Discoveries by Auckland University team give hope of finding suitable drug treatments.
Auckland neuro-scientists have made important discoveries in how Alzheimer's and Parkinson's may affect the brain, work which has prompted preliminary experiments with drugs they hope could help people who have the diseases.
The Auckland University team investigated how stem cells move around the brain and become connected to other cells, forming circuits for thinking, movement and other brain functions. Stem cells can develop into other types of cells to replace those damaged or destroyed.
To migrate in the brain, stem cells have to overcome the friction they face in moving through the organ's scaffolding. Even in the right place, friction gets them again, this time restricting their dendrites, long tentacles used to locate other brain cells to connect to.
The stem cells deal with this by putting a slippery substance - polysialic acid - on their surface.
"It's a bit like putting soap on your body before going down a hydroslide," said the director of the studies, Dr Maurice Curtis, of the university's Centre for Brain Research.
After its "hydroslide" ride, the cell has to suck up its slippery coat. This stops it moving and allows it to complete its hooking up with the brain's circuitry.
The researchers discovered what happens to the slippery substance - "the cell engulfs the polysialic acid and chops it up into pieces able to be re-used for something else," Dr Curtis said.
The group, whose findings are published in The Journal of Neurochemistry, also discovered in their laboratory dish experiments with human cell lines that high levels of insulin blocked the removal of polysialic acid from the cell surface.
Dr Curtis said that this "therefore affects the connectivity of cells within a circuit".
"This is important because we know that in people with Alzheimer's and Parkinson's, for example, they experience insulin insensitivity a little like in type 2 diabetes. They also become less sensitive to a number of other growth factors in the brain.
"The implication is this may be one of the ways high levels of insulin reduce the amount of connectivity or plasticity within the brain. If you can't get rid of the polysialic acid from those foraging dendrites, you can't get appropriate connections with what those dendrites are foraging to find.
"It is known that people with type 2 diabetes do have a much higher incidence of Alzheimer's. We don't really know why that is."
He said experiments had begun with new drug compounds that targeted polysialic acid removal, in the hope of improving neuron connectivity. If successful, it would still be some years before any trials in humans could be considered.
* More than 10,000 people affected in New Zealand.
* Symptoms include trembling, slow movement and depression.
* No cure.
* Drugs can control symptoms, but effectiveness can wane.
* Some have deep-brain stimulation, a surgical implant of an electrode.
* More than 20,000 in New Zealand.
* Symptoms include memory loss, cognitive decline, loss of movement control.
* No cure.
* Drug treatment can slow the disease.