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

Neuron-glia interactions

•  What are glia?

•  How are glia defined ?

•  How are neurogenesis and gliogenesis related?

•  How do glia affect neuronal differentiation & function?

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

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)

from Principle of Neuroscience 3rd edition Kandel, Schwartz and Jessell

Glia types by morphology

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.

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.

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

morphology can change with culture conditions

Identification relies on the context

morphology can change with experimental conditions

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.

?

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?

Sauvageot & Stiles (2002)

in rats

Neurogenesis proceeds gliogenesis

E11-15 precursor mitosis

cardiotrophin-1

CT-1

E15 - P0

Bernabe-Heider et al. (2005) Neuron

Neurons may induce/enhance gliogenesis

precursor

neuron astroglia

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

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

Ullian et al. (2001) Science 291:657-660 Figure 2

Studies in RGCs

Mauch et al. (2001) 294:1354-1357 Figure 1

Studies in RGCs

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

•  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

NM

DA

R

ECM

TSP TSP

Chol/ApoE

pre-synaptic

post-synaptic

TSP

extra-cellular matrix

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

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

Volterra & Meldolesi (2005) Nat. Rev. Neurosci.

Glia (mal)function in neuropathy

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

Haydon (2000) Current Biology 10:R712

glia stimulate synapse formation

synaptic activity stimulates gap junction formation

neuronal signaling enhances gap junction formation

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