The Neuron. • A memory is thought to be encoded by an interconnected network
of synapses. Psychology: An Introduction. Morris and Maisto 11th Edition ...
Little brains that think big: What model organisms are teaching us about memory
Dr Helen Fitzsimons Molecular Neurogenetics Group Institute of Fundamental Sciences Massey University
The Neuron • Memories are formed and stored in neurons • Neurons transmit information through synapses Psychology: An Introduction. Morris and Maisto 11th Edition
• A memory is thought to be encoded by an interconnected network of synapses
The Human Brain • Contains ~8.610 neurons (86 billion) • Each neuron has ~7000 synaptic connections with other neurons • ~1014 – 1015 synapses in an adult brain http://science.nationalgeographic.com/science/photos/brain/
A little bit of history… Eric Kandel’s pioneering work in Aplysia
http://dwb4.unl.edu/TheoryPaper/KandelAplysia.jpg
http://www.uoregon.edu/~joet/news/news.htm
Kandel’s Hypothesis: the molecular pathways involved in learning and memory are ancient and conserved
Why study Aplysia? • Small number of neurons (20,000) • Behaviours that can be modified by learning
Kandel, 2000
Aplysia Gill Reflex Withdrawal
Sensitisation = a type of learned fear where there is an increased response to an otherwise neutral stimulus
Short term memory Siphon
An increase in cAMP causes a transient increase neurotransmitter release i.e. a stronger connection between the cells
Sensory neuron
Tail shock
Modified from www.nobelprize.org
Long-term memory
• Requires repetition • A prolonged increase in cAMP results in new protein synthesis and growth of synaptic connections
• The cAMP pathway is conserved across invertebrates and vertebrates • Kandel’s hypothesis was correct www.nobelprize.org
A few key concepts to introduce: • The structure of chromatin • Histone acetylation and memory • Drosophila as a model for memory research • The courtship suppression assay • My current research and future directions
What is chromatin? DNA is tightly packaged in cells by interaction with histones
http://beyondthedish.wordpress.com
Histone acetylation regulates gene expression
McIntyre et al., 2007
How do cells remember who they are? Heart-specific genes ON Stomach-specific genes OFF
Stomach-specific genes ON Heart-specific genes OFF Signals during development switch on different sets of genes. This pattern is then maintained permanently in each cell by modifications to chromatin
Histone acetylation correlates with memory • Histone acetylation increases in the rat brain after a learning event • Loss of histone acetylation impairs long-term memory formation • Treatment with histone deacetylase inhibitors improves long-term memory • Histone acetylation is abnormal in rodent models of AD HDACs – negative regulators of memory??
My Research Questions Which histone deacetylases regulate memory formation and through what mechanism(s)?
Initial Strategy Manipulate the amount of each histone deacetylase in the adult brain and determine the effect on short and long-term memory
Drosophila as a model for memory research • Drosophila has been used widely as a model organism for studying genetics and development
Drosophila as a model for memory research • Shares many essential biological properties with humans (Conserved genome – 13,500 genes) • Great for genetic analysis • Relatively complex brain (200,000 neurons) • 75% of human disease genes are conserved in Drosophila • Displays reproducible behaviours that can be modified by learning
• Transgenics (GM flies) are easy to generate • Thousands of mutants available from stock centers • Can increase or decrease expression of almost any gene in almost any tissue at any time during development
Manipulating gene expression – the eye as an example Decrease the amount of a specific protein (Pax6)
Increase the amount of Pax6
Pax6 is also required for eye development in humans Aniridia Type II
Neethirajan et al. (2004)
The Drosophila brain
Heisenberg, 2003
The mushroom body
R Davis, Scripps Research Institute
Courtship Conditioning Assay Male flies remember that they have been previously rejected by a female
Courtship Behaviours
Sokolowski, 2001
Calculation of a Memory Index
Conservation of HDACs
Rpd3 (HDAC1/2)
• HDAC2 OE impairs long-term memory • HDAC2 is increased in the AD brain (mouse and human) • HDAC2 inhibitors – promising for drug development?
HDAC4 • Expressed in adult brain (mice and flies) – including at synapses • Subcellular localisation regulated by synaptic activity
• Interacts with MEF2 during muscle development
HDAC4 Project - Research Questions • What role does HDAC4 play in memory formation? • Which proteins does HDAC4 interact with to regulate memory formation? • Is HDAC4 expression altered in human Alzheimer patient brains?
Analysis of HDAC4 expression
Overexpression of HDAC4
Overexpression of HDAC4
HDAC4 overexpression impairs long-term memory
HDAC4 overexpression does not impair short-term memory
Colocalisation of HDAC4 and MEF2
HDAC4 knockdown impairs LTM
Future Directions • Identification of genes in the HDAC4 memory pathway – Enhancer/suppressor screen – Subcellular localisation screen – Identify gene targets of HDAC4
• Analyse this pathway in vertebrate memory • Examine HDAC4 in human AD patient brains
Acknowledgements Massey University Dr Jianyu Chen Fiona Given Melissa Rose North Carolina State University Assoc. Prof. Max Scott University of Auckland Assoc. Prof. Debbie Young Prof. Mike Dragunow Prof. Richard Faull Dr. Pritika Narayan
