Role of BMP Signaling for the Formation of Auditory Brainstem Nuclei and Large Auditory Relay Synapses

Abstract

Large excitatory synapses are found at specific points in the neuronal circuits of the auditory brainstem, to enable fast information transfer and the preservation of acoustic timing information. The extracellular cues and signaling mechanisms that lead to the development of these specialized synaptic connections, exemplified by the calyx of Held in the medial nucleus of the trapezoid body (MNTB), are still largely unknown. Here, we investigate the role of BMP signaling for the early development of the ventral cochlear nucleus (VCN) and MNTB, and for the initial formation of the calyx of Held synaptic connection. We used conditional alleles of two BMP type‐1 receptors in the background of a constitutive BMPR1b knock‐out (KO), or else a conditional allele of SMAD4. The conditional alleles were recombined by the Krox20Cre mouse line that is active around mid‐gestation in rhombomeres (r) 3 and 5 from which the VCN and MNTB are derived; alternatively, virus‐mediated Cre‐expression was performed early postnatally in the VCN. The data show that embryonic SMAD‐dependent BMP‐signaling in r3 and r5 contributes to the histogenesis of auditory brainstem nuclei. On the other hand, BMP‐receptor signaling early postnatally in presynaptic neurons of the calyx of Held projection is necessary for correct axon branch retraction, which suggests a cell‐autonomous role of presynaptic BMP‐receptors in synapse elimination at the developing calyx of Held. Thus, our work dissects developmentally early and late roles of BMP‐signaling for the formation of auditory brainstem nuclei, and the highly specialized synaptic connectivity in these structures.

Structure–function relation of the developing calyx of Held synapse in vivo

“…Even if we grant that these three cells demonstrate multi-calyceal competition, we identify, from our earlier (3/132 cells) and the current (0/32 cells) experiments, <10% of rat principal neurons with multi-calyceal innervation during development. These values are comparable to the 1% reported in a pre-hearing mouse study (Kronander et al 2019) and to our earlier study in P9 rats, in which 1 of 86 cells showed two large, perisomatic VGluT clusters (Rodríguez-Contreras et al 2006). Even though we cannot exclude having missed calyces due to low VGluT expression, we conclude that multi-calyceal competition is unlikely to significantly contribute to the development of the rat calyx of Held synapse.…”

“…These values are comparable to the 1% reported in a pre‐hearing mouse study (Kronander et al . 2019) and to our earlier study in P9 rats, in which 1 of 86 cells showed two large, perisomatic VGluT clusters (Rodríguez‐Contreras et al . 2006).…”

“…2013) in addition to a persistence of axonal branches with calyces and increased multi‐calyceal innervation (Kronander et al . 2019). Similarly, calyces fail to form when synaptic transmission is impaired by genetic deletion of dynamins (Fan et al .…”

“…In the fruit fly postsynaptic activity stimulates the release of bone morphogenetic protein (BMP) homologue, which promotes active zone development and NMJ growth (Berke et al 2013(Berke et al , 2019. Mice with conditionally deleted BMP receptor 1 and 2 show deficits in transmitter release and smaller calyces (Xiao et al 2013) in addition to a persistence of axonal branches with calyces and increased multi-calyceal innervation (Kronander et al 2019). Similarly, calyces fail to form when synaptic transmission is impaired by genetic deletion of dynamins (Fan et al 2016).…”

LTP of inhibition at PV interneuron output synapses requires developmental BMP signaling

“…In the mammalian CNS, BMP-receptors (BMPRs) and their ligands are expressed up to early adulthood [22][23][24] , suggesting that BMP signaling fulfills further roles in later brain development. Indeed, a role for BMP signaling in the elimination of excitatory synapses 25,26 and in the development of mono-innervation at a large excitatory connection in the auditory brainstem, has been reported 27,28 . Earlier genetic studies showed that BMP signaling drives the growth of motor nerve terminals at the Drosophila neuromuscular junction [29][30][31] .…”