Songbirds, Steroids, and Adult Neurogenesis

Brad WaltersBiological SciencesLehigh UniversitySeptember, 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

http://www.bcscience.com/bc9/pgs/quiz_section5.1.htm

[3H] Thymidine incorporates into dividing cells!

Altman, 1962

Some songbird brains change dramatically across seasons

Nottebohm, 1981

Brenowitz et al, 1991

Margoliash, 2005

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

But are they really neurons?

Paton and Nottebohm, 1984

Volta

ge (m

V)

Time (msec)

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.

Seasonal neurogenesis in songbirds suggests the involvement of steroid

hormones.

Smith et al. 1997

Spring

Fall

Kirn and Nottebohm, 1993

Testosterone increases neurogenesis

Bottjer and Dignan, 1988

However, Testosterone can be metabolized in the brain…

Testosterone

Dihydrotestosterone (DHT) 17β-Estradiol (Estrogen)

5-α Reductase Aromatase

So which steroid is it?

Tramontin et al. 2003

C T DHT E2 DHT+E2 C T DHT E2 DHT+E2

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

Tanapat et al. 1999

Estrogens enhance mammalian neurogenesis.

Proliferation

Differentiation

Migration

Perpetuation

Integration

Adapted from Gage, 2003

Which process or processes are being affected by Estrogen

provision?

• Functional Neurogenesis is comprised of…

1. Proliferation2. Migration3. Differentiation4. 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?

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

Perpetuation (Survival)

Saldanha et al. 2005

FAD FAD+E2

* *

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 (+)

Gag

e, 2

003

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

NIH (NINDS) 047267

PIColin Saldanha

CollaboratorsRyan Wynne

Work funded by: