Mechanosensitive Ion Channels
Dr. Farrugia's Cellular and Molecular Physiology of Gastrointestinal Disorders Lab studies the mechanosensitive ion channels that drive normal functions in the gastrointestinal tract. This work is in collaboration with Dr. Beyder's Gastrointestinal Mechanotransduction Laboratory.
To function normally, the gastrointestinal tract has to be an effective pump. Similar to other pumps in the body, such as the heart and bladder, the gastrointestinal tract uses transmembrane molecules called ion channels to generate electrical signals. The electrical activity generated by ion channels coordinates an influx of calcium required for smooth muscle contractions.
Ion channels are gated by ligands, voltage and force. Some ion channels are gated by specific primary stimuli such as voltage, but their function is also significantly modulated by other stimuli, such as ligands or force. In the gastrointestinal tract, where forces and electrical signals are critical for normal function, the voltage-gated mechanosensitive ion channels are especially relevant.
The lab uses conventional electrophysiology techniques and advanced techniques such as optogenetics, all-optical-electrophysiology and ultrafast pressure clamps to study the molecular mechanisms of ion channel mechanosensitivity and its effects on gastrointestinal physiology and pathophysiology.
By improving understanding of how mutations affect smooth muscle physiology and gastrointestinal function overall, Dr. Farrugia's team hopes to develop new treatments for functional gastrointestinal diseases such as irritable bowel syndrome (IBS) and slow transit constipation.
Cell perfusion
This video shows the relationship among a mechanosensitive calcium channel (shown in blue); a BK channel — a large conductance, calcium-activated potassium channel (shown in green); and the cell contractile state. As perfusion mechanically stimulates the mechanosensitive Ca2+ channel, calcium begins to enter the cell, calcium concentration increases (shown in red), the cell contracts and BK channels activate. This allows K+ to leave the cell, hyperpolarizing the cell and leading to cell relaxation.
Mechanoactivation in smooth muscle cells
Mechanoactivation in smooth muscle cells
Mechanoactivation in smooth muscle cells
Mechanoactivation of L-type Ca2+channels in human jejunal circular smooth muscle cells (A and B) increases Ca2+ entry and activates the contractile apparatus (C). Ca2+ entry subsequently activates large-conductance Ca2+ activated K+ channels (D), resulting in an increase in outward K+ current, membrane hyperpolarization (E) and relaxation (F).
The only mammalian ion channels thought to be mechanosensitive were nonvoltage-gated ion channels. In 1999, Dr. Farrugia's team showed that the main calcium entry pathway into gastrointestinal smooth muscle cells, the voltage-gated L-type calcium channel, is mechanosensitive, and mechanosensitivity is physiologically important as it regulates membrane potential by coupling calcium entry to potassium channels.
In 2002, Dr. Farrugia's lab showed that NaV1.5 is expressed in the human gut and that this voltage-gated ion channel is mechanosensitive. The lab's subsequent work dissected the mechanism of NaV1.5 mechanosensitivity at the molecular level and the relevance of NaV1.5 mechanosensitivity in GI physiology and pathophysiology.
At the molecular level, the lab found that there are specific NaV1.5 gating transitions that are affected by force and that in some cases, such as at the resting potential of the smooth muscle cell, this voltage-gated ion channel can actually be gated purely by force. Additionally, the lab found that NaV1.5 mechanosensitivity mechanism requires both the lipid bilayer and the cytoskeleton, but the details are still unclear.
Now, the Cellular and Molecular Physiology of Gastrointestinal Disorders Lab is focused on the molecular mechanism of NaV1.5 mechanosensitivity. Ranolazine and lipid-soluble local anesthetics effectively inhibit NaV1.5 mechanosensitivity by a mechanism that is different from the mechanism these drugs use to block voltage sensitivity. The NaV1.5 mechanosensitivity block appears to involve the lipid bilayer. Dr. Farrugia's research team is working to elucidate the mechanism of the block.
Colonic transit measured by scintigraphy shows a delay in colon transit in a patient with SCN5A mutation.
Colonic transit measured by scintigraphy shows a delay in colon transit in a patient with SCN5A mutation.
Colonic transit measured by scintigraphy shows a delay in colon transit in a patient with SCN5A mutation.
Top: Normal colonic transit. Labeled material is distributed among four colored sections, with an average position, or geometric center (GC), at 24 hours (GC24) being 2.1 and GC48 at 3.7. Bottom: Colonic transit in a patient with A997T-NaV1.5 mutation. The geometric center is significantly lower than normal for both GC24 (1.3) and GC48 (2.0). In this patient, the gastrointestinal transit was normalized by a treatment with a cardiac medication. Source: Gastroenterology. 2014;146:1659. Used with permission.
The lab's research is focused on why and how NaV1.5 is important for gastrointestinal physiology and pathophysiology. Dr. Farrugia's team has found that NaV1.5 mechanosensitivity is important for normal smooth muscle cell electrical and mechanical functions, and that a blockade of NaV1.5 mechanosensitivity by ranolazine decreases contractility, delays intestinal transit and results in constipation. Importantly, a subset of patients with IBS have SCN5A, a gene that codes for NaV1.5 mutations.
In this subset of patients, most of the mutations result in NaV1.5 channels that have functional abnormalities in both voltage-sensitivity and mechanosensitivity. However, Dr. Farrugia's team has found that restoration of NaV1.5 function in these cases normalizes gastrointestinal function. This finding is significant in that NaV1.5 is the first case of ion channelopathy in functional GI diseases.
In ongoing research, the lab aims to understand how IBS-associated SCN5A mutations affect smooth muscle physiology and gastrointestinal function overall, and how to pharmacologically target NaV1.5 mechanosensitivity in functional gastrointestinal diseases such as IBS and slow transit constipation.
Researchers studying mechanosensitive ion channels in Dr. Farrugia's lab work closely with the Gastrointestinal Mechanotransduction Laboratory of Arthur Beyder, M.D., Ph.D., at Mayo Clinic.