Cell death, DNA breaks and possible rearrangements: an alternative view

Mathematical modeling supports substantial mouse neural progenitor cell death

Background

Existing quantitative models of mouse cerebral cortical development are not fully constrained by experimental data.

Results

Here, we use simple difference equations to model neural progenitor cell fate decisions, incorporating intermediate progenitor cells and initially low rates of neural progenitor cell death. Also, we conduct a sensitivity analysis to investigate possible uncertainty in the fraction of cells that divide, differentiate, and die at each cell cycle.

Conclusion

We demonstrate that uniformly low-level neural progenitor cell death, as concluded in previous models, is incompatible with normal mouse cortical development. Levels of neural progenitor cell death up to and exceeding 50% are compatible with normal cortical development and may operate to prevent forebrain overgrowth as observed following cell death attenuation, as occurs in caspase 3-null mutant mice.

Cell cycle regulation in the postmitotic neuron: oxymoron or new biology?

Adult CNS neurons are typically described as permanently postmitotic but there is probably nothing permanent about the neuronal cell cycle arrest. Rather, it appears that these highly differentiated cells must constantly keep their cell cycle in check. Relaxation of this vigilance leads to the initiation of a cell cycle and entrance into an altered and vulnerable state, often leading to death. There is evidence that neurons which are at risk of neurodegeneration are also at risk of re-initiating a cell cycle process that involves the expression of cell cycle proteins and DNA replication. Failure of cell cycle regulation might be a root cause of several neurodegenerative disorders and a final common pathway for others.

Computational models of neocortical neuronogenesis and programmed cell death in the developing mouse, monkey, and human

This paper presents a computational model allowing quantitative simulations of acquisition of neocortical neuronal number across mammalian species. When extrapolating scientific findings from rodents to humans, it is particularly pertinent to acknowledge the importance of the accelerated enlargement of the neocortex during human evolution. Neocortex development is marked by discrete stages of neural progenitor cell proliferation and death, neuronal differentiation, and neuronal programmed cell death. We have developed computational models of human and rhesus monkey neocortical neuronal cell acquisition based on experimentally derived parameters of cell cycle length, commitment to cell cycle exit, and cell death. Our model results agree with independent stereological studies estimating neocortical neuron number in adult and developing rhesus monkey and human. Comparisons of our primate models with previously developed rodent models suggest correlations between the lengthening of the duration of the neuronogenesis period and a lengthening of the cellular processes of cell cycle progression and death can account for the vast increase in size of the primate neocortex. Furthermore, when compared with rodents, we predict that cell death may play a larger role in shaping the primate neocortex. Our mathematical models of the development and evolution of the neocortex provide a quantitative, biologically based construct for extrapolation between rodent and humans. These models can assist in focusing future experimental research on the differing mechanisms of rodent versus human neocortical development.

Neural progenitor number is regulated by nuclear factor-kappaB p65 and p50 subunit-dependent proliferation rather than cell survival

The number of cells generated by a proliferating stem or precursor cell can be influenced both by proliferation and by the degree of cell death/survival of the progeny generated. In this study, the extent to which cell survival controls progenitor number was examined by comparing the growth characteristics of neurosphere cultures derived from mice lacking genes for the death-inducing Bcl-2 homologue Hara Kiri (Hrk), apoptosis-associated protein 1 (Apaf1), or the prosurvival nuclear factor-kappaB (NFkappaB) subunits p65, p50, or c-rel. We found no evidence that Hrk or Apaf1, and by inference the mitochondrial cell death pathway, are involved in regulating the number of neurosphere-derived progeny. However, we identified the p65p50 NFkappaB dimer as being required for the normal growth and expansion of neurosphere cultures. Genetic loss of both p65 and p50 NFkappaB subunits resulted in a reduced number of progeny but an increased proportion of neurons. No effect on cell survival was observed. This suggests that the number and fate of neural progenitor cells are more strongly regulated by cell cycle control than survival.

The role of cell death during neocortical neurogenesis and synaptogenesis: implications from a computational model for the rat and mouse

We are quantitatively evaluating the acquisition of neocortical neurons through key stages of development including neurogenesis, migration, and synaptogenesis. Here we expand upon a previous computational model describing neocortical neurogenesis in the rat and mouse [Dev. Neurosci. 24 (2002) 467], to include the period of synaptogenesis (P0-P14) when programmed cell death (PCD) is known to play a major role in shaping the neocortex. We also quantitatively evaluate differing hypotheses on the role of cell death during neurogenesis. This new model construct allows prediction of acquisition of adult neuronal number in the rat and mouse neocortex from the beginning of neurogenesis through synaptogenesis. The mathematical model output is validated by independently derived stereologically determined neuron number estimates in the adult rat and mouse. Simulations suggest cell death during synaptogenesis reduces the neocortical neuronal population by 20-30%, while cell death of progenitor cells and newly formed neurons during neurogenesis may reduce output by as much as 24%. However, higher death rates during neurogenesis as suggested by some research would deplete the progenitor population, not allowing for the vast expansion that is the hallmark of the mammalian neocortex. Furthermore, our simulations suggest the clearance time of dying neurons labeled by TUNEL or pyknosis is relatively short, between 1 and 4 h, corroborating experimental research. This novel mathematical model for adult neocortical neuronal acquisition allows for in silico analysis of normal and perturbed states of neocortical development as well as interspecies and evolutionary analyses of neocortical development.

Neural stem cells and regulation of cell number

Normal CNS development involves the sequential differentiation of multipotent stem cells. Alteration of the numbers of stem cells, their self-renewal ability, or their proliferative capacity will have major effects on the appropriate development of the nervous system. In this review, we discuss different mechanisms that regulate neural stem cell differentiation. Proliferation signals and cell cycle regulators may regulate cell kinetics or total number of cell divisions. Loss of trophic support and cytokine receptor activation may differentially contribute to the induction of cell death at specific stages of development. Signaling from differentiated progeny or asymmetric distribution of specific molecules may alter the self-renewal characteristics of stem cells. We conclude that the final decision of a cell to self-renew, differentiate or remain quiescent is dependent on an integration of multiple signaling pathways and at each instant will depend on cell density, metabolic state, ligand availability, type and levels of receptor expression, and downstream cross-talk between distinct signaling pathways.

Neuronal differentiation and long-term survival of newly generated cells in the olfactory midbrain of the adult spiny lobster, Panulirus argus

The fate of continuously generated cells in the soma clusters of the olfactory midbrain of adult spiny lobsters, Panulirus argus, was investigated by in vivo pulse-chase experiments with the proliferation marker 5-bromo-2'-deoxyuridine (BrdU) combined with immunostainings for neuropeptides of mature neurons. A BrdU injection after a survival time (ST) of 14 h labeled about 100 nuclei in the lateral soma clusters (LC), comprised of projection neurons, and about 30 nuclei in the medial soma clusters (MC), comprised of local interneurons. The BrdU-positive nuclei were confined to small regions at the inside of these clusters, which also contain nuclei in different phases of mitosis and thus represent proliferative zones. After STs of 2 weeks or 3 months, the number of BrdU-positive nuclei was doubled, indicating a mitosis of all originally labeled cells. Dependent on ST, the BrdU-positive nuclei were translocated from the proliferative zones towards the outside of the clusters, where somata of mature neurons reside. Immunostainings with antibodies to the neuropeptides FMRFamide and substance P, both of which label a large portion of somata in the MC and a pair of giant neurons projecting into the LC, revealed that in both clusters the proliferative zones are surrounded by, but are themselves devoid of, labeling. In the MC, some BrdU-positive somata were double-labeled by the FMRFamide antibody after an ST of 3 months, and by the substance P antibody after STs of 6 and 11/14 months, but not after 3 months. In the LC, BrdU-positive somata after an ST of 3 months partially and after 6 and 11/14 months widely overlapped with the arborizations of the giant neurons, indicating the establishment of synaptic input. The experiments show that cells generated in proliferative zones in the LC and MC of adult spiny lobsters after a final mitosis differentiate into neurons within months, survive for at least 1 year, and are integrated into the circuitry of the olfactory midbrain. A new hypothesis about the mechanism of adult neurogenesis in the central olfactory pathway of decapod crustaceans is developed, linking it to neurogenesis during embryonic and larval development.

Elevated DNA double strand breaks and apoptosis in the CNS of scid mutant mice

Genetic approaches have provided evidence that DNA end-joining problems serve an essential role in neuronal survival during development of mammalian embryos. In the present study, we tested whether the DNA repair enzyme, DNA dependent protein kinase, plays an important role in the survival of cerebral cortical neurons in mice. DNA-PK is comprised of a DNA-binding subunit called Ku and a catalytic subunit called DNA-PKcs. In mice with the scid mutation, DNA-PKcs is truncated near the kinase domain, which causes loss of kinase activity. We compared the spatial and temporal aspects of neuronal cell death in scid versus isogenic wild-type embryos and found a significant increase in dying cells in scid mice, as assessed by nuclear changes, DNA fragmentation and caspase-3 activity. Additional biochemical and immunocytochemical studies indicated that of several DNA repair enzymes investigated, only PARP was increased in scid mice, possibly in response to elevated DNA strand breaks.