Christian Lohmann

Calcium signaling and the development of specific neuronal connections

During the development of the brain, synaptic connections between nerve cells are being established with remarkable specificity. This is achieved by a series of steps: first, axons grow to their terminal areas. Second, axons and dendrites contact each other and select among potential synaptic partners. Third, after synapses have become functional, the fine-tuning of synaptic connections optimizes emerging networks to perform their specific functions. Here, I summarize the evidence for a central role of intracellular calcium signaling in all three stages of the development of specific synaptic connections. In particular, calcium signaling has the capacity to integrate information from a wide array of extracellular factors that are known to regulate neuronal development, such as molecular cues or neuronal activity. Calcium signaling, in turn, directs structural as well as functional adaptations in individual neurons that underlie the establishment of synaptic specificity. Importantly, evidence is accumulating that errors in calcium-dependent network maturation are associated with neurodevelopmental disorders. Therefore, understanding the role of calcium in setting up brain networks may not only advance our insights into mechanisms of normal brain development, but also help identifying the causes of diseases such as autism or mental retardation.

Synaptic clustering during development and learning: the why, when, and how.

To contribute to a functional network a neuron must make specific connections and integrate the synaptic inputs that it receives in a meaningful way. Previous modeling and experimental studies have predicted that this specificity could entail a subcellular organization whereby synapses that carry similar information are clustered together on local stretches of dendrite. Recent imaging studies have now, for the first time, demonstrated synaptic clustering during development and learning in different neuronal circuits. Interestingly, this organization is dependent on synaptic activity and most likely involves local plasticity mechanisms. Here we discuss these new insights and give an overview of the candidate plasticity mechanisms that could be involved.

Gap Junctions in Developing Thalamic and Neocortical Neuronal Networks

The presence of direct, cytoplasmatic, communication between neurons in the brain of vertebrates has been demonstrated a long time ago. These gap junctions have been characterized in many brain areas in terms of subunit composition, biophysical properties, neuronal connectivity patterns, and developmental regulation. Although interesting findings emerged, showing that different subunits are specifically regulated during development, or that excitatory and inhibitory neuronal networks exhibit various electrical connectivity patterns, gap junctions did not receive much further interest. Originally, it was believed that gap junctions represent simple passageways for electrical and biochemical coordination early in development. Today, we know that gap junction connectivity is tightly regulated, following independent developmental patterns for excitatory and inhibitory networks. Electrical connections are important for many specific functions of neurons, and are, for example, required for the development of neuronal stimulus tuning in the visual system. Here, we integrate the available data on neuronal connectivity and gap junction properties, as well as the most recent findings concerning the functional implications of electrical connections in the developing thalamus and neocortex.

Calcium dynamics at developing synapses: mechanisms and functions.

During brain maturation, neurons form specific connections with each other to establish functional neuronal circuits. The processes underlying the development of connectivity, such as the selection of synaptic partners and the fine‐tuning of neuronal networks, act with single‐synapse precision. Calcium is an intracellular secondary messenger that operates with remarkable spatio‐temporal specificity and regulates functional and structural adaptations at the level of individual synapses. Although the structure, molecular composition and function of an emerging synapse changes dramatically during its development, the single‐synapse specificity of calcium signaling is maintained at every step of synapse formation: when the first contacts between axons and dendrites form, during the onset of synaptic function and later, when spine synapses emerge. Here, we describe the mechanisms that help developing neurons to confine calcium signaling to individual synapses, and discuss how these local calcium dynamics facilitate the development of accurate neuronal connections at each step of synapse maturation.

The Wiring of Developing Sensory Circuits-From Patterned Spontaneous Activity to Synaptic Plasticity Mechanisms

In order to accurately process incoming sensory stimuli, neurons must be organized into functional networks, with both genetic and environmental factors influencing the precise arrangement of connections between cells. Teasing apart the relative contributions of molecular guidance cues, spontaneous activity and visual experience during this maturation is on-going. During development of the sensory system, the first, rough organization of connections is created by molecular factors. These connections are then modulated by the intrinsically generated activity of neurons, even before the senses have become operational. Spontaneous waves of depolarizations sweep across the nervous system, placing them in a prime position to strengthen correct connections and weaken others, shaping synapses into a useful network. A large body of work now support the idea that, rather than being a mere side-effect of the system, spontaneous activity actually contains information which readies the nervous system so that, as soon as the senses become active, sensory information can be utilized by the animal. An example is the neonatal mouse. As soon as the eyelids first open, neurons in the cortex respond to visual information without the animal having previously encountered structured sensory input (Cang et al., 2005b; Rochefort et al., 2011; Zhang et al., 2012; Ko et al., 2013). In vivo imaging techniques have advanced considerably, allowing observation of the natural activity in the brain of living animals down to the level of the individual synapse. New (opto)genetic methods make it possible to subtly modulate the spatio-temporal properties of activity, aiding our understanding of how these characteristics relate to the function of spontaneous activity. Such experiments have had a huge impact on our knowledge by permitting direct testing of ideas about the plasticity mechanisms at play in the intact system, opening up a provocative range of fresh questions. Here, we intend to outline the most recent descriptions of spontaneous activity patterns in rodent developing sensory areas, as well as the inferences we can make about the information content of those activity patterns and ideas about the plasticity rules that allow this activity to shape the young brain.

Probing synaptic function in dendrites with calcium imaging

Calcium imaging has become a widely used technique to probe neuronal activity on the cellular and subcellular levels. In contrast to standard electrophysiological methods, calcium imaging resolves sub- and suprathreshold activation patterns in structures as small as fine dendritic branches and spines. This review highlights recent findings gained on the subcellular level using calcium imaging, with special emphasis on synaptic transmission and plasticity in individual spines. Since imaging allows monitoring activity across populations of synapses, it has recently been adopted to investigate how dendrites integrate information from many synapses. Future experiments, ideally carried out in vivo, will reveal how the dendritic tree integrates and computes afferent signals. For example, it is now possible to directly test the concept that dendritic inputs are clustered and that single dendrites or dendritic stretches act as independent computational units.

Mapping Synaptic Inputs of Developing Neurons Using Calcium Imaging

Studying changing synaptic activity patterns during development provides a wealth of information on how activity-dependent processes shape synaptic connectivity. In this chapter we introduce a method that combines whole-cell electrophysiology with calcium imaging to map functional synaptic sites on the dendritic tree and follow their activity over time. The key strength of this method lies in its ability to distinguish between synaptic and non-synaptic calcium signaling by their coincidence with synaptic currents measured at the soma. Next to the required materials and protocols that are necessary to perform these experiments, we thoroughly discuss how the acquired data can be analyzed. Since this method can be employed in many neuronal systems we believe that it can be a valuable tool to study developmental changes in synaptic connectivity.