Neurobiology of Ageing
Contact: Doug Smith
As we are all aware, most societies around the world are getting increasingly older. The impact of ageing populations is far reaching and encompasses many health, social and economic factors. It is predicted that 28% of the Australian population will be over 65 years by 2051, compared to 13% in 2004. This translates to an estimated 2.7 million people over the age of 85 years. Ageing is an inescapable process that, compared to many other biological processes, remains poorly understood. Given the above statistics, there is an obvious need to improve our understanding of the ageing process if we are to encourage a better quality of life for elderly individuals. Ageing affects most physiological functions, to varying extents, and the long-term goal of our studies is to determine how ageing impacts brain function. More specifically, we wish to elucidate why certain individuals seem to be relatively resistant to the effects of ageing, whereas others appear to age more rapidly. Widespread successful ageing, as it is called, would be a great benefit to society as it would lessen all the health, social and economic burdens currently associated with ageing populations. Importantly, individual differences in ageing are apparent in animal populations also, making them indispensable to the study of the ageing process.
MtDNA mutation study
There is increasing evidence that the cell's energy producers, the mitochondria, are instrumental in the ageing process. A number of studies have demonstrated elevated levels of mutated mitochondrial DNA (mtDNA) in aged tissue, including the brain. We are currently characterising the mtDNA mutation profiles in various brain regions, cell types (e.g. dopaminergic versus cholinergic neurons), and cellular compartments (cell body versus nerve terminal). This involves quantitative real-time PCR (qPCR) analysis of DNA extracted from relevant brain/cell regions of young and old animals. The qPCR results depicted in the graph show an approximate 90-fold increase in a particular mtDNA mutation (known as the common deletion) in old brains compared with young brains.
Loss of neuronal phenotype study
We are comparing gene expression levels between young and old animals. The initial genes of interest are those that are characteristic of dopaminergic neurons. Recent studies have shown that, contrary to earlier indications, there does not appear to be substantial age-related cell death of dopamine neurons, but rather a loss of the dopaminergic phenotype. We are determining whether such effects occur in our animal model using molecular techniques as described for the mtDNA studies.
One important limitation of many gene or protein expression studies concerns the dilution effects that occur when brain tissue is homogenized for nucleic acid or protein extraction. The brain is made up of many different cell types and the ability to isolate a known population would circumvent this problem. A number of microdissection techniques, such as Laser Capture Microdissection (LCM), have been developed that allow collection of identified cell populations. The images below show two dopamine neurons grown in culture, subsequently immunolabelled for tyrosine hydroxylase (a marker of dopamine neurons), and then collected by LCM. The isolated dopamine neurons (right panel) can then be used for RNA, DNA and protein analyses.
We are currently establishing microdissection techniques for use with brain sections in order to allow global gene expression analysis, which will permit the simultaneous quantification of the expression of all the transcribed genes (10s of 1000s) in a particular cell type.
Ageing successfully study
By comparing gene expression profiles in defined neuronal populations between animals deemed to have aged "successfully" and animals deemed to have aged "poorly" - meaning without and with functional impairment, respectively - we hope to be able to elucidate the mechanisms underpinning the ageing process. To do this in an animal model we will use a battery of behavioural tests that allow us to determine the effects of ageing on an animal's cognitive, movement, and emotion functions. For example, working memory is one cognitive function that appears to be particularly sensitive to the ageing process. Using a radial arm version of the Morris water maze, we can measure working memory performance and thereby categorise the individual animals as successful or poor agers.