Oligodendrocyte programmed cell death and central myelination deficiency induced in transgenic mice by synergism between c-Myc and Oct-6

RhoA-inhibiting NSAIDs promote axonal myelination after spinal cord injury

Nonsteroidal anti-inflammatory drugs (NSAIDs) are extensively used to relieve pain and inflammation in humans via cyclooxygenase inhibition. Our recent research suggests that certain NSAIDs including ibuprofen suppress intracellular RhoA signal and improve significant axonal growth and functional recovery following axonal injury in the CNS. Several NSAIDs have been shown to reduce generation of amyloid-beta42 peptide via inactivation of RhoA signal, supporting potent RhoA-repressing function of selected NSAIDs. In this report, we demonstrate that RhoA-inhibiting NSAIDs ibuprofen and indomethacin dramatically reduce cell death of oligodendrocytes in cultures or along the white matter tracts in rats with a spinal cord injury. More importantly, we demonstrate that treatments with the RhoA-inhibiting NSAIDs significantly increase axonal myelination along the white matter tracts following a traumatic contusion spinal cord injury. In contrast, non-RhoA-inhibiting NSAID naproxen does not have such an effect. Thus, our results suggest that RhoA inactivation with certain NSAIDs benefits recovery of injured CNS axons not only by promoting axonal elongation, but by enhancing glial survival and axonal myelination along the disrupted axonal tracts. This study, together with previous reports, supports that RhoA signal is an important therapeutic target for promoting recovery of injured CNS and that RhoA-inhibiting NSAIDs provide great therapeutic potential for CNS axonal injuries in adult mammals.

Astroglial c-Myc overexpression predisposes mice to primary malignant gliomas

Malignant astrocytomas are common human primary brain tumors that result from neoplastic transformation of astroglia or their progenitors. Here we show that deregulation of the c-Myc pathway in developing astroglia predisposes mice to malignant astrocytomas within 2-3 weeks of age. The genetically engineered murine (GEM) gliomas harbor a molecular signature resembling that of human primary glioblastoma multiforme, including up-regulation of epidermal growth factor receptor and Mdm2. The GEM gliomas seem to originate in an abnormal population of glial fibrillary acidic protein-expressing cells in the ventricular zone and, analogous to human glioblastomas, exhibit molecular and morphological heterogeneity. Levels of connexin 43 in the majority of the tumors are unaltered from normal tissue, indicating that GEM tumors have retained the capacity to establish syncytial networks. In line with this, individual glioma foci are composed of a mixture of actively proliferating cells expressing c-Myc and proliferating cell nuclear antigen and less dividing bystander cells that express glial fibrillary acidic protein and the broad complex tramtrack bric-a-brac/poxvirus and zinc finger domain protein HOF. A subset of the transgenic mice harbored, in addition to brain tumors, vestigial cerebellums in which granule cell migration and radial Bergman glial cell differentiation were disturbed. These observations argue for a window of vulnerability during astrocyte development where c-Myc overexpression is sufficient to trigger the neoplastic process, presumably by inducing the sustained growth of early astroglial cells. This is in contrast to most other transgenic studies in which c-Myc overexpression requires co-operating transgenes for rapid tumor induction.

Characterization of two novel nuclear BTB/POZ domain zinc finger isoforms. Association with differentiation of hippocampal neurons, cerebellar granule cells, and macroglia

BTB/POZ (broad complex tramtrack bric-a-brac/poxvirus and zinc finger) zinc finger factors are a class of nuclear DNA-binding proteins involved in development, chromatin remodeling, and cancer. However, BTB/POZ domain zinc finger factors linked to development of the mammalian cerebral cortex, cerebellum, and macroglia have not been described previously. We report here the isolation and characterization of two novel nuclear BTB/POZ domain zinc finger isoforms, designated HOF(L) and HOF(S), that are specifically expressed in early hippocampal neurons, cerebellar granule cells, and gliogenic progenitors as well as in differentiated glia. During embryonic development of the murine cerebral cortex, HOF expression is restricted to the hippocampal subdivision. Expression coincides with early differentiation of presumptive CA1 and CA3 pyramidal neurons and dentate gyrus granule cells, with a sharp decline in expression at the CA1/subicular border. By using bromodeoxyuridine labeling and immunohistochemistry, we show that HOF expression coincides with immature non-dividing cells and is down-regulated in differentiated cells, suggesting a role for HOF in hippocampal neurogenesis. Consistent with the postulated role of the POZ domain as a site for protein-protein interactions, both HOF isoforms are able to dimerize. The HOF zinc fingers bind specifically to the binding site for the related promyelocytic leukemia zinc finger protein as well as to a newly identified DNA sequence.

Disturbed oligodendrocyte development and recovery from hypomyelination in a c-myc transgenic mouse mutant

The complexity of interactions underlying the elaboration of myelin has been extensively demonstrated. We provide evidence that signals promoting myelination are not confined to the normal developmental time window for myelination and persist well into adult life. The 2-50 mutant, described previously, carries a c-myc transgene regulated by a myelin basic protein promoter. This mutant is characterised by severe hypomyelination and abnormal oligodendrocytes in early life, followed by loss of the phenotype and normal longevity. We show that c-myc expression in early oligodendrocyte development results in a substantial reduction of cells of this lineage. However, apparent complete recovery, associated with loss of c-myc expression, axonal survival, and gradual myelin accumulation, is observed by 4 months of age. Thus, stimulation of myelination continues during adult life until normal myelin levels are established. We propose that this mutant may contribute to the characterisation of oligodendrocyte responses to myelinating signals.

Expression of transcription factors during oligodendroglial development

Transcription factors coordinate the orderly changes of gene expression that underlie all developmental processes including those of oligodendrocytes. In comparison to other systems, relatively little is known about the role of transcription factors during oligodendrocyte development. However, recent years have seen the identification of oligodendroglial transcription factors that, not surprisingly, belong to the same gene families that are also important in other tissues or cell lineages. Some transcription factors such as the bHLH (basic helix-loop-helix) proteins Olig-1 and Olig-2 or the high-mobility-group protein Sox10 are expressed already early during development of the oligodendrocyte lineage, whereas expression of other transcription factors such as the homeodomain protein Gtx/Nkx6.2 only start at the time of terminal differentiation. Once turned on, expression of these proteins can either be permanent as in the above-mentioned cases or transient as exemplified by the POU domain proteins Tst-1/Oct6/SCIP, Brn-1 and Brn-2. Analyses of these transcription factors has already led to the identification of important principles such as functional redundancy between co-expressed proteins, unexpected divergence in the transcription factor repertoire of oligodendrocytes and Schwann cells, and equally unsuspected similarities in transcription factor usage between oligodendrocytes and neurons. Although so far only a small percentage of oligodendroglial transcription factors has been identified, these are excellent candidates for regulators of cell type specification, lineage progression, and terminal differentiation.