Songbirds, Steroids, and Adult Neurogenesis
Brad Walters Biological Sciences Lehigh University September, 2008 Songbirds, steroids, and neurogenesis… Some tissues regenerate well…
12
34
http://flickr.com/photos/museumoflondon/2392340925/ 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 Nucleus Do the changes in Axon Hillock volume result from Neurogenesis?
Axon Dendrites
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, Goldman and Nottebohm, 1983 D = dendrite,
But are they really neurons? Voltage (mV) Voltage 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 Average # of New HVC Neurons/Section Neurons/Section 18
200 16
* 14 *
150 12
10
100 8 HVC Neurons 6
50 [3H] Positive HVC Neurons N = 7 N = 8 4 N = 7 N = 8 2
0 0 No12 E E Replaced No12 E 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 1. Proliferation Neurogenesis is 2. Migration comprised of… 3. Differentiation 4. 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?
• Does E affect cell fate directly? • Does E affect factors
that affect cell fate? 200 BMP-2
150
100
50 Fold Change of mRNA expression Fold of mRNA Change
0 Brànnvall et al. 2002 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: HORMONES Glucocorticoids (-) NEUROTRANSMITTERS Serotonin (+) Testosterone (+) Norepinephrine (+) Progesterone (+) Glutamate (-)
BEHAVIOR Exercise (+) Enriched Environment (+) Gage, 2003 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