Bhlhb5::flpo allele uncovers a requirement for Bhlhb5 for the development of the dorsal cochlear nucleus

Abstract

Auditory information is initially processed in the cochlear nuclei before being relayed to the brain. The cochlear nuclei are subdivided into dorsal, anterior ventral, and posterior ventral domains, each containing several subtypes of neurons that are thought to play discrete roles in the processing of sound. However, the ontogeny of these neurons is poorly understood, and this gap in knowledge hampers efforts to understand the basic neural circuitry of this nucleus. Here, we reveal that Bhlhb5 is expressed in both excitatory (unipolar brush cells) and inhibitory neurons (cartwheel cells) of the DCN during development. To gain genetic access to Bhlhb5-expressing neurons in the DCN, we generated a Bhlhb5::flpo knockin allele. Using an intersectional genetic strategy, we labeled cartwheel cells, thereby providing proof of concept that subpopulations of Bhlhb5-expressing neurons can be genetically targeted. Moreover, fate-mapping experiments using this allele revealed that Bhlhb5 is required for the proper development of the DCN, since mice lacking Bhlhb5 showed a dramatically diminished number of neurons, including unipolar brush and cartwheel cells. Intriguingly, the Bhlhb5::flpo allele also genetically labels numerous other regions of the nervous system that process sensory input, including the dorsal horn, the retina, and the nucleus of the lateral olfactory tract, hinting at a more general role for Bhlhb5 in the development of neurons that mediate sensory integration.

Genes Involved in the Development and Physiology of Both the Peripheral and Central Auditory Systems

“…Spatiotemporal fate mapping for Neurog1-expressing cells with Neurog1-Cre BAC (bacterial artificial chromosome) transgenic mice has shown Neurog1 to be expressed in the CN, LL, and IC, and throughout the cortex and most of the thalamus (Kim et al 2011), but the mode of action of neurog1 in these brainstem auditory nuclei has yet to be investigated. Finally, the gene encoding the proneural bHLH transcription factor bhlhb5 is expressed in SGNs from E10.5 (Brunelli et al 2003), in the dorsal CN from E12 (Cai et al 2016), and in several primary sensory cortices, including the AC, soon after birth (Joshi et al 2008). Bhlhb5 knockout mice have very small numbers of dorsal CN neurons, such as cartwheel cells and unipolar brush cells (Cai et al 2016).…”

“…Finally, the gene encoding the proneural bHLH transcription factor bhlhb5 is expressed in SGNs from E10.5 (Brunelli et al 2003), in the dorsal CN from E12 (Cai et al 2016), and in several primary sensory cortices, including the AC, soon after birth (Joshi et al 2008). Bhlhb5 knockout mice have very small numbers of dorsal CN neurons, such as cartwheel cells and unipolar brush cells (Cai et al 2016). However, the functional roles of Bhlhb5 in the SGNs and the AC remain to be determined.…”

Gene, cell, and organ multiplication drives inner ear evolution

“…Chief among those is Ptf1a, a bHLH transcription factor that is essential for cerebellar (Pascual et al, 2007) (Millen et al, 2014) and dorsal cochlear nucleus development (Fujiyama et al, 2009) but other bHLH genes play a role as well (Cai et al, 2016). In addition, to be relevant for the formation of nuclei or brain areas, Ptf1a also plays an essential role in cell fate determination within a given nucleus defining excitatory and inhibitory neurons (Iskusnykh et al, 2016) Beyond formation of entire nuclei, several genes are now emerging that interact to define different cell types in different brainstem nuclei important for their normal function (Nothwang, 2016).…”

A single reporter mouse line for Vika, Flp, Dre, and Cre-recombination

“…For the mouse, the Cre/loxP system is the most widely used SSR for advanced genome engineering 32 . But with increased sophistication of experimental designs, the Dre/rox system and enhanced versions of Flp recombinase are now also routinely used in mice, with a number of Dre and Flp-deleter strains 16 , 17 , 22 , 43 , 44 and tissue-specific Dre-drivers 10 , 26 , 45 as well as tissue-specific Flp-drivers 46 – 51 demonstrating the utility of these enzymes in vivo . As an example, combinatorial use of tissue specific Dre and Cre recombinases in mice revealed that liver vasculature arises from the endocardium 26 .…”

“…Indeed, a panel of different recombinase-reporter strains for Cre, Flp and Dre have been reported 22 , 37 , 38 , 40 , 62 . The number of Cre-driver mice is constantly increasing 5 , 6 and a number of tissue-specific Dre-drivers 10 , 45 as well as tissue-specific Flp-drivers 46 – 51 also already exist. The prediction is that more will be made and that Vika will likely follow this trend.…”