Hirohide Takebayashi

Combinatorial Roles of Olig2 and Neurogenin2 in the Coordinated Induction of Pan-Neuronal and Subtype-Specific Properties of Motoneurons

Distinct classes of neurons are generated at defined times and positions during development of the nervous system. It remains elusive how specification of neuronal identity coordinates with acquisition of pan-neuronal properties. Here we show that basic helix-loop-helix (bHLH) transcription factors Olig2 and Neurogenin2 (Ngn2) play vital roles in the coordinated induction of pan-neuronal and subtype-specific properties of motoneurons. Olig2 and Ngn2 are specifically coexpressed in motoneuron progenitors. Misexpression studies in chick demonstrate the specific, combinatorial actions of Olig2 and Ngn2 in motoneuron generation. Our results further revealed crossregulatory interactions between bHLH and homeodomain transcription factors in the specification of motoneurons. We suggest that distinct classes of transcription factors collaborate to generate motoneurons in the ventral neural tube.

The Basic Helix-Loop-Helix Factor Olig2 Is Essential for the Development of Motoneuron and Oligodendrocyte Lineages

Sonic hedgehog (Shh), an organizing signal from ventral midline structures, is essential for the induction and maintenance of many ventral cell types in the embryonic neural tube. Olig1 and Olig2 are related basic helix-loop-helix factors induced by Shh in the ventral neural tube. Although expression analyses and gain-of-function experiments suggested that these factors were involved in motoneuron and oligodendrocyte development, they do not clearly define the functional differences between Olig1 and Olig2. We generated mice with a homozygous inactivation of Olig2. These mice did not feed and died on the day of birth. In the spinal cord of the mutant mice, motoneurons are largely eliminated and oligodendrocytes are not produced. Olig2(-/-) neuroepithelial cells in the ventral spinal cord failed to differentiate into motoneurons or oligodendrocytes and expressed an astrocyte marker, S100beta, at the time of oligodendrogenesis. Olig1 or Olig3, other family members, were expressed in the descendent cells that should have expressed Olig2. We concluded that Olig2 is an essential transcriptional regulator in motoneuron and oligodendrocyte development. Our data provide the first evidence that a single gene mutation leads to the loss of two cell types, motoneuron and oligodendrocyte.

Dynamic expression of basic helix-loop-helix Olig family members: implication of Olig2 in neuron and oligodendrocyte differentiation and identification of a new member, Olig3.

Basic helix-loop-helix (bHLH) transcription factors have been shown to be essential for specification of various cell types. Here, we describe a novel bHLH family consisting of three members, two of which (Olig1, Olig2) are expressed in a nervous tissue-specific manner, whereas the third, Olig3 is found mainly in non-neural tissues. Olig1 and Olig2, which recently have been implicated in oligodendrogenesis, are expressed in the region of the ventral ventricular zone of late embryonic spinal cord where oligodendrocyte progenitors appear. In the embryonic brain, the Olig2 expression domain is broader than that of Olig1 and does not overlap with an oligodendrocyte progenitor marker, CNP. Furthermore, Olig2 is expressed in most cells in the ventral half of the early embryonic spinal cord, which do not yet express an early neuronal marker TuJ1. These results indicate that Olig2 expression is not limited to the oligodendrocyte lineage but includes immature neuronal progenitors and multipotential neuron/glia progenitors as well as embryonic olfactory neurons.

Progeny of Olig2-Expressing Progenitors in the Gray and White Matter of the Adult Mouse Cerebral Cortex

Despite their abundance, still little is known about the rather frequent, constantly proliferating progenitors spread throughout the adult mouse brain parenchyma. The majority of these progenitors express the basic-helix-loop-helix transcription factor Olig2, and their number further increases after injury. Here, we examine the progeny of this progenitor population by genetic fate mapping using tamoxifen-inducible Cre-recombination in the Olig2 locus to turn on permanent reporter gene expression in the adult brain. Consistent with Olig2 expression in proliferating NG2+ progenitors, most reporter+ cells seen shortly after initiating recombination at adult stages incorporated BrdU and contained the proteoglycan NG2 in both the gray (GM) and the white matter (WM) of the cerebral cortex. However, at longer time points after induction, we observed profound differences in the identity of reporter+ cells in the WM and GM. Whereas most of the Olig2+ progenitors had generated mature, myelinating oligodendrocytes in the WM, hardly any reporter+ cells showing mature oligodendrocyte characteristics were detectable even up to 6 months after recombination in the GM. In the GM, most reporter+ cells remained NG2+, even after injury, but stopped proliferating rather soon after recombination. Thus, our results demonstrate the continuous generation of mature, myelinating oligodendrocytes in the WM, whereas cells in the GM generated mostly postmitotic NG2+ glia.

Physiologically Distinct Temporal Cohorts of Cortical Interneurons Arise from Telencephalic Olig2-Expressing Precursors

Inhibitory GABAergic interneurons of the mouse neocortex are a highly heterogeneous population of neurons that originate from the ventral telencephalon and migrate tangentially up into the developing cortical plate. The majority of cortical interneurons arise from a transient embryonic structure known as the medial ganglionic eminence (MGE), but how the remarkable diversity is specified in this region is not known. We have taken a genetic fate mapping strategy to elucidate the temporal origins of cortical interneuron subtypes within the MGE. We used an inducible form of Cre under the regulation of Olig2, a basic helix-loop-helix transcription factor highly expressed in neural progenitors of the MGE. We observe that the physiological subtypes of cortical interneurons are, to a large degree, unique to their time point of generation.

Anti-Human Olig2 Antibody as a Useful Immunohistochemical Marker of Normal Oligodendrocytes and Gliomas

Olig2 is a recently identified transcription factor involved in the phenotype definition of cells in the oligodendroglial lineage. The expression of Olig2 transcript has been demonstrated in human oligodendroglial tumors, although the protein expression has not been studied extensively. We developed a polyclonal antibody to human Olig2 and analyzed it immunohistochemically. The antibody depicted a single distinct band of predicted molecular weight by Western blotting, and did not cross-react with human Olig1. In normal human brain tissue, the nuclei of oligodendrocytes of interfascicular, perivascular, and perineuronal disposition were clearly labeled by the antibody. Similarly, the nuclei of oligodendroglial tumors were labeled. There was no apparent correlation between the staining intensity and histological grade. Astrocytic components within the tumors were generally less or not stained. Astrocytic tumors were also positive with the Olig2 antiserum to a lesser extent, and the difference between oligodendroglial and astrocytic tumors was demonstrated by a statistical analysis. Olig2 and glial fibrillary acidic protein were expressed in a mutually exclusive manner, and Olig2 expression was cell-cycle related. Neither central neurocytoma nor schwannoma cases were stained. Our antibody was demonstrated to be useful in recognizing normal oligodendrocytes on paraffin sections, and applicable in diagnosis of some brain tumors.

Genetic fate mapping of Olig2 progenitors in the injured adult cerebral cortex reveals preferential differentiation into astrocytes

Olig2 is a basic helix‐loop‐helix (bHLH) transcription factor essential for development of motoneurons and oligodendrocytes. It is known that Olig2+ cells persist in the central nervous system (CNS) from embryonic to adult stages and that the number of Olig2+ progenitors increases in the injured adult CNS. Recent studies have demonstrated an inhibitory action of Olig2 on neurogenesis in adult CNS, but the fate of Olig2+ cells in the injured state remains largely unknown. To trace directly the fate of Olig2 cells in the adult cerebral cortex after injury, we employed the CreER/loxP system to target the olig2 locus. In this genetic tracing study, green fluorescent protein (GFP) reporter‐positive cells labeled after cryoinjury coexpressed glial fibrillary acidic protein (GFAP), an astrocytic marker. Electron microscopy also showed that GFP+ cells have the ultrastructural characteristics of astrocytes. Furthermore, GFP+ cells labeled before injury, most of which had been NG2 cells, also produced bushy astrocytes. Here we show direct evidence that Olig2+ cells preferentially differentiate into astrocytes, which strongly express GFAP, in response to injury in the adult cerebral cortex. These results suggest that reactive astrocytes, known to be the main contributors to glial scars, originate, at least in part, from Olig2+ cells.

Evidence for a second wave of oligodendrogenesis in the postnatal cerebral cortex of the mouse.

The existing view is that cortical oligodendrocytes (OLs) in rodents are born from the cortical subventricular zone (SVZ) after birth, but recent data suggest that many forebrain oligodendrocyte progenitor cells (OPCs) are specified much earlier (between E9.5 and E13.5 in the mouse) in the ventricular zone of the ventral forebrain under the control of sonic hedgehog (Shh) and migrate into the cortex afterward. We examined expression of specific early OL markers (PDGFRα, PLP/DM20, Olig2, and NG2) in the developing forebrain to clarify this issue. We propose that OPCs colonize the developing cortex in two temporally distinct waves. The gray matter is at least partially populated by a first wave of OPCs that arises in the medial ganglionic eminence and the entopeduncular area and spreads into the cortex via the developing cortical plate. The cerebral cortex benefits from the second wave of OPCs coming from residential SVZ. In the second wave, there might be two different types of precursor cells: PLP/DM20+ cells populating only inner layers and PDGFRα+ cells, which might eventually myelinate the outer regions as well.

Regional- and temporal-dependent changes in the differentiation of Olig2 progenitors in the forebrain, and the impact on astrocyte development in the dorsal pallium.

Olig2 is a basic helix-loop-helix transcription factor essential for oligodendrocyte and motoneuron development in the spinal cord. Olig2-positive (Olig2+) cells in the ventricular zone of the ventral telencephalon have been shown to differentiate into GABAergic and cholinergic neurons. However, the fate of Olig2 lineage cells in the postnatal forebrain has not been fully described and Olig2 may regulate the development of both astrocytes and oligodendrocytes. Here, we examined the fate of embryonic Olig2+ progenitors using a tamoxifen-inducible Cre/loxP system. Using long-term lineage tracing, Olig2+ cells in the early fetal stage primarily differentiated into GABAergic neurons in the adult telencephalon, while those in later stages gave rise to macroglial cells, both astrocytes and oligodendrocytes. Olig2+ progenitors in the diencephalon developed into oligodendrocytes, as observed in the spinal cord, and a fraction developed into glutamatergic neurons. Olig2 lineage oligodendrocytes tended to form clusters, probably due to local proliferation at the site of terminal differentiation. In spite of the abundance of Olig2 lineage GABAergic neurons in the normal neocortex, GABAergic neurons seemed to develop at normal density in the Olig2 deficient mouse. Thus, Olig2 is dispensable for GABAergic neuron specification. In contrast, at the late fetal stage in the Olig2 deficient mouse, astrocyte development was retarded in the dorsal neocortex, but not in the basal forebrain. Olig2 functions, therefore, in gliogenesis in the dorsal pallium. Short-term lineage tracing experiments revealed that the majority of late Olig2+ cells were not direct descendants of early Olig2+ progenitors in the fetal forebrain. These observations indicate that embryonic Olig2+ progenitor cells change their differentiative properties during development, and also that Olig2 plays a role in astrocyte development in a region-specific manner.

Non-overlapping expression of Olig3 and Olig2 in the embryonic neural tube

Olig family is a novel sub-family of basic helix-loop-helix transcription factors recently identified. Olig1 and Olig2 were first reported to promote oligodendrocyte differentiation, and later Olig2 was reported to be involved in motoneuron specification as well. Olig3 was isolated as a third member of Olig family, but its precise expression pattern is poorly understood. Here, we describe detailed Olig3 expression analyses in the neural tube of embryonic mice. Olig3 was first detected in the dorsal neural tube from the midbrain/hindbrain boundary to the spinal cord. In E11.5 spinal cord, Olig3 was transiently expressed in the lateral margin of the subventricular zone as three ventral clusters at the level of the p3, p2 and p0 domains, as well as in the dorsal neural tube. Olig3 was co-expressed with Nkx2.2 in the lateral margin of the p3 domain. In forebrain, Olig3 was expressed in the dorsal thalamus while Olig2 was complementarily expressed in the ventral thalamus with an adjacent boundary at E12.5. Olig3 is specifically and transiently expressed in different types of progenitors of embryonic central nervous system and then disappears in the course of development.