̽��ֱ�� of Cambridge - Kristian Franze

Neurons feel the force – physical interactions control brain development

sc604 — Mon, 19 Sep 2016 15:00:00 +0000

Scientists have found that developing nerve cells are able to ‘feel’ their environment as they grow, helping them form the correct connections within the brain and with other parts of the body. ̽��ֱ��results, reported in the journal Nature Neuroscience, could open up new avenues of research in brain development, and lead to potential treatments for spinal cord injuries and other types of neuronal damage.

As the brain develops, roughly 100 billion neurons make over 100 trillion connections to send and receive information. For decades, it has been widely accepted that neuronal growth is controlled by small signalling molecules which are ‘sniffed’ out by the growing neurons, telling them which way to go, so that they can find their precise target. ̽��ֱ��new study, by researchers from the ̽��ֱ�� of Cambridge, shows that neuronal growth is not only controlled by these chemical signals, but also by the physical properties of their environment, which guide the neurons along complex stiffness patterns in the tissue through which they grow.

“ ̽��ֱ��fact that neurons in the developing brain not only respond to chemical signals but also to the mechanical properties of their environment opens many exciting new avenues for research in brain development,” said the study’s lead author Dr Kristian Franze, from Cambridge’s Department of Physiology, Development and Neuroscience. “Considering mechanics might also lead to new breakthroughs in our understanding of neuronal regeneration. For example, following spinal cord injuries, the failure of neurons to regrow through damaged tissue with altered mechanical properties has been a persistent challenge in medicine.”

We navigate our world guided by our senses, which are based on interactions with different facets of our environment — at the seaside you smell and taste the saltiness of the air, feel the grains of sand and the coldness of the water, and hear the crashing of waves on the beach. Within our bodies, individual neurons also sense and react to their environment – they ‘taste’ and ‘smell’ small chemical molecules, and, as this study shows, ‘feel’ the stiffness and structure of their surroundings. They use these senses to guide how and where they grow.

Using a long, wire-like extension called an axon, neurons carry electrical signals throughout the brain and body. During development, axons must grow along precisely defined pathways until they eventually connect with their targets. ̽��ֱ��enormously complex networks that result control all body functions. Errors in the neuronal ‘wiring’ or catastrophic severing of the connections, as occurs during spinal cord injury, may lead to severe disabilities.

A number of chemical signals controlling axon growth have been identified. Called ‘guidance cues,’ these molecules are produced by cells in the tissue surrounding growing axons and may either attract or repel the axons, directing them along the correct paths. However, chemical guidance cues alone cannot fully explain neuronal growth patterns, suggesting that other factors contribute to guiding neurons.

One of these factors turns out to be mechanics: axons also possess a sense of ‘touch’. In order to move, growing neurons must exert forces on their environment. ̽��ֱ��environment in turn exerts forces back, and the axons can therefore ‘feel’ the mechanical properties of their surroundings, such as its stiffness. “Consider the difference between walking on squelchy mud versus hard rock – how you walk, your balance and speed, will differ on these two surfaces,” said Franze. “Similarly, axons adjust their growth behaviour depending on the mechanical properties of their environment.” However, until recently it was not known what environments axons encounter as they grow, and Franze and his colleagues decided to find out.

They developed a new technique, based on atomic force microscopy, to measure the stiffness of developing Xenopus frog brains at high resolution – revealing what axons might feel as they grow through the brain. ̽��ֱ��study found complex patterns of stiffness in the developing brain that seemed to predict axon growth directions. ̽��ֱ��researchers showed that axons avoided stiffer areas of the brain and grew towards softer regions. Changing the normal brain stiffness caused the axons to get lost and fail to find their targets.

In collaboration with Professor Christine Holt’s research group, the team then explored how exactly the axons were feeling their environments. They found that neurons contain ion channels called Piezo1, which sit in the cell membrane: the barrier between cell and environment. These channels open only when a large enough force is applied, similar to shutter valves in air mattresses. Opening of these channels generates small pores in the membrane of the neurons, which allows calcium ions to enter the cells. Calcium then triggers a number of reactions that change how neurons grow.

When neuronal membranes were stiffened using a substance extracted from a spider venom, which made it harder to open the channels, neurons became ‘numb’ to environmental stiffness. This caused the axons to grow abnormally without reaching their target. Removing Piezo1 from the cells, similarly abolishing the axons’ capacity to feel differences in stiffness, had the same effect.

“We already understand quite a bit about the detection and integration of chemical signals” said Franze. “Adding mechanical signals to this picture will lead to a better understanding of the growth and development of the nervous system. These insights will help us answer critical questions in developmental biology as well as in biomedicine and regenerative biology.”

Reference:
David E Koser et al. ‘Mechanosensing is critical for axon growth in the developing brain.’ Nature Neuroscience (2016). DOI: 10.1038/nn.4394

Researchers have identified a new mechanism controlling brain development: that neurons not only ‘smell’ chemicals in their environment, but also ‘feel’ their way through the developing brain.

Considering mechanics might lead to new breakthroughs in our understanding of neuronal regeneration.

Kristian Franze

Eva Pillai

Brain of a frog embryo. ̽��ֱ��coloured structures are cell nuclei (containing DNA), the white structure in the center corresponds to the optic tract, which contains the neuronal axons studied.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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Surprising solution to fly eye mystery

gm349 — Thu, 11 Oct 2012 19:00:27 +0000

Fly eyes have the fastest visual responses in the animal kingdom, but how they achieve this has long been an enigma. A new study shows that their rapid vision may be a result of their photoreceptors - specialised cells found in the retina - physically contracting in response to light. ̽��ֱ��mechanical force then generates electrical responses that are sent to the brain much faster than, for example, in our own eyes, where responses are generated using traditional chemical messengers. ̽��ֱ��study was published today, in the journal Science.

It had been thought that the ion channels responsible for generating the photoreceptors’ electrical response were activated by chemical messengers as is usually the case in cell signalling pathways. However, these results suggest that the light-sensitive ion channels responsible for the photoreceptor’s electrical response may be physically activated by the contractions – a surprising solution to the mystery of light perception in the fly’s eye and a new concept in cellular signalling.

Professor Roger Hardie, lead author of the study from the ̽��ֱ�� of Cambridge’s Department of Physiology, Development and Neuroscience, said: “ ̽��ֱ��ion channel in question is the so-called ‘transient receptor potential’ (TRP) channel, which we originally identified as the light-sensitive channel in the fly in the 1990’s. It is now recognised as the founding member of one of the largest ion channel families in the genome, with closely related channels playing vital roles throughout our own bodies. As such, TRP channels are increasingly regarded as potential therapeutic targets for numerous pathological conditions, including pain, hypertension, cardiac and pulmonary disease, cancer, rheumatoid arthritis, and cerebral ischaemia. We are therefore hopeful that these new results may have significance well beyond the humble eye of the fly.”

A fly’s vision is so fast that it is capable of tracking movements up to five times faster than our own eyes. This performance is achieved using microvillar photoreceptor cells, in which the photo-receptive membrane is made up of tiny tubular membranous protrusions known as microvilli. In each photoreceptor cell, tens of thousands of these are packed together to form a long rod-like structure, which acts as a light-guide to absorb the incident light. Each microvillus also houses the biochemical machinery, which converts the energy of the absorbed light into the electrical responses that are sent to the brain – a process known as phototransduction.

As in all photoreceptors, phototransduction starts with absorption of light by a visual pigment molecule (rhodopsin). In microvillar photoreceptors this leads to activation of a specific enzyme known as phospholipase C (PLC). PLC is a ubiquitous and very well-studied enzyme, which cleaves a large piece from a specific lipid component of the cell membrane (“PIP₂”), leaving a smaller membrane lipid (DAG) in its place.

Somehow this enzymatic reaction leads to the opening of “ion channels” in the microvillus membrane; once opened, these allow positively charged ions such as Ca²⁺ and Na⁺ to flow into the cell thus generating the electrical response. This basic sequence of events has been established for over 20 years; but exactly how PLC’s enzymatic activity causes the opening of the channels has long remained a mystery and one of the major outstanding questions in sensory biology.

Professor Hardie added: “ ̽��ֱ��conventional wisdom would be that one of the products of this enzyme’s activity is a chemical ‘second messenger’ that binds to and activates the channel. However, years of research had previously failed to find compelling evidence for such a straightforward mechanism.”

̽��ֱ��new study, which was funded by the BBSRC and the Medical Research Council, using the fruitfly, Drosophila, now suggests a remarkable and unexpected resolution to this mystery. ̽��ֱ��key finding was that the photoreceptors physically contract in response to light flashes. ̽��ֱ��contractions were so small and fast that an “atomic force microscope” was needed to measure them. This revealed that the contractions were even faster than the cell’s electrical response and appeared to be caused directly by PLC activity.

̽��ֱ��researchers believe that the splitting of the membrane lipid PIP₂ by the enzyme PLC reduces the membrane area, thereby increasing tension in the membrane and causing each tiny microvillus to contract in response to light. ̽��ֱ��synchronised contraction of thousands of microvilli together then accounts for the contractions measured from the whole cell.

Dr Kristian Franze, co-author of the paper from the ̽��ֱ�� of Cambridge, said: “We propose that within each microvillus the increase in membrane tension acts directly on the light-sensitive channels. In other words, rather than using a traditional chemical 2^nd messenger, the channels were being activated mechanically.”

This concept was supported by experiments in which the native light-sensitive channels were eliminated by mutation and replaced with mechano-sensitive channels, which are known to open in response to membrane tension. Remarkably, these photoreceptors still generated electrical signals in response to light, but were now mediated by activation of the ectopic mechano-sensitive channels. To test whether the native light-sensitive channels could be affected by mechanical forces in the membrane, the microvillar membrane was stretched or compressed by changing the osmotic pressure. This simple experimental manipulation rapidly enhanced or suppressed channel openings in response to light as predicted.

These results suggest that PLC mediates its effects in the photoreceptors by changing the mechanical state of the membrane. ̽��ֱ��researchers suggest that it is the increase in the membrane tension (along with a pH change also resulting from PLC activity) that triggers the opening of the light-sensitive channels. Mechano-sensitive ion channels are actually well known, but normally involved in transducing mechanical stimuli – such as sound in the ears or pressure on the skin. One of their characteristics is that they can be activated extremely rapidly – perhaps an explanation for why fly photoreceptors have evolved this solution to phototransduction.

Professor Hardie said: “That a mechanical signal could be an intermediate signal -or ‘second messenger’- in an otherwise purely biochemical cascade is a novel concept that extends our understanding of cellular signalling mechanisms to a new level.”

For more information, please contact Genevieve Maul (genevieve.maul@admin.cam.ac.uk) at the ̽��ֱ�� of Cambridge Office of External Affairs and Communications.

Research provides insight into why flies have the fastest vision in the animal kingdom.

̽��ֱ��conventional wisdom would be that one of the products of this enzyme’s activity is a chemical ‘second messenger’ that binds to and activates the channel. However, years of research had previously failed to find compelling evidence for such a straightforward mechanism.

Professor Roger Hardie, lead author of the study from the ̽��ֱ�� of Cambridge’s Department of Physiology, Development and Neuroscience

Image H. Meinecke

Blowfly

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