Figure 7.

Overview of the effectors that may be involved in microglia dialog with other glial cells and neurons, upon exposure to unconjugated bilirubin (UCB). Extracellular increased levels of glutamate by UCB-induced secretion from astrocytes, microglia, and neurons, as well as by astrocytic uptake inhibition, together with the release of inflammatory mediators, such as interleukin (IL)-6, tumor necrosis factor (TNF)-α, and IL-1β from astrocytes and microglia (A), are included in cross-talk effectors. Also comprised are members of the oxidative stress, including the reactive oxygen species (ROS) produced by neurons and oligodendrocytes (OLGs), as well as nitrosative stress by nitric oxide (NO) generated by microglia, neurons, and OLGs, and elevated release of ATP from neurons, during exposure to UCB. (B) Phagocytosis upon short stimulation with UCB is visualized in microglia immunostained with an antibody against Iba1 (red) and using Hoechst 33258 stain to visualize the nucleus (blue), by counting the number of ingested fluorescent latex beads (green). Microglia intervenes in glutamate tissue homeostasis by both releasing and retaining glutamate when exposed to UCB. But microglia also reacts to UCB stimulus, very rapidly, through the activation of mitogen-activated protein kinases (MAPKs), mainly the c-Jun N-terminal kinases (JNK)1/2 and p38, and of the nuclear factor-kappaB (NF-κB) that is translocated to the nucleus, determining the activation of transcription factors (TF) and further release of cytokines. These pathways may mediate both the phagocytic and the inflammatory phenotypes and are related with a shift from an elongated morphology to a large and amoeboid shape (1). Longer exposure to UCB leas to fragmented and condensed cytoplasm, a feature of dystrophic microglia (2) indicative of cell degeneration and senescence. The multifaceted profile of microglia is determined by the stimulus type and duration, and dialog between cells. Although microglia activation may promote astrocytic reactivity, the release of pro-inflammatory cytokines, glutamate, and NO from these cells may exacerbate microglia reactivity, which by propagating inflammation will sensitize neurons and decrease their survival upon UCB exposure. Moreover, astrocytic deregulation and degeneration by UCB determine inaccurate neuron–astrocyte interactions, causing neuritic arborization and synaptic transmission impairment, favoring brain damage. OLGs do not play a role in promoting inflammation although, like neurons, are damaged by UCB and associated inflammatory processes.(C) OLGs and dorsal root ganglion neurons co-cultures incubated for 24 h with 100 μM human serum albumin (HSA) at 7 days in vitro evidence an increased number of myelinating OLGs. (D) Incubation with 50 μM UCB plus 100 μM HSA reveal a delayed myelination once a number of neurofilaments are still not myelinated when compared to the image in (C). To evaluate myelination, OLGs were fixed at 21 days in vitro and immunolabeled for myelin basic protein (MBP, red) and neurofilament (NF, green). Nuclei were counterstained with Hoechst 33258 dye (blue).