Astrocytes and the CNS

Question 1

General morphological features of astrocytes

Answer 1

• Glial fibrillary acidic protein (GFAP) staining reveals the traditional star shaped morophology. However, they do not stain finer processes. Other dye staining reveals that they have rather dense astrophytic processes.
• Histology reveals that astrocytes and synapses are closely associated, suggesting an important supportive role of the astrocytes.
• Astrocyte end feed are in close contact with blood vessels
• Astrocytes are closely connected to one another via gap junctions to create a network
• In rodents (less so in humans) they are highly territorial. When activated, their territories overlap more. In severe astrogliosis, they proliferate

Question 2

Five general functions of astrocytes

Answer 2

1. Developmental
2. Structural
3. Metabolic
4. Homeostatic
5. Signalling (controversial)

Question 3

Summarise the developmental functions of astrocytes

Answer 3

a) Regulation of neuro– and gliogenesis – astroglia are stem elements of the CNS
b) Neuronal path finding
c) Regulation of synaptogenesis (critical for some, or all, of synaptogenesis, including formation, pre-and post-synaptic function and elimination): this role may be retained in the adult CNS

Question 4

Summarise the structural functions of astroyctes

Answer 4

a) Astroglia divide the grey matter into independent territories and form neuro-vascular units (impairment of which could lead to cognitive impairment).
b) Astrocytes form anatomically segregated networks and integrate other neural cells into these networks
c. ) Formation of the glial-vascular interface and regulation of blood-brain barrier

Question 5

Summarise the metabolic functions of astrocytes

Answer 5

a) Regulation of cerebral microcirculation
b) Providing energy substrates for neurones through glucose-lactate shuttle
c) Recycling of glutamate and synthesis of glutamine (NB: neurons cannot synthetise glutamine)

Question 6

Summarise the homeostatic functions of microglia

Answer 6

a) Regulation of extracellular ion concentrations; in particular sequestration and redistributionof K+ following fluctuations associated with neuronal activity
b) Regulation of extracellular pH
c) Homeostasis of neurotransmitters and specifically glutamate
d) Brain water homeostasis

Question 7

Summarise the signalling functions of astrocytes

Answer 7

Signalling in astrocytes remains as a controverisal issue. Proposed functions include:

a) Modulation of synaptic transmission and synaptic plasticity
b) Release of gliotransmitters (controversial, particularly vesicular glutamate release)
c) Long-range signalling within the glial network
d) Integration of neuronal-glial networks : brain function would arise from the activity of a neuron-glia network

Question 8

In which functions are astrocytes heterogenous?

Answer 8

The list of functions is an extraordinary amount of functions for any one cell to perform. It is likely that, much like neurons, there are subgroups of astrocytes performing different functions. For simplicity, they are generally divided into grey matter astrocytes (protoplasmic) and white matter astrocytes (fibroplasmic).
However, it is likely that they differ in:

1. Morphology. Ten types identified thus far
2. Membrane currents
3. Transmitter receptor expression, particularly glutamate and glutamate transporters
4. Coupling of gap junctions
5. Ca++ signalling
6. Volume regulation.

These data however come from an older review - there is even newer data showing that astrocytes can be specialised in their functions etc to specific neuronal circuits (2017).

Question 9

What is the evidence for neural circuit specific astrocytes?

Answer 9

Astrocytes and neurons in striatal and hippocampal sections of mice have revealed that the striatum has a larger neuron to astrocyte ratio

Striatum is made up of small GABAergic neuron, and has many more neurons per astrocyte than the hippocampus, where there are mainly pyramidal glutamatergic cells.

Different neurons had different neurons accompanying them. For example, in dense GABAergic regions, astrocytes may not express many glutamate receptors compared to areas with high glutamate signalling.

Question 10

What is the general hypothesis of gliotransmission?

Answer 10

• NT release by neurons can bind to astocytes and elicit a reponse
• Ca++ increases in astrocytes elicit the regulated release from astrocytes of gliotransmitters e.g. glutamate, purines (ATP and adenosine), GABA, D-serine (this is not controversial, it is known they can release them)

•These trigger receptor-mediated currents in neurons

•It is likely that astrocytes express receptors and transporters for almost all NT (this is not controversial)

Question 11

What is currently controversial regarding the gliotransmission hypothesis?

Answer 11

It is well known and not contested that astrocytes can open channels and release glutamate under pathological conditions, such as reversal of EAA transporter, or swelling-induced opening of volume-regulated anion channels. Etc.

It is not contested that they can release glutamate. What is contested however is whether this is
A.) Simply and only a passive response with a linear relationship to neuronal activity e.g. the more activity in presynaptic neuron, the more Ca++ influx to the astrocyte and thus more glutamate release

B.) A pathological event

C.) Or whether astrocytes are capable of integrating information

Question 12

Why are astrocytes proposed to particpate in gliosis?

Answer 12

1. They have cellular excitability (they can respond with increase of Ca++ to sensory stimulation)
2. They can discriminate activity of different pathways: e.g., astrocytes in CA1 of the hippocampus do not respond to Glu released following stimulation of the alveus, but they respond to Glu released by Schaffer collateral (or to Ach released following alveus stimulation)
3. Ca++ signals show non-linear relationship with synaptic activity or exogenous application of neurotransmitters.

Question 13

Evidence for astrocyte role in working memory?

Answer 13

Han et al 2012 assessed the effect of cannabinoid-mediate deficit in working memory in mice using a maze test. They selectively blocked CB1 receptors on either the astrocytes, the glutamatergic neurons or the GABAergic neurons in the CA3-CA1 region of the hippocampus. Only KO of astrocyte CB1 prevented working memory deficit with cannabinoid exposure. This lead to the conclusion that astrocytes mediate cannabinoid deficits in spatial working memory.

Question 14

What is the hypothesised mechanism for astrocytes role in memory during cannabinoid exposure?

Answer 14

1. Cannabinoids bind to all CB1R (in black)
2. Astrocytes release glutamate (purple)
3. Glutamate binds to NMDAR
4. AMPAR is internalised
5. LTD is induced
6. The mouse is lost

content review needed

Question 15

What is the theory of astrocyte role in sleep regulation?

Answer 15

Astrocytes release ATP, which gets converted to adenosine in the extracellular space. This binds to A1R on presynpatic neurons and having an inhibitory effect, which may be involved in sleep-wake regulation.

Question 16

What are the main obstacles in astrocyte research?

Answer 16

Many emerging theories on astrocytes are difficult to prove, for several reasons:

1. There is no universal astrocyte marker. They are heterogenous, therefore it is difficult not only to label them (for example, GFAP does not label all astrocytes or fine processes), and also to knock out all astrocytes. This can allow for selectitivity too which is a benefit
2. Astrocytes are designed to react to any small changes, therefore any in-vitro studies involving removing astrocytes from brain are unlikely to reflex resting in-vivo morphology. Even culutred astrocytes are likely to be quite different. This makes the study of astrocytes unreliable and also correlative

Question 17

How can astrocytes contribute to CNS pathology?

Answer 17

Astrocytes can play primary or contributing roles to CNS pathology

Pathology can be linked to loss of normal astrocyte function or gain of abnormal effect

It is often difficult to establish whether their role is primary or secondary to neuronal dysfunction - for example, we do not know when time zero is for many disorders. We are only conducting research by time the disorder has already developed.

Question 18

What are the two main types of astrocyte pathology?

Answer 18

1. Asthenia/atrophy. This is a loss of function pathology. They shrink, reducing their synaptic coverage, their homestatic capability and their network connectivity.
2. Reactive astrogliosis. This can confer loss or gain of function. There are two subtypes - isomorphic, where domains are maintained [loss or gain of function] and anisomorphic, where domains are lost. This when an astriogliotic scar forms. Reactivity serves the biological function of having a protective effect - however the difference between protection and pathology is unclear. Reactive astrogliosis is a feature of many neurodegeneration.

Question 19

What are the four stages of astrogliosis?

Answer 19

1. Healthy tissue. Some express GFAP, little overlap, no proliferation.
2. Mild - moderate. Most express GFAP, little overlap, no proliferation
3. Severe diffuse. Most express GFAP, domain disruption, proliferation
4. Severe astrogliosis with compact scar formation. A scar forms around regions of tissue damage and inflammation. It serves the function of neuroprotective barrier to inflammatory cells and infectious agents, and reduce oedema, but can prevent axonal regeneration

Question 20

What are the main causes of severe astrogliosis with compact scar formation?

Answer 20

Trauma

Ischemia

Cytotoxicity

Infection

Autoimmune inflammation

Question 21

What is a case study disorder that indicates that astrocytes can be causative of a neurodegenerative disorder?

Answer 21

Alexanders disease

• Evidence that astrocytes can be the primary cause of CNS disease
• Genetic disorder of astrocytes
• Dominant, gain-of-function mutation of the GFAP gene.
• Astrocytes hypertrophic and GFAP-expressing
• Symptoms: macrocephaly, seizures, psychomotor disturbances and premature death

Question 22

What astrocyte pathologies and changes are observed in AD ( 5points)

Answer 22

1. Post mortem tissues show reactive astrogliosis, where scars are forming around plaques. Intensity of AD parrells this R.A. However, this represents a late stage of the diease.
2. Animal models and some human studies have also found atrophy of astroglia.
3. Glutamate transporters of astrocytes decline with disease progression
4. Reactive astrocytes contain substantial amounts of amyloid. It is unknown whether this is beneficial (e.g. protective) or determimental (e.g. causing damage)
5. They may lose ability to degrade AB and may begin to produce it.