The cellular code for mammalian thermosensation.

Mammalian somatosenory neurons respond to thermal stimuli and allow animals to reliably discriminate hot from cold and to select their preferred environments. Previously, we generated mice that are completely insensitive to temperatures from noxious cold to painful heat (-5 to 55°C) by ablating several different classes of nociceptor early in development. In the present study, we have adopted a selective ablation strategy in adult mice to study this phenotype and have demonstrated that separate populations of molecularly defined neurons respond to hot and cold. TRPV1-expressing neurons are responsible for all behavioral responses to temperatures between 40 and 50°C, whereas TRPM8 neurons are required for cold aversion. We also show that more extreme cold and heat activate additional populations of nociceptors, including cells expressing Mrgprd. Therefore, although eliminating Mrgprd neurons alone does not affect behavioral responses to temperature, when combined with ablation of TRPV1 or TRPM8 cells, it significantly decreases responses to extreme heat and cold, respectively. Ablation of TRPM8 neurons distorts responses to preferred temperatures, suggesting that the pleasant thermal sensation of warmth may in fact just reflect reduced aversive input from TRPM8 and TRPV1 neurons. As predicted by this hypothesis, mice lacking both classes of thermosensor exhibited neither aversive nor attractive responses to temperatures between 10 and 50°C. Our results provide a simple cellular basis for mammalian thermosensation whereby two molecularly defined classes of sensory neurons detect and encode both attractive and aversive cues.

Cell-Type-Specific Splicing of Piezo2 Regulates Mechanotransduction.

Piezo2 is a mechanically activated ion channel required for touch discrimination, vibration detection, and proprioception. Here, we discovered that Piezo2 is extensively spliced, producing different Piezo2 isoforms with distinct properties. Sensory neurons from both mice and humans express a large repertoire of Piezo2 variants, whereas non-neuronal tissues express predominantly a single isoform. Notably, even within sensory ganglia, we demonstrate the splicing of Piezo2 to be cell type specific. Biophysical characterization revealed substantial differences in ion permeability, sensitivity to calcium modulation, and inactivation kinetics among Piezo2 splice variants. Together, our results describe, at the molecular level, a potential mechanism by which transduction is tuned, permitting the detection of a variety of mechanosensory stimuli.

Coding and Plasticity in the Mammalian Thermosensory System.

Perception of the thermal environment begins with the activation of peripheral thermosensory neurons innervating the body surface. To understand how temperature is represented in vivo, we used genetically encoded calcium indicators to measure temperature-evoked responses in hundreds of neurons across the trigeminal ganglion. Our results show how warm, hot, and cold stimuli are represented by distinct population responses, uncover unique functional classes of thermosensory neurons mediating heat and cold sensing, and reveal the molecular logic for peripheral warmth sensing. Next, we examined how the peripheral somatosensory system is functionally reorganized to produce altered perception of the thermal environment after injury. We identify fundamental transformations in sensory coding, including the silencing and recruitment of large ensembles of neurons, providing a cellular basis for perceptual changes in temperature sensing, including heat hypersensitivity, persistence of heat perception, cold hyperalgesia, and cold analgesia.

Structural and functional divergence within the Dim1/KsgA family of rRNA methyltransferases.

The enzymes of the KsgA/Dim1 family are universally distributed throughout all phylogeny; however, structural and functional differences are known to exist. The well-characterized function of these enzymes is to dimethylate two adjacent adenosines of the small ribosomal subunit in the normal course of ribosome maturation, and the structures of KsgA from Escherichia coli and Dim1 from Homo sapiens and Plasmodium falciparum have been determined. To this point, no examples of archaeal structures have been reported. Here, we report the structure of Dim1 from the thermophilic archaeon Methanocaldococcus jannaschii. While it shares obvious similarities with the bacterial and eukaryotic orthologs, notable structural differences exist among the three members, particularly in the C-terminal domain. Previous work showed that eukaryotic and archaeal Dim1 were able to robustly complement for KsgA in E. coli. Here, we repeated similar experiments to test for complementarity of archaeal Dim1 and bacterial KsgA in Saccharomyces cerevisiae. However, neither the bacterial nor the archaeal ortholog could complement for the eukaryotic Dim1. This might be related to the secondary, non-methyltransferase function that Dim1 is known to play in eukaryotic ribosomal maturation. To further delineate regions of the eukaryotic Dim1 critical to its function, we created and tested KsgA/Dim1 chimeras. Of the chimeras, only one constructed with the N-terminal domain from eukaryotic Dim1 and the C-terminal domain from archaeal Dim1 was able to complement, suggesting that eukaryotic-specific Dim1 function resides in the N-terminal domain also, where few structural differences are observed between members of the KsgA/Dim1 family. Future work is required to identify those determinants directly responsible for Dim1 function in ribosome biogenesis. Finally, we have conclusively established that none of the methyl groups are critically important to growth in yeast under standard conditions at a variety of temperatures.