Activity-dependent thalamocortical axon branching

Beyond the cytoskeleton: The emerging role of organelles and membrane remodeling in the regulation of axon collateral branches

The generation of axon collateral branches is a fundamental aspect of the development of the nervous system and the response of axons to injury. Although much has been discovered about the signaling pathways and cytoskeletal dynamics underlying branching, additional aspects of the cell biology of axon branching have received less attention. This review summarizes recent advances in our understanding of key factors involved in axon branching. This article focuses on how cytoskeletal mechanisms, intracellular organelles, such as mitochondria and the endoplasmic reticulum, and membrane remodeling (exocytosis and endocytosis) contribute to branch initiation and formation. Together this growing literature provides valuable insight as well as a platform for continued investigation into how multiple aspects of axonal cell biology are spatially and temporally orchestrated to give rise to axon branches. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1293-1307, 2016.

Laminar and columnar development of barrel cortex relies on thalamocortical neurotransmission

A dynamic interplay between intrinsic regional molecular cues and extrinsic factors from the thalamus shape multiple features of early cortical development. It remains uncertain and controversial, however, whether the initial formation of cortical columns depends on neuronal activity, and there is little evidence that cortical lamination or neuronal differentiation is influenced by extrinsic activity. We examined the role of thalamic-derived factors in cortical development by selectively eliminating glutamatergic synaptic transmission from thalamocortical neurons in mice and found that eliminating thalamocortical neurotransmission prevented the formation of "barrel" columns in somatosensory cortex. Interestingly, based on cytoarchitectonic criteria and genetic markers, blocking thalamocortical neurotransmission also perturbed the development of superficial cortical lamina and the morphologic development of neurons. These experiments demonstrate that barrels and aspects of the layer-dependent pattern of cortical cytoarchitecture, gene expression, and neuronal differentiation depend on thalamocortical neurotransmission, extending the apparent influence of extrinsic, presumably activity-dependent factors, on cortical development.

Molecular control of axon branching

Axon branching is a complex morphological process, the regulation of which we are just beginning to understand. Many factors known to be important for axon growth and guidance have emerged as key regulators of axon branching. The extrinsic factors implicated in axon branching include traditional axon guidance cues such as the slits, semaphorins, and ephrins; neurotrophins such as BDNF; the secreted glycoprotein Wnt; the extracellular matrix protein anosmin-1; and certain transmembrane cell adhesion molecules--as well as sensory experience and neuronal activity. Although less is known about the intracellular control of axon branching, in recent years significant advances have been made in this area. Kinases and their regulators, Rho GTPases and their regulators, transcription factors, ubiquitin ligases, and several microtubule and actin-binding proteins are now implicated in the control of axon branching. It is likely that many more branching regulators remain to be discovered, as do the links between extrinsic cues and intracellular signaling proteins in the control of axon branching.

Many paths to synaptic specificity

The most impressive structural feature of the nervous system is the specificity of its synaptic connections. Even after axons have navigated long distances to reach target areas, they must still choose appropriate synaptic partners from the many potential partners within easy reach. In many cases, axons also select a particular domain of the postsynaptic cell on which to form a synapse. Thus, synapse formation is selective at both cellular and subcellular levels. Unsurprisingly, the nervous system uses multiple mechanisms to ensure proper connectivity; these include complementary labels, coordinated growth of synaptic partners, sorting of afferents, prohibition or elimination of inappropriate synapses, respecification of targets, and use of short-range guidance mechanisms or intermediate targets. Specification of any circuit is likely to involve integration of multiple mechanisms. Recent studies of vertebrate and invertebrate systems have led to the identification of molecules that mediate a few of these interactions.