Biomedical subjects

Sarita Garg

Publications and source records attributed to Sarita Garg.

3 recordsLinked to original sources

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.

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PubMed2004

Opioid receptor desensitization contributes to thermal hyperalgesia in infant rats.

Central nociceptive processing includes spinal and supraspinal neurons, but the supraspinal mechanisms mediating changes in pain threshold remain unclear. We investigated the role of forebrain neurons in capsaicin-induced hyperalgesia. Long-Evans rat pups at 21 days were randomized to undisturbed control group, or to receive tactile stimulation, saline injection (0.9% w/v) or capsaicin injection (0.01% w/v) applied to each paw at hourly intervals. Thermal paw withdrawal latency was measured 1 h later, forebrains were removed and purified forebrain neuronal membranes were assayed for adenylyl cyclase activity and opioid receptor function. Capsaicin-injected rats had decreased thermal latency (P < 0.0001) compared to the other groups. Neuronal membranes showed increased basal (P = 0.0003) and forskolin-stimulated (P=0.0002) adenylyl cyclase activity in the capsaicin group compared to other groups. The selective mu-opioid receptor agonist, [D-Ala2, N-Me-Phe4, Gly5-ol]enkephalin (DAMGO) was less effective in inhibiting adenylyl cyclase activity in the capsaicin group (P < 0.001) compared to other groups. These effects were naloxone-reversible and pertussis toxin-sensitive (P < 0.01) in the control, tactile stimulation and saline injection groups but not in the capsaicin group. Binding capacity and affinity for micro-opioid receptors were similar in all four groups, suggesting that receptor downregulation was not involved. Exposure to DAMGO increased [35S]GTPgammaS binding to neuronal membranes from the control, tactile and saline groups (P<0.001) in a naloxone-reversible and pertussis toxin-sensitive manner (P < 0.01) but not in the capsaicin group, suggesting mu-opioid receptor desensitization. Dose responses to systemic morphine were also reduced in the capsaicin group compared to the tactile group (P < 0.05). Capsaicin-induced hyperalgesia in 21-day-old rats was associated with an uncoupling of micro-opioid receptors in the forebrain. Opioid receptor desensitization in the forebrain may reduce opioidergic inputs to the descending inhibitory controls, associated with behavioral hyperalgesia and reduced responsiveness to morphine analgesia in capsaicin-injected young rats.

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