The effect of microglia on embryonic dopaminergic neuronal survival in vitro: diffusible signals from neurons and glia change microglia from neurotoxic to neuroprotective

Abstract

When embryonic dopaminergic neurons are transplanted into the adult brain, approximately 95% die within a few days. To assess whether microglia activated during transplantation might be responsible for this rapid death, we examined the effect of microglia on rat embryonic dopaminergic neurons in vitro. Conditioned medium from 7-day-old microglia was found to decrease the number of dopamine neurons surviving in primary culture, but activation of the microglia with N-formyl-methionyl-leucyl-phenylalanine (FMLP) or Zymosan A did not increase the toxicity of the conditioned medium. We next tested the effect of coculturing microglia and dopaminergic neurons by placing microglia in semipermeable well inserts over the neuronal cultures. The presence of microglia now increased dopaminergic neuronal survival, microglial activation again having no effect. To increase yet further the possible interactions between microglia and neurons, the mesencephalic cells and microglia were mixed together and placed as a tissue in three-dimensional culture, and here again the presence of microglia increased dopaminergic neuronal survival with no effect of activation. Contact of microglia with the mesencephalic cells therefore converted them from being toxic to dopaminergic neurons to promoting their survival. The change in microglial effect from toxic to protective was caused by soluble molecules secreted by cells in the neuronal cultures, as conditioned medium derived from microglia-neuronal cocultures also had a dopaminergic neuron survival effect, indicating that microglia in cocultures behave differently from microglia removed from neuronal and glial influence. Microglia cocultured with either neurons or astrocytes downregulated inducible nitric oxide synthase (iNOS), indicating a decrease in the production of nitric oxide and possibly other toxic molecules. These findings indicate that in their natural environment, microglia are likely to be beneficial for the survival of embryonic dopaminergic grafts.

Brain injury activates microglia that induce neural stem cell proliferation ex vivo and promote differentiation of neurosphere-derived cells into neurons and oligodendrocytes

“…The effects of microglia often differ between in vivo and in vitro models (Zietlow et al, 1999, Eskes et al, 2003. It is, therefore, important to create in vitro models that closely mimic the activation pattern of microglia in vivo and can replicate their effects on other cells in vivo.…”

Microglia expressing interleukin‐13 undergo cell death and contribute to neuronal survival in vivo

“…Activation of microglia has been proposed to play a pathological role in a variety of neurodegenerative diseases including Parkinson's disease, Alzheimer's disease, multiple sclerosis, AIDS dementia complex and amyotrophic lateral sclerosis (Raine, 1994;Vila et al, 2001;Minagar et al, 2002;Williams and Hickey, 2002). Activated microglia secrete many types of molecules, some classed as neuroprotective, others as neurotoxic (Zietlow et al, 1999;Kim et al, 2000;Ryu et al, 2002a). The roles of microglia-derived TNF-␣ and iNOS are well established in neuronal death (Kim et al, 2000;Ryu et al, 2002a;Choi et al, 2003), although the presence of iNOS in human microglia is disputed (Denis, 1994;Colasanti et al, 1995;Walker et al, 1995) and microglia in non-LPS-based neurodegeneration experimental models such as neurotrauma and excitotoxicity are not the main sources of TNF-␣ or iNOS (Lau and Yu, 2001;Acarin et al, 2002).…”

Microglia Kv1.3 Channels Contribute to Their Ability to Kill Neurons

“…Importantly, the remaining cells were astrocytes, with a very low incidence (Ͻ0.1%) of microglia. A key advantage of the present study is that, with the porous well inserts (Zietlow et al, 1999;Polazzi and Contestabile, 2002;Xie et al, 2002), we could separately treat either microglia or neurons, then wash them before coincubation. Moreover, after the cells had communicated via diffusible factors in the medium, the neurons and microglia could be separated for further analysis.…”