The dance between Glia and Neurons is critical to the development and maintenance to CNS. Did you know: • up to 90% of the cells in the vertebrate nervous system are “not neurons” • make > 50% of the brain volume. This is a great overview to anyone that wants to learn more.

1. Neuron-glia interactions
Background:
Eroglu, C., & Barres, B. A. (2010). Regulation of synaptic connectivity by
glia. Nature, 468(7321), 223–231. .
Discussion:
Eroglu et al. Gabapentin Receptor alpha2/delta1 Is a Neuronal
Thrombospondin Receptor Responsible for Excitatory CNS
Synaptogenesis. Cell (2009) vol. 139 (2) pp. 380-392

2. Neuron-glia interactions
• What are glia?
• How are glia defined ?
• How are neurogenesis and gliogenesis related?
• How do glia affect neuronal differentiation & function?

3. What are glia?
• up to 90% of the cells in the vertebrate nervous system are “not neurons”
• make > 50% of the brain volume
• Types of glia
• microglia ........................................ haematopoetic precursor?
• macroglia ....................................... neuroepithelium precursor
• PNS
• Schwann cells
• CNS
• Type 1 astroglia = astrocyte
• radial glia?
• oligodendrocyte
• type 2 astroglia

4. How are glia identified/defined?
• morphology
• reliable “if you know what you’re doing
• very subjective
• antigenic markers
• most defined by an antibody reactivity
• dependent on
• live vs. fixed
• fixation conditions
• species
• in vivo vs. in vitro
• culture conditions
• gene expression
• cell type specificity usually determined by morphology or antigenic marker
• function
• myelination
• guide migrating neurons (Bergmann & radial glia)

5. from Principle of Neuroscience 3rd edition Kandel, Schwartz and Jessell
Glia types by morphology

6. Glia types defined by antigentic marker
“Astrocyte differentiation in the CNS. Neuroepithelial stem cells (NEP), radial glia, and glial-restricted precursor cells (GRP)
have all been shown to generate astrocytes. Astrocyte-restricted precursor (ARP), as assessed by CD44 expression, might
be one of the intermediate precursors that give rise to mature astrocytes. Note that while radial glia do not normally express
CD44, we do not know whether they express CD44 as they mature to become astrocytes. CD44 overexpression prevents
glial precursors from differentiating into oligodendrocytes but does not prevent differentiation into astrocytes”
Liu et al. (2004) Dev. Biol. 276:31-46.

7. Question to think about:
Type 1 astrocytes grown in culture often lose GFAP expression.
What might explain this? Consider both normal biological and technique
related explanations.

8. Identifying glia by morphology can be difficult because...
cartoons are idealized

9. morphology can change
with culture conditions
Identification relies on
the context

10. morphology can change with experimental conditions

11. mp = mitotic precursor
n = neuron
A = type 1 astrocyte
RG = radial glia
neurons and glia can be derived from the same
mitotic precursors
from Development of the Nervous System Sanes, Reh and Harris eds.
?

12. Question to think about:
Design an experiment to determine if radial glia can differentiate into neurons.
Could you determine if radial glia always differentiate into neurons?

13. Sauvageot & Stiles (2002)
in rats
Neurogenesis proceeds gliogenesis

14. E11-15
precursor
mitosis
cardiotrophin-1
CT-1
E15 - P0
Bernabe-Heider et al. (2005) Neuron
Neurons may induce/enhance gliogenesis
precursor
neuron astroglia

15. Glia help ‘wire-up’ the nervous system
• support and firm the brain
• guide neuronal migration (radial & Bergmann glia)
• secrete matrix and/or guidance factors to guide axons and dendrites
• form the blood-brain barrier
• myelination
• induce and stabilize synapses
• ‘feed’ the brain

16. Glia: role in synapse formation & maturation
• in vivo, glia probably secrete the ECM (laminin, fibronectin, collagen etc..) that are the substrate
for neurite outgrowth.
• embryonic glia secrete sialic acid residues that organize ‘permissive’ laminin matrices
• postnatal glia organize ‘non-permissive’ laminin fibrillar webs that detach from cell
• Freire et al. (2004) J. cell Science 117:4067
• in culture, neurons grown in the absence of glia develop fewer, less active synapses
• glia conditioned media enhances/increases formation of morphologically normal synapses
• may be due to secretion of thrombospondins (TSP1 & TSP 2 )
• these synapses release & recycle vesicles = presynaptically “mature”, but...
• are postsynaptically “silent” because AMPAR are not on membrane
• Christopherson et al (2005) Cell 120:421
• co-culture with glia (even if physically separated) gives functional synapses
• may be due to cholesterol/ApoE2
• Mauch et al. (2001) Science 294:1354.
• neurons secrete a factor(s) that regulate glia myelin membrane production
• Trajkovic et al. (2006) JCB 172:937

17. Ullian et al. (2001) Science 291:657-660 Figure 2
Studies in RGCs

18. Mauch et al. (2001) 294:1354-1357
Figure 1
Studies in RGCs

19. Christopherson et al. (2005)
Cell 120:421-433
Figure 4
mEPSC
Figure
41
vesicle recycling assay
whole cell glutamate response
puncta = pre- & post-synaptic Ab stain
Studies in RGCs

20. • neuron-derived CT-1 regulates the onset of gliogenesis (in cortex)
• ECM secreted by glia is important in all stages of neuronal differentiation
• TSP - increases the number of presynaptically active, but postsynaptically silent synapses
• cholesterol - conflicting data
• Mauch et al (2001) - cholesterol increases the number of functional synapses
• Christoperson et al (2005) - cholesterol only increases quantal content
• ???- unknown factor in glia conditioned media induces morphologically mature synapses
to become fully functional/active
astrocyte
NMDAR
ECM
TSP TSP
Chol/ApoE
pre-synaptic
post-synaptic
TSP
extra-cellular matrix

21. Question to think about:
How might the density of neurons in culture alter their response to glia
conditioned media?

22. Glia maintain and ‘feed’ the brain
• scavengers (microglia) remove dead cells
• buffer K+ in the extra-cellular space
• can affect blood vessel dilation
• differential localization of metabolic enzymes reveals that neurons and glia
‘share’ metabolic duties
• take-up/recycle/produce neurotransmitter
• e.g. glutamate, probably GABA and aspartate
• store glycogen for ATP production via glycolysis
• neurons use lactate (from glycolysis) for mitochodrial production of ATP

23. Volterra & Meldolesi (2005) Nat. Rev. Neurosci.
Glia (mal)function in neuropathy

24. Glia are involved in neuronal signaling
• synaptic activity leads to glutamate uptake by glia
• up regulates connexin
• connexin forms gap junctions between astrocytes
• moves accumulated K+ between astrocytes
• polarized accumulation and release of K+ causes vascular dilation
• astrocytes propagate calcium waves
• calcium waves trigger glutamate release to neurons
• neurons take up, package into vesicles and release glutamate

25. Haydon (2000) Current Biology 10:R712
glia stimulate
synapse formation
synaptic activity stimulates
gap junction formation
neuronal signaling enhances gap junction formation

26. Haydon (2001) Nature Reviews Neuroscience 2:185
Summary neuron-glia interactions:
• neuronal survival & differentiation
• gliogenesis
• cardiotrophin-1
• synapse formation
• thrombospondin
• synapse maturation
• cholesterol/ApoE ?
• synapse function
• ATP/glutamate synthesis
• signal propagation
• gap junctions propagate Ca+2 waves