Biomedical subjects

Tammy Kielian

Publications and source records attributed to Tammy Kielian.

At least 19 recordsLinked to original sources

PubMed2007

Modulation of connexin expression and gap junction communication in astrocytes by the gram-positive bacterium S. aureus.

Gap junctions establish direct intercellular conduits between adjacent cells and are formed by the hexameric organization of protein subunits called connexins (Cx). It is unknown whether the proinflammatory milieu that ensues during CNS infection with S. aureus, one of the main etiologic agents of brain abscess in humans, is capable of eliciting regional changes in astrocyte homocellular gap junction communication (GJC) and, by extension, influencing neuron homeostasis at sites distant from the primary focus of infection. Here we investigated the effects of S. aureus and its cell wall product peptidoglycan (PGN) on Cx43, Cx30, and Cx26 expression, the main Cx isoforms found in astrocytes. Both bacterial stimuli led to a time-dependent decrease in Cx43 and Cx30 expression; however, Cx26 levels were elevated following bacterial exposure. Functional examination of dye coupling, as revealed by single-cell microinjections of Lucifer yellow, demonstrated that both S. aureus and PGN inhibited astrocyte GJC. Inhibition of protein synthesis with cyclohexamide (CHX) revealed that S. aureus directly modulates, in part, Cx43 and Cx30 expression, whereas Cx26 levels appear to be regulated by a factor(s) that requires de novo protein production; however, CHX did not alter the inhibitory effects of S. aureus on astrocyte GJC. The p38 MAPK inhibitor SB202190 was capable of partially restoring the S. aureus-mediated decrease in astrocyte GJC to that of unstimulated cells, suggesting the involvement of p38 MAPK-dependent pathway(s). These findings could have important implications for limiting the long-term detrimental effects of abscess formation in the brain which may include seizures and cognitive deficits.

Animals↗

PubMed2006

The role of Toll-like receptors in CNS response to microbial challenge.

The recent discovery of the family of Toll-like receptors has vastly expanded our understanding of the mechanisms by which the innate immune system recognizes and responds to a wide variety of microbial and endogenous pathogens. Toll-like receptors are transmembrane proteins that upon ligation with their cognate ligands trigger the production of cytokines, enzymes and other inflammatory agents. In the CNS Toll-like receptors are expressed predominantly by glial cells. In particular, the vastly abundant astrocytes are likely to be the major contributors to inflammatory responses within the CNS. Studies of the murine brain abscess model revealed that Toll-like receptor 2 plays a pivotal role in the generation of immune responses to Staphylococcus aureus. Although Toll-like receptor signaling is essential in antimicrobial defense, it may also lead to bystander injury of CNS tissue.

Animals↗

PubMed2006

Central role for MyD88 in the responses of microglia to pathogen-associated molecular patterns.

Microglia, the innate immune effector cells of the CNS parenchyma, express TLR that recognize conserved motifs of microorganisms referred to as pathogen-associated molecular patterns (PAMP). All TLRs identified to date, with the exception of TLR3, use a common adaptor protein, MyD88, to transduce activation signals. Recently, we reported that microglial activation in response to the Gram-positive bacterium Staphylococcus aureus was not completely attenuated following TLR2 ablation, suggesting the involvement of additional receptors. To assess the functional role of alternative TLRs in microglial responses to S. aureus and its cell wall product peptidoglycan as well as the Gram-negative PAMP LPS, we evaluated primary microglia from MyD88 knockout (KO) and wild-type mice. The induction of TNF-alpha, IL-12 p40, and MIP-2 (CXCL2) expression by S. aureus- and peptidoglycan-stimulated microglia was MyD88 dependent, as revealed by the complete inhibition of cytokine production in MyD88 KO cells. In addition, the expression of additional pattern recognition receptors, including TLR9, pentraxin-3, and lectin-like oxidized LDL receptor-1, was regulated, in part, via a MyD88-dependent manner as demonstrated by the attenuated expression of these receptors in MyD88 KO microglia. Microglial activation was only partially inhibited in LPS-stimulated MyD88 KO cells, suggesting the involvement of MyD88-independent pathways. Collectively, these findings reveal the complex mechanisms for microglia to respond to diverse bacterial pathogens, which occur via both MyD88-dependent and -independent pathways.

Adaptor Proteins, Signal Transducing↗

PubMed2006

Toll-like receptors in central nervous system glial inflammation and homeostasis.

Toll-like receptors (TLRs) are a family of pattern-recognition receptors expressed on cells of the innate immune system that allow for the recognition of conserved structural motifs on a wide array of pathogens, referred to as pathogen-associated molecular patterns, as well as some endogenous molecules. The recent emergence of studies examining TLRs in the central nervous system (CNS) indicates that these receptors not only play a role in innate immunity in response to infectious diseases but may also participate in CNS autoimmunity, neurodegeneration, and tissue injury. This review summarizes the experimental evidence demonstrating a role for TLRs in the context of CNS inflammation in both infectious and noninfectious conditions.

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PubMed2006

15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) and ciglitazone modulate Staphylococcus aureus-dependent astrocyte activation primarily through a PPAR-gamma-independent pathway.

Brain abscesses arise from a focal parenchymal infection by various pathogens, particularly Staphylococcus aureus. We have shown that astrocytes are activated upon exposure to S. aureus and may contribute to the excessive tissue damage characteristic of brain abscess. Therefore, modulating astrocyte activation may facilitate a reduction in brain abscess severity. Peroxisome proliferator activated receptor-gamma (PPAR-gamma) agonists are potent inhibitors of microglial activation; however, the effects of these compounds on S. aureus-dependent astrocyte activation have not yet been examined. Here, we demonstrate that two chemically distinct PPAR-gamma agonists, 15-deoxy-delta12,14-prostaglandin J2 (15d-PGJ2) and ciglitazone, suppress the production of several pro-inflammatory molecules in S. aureus-stimulated astrocytes including interleukin-1beta and nitric oxide (NO). Interestingly, 15d-PGJ2 attenuated Toll-like receptor 2 (TLR2) and inducible nitric oxide synthase expression, but failed to modulate macrophage inflammatory protein-2 (MIP-2/CXCL2) production, suggesting that 15d-PGJ2 is not a global inhibitor of astrocyte activation. Another novel finding of this study was the fact that both 15d-PGJ2 and ciglitazone were capable of attenuating pre-existing astrocyte activation, indicating their potential benefit in a therapeutic setting. Importantly, 15d-PGJ2 and ciglitazone were still capable of inhibiting S. aureus-induced pro-inflammatory mediator release in PPAR-gamma-deficient astrocytes, supporting PPAR-gamma-independent effects of these compounds. Collectively, these results suggest that 15d-PGJ2 and ciglitazone exert their anti-inflammatory actions on astrocytes primarily independent of the PPAR-gamma pathway.

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PubMed2005

Recognition of Staphylococcus aureus-derived peptidoglycan (PGN) but not intact bacteria is mediated by CD14 in microglia.

Recognition of Staphylococcus aureus and its cell-wall component peptidoglycan (PGN) by microglia is mediated, in part, by Toll-like receptor 2 (TLR2). However, the pattern recognition receptor (PRR) CD14 can also bind PGN and enhance TLR2-mediated signaling in macrophages, suggesting a similar phenomenon might occur in microglia. To assess the functional significance of CD14 on microglial activation, we evaluated the responses of primary microglia isolated from CD14 knockout (KO) and wild type (WT) mice. PGN-dependent microglial activation was partially CD14-dependent as demonstrated by the attenuated expression of TNF-alpha, macrophage inflammatory protein-2 (MIP-2/CXCL2), and the soluble PRR pentraxin-3 in CD14 KO microglia compared to WT cells. In contrast, microglial responses to intact S. aureus occurred primarily via a CD14-independent manner. Collectively, these findings reveal the complex nature of gram-positive bacterial recognition by microglia, which occurs, in part, via CD14.

Adaptor Proteins, Signal Transducing↗

PubMed2005

Toll-like receptor 2 (TLR2) is pivotal for recognition of S. aureus peptidoglycan but not intact bacteria by microglia.

Toll-like receptor 2 (TLR2) is a pattern recognition receptor that plays an important role in enabling cells of the innate immune system to recognize conserved structural motifs on a wide array of pathogens including gram-positive bacteria. Although microglia have recently been shown to express TLR2, the functional significance of this receptor in mediating microglial activation remains unknown. To ascertain the importance of TLR2 in microglial responses to S. aureus and its cell wall product peptidoglycan (PGN), we evaluated primary microglia from TLR2 knockout (KO) and wild-type (WT) mice. TLR2 was found to play a pivotal role in PGN recognition and subsequent activation in primary microglia, as demonstrated by the attenuated expression of TNF-alpha, IL-12 p40, MIP-2, and MCP-1 in PGN-treated TLR2 KO microglia compared with WT cells. In contrast, the responses of TLR2 KO and WT microglia to S. aureus were qualitatively similar, indicating that alternative receptors are responsible for recognizing intact bacteria. Microarray analysis confirmed that TLR2 plays a central role in PGN recognition by primary microglia. The expression of MyD88, a central adapter molecule in TLR-dependent signaling, was similar in both TLR2 KO and WT microglia, suggesting that the defect in PGN recognition by the former is not due to alterations in this key signaling intermediate. These findings reveal the complex nature of gram-positive bacterial recognition by microglia, which occurs, in part, through engagement of TLR2.

Adaptor Proteins, Signal Transducing↗

PubMed2005

Staphylococcus aureus-derived peptidoglycan induces Cx43 expression and functional gap junction intercellular communication in microglia.

Gap junctions serve as intercellular conduits that allow the exchange of small molecular weight molecules (up to 1 kDa) including ions, metabolic precursors and second messengers. Microglia are capable of recognizing peptidoglycan (PGN) derived from the outer cell wall of Staphylococcus aureus, a prevalent CNS pathogen, and respond with the robust elaboration of numerous pro-inflammatory mediators. Based on recent reports demonstrating the ability of tumor necrosis factor-alpha and interferon-gamma to induce gap junction coupling in macrophages and microglia, it is possible that pro-inflammatory mediators released from PGN-activated microglia are capable of inducing microglial gap junction communication. In this study, we examined the effects of S. aureus-derived PGN on Cx43, the major connexin in microglial gap junction channels, and functional gap junction communication using single-cell microinjections of Lucifer yellow (LY). Exposure of primary mouse microglia to PGN led to a significant increase in Cx43 mRNA and protein expression. LY microinjection studies revealed that PGN-treated microglia were functionally coupled via gap junctions, the specificity of which was confirmed by the reversal of activation-induced dye coupling by the gap junction blocker 18-alpha-glycyrrhetinic acid. In contrast to PGN-activated microglia, unstimulated cells consistently failed to exhibit LY dye coupling. These results indicate that PGN stimulation can induce the formation of a functional microglial syncytium, suggesting that these cells may be capable of influencing neuro-inflammatory responses in the context of CNS bacterial infections through gap junction intercellular communication.

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PubMed2005

Toll-like receptor 2 modulates the proinflammatory milieu in Staphylococcus aureus-induced brain abscess.

Toll-like receptor 2 (TLR2) is a pattern recognition receptor (PRR) that plays an important role in innate immune recognition of conserved structural motifs on a wide array of pathogens, including Staphylococcus aureus. To ascertain the functional significance of TLR2 in the context of central nervous system (CNS) parenchymal infection, we evaluated the pathogenesis of S. aureus-induced experimental brain abscess in TLR2 knockout (KO) and wild-type (WT) mice. The expression of several proinflammatory mediators, including inducible nitric oxide synthase, tumor necrosis factor alpha, and macrophage inflammatory protein-2, was significantly attenuated in brain abscesses of TLR2 KO mice compared to WT mice during the acute phase of infection. Conversely, interleukin-17 (IL-17), a cytokine produced by activated and memory T cells, was significantly elevated in lesions of TLR2 KO mice, suggesting an association between innate and adaptive immunity in brain abscess. Despite these differences, brain abscess severity in TLR2 KO and WT animals was similar, with comparable mortality rates, bacterial titers, and blood-brain barrier permeability, implying a role for alternative PRRs. Expression of the phagocytic PRRs macrophage scavenger receptor type AI/AII and lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) was increased in brain abscesses of both TLR2 KO and WT mice compared to uninfected animals. However, LOX-1 induction in brain abscesses of TLR2 KO mice was significantly attenuated compared to WT animals, revealing that the TLR2-dependent signal(s) influence LOX-1 expression. Collectively, these findings reveal the complex nature of gram-positive bacterial recognition in the CNS which occurs, in part, through engagement of TLR2 and highlight the importance of receptor redundancy for S. aureus detection in the CNS.

Animals↗

PubMed2004

Immunopathogenesis of brain abscess.

Brain abscess represents a significant medical problem despite recent advances made in detection and therapy. Due to the emergence of multi-drug resistant strains and the ubiquitous nature of bacteria, the occurrence of brain abscess is likely to persist. Our laboratory has developed a mouse experimental brain abscess model allowing for the identification of key mediators in the CNS anti-bacterial immune response through the use of cytokine and chemokine knockout mice. Studies of primary microglia and astrocytes from neonatal mice have revealed that S. aureus, one of the main etiologic agents of brain abscess in humans, is a potent stimulus for proinflammatory mediator production. Recent evidence from our laboratory indicates that Toll-like receptor 2 plays a pivotal role in the recognition of S. aureus and its cell wall product peptidoglycan by glia, although other receptors also participate in the recognition event. This review will summarize the consequences of S. aureus on CNS glial activation and the resultant neuroinflammatory response in the experimental brain abscess model.

Journal Article↗

PubMed2004

Microglia and chemokines in infectious diseases of the nervous system: views and reviews.

Microglia are one of the resident mononuclear phagocyte populations within the central nervous system (CNS). These cells share many phenotypical and functional characteristics with macrophages, suggesting that microglia participate in innate immune responses in the brain. As such, microglia are uniquely poised to provide an initial line of defense against invading pathogens into the CNS prior to peripheral leukocyte infiltration. Numerous studies have shown that microglia are capable of producing a wide array of chemokines that act to initiate or promote inflammatory processes in the CNS through facilitating the recruitment of peripheral immune cells into the CNS parenchyma. In addition, microglia also express numerous chemokine receptors that are involved in cell migration and serve as co-receptors for human immunodeficiency virus-1 (HIV-1) infection. The findings obtained from studies of chemokine expression in animal models of CNS infectious diseases as well as from patient populations highlight a marked promiscuity in cerebral chemokine expression patterns with simultaneous expression of multiple chemokines being the general rule. A detailed discussion regarding the profiles and implications of chemokine and chemokine receptor expression in the context of various CNS infectious diseases including HIV-1 encephalitis, other viral encephalitides, bacterial meningitis, and brain abscess is presented. Future studies dissecting the potential roles of individual chemokines and their receptors in the context of CNS infectious diseases may provide insights into the complex regulatory network dictating neuroinflammatory responses.

Animals↗

PubMed2004

Persistent immune activation associated with a mouse model of Staphylococcus aureus-induced experimental brain abscess.

We have established a mouse experimental brain abscess model using Staphylococcus aureus where lesion sites are greatly exaggerated compared to the localized area of initial infection, reminiscent of an overactive immune response. Here we demonstrate the prolonged expression of IL-1 beta, TNF-alpha, and macrophage inflammatory protein-2 (MIP-2/CXCL2), concomitant with a chronic disruption of the blood-brain barrier (BBB) in mice with S. aureus-induced brain abscess. These changes correlated with the continued presence of infiltrating neutrophils and macrophages/microglia. Collectively these findings suggest that the excessive tissue damage that often results from brain abscess may be mediated, in part, by the perpetuation of antibacterial immune responses that are not downregulated in a timely manner.

Animals↗

PubMed2004

Toll-like receptor 2 (TLR2) mediates astrocyte activation in response to the Gram-positive bacterium Staphylococcus aureus.

Astrocytes play an important role in initiating and regulating CNS immune responses through the release of proinflammatory cytokines and chemokines. Here we demonstrate that primary astrocytes are capable of recognizing the Gram-positive bacterium Staphylococcus aureus and its cell wall product peptidoglycan (PGN) and respond by producing numerous proinflammatory mediators including interleukin-1beta (IL-1beta), tumor necrosis factor-alpha (TNF-alpha), macrophage inflammatory protein-1beta (MIP-1beta), MIP-2, and monocyte chemoattractant protein (MCP-1). Astrocytes have recently been shown to express Toll-like receptor 2 (TLR2), a pattern recognition receptor important for recognizing structural components of various Gram-positive bacteria, fungi, and protozoa. However, the functional significance of TLR2 in mediating astrocyte activation remains unknown. Primary astrocytes from TLR2 knockout mice were used to evaluate the role of TLR2 in astrocyte responses to S. aureus and PGN. The results demonstrate that TLR2 is essential for maximal proinflammatory cytokine and chemokine production, but not phagocytosis, in primary astrocytes following S. aureus and PGN exposure. In addition, both stimuli led to a significant increase in TLR2 mRNA expression in wild-type astrocytes as assessed by real-time quantitative RT-PCR. These findings suggest that astrocytes may play a key role in the initial antibacterial immune response in the CNS through engagement of TLR2.

Animals↗

PubMed2004

IL-1 and TNF-alpha play a pivotal role in the host immune response in a mouse model of Staphylococcus aureus-induced experimental brain abscess.

Brain abscesses represent a significant medical problem despite recent advances made in detection and therapy. Using an established Staphylococcus aureus-induced brain abscess model, we have sought to define the functional importance of interleukin-1 (IL-1), tumor necrosis factor-alpha (TNF-alpha), and IL-6 in the host anti-bacterial immune response using cytokine gene knockout (KO) mice. Previous studies from our laboratory revealed that these cytokines are among the main proinflammatory mediators produced during the acute stage of brain abscess development. The results presented here demonstrate that although they share many redundant activities, IL-1 and TNF-alpha are important for containing bacterial infection in evolving brain abscesses as evident by increased mortality and bacterial burdens in IL-1 and TNF-alpha KO mice compared to wild type (WT) animals. In contrast, IL-6 was not found to be a major contributor to the host anti-bacterial immune response. Microarray analysis was used to evaluate the downstream consequences originating from the lack of IL-1 on subsequent proinflammatory mediator expression in brain abscesses from IL-1 KO and WT animals. Although numerous genes were significantly induced following S. aureus infection, only IL-1beta and 2 chemokines, CCL9 (macrophage inflammatory protein-1 gamma/MIP-1gamma) and CXCL13 (B lymphocyte chemoattractant/BLC), were differentially regulated in IL-1 KO versus WT animals. These results suggest that IL-1 and TNF-alpha play a pivotal role during the acute stage of brain abscess development through regulating the ensuing anti-bacterial inflammatory response.

Animals↗

PubMed2004

S. aureus-dependent microglial activation is selectively attenuated by the cyclopentenone prostaglandin 15-deoxy-Delta12,14- prostaglandin J2 (15d-PGJ2).

Microglial activation is a hallmark of brain abscess. The continual release of proinflammatory mediators by microglia following bacterial challenge may contribute, in part, to the destruction of surrounding normal tissue characteristic of brain abscess. Therefore, attenuating chronic microglial activation during the course of CNS bacterial infections may have therapeutic benefits. The purpose of this study was to evaluate the ability of the natural peroxisome proliferator-activated receptor (PPAR)-gamma agonist 15-deoxy-Delta12,14- prostaglandin J2 (15d-PGJ2) to modulate microglial activation in response to Staphylococcus aureus, one of the main etiologic agents of brain abscess in humans. 15d-PGJ2 was a potent inhibitor of proinflammatory cytokine (IL-1beta, TNF-alpha, IL-12 p40) and CC chemokine (MIP-1beta, MCP-1) production in primary microglia, but had no effect upon the expression of select CXC chemokines (MIP-2, KC). 15d-PGJ2 also selectively inhibited the S. aureus-dependent increase in microglial TLR2, CD14, MHC class II, and CD40 expression, whereas it had no effect on the co-stimulatory molecules CD80 and CD86. Microarray analysis revealed additional inflammatory mediators modulated by 15d-PGJ2 in primary microglia following S. aureus exposure, the majority of which were chemokines. These results suggest that suppressing microglial activation through the use of 15d-PGJ2 may lead to the sparing of damage to normal brain parenchyma that often results from brain abscess.

Animals↗

PubMed2003

Effects of peroxisome proliferator-activated receptor-gamma agonists on central nervous system inflammation.

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) plays a critical role in glucose and lipid metabolism. More recently, PPAR-gamma ligands have been reported to inhibit the expression of proinflammatory molecules by monocytes/macrophages. Of relevance to CNS disease is that PPAR-gamma agonists have been demonstrated to have similar effects on microglia. PPAR-gamma agonists also ameliorate experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis. This Mini-Review summarizes the effects of PPAR-gamma agonists in mediating immune responses and the potential of these agonists in the treatment of inflammatory disorders of the CNS.

Animals↗

PubMed2002

Characterization of microglial responses to Staphylococcus aureus: effects on cytokine, costimulatory molecule, and Toll-like receptor expression.

Microglia participate in innate immune responses in the central nervous system (CNS). This work demonstrates that microglia can recognize heat-inactivated Staphylococcus aureus and its cell wall product peptidoglycan (PGN) through the elaboration of numerous proinflammatory cytokines and chemokines. Microglia also exhibited S. aureus bactericidal activity. Microglia constitutively expressed low levels of Toll-like receptor 1 (TLR1), TLR2, TLR6, and CD14, all of which were enhanced following S. aureus exposure. Activation of resident microglia by S. aureus may serve to amplify the CNS antibacterial immune response through the release of cytokines, chemokines, and induction of bactericidal activity.

Animals↗

PubMed2002

MCP-1 expression in CNS-1 astrocytoma cells: implications for macrophage infiltration into tumors in vivo.

Gliomas are among the most resistant tumors to conventional anti-tumor therapy, and are typified by their highly infiltrative nature and ill-defined borders. Macrophages constitute a major proportion of the tumor cell mass in both primary human gliomas and as shown here, a CNS-1 glioma model. The objective of this study was to identify tumor-cell-derived chemotactic factor(s) which participate in macrophage recruitment into tumors in vivo. This study demonstrates the constitutive expression of monocyte chemoattractant protein-1 (MCP-1), a potent monocyte chemoattractant, by the rat astrocytoma cell line CNS-1. Characterization of cytokine expression by CNS-1 cells in vitro revealed the constitutive expression of TGF-beta but not other proinflammatory cytokines. However, numerous cytokines were detected in CNS-I tumors in vivo including Ltbeta, IL-1alpha, IL-1beta, TNF-alpha, TNF-beta, IL-10, and IFN-gamma. Attenuation of MCP- I release from CNS-1 cells using an anti-sense approach revealed no significant alterations in macrophage infiltration into tumors in vivo, suggesting redundancy in the signal(s) involved in macrophage recruitment. Depletion of peripheral macrophages using liposome-encapsulated clodronate revealed no significant differences in tumor growth or in the degree of macrophage infiltration into CNS-1 tumors in vivo. These results indicate that CNS-1 cells produce chemotactic factors which likely participate in macrophage recruitment into tumors in vivo. Whether or not macrophage recruitment confers a growth advantage for the tumor remains to be determined.

Animals↗