The Role of Glia in Neuroendocrinology

1. The Role of Glia in Neuroendocrinology Tamara Blutstein April 25, 2005 tblut001@umaryland.edu

2. Schwann Cell Microglia 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

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

4. “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

5. The Tripartite Synapse Volterra et al., 2002

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

7. Glia Facilitate Neuronal ProcessesThe SON • Magnocellular neurons • Produce oxytocin and vasopressin • Dehydration • Parturition • Suckling of young

8. 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

9. 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

10. 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!

11. Stimulation of the System Langle et al 2002 Ten fold increase in the percentage of neuronal somatic membrane in direct apposition

12. When stimulation ceases the astrocytic processes reappear and again separate the neuronal elements!

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

14. 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

15. 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

16. Cell-Cell Signaling • Members of the EGF family and their RTKs play key roles • TGF and neuregulins produced in hypothalamic astrocytes

17. 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

18. 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

19. 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

20. Cell-Cell Signaling Ojeda and Ma 1999

21. 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

22. 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

23. Glia are Responsive to Gonadal Steroids We have already seen evidence that gonadal steroids mediate changes in glial morphology in the developing brain……

24. 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

25. …..these morphological changes can also be seen in adulthood And they correlate with changes in neuronal morphology!

26. 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

27. The Adult Arcuate • This correlates with coordinate changes in axo-somatic synapse number Garcia-Segura et al1994

28. 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

29. 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

30. Hippocampus • High levels of estrogen correspond to decreased astrocytic volume Klintsova et al 1995

31. Hippocampus • Spine density exhibits coordinate changes during the estrous cycle Proestrous Estrus Woolley and McEwen 1992

32. Hippocampus • High levels of estrogen correspond to increases in spine density Estrogen Oil Woolley and McEwen 1992

33. 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.

34. Are these hormone mediated changes in astrocyte morphology associated with changes in astrocytic function? What is the functional significance of the morphological changes?

35. Microarray technology has become a useful tool for studying hormonal regulation of glial genes in vivo.

36. 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

37. What is GS? • A glial specific enzyme in the CNS • A key component of the glutamate-glutamine cycle Vardimon et al (1999)

38. We hypothesize that estrogen induced changes in GS expression reflect changes in astroglial function.

39. In-Situ Hybridization Results for GS * * * Estimatedgrains per cell

40. E2 Oil E2 Oil GS GAPDH 0.6 0.5 0.4 0.3 Calibrated O.D. 0.2 0.1 0 Oil E2 Hypothalamus *

41. Cortex E2 Oil E2 Oil GS GAPDH

42. 0.6 0.5 0.4 0.3 calibrated O.D. 0.2 0.1 0 Oil E2 Hippocampus E2 Oil E2 Oil GS * GAPDH

43. 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)

44. Largely unexaminedhas been the role hormonally-responsive astrocytes may play in the steroid mediated modulation of excitatory neurotransmission

45. 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

46. Increase Glutamate release -Increase dendritic spines -Increase ePSPs -Increase LTP E2 E2 Upregulation of GS E2 Changes in Morphology E2 E2 Acts at all Aspects of the Tripartite Synapse

47. 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.