Prof. Dr. Stephan Frings

Olfactory signal transduction

The chemosensory cilia of olfactory receptor neurons are exposed to the ambient air on the surface of the olfactory epithelium. They possess odorant receptors that drive a transduction cascade which eventually leads to electrical excitation of the neuron. This cascade has various unique features apart from the enormous variety of odorant receptors (~1200 different receptors in mouse). The second messenger of the olfactory cascade is cAMP which opens calcium-permeable ion channels in the ciliary membrane. The inflowing calcium opens chloride channel which appear to carry most of the receptor current during odor stimulation. In the absence of odorants, the cilia accumulate chloride in the cytoplasm. During stimulation, this chloride is discharged into the surrounding mucus, inducing an anion current that effectively depolarizes the cell. We study this unusual way of using chloride currents to generate receptor potentials.

Fig. 1  Schematic representation of the molecular components of olfactory signal transduction in a chemosensory cilium. Odorants dissolve in mucus at the surface of the olfactory epithelium and bind to odorant receptors (OR) in the ciliary membrane. The receptors activate the enzyme adenylyl cyclase (AC) to synthesize cAMP and open cyclic nucleotide-gated (CNG) cation channels. Calcium enters the cilium and opens chloride channels of the anoctamin (ANO2) type. Chloride is accumulated in the resting cilium through the chloride transporter NKCC1 which, in turn, is activated by a set of protein kinases (SPAK, OSR, WNK). Calcium is removed from the cilia by a sodium/ calcium exchanger (NaCaX) to terminate the recoetor current. (from Hengl et al., 2010)

Various questions concerning this transduction scheme have yet to be addressed. These concern the significance of the elaborate chloride mechanism for olfactory performance and the temporal responsiveness of the system. The latter point is particularly interesting because olfactory transduction must operate in a rapid oscillatory mode which is imposed by the sniffing frequency (5-8 Hz) of the animal. We study these questions using biophysical and biochemical methods in the rodent olfactory system.

Trigeminal modulation of olfactory information processing

The mammalian olfactory epithelium contains olfactory receptor neurons and trigeminal sensory endings. The former mediate odor detection, the latter the detection of irritants, the chemesthesis. The two apparently parallel chemosensory systems are in reality interdependent in various well documented ways. Psychophysical studies have shown that virtually all odorants can act as irritants, and that most irritants have an odor. Thus, the sensory perception of odorants and irritants is based on simultaneous input from both systems. Moreover, functional interactions between the olfactory system and the trigeminal system exist on both peripheral and central levels. We examine the impact of trigeminal stimulation on the odor response in collaboration with the group of Thomas Hummel at the TU Dresden. Using an odorant with low trigeminal potency (phenylethyl alcohol) and a non-odorous irritant (CO2), we explore this interaction in psychophysical experiments with human subjects and in EOG recordings from rats. We demonstrate that simultaneous activation of the trigeminal system attenuates the perception of odor intensity and distorts the EOG response.

Fig. 2  Results from a psychophysical study with co-application of PEA (rose odorant) and CO2 (an irritant) in human subjects (left). The stronger the perceived irritant (chemestesis) the weaker is the odor perception (olfaction). In the rat olfactory epithelium (center), co-application of the trigeminal neuropeptide CGRP with the rose odorant reduced the electrical response of the olfactory epithelium. The images on the right show trigeminal fibers crossing the olfactory epithelium (blue) right up to the tissue surface (red). (from Daiber et al.  2013)

On a molecular level, we identify a route for this cross-modal interaction. The neuropeptide calcitonin-gene related peptide (CGRP), which is released from trigeminal sensory fibers upon irritant stimulation, inhibits the odor response of olfactory receptor neurons. CGRP receptors expressed by these neurons mediate this neuromodulatory effect. This study demonstrates a site of trigeminal-olfactory interaction in the periphery. It reveals a pathway for trigeminal impact on olfactory signal processing that influences odor perception. We use an optogenetic approach to study the effect of trigeminal neuropeptides on olfactory signal processing in detail. In particular, we investigate the hypothesis that the peptides are involved in the formation of olfactory memory.
 

Ion channel structure-function studies

The transduction channels of sensory cells like olfactory receptor neurons are opened when the stimulus hits the sensory organelles - in our case the chemosensory cilia. The channel opening mechanism and various regulatory mechanisms involve second messengers, protein kinases, calcium/calmodulin and other factors like the membrane voltage. To understand these things, we use extensive mutagenesis studies on both olfactory transduction channels.

Fig. 3  Working models for the two olfactory transduction channels. These models have emerged from functional studies of many channel mutants and provide an explanation for the desensitization that occurs in both channels upon binding of calcium-calmodulin (from Adelman & Herson, 2004 and Vocke et al., 2013).

Such studies begin with bioinformatic analyses that help to identify possible regulatory domains in the channel structure. Targeted point mutations are then used to probe the significance of each individual domain for channel function. The read-out is usually a patch-clamp recording from a cell line that has been transfected with the mutant. Additional evidence comes from biochemical studies, especially for studies into the tertiary and quaternary structure as well as into protein-protein interactions.