Rapid structural remodeling of peripheral taste fibers is independent of taste cell turnover

Description

Taste bud cells are constantly replaced in taste buds as old cells die and new cells migrate into the bud. The perception of taste relies on new taste bud cells integrating with existing neural circuitry, yet how these new cells connect with a taste ganglion neuron is unknown. Do taste ganglion neurons remodel to accommodate taste bud cell renewal? If so, how much of the structure of taste axons is fixed and how much remodels? Here we measured the motility and branching of individual taste arbors (the portion of the axon innervating taste buds) over time with two-photon in vivo microscopy. Terminal branches of taste arbors continuously and rapidly remodel within the taste bud. This remodeling is faster than predicted by taste bud cell renewal, with terminal branches added and lost concurrently. Surprisingly, blocking entry of new taste bud cells with chemotherapeutic agents revealed that remodeling of the terminal branches on taste arbors does not rely on the renewal of taste bud cells. Although terminal branch remodeling was fast and intrinsically controlled, no new arbors were added to taste buds, and few were lost over 100 days. Taste ganglion neurons maintain a stable number of arbors that are each capable of high-speed remodeling. We propose that terminal branch plasticity permits arbors to locate new taste bud cells, while stability of arbor number supports constancy in the degree of connectivity and function for each neuron over time.

Steps to reproduce

Mice: TrkBCreER mice (Ntrk2tm3.1(cre/ERT2)Ddg; ; ISMR catalog #JAX:027214, RRID:IMSR_JAX:027214) were crossed with Cre-dependent tdTomato mice (68); RRID: IMSR_JAX:007914) to obtain TrkBCreER:tdTomato mice in which tdTomato is expressed following TrkB-driven Cre-mediated gene recombination. These mice were crossed with a Sox2GFP line (Sox2tm2Hoch; ISMR catalog #JAX:017592, RRID:IMSR_JAX:017592). These mice were injected with 1.5mg of tamoxifen to label a subset of arbors. Two-Photon imaging: In vivo 2PLSM imaging was performed using a Movable Objective Microscope (Sutter Instruments, Novato, CA). A Fidelity-2 1070-nm laser (Coherent, Silicon Valley was used to visualize tdTomato, and a Chameleon tunable laser (Coherent) set to 920 nm was used to visualize GFP. The excitation wavelength at the sample ranged from 20 51 milliWatts measured at the exit of the microscope objective. A custom-built tongue holder, based on a published design, was used to stabilize the anterior tongue for imaging of the dorsal surface (70, 71). Mice were anesthetized using a 0.6% isoflurane (Henry Schien, Melville, NY) and O2 mixture. A temperature controller was used to monitor and maintain body temperature at 36°C. Ophthalmic lubricant ointment (Henry Schien) was applied to the eyes. Taste bud maps were acquired using a 10 × 0.3 numerical aperture water immersion objective lens (Carl Zeiss, USA), and images of arbors were acquired using a 40 × 1.0 numerical aperture water immersion lens (Zeiss). An early version of ScanImage (Matlab) was used to collect images (72). Arbors were typically imaged to 70 microns in depth below the surface of the tongue, and 80 × 80 µm optical sections (512 × 512 pixels) were collected at 0.5 or 1 µm increments.

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