ChileAlzheimer's disease (AD) is a progressive neurodegenerative disorder, which is probably caused by the cytotoxic effect of the amyloid b-peptide (Ab). We report here molecular changes induced by Ab, both in neuronal cells in culture and in rats injected in the dorsal hippocampus with preformed Ab fibrils, as an in vivo model of the disease. Results indicate that in both systems, Ab neurotoxicity resulted in the destabilization of endogenous levels of b-catenin, a key transducer of the Wnt signaling pathway. Lithium chloride, which mimics Wnt signaling by inhibiting glycogen synthase kinase-3b promoted the survival of post-mitotic neurons against Ab neurotoxicity and recovered cytosolic b-catenin to control levels. Moreover, the neurotoxic effect of Ab fibrils was also modulated with protein kinase C agonists/inhibitors and reversed with conditioned medium containing the Wnt-3a ligand. We also examined the spatial memory performance of rats injected with preformed Ab fibrils in the Morris water maze paradigm, and found that chronic lithium treatment protected neurodegeneration by rescuing b-catenin levels and improved the deficit in spatial learning induced by Ab. Our results are consistent with the idea that Ab-dependent neurotoxicity induces a loss of function of Wnt signaling components and indicate that lithium or compounds that mimic this signaling cascade may be putative candidates for therapeutic intervention in Alzheimer's patients.

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Oxidative stress is a key mechanism in amyloid ␤-peptide (A␤)-mediated neurotoxicity; therefore, the protective roles of 17␤-estradiol (E 2 ) and antioxidants (Trolox and vitamin C) were assayed on hippocampal neurons. Our results show the following: 1) E 2 and Trolox attenuated the neurotoxicity mediated by A␤ and H 2 O 2 as measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction assays, quantification of apoptotic cells, and morphological studies of the integrity of the neurite network. 2) Vitamin C failed to protect neurons from A␤ toxicity. 3) A␤-mediated endoperoxide production, reported to induce cell damage, was decreased in the presence of E 2 and Trolox. 4) Two key Wnt signaling components were affected by E 2 and Trolox; in fact, the enzyme glycogen synthase kinase 3␤ was inhibited by both E 2 and Trolox, and both compounds were able to stabilize cytoplasmic ␤-catenin. 5) E 2 activated the expression of the Wnt-5a and Wnt-7a ligands, and at the same time, E 2 , through the ␣-estrogen receptor, was able to prevent the excitotoxic A␤-induced rise in bulk-free Ca 2؉as an alternative pathway to increase cell viability. 6) Finally, the Wnt-7a ligand protected against cytoplasmic calcium disturbances induced by A␤ treatment. Our results suggest that control of oxidative stress, regulation of cytoplasmic calcium, and activation of Wnt signaling may prevent A␤ neurotoxicity. Alzheimer disease (AD)1 is a neurodegenerative disease characterized by neuronal cell death, dystrophic neurites, neurofibrillary tangles, and senile plaques (1). Senile plaques are composed by the amyloid ␤-peptide (A␤), a 40 -42-amino acid peptide that originates from the proteolytic cleavage of the amyloid precursor protein (2). There is also evidence relating the etiopathology of AD with oxidative stress induced by A␤ in the brain of AD patients (3-6). A␤ increases the production of intraneuronal reactive oxygen species (ROS) and stimulates hydrogen peroxide (H 2 O 2 ) levels through metal ion reduction (7,8). Free radicals peroxidize membrane lipids (9) and oxidize proteins (10). In vitro experiments also support the observation that the neurotoxic effect of A␤ is mediated by free radical mechanisms (5, 11, 12) and alteration of Ca 2ϩ homeostasis (13). Furthermore, several studies have reported neuroprotection by antioxidants against A␤-mediated cytotoxicity (14 -16). Also, 17␤-estradiol (E 2 ; estrogen) treatment apparently has beneficial effects on AD (17, 18). In addition, E 2 prevents A␤-induced cell death by activation of the ␣-ER (19) and preserves neuronal viability and function in cortical neurons exposed to glutamate toxicity (20). Also, there is evidence that E 2 prevents morphological neurodegenerative changes in hippocampal neurons caused by A␤ deposits (21).On the other hand, neurofibrillary tangles are intracellular aggregates of paired helical filaments produced by hyperphosphorylation of the microtubule-associated protein tau (23). It has been proposed that glycogen synthase kinase-3␤ (GSK-3␤...

Summary In adult stem cell lineages, progenitor cells commonly undergo mitotic transit amplifying (TA) divisions before terminal differentiation, allowing production of many differentiated progeny per stem cell division. Mechanisms that limit TA divisions and trigger the switch to differentiation may protect against cancer by preventing accumulation of oncogenic mutations in the proliferating population. Here we show that the switch from TA proliferation to differentiation in the Drosophila male germline stem cell lineage is mediated by translational control. The TRIM-NHL tumor suppressor homolog Mei-P26 facilitates accumulation of the differentiation regulator Bam in TA cells. In turn, Bam and its partner Bgcn bind the mei-P26 3′UTR and repress translation of mei-P26 in late TA cells. Thus, germ cells progress through distinct, sequential regulatory states, from Mei-P26 on/Bam off to Bam on/Mei-P26 off. TRIM-NHL homologs across species facilitate the switch from proliferation to differentiation, suggesting a novel and conserved developmentally-programmed tumor suppressor mechanism.

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Dynamic regulation of cell adhesion receptors is required for proper cell migration in embryogenesis, tissue repair, and cancer. Integrins and Syndecan4 (SDC4) are the main cell adhesion receptors involved in focal adhesion formation and are required for cell migration. SDC4 interacts biochemically and functionally with components of the Wnt pathway such as Frizzled7 and Dishevelled. Non-canonical Wnt signaling, particularly components of the planar cell polarity branch, controls cell adhesion and migration in embryogenesis and metastasic events. Here, we evaluate the effect of this pathway on SDC4. We have found that Wnt5a reduces cell surface levels and promotes ubiquitination and degradation of SDC4 in cell lines and dorsal mesodermal cells from Xenopus gastrulae. Gain-and loss-of-function experiments demonstrate that Dsh plays a key role in regulating SDC4 steady-state levels. Moreover, a SDC4 deletion construct that interacts inefficiently with Dsh is resistant to Wnt5a-induced degradation. Non-canonical Wnt signaling promotes monoubiquitination of the variable region of SDC4 cytoplasmic domain. Mutation of these specific residues abrogates ubiquitination and results in increased SDC4 steadystate levels. This is the first example of a cell surface protein ubiquitinated and degraded in a Wnt/Dsh-dependent manner.Cell adhesion and migration are key processes for embryogenesis, inflammatory response, tissue repair, and cancer. Cell migration is a very dynamic process that requires the continuous assembly and disassembly of the cell surface complexes mediating cell-cell and cell-extracellular matrix adhesion (1, 2). Integrins and Syndecans are the main cell adhesion receptors involved in focal adhesion (FA) 5 formation and cell-extracellular matrix communication (3). FA formation of cells plated on fibronectin requires the specific engagement and synergism between ␣5␤1 integrin and Syndecan4 (SDC4)(3, 4).SDC4 is a cell surface heparan sulfate proteoglycan that activates intracellular signaling cascades through activation of PKC␣ (4). SDC4 overexpression or knockout compromises cell migration in normal (5) and tumorigenic cells (6). More recently it was demonstrated that SDC4 is required for localization of Rac1 and membrane protrusion formation at the leading edge to allow celldirected migration (7). In normal and tumorigenic cells, endocytosis and recycling of integrins modulate FA turnover and proper cell migration (8, 9). Which extracellular cues regulate the stability and turnover of FA receptors, in particular of SDC4, is a key question to understand cell migration.Wnt proteins constitute a large family of secreted glycoproteins that can activate diverse intracellular signaling pathways in a context-dependent manner. Wnt5a is known to promote polarized cell migration by activating a non-canonical Wnt pathway through the tyrosine kinase Ror2 receptor and c-Jun N-terminal kinase (10 -12). Wnt5a increases cell migration and invasiveness of tumorigenic cells by activation of FA kinase, PKC, Rac1, and promoti...

Alternative polyadenylation (APA) generates transcript isoforms that differ in the position of the 3′ cleavage site, resulting in the production of mRNA isoforms with different length 3′ UTRs. Although widespread, the role of APA in the biology of cells, tissues, and organisms has been controversial. We identified >500 Drosophila genes that express mRNA isoforms with a long 3′ UTR in proliferating spermatogonia but a short 3′ UTR in differentiating spermatocytes due to APA. We show that the stage-specific choice of the 3′ end cleavage site can be regulated by the arrangement of a canonical polyadenylation signal (PAS) near the distal cleavage site but a variant or no recognizable PAS near the proximal cleavage site. The emergence of transcripts with shorter 3′ UTRs in differentiating cells correlated with changes in expression of the encoded proteins, either from off in spermatogonia to on in spermatocytes or vice versa. Polysome gradient fractionation revealed >250 genes where the long 3′ UTR versus short 3′ UTR mRNA isoforms migrated differently, consistent with dramatic stage-specific changes in translation state. Thus, the developmentally regulated choice of an alternative site at which to make the 3′ end cut that terminates nascent transcripts can profoundly affect the suite of proteins expressed as cells advance through sequential steps in a differentiation lineage.

Extracellular regulation of growth factor signaling is a key event for embryonic patterning. Heparan sulfate proteoglycans (HSPG) are among the molecules that regulate this signaling during embryonic development. Here we study the function of syndecan1 (Syn1), a cell-surface HSPG expressed in the non-neural ectoderm during early development of Xenopus embryos. Overexpression of Xenopus Syn1 (xSyn1) mRNA is sufficient to reduce BMP signaling, induce chordin expression and rescue dorso-ventral patterning in ventralized embryos. Experiments using chordin morpholinos established that xSyn1 mRNA can inhibit BMP signaling in the absence of chordin. Knockdown of xSyn1 resulted in a reduction of BMP signaling and expansion of the neural plate with the concomitant reduction of the non-neural ectoderm. Overexpression of xSyn1 mRNA in xSyn1 morphant embryos resulted in a biphasic effect, with BMP being inhibited at high concentrations and activated at low concentrations of xSyn1. Interestingly, the function of xSyn1 on dorso-ventral patterning and BMP signaling is specific for this HSPG. In summary, we report that xSyn1 regulates dorso-ventral patterning of the ectoderm through modulation of BMP signaling.