Beta-amyloid and brain-derived neurotrophic factor, BDNF, up-regulate the expression of glutamate transporter GLT-1/EAAT2 via different signaling pathways utilizing transcription factor NF-kappaB

Malfunctioning of high-affinity glutamate transporters is believed to contribute to the accumulation of toxic concentrations of glutamate and, thus, trigger the cellular mechanisms of neurodegeneration. Emerging data point to the presence of excitotoxic component in Alzheimer's disease (AD) and aberrant expression of glutamate transporters in this neurodegenerative malady. Neuronal soluble factors are essential for differential expression and fine tuning of the astroglial glutamate transporters, GLT-1/EAAT2 and GLAST/EAAT1. However, the nature of factors specifically affecting glutamate uptake in AD is largely unknown. The overproduction of neurotoxic beta-amyloid peptide (Abeta), a major constituent of amyloid plaques, and marked down-regulation of BDNF, a neuroprotective factor, are hallmarks of AD pathophysiology. None of these typically neuronal factors was capable of changing the pattern of glutamate transporter expression in undifferentiated rat astrocytes that predominantly expressed GLAST. In differentiated astrocytes, BDNF and, to a lesser extent, subtoxic concentrations of Abeta 1-42 (1-5 microM) induced the expression of GLT-1 and increased glutamate uptake, whereas the GLAST levels were unaltered by these factors. The BDNF-dependent up-regulation of GLT-1 in differentiated astrocytes was partially antagonized by the activation of metabotropic glutamate receptor 4 (mGluR4), but not by group I or II mGluRs. Activation of transcription factor NF-kappaB appeared to be a shared essential, but not a sufficient molecular event in the BDNF- or Abeta-dependent induction of GLT-1. The BDNF-dependent activation of NF-kappaB and up-regulation of GLT-1 was critically dependent on the upstream activation of p42/p44 MAP kinase signaling, whereas the inhibition of these MAP kinases dramatically increased the Abeta-dependent activation of NF-kappaB and production of GLT-1. The capacity to up-regulate astroglial glutamate uptake system, that apparently represents a novel element in the neuroprotective repertoire of BDNF, can, however, provide adverse effect under certain insults when glutamate transporters start operating in reverse direction. The Abeta-dependent up-regulation of GLT-1/EAAT2, more pronounced under the deficit of MAP kinase signaling, may attenuate synaptic efficacy and, thus contribute to the impairment of neuroplasticity in AD.