Jack Rosenbluth

Neuregulin-1 Type III Determines the Ensheathment Fate of Axons

The signals that determine whether axons are ensheathed or myelinated by Schwann cells have long been elusive. We now report that threshold levels of neuregulin-1 (NRG1) type III on axons determine their ensheathment fate. Ensheathed axons express low levels whereas myelinated fibers express high levels of NRG1 type III. Sensory neurons from NRG1 type III deficient mice are poorly ensheathed and fail to myelinate; lentiviral-mediated expression of NRG1 type III rescues these defects. Expression also converts the normally unmyelinated axons of sympathetic neurons to myelination. Nerve fibers of mice haploinsufficient for NRG1 type III are disproportionately unmyelinated, aberrantly ensheathed, and hypomyelinated, with reduced conduction velocities. Type III is the sole NRG1 isoform retained at the axon surface and activates PI 3-kinase, which is required for Schwann cell myelination. These results indicate that levels of NRG1 type III, independent of axon diameter, provide a key instructive signal that determines the ensheathment fate of axons.

Axon-Glia Interactions and the Domain Organization of Myelinated Axons Requires Neurexin IV/Caspr/Paranodin

Myelinated fibers are organized into distinct domains that are necessary for saltatory conduction. These domains include the nodes of Ranvier and the flanking paranodal regions where glial cells closely appose and form specialized septate-like junctions with axons. These junctions contain a Drosophila Neurexin IV-related protein, Caspr/Paranodin (NCP1). Mice that lack NCP1 exhibit tremor, ataxia, and significant motor paresis. In the absence of NCP1, normal paranodal junctions fail to form, and the organization of the paranodal loops is disrupted. Contactin is undetectable in the paranodes, and K(+) channels are displaced from the juxtaparanodal into the paranodal domains. Loss of NCP1 also results in a severe decrease in peripheral nerve conduction velocity. These results show a critical role for NCP1 in the delineation of specific axonal domains and the axon-glia interactions required for normal saltatory conduction.

Intramembranous particle distribution at the node of Ranvier and adjacent axolemma in myelinated axons of the frog brain.

SummaryThe plasma membrane of myelinated axons in the frog brain has been examined by the freeze-fracture technique. The cytoplasmic leaflet of the axolemma contains numerous randomly distributed particles in nodal and internodal regions but relatively fewer particles in the axoglial junctional portion of the paranodal region. Particle distribution is even less uniform in the outer leaflet of the axolemma, which contains a low concentration of particles in the internodal region and a relatively high concentration at the node of Ranvier (∼1200 particles μm−2). The nodal particles tend to be larger than most intramembranous particles, approaching 200 Å diameter. The paranodal region of the leaflet is virtually devoid of such particles except in the narrow helical ‘groove’ which faces extracellular clefts between terminating glial processes. In places this pathway widens to form ‘lakes’ up to ∼0.3 μm2 area which contain large numbers of large particles resembling those at the node. The concentration of particles at the node is in the same range as the concentration of sodium channels estimated to be in this region and it is suggested on the basis of their location and concentration that these particles represent ionophores. The distribution of particles in the paranodal region suggests that the large intramembranous particles do not have free access to the axoglial junctional portion of the membrane and therefore the movement of such particles along the paranodal region of the membrane may occur primarily in the membrane of the ‘groove’ spiraling through this portion of the axolemma. Such a restriction in surface area for particle movements on either side of the node of Ranvier could result in trapping of particles at the node and thus contribute to their concentration in the nodal axolemma.

Sulfatide is essential for the maintenance of CNS myelin and axon structure.

Galactocerebroside (GalC) and sulfatide are abundant myelin lipids. In mice incapable of synthesizing these lipids, myelin is thin and regionally unstable and exhibits several subtle structural abnormalities. Although galactolipid‐null mice have been beneficial in the analysis of galactolipid function, it has not been possible to differentiate between the functions of GalC and sulfatide with these mice alone. In the present work, we have analyzed a murine model that forms normal levels of GalC but is incapable of synthesizing sulfatide. By comparing a plethora of morphological features between the galactolipid‐null and the sulfatide‐null mice, we have begun to differentiate between the specific functions of these closely related lipids. The most striking difference between these two mutants is the reduction of myelin developmental abnormalities (e.g., redundant and uncompacted myelin sheaths) in young adult sulfatide‐null mice as compared with the galactolipid‐null animals. Although sulfatide appears to play a limited role in myelin development, this lipid is essential for myelin maintenance, as the prevalence of redundant, uncompacted, and degenerating myelin sheaths as well as deteriorating nodal/paranodal structure is increased significantly in aged sulfatide‐null mice as compared with littermate wildtype mice. Finally, we show that the role played by sulfatide in CNS maintenance is not limited to the myelin sheath, as axonal caliber and circularity are normal in young adult mutant mice but are significantly altered in aged sulfatide‐null animals.

A critical role for the protein tyrosine phosphatase receptor type Z in functional recovery from demyelinating lesions

Several lines of evidence suggest that tyrosine phosphorylation is a key element in myelin formation, differentiation of oligodendrocytes and Schwann cells, and recovery from demyelinating lesions. Multiple sclerosis is a demyelinating disease of the human central nervous system, and studies of experimental demyelination indicate that remyelination in vivo requires the local generation, migration or maturation of new oligodendrocytes, or some combination of these. Failure of remyelination in multiple sclerosis could result from the failure of any of these processes or from the death of oligodendrocytes. Ptprz encodes protein tyrosine phosphatase receptor type Z (Ptpz, also designated Rptpβ), which is expressed primarily in the nervous system but also in oligodendrocytes, astrocytes and neurons. Here we examine the susceptibility of mice deficient in Ptprz to experimental autoimmune encephalomyelitis (EAE), a model of multiple sclerosis. We observe that mice deficient in Ptprz show impaired recovery from EAE induced by myelin oligodendrocyte glycoprotein (MOG) peptide. This sustained paralysis is associated with increased apoptosis of mature oligodendrocytes in the spinal cords of mutant mice at the peak of inflammation. We further demonstrate that expression of PTPRZ1, the human homolog of Ptprz, is induced in multiple sclerosis lesions and that the gene is specifically expressed in remyelinating oligodendrocytes in these lesions. These results support a role for Ptprz in oligodendrocyte survival and in recovery from demyelinating disease.

No obvious abnormality in mice deficient in receptor protein tyrosine phosphatase beta.

ABSTRACT The development of neurons and glia is governed by a multitude of extracellular signals that control protein tyrosine phosphorylation, a process regulated by the action of protein tyrosine kinases and protein tyrosine phosphatases (PTPs). Receptor PTPβ (RPTPβ; also known as PTPζ) is expressed predominantly in the nervous system and exhibits structural features common to cell adhesion proteins, suggesting that this phosphatase participates in cell-cell communication. It has been proposed that the three isoforms of RPTPβ play a role in regulation of neuronal migration, neurite outgrowth, and gliogenesis. To investigate the biological functions of this PTP, we have generated mice deficient in RPTPβ. RPTPβ-deficient mice are viable, are fertile, and showed no gross anatomical alterations in the nervous system or other organs. In contrast to results of in vitro experiments, our study demonstrates that RPTPβ is not essential for neurite outgrowth and node formation in mice. The ultrastructure of nerves of the central nervous system in RPTPβ-deficient mice suggests a fragility of myelin. However, conduction velocity was not altered in RPTPβ-deficient mice. The normal development of neurons and glia in RPTPβ-deficient mice demonstrates that RPTPβ function is not necessary for these processes in vivo or that loss of RPTPβ can be compensated for by other PTPs expressed in the nervous system.

Multiple functions of the paranodal junction of myelinated nerve fibers

Myelin sheaths include an extraordinary structure, the “paranodal axoglial junction” (PNJ), which attaches the sheath to the axon at each end of each myelin segment. Its size is enormous and its structure unique. Here we review past and current studies showing that this junction can serve multiple functions in maintaining reliable saltatory conduction. The present evidence points to three functions in particular. 1) It seals the myelin sheath to the axon to prevent major shunting of nodal action currents beneath the myelin sheath while still leaving a narrow channel interconnecting the internodal periaxonal space with the perinodal space. This pathway represents a potential route through which juxtaparanodal and internodal channels can influence nodal activity and through which nutrients, such as glucose, and other metabolites can diffuse to and from the internodal periaxonal space. 2) It serves as a mechanism for maintaining discrete, differentiated axolemmal domains at and around the node of Ranvier by acting as a barrier to the lateral movement of ion channel complexes within the axolemma, thus concentrating voltage‐gated sodium channels at the node and segregating fast voltage‐gated potassium channels to the juxtaparanode under the myelin sheath. 3) It attaches the myelin sheath to the axon on either side of the node and can thus maintain nodal dimensions in the face of mechanical stresses associated with stretch or other local factors that might cause disjunction. It is therefore the likely means for maintaining constancy of nodal surface area and electrical parameters essential for consistency in conduction.

Subtle myelin defects in PLP-null mice †

This study explores subtle defects in the myelin of proteolipid protein (PLP)‐null mice that could potentially underlie the functional losses and axon damage known to occur in this mutant and in myelin diseases including multiple sclerosis. We have compared PLP‐null central nervous system (CNS) myelin with normal myelin using ultrastructural methods designed to emphasize fine differences. In the PLP‐null CNS, axons large enough to be myelinated often lack myelin entirely or are surrounded by abnormally thin sheaths. Short stretches of cytoplasm persist in many myelin lamellae. Most strikingly, compaction is incomplete in this mutant as shown by the widespread presence of patent interlamellar spaces of variable width that can be labeled with ferricyanide, acting as an aqueous extracellular tracer. In thinly myelinated fibers, interlamellar spaces are filled across the full width of the sheaths. In thick myelin sheaths, they appear filled irregularly but diffusely. These patent spaces constitute a spiral pathway through which ions and other extracellular agents may penetrate gradually, possibly contributing to the axon damage known to occur in this mutant, especially in thinly myelinated fibers, where the spiral path length is shortest and most consistently labeled. We show also that the “radial component” of myelin is distorted in the mutant (“diagonal component”), extending across the sheaths at 45° instead of 90°. These observations indicate a direct or indirect role for PLP in maintaining myelin compaction along the external surfaces of the lamellae and to a limited extent, along the cytoplasmic surfaces as well and also in maintaining the normal alignment of the radial component.

Myelin structure in proteolipid protein (PLP)-null mouse spinal cord.

Fixed preparations of proteolipid protein (PLP)‐null mouse spinal cord show myelin sheaths which in some regions consist of typical alternating major dense lines (MDLs) and intermediate lines (ILs) with a repeat period of 10.3 nm. More commonly, the lamellar structure consists of what appears to be a single population of dense lines, having a repeat period of 5.2 nm. These apparently equivalent lines are, however, sometimes distinguishable as MDLs or ILs based on continuity with cytoplasmic or extracellular regions. Focal separations of lamellae at the intermediate line are common. MDLs too may be replaced focally by cytoplasmic pockets, sometimes in the same quadrant over several lamellae, resembling Schmidt‐Lanterman clefts. Occasional densities reminiscent of the “radial component” can be seen. Otherwise, this structure, which is prominent in wild‐type myelin, is conspicuously absent. Redundant folding of some lamellae but not others may occur in the same sheath. These observations conform to those made previously on the isolated myelin segments that occur in the myelin‐deficient rat central nervous system (CNS), which also lacks PLP. Thus, a compact lamellar structure can be seen in fixed PLP‐null myelin, but defects in the apposition of both the extracellular and the cytoplasmic surfaces of the myelin membranes are common. The abnormalities seen suggest a lack of firm intermembrane bonding, resulting in structural instability. PLP‐null myelin may therefore be more susceptible than normal myelin to disruption by mechanical or osmotic stresses. Although PLP is not essential for the formation of either major dense lines or intermediate lines, it may play a role in stabilizing the compact structure.

Myelin formation following transplantation of normal fetal glia into myelin-deficient rat spinal cord

SummaryStructurally normal myelin sheaths develop in the spinal cord of juvenile myelin-deficient rats (mdr) 11 days after transplantation of normal fetal spinal cord fragments or cultured cells that do not yet express galactocerebroside. Cultures result in more extensive myelin formation, and in both cases the myelin that forms is located primarily at or near the site of transplantation.Myelin formation also occurs after transplantation of postnatal donor tissue, but the extent diminishes with donor age, and none was seen after transplantation of adult donor tissue over the two-week period studied. Injection of killed tissue, tissue derived from mouse donors or an extract of myelin also did not lead to myelin formation. The results imply that myelin formed in the host following transplantation was generated by oligodendrocytes newly differentiated from donor precursor cells rather than by donor oligodendrocytes that were already mature at the time of transplantation or by host oligodendrocytes that took up components of the injected material.We conclude that exogenous fetal glial cell precursors are able to survive, differentiate and form myelin in the environment of the juvenile mdr spinal cord.