The role and mechanism of the up-regulation of fibrinolytic activity in painful peripheral nerve injury

Tissue plasminogen activator and neuropathy open the blood-nerve barrier with upregulation of microRNA-155-5p in male rats

The blood-nerve barrier (BNB) consisting of the perineurium and endoneurial vessels is sealed by tight junction proteins. BNB alterations are a crucial factor in the pathogenesis of peripheral neuropathies. However, barrier opening, e.g. by tissue plasminogen activator (tPA), can also facilitate topical application of analgesics. Here, we examined tPA both in the pathophysiology of neuropathy-induced BNB opening or via exogenous application and its effect on the cytoplasmatic tight junction protein anchoring protein, zona occludens-1 (ZO-1), the adherens molecule JAM-C and microRNA(miR)-155-5p. Specifically, we investigated whether tPA alone and barrier opening lead to pain behavioral changes, i.e. hyperalgesia, or whether these effects require further factors. Male Wistar rats underwent chronic constriction injury (CCI) or were treated by a single perisciatic application of recombinant (r)tPA. CCI elicited mechanical allodynia, tPA mRNA upregulation, macrophage invasion, BNB leakage for large molecule tracers, downregulation of ZO-1 and JAM-C mRNA/protein, and a loss of immunoreactivity of both in perineurium and endoneurial cells. Similarly, after perisciatic rtPA injection, ZO-1 and JAM-C mRNA as well as cytosolic/membrane protein and ZO-1 immunoreactivity were downregulated, and the BNB was opened. Neither mechanical hypersensitivity nor macrophage infiltration was observed after rtPA in contrast to CCI. Mechanistically, miR-155-5p, which is known to destabilize barriers and tight junction proteins like claudin-1 and ZO-1, was increased in CCI and to lesser extent after rtPA application. In summary, tPA transiently opens the BNB possibly via miR-155-5p. However, tPA does not provoke allodynia in the absence of a neuropathic stimulus like a ligation or inflammation.

Extracellular matrix hyaluronan signals via its CD44 receptor in the increased responsiveness to mechanical stimulation

We propose that the extracellular matrix (ECM) signals CD44, a hyaluronan receptor, to increase the responsiveness to mechanical stimulation in the rat hind paw. We report that intradermal injection of hyaluronidase induces mechanical hyperalgesia, that is inhibited by co-administration of a CD44 receptor antagonist, A5G27. The intradermal injection of low (LMWH) but not high (HMWH) molecular weight hyaluronan also induces mechanical hyperalgesia, an effect that was attenuated by pretreatment with HMWH or A5G27. Pretreatment with HMWH also attenuated the hyperalgesia induced by hyaluronidase. Similarly, intradermal injection of A6, a CD44 receptor agonist, produced hyperalgesia that was inhibited by HMWH and A5G27. Inhibitors of protein kinase A (PKA) and Src, but not protein kinase C (PKC), significantly attenuated the hyperalgesia induced by both A6 and LMWH. Finally, to determine if CD44 receptor signaling is involved in a preclinical model of inflammatory pain, we evaluated the effect of A5G27 and HMWH on the mechanical hyperalgesia associated with the inflammation induced by carrageenan. Both A5G27 and HMWH attenuated carrageenan-induced mechanical hyperalgesia. Thus, while LMWH acts at its cognate receptor, CD44, to induce mechanical hyperalgesia, HMWH acts at the same receptor as an antagonist. That the local administration of HMWH or A5G27 inhibits carrageenan-induced hyperalgesia supports the suggestion that carrageenan produces changes in the ECM that contributes to inflammatory pain. These studies define a clinically relevant role for signaling by the hyaluronan receptor, CD44, in increased responsiveness to mechanical stimulation.

A review of bioactive release from nerve conduits as a neurotherapeutic strategy for neuronal growth in peripheral nerve injury

Peripheral nerve regeneration strategies employ the use of polymeric engineered nerve conduits encompassed with components of a delivery system. This allows for the controlled and sustained release of neurotrophic growth factors for the enhancement of the innate regenerative capacity of the injured nerves. This review article focuses on the delivery of neurotrophic factors (NTFs) and the importance of the parameters that control release kinetics in the delivery of optimal quantities of NTFs for improved therapeutic effect and prevention of dose dumping. Studies utilizing various controlled-release strategies, in attempt to obtain ideal release kinetics, have been reviewed in this paper. Release strategies discussed include affinity-based models, crosslinking techniques, and layer-by-layer technologies. Currently available synthetic hollow nerve conduits, an alternative to the nerve autografts, have proven to be successful in the bridging and regeneration of primarily the short transected nerve gaps in several patient cases. However, current research emphasizes on the development of more advanced nerve conduits able to simulate the effectiveness of the autograft which includes, in particular, the ability to deliver growth factors.