Pro-inflammatory cytokines, such as Tumor Necrosis Factor-alpha (TNFalpha), Interlukin-1 (IL-1) and Interlukin-6 (IL-6), were first discovered in the context of cellular activation and cell-to-cell communication in the immune system. Early studies on the role of cytokines in the brain suggested that their expression and activity are induced in response to an infection, head trauma, stroke, or neurodegenerative diseases. A growing body of data suggests that hyperactivation of the immune system has been implicated in the pathophysiology of major depressive disorder. Indeed, pro-inflammatory cytokines are expressed in the brain where they have been shown to regulate synaptic plasticity. TNFalpha, which is secreted from glial cells, regulates amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) glutamate receptor trafficking and neuroplasticity. In this study we investigated the long-term effect of TNFalpha on regulation of membrane expression of Glutamate Receptor 1 (GluR1), Glutamate Receptor 2 (GluR2) and Glutamate Receptor 3 (GluR3) AMPA receptor subunits. Rat primary hippocampal cultures were treated with various doses of TNFalpha for different periods of time. At the end of the treatment, changes in surface expression of AMPA receptor subunits were determined by biotinylation assay and western blotting. Treatment of hippocampal neurons with TNFalpha (6 nM and 60 nM) for 20 min or 1 day increased expression of GluR1 subunit of AMPAR;however, no effect was observed after 3 days of treatment. Interestingly, preliminary data showed a trend of increase in GluR3 surface expression after 20 min of treatment with TNFalpha. TNFalpha applied for 20 min, 60 min, 1 day or 3 days had no effect on surface expression of GluR2 subunits. These findings suggest that TNFalpha-induced increase in AMPA receptors on the cell surface represents a novel molecular mechanism for neuron-glia interactions that might contribute to neurotoxicity in neuropathological conditions, associated with elevated levels of TNFalpha. We also found that treatment with TNFalpha at low doses for 1 day significantly enhanced the number of synapse and co-localization efficiency of GluR1 and the postsynaptic density marker PSD95 (post-synaptic density 95). This effect was sustained for up to 3 days. Together, these findings provide novel evidence that glial-derived TNFalpha promotes synaptogenesis in AMPA containing synapses, which might play a crucial role during traumatic brain injury, inflammation, and various psychiatric conditions.
| Khairova, Rushaniya A; Machado-Vieira, Rodrigo; Du, Jing et al. (2009) A potential role for pro-inflammatory cytokines in regulating synaptic plasticity in major depressive disorder. Int J Neuropsychopharmacol 12:561-78 |
