At Tohoku University, Japan, Dr Kumiko Hayashi explores the logistics of cargo transport within the cell. Taking a biophysics approach, Hayashi has developed a non-invasive method for measuring the physical properties of the cell’s ‘carriers’ – the motor proteins. Her elegant studies demonstrate that the number of motor proteins is vital, as is as their ability to carry cargo to where it is needed. Understanding these intricate details of cargo transport offer insights into neuronal diseases such as Alzheimer’s and Parkinson’s diseases.
Motor proteins are vital to complex life. Forming the transport infrastructure within our cells, they are intimately involved in cargo transport, cell motility, and the organisation of the cell’s own mechanical structure. The two main motor proteins, kinesin and dynein, are recognised as the ‘workhorses’ of the cell. They haul neurotransmitters, chromosomes, and other vital cargo along microtubules: the tiny network of molecular tracks within our cells.
Cellular cargo transport: The logistics of cells
Amazingly, there are microscopic highways within the cells of our bodies that carry out cellular cargo transport. The truck drivers are the motor proteins, carrying biomaterials – the goods to be delivered to retailers or customers – along a network of microtubules: the highways. Powered by energy in the form of adenosine triphosphate hydrolysis, motor proteins constantly transport cargo back and forth. This allows biomaterials made near the centre of the cell to be delivered to the specific areas where they are needed. There are around 40 different types of cellular drivers, each delivering specific types of cargo. We are only beginning to understand their individual roles and their importance.
At the Department of Applied Physics within the Graduate School of Engineering at Tohoku University, Japan, Dr Kumiko Hayashi is making groundbreaking advances in our understanding of how these motor proteins work. Hayashi set out to investigate the logistics of cargo transport within eukaryotic cells (cells containing a nucleus and microtubule networks). Her approach is unique: she applies the principles of physics of non-living things to these intricate biological processes. More specifically, Hayashi borrows principles from non-equilibrium statistical mechanics, the field of physics used to study transport phenomena, including electric currents and heat conduction. Applying these physical concepts of the transport of non-living things, she is deepening our understanding of cargo transport in live cells.
Her team’s discoveries shed insight into the understanding of molecular mechanism of neuronal diseases like Alzheimer’s, Parkinson’s, and Huntington’s disease.
From test tube to living organism
In most cases, the physical properties of motor proteins, such as transport velocity and force, have been estimated by performing in vitro (test tube) experiments on single molecules. Typically, these methods include optical tweezers (the technique awarded the Nobel prize for physics in 2018), using a highly focused laser beam to hold and move microscopic and sub-microscopic objects to measure force generated by motor proteins.
Such in vitro experiments have provided significant insights on motor proteins. However, recognising the significance of physical measurements of these motor proteins in cells – where many kinds of proteins fully exert their functions by interacting with each other – Hayashi sought to find out whether it was possible to perform physical measurements directly within the cell. Measuring the movement of motor proteins in living organisms and applying the principle of physics to them, however, is a challenge: there are variations in cargo shape, size, and viscosity, as well as complex and crowded environmental conditions within the cell in which there are many cytoskeletons and proteins. This state is called a non-equilibrium state, which is a different state of environment (an equilibrium state) in the case of test tube experiments. By using the knowledge of non-equilibrium statistical mechanics, developed for understanding of non-equilibrium phenomena, Hayashi estimates the transport force acting on cargos generated by motor proteins inside cells.
Many hands make light work
Hayashi has contributed new discoveries in this area of research. Her elegant studies demonstrate that multiple motor proteins work cooperatively to transport cellular cargo that is too large for a single motor protein to carry, because the number of motor proteins cooperatively hauling a single cargo can be estimated from the transport force generated by motors, noting that transport force monotonously increases for the number of carriers.
Hayashi explains, ‘The motor proteins in our neurons can be compared to a chain of ants carrying a piece of food that is too large for a single ant to bear.’
Having multiple motor proteins is key to the healthy functioning of neurons and facilitates stable and long-distance transport of biomaterials along axons. With many motor proteins working cooperatively to deliver synaptic cargo, neuronal activity remains healthy. Therefore, the number of motor proteins is an important indicator of neuronal health (and disease). See figure 1.
Neuronal cargo transport
Stable and long-distance cargo transport is particularly important for nerve cells (neurons), which have a very long nerve fibre, also called the axon. Neurons communicate with each other and transmit information to other nerve or muscle cells via the axons. Problems with the transport of synaptic cargo, the materials needed to form synapses, are strongly linked to neuronal diseases. As Hayashi explains, ‘In our society, the logistics of goods transport is disrupted when there is a shortage of truck drivers. It’s the same for neurons. The logistics of synaptic cargo transport breaks down if there are problems with the motor proteins.’
Neuronal disease related to the number of motors
Hayashi puts forward the concept that neuronal health may be connected to the number of motors hauling a cargo. Abnormal numbers of motor proteins are associated with a range of impairments to cargo transport, whereas healthy neurons have a tightly regulated number of motors. If motors become dysregulated in human neurons, this is considered to be related to neuronal disease, such as Alzheimer’s, Parkinson’s, and Huntington’s disease. In fact, a range of different impairments can occur with the alteration of the number of transport motors. When the number of motor proteins is changed, the transport becomes weak, or excessive, leading to abnormal synapse formation. This impairment can also be seen in the case of hereditary spastic paralysis, one of the kinesin-like protein KIF1A-associated neuronal disorders (KAND).
Calculating the number of motors using transport physics is helping to elucidate impairment mechanism of cargo transport in living organisms resulting from neuronal disease.
Transport physics for neuronal cargo transport is a powerful new tool for understanding the intricate details of cargo transport. Not only does the tool provide insight into neuronal diseases, it also has the potential to find application in many other complex biological systems.
- Hayashi, K, (2021) Intracellular force comparison of pathogenic KIF1A, KIF5, and dynein by fluctuation analysis. bioRxiv. doi.org/10.1101/2021.09.12.459977
- Hayashi, K, (2021) Effects of dynein inhibitor on the number of motor proteins transporting synaptic cargos. Biophysical Journal, 120, 1605–1614. doi.org/10.1016/j.bpj.2021.02.018
- Hasegawa, S, (2019) Investigation of multiple-dynein transport of melanosomes by non-invasive force measurement using fluctuation unit χ. Scientific Reports, 9:5099. doi.org/10.1038/s41598-019-41458-w
- Hayashi, K, (2018) Non-invasive force measurement reveals the number of active kinesins on a synaptic vesicle precursor in axonal transport regulated by ARL-8. Royal Society of Chemistry, 20, 3403–3410. doi.org/10.1039/c7cp05890j
- Hayashi, K, Tsuchizawa, Y, Iwaki, M, Okada, Y, (2018) Application of the fluctuation theorem for noninvasive force measurement in living neuronal axons. Mol Biol Cell, 29:25, 3017. doi.org/10.1091/mbc.E18-01-0022
Taking a biophysics approach, Kumiko Hayashi sheds insights into the physical mechanisms of neuronal diseases caused by deficits in cellular cargo transport.
- Japan Agency for Medical Research and Development (AMED)
- Japan Science and Technology Agency (JST)
Kumiko Hayashi completed her PhD in Philosophy at the University of Tokyo in 2006. Following this, she was a Research Fellow of the Japan Society for the Promotion of Science, and Assistant Professor of Applied Physics at Tohoku University. Overcoming the Great East Japan Earthquake hitting in Tohoku area in 2011, Hayashi was promoted to Associate Professor at the university in 2018. She is also PRESTO (Precursory Research for Embryonic Science and Technology) Researcher of the Japan Science and Technology Agency.
6-6-05 Aoba, Aramaki