"In vivo application of upconverting force sensors to elucidate neuromuscular pump action in C. elegans"

Who: PhD. Student Alice Lay, Stanford University, Department of materials science and engineering

Place: Donostia International Physics Center

Date: Wednesday, 22 November 2017, 12:00

The feeding behavior of C. elegans is a strong indicator of health; changes indicate environmental toxins, scarce or abundant food resources, aging, and neurodegenerative disease. In particular, the pharyngeal pump action is a rhythmic contraction and relaxation of muscles that allows the worm to pull in and concentrate bacteria, crush and chew them in the grinder, and then pass them through the intestinal tract [1]. Because the pump action is regulated by motor neurons (MC, M3, M4) [2-5], it serves as a model system for neuromuscular pumps like the heart. Here, we investigate the magnitude of forces exerted by muscles in the pharynx, combining extracellular electrophysiological recordings, or electrophargyngograms (EPGs), with optical force measurements from upconverting nanoparticles (UCNPs).  Sub-25 nm Mn2+-doped NaYF4:Er,Yb UCNPs provide a photostable and consistent color response to stress [6]. The nano- to micro-Newton sensitivity of these nanoparticles relies on the energetic coupling between the crystal field sensitive d-metal and upconverting lanthanides, which under stress, yields a positive or negative change in the red to green Er3+ emission ratio for cubic- and hexagonal-phase NaYF4, respectively. Further, we investigate new geometries (e.g. core-shell) for more efficient and force-sensitive nanoparticles. 

We demonstrate the first in vivo capabilities of these nanosensors to image and quantify forces exerted along the pharynx. First, we incubate the worms with water-soluble UCNPs (5 mg/mL) overnight for feeding, which yields no significant chronic cytotoxicity effects on their fertility. Then, we load the worms in a microfluidic device and collect upconversion spectra at key anatomical features.  Based on ratiometric differences in emission peaks, we find that forces exerted in the grinder (~10 uN) are nearly an order of magnitude higher than those exerted at the pharyngeal-intestinal valve (~1 uN). Furthermore, we compare these optical force measurements to muscle contraction and relaxation events, characterized by voltage spikes in the EPGs. We determine pump action parameters (e.g., duration, frequency, amplitude) and muscular forces in wild-type and neurotransmitter-treated (5 mM serotonin) worms. From these results, we work towards mapping neuromuscular pump dynamics and providing the first in-vivo determination of the forces required for healthy function in C. elegans.

[1] Fang-Yen, C., L. Avery, and S. Aravinthan. PNAS (2009)

[2] Raizen, D.M. and L. Avery. Neuron (1994)

[3] Niacaris, T. and L. Avery. Journal of Experimental Biology (2003)

[4] Trojanowski, N.F., D.M. Raizen, and C. Fang-Yen. Scientific reports (2016)

[5] Lee, K.S. et al. Nature Communications (2017)

[6] Lay, A. et al. Nano Letters (2017)

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