Geminin-Deficient Neural Stem Cells Exhibit Normal Cell Division and Normal Neurogenesis

Abstract

Neural stem cells (NSCs) are the progenitors of neurons and glial cells during both embryonic development and adult life. The unstable regulatory protein Geminin (Gmnn) is thought to maintain neural stem cells in an undifferentiated state while they proliferate. Geminin inhibits neuronal differentiation in cultured cells by antagonizing interactions between the chromatin remodeling protein Brg1 and the neural-specific transcription factors Neurogenin and NeuroD. Geminin is widely expressed in the CNS during throughout embryonic development, and Geminin expression is down-regulated when neuronal precursor cells undergo terminal differentiation. Over-expression of Geminin in gastrula-stage Xenopus embryos can expand the size of the neural plate. The role of Geminin in regulating vertebrate neurogenesis in vivo has not been rigorously examined. To address this question, we created a strain of Nestin-Cre/Gmnnfl/fl mice in which the Geminin gene was specifically deleted from NSCs. Interestingly, we found no major defects in the development or function of the central nervous system. Neural-specific GmnnΔ/Δ mice are viable and fertile and display no obvious neurological or neuroanatomical abnormalities. They have normal numbers of BrdU+ NSCs in the subgranular zone of the dentate gyrus, and GmnnΔ/Δ NSCs give rise to normal numbers of mature neurons in pulse-chase experiments. GmnnΔ/Δ neurosphere cells differentiate normally into both neurons and glial cells when grown in growth factor-deficient medium. Both the growth rate and the cell cycle distribution of cultured GmnnΔ/Δ neurosphere cells are indistinguishable from controls. We conclude that Geminin is largely dispensable for most of embryonic and adult mammalian neurogenesis.

Geminin Regulates the Transcriptional and Epigenetic Status of Neuronal Fate-Promoting Genes during Mammalian Neurogenesis

“…Here, we found that while Gem transiently repressed the expression of neuronal genes during mammalian neurogenesis, Gem was not sufficient to alter the differentiation potential of lineage-restricted neural precursor cells. These results are consistent with the recent observation that Gem loss in mammalian NSCs in a conditional mouse model did not affect their capability to self-renew or their differentiation potential in vivo or in vitro (19). We also found that while Gem increases PcG enrichment and otherwise alters the epigenetic status of neuronal genes in neural precursor cells, it did not alter the progression toward terminal neuronal or glial differentiation at either a phenotypic or molecular level.…”

“…Loss of geminin in cortical neural progenitor cells (NPCs) surprisingly resulted in increases in NPC cell number and in decreased production of neurons in ventricular zone regions, suggesting a positive role for geminin in promoting cortical neu-rogenesis (24). However, in another study, loss of geminin had no effect on the cycling NSC population in the subventricular region and hippocampal dentate gyrus, nor did it affect the ability of these NSCs to proliferate or further differentiate into neuronal and glial cell types in vitro (19). Another recent study reported complex actions of geminin during mammalian neurogenesis, where both knockdown and overexpression of geminin resulted in decreased numbers of Sox3-positive neural precursor cells in the mouse embryo.…”

Geminin Promotes an Epithelial-to-Mesenchymal Transition in an Embryonic Stem Cell Model of Gastrulation

“…Low levels of overexpression and downregulation obtained in that study may have created a state of chromatin readiness to respond to lineage differentiation factors, rather than neuronal differentiation per se. Recently, targeted deletion of Geminin in neural stem cells suggested that in vivo Geminin may have a primary role in cycle regulation rather than in lineage choice [29], although another analysis demonstrated that deletion expanded the neural precursor population, and overexpression promoted migration and premature neuronal differentiation [30].…”

Geminin is required for mitotic proliferation of spermatogonia

“…Studies of Geminin-deficient mice have not yet convincingly demonstrated that Geminin regulates cell differentiation. For example, mice carrying a neuron-specific deletion of Geminin appear to be neurologically normal, and cultured Gmnn D/D neural stem cells differentiate normally into neurons and glial cells (Schultz et al, 2011;Spella et al, 2011). The pattern of differentiation of Gmnn D/D T-lymphocytes is also completely normal (Karamitros et al, 2010).…”