Proliferation of hippocampal progenitors relies on p27-dependent regulation of Cdk6 kinase activity.

Neural stem cells give rise to granule dentate neurons throughout life in the hippocampus. Upon activation, these stem cells generate fast proliferating progenitors that complete several rounds of divisions before differentiating into neurons. Although the mechanisms regulating the activation of stem cells have been intensively studied, little attention has been given so far to the intrinsic machinery allowing the expansion of the progenitor pool. The cell cycle protein Cdk6 positively regulates the proliferation of hippocampal progenitors, but the mechanism involved remains elusive. Whereas Cdk6 functions primarily as a cell cycle kinase, it can also act as transcriptional regulator in cancer cells and hematopoietic stem cells. Using mouse genetics, we show here that the function of Cdk6 in hippocampal neurogenesis relies specifically on its kinase activity. The present study also reveals a specific regulatory mechanism for Cdk6 in hippocampal progenitors. In contrast to the classical model of the cell cycle, we observe that the Cip/Kip family member p27, rather than the Ink4 family, negatively regulates Cdk6 in the adult hippocampus. Altogether, our data uncover a unique, cell type-specific regulatory mechanism controlling the expansion of hippocampal progenitors, where Cdk6 kinase activity is modulated by p27.

Cycling or not cycling: cell cycle regulatory molecules and adult neurogenesis.

The adult brain most probably reaches its highest degree of plasticity with the lifelong generation and integration of new neurons in the hippocampus and olfactory system. Neural precursor cells (NPCs) residing both in the subgranular zone of the dentate gyrus and in the subventricular zone of the lateral ventricles continuously generate neurons that populate the dentate gyrus and the olfactory bulb, respectively. The regulation of NPC proliferation in the adult brain has been widely investigated in the past few years. Yet, the intrinsic cell cycle machinery underlying NPC proliferation remains largely unexplored. In this review, we discuss the cell cycle components that are involved in the regulation of NPC proliferation in both neurogenic areas of the adult brain.

Cdk6-dependent regulation of G(1) 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 G(1) 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 G(1) phase duration, reducing concomitantly the production of newborn neurons. Altogether, our data support G(1) 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.

Safety Profile of the Adjuvanted Recombinant Zoster Vaccine in Immunocompromised Populations: An Overview of Six Trials.

INTRODUCTION: The adjuvanted recombinant zoster vaccine (RZV) has demonstrated high efficacy against herpes zoster in older adults and immunocompromised populations. We present comprehensive safety data from six clinical trials in immunocompromised populations (autologous hematopoietic stem cell transplant and renal transplant recipients, patients with hematologic malignancies, patients with solid tumors, and human immunodeficiency virus-infected adults) who are at an increased risk of herpes zoster. METHODS: In all trials, immunocompromised adults ≥ 18 years of age were administered RZV or placebo. Safety was evaluated in the total vaccinated cohort. Solicited adverse events (AEs) were collected for 7 days and unsolicited AEs for 30 days after each dose. Serious AEs, fatal serious AEs, and potential immune-mediated diseases were collected from dose 1 until 12 months post-last dose or study end. Data were pooled for solicited AEs; unsolicited AEs, (fatal) serious AEs, and potential immune-mediated diseases were analyzed for each individual trial. All AEs were analyzed for sub-strata of adults 18-49 years of age and ≥ 50 years of age. RESULTS: In total, 1587 (RZV) and 1529 (placebo) adults were included in the pooled total vaccinated cohort. Solicited AEs were more common after RZV than placebo, were generally more common in the younger age stratum, and were mostly mild to moderate and resolved within 3 days (median duration). Unsolicited AEs and serious AEs were in line with underlying diseases and therapies. Across studies, the percentage of adults reporting one or more unsolicited AE was comparable between RZV and placebo, irrespective of age stratum. The percentage of adults reporting one or more serious AE, fatal serious AE, or potential immune-mediated diseases was generally similar for RZV and placebo, irrespective of age stratum. Overall, no safety concerns were identified. CONCLUSIONS: Recombinant zoster vaccine has a clinically acceptable safety profile. With the previously published vaccine efficacy and immunogenicity results, these data support a favorable benefit-risk profile of RZV vaccination in immunocompromised populations who are at an increased risk of herpes zoster.

Varicella zoster virus leads to chickenpox after primary infection and herpes zoster upon reactivation of the latent virus. Older adults and immunocompromised people, whose immune system is impaired because of the age-related decline in immunity and their underlying disease and/or treatment, respectively, are at an increased risk of herpes zoster and its complications. Recombinant zoster vaccine has been approved to prevent herpes zoster and its complications in adults aged ≥ 50 years in over 30 countries. In Europe, the vaccine has recently received approval to expand its use in adults aged 18 years or older who are at an increased risk of herpes zoster. We present an overview of the safety data from six clinical trials in immunocompromised patients vaccinated with recombinant zoster vaccine. We found that solicited adverse events were more common after the vaccine than placebo but that these were mild to moderate in intensity. Furthermore, the frequency of unsolicited adverse events was similar between the vaccine and placebo, and most of the reported adverse events and severe adverse events (e.g., infections or tumors) could be attributed to the pre-existent diseases and/or therapies. As such, no safety concern was identified following the review of the available clinical data. This overview, together with the published efficacy data in the prevention of herpes zoster and the vaccine immunogenicity, provides useful medical information and supports the use of the recombinant zoster vaccine in an immunocompromised population at an increased risk of herpes zoster.

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.

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

BACKGROUND: Newborn 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. RESULTS: In 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. CONCLUSION: Taken 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.

The adjuvanted recombinant zoster vaccine co-administered with a tetanus, diphtheria and pertussis vaccine in adults aged ≥50 years: A randomized trial.

BACKGROUND: This study evaluated immunogenicity and safety of the adjuvanted recombinant zoster vaccine (RZV) and the reduced-antigen-content diphtheria-tetanus-acellular pertussis vaccine (Tdap) when co-administered in adults aged ≥50 years. METHODS: In this open label, multi-center study (NCT02052596), participants were randomized 1:1 to the Co-Administration group (RZV dose 1 and Tdap at Day 0 [D0], RZV dose 2 at Month 2 [M2]) or Control group (Tdap at D0, RZV dose 1 at M2, RZV dose 2 at M4). Co-primary objectives were evaluation of the vaccine response rate (VRR) to RZV in the Co-Administration group, and demonstration of non-inferiority of the humoral responses to RZV and Tdap in the Co-Administration compared to Control group. Reactogenicity and safety of RZV and Tdap were also assessed. RESULTS: VRR to RZV was 97.8% in the Co-Administration group. The non-inferiority criterion was met for the humoral response to RZV and for 4 Tdap antigens, but was not met for the Tdap antigen pertactin. Occurrences of solicited, unsolicited and serious adverse events, and potential immune-mediated diseases were similar between groups. CONCLUSIONS: Co-administration of RZV and Tdap did not interfere with the humoral immune response to RZV or 4 of the 5 Tdap antigens. No safety concerns were identified.

Development of adjuvanted recombinant zoster vaccine and its implications for shingles prevention.

INTRODUCTION: GSK has developed a two-dose adjuvanted recombinant zoster vaccine (Shingrix, RZV) to protect people aged ≥50 years (50+) against herpes zoster (HZ) and its complications. RZV showed >90% efficacy against HZ, sustained over 4 years of follow-up, in all studied age groups. AREAS COVERED: This article reviews the scientific rationale underlying the design of RZV; the clinical evidence demonstrating immunogenicity, safety, and efficacy in persons 50+; and the public health implications and cost-effectiveness. EXPERT COMMENTARY: A decline in varicella zoster virus (VZV) immunity is associated with increased risk of HZ in adults 50+ and immunocompromised individuals. RZV was designed to restore levels of anti-VZV cellular and humoral immunity to prevent VZV reactivation. RZV includes the recombinant gE glycoprotein antigen, and Adjuvant System AS01B which promotes cellular and antibody responses. In two Phase III studies in subjects aged 50+ and 70+ years, RZV efficacy against HZ compared to placebo was >90% and ≥89% against post-herpetic neuralgia (PHN). RZV is expected to dramatically impact HZ morbidity including its complications, and associated health-care costs. In the US population aged 50+ years, vaccination with RZV can be cost-effective compared to no vaccination and cost-saving compared to the currently available live-attenuated HZ vaccine (Zostavax, Merck).

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 dispensable for adult hippocampal neurogenesis.

Granule neurons of the dentate gyrus (DG) of the hippocampus undergo continuous renewal throughout life. Among cell cycle regulators, cyclin-dependent kinase 2 (Cdk2) is considered as a major regulator of S-phase entry. We used Cdk2-deficient mice to decipher the requirement of Cdk2 for the generation of new neurons in the adult hippocampus. The quantification of cell cycle markers first revealed that the lack of Cdk2 activity does not influence spontaneous or seizure-induced proliferation of neural progenitor cells (NPC) in the adult DG. Using bromodeoxyuridine incorporation assays, we showed that the number of mature newborn granule neurons generated de novo was similar in both wild-type (WT) and Cdk2-deficient adult mice. Moreover, the apparent lack of cell output reduction in Cdk2(-/-) mice DG did not result from a reduction in apoptosis of newborn granule cells as analyzed by TUNEL assays. Our results therefore suggest that Cdk2 is dispensable for NPC proliferation, differentiation and survival of adult-born DG granule neurons in vivo. These data emphasize that functional redundancies between Cdks also occur in the adult brain at the level of neural progenitor cell cycle regulation during hippocampal neurogenesis.

The Yin and Yang of cell cycle progression and differentiation in the oligodendroglial lineage.

In white matter disorders such as leukodystrophies (LD), periventricular leucomalacia (PVL), or multiple sclerosis (MS), the hypomyelination or the remyelination failure by oligodendrocyte progenitor cells involves errors in the sequence of events that normally occur during development when progenitors proliferate, migrate through the white matter, contact the axon, and differentiate into myelin-forming oligodendrocytes. Multiple mechanisms underlie the eventual progressive deterioration that typifies the natural history of developmental demyelination in LD and PVL and of adult-onset demyelination in MS. Over the past few years, pathophysiological studies have mostly focused on seeking abnormalities that impede oligodendroglial maturation at the level of migration, myelination, and survival. In contrast, there has been a strikingly lower interest for early proliferative and differentiation events that are likely to be equally critical for white matter development and myelin repair. This review highlights the Yin and Yang principles of interactions between intrinsic factors that coordinately regulate progenitor cell division and the onset of differentiation, i.e. the initial steps of oligodendrocyte lineage progression that are obviously crucial in health and diseases.