It could almost be the negative of a view from a plane over a busy city at night. The lights of vehicles moving in and out, clusters of traffic; a city going about it’s business.
But this city is a brain cell. A neuron.
The roads are axons and dendrites; long slim projections that send (axons) and receive (dendrites) electrical impulses to and from the cell body. The lights aren’t cars, but proteins moving up and down the axons and dendrites. But they are definitely lights; of a sort.
Much like post at a large sorting office, cellular proteins are sorted into groups, depending on where they are needed. This is done by the ‘golgi apparatus‘, the cells sorting office. Proteins are packaged into vesicles and are targeted to different areas, be it axon, dendrite, or elsewhere. The packages are attached to a group of proteins called Myosins that carry cargo along a kind of scaffold inside the cell, the cytoskeleton. The cytokskelton is made up of two components, actin and microtubules and each has positive and negative ends. Different Myosins move in different directions along this.
The axons and dendrites have different properties, such as how the are charged electrically, their polarity, relative to each other.
The technique used in this video lets us see this happening in the cell, in real time.To make it, an imaginatively named protein, originally discovered in a bio-luminescent jellyfish, called green flourescent protein (GFP) was used. This shines bright green when lit by blue to UV light. In the video GFP is directly stuck to a neuronal protein, called Ng-CAM. This let the researchers track the movement of Ng-CAM through different parts of the cell.
But there was a problem. With so many different routes and cargo’s moving through the cell, the light from the GFP labelled cargo might be lost. To get around this the researchers ‘dammed up’ the labelled proteins, so that large groups of vesicles would accumulate around the golgi apparatus, they could then trigger their release; giving an unobstructed view of the trafficking.
The surprising result for the researchers was that proteins destined to different areas would move almost anywhere throughout the cell. But when, for example, a protein specifically destined for the dendrites got into the axons, they would hit a red light. They would either stay there, or turn around and head elsewhere, like a lost driver realising their mistake. Non-location specific proteins appeared to go almost anywhere within the cell.
The researchers think that a kind of filter made of actin – one component of the cytoskeleton – is present in the entrance to the axons. The filter lets different types of Myosin (Myosin Va or Myosin VI for example) determine if that’s where it’s cargo should be going. If it’s not, it stops, and turns around.
The filter may by the sought after end point for a series of reactions that people have previously described that lead to the polarising of a cell. But more than that, the researchers have now shown a new way to be able to zero in on, and visualise, specific pathways within a cell, something that could have far broader implications for cell biology.
Reference: Differential Trafficking of Transport Vesicles Contributes to the Localization of Dendritic Proteins, Sarmad Al-Bassam, Min Xu, Thomas J. Wandless, Don B. Arnold, Cell Reports – 26 July 2012 (Vol. 2, Issue 1, pp. 89-100)