BBI Glia.docx

**Glia** - Immune Cell Lineages: - Hematopoietic originating cells differentiate into red and white blood cells from the common myeloid progenitor - Neuroectodermal/Neuroepithelial Progenitor differentiates into macroglia and neurons from the ectoderm of the embryo...

**Glia** - Immune Cell Lineages: - Hematopoietic originating cells differentiate into red and white blood cells from the common myeloid progenitor - Neuroectodermal/Neuroepithelial Progenitor differentiates into macroglia and neurons from the ectoderm of the embryo - Macroglia differentiate into oligodendrocytes, astrocytes, and polydendrocytes - Microglia come from a different progenitor than other neural cells - Comes from common myeloid progenitor, during development from the extra-embryonic yolk sac myeloid cells - Microglia differentiate very early in development, a period where the cell is very vulnerable to damage - Do not proliferate/turn over very often; very very long lived - Therefore an impacted microglia will be affecting your brain for a long time - They are the resident macrophages of the brain - Microglia: - Similarities to Macrophages: - Express MHC-II - Can present antigen - Can phagocytosis - Can engulf debris and dead cells and have phagocytic cups that can nibble away at things to refine environment - Express T cell and B cell stimulating proteins - Can engage in immune activity in the brain \[not often\] - Can be difficult to differentiate from macrophages when looking at protein expression - Differences form Macrophages: - Look very different; microglia have long branching \[arborization\] and a smaller soma because they are not usually doing the same activities that macrophages - Relatively immunogenically quiet, have other purposes as well - Regulated by CNS environment - Have NT receptors - Role of microglia in quite not challenged brain: when immunogenically quiescent - Promote synaptogenesis - Produce and release neurotransmitters - Maintain synapses - Synaptic scaling - Regulate the blood brain barrier - Role of microglia during immune challenge: - Responsible for neuroinflammation \[pro-inflammatory cytokine production in the brain\] - Antigen presentation - Cytokine and chemokine \[cytokines that attract other cells\] synthesis - Phagocytosis of apoptotic debris - Important in sickness behavior - Have a dynamic morphology; their appearance can indicate what it is doing - Ramified: very small soma, thing long arborization; when immunogenically quiescent - Spectrum of shape - Amoeboid: when a neuronal injury or a pathogen is detected; increase soma are, pull in arborization - Hyper-ramified State: not well understood, but immunogenically active and super branched and pumping out more cytokines - Ratio of soma size to radius of branching indicates state of microglia - How do microglia know which state to take on? - Receptors that tell the microglia which state to change to - Toll-like Receptors (TLRs): a type of pattern recognition receptor - On immune cells - Pathogen Associated Molecular Patterns (PAMPs): unique to each pathogen; receptor recognizes these specific sequences - 10 human TLR have been identified and 13 rodent TLR - Different TLR bind to different PAMPs - TLR4: binds to LPS \[most common form\]; expressed frequently - TLR3: binds double stranded RNA; viral dsRNA often acts as the signal that the receptor picks up - When virus bursts or cell infected by virus bursts, dsRNA is present floating around - TLR5: binds flagellin, the protein strand that aids bacteria movement - Glucocorticoid Receptors: cortisol is a common glucocorticoid produced during a stress response - Fractalkine Receptor (CX3CR1): binds fractalkine - Fractalkine (CXCL1): a chemokine \[a cytokine that is chemoattractant\] - Expressed by neurons and sitting on the membrane \[tethered\] - A damaged neuron cleaves a portion of the fractalkine \[soluble\] - Tethered fractalkine binds to fractalkine receptor, this tells the microglia that the neuron is happy and healthy - Therefore, the microglia can enact quiescent roles - Decreases pro-inflammatory production - Soluble fractalkine binds to fractalkine receptor, this tells the microglia to become immunogenically active - Therefore, the microglia changes shape to be more amoeboid and pump out pro-inflammatory cytokines - Requires calcium for both processes - Both changes of microglia function are because of differing gene transcription following activation - Other chemoattractant - How are microglia identified? Immunohistochemistry - Methodology used to visualize microglia - Stain proteins that are known to be on surface of microglia - Take an antibody that has a light chain specific to known protein on microglia - Incubate primary antibody with secondary antibody \[has fluorescence\] that amplifies the primary antibody; thus identifying microglia - Proteins on Microglia that are Stained For: - Iba-1 (Ionized Calcium-Binding Adapter Molecule 1): associated with the membrane of the microglia on the cytoplasmic side; also associated with the cytoskeleton - Has one hand on the membrane and one on the cytoskeleton - Helpful in facilitating the change of the microglia's shape and movement - Staining for the Iba-1 gives an indication of the microglia's shape - P2Y12 (Purinergic Receptor): receptor for purines; receptor for released ADP from a cell when the cell is under stress; ADP released by damaged neural tissues - Expressed on the outside of the membrane - Allows for visualization of the branching - MHC-II: not used very often due to commonality among cells - CD11b (Cluster of Differentiation Molecule 11b): expressed in all microglia and in some astrocytes, bind to cell-adhesion molecules - Aid in the movement and attraction of the microglia to injury - Most common way microglia are Experimentally activated is using lipopolysaccharide (LPS) - Toxin produced by very many types of bacteria that humans are very used to - Therefore, our immune cells already have receptors for it on them - TLR4 is the receptor that binds LPS and is thus activated - LPS induces sickness behavior - LPS is a PAMP - When a neuronal injury or a pathogen is detected via TLRs - Microglia change their morphology to an amoeboid state - Pro-inflammatory cytokines are released - Promote oxidative stress through creation of ROS - ROS are produced as a result of energy metabolism by all cells - Can be damaging if too many are produced without antioxidant activity -\> oxidative stress when microglia overactivated - ROS are used for cell signaling - Need a balance of ROS and antioxidants - Microglial activation is needed for host defense and neuron survival but overactivation has deleterious and neurotoxic consequences - Immunologically Quiescent Roles of Microglia: - Processes are always receiving signals from neurons and other glia - Processes are enwrapping and interacting with the neuron - Refining synapses through their phagocytic cups - Nibble away at dendritic spines - Surveillance and maintenance of synapses through signals from neuron - Increased actin in the microglia processes changes the cytoskeleton shape of the microglia - Fractalkine Receptor: dependent on bound fractalkines - Neurotransmitter receptor on processes pick up signals from synapses - Astrocytes: originate from neuroectodermal or neuroepithelial progenitor; sister of neuron - Start in the embryo not embryonic yolk sac like the microglia - Have immunological properties; immunocompetent - Three times as many as microglia - Can migrate, but do not usually change shape - Visualized by staining with glial fibrillary acidic protein \[GFAP\] - Similar to Iba-1 - Associated with the membrane on the cytoplasmic side - Similar cytostructural job - Tripartite Synapse: - Pre-synaptic neuron - Post-synaptic neuron - Astrocyte - Main Functions: - Maintain and support blood brain barrier - In contact with capillary - Functional barriers at interfaces between the non-neuronal tissue and CNS neural parenchyma along blood vessels, meninges, and tissue lesions - Hang out at borders; meningeal \[covering of the brain\] border and lesion borders - Form scarring around the lesions - Have an endfoot where the end of the process spreads out; lay against the outside of the blood brain barrier of the capillary - Helps regulate the transport of ions, molecules, and potential toxins in and out of the blood - Structural support as well - Immune Modulation - Balance out the microglia; like NMDA cycle - Express cytokines; anti-inflammatory cytokines - Does not mean that it always induces a reduction in inflammation, just usually quells immune cell activity - Not binary - Can also produce pro-inflammatory cytokines - Tripartite synapse responsible for regulating glutamate in the synapse - Glutamate: released by vesicles of pre-synaptic cell, bind to receptors on post-synaptic cell -\> exciting the neuron to fire an action potential - Excitotoxicity can occur with too much glutamate activity - Can also occur if there is astrocytic dysfunction - Glutamate that stays in synaptic cleft for too long -\> continuously stimulate post-synaptic neuron - Astrocytes have glutamate reuptake molecules that suck up residual glutamate - The glutamate is then recycled into glutamine by the astrocyte - Glutamine is given back to pre-synaptic neuron - The neuron can then convert the pre-cursor glutamine into glutamate for re-use - Overactive astrocyte, thus overactive glutamate recycling, would lead to too little glutamate - Several neuropsychiatric illnesses associated with astrocytic and microglial dysfunction - Other brain macrophages \[brain resident macrophages around places where there is a lot of blood supply to the brain\] are also involved, they can infiltrate the brain and take on microglia function