Expression and function of microRNA-9 in the mid-hindbrain area of embryonic chick.

BACKGROUND: MiR-9 is a small non-coding RNA that is highly conserved between species and primarily expressed in the central nervous system (CNS). It is known to influence proliferation and neuronal differentiation in the brain and spinal cord of different vertebrates. Different studies have pointed to regional and species-specific differences in the response of neural progenitors to miR-9. METHODS: In ovo and ex ovo electroporation was used to overexpress or reduce miR-9 followed by mRNA in situ hybridisation and immunofluorescent stainings to evaluate miR- expression and the effect of changed miR-9 expression. RESULTS: We have investigated the expression and function of miR-9 during early development of the mid-hindbrain region (MH) in chick. Our analysis reveals a closer relationship of chick miR-9 to mammalian miR-9 than to fish and a dynamic expression pattern in the chick neural tube. Early in development, miR-9 is diffusely expressed in the entire brain, bar the forebrain, and it becomes more restricted to specific areas of the CNS at later stages. MiR-9 overexpression at HH9-10 results in a reduction of FGF8 expression and premature neuronal differentiation in the mid-hindbrain boundary (MHB). Within the midbrain miR-9 does not cause premature neuronal differentiation it rather reduces proliferation in the midbrain. CONCLUSION: Our findings indicate that miR-9 has regional specific effects in the developing mid-hindbrain region with a divergence of response of regional progenitors.

MicroRNA-9 directs late organizer activity of the midbrain-hindbrain boundary.

The midbrain-hindbrain boundary (MHB) is a long-lasting organizing center in the vertebrate neural tube that is both necessary and sufficient for the ordered development of midbrain and anterior hindbrain (midbrain-hindbrain domain, MH). The MHB also coincides with a pool of progenitor cells that contributes neurons to the entire MH. Here we show that the organizing activity and progenitor state of the MHB are co-regulated by a single microRNA, miR-9, during late embryonic development in zebrafish. Endogenous miR-9 expression, initiated at late stages, selectively spares the MHB. Gain- and loss-of-function studies, in silico predictions and sensor assays in vivo demonstrate that miR-9 targets several components of the Fgf signaling pathway, thereby delimiting the organizing activity of the MHB. In addition, miR-9 promotes progression of neurogenesis in the MH, defining the MHB progenitor pool. Together, these findings highlight a previously unknown mechanism by which a single microRNA fine-tunes late MHB coherence via its co-regulation of patterning activities and neurogenesis.

Further increase in thermostability of Moloney murine leukemia virus reverse transcriptase by mutational combination.

We previously generated a highly thermostable triple variant of Moloney murine leukemia virus reverse transcriptase, MM3 (E286R/E302K/L435R), by introducing positive charges by site-directed mutagenesis at positions that have been implicated in the interaction with template-primer (Yasukawa et al., (2010) J. Biotechnol., 150, 299-306). In this study, we attempted to further increase the thermostability of MM3. Twenty-nine mutations were newly designed, focusing on the number of surface charge, stabilization of hydrophobic core, and introduction of salt bridge. The corresponding 29 single variants were produced in Escherichia coli and characterized for activity and stability. Six mutations (A32V, L41D, L72R, I212R, L272E and W388R) were selected as the candidates for further stabilize MM3. Fifteen multiple variants were designed by combining two or more of the six mutations with the MM3 mutations, produced and characterized. The sextuple variant MM3.14 (A32V/L72R/E286R/E302K/W388R/L435R) exhibited higher thermostability than MM3.

Photoactivation of the CreER T2 recombinase for conditional site-specific recombination with high spatiotemporal resolution.

We implemented a noninvasive optical method for the fast control of Cre recombinase in single cells of a live zebrafish embryo. Optical uncaging of the caged precursor of a nonendogeneous steroid by one- or two-photon illumination was used to restore Cre activity of the CreER(T2) fusion protein in specific target cells. This method labels single cells irreversibly by inducing recombination in an appropriate reporter transgenic animal and thereby can achieve high spatiotemporal resolution in the control of gene expression. This technique could be used more generally to investigate important physiological processes (e.g., in embryogenesis, organ regeneration, or carcinogenesis) with high spatiotemporal resolution (single cell and 10-min scales).

her5 expression reveals a pool of neural stem cells in the adult zebrafish midbrain.

Current models of vertebrate adult neural stem cells are largely restricted to the rodent forebrain. To extract the general mechanisms of neural stem cell biology, we sought to identify new adult stem cell populations, in other model systems and/or brain areas. The teleost zebrafish appears to be an ideal system, as cell proliferation in the adult zebrafish brain is found in many more niches than in the mammalian brain. As a starting point towards identifying stem cell populations in this system, we used an embryonic neural stem cell marker, the E(spl) bHLH transcription factor Her5. We demonstrate that her5 expression is not restricted to embryonic neural progenitors, but also defines in the adult zebrafish brain a new proliferation zone at the junction between the mid- and hindbrain. We show that adult her5-expressing cells proliferate slowly, self-renew and express neural stem cell markers. Finally, using in vivo lineage tracing in her5:gfp transgenic animals, we demonstrate that the her5-positive population is multipotent, giving rise in situ to differentiated neurons and glia that populate the basal midbrain. Our findings conclusively identify a new population of adult neural stem cells, as well as their fate and their endogenous environment, in the intact vertebrate brain. This cell population, located outside the forebrain, provides a powerful model to assess the general mechanisms of vertebrate neural stem cell biology. In addition, the first transcription factor characteristic of this cell population, Her5, points to the E(Spl) as a promising family of candidate adult neural stem cell regulators.

Inhibition of neurogenesis at the zebrafish midbrain-hindbrain boundary by the combined and dose-dependent activity of a new hairy/E(spl) gene pair.

The intervening zone (IZ) is a pool of progenitor cells located at the midbrain-hindbrain boundary (MHB) and important for MHB maintenance, midbrain-hindbrain growth and the generation of midbrain-hindbrain neurons. Recently, we implicated the Hairy/E(spl) transcription factor Her5 in the formation of the medial (most basal) part of the IZ (MIZ) in zebrafish; the molecular bases for lateral IZ (LIZ) formation, however, remain unknown. We now demonstrate that her5 is physically linked to a new family member, him, displaying an identical MHB expression pattern. Using single and double knockdowns of him and her5, as well as a him+her5 deletion mutant background (b404), we demonstrate that Him and Her5 are equally necessary for MIZ formation, and that they act redundantly in LIZ formation in vivo. We show that these processes do not involve cross-regulation between Him and Her5 expression or activities, although Him and Her5 can heterodimerize with high affinity. Increasing the function of one factor when the other is depleted further shows that Him and Her5 are functionally interchangeable. Together, our results demonstrate that patterning and neurogenesis are integrated by the her5-him gene pair to maintain a progenitor pool at the embryonic MHB. We propose a molecular mechanism for this process where the global 'Him+Her5' activity inhibits ngn1 expression in a dose-dependent manner and through different sensitivity thresholds along the medio-lateral axis of the neural plate.

Intra- and interspecies interactions between prion proteins and effects of mutations and polymorphisms.

Recently, crystallization of the prion protein in a dimeric form was reported. Here we show that native soluble homogeneous FLAG-tagged prion proteins from hamster, man and cattle expressed in the baculovirus system are predominantly dimeric. The PrP/PrP interaction was confirmed in Semliki Forest virus-RNA transfected BHK cells co-expressing FLAG- and oligohistidine-tagged human PrP. The yeast two-hybrid system identified the octarepeat region and the C-terminal structured domain (aa90-aa230) of PrP as PrP/PrP interaction domains. Additional octarepeats identified in patients suffering from fCJD reduced (wtPrP versus PrP + 9OR) and completely abolished (PrP + 9OR versus PrP + 9OR) the PrP/PrP interaction in the yeast two-hybrid system. In contrast, the Met/Val polymorphism (aa129), the GSS mutation Pro102Leu and the FFI mutation Asp178Asn did not affect PrP/PrP interactions. Proof of interactions between human or sheep and bovine PrP, and sheep and human PrP, as well as lack of interactions between human or bovine PrP and hamster PrP suggest that interspecies PrP interaction studies in the yeast two-hybrid system may serve as a rapid pre-assay to investigate species barriers in prion diseases.

The 37 kDa/67 kDa laminin receptor is required for PrP(Sc) propagation in scrapie-infected neuronal cells.

The accumulation of PrP(Sc) in scrapie-infected neuronal cells has been prevented by three approaches: (i) transfection of ScMNB cells with an antisense laminin receptor precursor (LRP) RNA-expression plasmid, (ii) transfection of ScN2a cells and ScGT1 cells with small interfering RNAs (siRNAs) specific for the LRP mRNA, and (iii) incubation of ScN2a cells with an anti-LRP/LR antibody. LRP antisense RNA and LRP siRNAs reduced LRP/LR expression and inhibited the accumulation of PrP(Sc) in these cells. The treatments also reduced PrP(c) levels. The anti-LRP/LR antibody, W3, abolished PrP(Sc) accumulation and reduced PrP(c) levels after seven days of incubation. Cells remained free of PrP(Sc) after being cultured for 14 additional days without the antibody, whereas the PrP(c) level was restored. Our results demonstrate the necessity of the laminin receptor (LRP/LR) for PrP(Sc) propagation in cultured cells and suggest that LRP/LR-specific antibodies could be used as powerful therapeutic tools in the treatment of transmissible spongiform encephalopathies.

Different isoforms of the non-integrin laminin receptor are present in mouse brain and bind PrP.

The prion protein (PrP) plays a central role in prion diseases, and identifying its cellular receptor appears to be of crucial interest. We previously showed in the yeast two-hybrid system that PrP interacts with the 37 kDa precursor (LRP) of the high affinity 67 kDa laminin receptor (LR), which acts as the cellular receptor of PrP in cellular models. However, among the various isoforms of the receptor that have been identified so far, those which are present in the central nervous system and which bind PrP are still unknown. In this study, we have purified mouse brain fractions enriched in the laminin receptor and have performed overlay assays in order to identify those isoforms that interact with the prion protein. We demonstrate (i) the presence, in mouse brain, of several isoforms of the LRP/LR corresponding to different maturation states of the receptor (44, 60, 67 and 220 kDa) and (ii) the binding of all of these isoforms to PrP. Our data strongly support a physiological role of the laminin receptor/PrP interaction in the brain and highlight its relevance for transmissible spongiform encephalopathies.

Knock-down of the 37-kDa/67-kDa laminin receptor in mouse brain by transgenic expression of specific antisense LRP RNA.

The 37-kDa/67-kDa laminin receptor (LRP/LR) plays a major role in the propagation of PrPSc, the abnormal form of the prion protein. In order to ablate the expression of LRP/LR in mouse brain we generated transgenic mice ectopically expressing antisense LRP RNA in the brain under control of the neuron-specific enolase (NSE) promoter. Hemizygous transgenic mice TgN(NSEasLRP)2 showed a significant reduction of LRP/LR protein levels in hippocampal and cerebellar brain regions. These mice might act as powerful tools to investigate the role of the laminin receptor in scrapie pathogenesis.

High-level expression and characterization of a glycosylated covalently linked dimer of the prion protein.

There is evidence that prion protein dimers may be involved in the formation of the scrapie prion protein, PrP(Sc), from its normal (cellular) form, PrP(c). Recently, the crystal structure of the human prion protein in a dimeric form was reported. Here we report for the first time the overexpression of a human PrP dimer covalently linked by a FLAG peptide (PrP::FLAG::PrP) in the methylotrophic yeast Pichia pastoris. FLAG-tagged human PrP (aa1-aa253) (huPrP::FLAG) was also expressed in the same system. Treatment with tunicamycin and endoglycosidase H showed that both fusion proteins are expressed as various glycoforms. Both PrP proteins were completely digested by proteinase K (PK), suggesting that the proteins do not have a PrP(Sc) structure and are not infectious. Plasma membrane fractionation revealed that both proteins are transported to the plasma membrane of the cell. The glycosylated proteins might act as powerful tools for crystallization trials, PrP(c)/PrP(Sc) conversion studies and other applications in the life cycle of prions.