My first blog was about a transmissible form of cancer in dogs, Tasmanian devils and clams (Transmissible Cancer). This time I’m going to explain about the transmission of Alzheimer’s Disease but don’t worry, I don’t mean the transmission between people, that can’t happen. I mean the progression and spread of the disease to different parts of the brain. In my last post (Alzheimer’s Disease), I talked about the basic pathology of the disease and how it is thought the build-up of misfolded proteins (tau and amyloid β) cause the nasty memory loss symptoms. This build-up first occurs in areas important for short-term learning and memory, thinking and planning, namely the hippocampus, and it often happens before any symptoms are detected. In mild Alzheimer’s Disease, the damage spreads to other regions of the brain including areas important for spatial awareness and speech. In late stages of the disease, the pathology can be found in many areas. The associated cell death causes significant shrinkage in brain size.
But how do these misfolded proteins spread? Well… this has puzzled scientists for a while. Recently, I went to an Alzheimer’s Research UK day with really interesting talks from top researchers in this area. Dr Amy Pooler, who worked at King’s College London, explained about some of her research into how tau protein ‘jumps’ between neurons.
As you might already know, neurons signal to each other using chemicals called neurotransmitters, which are released from the end of one neuron, pass across a tiny gap (called a synapse) and are taken up by specific receptors on another neurone. Dr Pooler and her colleagues found that tau is released from neurons when they signal to each other. This tau is then somehow taken up by other cells. They discovered this when they stopped one neuron’s ability to produce its own tau. They then stimulated a signal from another neuron and found that, lo and behold, the first neuron contained tau even though it couldn’t produce its own. The mechanism is still unclear, but it does seem that the transmission of tau requires a functioning synapse. This might not seem significant, but it proves that a functioning neural connection is required for the disease to spread and it is not simply cell death and the bursting of cells. This is an important finding as it gets us one step closer to understanding how the pathology spreads to other brain regions.
However, this isn’t the full story. Professor Goedert from the University of Cambridge hypothesises that the spread is ‘prion like’. Let me explain what that means: a prion is basically an infectious protein. When an abnormal protein moves from one cell to another cell, the abnormal protein becomes the dominant force in the new cell. It seems to take over the normal version of the same protein, disrupting normal cell processes. There are still a lot of unknowns about why this happens. Is normal protein being converted into abnormal, or is abnormal protein being produced from scratch?
We already know about prion neurodegenerative diseases like Kuru and Creutzfeldt-Jakob disease which are ‘transmissible’ through the brain. Prof Goedert suspects that Alzheimer’s Disease and abnormal, bad tau spreads like this too (remember normal tau is still required for proper neuronal signalling, it’s the abnormal tau that is so destructive). He backed up this theory by injecting a misfolded tau protein into a mouse. It was found that the mouse began to develop tau pathology of its own in the injected brain region. Not only this, but over time the disease pathology spread to other parts of the brain. More needs to be learned about how this process occurs, but cracking the mechanisms could provide a great area for new treatments. Being able to stop the spread of the disease early on could mean vital areas of the brain are saved.
As per usual with most biology, the bottom line is that nobody knowwwwsss – yet! People have good ideas about the spread of Alzheimer’s around the brain, but a lot more firm evidence is required before we can understand how, why and where this happens. And also why only certain neurones are affected. But maybe I’ll leave that for the next post!