Sequences and structures required for recombination between virus-associated RNAs.

RNA recombination has been described for a number of viruses in the plant and animal kingdoms, but the mechanisms of selection of recombination sites are poorly understood. The nonrandom recombination between two subviral RNAs associated with turnip crinkle virus was used to study the requirement for specific sequences and structures in the generation of recombinant molecules. Single-base mutations that disrupted either the stem or the loop of one of the two computer-predicted stem-loop structures eliminated detectable recombinant molecules. However, recombinants were detected if compensatory mutations were generated that re-formed a stable hairpin structure. These results provide evidence for the necessity of specific structures in the formation of recombinant molecules in this system.

Role of founder cell deficit and delayed neuronogenesis in microencephaly of the trisomy 16 mouse.

Development of the neocortex of the trisomy 16 (Ts16) mouse, an animal model of Down syndrome (DS), is characterized by a transient delay in the radial expansion of the cortical wall and a persistent reduction in cortical volume. Here we show that at each cell cycle during neuronogenesis, a smaller proportion of Ts16 progenitors exit the cell cycle than do control, euploid progenitors. In addition, the cell cycle duration was found to be longer in Ts16 than in euploid progenitors, the Ts16 growth fraction was reduced, and an increase in apoptosis was observed in both proliferative and postmitotic zones of the developing Ts16 neocortical wall. Incorporation of these changes into a model of neuronogenesis indicates that they are sufficient to account for the observed delay in radial expansion. In addition, the number of neocortical founder cells, i.e., precursors present just before neuronogenesis begins, is reduced by 26% in Ts16 mice, leading to a reduction in overall cortical size at the end of Ts16 neuronogenesis. Thus, altered proliferative characteristics during Ts16 neuronogenesis result in a delay in the generation of neocortical neurons, whereas the founder cell deficit leads to a proportional reduction in the overall number of neurons. Such prenatal perturbations in either the timing of neuron generation or the final number of neurons produced may lead to significant neocortical abnormalities such as those found in DS.

Organotypic slice cultures for analysis of proliferation, cell death, and migration in the embryonic neocortex.

Dynamic cellular interactions during neocortical neurogenesis are critical for proper cortical development, providing both trophic and tropic support. Although cell proliferation and programmed cell death have been characterized in dissociated primary cell cultures, many in vivo processes during cortical neurogenesis depend on cell-cell interactions and therefore on the three-dimensional environment of the proliferating neuroblasts and their progeny. Here we describe a murine organotypic neocortical slice preparation that retains major morphological and functional in vivo characteristics of the developing neocortex and is viable (exhibits very low levels of cell death) for up to three days. Moreover, this slice preparation is amenable to direct experimental manipulation of potential diffusible regulators of neurogenesis. Using a variety of biochemical and physiological methods including time-lapse and quantitative confocal microscopy, we demonstrate that this system can be used effectively to investigate cellular mechanisms important for brain growth and maturation, including neurogenesis, apoptosis, and neuronal migration.

Differential modulation of proliferation in the neocortical ventricular and subventricular zones.

Recent studies have implicated the classical neurotransmitters GABA and glutamate in the regulation of neural progenitor proliferation. We now show that GABA and glutamate have opposite effects on the two neural progenitor populations in the ventricular zones (VZs) and subventricular zones (SVZs) of the embryonic cerebrum. Application of either molecule to organotypic slice cultures dramatically increases proliferation in the VZ by shortening the cell cycle, whereas proliferation in the SVZ is decreased. These disparate effects, measured both by bromodeoxyuridine uptake and the expansion of retrovirally labeled progenitor clones, are mimicked by the application of specific GABA and glutamate agonists and are blocked by antagonists. Thus, the relative contributions of the VZ and SVZ to neocortical growth may be regulated by differential responsiveness to GABA and glutamate.

The role of cell death in regulating the size and shape of the mammalian forebrain.

The size of the cerebral cortex is determined by the rate of production of neurons and glial cells in the proliferative ventricular and subventricular zones. Recent studies from targeted mutations of different death-effector gene families indicate that programmed cell death (PCD) plays an important role in cell production and early morphogenesis of the mammalian forebrain before the formation of neuronal connections. For example, disruption of the c/Jun N-kinase signaling pathway by double-targeted mutation of both Jnk1 and Jnk2 results in increased PCD in the forebrain leading to precocious degeneration of cerebral precursors. In contrast, disturbance of the caspase cascade by targeted disruption of either casp-9 or casp-3 leads to decreased PCD causing expansion and exencephaly of the forebrain as well as supernumerary neurons in the cerebral cortex. The supernumerary neurons in these knockout mice align radially and form an expanded cortical plate which begins to form cerebral convolutions. Thus, the precise coordination of different apoptotic signaling pathways during early stages of neurogenesis is crucial for regulation of the proper cortical size and shape.

Consequences of trisomy 16 for mouse brain development: corticogenesis in a model of Down syndrome.

We have studied abnormalities in the tangential and radial expansion of the cerebral cortex during fetal development in the trisomy 16 (Ts16) mouse, a model for human trisomy 21 (Down syndrome). Slowed tangential expansion of the neuroepithelium in Ts16 resulted in a reduction of final telencephalic size and is predicted to decrease the number of radial cortical units in the mature brain. In addition, radial growth of the Ts16 cortex was delayed at the time of peak cortical neurogenesis in normal mice, but by embryonic day 18 the cortex reached normal thickness. Because mouse chromosome 16 shares many genes with human chromosome 21, abnormalities in Ts16 brain development may parallel abnormalities in trisomy 21.

Epistatic and independent functions of caspase-3 and Bcl-X(L) in developmental programmed cell death.

The number of neurons in the mammalian brain is determined by a balance between cell proliferation and programmed cell death. Recent studies indicated that Bcl-X(L) prevents, whereas Caspase-3 mediates, cell death in the developing nervous system, but whether Bcl-X(L) directly blocks the apoptotic function of Caspase-3 in vivo is not known. To examine this question, we generated bcl-x/caspase-3 double mutants and found that caspase-3 deficiency abrogated the increased apoptosis of postmitotic neurons but not the increased hematopoietic cell death and embryonic lethality caused by the bcl-x mutation. In contrast, caspase-3, but not bcl-x, deficiency changed the normal incidence of neuronal progenitor cell apoptosis, consistent with the lack of expression of Bcl-X(L) in the proliferative population of the embryonic cortex. Thus, although Caspase-3 is epistatically downstream to Bcl-X(L) in postmitotic neurons, it independently regulates apoptosis of neuronal founder cells. Taken together, these results establish a role of programmed cell death in regulating the size of progenitor population in the central nervous system, a function that is distinct from the classic role of cell death in matching postmitotic neuronal population with postsynaptic targets.

Reduced apoptosis and cytochrome c-mediated caspase activation in mice lacking caspase 9.

Caspases are essential components of the mammalian cell death machinery. Here we test the hypothesis that Caspase 9 (Casp9) is a critical upstream activator of caspases through gene targeting in mice. The majority of Casp9 knockout mice die perinatally with a markedly enlarged and malformed cerebrum caused by reduced apoptosis during brain development. Casp9 deletion prevents activation of Casp3 in embryonic brains in vivo, and Casp9-deficient thymocytes show resistance to a subset of apoptotic stimuli, including absence of Casp3-like cleavage and delayed DNA fragmentation. Moreover, the cytochrome c-mediated cleavage of Casp3 is absent in the cytosolic extracts of Casp9-deficient cells but is restored after addition of in vitro-translated Casp9. Together, these results indicate that Casp9 is a critical upstream activator of the caspase cascade in vivo.

Requirement of the JIP1 scaffold protein for stress-induced JNK activation.

The c-Jun N-terminal kinase (JNK) signal transduction pathway is activated in response to the exposure of cells to environmental stress. Components of the JNK signaling pathway interact with the JIP1 scaffold protein. JIP1 is located in the neurites of primary hippocampal neurons. However, in response to stress, JIP1 accumulates in the soma together with activated JNK and phosphorylated c-Jun. Disruption of the Jip1 gene in mice by homologous recombination prevented JNK activation caused by exposure to excitotoxic stress and anoxic stress in vivo and in vitro. These data show that the JIP1 scaffold protein is a critical component of a MAP-kinase signal transduction pathway.