lecture 12: astrocyte pathophysiology

T/F: ~50% of cells in human brain are glial

true; astrocytes contribute to at least 1/3 of brain mass

what are the types of astrocytes, where are they, + what do they do (2)?

protoplasmic → grey matter (wrap synapses around neuronal cell bodies)

fibrous → white matter (wrap oligodendroglial cells + nodes of Ranvier)

what did a study by Han et al., find about astrocytes?

mice with transplanted human glial progenitor cells → gave rise to human astrocytes

• these mice w/ human astrocytes performed better in learning + memory tasks as well as displayed improved LTP (strengthening of synaptic connections)

what are 3 main aspects of astrocyte involvement?

1. modulation of synaptic function → tripartite synapse

2. astrocyte networks

3. glymphatic systems

how can we visualise astrocytes?

staining for GFAP

T/F: we can stain all astrocytes with GFAP

false; it only stains a proportion of their larger processes.

what are 5 main proposed astroglial functions?

1. developmental

2. structural

3. metabolic

4. homeostatic

5. signalling (info-conveying, instructional)

T/F: all astrocytes perform the same functions

false; they are heterogenous in multiple aspects

1. morphology

2. membrane currents

3. expression of receptors (glutamate or other NTs)

4. expression of GLUTs

5. coupling of DJs

6. Ca2+ signalling

7. vol reg

what is the tripartite synapse?

a synapse composed of 3 elements; a presynaptic neuron (1), post-synaptic neuron (2), and astrocyte process on the two

what is the tripartite synapse hypothesis?

during synaptic activity, neurons release NTs + astros respond to these w Ca2+ elevations

in turn, this controls neuronal excitability AND synaptic transmission via this Ca2+ dependent release of gliotransmitters, including

• GABA

• glutamate

• ATP

• adenosine

• D-serine

  • etc….

what led to this evidence for gliotransmission via astrocytes (2)?

astrocyte Ca2+ signalling in brain slices and in vivo experiments showed that astrocyte Ca2+ levels increased after

• electric stim of schaffer collaterals in hippocampus (brain slices)

• whisker stimulation in mice (in vivo)

this proved that astrocytes have cellular excitability

T/F: it has yet to be shown that although astrocytes have excitability, they do NOT integrate synaptic information

false; in hippocampus slices, a Ca2+ increase response triggered by Schaefer collateral stimulation was blocked by mGluR antagonists.

• a lack of correspondence was seen b/w observed + expected responses which was indicative of synaptic integration

overall, what did the tripartite synapse show?

1. astrocytes have cellular excitability (respond w increase of Ca2+ to sensory stim)

2. astrocytes can discriminate activity of diff pathways (e.g., astros in CA1 of hippocampus did NOT respond to Glu after stim of laves, but did respond to Glu released by Schaeffer collateral)

3. Ca2+ signals show non-linear relationship w synaptic activity Or exogenous application of NTs

how can astrocyte modulation @ tripartite synapse influence memory?

through LTP

• mice lacking CB1R (cannabinoid T1Rs) in astroglial cells OR glutamaterigc/GABAergic neurons were exposed to cannabinoids

• these mice lacking astrocytic CB1R had functioning spatial working memory AND LTD was not induced

  • but memory impairment + LTD induction was seen in mice lacking CB1R in neurons

Since Han et al showed cannabinoids can exert their action on working memory also in absence of receptors on neurons, was concluded that astrocytes mediate their action.

if CB1R knocked out on GABAergic axon, working memory impairment + LTD still occurred so CB1 action was not at these sites

• however if KO on astro, neither occurred (so CB1 important R is on astro, and astros have a role in this working memory)

how can astrocyte modulation @ tripartite synapse influence sleep reg?

astrocytes can release gliotransmitters via (eg exocytosis). when ATP is released, astrocytes can mod the behaviour via A1Rs in sleep reg via our circadian clock by conversion of ATP → Adenosine

by which mechs can astros release NTs (6)?

1. swelling-induced opening of v-gated ion channels

2. reverse operation of EAATs

3. Ca2+ dependent exocytosis

4. cystine-glutamate exchanger

5. connexion hemichannels

6. P2X7 Receptor

how can astrocytes form networks with each other?

via formation of gap junctions to form gap junction channels (composed of connexins)

what properties do GJs in astrocyte display?

1. selective permeability

2. voltage-dependent

T/F: astrocytes can respond to the stimulation of adjacent functional networks

false; they communicate within barrels but not w astrocytes in surrounding barrels

how do astrocytes DIRECTLY regulate neuronal activity?

by supporting propagation of Ca2+ waves which can lead to signalling via connected astrocytic networks (GJs) → release of ‘gliotransmitters’ to cause release of NTs

how can astrocytes have a role in epilepsy?

by astrocytic network reg of neuronal activity via release of NT and/or K+ buffering; extracellular accumulation of K+ follloing repeated/prolonged activation of synapses can depolarise the neuron making them hyper excitable

what happens to astrocytes in injury/infection/diseased states?

they exhibit reactive astrogliosis, can respond to signals from a variety of sources, and can release a mix of neurotoxic/protective molecules.

chronić astrocyte reactivity can lead to…

loss of normal or gain of abnormal function → exacerbate disease state w dysfunctional reactivity to worsen disorder outcome

T/F: genetic mutations + polymorphisms can lead to cell-autonomous dysfunctions in astrocytes → nonreactive disease states

true

what does the dual role of homeostatic cascades expressed in astrocytes control?

1. physiological conditions

   1. K+ buffering

   2. reg + dist of H2O

   3. control of extracellular [NTs]

2. pathological conditions

   1. failure in H2O transport (brain oedema)

   2. glutamate excitoxicity (several of NT transporters w Ca2+ dependent exocytosis)

   3. inadequate K+ buffering (excitation)

   4. release of ROS + pro-inflammatory factors

T/F: reactive astrogliosis (assoc. w neuroinflammation) is present in several neurodegenerative diseases

true

which main neurological conditions are astrocytes implicated in (3)?

1. alexander disease

2. alzheimer’s disease

3. epilepsy

explain the role of astrocytes in alexander disease

they are the primary cause; a genetic disorder of astrocytes (dominant, gain-of-function mutation in GFAP gene)

• loss of fine processes

• heterogeneous reactive phenotype

• reduced levels of AQP channels

• not coupled @ GJs + connexins are decreased

• reduced buffering of glutamate + accumulation of GABA

• lower levels of K+ channels

  • seizures

  • psychomotor disturbances

  • premature death

what is the proposed role of astrocytes in AD?

reactive astrogliosis around amyloid plaques OR diffuse amyloid deposits has been present but not clear if it precedes disease onset

• also, atrophic changes in astroglia have been observed

  • potential EAAT2 decline w progression; excitotoxicity?)

what did Chun et al observe abt H2I2 mediated brain atrophy by induced astrocytosis?

• showed importance of astrocytic reactivity on pathogenesis of AD using GiD model of reactive astrocytes

  • GiDm (mild astrocyte reactivity) vs GiDs (severe)

  • H2O2 originates from MOA-B in severe reactive astros → glial activation, tauopathy, brain atrophy, eventual death

    • H2O2 induced pathological features of AD in GiDs are consistent

H2O2 in severe but not mild reactive astrocytes is a key determinant of neurodegeneration in AD

what is proposed for astrocyte implications in the glymphatic system ?

regarding Aß clearance, in sporadic AD…

• with ageing, lymphatic function is reduced

• may be due to astros becoming reactive

  • accumulation of ß-amyloid may be due to impairment of glymphatic system

how can astrocytes contribute to seizure generation ?

• recurrent spontaneous seizures

◦ due to hyperexcitability + hypersynchrony of brain neurons

◦ do astros have a primary role in epileptogenesis and/or seizure generation?

‣ can disrupt K+ buffering

‣ can disrupt E supply --> neuron

a wide range of astro abnormalities could contribute to seizure generation

what are the main epilepsy-associated alterations of functional properties in astrocytes (3)?

K+ channel

AQP channel (important for iron balance)

Glutamate release + neuronal synchronisation

T/F: • astroglial Glutamine Synthase deficiency is sufficient to cause epileptic seizures

true