Lecture 13: Microglia and Ependymal Cells Flashcards
microglia can be a
friend and a foe
ependymal cells are important …
important defence mechanism in looking after our internal environment
Microglia features
small cells, highly branched,
each normally own domain (15-30μm)
constitute between 5-20% of all glial cell
important for surveying and reacting to the internal environment
Identifying microglia
Difficult to identify
Immuno-cytochemical identification
Iba1 - actin binding proteins therefore good for marking microglia
Functions of microglia
homeostasis – routinely monitor extracellular environment
activity dependent synapse elimination - can manage synapses and if they need to be removed they can do that
phagocytosis of surplus neural precursor cells
defence function -> normal (-> exacerbate inflammation) (can go wrong and this can cause disease)
many roles in disease processes
Have different functions depending on the stage of life, region of the CNS and the environment (whether it is health or disease) ultimately they act by sensing and regulating the environment of the CNS, they eliminate certain structures such as pathogens, dead cells and protein aggregates and others and can secrete cytokines and neurotrophic factors for this purpose and for other immune functions too, during life they can contribute to neurogenesis, neuronal circuit shaping, vascular shaping, and homeostasis
Distribution of microglia
varies dependent on region
greater in grey matter
around synapses
with astrocytes
Microglial morphology - embryonic development
Invade CNS in late embryonic development
́development from haemopoietic cells of bone marrow
́Myeloid origin (not ectodermal) ́
Microglial morphology - in adult
In adult
́Variable shape – rearrangements of actin cytoskeleton ́Iba1– shape changes (protein that contributes to the shape change)
́Can divide - can divide quite a lot but they can also slowly renew themselves, can be around for a long time (2 decades) but also can replicate in response to an immune or toxin attack as well
́Renew slowly at a median rate of 28% per year, and some microglia last for more than two decades.
Microglial morphology
Invade CNS in late embryonic development
́development from haemopoietic cells of bone marrow
́Myeloid origin (not ectodermal) ́
In adult
́Variable shape – rearrangements of actin cytoskeleton ́Iba1– shape changes
́Can divide
́Renew slowly at a median rate of 28% per year, and some microglia last for more than two decades.
Microglial origin
Not derived from the same embryonic lines as neurons and astrocytes, they actually share the same origin as macrophages and other haemopoetic cells so really they originate from those cells that are scavenging CNS for plaque, damaged neurons and infectious agents as well
develop from haemopoetic cells of bone marrow - myeloid origin rather than ectodermal
Ultrastructure of microglia
elongated nuclei “bean” shaped with peripheral heterochromatin
́ scattered cisternae of rough endoplasmic reticulum and Golgi complexes at both poles
́ a microglial cell is adjacent to a neuron there is usually a thin astrocytic process (arrow)
Has dark cytoplasm and it has granules of rER and Golgi complexes at both poles so its nuclei takes on a bean like shape
Two states of microglia (remember form depends on the function)
resting state and activated state
from resting go through morphological changes to become activated, structure becomes quite different
Resting state of microglia overall function
(not moving)
but
Ramified (branched) processes survey the microenvironment (Surveying microglia) - can survey all the time to see if they need to react to something that is in the brain
Maintain a constant level of available microglia to rapidly detect and fight infection - guardians of the brain
IBA1
actin binding protein, used to identify microglia
Activated state of microglia overall function
Activated Ameoboid free movement throughout the neural tissue =scavenging, phagocytose debris, Development/pruning
why are we interested in microglial function?
Resting = Surveillant
́sensing the condition of the extracellular milieu
́synaptic pruning (prune if required) and developmental apoptosis, neurogenesis (influences neurogenesis)
assessing synapses
́Detect injury
́Receptors in the microglial cell membrane
́initiate the process of active response
Response to injury
́synthesis and release of chemokines – attract other microglia (can make chemokine etc so it can elicit a response)
Proliferation or entry of monocytes via BBB (often faulty in disease)
Become motile, apoptosis, phagocytic
activity is wide and varied in response to toxicity
resting = surveillant …
Resting = Surveillant
́sensing the condition of the extracellular milieu
́synaptic pruning (prune if required) and developmental apoptosis, neurogenesis (influences neurogenesis)
assessing synapses
Microglia - detect injury
́Detect injury
́Receptors in the microglial cell membrane
́initiate the process of active response
Microglia - response to injury
Response to injury
́synthesis and release of chemokines – attract other microglia (can make chemokine etc so it can elicit a response)
Proliferation or entry of monocytes via BBB (often faulty in disease)
Become motile, apoptosis, phagocytic
By the resting (surveillant) microglia the entire volume of the brain is examined every
4 to 5 hours
Four main themes of function for microglia …
1 - neural development - really important in programmed cell death
2 - homeostasis of synapses/synaptic interactions - can also survey, monitor and prune presynaptic terminals and dendritic spines to maintain homeostasis of the synapses, phagocytosis, synaptic plasticity
3 - adult neurogenesis - includes phagocytosis adult newborn cells
4 - neurological and psychiatric disorders - associated with these disorders if the physiological functions have been affected in these microglia
Microglia actively
survey and shape neuronal circuit structure and function
Microglia and neural development
Cytokines/inflammation ….
inflammatory molecules
cytokines
apoptosis can release inflammatory molecules cytokines so that they can assist in synaptic pruning and apoptosis
Growth factors …
Growth factors
promote synaptic plasticity and neurogenesis
the same microglia have the capacity to release growth factors which likely promote synaptic plasticity
Microglia and homeostasis of synapses
microglia affect basal neurotransmission and synaptic plasticity i.e. long term potentiation
- Microglia - sense defunct synapses and phagocytose them in normal brain
- Via fractalkine
- Synaptic pruning by microglia is essential for the remodeling of synaptic circuits synaptic plasticity
In the adult brain we have these very soluble factors such as BDNF and TNFalpha that can be released by microglia which affects the basal transmission and plasticity and they can also act directly on astrocytes and affect it as well
In addition, have fractalkine signalling via the soluble fractalkine which are released by neurons and microglia have fractalkine receptors which can modulate the microglia and synapse interactions which can affect LTP behaviour in the mature CNS, the neurons can talk to the microglia and the microglia can also talk to the neurons and affect synaptic plasticity and basal neurotransmission
Microglia are important for basal level functioning in the brain
Microglia and neurogenesis
microglia regulation of neurogenesis during lifespan
Two regions of the brain where we make new neurons as adults and one is the subventricukqr zone near the lateral ventricle and the other is the subgranular zone which is in the denture gyrus of the hippocampus
Neuro genesis doesn’t replace all the neurons in the brain, only a few that become new neurons, figuring out how to up regrulate theses in neurodegenerative disorders
Important regulation of neuro genesis over age as well, When your have the first* neuronal stem cells hey can phagocytise not very good cells (spoptotic and dysfunctional progenitors), can support progenitor migration, synaptic maintenance, secretion of tropic factors
Over time, disease processes such as Parkinson’s or Alzheimer’s and other modulators factors come into play such as signalling factors, Eli genetic factors, genetic factors and environmental factors
Pathologically activated microglia, has a huge effect on neurogenesis
Can do the opposite of healthy - can impair phagocytosis of apoptosis and dysfunctional progenitors, dysfunctional progenitors migration or altered, failure of synaptic maintenance, secretion of cytotoxic factors
The good and the bad of microglia
injury to the brain ….
Proliferation and hypertrophy -> GOOD or BAD
Good response by microglia to local injury - how does it start?
microglia branches quickly hone in on the injury site and then the microglia surround it and form a barrier to protect it, first response to injury that is quire fast so that healing can start
Good response by microglia to local injury - PRRs and TLRs
Microglia actively survey the environment with pattern recognition receptors (PRRs) - recognise pathogen associated patterns , molecular patterns of pathogens
TLR: Toll-like receptors
= type I transmembrane receptors with extracellular domain
Extracellular side of TLR-recognition of the microbial product
The extracellular side of the TLR recognises the microbial product which sends a message to the cytoplasmic side of the toll like receptor so there it can recruit signal,ing molecules and alter kinase activation/transcription factors and finally it can modify gene expression and can tailor the immune response to the pathogen of origin (reacts to what the pathogen is) therefore it is important for inflammation/defence
Cytoplasmic side of TLR - TIR domain
- recruit signalling molecules
- alter kinase activation / transcription factors
- > Modify gene expression
Tailors immune response to the specific pathogen
Inflammation / defense
PRRs =
pattern recognition receptors
TLRs =
toll-like receptors
TLR2 and TLR4
bacterial lipids
TLR3
viral DNA
TLR9
Bacterial DNA
ROS
reactive oxygen species = neurotoxic
Microglial activation is good but can become over activated …
Ligand recognized – internalized – eliminated (good)
BUT
́Microglial can become overactivated (BAD)
́produce cytotoxic factors = neurotoxic
́Superoxide, nitric oxide, tumour necrosis factor-α (some are good and actually protect the environment but it about the over activation and overproduction of these factors that makes them bad)
́Causes of overactivation -not well understood
́Environmental toxins - pesticide (environmental toxins such as pesticides can actually activate our microglia)
́Neurodegenerative disease -> many microglia (many are over activated)
́Alzheimer’s disease – microglial activation increases as disease progresses
Microglia actively survey the environment with pattern recognition receptors (PRRs) - TLR activated - inflammation/defense over exaggerated/overactivated….
Inflammation/defense (over exaggeration causes the problem) = too much ROS (microglial apoptotic cell death or - too much pro inflammatory gene expression
- > overactivity ROS extracellular
- > neurotoxicity
ROS can actually be good for signalling but it is the over activation that causes the problems
ROS leads to
neurodegeneration
Toxins can cause neurotoxicity through activation of ___________ -> increased _______
NADPH oxidase
ROS (reactive oxygen species)
Toxins examples can cause neurotoxicity through activation of NADPH oxidase -> increased ROS
Lipopolysaccahride (E coli), paraquat (chemical herbicide), MPTP (synthetic heroin -> acute Parkinsonian symptoms), amyloid beta, thrombin etc.
NADPH oxidase activation to produce ROS is activated in….
NADPH oxidase activation to produce ROS is activated in; Alzheimer’s disease
Parkinson’s disease
Involved in the neural damage in response to cerebral vascular
dysfunction
(experimentally - Ischaemic stroke is reduced in mice lacking functional NADPH oxidase)
becoming activated microglia with ROS
activated microglia when they start becoming activated, if there is inflammation we can have kinase activation, transcription factor activation and intracellular ROS concentration increases
need to limit the level of response in the microglia because if there are too many microglia they can get damaged from the ROS as well
Microglia can act on other cells … astrocytes…
Microglia -> astrocyte dysfunction -> damage
activation of microglia
release cytokines etc
alter astrocyte behaviour impaired glutamate uptake activation of postsynaptic inotropic glut receptors Ca2+ influx Excitotoxicity - neuron death
Environment is important at cellular level
Remember in disease - many things are happening at once
Astrocytes are important in mopping up glutamate (another neurotransmitter at synapse), what can happen with microglia is that the production of the ROS in the microglia these can inhibit the glutamate transporters that are in the astrocytes so you have glutamate that is not being mopped up by astrocytes and they are in the extracellular environment in the synapse which will cause a lot of problems with the neurons because it going the bind to extracellular and extrasynaltic receptors which potentially causes increase in calcium concentration in the neurons causing neurotoxicity and killing them - misbehaving microglia can directly affect astrocytes which can then directly affect neurons because of the extra amount of glutamate around and causing that extra toxicity to be around the neurons and therefore you are not going to get the prime depolarisation either
Environmental activation of microglia - air pollution
Air pollution - pervasive and harmful environmental toxicants modern world,
Environmental activation of microglia - ASD
Air pollution - pervasive and harmful environmental toxicants modern world,
́ Large scale epidemiological studies - prenatal air pollution exposure - increased risk of neurodevelopmental disorders – autism spectrum disorder (ASD). - link between environmental toxins, microglia and potentially ASD as well
Environmental activation of microglia - diesel exhaust particles (DEP)
́ Diesel exhaust particles (DEP) = primary toxic component of air pollution -> activate microglia in vitro and in vivo in adult rodents
Impact of gestational exposure to DEP on microglial morphology - developing brains, male and female mice.
́ increased inflammatory cytokine protein
́ altered the morphology of microglia, = activation or a delay in maturation, only male mice
Microglia and neurons overlap in juvenile offspring prenatally exposed to DEP
increased overlap between microglia and neurons in juvenile offspring prenatally exposed to DEP
overlap affects how the neuron works
volume of microglia is bigger with exposure to DEP particles compared to normal
Male overlap between microglia and neuron is bigger in the exposed vs in the not exposed so they could say that the exposure of the environmental toxin prenatal could affect male mice
does affect mice developmentally
Microglial can become over activated and cause neurotoxicity
Have a toxic trigger and then release neuro toxic factors and have this direct neuro toxic insult and the neuron responds and you may get this microglia activated from this neuron and then you get a self-perpetuating neurotoxicity cycle happen with these neurons
PET images - AD vs normal patient
more microglial activation in AD
Targeting microglia could be a potential therapy for AD
Since we know it is toxic
Tangles of tau protein form inside neurons as Alzheimer’s disease develops. The tangles attract immune cells known as microglia. The protein APOE collects inside microglia, helping the immune cells transform into their activated form. Activated microglia attach and injure neurons, causing brain damage and leading to cognitive decline
If you can eliminate the microglia if they are in this activated form then you can potentially inhibit the death of the neurons
DAM - disease microglia
a unique microglia type associated with restricting development of AD
How they found
Found a disease microglia -initially have an early stage change in the microglia cells by gene expression and then have a decrease in the immune checkpoints (immune response genes are changing) to cause it to increase phagocytosis and can see this change in the microglia as the genes progress
DAM - how they were found
How they were found…
Wildtype vs AD model
Single cell RNA sequencing of all immune cells - sequenced all the single cells of the immune cells within the mouse models and looked specifically at the microglia
Found a disease microglia -initially have an early stage change in the microglia cells by gene expression and then have a decrease in the immune checkpoints (immune response genes are changing) to cause it to increase phagocytosis and can see this change in the microglia as the genes progress
Gene expression profiles of microglia reveal how microglia…
…progressively switch states with disease
homeostatic microglia - stage 1 DAM - stage 2 DAM
Hypothetical dual role for disease associated microglia (DAM) in the procession of disease
if we can understand the gene expression and understand that there is this progression in the changes of microglia through the activated state then we can actually target this microglia
microglia can produce …
cytokines (both pro-inflammatory and anti- inflammatory cytokines)
Also:
growth factors, chemokines and neurotrophins.
Ependymal cells line
the fluid filled cavity of the brain which is the ventricular system, line the ventricular system of the CNS
Ependymal cells are within the ventricle and choroid plexus makes the cerebrospinal fluid (protective from impact), ependymal cells are lining the ventricle where you have the choroid plexus as well and they have cilia which can move cilia, form a bit of a barrier between the CSF and the brain exchanges
Ependymal cell shape
cuboidal -columnar shape
apical microvilli, cila
contain intermediate filaments
Function of ependymal cells
Ependymal epithelium: interface between brain and CSF
́Brain-CSF barrier and filter - filtration barrier
́Inflammatory response - can release factors and protect the cells that surround them
́A regulator of osmotic pressure
́ Control the concentrations of regulatory peptides - ensure that their are not too many, regulation
control of CSF flow - can move the CSF through the brain by the cilia (effective stroke pushes fluid and then recovery stroke puts it back in the position to do an effective stroke again)
Trophic and metabolic support
́In specialized locations secrete cerebrospinal fluid
́Function as neural stem cells, (self-renewal and multipotency)? - increased cell proliferation and neurogenesis in the adult human Huntington’s disease brain - The demonstration of endogenous stem/progenitor cells in the hippocampus and the subependymal layer (SEL) of the basal ganglia in the adult mammalian brain has raised the exciting possibility that these undifferentiated cells may be able to generate neurons for cell replacement in neuro- degenerative diseases such as HD.
‘Brain-washing’
The brain nightly rinses itself though csf so a good night sleep is essential for brain health which might help to clear the brain of the toxic waste, rhythmic waves of blood and csf
