Glial transplants: an in vivo analysis of extrinsic and intrinsic determinants of dysmyelination in genetic variants

Building brains: neural chimeras in the study of nervous system development and repair

The ability to isolate multipotential neuroepithelial precursor cells from the mammalian nervous system provides exciting perspectives for the in vitro analysis of early nervous system development and the generation of donor cells for neural repair. New models are needed to study the properties of these cells in vivo. Neural chimeras have revealed a remarkable degree of plasticity in the developmental potential of neuroepithelial precursor cells. Following transplantation into the cerebral ventricle of embryonic hosts, precursors derived from various brain regions and developmental stages participate in host brain development and undergo region-specific differentiation into neurons and glia. These findings indicate that in the developing nervous system, migration and differentiation of neural precursors cells are regulated to a large extent by extrinsic signals. Neural chimeras composed of genetically modified cells will permit the study of the molecular mechanisms underlying these guidance cues, which may eventually be exploited for cell replacement strategies in the adult brain. A key problem in neural transplantation is the availability of suitable donor tissue. Neural chimeras composed of embryonic stem (ES) cell-derived neurons and glia depict ES cells as a versatile and virtually unlimited donor source for neural repair. Generation of interspecies neural chimeras composed of human and rodent cells facilitates the translation of these advances into clinical strategies for human nervous system repair.

Chimeric brains generated by intraventricular transplantation of fetal human brain cells into embryonic rats

Limited experimental access to the central nervous system (CNS) is a key problem in the study of human neural development, disease, and regeneration. We have addressed this problem by generating neural chimeras composed of human and rodent cells. Fetal human brain cells implanted into the cerebral ventricles of embryonic rats incorporate individually into all major compartments of the brain, generating widespread CNS chimerism. The human cells differentiate into neurons, astrocytes, and oligodendrocytes, which populate the host fore-, mid-, and hindbrain. These chimeras provide a unique model to study human neural cell migration and differentiation in a functional nervous system.

Phenotypic severity of murine Plp mutants reflects in vivo and in vitro variations in transport of PLP isoproteins

Mutations of the major myelin gene, proteolipid protein (Plp), cause Pelizaeus-Merzbacher disease and some forms of spastic paraplegia in man and dysmyelinating phenotypes in animals. The clinical severity is markedly heterogeneous, ranging from relatively mild to severe and fatal. Point mutations, or frame shifts, which are predicted to result in translation of structurally altered proteins account for many of these cases, including 3 of the allelic murine conditions. Plp(jp-rsh), Plp(jp-msd), and Plp(jp) represent an increasing severity of clinical and pathological phenotypes, respectively. In this study we determined whether there was any correlation between the severity of phenotype and the transport of the predicted abnormal protein. We examined the ability of the two products of the Plp gene, PLP and DM20, to insert into the plasma membrane of transfected BHK or COS-7 cells, and into the myelin sheath of oligodendrocytes. With these complementary in vitro and in vivo approaches we find that proteins of Plp(jp-rsh), associated with the mildest phenotype, have a far greater ability to insert into the cell membrane or myelin than those associated with the severe phenotypes. Additionally, altered DM20 is more readily transported to the cell surface and to myelin than the PLP isoprotein. Interestingly, the two clonal cell lines chosen for transient transfection differ in their ability to fold DM20 from Plp(jp-rsh) and Plp(jp-msd) mice correctly, as inferred by staining for the conformation-sensitive O10 epitope. In the case of Plp(jp), which is associated with the most severe phenotype, no PLP or O10 staining is present at the cell surface or in myelin. The perturbation in trafficking observed for altered Plp(jp) PLP and DM20 in oligodendrocytes does not extend to other myelin membrane proteins, such as MAG and MOG, nor to wild type PLP co-expressed in the same cell, all of which are correctly inserted into myelin. As Plp-knockout mice do not have a dysmyelinating phenotype it seems unlikely that absence of PLP and/or DM20 in the membrane is responsible for the pathology. It remains to be determined whether the perturbation in protein trafficking is associated with the dysmyelination, or if the altered product of the mutant alleles acquire a novel function which is deleterious to myelin production by oligodendrocytes.

Myelin mutants: model systems for the study of normal and abnormal myelination

Spontaneous mutations that perturb myelination occur in a range of species including man, and together with engineered mutations have been used to study disease, normal myelination and axon/glial inter-relationships. Only a minority of the currently defined mutations have an apparently simple pathogenesis due to lack of a functional protein. Mutations in the myelin basic protein gene lead to a lack of protein, resulting in changes in the structure of myelin, which can be rescued by transgenic complementation. The pathogenesis of autosomal dominant and X-linked mutations affecting either oligodendrocytes or Schwann cells is more complex. Point mutations may act in a dominant negative manner and gene dosage is clearly linked to phenotypic change. Mutations in regulatory genes, such as those encoding transcription factors, can also disturb myelination by selected cell types. Other less-well studied and unexpected consequences of myelin mutations, such as seizures in mutations affecting genes expressed in Schwann cells and axonal changes associated with dysmyelination, are also considered. With the major developments in gene mapping and cloning it is now relevant to study mutations in a variety of species with the real prospect of defining their molecular basis. Examples are given of unusual, but potentially useful, uncharacterized mutations in dog and bovine.

Regulation of oligodendrocyte development and CNS myelination by growth factors: prospects for therapy of demyelinating disease

Multiple sclerosis (MS), the most common neurological disorder diagnosed in young adults, is characterized by autoimmune demyelination in the central nervous system (CNS). Promotion of remyelination in the brain and spinal cord is a potential strategy for therapeutic intervention in MS and other demyelinating diseases. Recent studies have shown that the development of oligodendrocytes, the myelin-forming cells of the CNS, is extensively controlled by growth factors. These factors regulate the proliferation, migration, differentiation, survival and regeneration of oligodendroglial cells and the synthesis of myelin, and often interact in a complex manner. Moreover, insulin-like growth factor I (IGF-I) has proven effective for therapy of experimental autoimmune encephalomyelitis (EAE), an animal model of autoimmune demyelination. In this review we summarize recent findings on the regulation of oligodendrocyte development and CNS myelination by growth factors, and discuss these findings in the context of possible clinical application for the therapy of neurological disease in humans.