Newsletter No. 119

CUHK Newsletter No. 119 4thJanuary 1998 3 'Excellent' Research Project Touching: What Goes on Under the Skin? Investigating the Role of Merkel Cells in the Sensory Perception of Touch Our life is normally dominated by our visual and aural senses. We believe only what we see and hear, often neglecting our sense of touch. Yet, new born mammals depend on touch to find their mother's milk, nocturnal animals and the blind feel their way around their surroundings. Our sense of touch is extremely sensitive. The touch receptors in the skin of our finger tips respond to the slightest stimuli on the skin's surface. How Touch I s Sensed How are stimuli on the skin transmitted to the central nervous system? Below the epidermis are different types of mechano-receptors important for the sensory perception of touch. When stimulated, they convert the stimulus into electrical nerve signals which pass through the afferent nerve fibres into the central nervous system. Do Merkel Cells Relay Messages? Located just below the epidermis are also clusters of Merkel cells, which form a system with afferent nerve fibres. Together they are known as Merkel cell mechano- receptors. The junctions between Merkel cells and nerve terminals show all characteristic features of synapses —structures that transmit information from one cell to another within the nervous system. It has therefore been postulated that Merkel cells are the actual sites of mechano-electrical transduction converting stimuli into action potential. The well-protected location of Merkel cells — between the horny epidermis and the firm collagenous fibres of the cutis and subcutaneous tissues — has made them virtually inaccessible for direct observation or measurement with microelectrodes. As a result there has been much controversy about the role of Merkel cells. Insight from Mechanisms of Sound Transmission In the inner ear the mechanical vibrations of sound are converted into nerve signals in specialized cells called hair cells, which have a number of similarities with Merkel cells. It has been found that in the transduction process, voltage gated channels are opened up in the hair cells to allow the entrance of Ca 2+ ions. This and other findings have prompted Prof. Klaus Baumann and Prof. Yung Wing-ho of the Department of Physiology to investigate the role of intracellular in Merkel cell receptor function. They formed a research team with Ph.D. students Eliza Chan and Solomon Senok and embarked on the project 'Molecular Mechanisms in the Transduction Process in Mammalian Merkel Cell Mechano-receptors in the Skin'. The project received a grant of HK$727,000 from the Research Grants Council in 1992 and was completed in 1995. Its findings verified the functions of Merkel cell mechano-receptors. The research was rated 'excellent' by the RGC in late 1996. Successful Isolation of Merkel Cell Mechanoreceptors for Investigation Prof. Yung said, 'The greatest difficulty we had to overcome was isolating normal and functioning Merkel cell mechano-receptors from mammals for experiments. It had never been done before.' The team found the solution to their problem in rats whose whiskers or sinus hairs are abundant in Merkel cell mechano-receptors. Moreover, Merkel cell mechano-receptors in whiskers are, comparatively speaking, easier to isolate once the whisker including the root has been carefully excised from the animal. Despite the absence of research literature on the subject, Chan succeeded after repeated attempts in removing the outer layers of the whisker's root with the help of a dissection microscope. The isolated Merkel cells and relevant nerve fibres are covered only by a thin glassy membrane which does not interfere with optical measurements (figure 1). Prof. Yung believed that the development of this novel preparation is one of the factors contributing to the project's excellent rating. Increased Calcium Concentration in Cells indicates Transduction Function The team then measured the concentration of Ca 2+ in Merkel cells using a technique called microfluorimetry. Merkel cells were loaded with Ca 2+ . sensitive dyes and then stimulated mechanically. The difference in Ca 2+ concentration before and after the stimuli was measured by the dyes' shift on the absorption or emission spectrum in the ultraviolet or visible ranges . At the same time, to confirm the receptor function of Merkel cell mechano- receptors, Senok made electrophysiological recordings of receptor responses. Samples were also sent to the University of Hamburg to observe whether the ultrastructure of the isolated Merkel cells was intact. Test results demonstrated that the Ca 2+ concentration in Merkel cells increased after direct mechanical stimulation (figure 2), and was associated with receptor responses. These preliminary findings serve as evidence for the direct involvement of Merkel cells in the mechano-electric transduction process. Future Studies The responses of the Merkel cells to mechanical stimulation vary greatly. Prof. Yung explained that this is not surprising because it is difficult to apply the same pressure to each of the few cells under the microscope. Besides, such mechanically induced increases in calcium may be brief and very localized. To achieve greater accuracy, the researchers replaced mechanical stimulation with stimulation by Adenosine Triphosphate (ATP) or high concentrations of Potassium Chloride (KC1). The conditions induced by these chemicals better mimic the effects of natural stimulation. The results are found to be consistent with those obtained by mechanical stimulation (figure 2). Cells within the nervous system are shown to transmit information through changes in ion currents in the cell's membrane. The role of Merkel cells in mechano-electrical transduction may be proved unequivocally by measuring transmembrane ion currents in Merkel cells. To this end Prof. Baumann and Prof. Yung began a follow-up project, 'Mechano-electric Transduction Currents in Merkel Cell Receptors', which received a grant of HK$531,000 from the RGC in 1995. Figure 1 Source : J. Comp. Neurol, 337,36 6 (1993 ) Figure 2 Prof. W.H. Yung graduated with first class honours from the Department of Biology of The Chinese University in 1 9 8 5, and obtained his M . P h i l . in physiology from CUHK two years later. He then received a Commonwealth Scholarship to pursue doctoral studies at Oxford University, receiving his D . P h i l . in 1990. He was subsequently granted a Croucher Foundation Fellowship to stay an extra year as postdoctoral fellow. Prof. Yung joined the University's Department of Physiology as lecturer in 1991. His main research interests lie in the receptor function of Merkel cells, and synaptic physiology in the central nervous system. Prof. K.I. Baumann obtained his MD from the University of Hamburg, Germany, in 1971, and his Habilitation (or Ph.D.) from the same university in 1978. He joined The Chinese University as senior lecturer in 1981, becoming reader in 1991. Prof. Baumann left the University and returned to Hamburg University in 1996,