FM Regulation text

Frequency-modulation for the regulation of ciliary activity

A common scheme for the interpreting regulatory signals is analog-modulation (AM). In this scheme, the magnitude of response of the system is generally in proportion to, and in synchrony with, the magnitude of the regulatory signal. A second scheme of regulation is frequency-modulation (FM). In this scheme, the magnitude of the response is proportional to the frequency of equal-sized stimulatory pulses or signals.

The effect of the frequency of Ca²⁺ Oscillations on ciliary beat frequency
The possibility that ciliary beat frequency is regulated by FM was suggested by two observations. First, repetitive Ca²⁺ oscillations that elevated the [Ca²⁺]_i ~ 300 mM were capable of maintaining the beat frequency at a rate higher than that induced by a much larger Ca²⁺ increase. Second, there is a hysteresis associated with the decrease in beat frequency.

Experimental data (above) for regulation of ciliary activity by FM was found by comparing the beat frequencies of cilia in cells that experienced Ca²⁺oscillations at two different frequencies (Period ~ 8 seconds or 5.5 seconds). The increase and decrease in beat frequency with respect to changes in Ca²⁺ for both Ca²⁺ oscillation frequencies had a similar pattern (left, red symbols = increases in cbf, white symbols = decrease in cbf: period triangles ~ 5.5 sec, circles ~ 8 s).

However, the magnitudes of the two responses were not equal (right). Although a similar maximal beat frequency was induced at slightly different [Ca²⁺]_iby the two Ca²⁺ oscillations of different periods, the minimal beat frequency during the Ca²⁺ oscillations was lower for the slower oscillation (right), even though this occurred at similar minimal [Ca²⁺]_i.

Hypothesis of mechanism
A simple explanation for the preceding results is that the de-activation of beat frequency occurs more slowly than the activation of beat frequency. A process that is required for FM - regulation.

Data shows that activation of beat frequency is a Ca²⁺ dependent - process but, because the deactivation process is slower, this action of Ca²⁺ would seem to be indirect. A common way of regulating biological process is through phosphorylation. We propose that Ca²⁺ activates a kinase (perhaps CAM kinase) to phosphorylate the axoneme. A phosphorylation event would help to explain the near-maximal responses to relatively low Ca²⁺increases. De-phosphorylation would be required to reverse the increase in beat frequency. This process may be Ca²⁺ independent and may occur at a steady, but slower rate than phosphorylation. As a result, the time between the Ca²⁺ peaks of oscillations will determine the amount by which beat frequency declines. In other words, oscillations with a shorter time period will result in a more stable beat frequency.

A Working Model: Extracellular ATP may reach the airway lumen as a result of constituent release from epithelial cells via the cystic fibrosis transmembrane regulator (CFTR). Alternatively, ATP is often contained in secretory granules and may be released with mucus. This would seem ideal if the role of ATP is to regulate beat frequency in the vicinity of mucus. ATP acts on P2 receptors to activate PLC to produce IP₃which in turn binds to IP₃ receptors of the endoplasmic reticulum (ER) to stimulate the release of internal stores of Ca²⁺. The negative feedback of the Ca²⁺ on the IP₃R contributes to the generation of Ca²⁺ oscillations. The Ca²⁺ interacts with calmodulin to activate CAM-kinase to phosphorylate the axoneme. A slower de-phosphorylation reverses the action of Ca²⁺.