Sense to Synapse:
Biophysical Mechanisms of Perception

April 19, 2012
9:00 AM - 4:15 PM
Davis Auditorium (Schapiro CESPR Bldg.)
Columbia University, New York City

Schedule/Abstracts

• 9:00 - Welcome


• 9:05 - Introduction - Elizabeth Olson (Columbia University)


• 9:15 - Olfaction Keynote - Stuart Firestein (Columbia University)
  → Making Sense of Scents: Mammalian Olfaction


• 10:00 - Julia Tsitron (Rutgers University)
  → A Physical Model and Bayesian Inference Guides the Design and Analysis of Chemosensory Arrays
  The first step in olfactory transduction involves the binding of odor molecules from the environment to G protein-coupled receptors (GPCRs) located on the surface of olfactory sensory neurons (OSNs). OSNs are long bipolar cells with one end specialized for chemical sensing and the other for signaling to higher brain areas. At the sensing end, detection of odorants is mediated by a combinatorial response of the GPCRs whereby an enormous repertoire of odors can be discriminated using a relatively small number of receptors. Inspired by these principles, arrays of cross-specific receptors have been proposed as a means to create sensitive, inexpensive chemosensory assays, or 'artificial noses'. However, the complex pattern of receptor response to even a single analyte and moreover, to mixtures of compounds has made analysis of array readout a challenging task. We use a biophysical model of receptor-ligand interactions and a Bayesian inference algorithm to make quantitative predictions of compound concentrations in mixtures of highly related sugar nucleotides using output from four engineered GPCRs. In addition, our in silico studies shed light on the design principles of combinatorial sensor arrays that optimize the discriminatory powers of a set of receptors. For example, we show that the number of receptors needed to successfully discriminate a mixture should be at least one half of the number of components in that mixture. We also find that optimal binding affinities, efficacies and other sensor parameters show weak dependence on the relative concentrations of compounds in a mixture. Most interestingly, our computational analysis shows that antagonistic response at the receptor level, i.e., where a receptor is bound by a ligand but there is no downstream reporter activity - also known to play a role in biological olfactory systems - proves to be necessary for accurate decomposition of chemical mixtures.


• 10:10 - Saul Kato (Columbia University)
  → A Response Dynamics of C. Elegans Chemosensory Neurons
  Sensory responses have been qualitatively described in the nematode C. elegans, but it has been an open question as to how amenable the graded response dynamics of C. elegans neurons are to quantitative analysis. Here we show that the worm's sensory neurons are capable of faithfully transducing complex temporal patterns of chemical stimuli with extremely low trial-to-trial and animal-to-animal variability, and possess sophisticated functional abilities such as dynamic range control and multiplexing of sensory information on multiple timescales. We then show that response properties of these neurons can be well captured by simple mathematical models, that when coupled with genetic dissection, can provide insight into the source of functional capabilities, including ubiquitous biophysical mechanisms such as G protein-receptor interactions.


• 10:20 - Coffee Break


• 10:45 - Vision Keynote - Eero Simoncelli (NYU)
  Visual transformations and the loss of peripheral information


• 11:30 - Adema Ribic (Yale University)
  → A SynCAM 1 Contributes to Synapse Organization and Function in the Retina
  Visual cortex is the main site for processing of visual information. However, perception and processing of visual stimuli begins in the retina. Function of both (as well as the rest of the nervous system) largely depends on precise synapse formation and remodeling. Recent research has pointed to select adhesion molecules as mediators of these processes. One of these is synaptic cell adhesion molecule 1 (SynCAM 1), involved in hippocampal excitatory synapse formation and maintenance both in vitro and in vivo (Robbins et al., 2010). SynCAM 1 is also expressed in the retina in a developmentally regulated manner, with expression low in early stages of mouse postnatal development and increasing during and after eye opening (P14). SynCAM 1 is particularly enriched in the photoreceptor neurons and their terminals in the outer plexiform layer (OPL), where it mainly associates with rod photoreceptors. Although gross retinal structure is normal in SynCAM 1 knockout (KO) mice, fine structure of the OPL is impaired. Electron microscopy highlighted subtle defects in the structure of ribbon synapses formed between rod photoreceptors, their target bipolar cells and feedback-providing horizontal cells. Functionally, SynCAM 1 KO mice displayed increased photoreceptor hyperpolarization and delayed response of rod bipolar cells to light stimuli, as assessed with electroretinogram (ERG) recordings. Our study for the first time implicates SynCAM 1 in proper structural development of rod ribbon synapses and in the functioning of mouse rod visual pathway.


• 11:40 - Joriene de Nooij (Columbia University)
  → A Function for Whirlin in Proprioceptor Mechanotransduction
  Muscle length and tension are critical to inform the CNS of the position of body and limbs in space, a necessity for coordinated motor behavior. This proprioceptive sense is mediated by specialized mechanoreceptors that are located within the muscle: the muscle spindle (responsive to changes in muscle stretch) and Golgi tendon organ (responsive to changes in muscle tension). Anatomical and physiological analysis provided many insights into the properties of the proprioceptive muscle afferents that innervate these mechanoreceptors but the molecular mechanism that underlies their sensory transduction process remains largely unknown. We have found that the PDZ-scaffold protein Whirlin is expressed in proprioceptive neurons in dorsal root ganglia and localizes to their peripheral sensory endings. Whirlin previously has been implicated in hair cell and photoreceptor sensory transduction, raising the possibility that Whirlin also functions in the proprioceptive mechanotransduction process. Using an in vitro muscle/nerve preparation, we find that the activation of spindle afferents by mechanical stretch is diminished in whirlin (wi) mutant mice when compared to their wild type (wt) litter mates. Application of exogenous glutamate restores afferent excitability in response to stretch, however spike frequency in wi mutants never reaches the frequency observed in wt mice. In addition, amiloride (an inhibitor of DEG/ENaC channels) and PCCG-13 (blocking atypical metabotropic glutamate receptors) reduce stretch-evoked activity in wt animals, but have comparatively less effect on the stretch-evoked spike frequency in wi mutants. Together, these observations indicate that essential components of proprioceptor mechanotransduction machinery are present but operate inefficiently in the absence of Whirlin function. Whirlin may serve to recruit and/or ensure the proper localization of transduction molecules in proprioceptive sensory terminals.


• 11:50 - Lunch


• 12:50 - Taste Keynote - M. Hakan Ozdener (Monell Center, Temple University)
  → Human taste cell culture: To study taste


• 1:35 - Ewan St. John Smith (NYU)
  → The Molecular Basis of Acid Insensitivity in the African Naked Mole-Rat
  The African naked mole-rat (NMR, Heterocephalus glaber) lives in subterranean colonies of <300 animals and is thus exposed to a low-O2, high-CO2 environment. Evolutionary adaptation to hypoxic/hypercapnic conditions has resulted in NMR brains that are hypoxia resistant, a lack of CO2 avoidance, no nocifensive avoidance behavior to extreme tissue acidosis and acid-insensitive cutaneous sensory fibers (C-fiber nociceptors). Acid evokes nocifensive behavior by depolarizing C-fiber nociceptors and evoking action potentials (APs). Acid insensitivity could not be explained by a lack of acid-gated depolarizing currents as essentially identical currents were measured in cell bodies of nociceptors isolated from dorsal root ganglia (DRG) of acid sensitive mice and NMRs. In neither mouse nor NMR DRG neurons did the magnitude of acid-gated inward currents correlate with AP generation. AP initiation following depolarization is mediated by voltage-gated sodium channels (NaV) that are blocked by acid. Voltage-gated inward currents in mouse DRG neurons were inhibited by acid (IC50 = pH6) and pH6 inhibited Gmax by 45%. In NMR DRG neurons voltage-gated inward currents were significantly more sensitive to acid: pH6 caused 63% inhibition (p<0.01). Mechanically-evoked APs in NMR C-fibers were also significantly more inhibited by acid compared to mouse (p<0.05). The NaV isoform NaV1.7 primarily determines AP threshold in nociceptors. We identified two amino-acid variations in NMR NaV1.7 not found in other mammals, in a region known to govern NaV acid sensitivity. We introduced the NMR sequences into human NaV1.7 and found that pH6 inhibited WT hNaV1.7 by 57%, but mut hNaV1.7 was significantly more acid-sensitive, Gmax being inhibited by 71% (p<0.001). Greater inhibition of NaV1.7 explains the lack of acid-induced nocifensive behavior in NMRs: acid induces inward currents, but increased NaV1.7 inhibition prevents AP generation. Increased acid-sensitivity of NMR NaV1.7 may have evolved to negate the noxious effects of living in high levels of CO2.


• 1:45 - Michael Nitabach (Yale University)
  → Expression Cloning of a High-Affinity TRPA1 Antagonist using a Recombinant Membrane-tethered Spider Toxin Library
  We screened a novel recombinant library of GPI membrane-tethered inhibitory cystine knot spider toxins against TRPA1, a stimulus-activated nociceptor ion channel, and thereby identified 35 amino acid protoxin-I (ProTx-I) from the venom of the Peruvian green-velvet tarantula as a high-affinity TRPA1 antagonist. ProTx-I was previously identified as a voltage-gated ion channel blocker, and we determined that ProTx-I blocks human Nav1.2 and Drosophila para voltage-gated sodium channel, but not Kir4.1 inward-rectifier K+ channel. Saturating concentration of chemically synthesized in vitro folded soluble ProTx-I almost completely suppresses TRPA1 current induced by mustard oil (MO) ligand, with half-maximal inhibitory concentration (IC50) 389 ű 77 nM. Consistent with previous evidence that ProTx-I binds to the S1-S4 voltage-sensing domain of voltage-gated channels, we found that prolonged membrane depolarization results in time-dependent unbinding of ProTx-I from para voltage-gated Na+ channel. In order to identify the surfaces for ProTx-I by which it binds to voltage-gated and TRPA1 channels, we generated alanine mutants of ProTx-I and tested them for activity against Nav1.2 and TRPA1. This alanine-scanning mutagenesis revealed a surface comprising ten amino acid side chains for binding to Nav1.2, including both hydrophobic and charged residues, and a surface of five side chains for binding to TRPA1, including only hydrophobic residues. Three side chains are shared between the Nav1.2 and TRPA1 binding surfaces. These studies suggest that ProTx-I inhibits TRPA1 activity by binding to its S1-S4 gating domain, and thus ProTx-I will be useful for mechanistic studies of 3 TRPA1 gating and as a lead compound for optimization and therapeutic validation.


• 1:55 - Coffee Break


• 2:20 - Hearing Keynote - A.J. Hudspeth (Rockefeller University)
  → Making an Effort to Listen: Mechanical Amplification by Myosin Molecules and Ion Channels in Hair Cells of the Inner Ear


• 3:05 - Jeremie Barral (NYU)
  → Phantom Tones and Suppressive Masking by Active Nonlinear Oscillation of the Hair-Cell Bundle
  Processing of two-tone stimuli by the auditory system introduces prominent masking of one frequency component by the other as well as additional "phantom" tones that are absent in the sound input. These phenomena are thought to originate in sensory hair cells from the intrinsic nonlinearity associated with the direct gating of mechano-electrical transduction channels by mechanical force. However, linking the rich phenomenology of two-tone interferences in hearing with hair-cell biophysics remains a central question in auditory physiology. Here, we study the mechanical response to two-tone stimuli of single hair-cell bundles, under natural conditions for which the hair bundle oscillates spontaneously. We report two-tone suppression and distortions that are shaped by active nonlinear amplification of periodic stimuli near the characteristic frequency of spontaneous oscillations. When both stimulus frequencies enter the bandwidth of the hair-bundle amplifier, two-tone interferences display level functions that are characteristic both of human psychoacoustics and of in vivo mechanical measurements in the mammalian and nonmammalian auditory organs. Our work distinguishes the nonlinearity that emerges from the active process that drives the hair bundle into spontaneous oscillations from the passive nonlinear compliance associated with channel gating. Numerical simulations based on a generic description of an active dynamical system poised near an oscillatory instability - a Hopf bifurcation - accounts quantitatively for our experimental observations. In return, we conclude that the properties of two-tone interferences in hearing betray the workings of self-sustained "critical" oscillators that function as nonlinear amplifying elements in the inner ear.


• 3:15 - Touch Keynote - Ellen Lumpkin (Columbia University)
  → Mechanosensory Mechanisms in a Mammalian Touch Receptor


• 4:00 - End

Additional Information

• Event flyer can be downloaded here
• Directions to Davis Auditorium can be found here
• Contact: sense2synapse {at} gmail.com