songbirds, steroids, and adult neurogenesisinbios21/pdf/fall2008/walters_09082008.pdf · songbirds,...
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Songbirds, Steroids, and Adult Neurogenesis
Brad WaltersBiological SciencesLehigh UniversitySeptember, 2008
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Songbirds, steroids, and neurogenesis…
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Some tissues regenerate well…
http://flickr.com/photos/museumoflondon/2392340925/
1 2
3 4
http://www.anat.ucl.ac.uk/business/becker1.shtml
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Others, not so good…http://commons.wikimedia.org/wiki/Image:MCAO-sheep.jpg
The absence of evidence is not evidence of absence!!!
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Visualizing Neurogenesis: nucleotides (e.g. thymidine) get incorporated into
DNA strands during replication
http://www.bcscience.com/bc9/pgs/quiz_section5.1.htm
[3H] Thymidine incorporates into dividing cells!
Altman, 1962
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Some songbird brains change dramatically across seasons
Nottebohm, 1981
Brenowitz et al, 1991
Margoliash, 2005
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A neuron stained with [3H]thymidine, counterstained with methylene blue. Bar = 50µm
Electron micrograph of a new neuron in HVC. N = nucleus, A = axonal hillock, D = dendrite, Goldman and Nottebohm, 1983
Do the changes in volume result from
Neurogenesis?
Nucleus
Axon Hillock
Dendrites Axon
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But are they really neurons?
Paton and Nottebohm, 1984
Volta
ge (m
V)
Time (msec)
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No… But seriously… Are they really neurons?
Kirn and Nottebohm, 1991
The retrograde tracer, Fluorogold was injected into RA, a nucleus known
to be innervated by axons projecting from HVC.
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Seasonal neurogenesis in songbirds suggests the involvement of steroid
hormones.
Smith et al. 1997
Spring
Fall
Kirn and Nottebohm, 1993
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Testosterone increases neurogenesis
Bottjer and Dignan, 1988
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However, Testosterone can be metabolized in the brain…
Testosterone
Dihydrotestosterone (DHT) 17β-Estradiol (Estrogen)
5-α Reductase Aromatase
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So which steroid is it?
Tramontin et al. 2003
C T DHT E2 DHT+E2 C T DHT E2 DHT+E2
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Estrogen increases neurogenesis in adult songbirds…
adapted from Hidalgo et al. 1995
1 2
HVC
Neu
rons
0
50
100
150
200
Average # of HVCNeurons/Section
No E E Replaced
N = 7 N = 8
*
1 2
[3H
] Pos
itive
HVC
Neu
rons
0
2
4
6
8
10
12
14
16
18
No E E Replaced
N = 7 N = 8
*
Average # of New HVCNeurons/Section
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Tanapat et al. 1999
Estrogens enhance mammalian neurogenesis.
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Proliferation
Differentiation
Migration
Perpetuation
Integration
Adapted from Gage, 2003
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Which process or processes are being affected by Estrogen
provision?
• Functional Neurogenesis is comprised of…
1. Proliferation2. Migration3. Differentiation4. Perpetuation
(Survival)5. Integration
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Does E affect neural stem cell proliferation?
Two main questions:– Do estrogens increase the number of cells entering
the cell cycle at any given point in time?
– Do estrogens increase the rate at which cells proceed through the cell cycle?
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Does E affect neuronal migration?
• E acts on cells involved in the migration process.
• E influences migration away from VZ.
Williams et al., 1999
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Does E affect Differentiation?
Fold
Cha
nge
of m
RN
A ex
pres
sion
0
50
100
150
200 BMP-2
Fadrozole SalineModified from Walters and Saldanha, 2008
• Does E affect cell fate directly?
• Does E affect factors that affect cell fate?
Brànnvall et al. 2002
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Perpetuation (Survival)
Saldanha et al. 2005
FAD FAD+E2
* *
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With regard to adult neurogenesis, estrogen affects…
• Proliferation: tentative yes
• Migration: tentative yes
• Differentiation: maybe
• Survival: yes
• Integration: ??? This is the next big question!!!
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OTHER FACTORS KNOWN TO INFLUENCE NEUROGENESIS INCLUDE:HORMONES Glucocorticoids (-) NEUROTRANSMITTERS Serotonin (+)
Testosterone (+) Norepinephrine (+)Progesterone (+) Glutamate (-)
BEHAVIOR Exercise (+)Enriched Environment (+)
Gag
e, 2
003
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Summary• Brain Injury and Neurodegenerative diseases can be devastating
to the adult brain.
• However, neurogenesis in the CNS does occur throughout life.
• Thymidine analogs are a powerful, but limited, tool for measuring this process.
• Songbirds are an indispensable model organism in this field.
• Steroid hormones affect neurogenesis.
• Estrogen increases neurogenesis most likely by increasing survival as well as via proliferation (probably), and possibly by affecting cell fate and/or migration.
• There are many factors, often working together, that affect neurogenesis.
• Clinical applications will depend heavily on the site specific migration and integration of new neurons.
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Acknowledgements
NIH (NINDS) 047267
PIColin Saldanha
CollaboratorsRyan Wynne
Work funded by: