Snapshot: What is Neurofilament light protein (NfL)?

Nerve cells (aka neurons) are unique cells in that they have long, and thin extensions called axons which form connections with and talk to other neurons. This particular shape of neurons determines how quickly they can get their messages to other cells. You can think of the axons in the brain like the wires connecting all the components of a dense electrical network.

NfL stands for Neurofilament light protein (Not to be confused with the national football league!). Neurofilaments are proteins found in our neurons. They are important for helping these cells hold their structure and size. We know this is important for their ability to send messages to other neurons. NfL is the smallest unit of three types of neurofilaments (light, medium and heavy). There is a lot of NfL found in the axons of neurons.

A large neuron with long interconected axons
A cortical neuron stained green with antibody to NfL. Image courtesy of GerryShaw on Wikimedia.

How do you measure NfL levels?

Like other proteins, NfL levels can be measured in fluids using tools known as immunoassays. These tools make use of antibodies generated by the immune systems to capture and count the protein of interest. It has been possible to measure NfL in cerebrospinal fluid (CSF) – the clear fluid that surrounds the brain and has lots of brain proteins – since 2005. In recent years, immunoassay technology has improved significantly, permitting the quantification of proteins previously too low in concentration to detect. One of these technologies is Single Molecule Array (Simoa) and has made it possible to measure NfL reliably in blood.

Why is NfL used as a biomarker?

Biomarkers are biological characteristics that can be measured and that tell us about a particular biological or disease process or response to a therapy. They can be used to make drug development more efficient. NfL is released into CSF after brain injury and also in many neurodegenerative diseases. This makes it a biomarker of neuronal injury. The problem with CSF is that it requires a safe but relatively invasive medical procedure called a lumbar puncture or spinal tap to collect. It would be a lot easier for both patients and doctors if we could get the same information from a blood test. Being able to quantify NfL – a brain protein – in blood, and more importantly, that it reflected what was happening in the brain was very exciting for many diseases.

In neurodegenerative diseases with effective disease modifying therapies (such as Multiple Sclerosis and Spinal Muscular Atrophy), a lowering of NfL reflects the clinical benefit in response to these therapies. In another genetic neurodegenerative disease caused by a CAG expansion, Huntington’s disease, NfL increase has been shown to be the earliest detectable change in asymptomatic gene carriers who are very far from their predicted age of disease onset. Many results like these suggest that NfL could help monitor disease even before symptoms appear, decide when to start therapies, and tell us if a drug is improving the health of neurons.

What NfL research is being done in ataxia research?

So what about ataxias? You will be pleased to know that Ataxia researchers have also jumped on the NfL band wagon. We previously wrote an article on two independently published studies in SCA3 which showed in many patients that NfL levels increased as Ataxia severity got worse, they were correlated with a measure of clinical severity (SARA) and increased with the level of brain loss (atrophy). One of the studies showed NfL levels increased with a higher number of CAG repeats in someone’s SCA3 mutation. There is also work using mouse models of SCA3 to understand this biomarker further. Two studies have now shown that NfL is also increased in Friedreich’s ataxia. With more research, NfL could potentially be used to design better clinical trials for ataxias and to monitor disease.

If you would like to learn more about NfL, take a look at this article by NeurologyLive.

Snapshot written by Dr. Lauren Byrne and edited by Dr. Gülin Öz.

Snapshot: What are Astrocytes?

The human brain contains about 170 billion cells. Half of these are neurons and the other half are lesser known cells called glia. Glial cells include astrocytes, oligodendrocytes and microglia. Astrocytes tile the entire brain and interact closely with neurons. Astrocytes are very important for neuronal function, in many ways playing a parenting-like role. They provide energy and support to neurons, and they clean after them. Astrocytes make sure that neuronal surroundings are “just right” for optimal function of neurons. They can also actively influence neuronal activity.

An astrocyte cell grown in tissue culture stained with antibodies to GFAP and vimentin. The GFAP is coupled to a red fluorescent dye and the vimentin is coupled to a green fluorescent dye. Both proteins are present in large amounts in the intermediate filaments of this cell, so the cell appears yellow, the result of combining strong red and green signals. The blue signal is DNA revealed with DAPI, and shows the nucleus of the astrocyte and of other cells in this image.
An astrocyte cell imaged using a microscope and colored antibodies. Imaged courtesy of Wikimedia.

Similar to neurons, there are important differences in astrocytes from different brain regions. For instance, in the cerebellum there are about 5 times more neurons than astrocytes. Meanwhile in the cortex there are 10 times more astrocytes than neurons. In addition, astrocytes in the cerebellum and hippocampus (a brain region that plays an important role in memory) express different sets of proteins. These brain region differences can contribute to the role that astrocytes play in neuronal function in health as well as in disease.

Bergman Glia: An Important type of Astrocyte in Ataxia

In the cerebellum, there is a special type of astrocyte called Bergman glia that are very closely connected with Purkinje cells, neurons that are often vulnerable in ataxia. In fact, the relationship between Purkinje cells and Bergmann glia is often referred to as the most intimate neuron-astrocyte relationship in the brain. This is important as in brain injury and neurodegenerative diseases astrocytes undergo process called gliosis that changes their function. Gliosis can make them either more neuroprotective (helpful) or harmful.

For instance, when there is disease Bergmann glia can increase their support to help Purkinje neurons maintain their function and delay onset of disease symptoms. But also, Bergmann glia can become harmful worsening the dysfunction of Purkinje neurons and more severe disease symptoms.

Bergmann glia  help support Purkinje cells early on in ataxia, but as the disease progresses they can actual make symptoms worse.
Illustrating intimate relationship between Bergmann glia (BG) and Purkinje cells (PC). Bergmann glia may increase their support to Purkinje cells early in disease. However, they can become harmful with disease progression. Image desgined by Marija Cvetanovic.

It is important to learn more about how astrocytes are altered in ataxia for these reasons. We can use that knowledge about astrocytes to develop novel therapies to delay onset of ataxia symptoms and their severity.

If you would like to learn more about astrocytes, take a look at these resources by Khan Academy and Tempo Bioscience.

Snapshot written by Dr. Marija Cvetanovic and edited by David Bushart.

Snapshot: What is an ion channel?

One of the most important features of neurons (Purkinje cells, for example), is that they are capable of electrical communication. Think of the last time you saw a TV intro or movie montage with a depiction of the brain on a microscopic level – though it’s technically invisible to the naked eye, that ‘spark’ you can see traveling down a portion of the neuron is actually not too far from reality. One of the most common ways to describe an active neuron, in fact, is to say that it’s “firing.” Essentially, when a neuron is activated, it ‘fires off’ an electrical impulse that is transmitted down a long, slender extension known as the axon. The axon ends where the next neuron in the circuit begins, and when the impulse arrives at that point, it initiates a series of events that allows the signal to jump to the next cell.

cartoon of neuron delivering an electrical impulse
An electrical impulse traveling down a neuron. Photo courtesy of Wikimedia.

This electrical signal is made possible by molecular machines known as ion channels. These proteins span the cell membrane, which is the barrier between the interior and exterior of the cell. When they receive a certain signal, the channel opens, allowing ions – atoms that carry an electrical charge, such as sodium, potassium, and calcium – to pass through. There are many types of proteins that allow the transport of small molecular components, but the source of a neuron’s electrical capabilities is that its channels specifically allow ions to pass into or out of the cell. Though a single ion’s charge is quite small, the large number of ions that are exchanged when a neuron’s channels open makes for a significant electrical effect – enough to produce an electrical signal that allows neurons to communicate with one another, giving us the ability to think, move, and interact with our environment.

diagram of an ion channgel in the closed, open, and inactivated state.
Cartoon of an ion channel in different states. Photo courtesy of Wikimedia.

Though the mutations that cause SCAs typically occur in genes that are expressed in every cell of the body, disease is usually restricted to the brain. One theory about why this is the case is that these SCA-related genes are necessary for the health and maintenance of ion channels in certain brain tissues – namely, the cerebellum and brainstem. At any rate, there is evidence that the electrical activity of these brain regions is abnormal in many SCAs, which strongly suggests that ion channels play a critical role in these disorders.

If you would like to learn more about ion channels, take a look at this Encyclopaedia Britannica article.

Snapshot written by Logan Morrison edited by Dr. David Bushart