Marcela Colombres

Wnt signaling in neuroprotection and stem cell differentiation.

In the past several years, we postulated that the loss of Wnt signaling was implicated in the pathology of Alzheimers disease (AD). Since then, our lab and other groups have confirmed the involvement of the Wnt signaling in some aspects of AD. So far, we have demonstrated that activation of Wnt signaling protects neurons against neurotoxic injuries, including both amyloid-beta (Abeta) fibrils and Abeta oligomers by using either lithium, an inhibitor of the glycogen-synthase-kinase-3beta (GSK-3beta), or different Wnt ligands. Also, we have found that several molecules which activate well known neurotransmitter systems and other signaling system, are able by crosstalk to activate Wnt/beta-catenin signaling in order to protect neurons against both Abeta fibrils or Abeta oligomers. In particular, the activation of non-canonical Wnt signaling was able to protect postsynaptic regions and dendritic spines against Abeta oligomers. Furthermore Wnt signaling ligands also affect stem cells, and they are also involved in cell fate decision during neurogenesis and embryonic development as well as in adult stem cells differentiation in the nervous system. The Wnt signaling plays a key role modulating their cell differentiation or proliferation states. Altogether, these findings in both stem cell biology and neuroprotection, may introduce new approaches in the treatment of neurodegenerative diseases, including drug screening and therapies against neurodegenerative diseases which activates the Wnt signaling pathway.

Wnt-7a Induces Presynaptic Colocalization of α7-Nicotinic Acetylcholine Receptors and Adenomatous Polyposis Coli in Hippocampal Neurons

Nicotinic acetylcholine receptors (nAChRs) contribute significantly to hippocampal function. α7-nAChRs are present in presynaptic sites in hippocampal neurons and may influence transmitter release, but the factors that determine their presynaptic localization are unknown. We report here that Wnt-7a, a ligand active in the canonical Wnt signaling pathway, induces dissociation of the adenomatous polyposis coli (APC) protein from the β-catenin cytoplasmic complex and the interaction of APC with α7-nAChRs in hippocampal neurons. Interestingly, Wnt-7a induces the relocalization of APC to membranes, clustering of APC in neurites, and coclustering of APC with different, presynaptic protein markers. Wnt-7a also increases the number and size of coclusters of α7-nAChRs and APC in presynaptic terminals. These short-term changes in α7-nAChRs occur in the few minutes after ligand exposure and involve translocation to the plasma membrane without affecting total receptor levels. Longer-term exposure to Wnt-7a increases nAChR α7 subunit levels in an APC-independent manner and increases clusters of α7-nAChRs in neurites via an APC-dependent process. Together, these results demonstrate that stimulation through the canonical Wnt pathway regulates the presynaptic localization of APC and α7-nAChRs with APC serving as an intermediary in the α7-nAChR relocalization process. Modulation by Wnt signaling may be essential for α7-nAChR expression and function in synapses.

Calcium/calmodulin-dependent protein kinase type IV Is a target gene of the Wnt/β-catenin signaling pathway

Calcium/calmodulin‐dependent protein kinase IV (CaMKIV) plays a key role in the regulation of calcium‐dependent gene expression. The expression of CaMKIV and the activation of CREB regulated genes are involved in memory and neuronal survival. We report here that: (a) a bioinformatic analysis of 15,476 promoters of the human genome predicted several Wnt target genes, being CaMKIV a very interesting candidate; (b) CaMKIV promoter contains TCF/LEF transcription motifs similar to those present in Wnt target genes; (c) biochemical studies indicate that lithium and the canonical ligand Wnt‐3a induce CaMKIV mRNA and protein expression levels in rat hippocampal neurons as well as CaMKIV promoter activity; (d) treatment of hippocampal neurons with Wnt‐3a increases the binding of β‐catenin to the CaMKIV promoter: (e) In vivo activation of the Wnt signaling improve spatial memory impairment and restores the expression of CaMKIV in a mice double transgenic model for Alzheimers disease which shows decreased levels of the kinase. We conclude that CaMKIV is regulated by the Wnt signaling pathway and that its expression could play a role in the neuroprotective function of the Wnt signaling against the Alzheimers amyloid peptide. J. Cell. Physiol. 221: 658–667, 2009.

Synaptotoxicity in Alzheimer's disease: The Wnt signaling pathway as a molecular target

Recent evidence supports a role of the Wnt pathway in neurodegenerative disorders such as Alzheimers disease (AD). A relationship between amyloid‐β‐peptide (Aβ)‐induced neurotoxicity and a decrease in the cytoplasmatic levels of β‐catenin has been proposed. Also, the inhibition of glycogen synthase kinase (GSK‐3β), a central modulator of the pathway, protects rat hippocampal neurons from Aβ‐induced damage. Interestingly, during the progression of AD, it has been described that active GSK‐3β is found in neuronal cell bodies and neurites, co‐localizing with pre‐neurofibrillary tangles observed in disease brains. Since Aβ oligomers are associated with the post‐synaptic region and we have found that the non‐canonical Wnt signaling modulates PSD‐95 and glutamate receptors, we propose that the synaptic target for Aβ oligomers in AD is the postsynaptic region and at the molecular level is the non‐canonical Wnt signaling pathway. Altogether, our evidence suggests that a sustained loss of Wnt signaling function may be involved in the Aβ‐dependent neurodegeneration observed in AD brains and that the activation of this signaling pathway could be of therapeutic interest in AD.

An Overview of the Current and Novel Drugs for Alzheimers Disease with Particular Reference to Anti-Cholinesterase Compounds

Several cellular processes could be targeted if the complex nature of Alzheimers disease (AD) was already understood. Most of AD treatments have been focused on the inhibition of acetylcholinesterase (AChE) in order to raise the levels of its substrate, i.e. the neurotransmitter acetylcholine (ACh), to augment cognitive functions of affected patients. Effectiveness in AChE inhibition and side-effect issues of clinical (tacrine, donepezil, galanthamine and rivastigmine) as well as of novel inhibitors is reviewed here. Novel design methods for the inhibition of AChE include the use of in silico tools to predict the interactions between AChE and the desired compound, both at the active site of the enzyme, responsible of hydrolysing ACh and with the peripheral anionic site (PAS), which has been described as a promoting agent of the amyloid beta-peptide (A beta) aggregation present in the senile plaques of the brain of AD individuals.

Genome-wide identification of new Wnt/β-catenin target genes in the human genome using CART method

BackgroundThe importance of in silico predictions for understanding cellular processes is now widely accepted, and a variety of algorithms useful for studying different biological features have been designed. In particular, the prediction of cis regulatory modules in non-coding human genome regions represents a major challenge for understanding gene regulation in several diseases. Recently, studies of the Wnt signaling pathway revealed a connection with neurodegenerative diseases such as Alzheimers. In this article, we construct a classification tool that uses the transcription factor binding site motifs composition of some gene promoters to identify new Wnt/β-catenin pathway target genes potentially involved in brain diseases.ResultsIn this study, we propose 89 new Wnt/β-catenin pathway target genes predicted in silico by using a method based on multiple Classification and Regression Tree (CART) analysis. We used as decision variables the presence of transcription factor binding site motifs in the upstream region of each gene. This prediction was validated by RT-qPCR in a sample of 9 genes. As expected, LEF1, a member of the T-cell factor/lymphoid enhancer-binding factor family (TCF/LEF1), was relevant for the classification algorithm and, remarkably, other factors related directly or indirectly to the inflammatory response and amyloidogenic processes also appeared to be relevant for the classification. Among the 89 new Wnt/β-catenin pathway targets, we found a group expressed in brain tissue that could be involved in diverse responses to neurodegenerative diseases, like Alzheimers disease (AD). These genes represent new candidates to protect cells against amyloid β toxicity, in agreement with the proposed neuroprotective role of the Wnt signaling pathway.ConclusionsOur multiple CART strategy proved to be an effective tool to identify new Wnt/β-catenin pathway targets based on the study of their regulatory regions in the human genome. In particular, several of these genes represent a new group of transcriptional dependent targets of the canonical Wnt pathway. The functions of these genes indicate that they are involved in pathophysiology related to Alzheimers disease or other brain disorders.

β-Amyloid Oligomers Affect the Structure and Function of the Postsynaptic Region: Role of the Wnt Signaling Pathway

Background: Alzheimer’s disease (AD) is the most prevalent neurodegenerative disease in the growing population of elderly people. Synaptic dysfunction is an early manifestation of AD. The cellular mechanism by which β-amyloid peptide (Aβ) affects synapses remains unclear. Aβ oligomers target synapses in cultured rat hippocampal neurons suggesting that they play a key role in the regulation of synapses. Objective: The aim of this work is to study the effect of Aβ oligomers on the central synapses and the possible role of the Wnt signaling pathway in preventing the Aβ effects. Methods: We used rat hippocampal neurons, immunofluorescence and western blot procedures to detect synaptic proteins. Results: Aβ oligomers induced a reduction of the postsynaptic density protein 95 (PSD-95) and the NMDA glutamate receptors. We found that Wnt-5a, a noncanonical Wnt ligand, prevents the decrease triggered by Aβ oligomers in the glutamate receptor and PSD-95. Conclusion: Altogether, our results suggest that Aβ oligomers decrease the synaptic responses by affecting the postsynaptic region at different levels. The Wnt signaling activation prevents synaptic damage induced by Aβ, which raises the possibility of a new therapeutic intervention for the treatment of synaptic changes observed in AD.

Acetylcholinesterase (AChE) - Amyloid-β-Peptide Complexes in Alzheimers Disease. The Wnt Signaling Pathway

Alzheimers disease (AD) is characterized by selective neuronal cell death, which is probably caused by amyloid beta-peptide (Abeta) oligomers and fibrils. We have found that acetylcholinesterase (AChE), a senile plaque component, increases amyloid fibril assembly with the formation of highly toxic complexes (Abeta-AChE). The neurotoxic effect induced by Abeta-AChE complexes was higher than that induced by the Abeta peptide alone as shown both in vitro (hippocampal neurons) and in vivo (rats injected with Abeta peptide in the dorsal hippocampus). Interestingly, treatment with Abeta-AChE complexes decreases the cytoplasmic beta-catenin level, a key component of Wnt signaling. Conversely, the activation of this signaling pathway by Wnt-3a promotes neuronal survival and rescues changes in Wnt components (activation or subcellular localization). Moreover Frzb-1, a Wnt antagonist reverses the Wnt-3a neuroprotection effect against Abeta neurotoxicity. Compounds that mimic the Wnt signaling or modulate the cross-talking with this pathway could be used as neuroprotective agents for therapeutic strategies in AD patients.