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

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 Krox20(Cre) 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.

BMP signaling specifies the development of a large and fast CNS synapse

Enida Gjoni, Graham Knott, Elin Kristina Kronander, Ralf Schneggenburger, Le Xiao

excitatory synapses with multiple active zones ensure reliable and fast information transfer at specific points in neuronal circuits. However, the mechanisms that determine synapse size in CNS circuits are largely unknown. Here we use the calyx of Held synapse, a major relay in the auditory system, to identify and study signaling pathways that specify large nerve terminal size and fast synaptic transmission. Using genome-wide screening, we identified bone morphogenetic proteins (BMPs) as candidate signaling molecules in the area of calyx synapses. Conditional deletion of BMP receptors in the auditory system of mice led to aberrations of synapse morphology and function specifically at the calyx of Held, with impaired nerve terminal growth, loss of monoinnervation and less mature transmitter release properties. Thus, BMP signaling specifies large and fast-transmitting synapses in the auditory system in a process that shares homologies with, but also extends beyond, retrograde BMP signaling at Drosophila neuromuscular synapses.

Nature Publishing Group2013

Genetic dissection of the role of BMP signaling for the development of the auditory brainstem nuclei and the large calyx of Held synapse

Elin Kristina Kronander

Synapses are key elements for communication within the complex network of neurons that make up our brain. Interestingly, synaptic connections can differ greatly in form and function, depending on the specific circuit into which they are embedded. In this PhD thesis, I use a large excitatory synaptic connection in the auditory brainstem, the calyx of Held, as a model to study the molecular mechanisms of synapse development. We are especially interested in the role of bone morphogenetic protein (BMP) signaling in this process. BMPs are extracellular morphogens with signaling roles early in the development of multicellular animals, and in brain development. In the first part of this PhD thesis, I have established an organotypic culture model of the auditory brainstem, in which large calyx-like synapses form in vitro. I could demonstrate the usefulness of this culture system for studying molecular mechanisms of nerve terminal growth, by showing that the BMP-receptor inhibitor, LDN-193189, leads to a reduction in the size of nerve terminals forming in vitro. In the second part, I have used mouse genetics, especially floxed alleles of BMP-receptors and the SMAD4 protein, coupled with morphological analysis and electrophysiological measurements, to investigate the role of BMP signaling in the development of large calyx of Held nerve terminals. Genetic inactivation of BMP type I receptors early-on (~ embryonic day 9 (E9) led to a significantly smaller ventral cochlear nucleus (VCN) and medial nucleus of the trapezoid body (MNTB), indicating that BMP signaling is involved in the histogenesis of these auditory brainstem nuclei. These deficits were in part caused by the intracellular SMAD - signaling pathway, as revealed by conditional inactivation of the SMAD4 gene. To circumvent morphological changes observed upon the early inactivation of BMP receptors, I investigated the onset of Cre expression in a Parvalbumin (PV) Cre mouse line where recombination happens in presynaptic VCN neurons at ~ E15, and in postsynaptic MNTB neurons at postnatal day (P) 6. Conditional inactivation of BMP-receptors using the PVCre mouse line did not lead to morphological alterations. However, in these conditional KO mice, the functional development of the calyx of Held synapse was not significantly altered when measured at P6 (only the presynaptic receptors should be removed), nor at P14 or at P21, at which both pre- and postsynaptic receptors should be removed. Thus, presynaptic BMP-receptor activation alone seems unlikely to be necessary for calyx of Held growth. Also, ongoing BMP-signaling in both pre- and postsynaptic compartments seems dispensable for later maturation processes and the maintenance of the calyx of Held synapse. Finally, I removed BMP receptors specifically in the presynaptic neuron pool via virus-mediated expression of Cre-recombinase in newborn mice. I then observed subtle changes in the branching pattern of presynaptic axons and a slight increase in double-innervations. Taken together, this study shows that BMP signaling plays a role in the histogenesis of auditory brainstem structures and partially in the growth of calyx of Held synapses and elimination of competing synaptic connections. This work also offers a deeper understanding into the use of mouse genetic tools for studying the development of specific synaptic connections and establishes a novel culture model for studying such connections in vitro.

EPFL2016

Structure and dynamics of the neocortical microcircuit connectivity

Jean-Vincent Le Bé

The neocortex is the most computationally advanced portion of the brain. It is currently assumed to be composed of a large number of "cortical columns" – intricate arrangements of cortical neurons approximately 300-500 µm in diameter and 2-5 mm in height in humans – that might serve as the elementary computational unit of the neocortex. Understanding the computation performed by this microcircuit is one of the keys to our comprehension of the brain. The so-called cortical column is not a static entity, however, and it evolves throughout a lifetime and continually adapts to the information from its cortical environment. Despite the differences between cortical columns across the cortex, a number of common features have been identified: a laminar structure, the dynamics of connections between identified neurons or the mechanisms for these connections to be modified (that give its specificity to each microcircuit). This thesis presents the description of the differential connectivity and synaptic dynamics across cell populations and the long term neuronal rewiring in a particular neuronal population within the cortical column. Somatic whole cell recordings have been performed to probe the connectivity, synaptic dynamics and plasticity of the connections in the rat neocortex. Two populations of layer V pyramidal neurons were studied in particular: cortico-callosally projecting pyramidal cells (CCPs) and thick tufted pyramidal cells (TTCs). The first major results from this work revealed the degree of connectivity and the linear dynamics of the CCPs population when compared to the TTCs. CCPs have nearly 4 times fewer interconnections and subsequent post-synaptic potentials were less decreasing in amplitude along a pre-synaptic series of action potentials. Long term configuration of TTC networks was explored. These experiments show for the first time the emergence of new functional synaptic connections between TTCs within hours. Activation of the slice by glutamate greatly increases their rate of emergence and this work demonstrated that metabotropic glutamate receptor 5 (mGluR5) activation and action potential firing are required for new connections to be formed in this experimental protocol. Newly formed connections respond in a more linear fashion and have weaker post-synaptic influence than already existing connections. Pre-existing connections are also modified after stimulation, requiring mGluR5, action potentials as well as α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptors activation. The activation of group III metabotropic glutamate receptors (mGluRs) however results in a decrease in the strength of connections. Finally, the influence of inhibitory interneurons on the activity and connectivity between TTCs was also investigated. The results of this study show that firing of inhibitory interneurons can be triggered by the input of only one pyramidal cell. They further show that stimulation of a single TTC can result in an hyperpolarization of the post-synaptic TTC mediated by an interneurone. When the pre-synaptic neuron is also directly connected to the post-synaptic neuron with an excitatory synapse, the indirect inhibitory connection serves to curtail the excitatory response. This work has provided a new insight into the dynamic nature of the cortical microcircuitry, showing that it evolves rapidly and can adapt, reconfigure and rewire itself in remarkably short time-spans. It also describes the variety of dynamics exhibited by the different types of pyramidal cells, due to either the projecting site specificity or to the action of an intermediate interneuron.