Renaud Vandenbosch

Cell "circadian" cycle: new role for mammalian core clock genes.

In mammals, 24 hours rhythms are organized as a biochemical network of molecular clocks that are operative in all tissues, with the master clock residing in the hypothalamic suprachiasmatic nucleus (SCN). The core pacemakers of these clocks consist of auto-regulatory transcriptional/post-transcriptional feedback loops. Several lines of evidence suggest the existence of a crosstalk between molecules that are responsible for the generation of circadian rhythms and molecules that control the cell cycle progression. In addition, highly specialized cell cycle checkpoints involved in DNA repair after damage seem also, at least in part, mediated by clock proteins. Recent studies have also highlighted a putative connection between clock protein dysfunction and cancer progression. This review discusses the intimate relation that exists between cell cycle progression and components of the circadian machinery.

Cdk2 is critical for proliferation and self-renewal of neural progenitor cells in the adult subventricular zone

We investigated the function of cyclin-dependent kinase 2 (Cdk2) in neural progenitor cells during postnatal development. Chondroitin sulfate proteoglycan (NG2)–expressing progenitor cells of the subventricular zone (SVZ) show no significant difference in density and proliferation between Cdk2−/− and wild-type mice at perinatal ages and are reduced only in adult Cdk2−/− mice. Adult Cdk2−/− SVZ cells in culture display decreased self-renewal capacity and enhanced differentiation. Compensatory mechanisms in perinatal Cdk2−/− SVZ cells, which persist until postnatal day 15, involve increased Cdk4 expression that results in retinoblastoma protein inactivation. A subsequent decline in Cdk4 activity to wild-type levels in postnatal day 28 Cdk2−/− cells coincides with lower NG2+ proliferation and self-renewal capacity similar to adult levels. Cdk4 silencing in perinatal Cdk2−/− SVZ cells abolishes Cdk4 up-regulation and reduces cell proliferation and self- renewal to adult levels. Conversely, Cdk4 overexpression in adult SVZ cells restores proliferative capacity to wild-type levels. Thus, although Cdk2 is functionally redundant in perinatal SVZ, it is important for adult progenitor cell proliferation and self-renewal through age-dependent regulation of Cdk4.

Period 2 regulates neural stem/progenitor cell proliferation in the adult hippocampus

BackgroundNewborn granule neurons are generated from proliferating neural stem/progenitor cells and integrated into mature synaptic networks in the adult dentate gyrus of the hippocampus. Since light/dark variations of the mitotic index and DNA synthesis occur in many tissues, we wanted to unravel the role of the clock-controlled Period2 gene (mPer2) in timing cell cycle kinetics and neurogenesis in the adult DG.ResultsIn contrast to the suprachiasmatic nucleus, we observed a non-rhythmic constitutive expression of mPER2 in the dentate gyrus. We provide evidence that mPER2 is expressed in proliferating neural stem/progenitor cells (NPCs) and persists in early post-mitotic and mature newborn neurons from the adult DG. In vitro and in vivo analysis of a mouse line mutant in the mPer2 gene (Per2Brdm1), revealed a higher density of dividing NPCs together with an increased number of immature newborn neurons populating the DG. However, we showed that the lack of mPer2 does not change the total amount of mature adult-generated hippocampal neurons, because of a compensatory increase in neuronal cell death.ConclusionTaken together, these data demonstrated a functional link between the constitutive expression of mPER2 and the intrinsic control of neural stem/progenitor cells proliferation, cell death and neurogenesis in the dentate gyrus of adult mice.

Ototoxic drugs: Difference in sensitivity between mice and guinea pigs

The development of experimental animal models has played an invaluable role in understanding the mechanisms of neurosensory deafness and in devising effective treatments. The purpose of this study was to develop an adult mouse model of ototoxic drug-induced hearing loss and to compare the ototoxicity in the adult mouse to that in the well-described guinea pig model. Mice are a powerful model organism, especially due to the large availability of antibodies, probes and genetic mutants. In this study, mice (n=114) and guinea pigs (n=35) underwent systemic treatment with either kanamycin or cisplatin. Auditory brainstem responses showed a significant threshold shift in guinea pigs 2 weeks after the beginning of the ototoxic treatment, while there was no significant hearing impairment recorded in mice. Hair cells and neuronal loss were correlated with hearing function in both guinea pigs and mice. These results indicate that the mouse is not a good model for ototoxicity, which should be taken into consideration in all further investigations concerning ototoxicity-induced hearing loss.

Cdk6-Dependent Regulation of G1 Length Controls Adult Neurogenesis†‡§

The presence of neurogenic precursors in the adult mammalian brain is now widely accepted, but the mechanisms coupling their proliferation with the onset of neuronal differentiation remain unknown. Here, we unravel the major contribution of the G1 regulator cyclin‐dependent kinase 6 (Cdk6) to adult neurogenesis. We found that Cdk6 was essential for cell proliferation within the dentate gyrus of the hippocampus and the subventricular zone of the lateral ventricles. Specifically, Cdk6 deficiency prevents the expansion of neuronally committed precursors by lengthening G1 phase duration, reducing concomitantly the production of newborn neurons. Altogether, our data support G1 length as an essential regulator of the switch between proliferation and neuronal differentiation in the adult brain and Cdk6 as one intrinsic key molecular regulator of this process. STEM Cells 2011;29:713–724

Opposing Regulation of Sox2 by Cell-Cycle Effectors E2f3a and E2f3b in Neural Stem Cells

The mechanisms through which cell-cycle control and cell-fate decisions are coordinated in proliferating stem cell populations are largely unknown. Here, we show that E2f3 isoforms, which control cell-cycle progression in cooperation with the retinoblastoma protein (pRb), have critical effects during developmental and adult neurogenesis. Loss of either E2f3 isoform disrupts Sox2 gene regulation and the balance between precursor maintenance and differentiation in the developing cortex. Both isoforms target the Sox2 locus to maintain baseline levels of Sox2 expression but antagonistically regulate Sox2 levels to instruct fate choices. E2f3-mediated regulation of Sox2 and precursor cell fate extends to the adult brain, where E2f3a loss results in defects in hippocampal neurogenesis and memory formation. Our results demonstrate a mechanism by which E2f3a and E2f3b differentially regulate Sox2 dosage in neural precursors, a finding that may have broad implications for the regulation of diverse stem cell populations.

Cdk2 loss accelerates precursor differentiation and remyelination in the adult central nervous system.

Cdk2 is not essential for oligodendrocyte maturation and myelination during development, but in response to demyelination, it is required for oligodendrocyte precursor cell proliferation (OPC), and its loss accelerates OPC differentiation and remyelination.

New insights into peripherin expression in cochlear neurons.

Peripherin is an intermediate filament protein that is expressed in peripheral and enteric neurons. In the cochlear nervous system, peripherin expression has been extensively used as a differentiation marker by preferentially labeling the type II neuronal population at adulthood, but yet without knowing its function. Since the expression of peripherin has been associated in time with the process of axonal extension and during regeneration of nerve fibers in other systems, it was of interest to determine whether peripherin expression in cochlear neurons was a static phenotypic trait or rather prone to modifications following nerve injury. In the present study, we first compared the expression pattern of peripherin and beta III-tubulin from late embryonic stages to the adult in rat cochlea. The staining for both proteins was seen before birth within all cochlear neurons. By birth, and for 2 or 3 days, peripherin expression was gradually restricted to the type II neuronal population and their projections. In contrast, from postnatal day (P) 10 onwards, while the expression of beta III-tubulin was still found in projections of all cochlear neurons, only the type I population had beta III-tubulin immunoreactivity in their cell bodies. We next investigated the expression of peripherin in axotomized cochlear neurons using an organotypic explant model. Peripherin expression was surprisingly re-expressed in a vast majority of neurons after axotomy. In parallel, the expression and localization of beta III-tubulin and peripherin in dissociated cultures of cochlear neurons were studied. Both proteins were distributed along the entire neuronal length but exhibited complementary distribution, especially within the projections. Moreover, peripherin immunoreactivity was still abundant in the growth cone, whereas that of beta III-tubulin was decreasing at this compartment. Our findings are consistent with a model in which peripherin plays an important structural role in cochlear neurons and their projections during both development and regenerative processes and which is compatible with the assumption that frequently developmentally regulated factors are reactivated during neuronal regeneration.

The Retinoblastoma Protein Is Essential for Survival of Postmitotic Neurons

The retinoblastoma protein (Rb) family members are essential regulators of cell cycle progression, principally through regulation of the E2f transcription factors. Growing evidence indicates that abnormal cell cycle signals can participate in neuronal death. In this regard, the role of Rb (p105) itself has been controversial. Germline Rb deletion leads to massive neuronal loss, but initial reports argue that death is non-cell autonomous. To more definitively resolve this question, we generated acute murine knock-out models of Rb in terminally differentiated neurons in vitro and in vivo. Surprisingly, we report that acute inactivation of Rb in postmitotic neurons results in ectopic cell cycle protein expression and neuronal loss without concurrent induction of classical E2f-mediated apoptotic genes, such as Apaf1 or Puma. These results suggest that terminally differentiated neurons require Rb for continuous cell cycle repression and survival.

Adult Neurogenesis and the Diseased Brain

For a long time it was believed that the adult mammalian brain was completely unable to regenerate after insults. However, recent advances in the field of stem cell biology, including the identification of adult neural stem cells (NSCs) and evidence regarding a continuous production of neurons throughout life in the dentate gyrus (DG) and the subventricular zone of the lateral ventricles (SVZ), have provided new hopes for the development of novel therapeutic strategies to induce regeneration in the damaged brain. Moreover, proofs have accumulated this last decade that endogenous stem/progenitor cells of the adult brain have an intrinsic capacity to respond to brain disorders. Here, we first briefly summarize our current knowledge related to adult neurogenesis before focusing on the behaviour of adult neural stem/progenitors cells following stroke and seizure, and describe some of the molecular cues involved in the response of these cells to injury. In the second part, we outline the consequences of three main neurodegenerative disorders on adult neurogenesis and we discuss the potential therapeutic implication of adult neural stem/progenitors cells during the course of these diseases.