Cumulative evidence indicates that neuronal cell cycle re-entry represents an early and critical event in AD, recapitulating known hallmarks of the disease including tau hyperphosphorylation and production of Aβ peptide-containing plaques. Neurons that duplicate their DNA are rarely observed to undergo mitosis, and they remain for long time as tetraploid cells, in accordance with the chronic course of the disease. We have recently shown that cell cycle re-entry and somatic tetraploidization occurs during normal development in a subpopulation of RGCs, giving rise to enlarged neurons with extensive dendritic trees. Tetraploization in these neurons occurs in response to the activation of the neurotrophin receptor p75NTR by an endogenous source of NGF. In contrast, BDNF inhibits G2/M transition in tetraploid RGCs, preventing their death by apoptosis. In AD both proNGF and p75NTR are overexpressed, and AD-associated oxidative conditions have been shown to enhance proNGF function. This suggests that p75NTR could be a trigger for neuronal tetraploidization in AD, being the p75NTR-mediated pathway a putative target for therapeutical intervention. Functional changes in affected neurons, derived from tetraploidy-associated hypertrophy, could compromise neuronal viability. The known decline of BDNF/TrkB expression in AD could facilitate G2/M transition and apoptosis in tetraploid neurons.

A subpopulation of chick retinal projection neurons becomes tetraploid during development, an event prevented by blocking antibodies against p75 neurotrophin receptor (p75^NTR). We have used an optimized flow cytometric assay, based on the analysis of unfixed brain cell nuclei, to study whether p75^NTR-dependent neuronal tetraploidization takes place in the cerebral cortex, giving rise to projection neurons as well. We show that 3% of neurons in both murine neocortex and chick telencephalic derivatives are tetraploid, and that in the mouse ∼85% of these neurons express the immediate early genes Erg-1 and c-Fos, indicating that they are functionally active. Tetraploid cortical neurons (65–80%) express CTIP2, a transcription factor specific for subcortical projection neurons in the mouse neocortex. During the period in which these neurons are born, p75^NTRis detected in differentiating neurons undergoing DNA replication. Accordingly, p75^NTR-deficient mice contain a reduced proportion of both NeuN and CTIP2-positive neocortical tetraploid neurons, thus providing genetic evidence for the participation of p75^NTRin the induction of neuronal tetraploidy in the mouse neocortex. In the striatum tetraploidy is mainly associated with long-range projection neurons as well since ∼80% of tetraploid neurons in this structure express calbindin, a marker of neostriatal-matrix spiny neurons, known to establish long-range projections to the substantia nigra and globus pallidus. In contrast, only 20% of tetraploid cortical neurons express calbindin, which is mainly expressed in layers II–III, where CTIP2 is absent. We conclude that tetraploidy mainly affects long-range projection neurons, being facilitated by p75^NTRin the neocortex.

In mammals, the type II melanoma antigen (Mage) protein family is constituted by at least 10 closely related members that are expressed in different tissues, including the nervous system. These proteins are believed to regulate cell cycle withdrawal, neuronal differentiation, and apoptosis. However, the analysis of their specific function has been complicated by functional redundancy. In accordance with previous studies in teleosts and Drosophila, we present evidence that only one mage gene exists in genomes from protists, fungi, plants, nematodes, insects, and nonmammalian vertebrates. We have identified the chicken mage gene and cloned the cDNA encoding the chick Mage protein (CMage). CMage shares close homology with the type II Mage protein family, and, as previously shown for the type II Mage proteins Necdin and Mage-G1, it can interact with the transcription factor E2F-1. CMage is expressed in specific regions of the developing nervous system including the retinal ganglion cell layer, the ventral horn of the spinal cord, and the dorsal root ganglia, coinciding with the expression of the neurotrophin receptor p75 (p75^NTR) in these regions. We show that the intracellular domain of p75^NTRcan interact with both CMage and Necdin, thus preventing the binding of the latter proteins to the transcription factor E2F-1, and facilitating the proapoptotic activity of E2F-1 in N1E-115 differentiating neurons. The presence of a single mage gene in the chicken genome, together with the close functional resemblance between CMage and Necdin, makes this species ideal to further analyze signal transduction through type II Mage proteins.

In response to different stress signals, the c-Jun NH₂-terminal kinase (JNK) can trigger cell death. However, JNK also facilitates the survival and cell cycle progression of tumor cells by mechanisms that are poorly defined. Here, we show that schwannoma RN22 cells can survive and proliferate under serum-free conditions although serum withdrawal rapidly induces mitochondrial fission and swelling. Although the morphologic changes observed in the mitochondria did not trigger cytochrome c release, they were accompanied by an increase in the mitochondrial membrane potential (ΔΨ_M) and of immunoreactivity for active JNK in these organelles. Pharmacologic inhibition of JNK provoked a further increase of the ΔΨ_M, an increase in reactive oxygen species (ROS) production, and a sustained decrease in cell viability due to necrosis. This increase in necrosis was prevented by the presence of ROS scavengers. Immunoreactivity for active JNK was also observed in the mitochondria of neuroblastoma 1E-115 and neuroblastoma 2a neuroblastoma cell lines on serum withdrawal, whereas active JNK was barely detected in serum-deprived fibroblasts. Accordingly, the reduction in neural tumor cell viability induced by JNK inhibition was largely attenuated in serum-deprived fibroblasts. These data indicate that local activation of JNK in the mitochondria can protect against necrotic cell death associated with ROS production, facilitating the growth of neural tumor cells subjected to serum deprivation. (Mol Cancer Res 2007;5(1):47–60)