Download Songbirds, Steroids, and Adult Neurogenesis

Songbirds, Steroids, and Adult
Neurogenesis
Brad Walters
Biological Sciences
Lehigh University
September, 2008
Songbirds, steroids, and
neurogenesis…
Some tissues regenerate well…
http://flickr.com/photos/museumoflondon/2392340925/
1
2
3
4
http://www.anat.ucl.ac.uk/business/becker1.shtml
Others, not so good…
http://commons.wikimedia.org/wiki/Image:MCAO-sheep.jpg
The absence of evidence is not evidence of absence!!!
Visualizing Neurogenesis:
nucleotides (e.g. thymidine) get incorporated into
DNA strands during replication
Altman, 1962
[3H] Thymidine
incorporates into dividing cells!
http://www.bcscience.com/bc9/pgs/quiz_section5.1.htm
Some songbird brains change
dramatically across seasons
Brenowitz et al, 1991
Margoliash, 2005
Nottebohm, 1981
Do the changes in
volume result from
Neurogenesis?
Nucleus
Axon Hillock
Dendrites
A neuron stained with [3H]thymidine,
counterstained with methylene blue.
Bar = 50µm
Goldman and Nottebohm, 1983
Axon
Electron micrograph of a new neuron in
HVC. N = nucleus, A = axonal hillock,
D = dendrite,
Voltage (mV)
But are they really neurons?
Time (msec)
Paton and Nottebohm, 1984
No… But seriously… Are they
really neurons?
The retrograde tracer, Fluorogold was
injected into RA, a nucleus known
to be innervated by axons
projecting from HVC.
Kirn and Nottebohm, 1991
Seasonal neurogenesis in songbirds
suggests the involvement of steroid
hormones.
Spring
Fall
Kirn and Nottebohm, 1993
Smith et al. 1997
Testosterone increases
neurogenesis
Bottjer and Dignan, 1988
However, Testosterone can be metabolized
in the brain…
5-α Reductase
Aromatase
Testosterone
Dihydrotestosterone (DHT)
17β-Estradiol (Estrogen)
So which steroid is it?
C
T
DHT
E2 DHT+E2
C
T
DHT
E2 DHT+E2
Tramontin et al. 2003
Estrogen increases neurogenesis
in adult songbirds…
Average # of HVC
Neurons/Section
18
200
HVC Neurons
150
100
N=7
N=8
[3H] Positive HVC Neurons
16
*
50
Average # of New HVC
Neurons/Section
14
*
12
10
8
6
4
N=7
N=8
2
0
0
No1 E
2
E Replaced
No1 E
2
E Replaced
adapted from Hidalgo et al. 1995
Estrogens enhance mammalian neurogenesis.
Tanapat et al. 1999
Proliferation
Migration
Integration
Perpetuation
Differentiation
Adapted from Gage, 2003
Which process or processes are
being affected by Estrogen
provision?
• Functional
Neurogenesis is
comprised of…
1.
2.
3.
4.
Proliferation
Migration
Differentiation
Perpetuation
(Survival)
5. Integration
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?
Does E affect neuronal migration?
• E acts on cells involved in the migration
process.
• E influences migration away from VZ.
Williams et al., 1999
Does E affect Differentiation?
200
Fold Change of mRNA expression
• Does E affect cell fate
directly?
• Does E affect factors
that affect cell fate?
150
100
50
0
Brànnvall et al. 2002
BMP-2
Fadrozole
Saline
Modified from Walters and Saldanha, 2008
Perpetuation (Survival)
*
*
FAD
FAD+E2
Saldanha et al. 2005
With regard to adult neurogenesis,
estrogen affects…
• Proliferation: tentative yes
• Migration: tentative yes
• Differentiation: maybe
• Survival: yes
• Integration: ??? This is the next big
question!!!
OTHER FACTORS KNOWN TO INFLUENCE NEUROGENESIS INCLUDE:
Glucocorticoids (-)
Testosterone (+)
Progesterone (+)
BEHAVIOR
Exercise (+)
Enriched Environment (+)
NEUROTRANSMITTERS
Serotonin (+)
Norepinephrine (+)
Glutamate (-)
Gage, 2003
HORMONES
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.
Acknowledgements
PI
Colin Saldanha
Collaborators
Ryan Wynne
Work funded by:
NIH (NINDS) 047267