M. Tekaya

Bmi1 Loss Produces an Increase in Astroglial Cells and a Decrease in Neural Stem Cell Population and Proliferation

The polycomb transcriptional repressor Bmi1 promotes cell cycle progression, controls cell senescence, and is implicated in brain development. Loss of Bmi1 leads to a decreased brain size and causes progressive ataxia and epilepsy. Recently, Bmi1 was shown to control neural stem cell (NSC) renewal. However, the effect of Bmi1 loss on neural cell fate in vivo and the question whether the action of Bmi1 was intrinsic to the NSCs remained to be investigated. Here, we show that Bmi1 is expressed in the germinal zone in vivo and in NSCs as well as in progenitors proliferating in vitro, but not in differentiated cells. Loss of Bmi1 led to a decrease in proliferation in zones known to contain progenitors: the newborn cortex and the newborn and adult subventricular zone. This decrease was accentuated in vitro, where we observed a drastic reduction in NSC proliferation and renewal because of NSC-intrinsic effects of Bmi1 as shown by the means of RNA interference. Bmi1-/- mice also presented more astrocytes at birth, and a generalized gliosis at postnatal day 30. At both stages, colocalization of bromodeoxyuridine and GFAP demonstrated that Bmi1 loss did not prevent astrocyte precursor proliferation. Supporting these observations, Bmi1-/- neurospheres generate preferentially astrocytes probably attributable to a different responsiveness to environmental factors. Bmi1 is therefore necessary for NSC renewal in a cell-intrinsic mode, whereas the altered cell pattern of the Bmi1-/- brain shows that in vivo astrocyte precursors can proliferate in the absence of Bmi1.

High Yield of Cells Committed to the Photoreceptor Fate from Expanded Mouse Retinal Stem Cells

The purpose of the present work was to generate, from retinal stem cells (RSCs), a large number of cells committed toward the photoreceptor fate in order to provide an unlimited cell source for neurogenesis and transplantation studies. We expanded RSCs (at least 34 passages) sharing characteristics of radial glial cells and primed the cells in vitro with fibroblast growth factor (FGF)‐2 for 5 days, after which cells were treated with the B27 supplement to induce cell differentiation and maturation. Upon differentiation, cells expressed cell type‐specific markers corresponding to neurons and glia. We show by immunocytochemistry analysis that a subpopulation of differentiated cells was committed to the photoreceptor lineage given that these cells expressed the photoreceptor proteins recoverin, peripherin, and rhodopsin in a same ratio. Furthermore, cells infected during the differentiation procedure with a lentiviral vector expressing green fluorescent protein (GFP) under the control of either the rhodopsin promoter or the interphotoreceptor retinoid‐binding protein (IRBP) promoter, expressed GFP. FGF‐2 priming increased neuronal differentiation while decreasing glia generation. Reverse transcription‐polymerase chain reaction analyses revealed that the differentiated cells expressed photoreceptor‐specific genes such as Crx, rhodopsin, peripherin, IRBP, and phosphodiesterase‐α. Quantification of the differentiated cells showed a robust differentiation into the photoreceptor lineage: Approximately 25%–35% of the total cells harbored photoreceptor markers. The generation of a significant number of nondifferentiated RSCs as well as differentiated photoreceptors will enable researchers to determine via transplantation studies which cells are the most adequate to integrate a degenerating retina.

Retinal Degeneration Progression Changes Lentiviral Vector Cell Targeting in the Retina

In normal mice, the lentiviral vector (LV) is very efficient to target the RPE cells, but transduces retinal neurons well only during development. In the present study, the tropism of LV has been investigated in the degenerating retina of mice, knowing that the retina structure changes during degeneration. We postulated that the viral transduction would be increased by the alteration of the outer limiting membrane (OLM). Two different LV pseudotypes were tested using the VSVG and the Mokola envelopes, as well as two animal models of retinal degeneration: light-damaged Balb-C and Rhodopsin knockout (Rho-/-) mice. After light damage, the OLM is altered and no significant increase of the number of transduced photoreceptors can be obtained with a LV-VSVG-Rhop-GFP vector. In the Rho-/- mice, an alteration of the OLM was also observed, but the possibility of transducing photoreceptors was decreased, probably by ongoing gliosis. The use of a ubiquitous promoter allows better photoreceptor transduction, suggesting that photoreceptor-specific promoter activity changes during late stages of photoreceptor degeneration. However, the number of targeted photoreceptors remains low. In contrast, LV pseudotyped with the Mokola envelope allows a wide dispersion of the vector into the retina (corresponding to the injection bleb) with preferential targeting of Müller cells, a situation which does not occur in the wild-type retina. Mokola-pseudotyped lentiviral vectors may serve to engineer these glial cells to deliver secreted therapeutic factors to a diseased area of the retina.

Lentiviral Gene Transfer-Mediated Cone Vision Restoration in RPE65 Knockout Mice

In photoreceptors, the photo-isomerization of 11-cis-retinal chromophore into alltrans-retinal is the first step of the photo-transduction cascade. To regain light sensitivity after light exposure, photoreceptors need to recycle their chromophore. This is done through the retinoid visual cycle, which involves a series of enzymatic reactions taking place in photoreceptors as well as in the cells of the retinal-pigmented epithelium (RPE) (reviewed by JC Saari (Saari, 2000)). Rpe65, a gene specifically expressed in RPE, has recently been identified as the isomerohydrolase (Jin et al., 2005; Moiseyev et al., 2005), one of the key enzymes of the retinoid visual cycle. As a consequence, when Rpe65 is mutated, the retinoid visual cycle is disrupted, and the sensitivity to light of the photoreceptors is drastically reduced. In humans, mutations in the RPE65 gene lead to Leber congenital amaurosis (LCA), the most severe form of retinitis pigmentosa (Gu et al., 1997; Hanein et al., 2004). Several animal models of these mutations have already been characterized, and, among them, the RPE65 deficient Briard dog (Aguirre et al., 1998), and the Rpe65-/(Redmond et al., 1998) mouse have been broadly used for experimental studies. The restricted expression of RPE65 in RPE cells, the possibility to efficiently target these cells with viral vectors and the availability of animal models for RPE65 deficiency have made of RPE65 gene therapy a widely studied field of experimental medicine (reviewed by JW Bainbridge (Bainbridge et al., 2006)). Adeno-associated virus vectors (AAV) have been the most extensively used vectors for Rpe65 gene transfer. Several teams have demonstrated that AAV gene transfer of Rpe65 leads to restoration of the retinal function and vision rescue in the RPE65 deficient dog (Acland et al., 2001; Narfstrom et al., 2003; Le Meur et al., 2007). Furthermore, long-term studies in this canine model have shown that

BMI1 loss delays photoreceptor degeneration in Rd1 mice. Bmi1 loss and neuroprotection in Rd1 mice.

Retinitis pigmentosa (RP) is a heterogeneous group of genetic disorders leading to blindness, which remain untreatable at present. Rd1 mice represent a recognized model of RP, and so far only GDNF treatment provided a slight delay in the retinal degeneration in these mice. Bmi1, a transcriptional repressor, has recently been shown to be essential for neural stem cell (NSC) renewal in the brain, with an increased appearance of glial cells in vivo in Bmi1 knockout (Bmi1-/-) mice. One of the roles of glial cells is to sustain neuronal function and survival. In the view of a role of the retinal Miller glia as a source of neural protection in the retina, the increased astrocytic population in the Bmi1-/- brain led us to investigate the effect of Bmi1 loss in Rd1 mice. We observed an increase of Müller glial cells in Rd1-Bmi1-/- retinas compared to Rd1. Moreover, Rd1-Bmi1-/- mice showed 7-8 rows of photoreceptors at 30 days of age (P30), while in Rd1 littermates there was a complete disruption of the outer nuclear layer (ONL). Preliminary ERG results showed a responsiveness of Rd1-Bmi1-/- mice in scotopic vision at P35. In conclusion, Bmi1 loss prevented, or rescued, photoreceptors from degeneration to an unanticipated extent in Rd1 mice. In this chapter, we will first provide a brief review of our work on the cortical NSCs and introduce the Bmi1 oncogene, thus offering a rational to our observations on the retina.