NEURAL cells = neurons and glia Neuronal cells = neuron GLIA = Macroglia and Microglia MACROGLIA ASTROCYTES RADIAL GLIA OLIGODENDROGLIA (CNS myelin) SCHWANN CELLS (PNS myelin)

Embryonic Origin

Neurons + Astrocytes and Oligodenrocytes from the neural tubeSchwann Cells are from the Neural CrestMicroglia derived from macrophages (ie blood)

Gliogenesis

Glia of The CNS

Microglia

1. Nucleus of microglia 2. Process of microglia3. Lysosome4. Capillary5. Pericyte

Evidence for the monocytic origin of microglia

Bone marrow chimeras show that the brain becomes populated by donor specific cells of the haemopoietic origin Exogenously labelled monocytes enter the developing CNS and immunocytochemical studies show the changes in morphology as monocytes transform to microglia. Microglia share many cell surface and cytoplasmic antigens exclusively restricted to macrophages Immunocytochemical studies of transplanted CNS tissue show that the transplanted CNS tissue becomes populated by microglia bearing host specific makers for myelomonocytic cells.

Types of Microglia

Ameboid. Round cells - clustered, found in development. Develop in cultures of glia derived from neonatal brain. Concentrated in the corpus collosum. Proliferate. Ramified Adult form found in the brain - having numerous, fine processes. Found throughout the brain - resting. Dont proliferate when restingActivation states of adult microglia can identified by the structure and also the antigens expressed by them. Resting - fine processes OX-42 Activated - thick processes, vimentin, OX-42, proliferate, migrate Phagocytic - globular, vimentin, OX-42, OX-8, proliferate

- Resting microglia in normal brain- Activated microglia in diseased brain(marked induction of indolamine 2,3-dioxygenase)- Phagocytic macrophages (marked induction of indolamine 2,3-dioxygenase)

Microglial Function

Resting cells provide support and protect neurones. Growth factors released by microglia. Presence of viral or other antigens result in activation of microglia and migration towards the source of antigents. Secrete substance that kill, superoxide, nitric oxide, pro-inflammatory cytokines (TNF-alpha). Respond to degraded neurones to attack and remove damaged neurones. Myelin fragments activate microglia and result in them converting to a phagocytic or macrophage-like state. Consume cellular debris from neurones that have died.

Microglia and Cell Death

Differing views on the role played by microglia in neurodegenerative disease. Cell culture studies show that certain proteins produced by neurones during neurodegenerative diseases e.g. amyloid, cause microglia to become activated and kill neurones by the production of reactive oxygen species (e.g. peroxynitrate). Activation of microglia is different in culture as compared to in a brain. Some suggest that microglia do not play an active role in killing otherwise healthy cells in the brain. Microglia respond to damaged neurones by killing them Other viewed suggest that this also does not occur and that microglia only become activated in response to dead cells to remove debris.

Astrocytes

Out number Neurons 50:1

Protoplasmic Astrocyte: Gray Matter

Maintain blood brain barrier, interstitial environment Transport processes for K+, glucose

Fibrous Astrocute: White Matter

Support, structure Form scar tissue

Processes

GFAP glial fibrillary acidic protein intermediate filament Perivascular Feet (Foot Process, Vascular End-Feet) surround blood vessels

Astrocyte Function 1:Pathway

During development astrocytes provide the pathways used by neurones to migrate to their appropriate sites. Role in axonal guidance (the ability of the axon of a neurone to connect to its correct target Create boundaries or decision points for axonal movement. Proteins involved in these axonal pathfinding included CAMs, cadherins, integrins, selectins (more in later lectures).

Astrocyte Function 2: Support

Astrocyte Function 3(Growth, death)

Release large number of different factors Growth factorsCytokines - Important for survival and development of neurons During axonal damage, astrocytes proliferate around the site of the damage. This could be to aid repair but in extreme cases the astrocytes form a glial scar that can inhibit axonal regeneration.

Astrocyte Function 4: Calcium Signalling

Gap junctions between astrocytes that allow them to communicate rapidly using calcium. In culture calcium waves can be observed sweeping through astrocytes very rapidly

During a calcium wave, the synaptic environment changes dramatically. The astrocytic calcium wave reduces Ca2+ in the cleft. Decreased [Ca2+] in the cleft inhibits further neurotransmitter release, despite the arrival of action potentials. Only with termination of the astrocytic calcium wave will Ca2+ return to its original level in the synaptic cleft allowing neurotransmitter release at high levels. By simultaneously regulating neurotransmission in all of the synapses an astrocyte has enveloped, the astrocytic calcium wave may coordinate synapses into synchronously firing groups. Thus, all of the synapses enveloped or partially enveloped by an astrocyte may be within that astrocyte's domain of synaptic influence. In effect, one group of neurons could possibly influence another distant group of neurons through strictly astrocytic pathways. The implications of this are enormous; entire models of cognitive functioning could possibly be infuenced by these astrocyte to neuron communications.-Filipodial extensions..endfeet moving up neurotransmitter gradienthistory of use increases probability of astrocytic control

Astrocyte Function 5(glutamate)

- clear released glutamate. - GLAST, GLT-1. important in preventing excitotoxic death. Glutamate converted to glutamine and returned to neurones.

Astrocyte Function 6: BBB

The BBB normally excludes circulating immune effector molecules such as immunoglobulin and complement. Ediden (1972) reported that MHC antigens were not expressed in brain. Minimal neutrophil recruitment following excitotoxin-mediated neuronal degeneration. Naive T cells are unable to cross the BBB.

Astrocyte Function 7: K+ and Glucose Transport

GLUT= glucose transportor

Immunological privilege

In most parts of the body allografts (transplanted tissue from a donor) are rejected because of lack of immunological compatability (MHCs). Allografts placed in the CNS do not reject Oligodendrocytes transplanted into the CNS of some mice do not reject (Gumpel 1983). Transplantation of embryonic nigral tissue ameliorates functional deficiencies in Parkinson disease (Lindvall 1994).

Astrocyte Function 8(channels and receptors)

Ion channels, and neurotransmitter receptors- Express ion channels for Ca2+ and K+- Important to maintain interstitial level of ions- Express neurotransmitter receptors

Astrocyte Function 9(extracellular volume)

Determinant of the volume fraction of the extracellular space. Astrocytic swelling has been shown to occur as a result of both glutamate and adenosine receptor stimulation. Filopodial extension, or outgrowth of membrane, can be induced in astrocytes by focal application of glutamate. alters size and morphology altering extracellular space. Synaptic function is sensitive to the extracellular volume fraction. volume fraction decreases = increased concentration of extracellular molecules Ie increases effective concentrations of neurotransmitters astrocytes, through modulation of their size, can influence the excitability of neurons.

Astrocyte Function: ???

Possibilities for influencing neuronal excitability- Possible influence of change in astrocyte membrane potential- Effects on calcium concentration Research has shown that developing neurons in culture do not achieve fully functional synapses until glia are introduced though the mechanism of action for this is not known. Regulation of Metals - can uptake metals which influence neuronal activity

Cytokines

Produced by both astrocytes and microglia and by neuronsIn the nervous system these protein can alter cell survival and proliferation. either a pro-inflammitory or anti-inflammatory response.

Cytokines can have different actions depending on: Their point of release, The cells they act upon Concentration.

Include, interferons (IL), interleukins(IFN), tumor necrosis factor (TNF)family, tranforming growth factor family (TGF), chemokines, colony stimulating factors (CSF) variety of related protein which can also act as growth factors such cilliary neurotrophic factor (CNTF).

Structure of Cytokines

Molecular weight less than 30 kDa One of the hematopoietin family, interferon family, tumor necrosis factor family or chemokine family High degree of -helix structure Little or no -sheet structure Share similar polypeptide fold

Cytokine Receptors

There are specific receptors for cytokines that can be activated by a variety of cytokines. Microglia, astrocytes and oligodendrocytes express receptors for many cytokines IL-1, IL-4, IL-10, TNF-R1, TNF-R2 TGF-b, IFN receptors, gp130 and CNTR. Microglia also express IL-6 receptor Gp130 can bind a variety of cytokines including LIF and IL-6.

Three subfamilies:GM-CSF receptor subfamily, IL-6 receptor subfamily, and IL-2 receptor subfamily

Signal transduction mediated by cytokine receptors

Cytokines Produced (neurones)

Stimulus Stresses Hypoxia, Axotomy, other damagesCytokine FunctionM-CSFmicroglia activationchemokinesmicroglia recruitment

Cytokines Produced (Astrocyte)

Stimulus Viruses, microglia, macrophage IL-1beta, TNF, TGF-beta Cytokine FunctionIFNa/bAntiviral activityIL-6Inflammation, glial activation, acute phase responseTGF-bDownregulation if inflammation, neuroprotectionM-CSF, GM-CSFMicroglial proliferation, activationChemokinesLeukocyte recruitment, glial activationCNTF, LIFNeuroprotection

Cytokines Produced (Microglia)

Stimulus Damaged cell, Virus bacteria, Pathogens molecules, Activated T cellsCytokine FunctionIL-1 TNFTissue breakdown/repair, glial activationIL-6inflammation, glial activation, acute phase responseTGF-bDownregulation if inflammation, neuroprotectiontissue repairIL-10Downregulation if inflammation, neuroprotectionIL-12, IL-15, IL-16Stimulates cell mediated immune responseChemokinesLeukocyte recruitment, glial activation

Other Glial Types

Radial Glia - a transitory population of cells found during development. Act as a scaffolding for neuronal development. Have long projections that extend from the inner to the outer surface of the cortex. Neuronal progenitor cells migrate along them during development. Can develop into other glial types or even neurones. Bergmann Glia, radial astrocytes of the cerebellum. Lie in the Purkinje cell layer and have long projections that extend into the molecular layer. Each Bergmann glia ensheaths 2000-6000 Purkinje cell synapses. Mller Glia, radial glia of the retina, with microglia like activity. Have projections that contact every layer of the retina.

Radial glia

Embryonic scaffold throughout CNS Guides for radial migration of neurons Produce matrix and adhesion proteins