Tracking neuron cargo

4 August 2016

In three months, Ms Christy Hung, 2013 Croucher scholar, will have completed her PhD at The University of Cambridge. Her research investigates a potential and much-debated cause of Alzheimer’s disease, and its implications could lead to the development of more successful treatments for neurodegenerative diseases.

Hung’s investigation focuses on neuron cells; exploring axonal, or axoplasmic, transport, the process of transporting proteins from the cell body to the axon terminal.

Axonal transport has been found to be impaired in patients with neurodegenerative diseases, causing axonal accumulation, which can lead to the death of neuron cells. For example, in Alzheimer’s disease, over-expression of amyloid precursor proteins (APP) has been shown to cause axonal blockage in both drosophila and mouse models, suggesting axonal transport impairment may play a key role in the disease process.

Amyloid β (Aβ) peptide deposition is one of the major pathological hallmarks of Alzheimer’s disease. This plaque is produced when enzyme BACE1 cleaves APP.

Alzheimer’s and Parkinson’s diseases are potentially the most devastating diseases a person could get. It’s sad to think that you could forget your children or grandchildren.

Preexisting drugs already target BACE1 enzymes, however these drugs appear to be inefficient due to a lack of understanding about the normal physiological functions of the enzyme, such as where it meets APP. Previous studies have suggested that BACE1 and APP are transported to the axon terminals together, and that this is when the plaque is produced.

However, the location where the two meet is actually controversial, as contradictory studies suggest that BACE1 and APP are typically separate. Since abnormal accumulation of BACE1 at the presynaptic terminals has been reported in the brains of patients with Alzheimer’s disease post-mortem, understanding axonal BACE1 transport is vital to tackling the disease. These findings suggest that elevation of local BACE1 levels could promote the generation of Aβ, potentially contributing to the development of Alzheimer’s disease. However, the molecular components involved in BACE1 transport in the axons are still not fully identifiable.

Transport in the axons

Hung’s research focused on kinesin KIF1A. Kinesins are responsible for the transportation of various different cargoes, including synaptic vesicles, to the axon terminals. Hung made use of live-cell imaging to explore sympathetic neurons from mice.

In order to discover which cargoes co-migrate with KIF1A, a range of cargoes were fluorescently tagged, and dissociated superior cervical ganglions (SCG) cultures were microinjected with low levels of KIF1A-GFP/Cargo candidates-RFP. Subsequent transport in the axons was visualised via fluorescent labels, and the percentage of co-migration in the axon was analysed by comparing pairs of kymographs generated for each fluorescently tagged cargo.

Hung chose to use live-cell imaging as opposed to immunoprecipitation due to some of the limitations associated with the latter, such as this approach alone not proving that a motor actually moves cargo associated with it.

The study revealed that KIF1A had the highest percentage of co-migration with BACE1, while its co-migration levels with APP were considerably lower.

This discovery suggests, in this neuron subtype at least, that over-expressed BACE1 and APP would not meet, and that a mechanism of Alzheimer’s disease might place the two together, which would subsequently create the plaque that Hung describes as, ‘the pathological hallmark of the disease’.

Her results propose that changes in KIF1A could influence the intracellular site and rate of processing of APP. Hung highlights that altering the different components of axonal transport may have different effects on Alzheimer’s disease pathogenesis.

A bright outlook

Hung acknowledges that there are some limitations to her research, which has involved sympathetic neurons as opposed to the central nervous system-derived neuronal cells. Although APP is expressed ubiquitously, aggregation of Aβ only occurred in specific regions of the brains of Alzheimer’s patients.

Hung chose to use sympathetic neurons as working within the central nervous system would require more complex systems, including one to differentiate between axons and dendrites, which are easily identifiable in sympathetic neurons. However, this does not negate the importance of the discovery, as Hung is confident that her research could be extended not only to the central nervous system, but to in vivo models.

Protein trafficking in neurons

As it is now possible to follow and record the movement of fluorescently labelled cargo within axons in vivo in real time, in the future, the research could conceivably cross the transgenic mice expressing fluorescently tagged BACE1 with other transgenic models to simultaneously report on multiple axonal transport motors. In this way, it would be possible to obtain a more comprehensive view of how axonal transport is affected in models of neurodegenerative diseases.

Hung has long been interested in the field of neurodegenerative diseases as a whole, saying, “I think that neurodegenerative diseases, such as Alzheimer’s and Parkinson’s diseases are potentially the most devastating diseases a person could get. It’s sad to think that you could forget your children or grandchildren, and even forget where you are. These are very sad and depressing diseases.”

After completing her PhD, she intends to continue researching in the neuroscience field, and hopes to stay focused on neurodegenerative diseases in particular. She also plans on continuing her research within the UK, as it provides many opportunities to work in this area. One of her long-term aims is to identify the kinesin responsible for the transport of another important protein named NMNAT2, an NAD-synthesizing enzyme that is now well established as an essential survival factor that prevents spontaneous degeneration of healthy axons.

In 2013, Hung received her Bachelor’s degree in Biochemistry from The University of Hong Kong (HKU). While at HKU, she was the recipient of the Patrick Chow Lum Wong Memorial Prize for Biochemistry, as well as a number of scholarships, including the HSBC Hong Kong Scholarship and the HKU-Pembroke-Kings Scholarship. For her PhD research, undertaken at The University of Cambridge, she moved into the field of neuroscience. Her current work aims to increase our understanding of neurodegenerative diseases by exploring axonal transportation. The research, due to be completed this year, is supported by both the Croucher Cambridge International Scholarship and the Cambridge International Scholarship. Eventually, Hung hopes findings from this research could be applied to the field of drug development. Her study on the mediation of the axonal transport of BACE1 by KIF1A is due to be published in the journal Traffic.

To view Hung’s personal Croucher profile, please click here.