IGF2 interacts with the imprinted gene Cdkn1c to promote terminal differentiation of neural stem cells.

Adult neurogenesis is supported by multipotent neural stem cells (NSCs) with unique properties and growth requirements. Adult NSCs constitute a reversibly quiescent cell population that can be activated by extracellular signals from the microenvironment in which they reside in vivo. Although genomic imprinting plays a role in adult neurogenesis through dose regulation of some relevant signals, the roles of many imprinted genes in the process remain elusive. Insulin-like growth factor 2 (IGF2) is encoded by an imprinted gene that contributes to NSC maintenance in the adult subventricular zone through a biallelic expression in only the vascular compartment. We show here that IGF2 additionally promotes terminal differentiation of NSCs into astrocytes, neurons and oligodendrocytes by inducing the expression of the maternally expressed gene cyclin-dependent kinase inhibitor 1c (Cdkn1c), encoding the cell cycle inhibitor p57. Using intraventricular infusion of recombinant IGF2 in a conditional mutant strain with Cdkn1c-deficient NSCs, we confirm that p57 partially mediates the differentiation effects of IGF2 in NSCs and that this occurs independently of its role in cell-cycle progression, balancing the relationship between astrogliogenesis, neurogenesis and oligodendrogenesis.

Differential contribution of TrkB and p75NTR to BDNF-dependent self-renewal, proliferation, and differentiation of adult neural stem cells.

Alterations in adult neurogenesis are a common hallmark of neurodegenerative diseases. Therefore, understanding the molecular mechanisms that control this process is an indispensable requirement for designing therapeutic interventions addressing neurodegeneration. Neurotrophins have been implicated in multiple functions including proliferation, survival, and differentiation of the neural stem cells (NSCs), thereby being good candidates for therapeutic intervention. Brain-derived neurotrophic factor (BDNF) belongs to the neurotrophin family and has been proven to promote neurogenesis in the subgranular zone. However, the effects of BDNF in the adult subventricular zone (SVZ) still remain unclear due to contradictory results. Using in vitro cultures of adult NSCs isolated from the mouse SVZ, we show that low concentrations of BDNF are able to promote self-renewal and proliferation in these cells by activating the tropomyosin-related kinase B (TrkB) receptor. However, higher concentrations of BDNF that can bind the p75 neurotrophin receptor (p75NTR) potentiate TrkB-dependent self-renewal and proliferation and promote differentiation of the adult NSCs, suggesting different molecular mechanisms in BDNF-promoting proliferation and differentiation. The use of an antagonist for p75NTR reduces the increment in NSC proliferation and commitment to the oligodendrocyte lineage. Our data support a fundamental role for both receptors, TrkB and p75NTR, in the regulation of NSC behavior.

E2F4DN Transgenic Mice: A Tool for the Evaluation of E2F4 as a Therapeutic Target in Neuropathology and Brain Aging.

E2F4 was initially described as a transcription factor with a key function in the regulation of cell quiescence. Nevertheless, a number of recent studies have established that E2F4 can also play a relevant role in cell and tissue homeostasis, as well as tissue regeneration. For these non-canonical functions, E2F4 can also act in the cytoplasm, where it is able to interact with many homeostatic and synaptic regulators. Since E2F4 is expressed in the nervous system, it may fulfill a crucial role in brain function and homeostasis, being a promising multifactorial target for neurodegenerative diseases and brain aging. The regulation of E2F4 is complex, as it can be chemically modified through acetylation, from which we present evidence in the brain, as well as methylation, and phosphorylation. The phosphorylation of E2F4 within a conserved threonine motif induces cell cycle re-entry in neurons, while a dominant negative form of E2F4 (E2F4DN), in which the conserved threonines have been substituted by alanines, has been shown to act as a multifactorial therapeutic agent for Alzheimer's disease (AD). We generated transgenic mice neuronally expressing E2F4DN. We have recently shown using this mouse strain that expression of E2F4DN in 5xFAD mice, a known murine model of AD, improved cognitive function, reduced neuronal tetraploidization, and induced a transcriptional program consistent with modulation of amyloid-ß (Aß) peptide proteostasis and brain homeostasis recovery. 5xFAD/E2F4DN mice also showed reduced microgliosis and astrogliosis in both the cerebral cortex and hippocampus at 3-6 months of age. Here, we analyzed the immune response in 1 year-old 5xFAD/E2F4DN mice, concluding that reduced microgliosis and astrogliosis is maintained at this late stage. In addition, the expression of E2F4DN also reduced age-associated microgliosis in wild-type mice, thus stressing its role as a brain homeostatic agent. We conclude that E2F4DN transgenic mice represent a promising tool for the evaluation of E2F4 as a therapeutic target in neuropathology and brain aging.

Aberrations of Genomic Imprinting in Glioblastoma Formation.

In human glioblastoma (GBM), the presence of a small population of cells with stem cell characteristics, the glioma stem cells (GSCs), has been described. These cells have GBM potential and are responsible for the origin of the tumors. However, whether GSCs originate from normal neural stem cells (NSCs) as a consequence of genetic and epigenetic changes and/or dedifferentiation from somatic cells remains to be investigated. Genomic imprinting is an epigenetic marking process that causes genes to be expressed depending on their parental origin. The dysregulation of the imprinting pattern or the loss of genomic imprinting (LOI) have been described in different tumors including GBM, being one of the earliest and most common events that occurs in human cancers. Here we have gathered the current knowledge of the role of imprinted genes in normal NSCs function and how the imprinting process is altered in human GBM. We also review the changes at particular imprinted loci that might be involved in the development of the tumor. Understanding the mechanistic similarities in the regulation of genomic imprinting between normal NSCs and GBM cells will be helpful to identify molecular players that might be involved in the development of human GBM.

Dlk1 dosage regulates hippocampal neurogenesis and cognition.

Neurogenesis in the adult brain gives rise to functional neurons, which integrate into neuronal circuits and modulate neural plasticity. Sustained neurogenesis throughout life occurs in the subgranular zone (SGZ) of the dentate gyrus in the hippocampus and is hypothesized to be involved in behavioral/cognitive processes such as memory and in diseases. Genomic imprinting is of critical importance to brain development and normal behavior, and exemplifies how epigenetic states regulate genome function and gene dosage. While most genes are expressed from both alleles, imprinted genes are usually expressed from either the maternally or the paternally inherited chromosome. Here, we show that in contrast to its canonical imprinting in nonneurogenic regions, Delta-like homolog 1 (Dlk1) is expressed biallelically in the SGZ, and both parental alleles are required for stem cell behavior and normal adult neurogenesis in the hippocampus. To evaluate the effects of maternally, paternally, and biallelically inherited mutations within the Dlk1 gene in specific behavioral domains, we subjected Dlk1-mutant mice to a battery of tests that dissociate and evaluate the effects of Dlk1 dosage on spatial learning ability and on anxiety traits. Importantly, reduction in Dlk1 levels triggers specific cognitive abnormalities that affect aspects of discriminating differences in environmental stimuli, emphasizing the importance of selective absence of imprinting in this neurogenic niche.

TET3 prevents terminal differentiation of adult NSCs by a non-catalytic action at Snrpn.

Ten-eleven-translocation (TET) proteins catalyze DNA hydroxylation, playing an important role in demethylation of DNA in mammals. Remarkably, although hydroxymethylation levels are high in the mouse brain, the potential role of TET proteins in adult neurogenesis is unknown. We show here that a non-catalytic action of TET3 is essentially required for the maintenance of the neural stem cell (NSC) pool in the adult subventricular zone (SVZ) niche by preventing premature differentiation of NSCs into non-neurogenic astrocytes. This occurs through direct binding of TET3 to the paternal transcribed allele of the imprinted gene Small nuclear ribonucleoprotein-associated polypeptide N (Snrpn), contributing to transcriptional repression of the gene. The study also identifies BMP2 as an effector of the astrocytic terminal differentiation mediated by SNRPN. Our work describes a novel mechanism of control of an imprinted gene in the regulation of adult neurogenesis through an unconventional role of TET3.

Beyond protein-coding genes.

A long non-coding RNA called lnc-NR2F1 regulates several neuronal genes, including some involved in autism and intellectual disabilities.

Genomic Imprinting and the Regulation of Postnatal Neurogenesis.

Most genes required for mammalian development are expressed from both maternally and paternally inherited chromosomal homologues. However, there are a small number of genes known as "imprinted genes" that only express a single allele from one parent, which is repressed on the gene from the other parent. Imprinted genes are dependent on epigenetic mechanisms such as DNA methylation and post-translational modifications of the DNA-associated histone proteins to establish and maintain their parental identity. In the brain, multiple transcripts have been identified which show parental origin-specific expression biases. However, the mechanistic relationship with canonical imprinting is unknown. Recent studies on the postnatal neurogenic niches raise many intriguing questions concerning the role of genomic imprinting and gene dosage during postnatal neurogenesis, including how imprinted genes operate in concert with signalling cues to contribute to newborn neurons' formation during adulthood. Here we have gathered the current knowledge on the imprinting process in the neurogenic niches. We also review the phenotypes associated with genetic mutations at particular imprinted loci in order to consider the impact of imprinted genes in the maintenance and/or differentiation of the neural stem cell pool in vivo and during brain tumour formation.