Musashi1 (Msi1) is a mammalian neural RNA-binding protein highly enriched in neural precursor cells that are capable of generating both neurons and glia during embryonic and postnatal CNS development. Here, we identified Musashi2 (Msi2), a novel mammalian RNA-binding protein that exhibits high sequence similarity to Msi1. The Msi2 transcript appeared to be distributed ubiquitously in a wide variety of tissues, consistent with the mRNA distribution of its Xenopus homolog, xrp1. However, the present study revealed cell type-specific and developmentally regulated expression of Msi2 in the mammalian CNS. Interestingly, Msi2 was expressed prominently in precursor cells in the ventricular zone and subventricular zone with the same pattern as Msi1 throughout CNS development. In the postnatal and adult CNS, this concurrent expression of Msi2 and Msi1 was seen in cells of the astrocyte lineage, including ependymal cells, a possible source for postnatal CNS stem cells. During neurogenesis, the expression of both Msi2 and Msi1 was lost in most postmitotic neurons, whereas Msi2 expression persisted in a subset of neuronal lineage cells, such as parvalbumin-containing GABA neurons in the neocortex and neurons in several nuclei of the basal ganglia. Msi2 may have a unique role that is required for the generation and/or maintenance of specific neuronal lineages. Furthermore, in vitro studies showed that Msi2 and Msi1 have similar RNA-binding specificity. These two RNA-binding proteins may exert common functions in neural precursor cells by regulating gene expression at the post-transcriptional level.
There is an increasing interest in the role of RNA-binding proteins during neural development. Mouse-Musashi-1 (m-Msi-1) is a mouse neural RNA-binding protein with sequence similarity to Drosophila musashi (d-msi), which is essential for neural development. m-Msi-1 is highly enriched in neural precursor cells that are capable of generating both neurons and glia during embryonic CNS development. The present study characterized m-Msi-1-expressing cells in the postnatal and adult CNS. Postnatally, m-Msi-1 was expressed in proliferative neuronal precursors in the external granule cell layer of the cerebellum and in the anterior corner of the subventricular zone of the lateral ventricles. In gliogenesis, the persistent expression of m-Msi-1 was observed in cells of the astrocyte lineage ranging from proliferative glial precursors in the subventricular zone (SVZ) to differentiated astrocytes in the parenchyma. In addition, we showed that m-Msi-1 was still expressed in proliferating cells in the adult SVZ, which may contain neural precursor or stem cells. Another neural RNA-binding protein Hu (the mammalian homolog of a Drosophila neuronal RNAbinding protein Elav) was present in postmitotic neurons throughout the development of the CNS, and its pattern of expression was compared with that of m-Msi-1. These observations imply that these two RNA-binding proteins may be involved in the development of neurons and glia by regulating gene expression at the post-transcriptional level. Key words: RNA-binding proteins; postnatal CNS; mouseMusashi-1 (m-Msi-1); Hu; Drosophila musashi; neuronal and glial precursor cells; astrocyte lineageA number of transcription factors that f unction in the proliferation and differentiation of neural precursor cells have been identified. However, the recent discovery of neural-specific RNAbinding proteins raises the possibility that the development of neural cells from the precursors may also be regulated at the post-transcriptional level. We believe that at least two gene families are represented by the neural RNA-binding proteins found in both invertebrates and vertebrates (Okano, 1995;Sakakibara et al., 1996). One, the Elav family, includes Drosophila elav genes (Yao et al., 1993) and their mammalian homologs, the Hu genes (Szabo et al., 1991). Members of this family share extensive sequence similarity with one another, and they seem to be expressed mainly in postmitotic neurons (Okano and Darnell, 1997). The other Msi family is composed of Drosophila musashi (Nakamura et al., 1994), Xenopus laevis nrp-1 (Richter et al., 1990), and their mouse homolog mouse-musashi-1 (m-msi-1) (Sakakibara et al., 1996). d-Msi is required for the two successive asymmetric cell divisions of sensory organ precursors (Nakamura et al., 1994). In contrast to the Elav family, the members of the Msi family are expressed in the neural precursor cells at least during embryonic C NS development (Sakakibara et al., 1996). Although the molecular f unctions and the in vivo RNA targets of Msi and Elav families remain largel...
Hes1 is one of the basic helix-loop-helix transcription factors that regulate mammalian CNS development, and its loss- and gain-of-function phenotypes indicate that it negatively regulates neuronal differentiation. Here we report that Hes1(-/-) mice expressed both early (TuJ1 and Hu) and late (MAP2 and Neurofilament) neuronal markers prematurely, and that there were approximately twice the normal number of neurons in the Hes1(-/-) brain during early neural development. However, immunochemical analyses of sections and dissociated cells using neural progenitor markers, including nestin, failed to detect any changes in Hes1(-/-) progenitor population. Therefore, further characterization of neural progenitor cells that discriminated between multipotent and monopotent cells was performed using two culture methods, low-density culture, and a neurosphere assay. We demonstrate that the self-renewal activity of multipotent progenitor cells was reduced in the Hes1(-/-) brain, and that their subsequent commitment to the neuronal lineage was accelerated. The Hes1(-/-) neuronal progenitor cells were functionally abnormal, in that they divided, on average, only once, and then generated two neurons, (instead of one progenitor cell and one neuron), whereas wild-type progenitor cells divided more. In addition, some Hes1(-/-) progenitors followed an apoptotic fate. The overproduction of neurons in the early Hes1(-/-) brains may reflect this premature and immediate generation of neurons as well as a net increase in the number of neuronal progenitor cells. Taken together, we conclude that Hes1 is important for maintaining the self-renewing ability of progenitors and for repressing the commitment of multipotent progenitor cells to a neuronal fate, which is critical for the correct number of neurons to be produced and for the establishment of normal neuronal function.
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