts with immunoglobulin domain-containing proteins known to regulate tight/adherens junctions and/or transmembrane adhesions, including junctional adhesion molecule-C (JAM-C) and myelin-associated glycoprotein (MAG). Deletion of Jam-c or Mag in mice recapitulates pathology in HNPP.

INTERPRETATION

Our study reveals a novel mechanism by which PMP22 deficiency affects nerve conduction not through removal of myelin, but through disruption of myelin junctions.

Assessing the permeability of the rat sciatic nerve epineural sheath against compounds with local anesthetic activity: an ex vivo electrophysiological study

Abstract Studies have shown that the sciatic nerve epineural sheath acts as a barrier and has a delaying effect on the diffusion of local anesthetics into the nerve fibers and endoneurium. The purpose of this work is to assess and to quantify the permeability of the epineural sheath. For this purpose, we isolated the rat sciatic nerve in a three-chamber recording bath that allowed us to monitor the constant in amplitude evoked nerve compound action potential (nCAP) for over 24 h. For nerves exposed to the compounds under investigation, we estimated the IT50 the time required to inhibit the nCAP to 50% of its initial value. For desheathed nerves, the half-vitality time was denoted as IT50(-) and for the ensheath normal nerves as IT50(+). There was no significant difference between the IT50 of desheathed and ensheathed nerves exposed to normal saline. The IT50(-) for nerves exposed to 40 mM lidocaine was 12.1 ± 0.95 s (n=14) and the IT50(+) was 341.4 ± 2.49 s (n=6). The permeability (P) coefficient of the epineural sheath was defined as the ratio IT50(+)/IT50(-). The P coefficient for 40 mM lidocaine and linalool was 28.2 and 3.48, correspondingly, and for 30 mM 2-heptanone was 4.87. This is an indication that the epineural sheath provided a stronger barrier against lidocaine, compared to natural local anesthetics, linalool and 2-heptanone. The methodology presented here is a useful tool for studying epineural sheath permeability to compounds with local anesthetic properties.

A peptidomimetic tight junction modulator to improve regional analgesia

The paracellular flux of solutes through tissue barriers is limited by transmembrane tight junction proteins. Within the family of tight junction proteins, claudin-1 seems to be a key protein for tightness formation and integrity. In the peripheral nervous system, the nerve fibers are surrounded with a barrier formed by the perineurium which expresses claudin-1. To enhance the access of hydrophilic pharmaceutical agents via the paracellular route, a claudin-1 specific modulator was developed. For this purpose, we designed and investigated the claudin-1 derived peptide C1C2. It transiently increased the paracellular permeability for ions and high and low molecular weight compounds through a cellular barrier model. Structural studies revealed a β-sheet potential for the functionality of the peptide. Perineurial injection of C1C2 in rats facilitated the effect of hydrophilic antinociceptive agents and raised mechanical nociceptive thresholds. The mechanism is related to the internalization of C1C2 and to a vesicle-like distribution within the cells. The peptide mainly colocalized with intracellular claudin-1. C1C2 decreased membrane-localized claudin-1 of cells in culture and in vivo in the perineurium of rats after perineurial injection. In conclusion, a novel tool was developed to improve the delivery of pharmaceutical agents through the perineurial barrier by transient modulation of claudin-1.

A modified technique for the impregnation of lanthanum tracer to study the integrity of tight junctions on cells grown on a permeable substrate

Ionic lanthanum is commonly used to trace permeability pathways across epithelia and endothelia in biological electron microscopy. A method for obtaining a uniformly dense precipitate of lanthanum is described. The method, which is a modification of the technique described by Shaklai and Tavassoli (1977) was suitable for fixation of cell cultures grown on permeable filter inserts and was successfully applied to study opening of tight junctions by hypertonic solutions in the airway epithelial cell line 16HBE14o(-). The preparation method formed the basis for a semiquantitative morphological determination in which the tight junctions were subdivided as "intact," "weakened," and "open." By using this modified technique, it could be demonstrated that opening of tight junctions in airway epithelial cells increased, with increasing osmolarity with electrolytes having a stronger effect than nonelectrolytes. A significant linear relationship was found between the osmolarity of the medium and the open state of the tight junctions (as determined by the semiquantitative morphological technique) or the transepithelial electrical resistance.

Tight junction proteins ZO-1, occludin, and claudins in developing and adult human perineurium

In peripheral nerves, groups of Schwann cell-axon units are isolated from the adjacent tissues by the perineurium, which creates a diffusion barrier responsible for the maintenance of endoneurial homeostasis. The perineurium is formed by concentric layers of overlapping, polygonal perineurial cells that form tight junctions at their interdigitating cell borders. In this study, employing indirect immunofluorescence and immunoelectron microscopy, we demonstrate that claudin-1 and -3, ZO-1, and occludin, but not claudin-2, -4, and -5, are expressed in the perineurium of adult human peripheral nerve. We also describe the expression of occludin, ZO-1, claudin-1, -3, and -5 in the developing human perineurium, showing that the expressions of claudin-1 and -3, ZO-1, and occludin follow similar spatial developmental expression patterns but follow different timetables in achieving their respective adult distributions. Specifically, claudin-1 is already largely restricted to perineurium-derived structures at 11 fetal weeks, whereas claudin-3 and occludin are weakly expressed in the perineurial structures at this age and acquire a well-defined perineurial distribution only between 22 and 35 fetal weeks. ZO-1 appears to acquire its mature profile even later during the third trimester. The results of the present and previous studies show that the perineurial diffusion barrier matures relatively late during human peripheral nerve development.

Excitatory Neurotoxic Properties of Pontamine Sky Blue Make It a Useful Tool for Examining the Functions of Focal Brain Parts

Pontamine sky blue (PSB) is used in brain studies to mark the position of microelectrode and micropipette tips. However, few studies have been made on the effects of PSB on neurons; therefore we examined these effects. When puffed on isolated sensory ganglion cells of rats, PSB increased membrane conductance, depolarized membrane potential, and reduced the amplitude of action potentials. When dripped on frog sympathetic ganglion, much like hexamethonium, PSB decreased the amplitude of compound action potentials of the postganglionic strand. A bath application of PSB to sartorius muscle fibers that had been treated with tetrodotoxin depolarized the membrane potential and increased the frequency and amplitude of miniature end-plate potentials. All these effects were reversible. When injected into the rat's pontine part corresponding to the location of the canine pontine defecation reflex center, PSB produced repetitive colorectal contractions and irreversibly abolished them in response to anal-canal stimulation. The excitatory and blocking effects of PSB and its staining ability make it a useful tool for examining the functions of focal brain parts.