The role of DKK3 in synapse dynamics in the adult hippocampus and in Alzheimer’s Disease

Synaptic loss highly correlates with cognitive decline in Alzheimer’s Disease (AD). Accumulating evidence implicates deregulation of Wnt signalling in AD pathology and synapse dysfunction. Wnt signalling plays an important role in synapse formation during development and synapse plasticity and maintenance in the adult brain. Recent studies showed that Dickkopf-3 (DKK3), an abundantly expressed Wnt antagonist, is elevated in the cerebral spinal fluid and accumulates in Amyloid beta (Aβ) plaques in AD patients. However, the role of DKK3 in the brain is mostly unexplored. I examined the function of DKK3 in mature synapses and in AD by performing gain- and loss-of-function studies, and using biochemical, molecular and imaging techniques. I showed that DKK3 is present in neurons, synapses, and astrocytes of the adult hippocampus. DKK3 gain-of-function leads to a decrease in excitatory to inhibitory (E/I) synapse ratio, whereas loss-of-function of DKK3 leads to an increase in E/I synapse ratio in the hippocampus. Exploring the role of DKK3 in E/I synaptic changes after long-term depression (LTD), I discovered that DKK3 secretion is increased and that DKK3 is required for the E/I synapse reorganisation after NMDAR-mediated LTD. These findings reveal a previously unknown role of DKK3 in regulating E/I synapse density and advances our knowledge of E/I synapse balance regulation in the adult hippocampus. Investigating the role of DKK3 in AD, I found that DKK3 secretion is increased in the J20 hippocampus, which exhibits decreased E/I synapse ratio. In addition, I showed that DKK3 accumulates in dystrophic neurites around plaques and promotes plaque expansion, possibly through microglia regulation. My results suggest that DKK3 contributes to AD pathology by controlling plaques growth and may further contribute by affecting E/I synapse imbalance in the hippocampus. In summary, these results demonstrate a novel mechanism, by which deficient Wnt signalling contributes to synapse vulnerability and pathology in AD

Restoring synapse integrity and memory in Alzheimer’s disease by downregulation of the Wnt antagonist Dickkopf-3

Increasing evidence supports a role of deficient Wnt signaling in Alzheimer’s disease (AD). Recent studies reveal that the secreted Wnt antagonist Dickkopf-3 (DKK3) is elevated in the human AD brain. Here, we investigate the contribution of DKK3 to synapse integrity in the healthy and AD brain. We uncover a novel genetic link between DKK3 gene variants and AD risk. Our findings show that DKK3 protein is increased in different human brain fractions consistent with disease progression. In the hAPP-J20 and hAPPNL-G-F/NL-G-F AD models, DKK3 accumulates at plaques in the brain. Oligomers of amyloid-β enhance the secretion of DKK3 from cultured neurons and DKK3 secretion is also increased in hippocampal slices of hAPP-J20 mice. In addition, gain-of-function experiments revealed that DKK3 decreases the density of excitatory synapses through inhibition of the canonical Wnt/GSK3β pathway but increases inhibitory synapse density through activation of the Wnt/JNK pathway. Our studies demonstrate that in vivo DKK3 downregulation restores synapse number in hAPP-J20 mice. Importantly, DKK3 knockdown improves memory in this AD model. Collectively, our findings identify DKK3 as a novel driver of synapse defects and memory impairment in AD

Rational design of a conformation-specific antibody for the quantification of Aβ oligomers.

Protein misfolding and aggregation is the hallmark of numerous human disorders, including Alzheimer's disease. This process involves the formation of transient and heterogeneous soluble oligomers, some of which are highly cytotoxic. A major challenge for the development of effective diagnostic and therapeutic tools is thus the detection and quantification of these elusive oligomers. Here, to address this problem, we develop a two-step rational design method for the discovery of oligomer-specific antibodies. The first step consists of an "antigen scanning" phase in which an initial panel of antibodies is designed to bind different epitopes covering the entire sequence of a target protein. This procedure enables the determination through in vitro assays of the regions exposed in the oligomers but not in the fibrillar deposits. The second step involves an "epitope mining" phase, in which a second panel of antibodies is designed to specifically target the regions identified during the scanning step. We illustrate this method in the case of the amyloid β (Aβ) peptide, whose oligomers are associated with Alzheimer's disease. Our results show that this approach enables the accurate detection and quantification of Aβ oligomers in vitro, and in Caenorhabditis elegans and mouse hippocampal tissues

Reversal of Synapse Degeneration by Restoring Wnt Signaling in the Adult Hippocampus

Synapse degeneration occurs early in neurodegenerative diseases and correlates strongly with cognitive decline in Alzheimer's disease (AD). The molecular mechanisms that trigger synapse vulnerability and those that promote synapse regeneration after substantial synaptic failure remain poorly understood. Increasing evidence suggests a link between a deficiency in Wnt signaling and AD. The secreted Wnt antagonist Dickkopf-1 (Dkk1), which is elevated in AD, contributes to amyloid-β-mediated synaptic failure. However, the impact of Dkk1 at the circuit level and the mechanism by which synapses disassemble have not yet been explored. Using a transgenic mouse model that inducibly expresses Dkk1 in the hippocampus, we demonstrate that Dkk1 triggers synapse loss, impairs long-term potentiation, enhances long-term depression, and induces learning and memory deficits. We decipher the mechanism involved in synapse loss induced by Dkk1 as it can be prevented by combined inhibition of the Gsk3 and RhoA-Rock pathways. Notably, after loss of synaptic connectivity, reactivation of the Wnt pathway by cessation of Dkk1 expression completely restores synapse number, synaptic plasticity, and long-term memory. These findings demonstrate the remarkable capacity of adult neurons to regenerate functional circuits and highlight Wnt signaling as a targetable pathway for neuronal circuit recovery after synapse degeneration

Downregulation of Dickkopf-3, a Wnt antagonist elevated in Alzheimer’s disease, restores synapse integrity and memory in a disease mouse model

Increasing evidence supports a role for deficient Wnt signaling in Alzheimer’s disease (AD). Studies reveal that the secreted Wnt antagonist Dickkopf-3 (DKK3) colocalizes to amyloid plaques in AD patients. Here, we investigate the contribution of DKK3 to synapse integrity in healthy and AD brains. Our findings show that DKK3 expression is upregulated in the brains of AD subjects and that DKK3 protein levels increase at early stages in the disease. In hAPP-J20 and hAPPNL-G-F/NL-G-F mouse AD models, extracellular DKK3 levels are increased and DKK3 accumulates at dystrophic neuronal processes around plaques. Functionally, DKK3 triggers the loss of excitatory synapses through blockade of the Wnt/GSK3β signaling with a concomitant increase in inhibitory synapses via activation of the Wnt/JNK pathway. In contrast, DKK3 knockdown restores synapse number and memory in hAPP-J20 mice. Collectively, our findings identify DKK3 as a novel driver of synaptic defects and memory impairment in AD

Wnt7b signalling through Frizzled-7 receptor promotes dendrite development by coactivating CaMKII and JNK

The formation of complex dendritic arbors is crucial for the assembly of functional networks as abnormal dendrite formation underlies several neurodevelopmental and psychiatric disorders. Many extracellular factors have been postulated as regulators of dendritic growth. Wnt proteins play a critical role in neuronal development and circuit formation. We previously demonstrated that Wnt7b acts through the scaffold protein dishevelled 1 (Dvl1) to modulate dendrite arborisation by activating a non-canonical Wnt signalling pathway. Here,we identify the seven-transmembrane frizzled-7 (Fz7, also known as FZD7) as the receptor for Wnt7b-mediated dendrite growth and complexity. Importantly, Fz7 is developmentally regulated in the intact hippocampus, and is localised along neurites and at dendritic growth cones, suggesting a role in dendrite formation andmaturation. Fz7 lossof- function studies demonstrated that Wnt7b requires Fz7 to promote dendritic arborisation. Moreover, in vivo Fz7 loss of function results in dendritic defects in the intact mouse hippocampus. Furthermore, our findings reveal that Wnt7b and Fz7 induce the phosphorylation of Ca2 +/calmodulin-dependent protein kinase II (CaMKII) and JNK proteins, which are required for dendritic development. Here, we demonstrate that Wnt7b-Fz7 signals through two non-canonical Wnt pathways to modulate dendritic growth and complexity.Fil: Ferrari, María Edith. Universidad Nacional de Rosario. Facultad de Ciencias Bioquímicas y Farmacéuticas. Laboratorio de Toxicología Experimental; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario; ArgentinaFil: Bernis, Maria Eugenia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Química Biológica de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Centro de Investigaciones en Química Biológica de Córdoba; ArgentinaFil: McLeod, Faye. Colegio Universitario de Londres; Reino UnidoFil: Podpolny, Marina. University College London; Estados UnidosFil: Coullery, Romina Paola. Universidad Nacional de Rosario. Facultad de Ciencias Bioquímicas y Farmacéuticas. Laboratorio de Toxicología Experimental; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario; ArgentinaFil: Casadei, Inelia Mailín Iara. Universidad Nacional de Rosario. Facultad de Ciencias Bioquímicas y Farmacéuticas. Laboratorio de Toxicología Experimental; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario; ArgentinaFil: Salinas, Patricia. Colegio Universitario de Londres; Reino UnidoFil: Rosso, Silvana Beatriz. Universidad Nacional de Rosario. Facultad de Ciencias Bioquímicas y Farmacéuticas. Laboratorio de Toxicología Experimental; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario; Argentin