Mechanosensitive Ion Channels
Mechanosensitive ion channels are ion channels whose gating can be altered by mechanical forces. Mechanical stress is subsequently transformed into an electrical response. A major interest of Dr. Farrugia's laboratory has been the study of mechanosensitive ion channels in human jejunal circular smooth muscle cells, which appear to express a mechanosensitive L-type calcium channel.
The presence of mechanosensitive ion channels in gastrointestinal smooth muscle suggests that this muscle is not only a motor organ, but also a sensory organ that is able to sense and directly respond to changes in its environment.
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.
Figure 1. Effect of applying intracellular pressure.
Mechanosensitive calcium and potassium channels
The response of voltage-dependent Ca2+ channels to mechanical activation can be demonstrated by applying intracellular pressure (figure 1).
The lab also has tools to apply mechanical activation by altering the perfusion rate of extracellular solution and to study this effect in patches of membranes pulled from the cells. The sensitivity of the large conductance Ca2+-activated K+ channels to mechanoactivation can also be demonstrated, which has resulted in the development of a model (figure 2).
Mechanosensitive sodium channel
The mechanosensitivity of the TTX-resistant Na+ channel Nav1.5 (also known as SCN5A or hH1) has been intensively studied in Dr. Farrugia's laboratory. This protein is expressed in human (but not mouse) interstitial cells of Cajal and smooth muscle cells and plays a role in the electrical slow wave and human gastrointestinal motility.
Figure 2. 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).
Abnormal function of this channel is also associated with severe cardiac arrhythmias, as demonstrated by the laboratory of Michael Ackerman, M.D., Ph.D., with which Dr. Farrugia's lab collaborates. At the single channel level, the lab has demonstrated fundamental biophysical properties of this protein. In humans, the lab has identified a subset of people with functional motility disorders who have mutations in the channel.
Therapeutic agents with effects on mechanosensitive ion channels in gastrointestinal smooth muscle
Mechanosensitive ion channels are attractive targets for modulation of gastrointestinal motility. The lab has examined in detail the effects of several drugs on mechanosensitive, voltage-gated cation channels in gastrointestinal smooth muscle cells. This includes the weakly selective T-type Ca2+ channel inhibitor; mibefradil; otilonium bromide, an antispasmodic that is sometimes used to treat irritable bowel syndrome; and ranolazine, the regulator of voltage-gated Na+- channels.