Faculty Bibliography

The polarized domains of myelinated axons are specifically organized to maximize the efficiency of saltatory conduction. The paranodal region is directly adjacent to the node of Ranvier and contains specialized septate-like junctions that provide adhesion between axons and glial cells and that constitute a lateral diffusion barrier for nodal components. To complement and extend earlier studies on the peripheral nervous system, electron tomography was used to image paranodal regions from the central nervous system (CNS). Our three-dimensional reconstructions revealed short filamentous linkers running directly from the septate-like junctions to neurofilaments, microfilaments, and organelles within the axon. The intercellular spacing between axons and glia was measured to be 7.4 +/- 0.6 nm, over twice the value previously reported in the literature (2.5-3.0 nm). Averaging of individual junctions revealed a bifurcated structure in the intercellular space that is consistent with a dimeric complex of cell adhesion molecules composing the septate-like junction. Taken together, these findings provide new insight into the structural organization of CNS paranodes and suggest that, in addition to providing axo-glial adhesion, cytoskeletal linkage to the septate-like junctions may be required to maintain axonal domains and to regulate organelle transport in myelinated axons. (c) 2010 Wiley-Liss, Inc

Members of the neuregulin-1 (Nrg1) growth factor family play important roles during Schwann cell development. Recently, it has been shown that the membrane-bound type III isoform is required for Schwann cell myelination. Interestingly, however, Nrg1 type II, a soluble isoform, inhibits the process. The mechanisms underlying these isoform-specific effects are unknown. It is possible that myelination requires juxtacrine Nrg1 signaling provided by the membrane-bound isoform, whereas paracrine stimulation by soluble Nrg1 inhibits the process. To investigate this, we asked whether Nrg1 type III provided in a paracrine manner would promote or inhibit myelination. We found that soluble Nrg1 type III enhanced myelination in Schwann cell-neuron cocultures. It improved myelination of Nrg1 type III(+/-) neurons and induced myelination on normally nonmyelinated sympathetic neurons. However, soluble Nrg1 type III failed to induce myelination on Nrg1 type III(-/-) neurons. To our surprise, low concentrations of Nrg1 type II also elicited a similar promyelinating effect. At high doses, however, both type II and III isoforms inhibited myelination and increased c-Jun expression in a manner dependent on Mek/Erk (mitogen-activated protein kinase kinase/extracellular signal-regulated kinase) activation. These results indicate that paracrine Nrg1 signaling provides concentration-dependent bifunctional effects on Schwann cell myelination. Furthermore, our studies suggest that there may be two distinct steps in Schwann cell myelination: an initial phase dependent on juxtacrine Nrg1 signaling and a later phase that can be promoted by paracrine stimulation.

Myelin is a multilamellar structure deriving from the spiral wrapping around the axons of the myelin forming glia, oligodendrocytes (OL) in the central nervous system (CNS) and Schwann cells (SC) in the peripheral nervous system (PNS). Several studies have shown that the communication between glial cells and neurons is important for proper neuronal development. In the absence of glial cells neurons die, demonstrating that the survival of these two cell types in developing nerves is strictly linked. Axons promote the proliferation, survival and differentiation of myelinating glia. In the PNS axons also determine whether SC ensheathe multiple smaller axons or segregate individual axons into a 1:1 relationship and myelinate them. In addition, axons also determine the amount of myelin formed. Glial cells, in turn, participate in normal development, long-term survival of the axon and the formation of domains. This is particularly important in demyelinating disorders whose morbidity is determined by axonal loss. Neuregulin1 (NRG1) is a key axonal signal that controls many aspects of PNS development, including glial cell survival and proliferation. Several NRG1 isoforms are known - type III NRG1 is the major isoform expressed by neurons.We previously showed that NRG1 type III is an essential instructive signal for PNS myelination. Above a threshold level of expression, axons are myelinated, and the amount of myelin and its thickness, are graded to the levels of type III NRG1 on the axon. Several studies indicate that NRG1 are proteolytically cleaved in their extracellular domain. Upon cleavage in the juxtamembrane region type I and type II are released, whereas type III NRG1 remains tethered on the axonal surface and acts as a juxtacrine signal. Previous studies have reported that the beta-secretase BACE-1, by cleaving NRG1 type III in the juxtamembrane region promotes myelination. We now report that NRG1 type III is also cleaved by the alpha-secretase TACE and that this cleavage event inhibits myelination. Lentiviral shRNA mediated-knock down of TACE in DRG neurons, in a myelinating SCneuronal coculture system, induces earlier activation of myelination and increases the amount of type III NRG1 on the axon. In agreement, conditional ablation of TACE in vivo in transgenic animals enhances myelination. Our results underscore the existence of several components controlling the myelination process, implicate secretases as key determinants in controlling type III NRG1 expression on the axon and indicate a novel level of regulation of myelination in the PNS

Myelinated axons are organized into a series of specialized domains with distinct molecular compositions and functions. These domains, which include the node of Ranvier, the flanking paranodal junctions, the juxtaparanodes, and the internode, form as the result of interactions with myelinating Schwann cells. This domain organization is essential for action potential propagation by saltatory conduction and for the overall function and integrity of the axon

The myelin sheath forms by the spiral wrapping of a glial membrane around the axon. The mechanisms responsible for this process are unknown but are likely to involve coordinated changes in the glial cell cytoskeleton. We have found that inhibition of myosin II, a key regulator of actin cytoskeleton dynamics, has remarkably opposite effects on myelin formation by Schwann cells (SC) and oligodendrocytes (OL). Myosin II is necessary for initial interactions between SC and axons, and its inhibition or down-regulation impairs their ability to segregate axons and elongate along them, preventing the formation of a 1:1 relationship, which is critical for peripheral nervous system myelination. In contrast, OL branching, differentiation, and myelin formation are potentiated by inhibition of myosin II. Thus, by controlling the spatial and localized activation of actin polymerization, myosin II regulates SC polarization and OL branching, and by extension their ability to form myelin. Our data indicate that the mechanisms regulating myelination in the peripheral and central nervous systems are distinct

Schwann cells are remarkably plastic cells that can both form and stably maintain myelin sheaths around axons and also rapidly dedifferentiate upon injury. New findings (Parkinson, D.B., A. Bhaskaran, P. Arthur-Farraj, L.A. Noon, A. Woodhoo, A.C. Lloyd, M.L. Feltri, L. Wrabetz, A. Behrens, R. Mirsky, and K.R. Jessen. 2008. J. Cell Biol. 181:625-637) indicate that the transition between these distinct states of differentiation is directed by the transcription factor Krox-20, which promotes and maintains myelination, and c-Jun, which antagonizes it. Cross-inhibition of these transcription factors serves to switch Schwann cells between the myelinated and dedifferentiated phenotypes, respectively