Helicobacter pylori Urease: Potential Contributions to Alzheimer's Disease.

Alzheimer's disease (AD) causes dementia and memory loss in the elderly. Deposits of beta-amyloid peptide and hyperphosphorylated tau protein are present in a brain with AD. A filtrate of Helicobacter pylori's culture was previously found to induce hyperphosphorylation of tau in vivo, suggesting that bacterial exotoxins could permeate the blood-brain barrier and directly induce tau's phosphorylation. H. pylori, which infects ~60% of the world population and causes gastritis and gastric cancer, produces a pro-inflammatory urease (HPU). Here, the neurotoxic potential of HPU was investigated in cultured cells and in rats. SH-SY5Y neuroblastoma cells exposed to HPU (50-300 nM) produced reactive oxygen species (ROS) and had an increased [Ca2+]i. HPU-treated BV-2 microglial cells produced ROS, cytokines IL-1ß and TNF-α, and showed reduced viability. Rats received daily i.p., HPU (5 µg) for 7 days. Hyperphosphorylation of tau at Ser199, Thr205 and Ser396 sites, with no alterations in total tau or GSK-3ß levels, and overexpression of Iba1, a marker of microglial activation, were seen in hippocampal homogenates. HPU was not detected in the brain homogenates. Behavioral tests were performed to assess cognitive impairments. Our findings support previous data suggesting an association between infection by H. pylori and tauopathies such as AD, possibly mediated by its urease.

Cellular mechanisms of contextual fear memory reconsolidation: Role of hippocampal SFKs, TrkB receptors and GluN2B-containing NMDA receptors.

Memories are stored into long-term representations through a process that depends on protein synthesis. However, a consolidated memory is not static and inflexible and can be reactivated under certain circumstances, the retrieval is able to reactivate memories and destabilize them engaging a process of restabilization known as reconsolidation. Although the molecular mechanisms that mediate fear memory reconsolidation are not entirely known, so here we investigated the molecular mechanisms in the hippocampus involved in contextual fear conditioning memory (CFC) reconsolidation in male Wistar rats. We demonstrated that the blockade of Src family kinases (SFKs), GluN2B-containing NMDA receptors and TrkB receptors (TrkBR) in the CA1 region of the hippocampus immediately after the reactivation session impaired contextual fear memory reconsolidation. These impairments were blocked by the neurotrophin BDNF and the NMDAR agonist, D-Serine. Considering that the study of the link between synaptic proteins is crucial for understanding memory processes, targeting the reconsolidation process may provide new ways of disrupting maladaptive memories, such as those seen in post-traumatic stress disorder. Here we provide new insights into the cellular mechanisms involved in contextual fear memory reconsolidation, demonstrating that SFKs, GluN2B-containing NMDAR, and TrkBR are necessary for the reconsolidation process. Our findings suggest a link between BDNF and SFKs and GluN2B-containing NMDAR as well as a link between NMDAR and SFKs and TrkBR in fear memory reconsolidation. These preliminary pharmacological findings provide new evidence of the mechanisms involved in the reconsolidation of fear memory and have the potential to contribute to the development of treatments for psychiatric disorders involving maladaptive memories.

Role of Hippocampal Wnt Signaling Pathways on Contextual Fear Memory Reconsolidation.

Memories already consolidated when reactivated return to a labile state and can be modified, this process is known as reconsolidation. It is known the Wnt signaling pathways can modulate hippocampal synaptic plasticity as well as learning and memory. Yet, Wnt signaling pathways interact with NMDA (N-methyl-D-aspartate) receptors. However, whether canonical Wnt/ß-catenin and non-canonical Wnt/Ca2 + signaling pathways are required in the CA1 region of hippocampus for contextual fear memory reconsolidation remains unclear. So, here we verified that the inhibition of canonical Wnt/ß-catenin pathway with DKK1 (Dickkopf-1) into CA1 impaired the reconsolidation of contextual fear conditioning (CFC) memory when administered immediately and 2 h after reactivation session but not 6 h later, while the inhibition of non-canonical Wnt/Ca2+ signaling pathway with SFRP1 (Secreted frizzled-related protein-1) into CA1 immediately after reactivation session had no effect. Moreover, the impairment induced by DKK1 was blocked by the administration of the agonist of the NMDA receptors glycine site, D-Serine, immediately and 2 h after reactivation session. We found that hippocampal canonical Wnt/ß-catenin is necessary to the reconsolidation of CFC memory at least two hours after reactivation, while non-canonical Wnt/Ca2+ signaling pathway is not involved in this process and, that there is a link between Wnt/ß-catenin signaling pathway and NMDA receptors. In view of this, this study provides new evidence regarding the neural mechanisms underlying contextual fear memory reconsolidation and contributes to provide a new possible target for the treatment of fear related disorders.

Memory persistence induced by environmental enrichment is dependent on different brain structures.

Environmental enrichment (EE) has been demonstrated to have a beneficial effect on different functions of the central nervous system in several mammal species, being used to improve behavior and cell damage in various neurological and psychiatric diseases. However, little has been investigated on the effect of EE in healthy animals, particularly regarding its impact on memory persistence and the brain structures involved. Therefore, here we verified in male Wistar rats that contextual fear conditioning (CFC) memory persistence, tested 28 days after the CFC training session, was facilitated by 5 weeks of exposure to EE, with no effect in groups tested 7 or 14 days after CFC training. However, a two-week exposure to EE did not affect memory persistence. Moreover, we investigated the role of specific brain regions in mediating the effect of EE on memory persistence. We conducted inactivation experiments using the GABAergic agonist Muscimol to target the basolateral amygdala (BLA), medial prefrontal cortex (mPFC), and CA1 region of the hippocampus (CA1). Inactivation of the BLA immediately and 12 h after CFC training impaired the effect of EE on memory persistence. Similarly, inactivation of the CA1 region and mPFC 12 h after training, but not immediately, also impaired the effect of EE on memory persistence. These results have important scientific implications as they shed new light on the effect of an enriched environment on memory persistence and the brain structures involved, thereby helping elucidate how an environment rich in experiences can modify the persistence of learned information.

PKMζ Maintains Remote Contextual Fear Memory by Inhibiting GluA2-Dependent AMPA Receptor Endocytosis in the Prelimbic Cortex.

Fear memories allow animals to recognize and adequately respond to dangerous situations. The prelimbic cortex (PrL) is a crucial node in the circuitry that encodes contextual fear memory, and its activity is central for fear memory expression over time. However, while PrL has been implicated in contextual fear memory storage, the molecular mechanisms underlying its maintenance remain unclear. Protein kinase M zeta (PKMζ) is a persistently active enzyme which has been shown to maintain many forms of memories by inhibiting the endocytosis of GluA2-containing AMPA receptors. Therefore, we hypothesized that PKMζ action upon GluA2-containing AMPARs could be a mechanism for contextual fear memory maintenance in the PrL. To test this hypothesis, we trained rats in a contextual fear conditioning (CFC) paradigm and administered intra-PrL infusions of the PKMζ inhibitor ZIP, the GluA2-dependent endocytosis inhibitor GluA23Y or the inactive peptide GluA23Y(s), either two or twenty days after conditioning, and assessed long-term memory retention twenty-four hours later. We found that acute inhibition of GluA2-dependent AMPAR endocytosis in the PrL does not affect recent or remote contextual fear memory maintenance. Also, PKMζ inhibition in the PrL does not impair the maintenance of recent contextual fear memory. However, we found that inhibition of prelimbic PKMζ at a remote time point disrupts contextual fear memory maintenance, and that blocking GluA2-dependent removal of AMPARs prevents this impairment. Our results confirm the central role of PrL in fear memory and identify PKMζ-induced inhibition of GluA2-containing AMPAR endocytosis as a key mechanism governing remote contextual fear memory maintenance.

Involvement of medial prefrontal cortex canonical Wnt/ß-catenin and non-canonical Wnt/Ca2+ signaling pathways in contextual fear memory in male rats.

Wnt proteins activate different signaling pathways, such as the canonical Wnt/ß-catenin signaling pathway and non-canonical ß-catenin-independent signaling pathway and have been related to several functions in central nervous system, including learning and memory. However, whether these signaling pathways are required in the medial prefrontal cortex (mPFC) for fear memory acquisition, consolidation and retrieval remains unclear. To address this question, we submitted male rats to a contextual fear conditioning (CFC) paradigm, and administered canonical Wnt/ß-catenin and non-canonical Wnt/Ca2+ signaling pathways inhibitors, DKK1 and SFRP1, respectively, into the prelimbic (PrL) subdivision of the mPFC at different moments and evaluated short-term and long-term memory acquisition, consolidation and retrieval. We found that blocking canonical Wnt/ß-catenin and non-canonical Wnt/Ca2+ signaling pathways 15 min before or immediately after CFC training had no effect on STM and LTM of CFC, while their blockade 15 min before the retention test prevented the retrieval of STM and LTM of CFC. These results highlight the importance of the mPFC in fear memory retrieval demonstrating that both canonical Wnt/ß-catenin and non-canonical Wnt/Ca2+ signaling pathways participate in this process. To understand how brain systems act on fear memories could provide a new target for the treatment of fear related disorders such as post-traumatic stress disorder and other anxiety disorders.

Molecular Mechanisms in Hippocampus Involved on Object Recognition Memory Consolidation and Reconsolidation.

Acquired information is stabilized into long-term memory through a process known as consolidation. Though, after consolidation, when stored information is retrieved they can be again susceptible, allowing modification, updating and strengthening and to be re-stabilized they need a new process referred to as memory reconsolidation. However, the molecular mechanisms of recognition memory consolidation and reconsolidation are not fully understood. Also, considering that the study of the link between synaptic proteins is key to understanding of memory processes, we investigated, in male Wistar rats, molecular mechanisms in the hippocampus involved on object recognition memory (ORM) consolidation and reconsolidation. We verified that the blockade of AMPA receptors (AMPAr) and L-VDCCs calcium channels impaired ORM consolidation and reconsolidation when administered into CA1 immediately after sample phase or reactivation phase and that these impairments were blocked by the administration of AMPAr agonist and of neurotrophin BDNF. Also, the blockade of CaMKII impaired ORM consolidation when administered 3 h after sample phase but had no effect on ORM reconsolidation and its effect was blocked by the administration of BDNF, but not of AMPAr agonist. So, this study provides new evidence of the molecular mechanisms involved on the consolidation and reconsolidation of ORM, demonstrating that AMPAr and L-VDCCs are necessary for the consolidation and reconsolidation of ORM while CaMKII is necessary only for the consolidation and also that there is a link between BDNF and AMPAr, L-VDCCs and CaMKII as well as a link between AMPAr and L-VDCCs on ORM consolidation and reconsolidation.