Hyunkyoung Lee

Role of microglial IKKβ in kainic acid-induced hippocampal neuronal cell death

Microglial cells are activated during excitotoxin-induced neurodegeneration. However, the in vivo role of microglia activation in neurodegeneration has not yet been fully elucidated. To this end, we used Ikkbeta conditional knockout mice (LysM-Cre/Ikkbeta(F/F)) in which the Ikkbeta gene is specifically deleted in cells of myeloid lineage, including microglia, in the CNS. This deletion reduced IkappaB kinase (IKK) activity in cultured primary microglia by up to 40% compared with wild-type (Ikkbeta(F/F)), and lipopolysaccharide-induced proinflammatory gene expression was also compromised. Kainic acid (KA)-induced hippocampal neuronal cell death was reduced by 30% in LysM-Cre/Ikkbeta(F/F) mice compared with wild-type mice. Reduced neuronal cell death was accompanied by decreased KA-induced glial cell activation and subsequent expression of proinflammatory genes such as tumour necrosis factor (TNF)-alpha and interleukin (IL)-1beta. Similarly, neurons in organotypic hippocampal slice cultures (OHSCs) from LysM-Cre/Ikkbeta(F/F) mouse brain were less susceptible to KA-induced excitotoxicity compared with wild-type OHSCs, due in part to decreased TNF-alpha and IL-1beta expression. Based on these data, we concluded that IKK/nuclear factor-kappaB dependent microglia activation contributes to KA-induced hippocampal neuronal cell death in vivo through induction of inflammatory mediators.

Dynamic states of a continuum traffic equation with on-ramp

We study the phase diagram of the continuum traffic flow model of a highway with an on-ramp. Using an open boundary condition, traffic states and metastabilities are investigated numerically for several representative values of the upstream boundary flux f(up) and for the whole range of the on-ramp flux f(rmp). An inhomogeneous but time-independent traffic state (standing localized cluster state) is found and related to a recently measured traffic state. Due to the density gradient near the on-ramp, a traffic jam can occur even when the downstream density is below the critical density of the usual traffic jam formation in homogeneous highways, and its structure varies qualitatively with f(rmp). The free flow, the recurring hump (RH) state, and the traffic jam can all coexist in a certain metastable region where the free flow can undergo phase transitions either to the RH state or to the traffic jam state. We also find two nontrivial analytic solutions. These solutions correspond to the standing localized cluster state and the homogeneous congested traffic state (one form of the traffic jam), which are observed in numerical simulations.

Toll-Like Receptors: Sensor Molecules for Detecting Damage to the Nervous System

Toll-like receptors (TLRs) are type I transmembrane signaling molecules that are expressed in cells of the innate immune system. In these cells, TLRs function as pattern recognition receptors (PRR) that recognize specific molecular patterns derived from microorganisms. Upon activation, TLRs trigger a cascade of intracellular signaling pathways in innate immune cells, leading to the induction of inflammatory and innate immune responses, which in turn regulate adaptive immune responses. In the nervous system, different members of the TLR family are expressed on glial cells (astrocytes, microglia, oligodendrocytes, and Schwann cells) and neurons. Recently, increasing evidence has supported the idea that TLRs also recognize endogenous molecules that are released from damaged tissue, thereby regulating inflammatory responses and subsequent tissue repair. These findings imply that TLRs on glial cells may also be involved in the inflammatory response to tissue damage in the nervous system. In this review, we discuss recent studies on TLR expression in the cells of the nervous system and their roles in acute neurological disorders involving tissue damage such as strokes, traumatic spinal cord and brain injuries, and peripheral nerve injuries.

TLR4 enhances histamine-mediated pruritus by potentiating TRPV1 activity

BackgroundRecent studies have indicated that Toll-like receptor 4 (TLR4), a pathogen-recognition receptor that triggers inflammatory signals in innate immune cells, is also expressed on sensory neurons, implicating its putative role in sensory signal transmission. However, the possible function of sensory neuron TLR4 has not yet been formally addressed. In this regard, we investigated the role of TLR4 in itch signal transmission.ResultsTLR4 was expressed on a subpopulation of dorsal root ganglia (DRG) sensory neurons that express TRPV1. In TLR4-knockout mice, histamine-induced itch responses were compromised while TLR4 activation by LPS did not directly elicit an itch response. Histamine-induced intracellular calcium signals and inward currents were comparably reduced in TLR4-deficient sensory neurons. Reduced histamine sensitivity in the TLR4-deficient neurons was accompanied by a decrease in TRPV1 activity. Heterologous expression experiments in HEK293T cells indicated that TLR4 expression enhanced capsaicin-induced intracellular calcium signals and inward currents.ConclusionsOur data show that TLR4 on sensory neurons enhances histamine-induced itch signal transduction by potentiating TRPV1 activity. The results suggest that TLR4 could be a novel target for the treatment of enhanced itch sensation.

TLR2-induced astrocyte MMP9 activation compromises the blood brain barrier and exacerbates intracerebral hemorrhage in animal models.

BackgroundThe innate immune response plays an important role in the pathogenesis of intracerebral hemorrhage (ICH). Recent studies have shown that Toll-like receptor 2 (TLR2) is involved in the innate immune response in various neurological diseases, yet neither its role in ICH nor the mechanisms by which it functions have yet been elucidated. We examined these in this study using a collagenase-induced mouse ICH model with TLR2 knock-out (KO) mice.ResultsTLR2 expression was upregulated in the ipsilateral hemorrhagic tissues of the collagenase-injected mice. Brain injury volume and neurological deficits following ICH were reduced in TLR2 KO mice compared to wild-type (WT) control mice. Heterologous blood-transfer experiments show that TLR2 signaling in brain-resident cells, but not leukocytes, contributes to the injury. In our study to elucidate underlying mechanisms, we found that damage to blood–brain barrier (BBB) integrity following ICH was attenuated in TLR2 KO mice compared to WT mice, which may be due to reduced matrix metalloproteinase-9 (MMP9) activation in astrocytes. The reduced BBB damage accompanies decreased neutrophil infiltration and proinflammatory gene expression in the injured brain parenchyma, which may account for the attenuated brain damage in TLR2 KO mice after ICH.ConclusionsTLR2 plays a detrimental role in ICH-induced brain damage by activating MMP9 in astrocytes, compromising BBB, and enhancing neutrophils infiltration and proinflammatory gene expression.

Vasoactive intestinal peptide inhibits toll-like receptor 3-induced nitric oxide production in Schwann cells and subsequent sensory neuronal cell death in vitro

We have previously reported that polyinosinic‐polycytidylic acids [poly(I:C)], a synthetic toll‐like receptor 3 (TLR3) agonist, induce Schwann cell activation, which exerts neurotoxic effects on sensory neurons. In this study, we investigated the effects of vasoactive intestinal peptide (VIP), a neuropeptide implicated in nerve regeneration, on TLR3‐induced Schwann cell activation. VIP receptors VPAC1 and VPAC2 were constitutively expressed in rat Schwann cells. VIP pretreatment inhibited TLR3‐induced inducible nitric oxide synthase (iNOS) gene expression and NO production in Schwann cells. Studies on the intracellular signal transduction pathways indicate that the VIP effect is mediated by protein kinase A activation. VIP also inhibited the poly(I:C)‐induced p38 activation that is responsible for the iNOS gene expression in Schwann cells. Finally, VIP inhibited dorsal rooyt ganglion neuronal cell death caused by NO produced in activated Schwann cells. Taken together, our data suggest that VIP exerts a neuroprotective effect by inhibiting neurotoxic Schwann cell activation.

Gastric Lesions and Immune Responses caused by Long-term Infection with Helicobacter heilmannii in C57BL/6 Mice

Helicobacter heilmannii is a gastric micro-organism that can induce gastritis and B-cell MALT (mucosa-associated lymphoid tissue) lymphoma in mice, in a host-dependent manner. The present study was designed to examine gastric lesions and immune responses caused by intragastric H. heilmannii infection of an inbred mouse strain, C57BL/6. Long-term infection led to the formation of gastric nodules and increased mucosal thickness of the stomach, due to gastric epithelial proliferation. Infection also induced the formation of lymphoid follicles in the corpus mucosa and submucosa. The follicular cells were mainly CD45R+ cells that did not produce immunoglobulin. However, scattered in the lamina propria and corpus submucosa, numerous IgA+ cells were found in infected mice, but not in control mice. RT-PCR results showed that H. heilmannii infection led to increased mRNA expression for IFN-gamma (a Th1 cytokine) and IL-10 (a Th2 cytokine) in the mouse stomach, suggesting that both Th1 and Th2 responses are associated with H. heilmannii infection. The mRNA of other cytokines and chemokines (IL-1beta, IL-12p40, TNF-alpha, MCP-1, KC and MIP-2) was also increased by infection.

IKKβ-mediated inflammatory myeloid cell activation exacerbates experimental autoimmune encephalomyelitis by potentiating Th1/Th17 cell activation and compromising blood brain barrier

BackgroundThe inflammatory myeloid cell activation is one of the hallmarks of experimental autoimmune encephalomyelitis (EAE), yet the in vivo role of the inflammatory myeloid cell activation in EAE has not been clearly resolved. It is well-known that IKK/NF-κB is a key signaling pathway that regulates inflammatory myeloid activation.MethodsWe investigated the in vivo role of inflammatory myeloid cell activation in myelin oligodendrocyte glycoprotein (MOG) peptides-induced EAE using myeloid cell type-specific ikkβ gene conditional knockout-mice (LysM-Cre/IkkβF/F).ResultsIn our study, LysM-Cre/IkkβF/F mice had alleviated clinical signs of EAE corresponding to the decreased spinal demyelination, microglial activation, and immune cell infiltration in the spinal cord, compared to the wild-type mice (WT, IkkβF/F). Myeloid ikkβ gene deletion significantly reduced the percentage of CD4+/IFN-γ+ (Th1) and CD4+/IL-17+ (Th17) cells but increased the percentages of CD4+/CD25+/Foxp3+ (Treg) cells in the spinal cord and lymph nodes, corresponding to the altered mRNA expression of IFN-γ, IL-17, IL-23, and Foxp3 in the spinal cords of LysM-Cre/IkkβF/F EAE mice. Also, the beneficial effect of myeloid IKKβ deletion in EAE corresponded to the decreased permeability of the blood brain barrier (BBB).ConclusionsOur findings strongly suggest that IKK/NF-kB-induced myeloid cell activation exacerbates EAE by activating Th1 and Th17 responses and compromising the BBB. The development of NF-κB inhibitory agents with high efficacy through specific targeting of IKKβ in myeloid cells might be of therapeutic potential in MS and other autoimmune disorders.

IKK/NF-κB-dependent satellite glia activation induces spinal cord microglia activation and neuropathic pain after nerve injury

Abstract Increasing evidence indicates that both microglia and satellite glial cell (SGC) activation play causal roles in neuropathic pain development after peripheral nerve injury; however, the activation mechanisms and their contribution to neuropathic pain remain elusive. To address this issue, we generated Ikk&bgr; conditional knockout mice (Cnp-Cre+/−/Ikk&bgr;f/f; cIkk&bgr;−/−) in which IKK/NF-&kgr;B-dependent proinflammatory SGC activation was abrogated. In these mice, nerve injury–induced spinal cord microglia activation and pain hypersensitivity were significantly attenuated compared to those in control mice. In addition, nerve injury–induced proinflammatory gene expression and macrophage infiltration into the dorsal root ganglion (DRG) were severely compromised. However, macrophages recruited into the DRG had minimal effects on spinal cord microglia activation, suggesting a causal effect for SGC activation on spinal cord microglia activation. In an effort to elucidate the molecular mechanisms, we measured Csf1 expression in the DRG, which is implicated in spinal cord microglia activation after nerve injury. In cIkk&bgr;−/− mice, nerve injury–induced Csf1 upregulation was ameliorated indicating that IKK/NF-&kgr;&Bgr;-dependent SGC activation induced Csf1 expression in sensory neurons. Taken together, our data suggest that nerve injury–induced SGC activation triggers Csf1 induction in sensory neurons, spinal cord microglia activation, and subsequent central pain sensitization.

Empirical Phase Diagram of Traffic Flow on Highways with On-Ramps

Multiple congested traffic states are found from empirical data. A preliminary empirical phase diagram of congested traffic flow is constructed. This paper also discusses connections between observations and theoretical analysis.