Role of glial cells in manganese neurotoxicity

The objectives of this focused review are to (i) provide a systematic overview of recent advances pertaining to the role of glia, namely microglia and astrocytes, in the neuropathology associated with excessive exposure to manganese (Mn), (ii) highlight possible mechanisms and factors involved in Mn-modulated, glia-derived neuroinflammation, and (iii) discuss the implications of excessive neuroinflammation on neuronal injury within the context of Mn overexposure. As this is not meant to be a comprehensive review on the topic of Mn neurotoxicity, the reader may wish to refer to several broader and more comprehensive reviews. After a brief introduction to Mn neurotoxicity, we first discuss the role of glial cells in neurodegeneration. Next, we review existing in vitro and in vivo studies that implicate Mn as a modulator of glial activation and ensuing neuroinflammation. This is followed by an examination of recognized and potential mechanisms that are involved in the modulation of glial inflammatory output by Mn; here the common pathways activated by Mn in glial and neuronal cells, including outcomes of such activation, are also addressed. We finish with a discussion of the implications of Mn-modulated glial activation for neuronal survival and with a list of data gaps in the topic that need to be filled in the future.

Manganese modulation of MAPK pathways: effects on upstream mitogen activated protein kinase kinases and mitogen activated kinase phosphatase-1 in microglial cells

Multiple studies demonstrate that manganese (Mn) exposure potentiates inflammatory mediator output from activated glia; this increased output is associated with enhanced mitogen activated protein kinase (MAPK: p38, ERK and JNK) activity. We hypothesized that Mn activates MAPK by activating the kinases upstream of MAPK, i.e. MKK-3/6, MKK-1/2 and MKK-4 (responsible for activation of p38, ERK, and JNK, respectively), and/or by inhibiting a major phosphatase responsible for MAPK inactivation, MKP-1. Exposure of N9 microglia to Mn (250 µm), LPS (100 ng ml⁻¹) or Mn + LPS increased MKK-3/6 and MKK-4 activity at 1 h; the effect of Mn + LPS on MKK-4 activation was greater than the rest. At 4 h, Mn, LPS, and Mn + LPS increased MKK-3/6 and MKK-1/2 phosphorylation, whereas MKK-4 was activated only by Mn and Mn + LPS. Besides activating MKK-4 via Ser257/Thr261 phosphorylation, Mn (4 h) prevented MKK-4's phosphorylation on Ser80, which negatively regulates MKK-4 activity. Exposure to Mn or Mn + LPS (1 h) decreased both mRNA and protein expression of MKP-1, the negative MAPK regulator. In addition, we observed that at 4 h, but not at 1 h, a time point coinciding with increased MAPK activity, Mn + LPS markedly increased TNF-α, IL-6 and Cox-2 mRNA, suggesting a delayed effect. The fact that all three major groups of MKKs, MKK-1/2, MKK-3/6 and MKK-4, are activated by Mn suggests that Mn-induced activation of MAPK occurs via traditional mechanisms, which perhaps involve the MAPKs furthest upstream, MKKKs (MAP3Ks). In addition, for all MKKs, Mn-induced activation was persistent at least for 4 h, indicating a long-term effect.