Lieberam Lab - our Research

Background

Motor Neurons

From the nod of your head to the wriggle of your toes, the external and internal movements of your body are controlled by signals sent from your CNS to your muscles via nerve cells called motor neurons. There are around 500,000 motor neurons carrying information from your CNS to muscles, organs, and glands. Some motor neurons transmit signals from the spinal cord to relay points call sympathetic ganglia, which in turn connect to smooth muscles, such as those in the blood vessels that control blood pressure. Other motor neurons connect the CNS with skeletal muscles, such as those in your legs that contract voluntarily so your limbs can move.

The structure of a motor neuron can be split into three key parts:

Neuromuscular junctions

All neurons send electrical signals called action potentials along their axons. This is mediated by charged particles flow in and out of the axon, causing a change in the voltage. The action potential is carried from the cell body down the axon down to the nerve endings called synapses. A synapse is a small gap between two neurons where they can pass information to one another. This communication can be electrical, passing a positive electrical signal to the next neuron, or chemical, releasing chemical messengers called neurotransmitters. A neural circuit is a group of many neurons connected by synapses.

There is a special kind of synapse between motor neurons and muscle called the neuromuscular junction. It is a chemical synapse releasing the neurotransmitter acetylcholine (ACh) from the nerve endings of motor neurons to the muscle, where Ach binds to sensor molecules called receptors and causes muscle cells to contract.

Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis (ALS), sometimes known as Lou Gehrig’s Disease, is a neurological disorder caused by the progressive degeneration and eventual death of both upper and lower motor neurons. Most people who develop ALS are between 40 and 70 years of age and early symptoms include:

• Muscle twitches in the arm, leg, shoulder, or tongue
• Muscle cramps
• Muscle weakness affecting an arm, a leg, the neck, or diaphragm
• Slurred and nasal speech

As the disease progresses, muscle weakness and wasting becomes more severe and spreads to other parts of the body and will eventually have difficulty swallowing, speaking, and breathing. Most people with ALS will die from respiratory failure 3 to 5 years after the onset of symptoms.

5 to 10% of people with ALS have familial ALS. They have a family history of ALS or a condition called frontotemporal dementia. Mutations in several genes are known to contribute to the development of familial ALS.

Each of these genes is involved in the normal functioning of motor neurons and researchers are trying to find out how mutations cause the death of motor neurons. A common theme of genes linked to ALS is that many are either involved in handling RNA (the intermediate code between DNA and protein) or the removal of molecular waste material.

The majority of people have sporadic ALS which has no clear cause, although it is thought to be a combination of genetic risk factors, like those above and other undiscovered mutations, and environmental risk factors such as:

• Smoking
• Environmental toxin exposure
• Military service

New research suggests that abnormal development of motor neuron circuits during embryonic development or just following birth play a role in the development of ALS. The Lieberam lab and our collaborators are interested in how and why anormal events during development arise and lead to ALS. By understanding these early changes, we can attempt to prevent motor neuron degeneration before it begins.

Our questions:

We are trying to answer a few key questions:

• How can we use stem cells models to understand the function and dysfunction of motor neuron circuits?
• How can we use stem cell models of motor neurons and muscle to study the development and progression of conditions such as ALS?
• How can stem cell models be used to develop drugs which restore motor function in patients suffering from neuromuscular diseases and other movement disorders?

Modelling ALS in a Dish

When studying disorders of the central nervous system, it is not possible to use motor neurons extracted directly from humans as they are difficult to access and are already badly deteriorated by the time neurodegenerative disorders are diagnosed. Many researchers use experimental animals with mutations that produce similar cellular changes and symptoms as the disease being studied. Such animals can be very useful but do not always mimic the human disease, and treatments developed in such animal models often do not work in humans.

To overcome these issues, we use human stem cell-derived models grown in a culture dish. Stem cells are cells which can renew themselves and specialise into other cell types (differentiate). In the Lieberam lab we use a special kind of stem cell called induced pluripotent stem cells (iPSCs). These cells are derived from adult tissue, such as skin or blood, but their cell identity is reset such that they acquire the properties of embryonic cells. As a consequence, many different types of somatic cells, including neurons, glia and muscle cells, can be derived from iPSCs in cell culture.

In collaboration with Virgile Viasnoff (MBI/NUS), we have created a microdevice which allows us to grow iPSC-derived motor neurons and their supporting cells (astrocytes) in one chamber and muscle fibre cells in a separate chamber, connected to the first one. The neurons grow axons through small channels, reach the second chamber, and innervate the muscle cells. This creates neuromuscular circuits that resemble what we see in the human body.

Together with Wenhui Song (UCL), we have adapted human iPSC-derived neuromuscular cultures to a multi-well format suitable for high content imaging screens. Myofiber collapse, an important engineering challenge in these kind of co-culture systems, is prevented by a scaffold of suspended elastomer nanofibers, which stabilize muscle fibers and mimic connective tissue found muscle.

The two models also incorporate a technology known as optogenetics which allows us to activate motor neurons in response to light stimulation and observe how muscle contraction triggered by motor neurons is affected in ALS.

Altogether, these model allows us to study how motor neuron function and deterioration is affected by gene mutations and trial small molecule drugs and gene therapies to prevent or reverse motor neuron degeneration in ALS and other neuromuscular diseases.

Hyperexcitability

New research suggests that the ALS disease process may begin with unusually high electrical signalling in motor neurons.

In collaboration with Juan Burrone (KCL), we have identified a change in the axon of motor neurons made from iPSCs of patients with TDP-43 mutations. These ALS-related motor neurons fire more action potentials than normal motor neurons and are unable to tune down their activity.

If we can understand why this and other early neuronal changes happen and identify them before the onset of ALS symptoms, we may be able to develop therapies which can prevent motor neuron loss by normalizing their activity early in the disease process.