Induction of presynaptic reexpression of an adhesion protein in lamina II after dorsal root deafferentation in adult rat spinal cord

Tissue type plasminogen activator induced in rat dorsal horn astrocytes contributes to mechanical hypersensitivity following dorsal root injury

Dorsal root injury is known to induce alteration of the extracellular environment in the spinal cord and synaptic reorganization with degradation of injured primary afferent and sprouting of spared terminal. These changes affect behavioral sensitivity and sometimes lead to neuropathic pain. We have hypothesized that changes in extracellular proteolysis in the dorsal horn is involved in neuroplastic changes in the dorsal horn after nerve injury. Tissue type plasminogen activator (tPA) is a well-known extracellular serine protease and is involved in the modification of the extracellular matrix, which leads to neuroplastic changes such as long-term potentiation in the hippocampus. In the present study, we found a marked induction of tPA in activated astrocytes following L4/5 root injury and a resultant increase of proteolytic enzymatic activity in the dorsal horn. We also examined the involvement of tPA activity on mechanical hypersensitivity using a root ligation model which has been used for investigating radiculopathy pain behavior. Intrathecal and continuous administration of tPA inhibitor, tPA-STOP, suppressed root ligation-induced mechanical allodynia in a dose-dependent manner during an early stage of injury (0-4 days). In contrast, the delayed administration of tPA-STOP during the chronic stage of injury (10 days) did not affect pain behavior. These data suggest an important contribution of astrocytes in the dorsal horn to the pathophysiology of radiculopathy pain, and astrocyte-derived tPA and the proteolytic activity in the dorsal horn may be one of the essential factors involved in pain following root injury.

Distribution and injury-induced plasticity of cadherins in relationship to identified synaptic circuitry in adult rat spinal cord

Cadherins are synaptically enriched cell adhesion and signaling molecules. In brain, they function in axon targeting and synaptic plasticity. In adult spinal cord, their localization, synaptic affiliation, and role in injury-related plasticity are mostly unexplored. Here, we demonstrate in adult rat dorsal horn that E- and N-cadherin display unique patterns of localization to functionally distinct types of synapses of intrinsic and primary afferent origin. Within the nociceptive afferent pathway to lamina II, nonpeptidergic C-fiber synapses in the deeper half of lamina II (IIi) contain E-cadherin but mostly lack N-cadherin, whereas the majority of the peptidergic C-fiber synapses in the outer half of lamina II (IIo) contain N-cadherin but lack E-cadherin. Approximately one-half of the Abeta-fiber terminations in lamina III contain N-cadherin; none contain E-cadherin. Strikingly, the distribution and levels of these cadherins are differentially affected by sciatic nerve axotomy, a model of neuropathic pain in which degenerative and regenerative structural plasticity has been implicated. Within the first 7 d after axotomy, E-cadherin is rapidly and completely lost from the dorsal horn synapses with which it is affiliated, whereas N-cadherin localization and levels are unchanged; such patterns persist through 28 d postlesion. The loss of E-cadherin thus occurs before the onset of mechanical hyperalgesia (approximately 10-21 d postlesion), as reported previously. Together, the synaptic specificity displayed by these cadherins, coupled with their differential response to injury, suggests that they may proactively contribute to the maintenance of some, and incipient dismantling of other, synaptic circuits in response to nerve injury. Speculatively, such changes may ultimately contribute to subsequently emerging abnormalities in pain perception.

Complementary expression and heterophilic interactions between IgLON family members neurotrimin and LAMP

Neurotrimin (Ntm) and the limbic system-associated membrane protein (LAMP) are members of the IgLON (LAMP, OBCAM, Ntm) family of glycorylphosphatidylinositol anchored neural cell adhesion molecules. We previously reported that LAMP and Ntm promote adhesion and neurite outgrowth via a homophilic mechanism, suggesting that these proteins promote the formation of specific neuronal circuits by homophilic interactions. In this report, we have further characterized the expression and binding specificity of Ntm. Using a newly generated monoclonal antibody to Ntm, we demonstrated that this protein is largely expressed in a complementary pattern to that of LAMP in the nervous system, with co-expression at a few sites. Ntm is expressed at high levels in sensory-motor cortex and, of particular note, is transiently expressed in neurons of cortical barrel fields and corresponding thalamic "barreloids." Binding of a recombinant, soluble form of Ntm to CHO cells expressing either Ntm or LAMP demonstrates that Ntm and LAMP interact both homophilically and heterophilically. In contrast to conventional growth-promoting activity of Ig superfamily members, LAMP strongly inhibits the outgrowth of Ntm-expressing dorsal root ganglion (DRG) neurons in a heterophilic manner. These anatomical and functional data support the concept that homophilic and heterophilic interactions between IgLON family members are likely to play a role in the specification of neuronal projections via growth promoting and inhibiting effects, respectively.