Regional and developmental variations of GFAP and actin mRNA levels in the CNS of jimpy and shiverer mutant mice

Drebrin regulates cytoskeleton dynamics in migrating neurons through interaction with CXCR4

Stromal cell-derived factor-1 (SDF-1) and chemokine receptor type 4 (CXCR4) are regulators of neuronal migration (e.g., GnRH neurons, cortical neurons, and hippocampal granule cells). However, how SDF-1/CXCR4 alters cytoskeletal components remains unclear. Developmentally regulated brain protein (drebrin) stabilizes actin polymerization, interacts with microtubule plus ends, and has been proposed to directly interact with CXCR4 in T cells. The current study examined, in mice, whether CXCR4 under SDF-1 stimulation interacts with drebrin to facilitate neuronal migration. Bioinformatic prediction of protein-protein interaction highlighted binding sites between drebrin and crystallized CXCR4. In migrating GnRH neurons, drebrin, CXCR4, and the microtubule plus-end binding protein EB1 were localized close to the cell membrane. Coimmunoprecipitation (co-IP) confirmed a direct interaction between drebrin and CXCR4 using wild-type E14.5 whole head and a GnRH cell line. Analysis of drebrin knockout (DBN1 KO) mice showed delayed migration of GnRH cells into the brain. A decrease in hippocampal granule cells was also detected, and co-IP confirmed a direct interaction between drebrin and CXCR4 in PN4 hippocampi. Migration assays on primary neurons established that inhibiting drebrin (either pharmacologically or using cells from DBN1 KO mice) prevented the effects of SDF-1 on neuronal movement. Bioinformatic prediction then identified binding sites between drebrin and the microtubule plus end protein, EB1, and super-resolution microscopy revealed decreased EB1 and drebrin coexpression after drebrin inhibition. Together, these data show a mechanism by which a chemokine, via a membrane receptor, communicates with the intracellular cytoskeleton in migrating neurons during central nervous system development.

Modulation of the Innate Immune Response by Human Neural Precursors Prevails over Oligodendrocyte Progenitor Remyelination to Rescue a Severe Model of Pelizaeus-Merzbacher Disease

Pelizaeus-Merzbacher disease (PMD) results from an X-linked misexpression of proteolipid protein 1 (PLP1). This leukodystrophy causes severe hypomyelination with progressive inflammation, leading to neurological dysfunctions and shortened life expectancy. While no cure exists for PMD, experimental cell-based therapy in the dysmyelinated shiverer model suggested that human oligodendrocyte progenitor cells (hOPCs) or human neural precursor cells (hNPCs) are promising candidates to treat myelinopathies. However, the fate and restorative advantages of human NPCs/OPCs in a relevant model of PMD has not yet been addressed. Using a model of Plp1 overexpression, resulting in demyelination with progressive inflammation, we compared side-by-side the therapeutic benefits of intracerebrally grafted hNPCs and hOPCs. Our findings reveal equal integration of the donor cells within presumptive white matter tracks. While the onset of exogenous remyelination was earlier in hOPCs-grafted mice than in hNPC-grafted mice, extended lifespan occurred only in hNPCs-grafted animals. This improved survival was correlated with reduced neuroinflammation (microglial and astrocytosis loads) and microglia polarization toward M2-like phenotype followed by remyelination. Thus modulation of neuroinflammation combined with myelin restoration is crucial to prevent PMD pathology progression and ensure successful rescue of PMD mice. These findings should help to design novel therapeutic strategies combining immunomodulation and stem/progenitor cell-based therapy for disorders associating hypomyelination with inflammation as observed in PMD.

GFAP expression as an indicator of disease severity in mouse models of Alexander disease

AxD (Alexander disease) is a rare disorder caused by heterozygous mutations in GFAP (glial fibrillary acidic protein) resulting in accumulation of the GFAP protein and elevation of Gfap mRNA. To test whether GFAP itself can serve as a biomarker of disease status or progression, we investigated two independent measures of GFAP expression in AxD mouse models, one using a genetic reporter of promoter activity and the other quantifying GFAP protein directly in a manner that could also be employed in human studies. Using a transgenic reporter line that expresses firefly luciferase under the control of the murine Gfap promoter (Gfap-luc), we found that luciferase activity reflected the regional CNS (central nervous system) variability of Gfap mRNA in Gfap^+/+ mice, and increased in mice containing a point mutation in Gfap that mimics a common human mutation in AxD (R239H in the human sequence, and R236H in the murine sequence). In a second set of studies, we quantified GFAP protein in CSF (cerebrospinal fluid) taken from three different AxD mouse models and littermate controls. GFAP levels in CSF were increased in all three AxD models, in a manner corresponding to the concentrations of GFAP in brain. These studies demonstrate that transactivation of the Gfap promoter is an early and sustained indicator of the disease process in the mouse. Furthermore, GFAP in CSF serves as a potential biomarker that is comparable between mouse models and human patients.

Loss of Testicular Orphan Receptor 4 Impairs Normal Myelination in Mouse Forebrain

Testicular orphan nuclear receptor 4 (TR4) has been suggested to play important roles in the development and functioning of the central nervous system (CNS). We find reduced myelination in TR4 knockout (TR4(-/-)) mice, which is particularly obvious in forebrains and in early developmental stages. Further analysis reveals that CC-1-positive (CC-1+) oligodendrocytes are decreased in TR4(-/-) forebrains. The O4+ signals are also reduced in TR4(-/-) forebrains when examined at postnatal d 7. However, the number and proliferation rate of platelet-derived growth factor receptor alpha-positive (PDGFalphaR+) oligodendrocyte precursor cells (OPCs) remain unaffected in these regions, suggesting that loss of TR4 interrupts oligodendrocyte differentiation. This is further supported by the observation that CC-1+ oligodendrocytes derived from 5-bromo-2'-deoxyuridine incorporating OPCs are significantly reduced in TR4(-/-) forebrains. We also find higher Jagged1 expression levels in axon fiber-enriched regions in TR4(-/-) forebrains, suggesting a more activated Notch signaling in these regions that correlates with previous reports showing that Notch activation inhibits oligodendrocyte differentiation. Together, our results suggest that TR4 is required for proper myelination in the CNS and is particularly important for oligodendrocyte differentiation and maturation in the forebrain regions. The altered Jagged1-Notch signaling in TR4(-/-) forebrain underlies a potential mechanism that contributes to the reduced myelination in the forebrain.

Immunohistochemical and morphometrical studies on myelin breakdown in the demyelination (dmy) mutant rat

The demyelination (dmy) rat is a unique mutant exhibiting severe myelin breakdown in the central nervous system (CNS). In this study, we conducted immunohistochemical and morphometrical investigations in the dmy rat. From around 6 weeks of age, the affected rats developed ataxia especially in the hindlimbs. Afterwards, ataxia worsened rapidly, resulting in complete paralysis of the hindlimbs and recumbency. Histopathology at 7 to 10 weeks of age revealed myelin destruction throughout the white matter of the CNS in the dmy rats. The most severely affected lesions were distributed in the corpus callosum, capsula interna, striatum, subcortical white matter, cerebellar peduncle, and ventral and lateral parts of the spinal cord. Immunohistochemistry demonstrated prominent astrogliosis and many ED-1 positive macrophages in the myelin-destructed areas. Until the 4th week, no significant differences in myelin thickness and fiber diameter were found between dmy and control rats. However, from 5 weeks of age, myelin thickness of residual myelinated fibers in dmy rats became significantly less than that in controls. These data indicated that the dmy phenotype shows a prolonged period of myelin destruction, suggesting that dmy mutation affects the adequate maintenance of myelin.

Recovery of myelin after induction of oligodendrocyte cell death in postnatal brain

A transgenic mouse line (Oligo-TTK) was established to monitor oligodendrocyte cell death and myelin formation in the CNS. The expression of a conditionally toxic gene, the herpes simplex virus-1 thymidine kinase (HSV1-TK), was made under control of the MBP (myelin basic protein) gene promoter. A truncated form of the HSV1-TK (TTK) gene was used to avoid both bystander effect resulting from leaking in thymidine kinase activity and sterility in transgenic males observed in previous transgenic mice. The transgene was expressed in the CNS with a restricted localization in oligodendrocytes. Oligodendrocyte proliferation and myelin formation are therefore tightly controlled experimentally by administration of ganciclovir (GCV) via the induction of oligodendrocyte cell death. The most severe and irreversible hypomyelination was obtained when GCV was given daily from postnatal day 1 (P1) to P30. Oligodendrocyte plasticity and myelin recovery were analyzed in another phenotype generated by GCV treatment from P1 to P15. In this model, after dysmyelination, an apparent normal behavior was restored with no visible pathological symptoms by P30. Proliferating cells, which may be implicated in myelin repair in this model, are detected primarily in myelin tracts expressing the oligodendrocyte phenotype. Therefore, the endogenous potential of oligodendrocytes to remyelinate was clearly demonstrated in the mice of this study.

A transgenic mouse model for inducible and reversible dysmyelination

Oligodendrocytes are glial cells devoted to the production of myelin sheaths. Myelination of the CNS occurs essentially after birth. To delineate both the times of oligodendrocyte proliferation and myelination, as well as to study the consequence of dysmyelination in vivo, a model of inducible dysmyelination was developed. To achieve oligodendrocyte ablation, transgenic animals were generated that express the herpes virus 1 thymidine kinase (HSV1-TK) gene under the control of the myelin basic protein (MBP) gene promoter. The expression of the MBP-TK transgene in oligodendrocytes is not toxic on its own; however, toxicity can be selectively induced by the systemic injection of animals with nucleoside analogs, such as FIAU [1-(2-deoxy-2-fluoro-beta-delta-arabinofuranosyl)-5-iodouracil]. This system allows us to control the precise duration of the toxic insult and the degree of ablation of oligodendrocytes in vivo. We show that chronic treatment of MBP-TK mice with FIAU during the first 3 postnatal weeks triggers almost a total depletion of oligodendrocytes in the CNS. These effects are accompanied by a behavioral phenotype characterized by tremors, seizures, retarded growth, and premature animal death. We identify the period of highest oligodendrocytes division in the first 9 postnatal days. Delaying the beginning of FIAU treatments results in different degrees of dysmyelination. Dysmyelination in MBP-TK mice is always accompanied by astrocytosis. Thus, this transgenic line provides a model to study the events occurring during dysmyelination of various intensities. It also represents an invaluable tool to investigate remyelination in vivo.

Regional and temporal progression of reactive astrocytosis in the brain of the myelin mutant taiep rat

Reactive astrocytosis in taiep rats was shown by glial fibrillary acidic protein (GFAP) immunoreactivity measured by means of enzyme-linked immunosorbent assay and indirect immunofluorescence. Increased GFAP immunoreactivity was first observed in the brainstem of 15-day-old taiep rats and was widespread throughout all brain regions at 6 months of age. Characteristically, astrocytes were hypertrophic and displayed strong GFAP fluorescence. The pattern of these reactive cells may correlate with the process of dysmyelination in the taiep rat.

Understanding glial abnormalities associated with myelin deficiency in the jimpy mutant mouse

Jimpy is a shortened life-span murine mutant showing recessive sex-linked inheritance. The genetic defect consists of a point mutation in the PLP gene and produces a severe CNS myelin deficiency that is associated with a variety of complex abnormalities affecting all glial populations. The myelin deficiency is primarily due to a failure to produce the normal amount of myelin during development. However, myelin destruction and oligodendrocyte death also account for the drastic myelin deficit observed in jimpy. The oligodendroglial cell line shows complex abnormalities in its differentiation pattern, including the degeneration of oligodendrocytes through an apoptotic mechanism. Oligodendrocytes seem to be the most likely candidate to be primarily altered in a disorder affecting myelination, but disturbances affecting astrocytes and microglia are also remarkable and may have a crucial significance in the development of the jimpy disorder. In fact, the jimpy phenotype may not be attributed to a defect in a single cell but rather to a deficiency in the normal relations between glial cells. Evidences from a variety of sources indicate that the jimpy mutant could be a model for disturbed glial development in the CNS. The accurate knowledge of the significance of PLP and its regulation during development must be of vital importance in order to understand glial abnormalities in jimpy.

In vitro death of jimpy oligodendrocytes: correlation with onset of DM-20/PLP expression and resistance to oligodendrogliotrophic factors

Severe hypomyelination in the jimpy (jp) mouse mutation results from premature death of most oligodendrocytes (OCs). We have applied an immunopanning technique to successfully purify oligodendroblasts (OBs) directly from neonatal jp brainstem in order to determine if their death during differentiation into OCs is preventable in culture by diffusible oligodendrogliotrophic factors. No significant differences in the yield (0.9-1.1 x 10(5) cells/brainstem) or viability (approximately 90%) of OB populations from jp and wild-type (wt) littermates were observed, indicating that cell death occurs at a later stage in the mutant lineage. When cultured in a basally defined, insulin-containing medium, wt and jp OBs died 1-2 days later as their differentiation into GalC+ OCs began. Survival was not enhanced by known trophic factors (ciliary neurotrophic factor, leukemia inhibitory factor, neurotrophin-3) for differentiating rat OCs. In medium conditioned by neonatally derived rat or wt mouse astrocytes, however, wt OBs survived terminal OC differentiation, expressing first GalC, then DM-20/PLP on their surface 1-2 days later, before elaborating myelin-like membrane. By contrast, jp OBs in sister cultures survived differentiation initially as well as their normal counterparts did but rapidly died thereafter, beginning at the time when PLP/DM-20 immunoreactivity became detectable on premature wt GalC+ OCs. Additionally under these conditions, there survived a minor population (<5%) of jp cells, including mature OCs, which expressed stunted membranes and DM-20/PLP immunoreactivity in their cytoplasm, and undifferentiated progenitors. This model supports the concept that OC death in jp is effected by an intrinsic program, one mechanistically related to jp PLP/DM-20 gene expression and refractory to trophic cues in the environment.

Distribution of the messenger RNA for the extracellularly regulated kinases 1, 2 and 3 in rat brain: effects of excitotoxic hippocampal lesions

Neurofibrillary tangles in Alzheimer's disease are composed of hyperphosphorylated forms of the microtubule-associated protein tau. Based on biochemical criteria, several enzymes have emerged as potential tau protein kinases, including the extracellularly regulated kinases 1, 2 and 3. In situ hybridization was used to map the messenger RNA distribution of extracellularly regulated kinase 1, 2 and 3 in the adult rat brain and their response to excitotoxic hippocampal lesions was examined. Extracellularly regulated kinase 1 messenger RNA was uniformly expressed by glia, but was also present in the dentate gyrus and some other neuronal populations. Extracellularly regulated kinase 2 was exclusively neuronal and concentrated within the cortical laminae and the CA subfields of the hippocampal formation. Extracellularly regulated kinase 3 messenger RNA expression was similar to extracellularly regulated kinase 2 and was also present in neurons but the level of expression was lower. Extracellularly regulated kinases 2 and 3 messenger RNA expression was lost following excitotoxic injury, further supporting a neuronal localization. Extracellularly regulated kinase 1 messenger RNA expression appeared unaltered, suggesting a non-neuronal localization and lack of responsiveness to lesion at the level of transcription. By contrast, messenger RNA of sgk, a recently described serine/threonine kinase, was up-regulated by glial cells following excitotoxic injury. Based on their messenger RNA distribution, cellular localization and response to lesion, it is clear that each kinase may function differently in various signaling pathways. Extracellularly regulated kinase 2, however, is the only kinase with the proper messenger RNA distribution to contribute to neurofibrillary tangle formation in Alzheimer's disease.

Contribution of transplantations to the understanding of the role of the PLP gene

We present an overview of the results obtained in a cross-transplantation system using respectively controls, jimpy (jp), and shiverer mutant mice as donors and recipients. Homochronic transplantations (O days into O days) demonstrated that jp environment is non-toxic for non-jp cells and that, contrary to in vitro, jp oligodendrocytes phenotype cannot be modified by environmental factors at this age. Transplantations of embryonic fragments into the newborn brain demonstrated that in contrast to oligodendrocyte precursors contained in fragments of newborn tissue, jimpy embryonic stem cells are sensitive to environmental factors able to modulate the proportion of surviving oligodendrocytes. In addition, these series evidenced a disjunction between the surviving and the myelinating capacity of jp cells demonstrating a pleiotropic effect of the jp mutation on oligodendrocyte biology. Results are discussed with regards to the recent molecular biological finding on the role of the DM20/PLP gene.