Duk Joon Suh

Actin Polymerization Is Essential for Myelin Sheath Fragmentation during Wallerian Degeneration

The mechanisms that trigger Wallerian degeneration (WD) of peripheral nerves after injury are not well understood. During the early period of WD, fragmentation of myelin into ovoid structures occurs near the Schmidt-Lantermann incisures (SLI), a noncompact region of the myelin sheath containing autotypical adherens junction. In this study, we found that new filamentous actin polymerization occurs in the SLI of mouse sciatic nerves after injury and that its inhibition prevented not only the degradation of E-cadherin in the SLI but also myelin ovoid formation. However, the inhibition of actin polymerization could not block Schwann cell dedifferentiation. The activation of Rac GTPase was observed in the distal stump of the injured nerves, and a specific Rac inhibitor, a dominant-negative Rac, and Rac1-RNA interference blocked myelin ovoid formation. Together, these findings suggest that dynamic changes in actin in the SLI are essential for initiation of demyelination after peripheral nerve injury.

Interleukin‐6 is required for the early induction of glial fibrillary acidic protein in Schwann cells during Wallerian degeneration

Signal transducer and activator of transcription 3 (STAT3) regulates gene transcription in response to cytokines and growth factors. In the central nervous system, STAT3 plays a role in neuroprotection and reactive gliosis after lesions. During peripheral nerve regeneration, a nerve injury‐induced up‐regulation of cytokines and growth factors accompanies STAT3 activation in sensory neurons and Schwann cells (SCs) even though its molecular details and functions are unknown. We then analyzed the ligands and functions of STAT3 activation in RT4 schwannoma cells and adult SCs in vitro and in vivo. We have identified that interleukin‐6 (IL‐6), but not ciliary neurotrophic factor, leukemia inhibitory factor, or ligands for receptor tyrosine kinases, activates STAT3 in SCs. The IL‐6/STAT3 signaling in primary SCs and RT4 cells induced the gene expression of glial fibrillary acidic protein (GFAP), which is known to be required for the proper regeneration of the injured nerves. Finally, the GFAP induction in the sciatic nerves after injury was significantly delayed in IL‐6‐deficient mice. These findings indicate that IL‐6 plays an important role in STAT3‐dependent GFAP induction in SCs during peripheral nerve regeneration.

Netrin induces down-regulation of its receptor, Deleted in Colorectal Cancer, through the ubiquitin–proteasome pathway in the embryonic cortical neuron

The proper regulation of temporal and spatial expression of the axon guidance cues and their receptors is critical for the normal wiring of nervous system during development. Netrins, a family of secreted guidance cues, are involved in the midline crossing of spinal commissural axons and in the guidance of cortical efferents. Axons normally lose the responsiveness to their attractants when they arrive at their targets, where the attractant is produced. However the molecular mechanism is still unknown. We investigated the molecular mechanism of down‐regulation of netrin‐1 signaling in the embryonic cortical neurons. Netrin‐1 induced the ubiquitination and proteolytic cleavage of Deleted in Colorectal Cancer (DCC), a transmembrane receptor for netrin, in dissociated cortical neurons. A dramatic decrease of DCC level particularly on the cell surface was also observed after netrin‐1 stimulation. Specific ubiquitin–proteasome inhibitors prevented the netrin‐induced DCC cleavage and decrease of cell surface DCC. We suggest that the ligand‐mediated down‐regulation of DCC might participate in the loss of netrin‐responsiveness in the developing nervous system.

The Neuregulin‐Rac‐MKK7 pathway regulates antagonistic c‐jun/Krox20 expression in Schwann cell dedifferentiation

Schwann cells respond to nerve injury by dedifferentiating into immature states and producing neurotrophic factors, two actions that facilitate successful regeneration of axons. Previous reports have implicated the Raf‐ERK cascade and the expression of c‐jun in these Schwann cell responses. Here we used cultured primary Schwann cells to demonstrate that active Rac1 GTPase (Rac) functions as a negative regulator of Schwann cell differentiation by upregulating c‐jun and downregulating Krox20 through the MKK7‐JNK pathway, but not through the Raf‐ERK pathway. The activation of MKK7 and induction of c‐jun in sciatic nerves after axotomy was blocked by Rac inhibition. Microarray experiments revealed that the expression of regeneration‐associated genes, such as glial cell line‐derived neurotrophic factor and p75 neurotrophin receptor, after nerve injury was dependent on Rac but not on ERK. Finally, the inhibition of ErbB2 signaling prevented MKK7 activation, c‐jun induction, and Rac‐dependent gene expression in sciatic nerve explant cultures. Taken together, our results indicate that the neuregulin‐Rac‐MKK7‐JNK/c‐jun pathway regulates Schwann cell dedifferentiation following nerve injury.

Pathological adaptive responses of Schwann cells to endoplasmic reticulum stress in bortezomib-induced peripheral neuropathy.

Bortezomib, a proteasome inhibitor, has been considered as a promising anticancer drug in the treatment of recurrent multiple myeloma and some solid tumors. The bortezomib‐induced peripheral neuropathy (BIPN) is a prominent cause of dose‐limiting toxicities after bortezomib treatment. In this study, we found that BIPN in a mouse model is characterized by acute but transient endoplasmic reticulum (ER) damages to Schwann cells. These damaged Schwann cells exhibit abnormal outcomes from healing processes such as the myelination of Remak bundles. A morphometric analysis of polymyelinated Remak bundles revealed that the pathological myelination was not related to the axonal parameters that regulate the normal myelination process during development. In addition, demyelinating macrophages were focally infiltrated within endoneurium of the sciatic nerve. To identify the mechanism underlying these pathologies, we applied a gene microarray analysis to bortezomib‐treated primary Schwann cells and verified the changes of several gene expression in bortezomib‐treated sciatic nerves. The analysis showed that bortezomib‐induced ER stress was accompanied by the activation of several protective molecular chaperones and the down‐regulation of myelin gene expression. ER stress inducers such as thapsigargin and bredelfin A also suppressed the mRNA expression of myelin gene P0 at transcriptional levels. In addition, the expression of chemokines such as the macrophage chemoattractants Ccl3 and Cxcl2 was significantly increased in Schwann cells in response to bortezomib and ER stress inducers. Taken together, these observations suggest that the pathological adaptive responses of Schwann cells to bortezomib‐induced ER stress may, in part, participate in the development of BIPN.

A novel mechanism of methylglyoxal cytotoxicity in neuroglial cells.

Methylglyoxal (MGO) is an endogenous dicarbonyl compound that is highly produced in hyperglycemic conditions. It forms advanced glycation endproducts that are believed to contribute, as etiological factors, to the pathophysiology of diabetic complications. In addition, MGO suppresses cell viability through the induction of apoptosis in vitro. In this study, we have, for the first time, demonstrated the effect of MGO on the gp130 cytokine‐induced signal transducer and activator of transcription 3 (STAT3) responses in RT4 schwannoma, PC12 pheochromocytoma and U87MG glioma cells. At dose that very mildly affects cell viability, MGO rapidly induces endocytotic degradation of gp130, which involves the di‐leucine internalization motif in the cytoplasmic domain of gp130, without affecting other growth factor receptors. Concomitant inhibition of basal and interleukin‐6‐induced STAT3 activation was observed following pre‐treatment with MGO. The inhibitory effect of MGO on the gp130/STAT3 signaling was prevented by the pre‐treatment with an advanced glycation endproduct scavenger aminoguanidine. Finally, these deleterious effects of MGO on STAT3 signaling led to down‐regulation of a STAT3 target gene, Bcl‐2, and sensitized cellular toxicity induced by H2O2 and etoposide. Our data indicate that MGO affects cell viability via desensitization of gp130/STAT3 signaling, which is the key signaling pathway for cell survival, and thereby promotes cytotoxicity.

Complementary and alternative medicine in the undergraduate medical curriculum: A survey of Korean medical schools

BACKGROUND The current status of complementary and alternative medicine (CAM) education in Korean medical schools is still largely unknown, despite a growing need for a CAM component in medical education. The prevalence, scope, and diversity of CAM courses in Korean medical school education were evaluated. DESIGN Participants included academic or curriculum deans and faculty at each of the 41 Korean medical schools. A mail survey was conducted from 2007 to 2010. Replies were received from all 41 schools. RESULTS CAM was officially taught at 35 schools (85.4%), and 32 schools (91.4%) provided academic credit for CAM courses. The most common courses were introduction to CAM or integrative medicine (88.6%), traditional Korean medicine (57.1%), homeopathy and naturopathy (31.4%), and acupuncture (28.6%). Educational formats included lectures by professors and lectures and/or demonstrations by practitioners. The value order of core competencies was attitude (40/41), knowledge (32/41), and skill (6/41). Reasons for not initiating a CAM curriculum were a non-evidence-based approach in assessing the efficacy of CAM, insufficiently reliable reference resources, and insufficient time to educate students in CAM. CONCLUSIONS This survey reveals heterogeneity in the content, format, and requirements among CAM courses at Korean medical schools. Korean medical school students should be instructed in CAM with a more consistent educational approach to help patients who participate in or demand CAM.

Characterization of K + Channels in the Basolateral Membrane of Rat Tracheal Epithelia

Abstract. To study K+ channels in the basolateral membrane of chloride-secreting epithelia, rat tracheal epithelial monolayers were cultured on permeable filters and mounted into an Ussing chamber system. The mucosal membrane was permeabilized with nystatin (180 μg/ml) in the symmetrical high K+ (145 mm) Ringer solution. During measurement of the macroscopic K+ conductance properties of the basolateral membrane under a transepithelial voltage clamp, we detected at least two types of K+ currents: one is an inwardly rectifying K+ current and the other is a slowly activating outwardly rectifying K+ current. The inwardly rectifying K+ current is inhibited by Ba2+. The slowly activating K+ current was potentiated by cAMP and inhibited by clofilium, phorbol 12-myristae 13-acetate (PMA) and lowering temperature. This is consistent with the biophysical characteristics of ISK channel. RT-PCR analysis revealed the presence of ISK cDNA in the rat trachea epithelia. Although 0.1 mm Ba2+ only had minimal affect on short-circuit current (Isc) induced by cAMP in intact epithelia, 0.1 mm clofilium strongly inhibited it. These results indicate that ISK might be important for maintaining cAMP-induced chloride secretion in the rat trachea epithelia.

Grb2-Associated Binder-1 Is Required for Neuregulin-1-Induced Peripheral Nerve Myelination

Grb2-associated binders (Gabs) are scaffolding proteins implicated in cell signaling via receptor tyrosine kinases including neuregulin-1(NRG1)-ErbB receptor signaling, which is essential for peripheral nerve myelination. Here, we show that the conditional removal of Gab1 from Schwann cells resulted in hypomyelination and abnormal development of Remak bundles. In contrast, hypomyelination was not observed in conventional Gab2 knock-out mice. Tyrosine phosphorylation of Gab1, but not Gab2, in sciatic nerves was upregulated during the myelination period and was found to be suppressed in NRG1-type III(+/−) mice, which display a hypomyelinated phenotype similar to that observed in Gab1 knock-out mice. Gab1 knock-out and NRG1-type III(+/−) mice both exhibited reduced extracellular signal-regulated kinase activity in myelinating nerves. In addition, Krox20, a transcription factor that is critical for myelination, has been identified as a target of the NRG1-Gab1 pathway during the myelination process. Our findings suggest that Gab1 is an essential component of NRG1-type III signaling during peripheral nerve development.

Transient lysosomal activation is essential for p75 nerve growth factor receptor expression in myelinated Schwann cells during Wallerian degeneration

Myelinated Schwann cells in the peripheral nervous system express the p75 nerve growth factor receptor (p75NGFR) as a consequence of Schwann cell dedifferentiation during Wallerian degeneration. p75NGFR has been implicated in the remyelination of regenerating nerves. Although many studies have shown various mechanisms underlying Schwann cell dedifferentiation, the molecular mechanism contributing to the re-expression of p75NGFR in differentiated Schwann cells is largely unknown. In the present study, we found that lysosomes were transiently activated in Schwann cells after nerve injury and that the inhibition of lysosomal activation by chloroquine or lysosomal acidification inhibitors prevented p75NGFR expression at the mRNA transcriptional level in an ex vivo Wallerian degeneration model. Lysosomal acidification inhibitors suppressed demyelination, but not axonal degeneration, thereby suggesting that demyelination mediated by lysosomes may be an important signal for inducing p75NGFR expression. Tumor necrosis factor-α (TNF-α) has been suggested to be involved in regulating p75NGFR expression in Schwann cells. In this study, we found that removing TNF-α in vivo did not significantly suppress the induction of both lysosomes and p75NGFR. Thus, these findings suggest that lysosomal activation is tightly correlated with the induction of p75NGFR in demyelinating Schwann cells during Wallerian degeneration.