Embryonic taste bud primordia are specified as taste placodes on the tongue surface and differentiate into the first taste receptor cells (TRCs) at birth. Throughout adult life, TRCs are continually regenerated from epithelial progenitors. Sonic hedgehog (SHH) signaling regulates TRC development and renewal, repressing taste fate embryonically, but promoting TRC differentiation in adults. Here, using mouse models, we show TRC renewal initiates at birth and coincides with onset of SHHs pro-taste function. Using transcriptional profiling to explore molecular regulators of renewal, we identified Foxa1 and Foxa2 as potential SHH target genes in lingual progenitors at birth, and show SHH overexpression in vivo alters FoxA1 and FoxA2 expression relevant to taste buds. We further bioinformatically identify genes relevant to cell adhesion and cell locomotion likely regulated by FOXA1;FOXA2, and show expression of these candidates is also altered by forced SHH expression. We present a new model where SHH promotes TRC differentiation by regulating changes in epithelial cell adhesion and migration.
A microfluidic
gradient chamber (MGC) and a homogeneous batch culturing
system were used to evaluate whether spatial concentration gradients
of the antibiotic ciprofloxacin allow development of greater antibiotic
resistance in Escherichia coli strain
307 (E. coli 307) compared to exclusively
temporal concentration gradients, as indicated in an earlier study.
A linear spatial gradient of ciprofloxacin and Luria–Bertani
broth (LB) medium was established and maintained by diffusion over
5 days across a well array in the MGC, with relative concentrations
along the gradient of 1.7–7.7× the original minimum inhibitory
concentration (MICoriginal). The E. coli biomass increased in wells with lower ciprofloxacin concentrations,
and only a low level of resistance to ciprofloxacin was detected in
the recovered cells (∼2× MICoriginal). Homogeneous
batch culture experiments were performed with the same temporal exposure
history to ciprofloxacin concentration, the same and higher initial
cell densities, and the same and higher nutrient (i.e., LB) concentrations
as in the MGC. In all batch experiments, E. coli 307 developed higher ciprofloxacin resistance after exposure, ranging
from 4 to 24× MICoriginal in all replicates. Hence,
these results suggest that the presence of spatial gradients appears
to reduce the driving force for E. coli 307 adaptation to ciprofloxacin, which suggests that results from
batch experiments may over predict the development of antibiotic resistance
in natural environments.
Head and neck cancer patients receiving conventional repeated, low dose radiotherapy (fractionated IR) suffer from taste dysfunction that can persist for months and often years after treatment. To understand the mechanisms underlying functional taste loss, we established a fractionated IR mouse model to characterize how taste buds are affected. Following fractionated IR, we found as in our previous study using single dose IR, taste progenitor proliferation was reduced and progenitor cell number declined, leading to interruption in the supply of new taste receptor cells to taste buds. However, in contrast to a single dose of IR, we did not encounter increased progenitor cell death in response to fractionated IR. Instead, fractionated IR induced death of cells within taste buds. Overall, taste buds were smaller and fewer following fractionated IR, and contained fewer differentiated cells. In response to fractionated IR, expression of Wnt pathway genes, Ctnnb1, Tcf7, Lef1 and Lgr5 were reduced concomitantly with reduced progenitor proliferation. However, recovery of Wnt signaling post-IR lagged behind proliferative recovery. Overall, our data suggest carefully timed, local activation of Wnt/β-catenin signaling may mitigate radiation injury and/or speed recovery of taste cell renewal following fractionated IR.
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