XI European Meeting on Glial Cell Function in Health and Disease

Overview Session Overview Sessionprint print  

July 3, 2013 - Room 1 3:15pm - 5:15pm
Symposium 2: Axoglial Interactions in the Assembly and Stability of Axonal Domains Essential for Rapid Nerve Impulse Conduction

Peter Brophy (Edinburgh)


Axonal domains in myelinated nerves: assembly and function

*Peter Brophy 1
1 University of Edinburgh, Centre for Neuroregeneration, Edinburgh, United Kingdom
Abstract text :

Myelination of axons by oligodendrocytes in the CNS and Schwann cells in the PNS is a key event that stimulates the clustering of voltage gated sodium channels at nodes of Ranvier. The concentration of these sodium channels with associated proteins at nodes promotes rapid propagation of the action potential. A second key axonal domain is the axon initial segment, which is a proximal region of the axon close to the neuronal cell body. Because the axon initial segment is also rich in voltage-gated sodium channels it has been assumed that it initiates the action potential. Hence these two domains are essential for the propagation of rapid nerve impulse transmission in vertebrate nerves.Over the last 5-7 years it has become clear that these two domains have a similar protein composition, not least in the presence of voltage-gated sodium channels. Hence, there has been great interest in how these two domains achieve the clustering and concentration of these key channels required to generate and propagate the action potential and the role of glia in determining these processes.


Functional organization of the axon initial segment

*Bénédicte Dargent 1
1 Aix Marseille University - CNRS, UMR 7286, Faculté de Médecine-Nord, Marseille, France
Abstract text :

The axonal initial segment (AIS) is a unique sub-domain that plays a central role in the physiology of the neuron, as it orchestrates both electrogenesis and the maintenance of neuronal polarity. Recent findings highlighted that the AIS is capable of homeostatic plasticity through an activity –dependent change in either its location or morphology. The molecular organization of the AIS involves structural components like ankyrin G and beta-IV spectrin, which anchor ion channels responsible for the generation of the action potential. In line with current reports, we gathered new evidence of a complex organization of ion channels at the AIS in separate sub-domains. Our recent findings showed that complex protein-protein interaction between AIS components is regulated by distinct kinases. We showed that protein kinase CK2 and cyclin-dependant kinases, regulate the AIS trafficking and assembly of sodium (Nav1) and potassium (Kv1) channel complexes, respectively. The functional organization of the AIS also requires intracellular connections between ankyrin G and the microtubules. Indeed, we discovered that the microtubule plus-end-binding (EB) proteins EB1 and EB3 participate in AIS maintenance through direct interaction with ankyrin G. In addition, AIS disassembly leads to cell- wide up-regulation of EB3 and EB1 comets. Thus, EB3 and EB1 coordinate a molecular and functional interplay between ankyrin G and the AIS microtubules. To date, little information is available on the cellular and molecular mechanisms accounting for the precise cellular location of the AIS along the proximal axon. The molecular machinery that regulates AIS distal position has not been addressed yet.  We recently obtained convergent findings uncovering a novel player of the AIS molecular complex that shapes the cellular morphology of the AIS. These novel findings will provide a novel conceptual framework for the functional organization of the AIS. Altogether, our findings open new paths for a better understanding of the molecular and cellular mechanisms accounting for the homeostatic plasticity of the AIS.


Proteolytic processing of gliomedin regulates sodium channel clustering at the developing nodes of Ranvier

Yael Eshed-Eisenbach 1 , *Elior Peles 1
1 Weizmann Institute of Science, , Rehovot, Israel
Abstract text :

During the development of myelinated peripheral nerves, the Schwann cell-derived protein gliomedin initiates the clustering of sodium channels at the edges of each growing myelin segment by binding to the axonal cell adhesion molecule neurofascin 186.  The extracellular domain of gliomedin contains several distinct domains, including a coiled-coil sequence that mediates its multimerization, two collagen repeats that facilitate the formation of triple helix conformation, and mediate binding of gliomedin to heparan sulfate proteoglycans (HSPGs), and a C-terminal olfactomedin domain that mediates binding of gliomedin to NF186 and NrCAM. The clustering activity of gliomedin is tightly regulated by two distinct and functionally opposing proteolytic enzymes: Furin, a proprotein convertase that act at the trans Golgi network or at the cell surface, and is required for the activation of several membrane-associated collagens, and BMP-1/tolloid-like (TLD) metalloproteinase that separates between the olfactomedin domain and the N-terminal coiled-coil and collagen domains of gliomedin. While the clustering of Na channels by gliomedin depends on the shedding of the latter from the surface of Schwann cells by a furin protease, its activity is negatively regulated by BMP1/TLD, which is enriched at the paranodes that flank the forming nodes. Cleavage by BMP1/TLD restricts the activity of gliomedin to the nodal area and prevents the formation of ectopic clusters along axons that are devoid of myelin segments.  We propose that proteolytic processing of gliomedin facilitates, yet limits, the clustering of Na channels to specific sites along the axon.



‘Ankyrin’ the paranode.

Kae-jiun Chang 1 , Daniel Zollinger 1 , Keiichiro Susuki 1 , Tammy Ho 1 , Edward C. Cooper 1 , Peter J. Mohler 2 , Vann Bennett 3 , *Matthew N.  Rasband 1
1 Baylor College of Medicine , , Houston, TX, United States
2 Ohio State University, , Columbus, OH, United States
3 Duke University, , Durham, NC, United States
Abstract text :

In myelinated axons action potentials are regenerated at nodes of Ranvier. Paranodal junctions flank nodes and consist of axonal and glial cell adhesion molecules (CAMs). Paranodal junctions function as diffusion barriers. Mice lacking these cell adhesion molecules have disrupted paranodal junctions, reduced myelinated nerve conduction velocity, tremors and ataxia. In addition to the CAMs, several cytoskeletal scaffolds have also been identified at paranodes: 4.1B, αII/βII spectrins and ankyrinB (AnkB).  However, the functions of these proteins are unknown.  Here, we sought to determine the function of paranodal AnkB. In contrast to previous reports, we found paranodal AnkB is a glial paranodal protein in Schwann cells. Conditional knockouts of AnkB using CNP-Cre in myelinating glia revealed that paranodal AnkB is not essential for the integrity of paranodal junctions or their diffusion barrier function. In addition to AnkB, we also found the nodal cytoskeletal scaffold AnkG highly enriched at paranodes, but only in the CNS. This suggests a conservative scenario where Schwann cells in the PNS accumulate AnkB at paranodes, whereas oligodendrocytes in the CNS have AnkG at paranodes. Conditional knockouts of AnkG using CNP-Cre mice confirmed its presence in oligodendrocytes. Loss of AnkG significantly impaired paranode formation during early development.  Interestingly, AnkB accumulated at some paranodes in the absence of AnkG. Double conditional knockouts of AnkG and AnkB using CNP-Cre mice showed more severe defects in paranodal junctions and significant reductions in conduction velocity. Our results show that AnkB and AnkG are located at paranodes in myelinating glia and that paranodal ankyrins play a role in efficient formation of CNS paranodal junctions.