The Role of Glia in Neuroendocrinology

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The Role of Glia in Neuroendocrinology

• Tamara Blutstein
• April 25, 2005
• tblut001_at_umaryland.edu

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What are Glia?

• Non-neuronal cells
• More abundant than neurons
• In CNS and PNS
• PNS-Schwann cells
• Myelination
• CNS- Macroglia vs Microglia
• Microglia
• Macrophage type cells
• Sensors of pathology

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Macroglia

• Ependymal Cells
• Line ventricle
• Oligodendrocytes
• Myelinating axons
• Astrocytes
• Most heterogeneous
• Classical role as silent partners
• Structure and nutrients to
• neuron

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glia cells as humble servants laboring to cover
neuronal needs (Garcia Segura et al 1996)

• Express receptors for classical neurotransmitters
  
• Excitability via calcium
• Integration centers
• Release neurotransmitters
• Regulators of ions, metabolites and
  neurotransmitters in the synaptic cleft

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The Tripartite Synapse
Volterra et al., 2002
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Glia and Neuroendocrinology

• Glia facilitate neuronal processes
• Glia modulate hormone release
• Glia are responsive to gonadal steroids
• Functional role for hormonally responsive glia

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Glia Facilitate Neuronal ProcessesThe SON

• Magnocellular neurons
• Produce oxytocin and vasopressin
• Dehydration
• Parturition
• Suckling of young

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OT System

• Occur in tightly packed clusters or pairs
• Remain separated by fine processes of astrocytes
• 1 of total neuronal membrane in direct
  apposition

Langle et al 2002
Stereotypical astrocytic coverage of neuronal
tissue
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The SON

• When stimulated OT neurons distinct patterns of
  activity
• Increase firing rates from slow and irregular to
  a fast continuous pattern
• Regular reduced interspike intervals
• Morphology is modified
• Hypertrophy
• Shortened dendrites
• Enlarged axons
• Retraction of glial processes

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Stimulation of the System

• Glial coverage of OT somata and dendrites
  significantly diminishes
• Surfaces are left juxtaposed
• Increase number of synapses
• Increase in gap junctions between the neurons
• Reduced glial uptake of transmitter
• Higher Concentrations of K extracellularly

Establishing a basis for synchronous firing of OT
neurons that proceeds the milk-ejection reflex!
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Stimulation of the System
Langle et al 2002
Ten fold increase in the percentage of neuronal
somatic membrane in direct apposition
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When stimulation ceases the astrocytic processes
reappear and again separate the neuronal elements!
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Significance

• Remarkable plasticity that facilitates an
  essential neuroendocrine process
• Example of how glial cells can alter
  neurotransmission in a significant way

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Glia Regulate Hormone ReleaseLHRH

• Glial cells play an important role in the
  initiation of female puberty
• Particularly by influencing LHRH neurons in the
  POA and ME

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Glia and LHRH

• LHRH neurons have sparse synaptic input
• 2 of the cell membrane is innervated
• Numerous associations with glial cells

Ojeda and Ma 1999
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Cell-Cell Signaling

• Members of the EGF family and their RTKs play key
  roles
• TGF? and neuregulins produced in hypothalamic
  astrocytes

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TGF?

• Member of a large family of membrane anchored
  peptides that promote adhesion, migration,
  survival and differentiation
• Gene expression in hypothalamus increases at time
  of female puberty
• E2 treatment increases expression in the POA
• Over expression in ME or POA results in sexual
  precocity

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Cell-Cell Signaling

• Members of the EGF family and their RTKs play key
  roles
• TGF? and neuregulins produced in hypothalamic
  astrocytes
• Activate erbB1/erbB2 and erbB4/erbB2 receptor
  complexes on astrocytes
• Leads to the release of PGE2

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Cell-Cell Signaling

• PGE2 binds to EP-1 and EP-3 receptors on LHRH
  neurons
• EP1R calcium mobilization and phosphatidyl
  inositol turnover
• EP3R cAMP
• Elicits LHRH secretion

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Cell-Cell Signaling
Ojeda and Ma 1999
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Role of Gonadal Steroids

• Increase astrocytic gene expression of TGF?
• Increase expression of erbB2 and erbB4
• Upregulate EPR expression in the LHRH neurons
• EGF and TGF? can activate the ER in a
  ligand-independent manner

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This neuronal-glia interaction, mediated by
TGF?/erbB signaling and PGE2, seems to play an
important role in the onset of puberty by
affecting LHRH release at the level of the POA
and ME
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Glia are Responsive to Gonadal Steroids
We have already seen evidence that gonadal
steroids mediate changes in glial morphology in
the developing brain
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Astrocytes on Steroids!
E2
Astrocytes exposed to estrogen in vivo display
more complex morphology as measured by
significant increases in process length and
number!
Mong and McCarthy 1999
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• ..these morphological changes can also be seen
  in adulthood

And they correlate with changes in neuronal
morphology!
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The Adult Arcuate

• Exhibits changes in glial morphology that
  correlates with the phase of the estrus cycle
• High E2 increase glial extension

Garcia-Segura et al1994
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The Adult Arcuate

• This correlates with coordinate changes in
  axo-somatic synapse number

Garcia-Segura et al1994
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As astrocytic morphology becomes more elaborate
the number of axosomatic synapses decreases.
These changes are mediated by fluctuations in
circulating hormones in the cycling female rat
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Primate Arcuate

• OVX E induces an increase in the of neuronal
  membrane covered by glia
• This coincides with a decrease in the of
  axosomatic synapses

Garcia-Segura et al1994
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Hippocampus

• High levels of estrogen correspond to decreased
  astrocytic volume

Klintsova et al 1995
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Hippocampus

• Spine density exhibits coordinate changes during
  the estrous cycle

Proestrous
Estrus
Woolley and McEwen 1992
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Hippocampus

• High levels of estrogen correspond to increases
  in spine density

Estrogen
Oil
Woolley and McEwen 1992
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In both the Arcuate and Hippocampus estrogen
mediates changes in glial morphology and neuronal
morphology.
There is an inverse relationship between glial
and neuronal morphology.
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Are these hormone mediated changes in astrocyte
morphology associated with changes in astrocytic
function?
What is the functional significance of the
morphological changes?
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Microarray technology has become a useful tool
for studying hormonal regulation of glial genes
in vivo.
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Previous Study

• Compared gene regulation in the hypothalamus of
  OVX adult female mice treated with either E2
  (10ug) or Oil Vehicle and collected 2 or 24 hours
  after injection
• Found a variety of genes being regulated
  including Glial Specific Genes (such as PGDS and
  PEA-15)
• GLUTAMINE SYNTHETASE
• 2hrs 3.4 fold change
• 24hrs 2.0 fold change

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What is GS?

• A glial specific enzyme in the CNS
• A key component of the glutamate-glutamine cycle

Vardimon et al (1999)
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We hypothesize that estrogen induced changes in
GS expression reflect changes in astroglial
function.
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In-Situ Hybridization Results for GS



Estimated grains per cell
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Hypothalamus



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Cortex
E2
E2
Oil
Oil
GS

GAPDH
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Hippocampus
E2
E2
Oil
Oil



GS

GAPDH
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Estrogen Modulation of Glutamatergic
Neurotransmission

• Increases number of dendritic spines
• Upregulation of NMDA-2D expression (Wantabe et
  al., 1999)
• prolongs excitatory post synaptic potentials and
  cause greater degrees of long-term potentiation
  (Wong and Moss, 1992 Warren et al., 1995,
  Woolley et al., 1997)
• Presynaptically potentiates glutamate release
  (Yokomaku et al., 2003)

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Largely unexamined has been the role
hormonally-responsive astrocytes may play in the
steroid mediated modulation of excitatory
neurotransmission
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Potential Significance of E2 Mediated Increases
in GS ExpressionSynaptic Transmission

• Neurons absolutely require glutamine from
  astrocytes to replenish releasable pools of GABA
  and Glutamate
• The regulation of GS by E2 may affect
  neurotransmitter availability and thus synaptic
  transmission

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E2 Acts at all Aspects of the Tripartite Synapse
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We hypothesize that estrogen induced changes in
GS expression reflect changes in astroglial
function and that in turn will lead to overall
changes in synaptic function.