Hippocampal Wnt Signaling: Memory Regulation and Hormone Interactions

Abstract

Wnt signaling has emerged in recent years as a major player in both nervous system development and adult synaptic plasticity. Of particular relevance to researchers studying learning and memory, Wnt signaling is critical for normal functioning of the hippocampus, a brain region that is essential for many types of memory formation and whose dysfunction is implicated in numerous neurodegenerative and psychiatric conditions. Impaired hippocampal Wnt signaling is implicated in several of these conditions, however, little is known about how Wnt signaling mediates hippocampal memory formation. This review will provide a general overview of Wnt signaling and discuss evidence demonstrating a key role for Wnt signaling in hippocampal memory formation in both normal and disease states. The regulation of Wnt signaling by ovarian sex steroid hormones will also be highlighted, given that the neuroprotection afforded by Wnt-hormone interactions may have significant implications for cognitive function in aging, neurodegenerative disease, and ischemic injury.

Keywords: Alzheimer’s disease; GSK3β; estradiol; hippocampus; progesterone; β-catenin.

Figure 1.

Wnt signaling is mediated by canonical and non-canonical pathways. (A) Canonical Wnt/β-catenin signaling is activated when a canonical Wnt ligand binds to a frizzled (Fzd) receptor and its co-receptor LRP 5/6. This leads Dishevelled (Dvl) to phosphorylate GSK3β on Serine 9, which inactivates GSK3β. This inactivation decreases phosphorylation of the transcriptional activator β-catenin and increases nuclear translocation of β-catenin, which then interacts with TCF/LEF transcriptional complexes to increase expression of downstream target genes such as Cyclin D1 and c-myc. (B) Wnt ligands binding to Fzd receptors can also activate the non-canonical JNK/planar cell polarity (PCP) or the Wnt/Ca²⁺ pathways. In the JNK/PCP pathway, activation of Dvl signals to Rho and Rac GTPases to activate ROCK and JNK, respectively, to regulate cytoskeletal reorganization. In the Wnt/Ca²⁺ pathway, G-protein mediated activation of phospholipase C increases intracellular Ca²⁺ and diacylglycerol (DAG). The rise in intracellular Ca²⁺ activates CaMKII and calcineurin to modulate transcriptional activity through regulation of CREB and NFAT, respectively. DAG activates the enzyme protein kinase C (PKC).

Figure 2.

Object training activates canonical Wnt/β-catenin-dependent signaling in the dorsal hippocampus of male mice. (A) Protein levels of Wnt7a were significantly increased relative to controls 5 minutes after training. (B) Phospho-GSK3β protein levels were significantly increased relative to controls 30 minutes after training. (C, D) At all time points following training, protein levels of β-catenin (C) and Cyclin D1 (D) were significantly increased relative to controls. Protein levels were normalized to β-actin. Each bar represents the mean ± SEM percent change from vehicle (*P ≤ 0.05 relative to controls). Insets show representative Western blots. Adapted with permission from Fortress and others (2013a).

Figure 3.

Inhibition of canonical Wnt/β-catenin-dependent signaling in the dorsal hippocampus prevents object recognition memory consolidation and downstream cell signaling. (A) Behavioral paradigm used to test object recognition. Immediately after accumulating 30 seconds exploring two identical objects (training), mice received bilateral dorsal hippocampal infusions of vehicle or one of three doses of the canonical Wnt/β-catenin inhibitor Dkk-1. Twenty-four hours later, a time at which vehicle mice remember the familiar object, memory for the familiar object was tested by allowing mice to accumulate 30 seconds of time with a familiar and novel object. Because mice are drawn to novelty, they will explore the novel object more than chance (15 seconds) if they remember the familiar training objects. (B) Twenty-four hours after training, mice infused with vehicle, but not any dose of Dkk-1 (50 ng, 100 ng, 200 ng/hemisphere), spent significantly more time than chance (dashed line at 15 seconds) with the novel object (**P < 0.01 compared to chance). These data indicate that inhibition of Wnt signaling impaired memory consolidation. Bilateral dorsal hippocampal infusion of 50 ng Dkk-1 significantly decreased dorsal hippocampal Wnt7a (C) 5 minutes after infusion and increased GSK3β (D) protein levels 4 hours after infusion (*P ≤ 0.05 relative to controls). Similarly, bilateral dorsal hippocampal infusion of 50 ng of Dkk-1 significantly decreased β-Catenin (E) protein levels in the dorsal hippocampus 4 hours after infusion. Protein levels were normalized to β-actin. Each bar represents the mean ± SEM percent change from vehicle (*P ≤ 0.05 relative to vehicle-infused mice). Insets show representative Western blots. Adapted with permission from Fortress and others (2013a).

Figure 4.

Classical and non-classical 17β-estradiol (E₂) signaling mechanisms. In the classical response (left), E₂ binds ERα and ERβ, which then translocate into the nucleus, bind to the estrogen response element (ERE) on DNA, and interact with co-regulatory proteins (including histone acetyltransferases, HAT) to influence transcription. In a non-classical response (center), E₂ may affect cell signaling in several ways. It can bind to ERs that interact with metabotropic glutamate receptors (mGluRs) at the membrane and activate extracellular regulated kinase (ERK) signaling. E₂ can also interact with NMDA receptors and membrane-bound ERs (mER) to activate the protein kinase A (PKA), phosphoinositol-3-kinase (PI3K), and mammalian target of rapamycin (mTOR) signaling pathways. mTOR signaling regulates the protein synthesis necessary for memory formation. Activation of ERK increases histone H3 acetylation (Ac). Both H3 acetylation and DNA methylation (Me) are necessary for E₂ to enhance memory consolidation. Adapted with permission from Fortress and Frick (2014).

Figure 5.

Diagram of hypothesized progesterone (P₄)-mediated cell signaling mechanisms in the dorsal hippocampus. Our data suggest that P₄ activates membrane or intracellular receptors to activate two different signaling cascades. P₄, or the membrane-associated progesterone receptor (PR) agonist BSA-P, binds to PRs such as the PGMRCs or the mPRα–ε to trigger signaling through the ERK-dependent mTOR pathway (left). Alternatively, P₄, or the intracellular agonist R5020, can bind to PR-A and PR-B to increase Wnt7a protein levels (center). The rapid rise in Wnt7a protein levels may then activate downstream canonical Wnt/β-catenin-dependent signaling (right).

Figure 6.

Progesterone (P₄)-mediated activation of canonical Wnt/β-catenin-dependent signaling. Bilateral dorsal hippocampal infusion of P₄ or the intracellular PR agonist R5020 significantly increased Wnt7a (A) and β-catenin (B) protein levels 5 minutes after infusion. Protein levels of Wnt7a and β-catenin were not affected by the membrane-associated PR agonist BSA-P, suggesting a role of intracellular PRs in activating Wnt signaling. Bilateral dorsal hippocampal infusion of P₄ also significantly increased c-myc (C) protein levels 5 minutes after infusion. Protein levels were normalized to β-actin. Each bar represents the mean ± SEM percent change from vehicle (*P ≤ 0.05 relative to vehicle-infused mice). Insets show representative Western blots. Adapted with permission from Fortress and others (2014a).