Oct4‐induced oligodendrocyte progenitor cells enhance functional recovery in spinal cord injury model

The generation of patient‐specific oligodendrocyte progenitor cells (OPCs) holds great potential as an expandable cell source for cell replacement therapy as well as drug screening in spinal cord injury or demyelinating diseases. Here, we demonstrate that induced OPCs (iOPCs) can be directly derived from adult mouse fibroblasts by Oct4‐mediated direct reprogramming, using anchorage‐independent growth to ensure high purity. Homogeneous iOPCs exhibit typical small‐bipolar morphology, maintain their self‐renewal capacity and OPC marker expression for more than 31 passages, share high similarity in the global gene expression profile to wild‐type OPCs, and give rise to mature oligodendrocytes and astrocytes in vitro and in vivo. Notably, transplanted iOPCs contribute to functional recovery in a spinal cord injury (SCI) model without tumor formation. This study provides a simple strategy to generate functional self‐renewing iOPCs and yields insights for the in‐depth study of demyelination and regenerative medicine.     

Delayed cell cycle pathway modulation facilitates recovery after spinal cord injury

Traumatic spinal cord injury (SCI) causes tissue loss and associated neurological dysfunction through mechanical damage and secondary biochemical and physiological responses. We have previously described the pathobiological role of cell cycle pathways following rat contusion SCI by examining the effects of early intrathecal cell cycle inhibitor treatment initiation or gene knockout on secondary injury. Here, we delineate changes in cell cycle pathway activation following SCI and examine the effects of delayed (24 h) systemic administration of flavopiridol, an inhibitor of major cyclin-dependent kinases (CDKs), on functional recovery and histopathology in a rat SCI contusion model. Immunoblot analysis demonstrated a marked upregulation of cell cycle-related proteins, including pRb, cyclin D1, CDK4, E2F1 and PCNA, at various time points following SCI, along with downregulation of the endogenous CDK inhibitor p27. Treatment with flavopiridol reduced induction of cell cycle proteins and increased p27 expression in the injured spinal cord. Functional recovery was significantly improved after SCI from day 7 through day 28. Treatment significantly reduced lesion volume and the number of Iba-1⁺ microglia in the preserved tissue and increased the myelinated area of spared white matter as well as the number of CC1⁺ oligodendrocytes. Furthermore, flavopiridol attenuated expression of Iba-1 and glactin-3, associated with microglial activation and astrocytic reactivity by reduction of GFAP, NG2, and CHL1 expression. Our current study supports the role of cell cycle activation in the pathophysiology of SCI and by using a clinically relevant treatment model, provides further support for the therapeutic potential of cell cycle inhibitors in the treatment of human SCI.     

Targeted overexpression of a golli–myelin basic protein isoform to oligodendrocytes results in aberrant oligodendrocyte maturation and myelination

Recently, several in vitro studies have shown that the golli–myelin basic proteins regulate Ca²⁺ homoeostasis in OPCs (oligodendrocyte precursor cells) and immature OLs (oligodendrocytes), and that a number of the functions of these cells are affected by cellular levels of the golli proteins. To determine the influence of golli in vivo on OL development and myelination, a transgenic mouse was generated in which the golli isoform J37 was overexpressed specifically within OLs and OPCs. The mouse, called JOE (J37-overexpressing), is severely hypomyelinated between birth and postnatal day 50. During this time, it exhibits severe intention tremors that gradually abate at later ages. After postnatal day 50, ultrastructural studies and Northern and Western blot analyses indicate that myelin accumulates in the brain, but never reaches normal levels. Several factors appear to underlie the extensive hypomyelination. In vitro and in vivo experiments indicate that golli overexpression causes a significant delay in OL maturation, with accumulation of significantly greater numbers of pre-myelinating OLs that fail to myelinate axons during the normal myelinating period. Immunohistochemical studies with cell death and myelin markers indicate that JOE OLs undergo a heightened and extended period of cell death and are unable to effectively myelinate until 2 months after birth. The results indicate that increased levels of golli in OPC/OLs delays myelination, causing significant cell death of OLs particularly in white matter tracts. The results provide in vivo evidence for a significant role of the golli proteins in the regulation of maturation of OLs and normal myelination.     

GM1 ganglioside contributes to retain the neuronal conduction and neuronal excitability in visceral and baroreceptor afferents

GM1 ganglioside has a great impact on the function of nodes of Ranvier on myelinated fiber, suggesting its potential role to maintain the electrical and neuronal excitability of neurons. Here we first demonstrate that visceral afferent conduction velocity of myelinated and unmyelinated fibers are reduced significantly by tetrodotoxin (TTX) or cholera toxin-B subunits (CTX-B), and only the effects mediated by CTX-B are prevented by GM1 pre-treatment. At soma of myelinated A and unmyelinated C-type nodose ganglion neurons (NGNs), the action potential spike frequency reduced by CTX-B is also prevented by GM1. Additionally, the current density of both TTX-sensitive (TTX-S) and TTX-resistant (TTX-R) Na⁺ channels were significantly decreased by CTX-B without changing the voltage-dependent property. These data confirm that endogenous GM1 may play a dominant role in maintaining the electrical and neuronal excitability via modulation of sodium (Na⁺) channel around nodes and soma as well, especially TTX-S Na⁺ channel, which is also confirmed by the reduction of spike amplitude and depolarization. Similar data are also extended to fluorescently identified and electrophysiologically characterized aortic baroreceptor neurons. These findings suggest that GM1 plays an important role in the neural modulation of electric and neuronal excitability in visceral afferent system.     

Mutations in the cytoplasmic domain of P0 reveal a role for PKC-mediated phosphorylation in adhesion and myelination.

Mutations in P0 (MPZ), the major myelin protein of the peripheral nervous system, cause the inherited demyelinating neuropathy Charcot-Marie-Tooth disease type 1B. P0 is a member of the immunoglobulin superfamily and functions as a homophilic adhesion molecule. We now show that point mutations in the cytoplasmic domain that modify a PKC target motif (RSTK) or an adjacent serine residue abolish P0 adhesion function and can cause peripheral neuropathy in humans. Consistent with these data, PKCalpha along with the PKC binding protein RACK1 are immunoprecipitated with wild-type P0, and inhibition of PKC activity abolishes P0-mediated adhesion. Point mutations in the RSTK target site that abolish adhesion do not alter the association of PKC with P0; however, deletion of a 14 amino acid region, which includes the RSTK motif, does abolish the association. Thus, the interaction of PKCalpha with the cytoplasmic domain of P0 is independent of specific target residues but is dependent on a nearby sequence. We conclude that PKC-mediated phosphorylation of specific residues within the cytoplasmic domain of P0 is necessary for P0-mediated adhesion, and alteration of this process can cause demyelinating neuropathy in humans.     

Analogous standard motifs in myelin basic protein and in MARCKS

Myelin basic protein (MBP) and myristoylated alanine-rich C-kinase substrate (MARCKS) are similar in terms of having extended conformations regulated by their environment (i.e., solubilised or lipid-associated), N-terminal modifications, a dual nature of interactions with lipids, binding to actin and Ca²⁺-calmodulin, and being substrates for different kinds of protein kinases. The further sequence similarities of segments of MBP with lipid effector regions of MARCKS, and numerous reports in the literature, support the thesis that some developmental isoform of MBP functions in signal transduction.     

Generation of Transgenic Mice Expressing Insulin-Like Growth Factor-1 Under the Control of the Myelin Basic Protein Promoter: Increased Myelination and Potential for Studies on the Effects of Increased IGF-1 on Experimentally and Genetically Induced Demyelination

In order to investigate a role for insulin-like growth factor-1 (IGF-1) in ameliorating the effects of demyelinating events and potentiating remyelination, we have generated transgenic (tg) mice expressing IGF-1 under the control of the myelin basic protein promoter. Heterozygous tg mice expressed the highest levels of IGF-1 in brain during the most active periods of myelination as determined by Western and Northern blotting. A high level of expression was found throughout the lives of the tg mice. There was no increased expression of IGF-1 in other organs. The brains of heterozygous mice were larger than those of normal mice by 2 weeks of age, and they continued to increase in size for several months. Light and electron microscopy showed extensive myelination of axons. Behavioral studies of the older heterozygous mice documented difficulty with balance. This new tg mouse model can be bred to mice that are heterozygous for genetic leukodystrophies to produce eventually mice that are affected with a given leukodystrophy but overexpress IGF-1 during myelination and remyelination. It will be interesting to see if overexpression of IGF-1 can modulate the pathological and clinical features of the inherited leukodystrophies with or without supplemental therapies.