Bioactive lipids and metabolic syndrome-a symposium report.

Recent research has shed light on the cellular and molecular functions of bioactive lipids that go far beyond what was known about their role as dietary lipids. Bioactive lipids regulate inflammation and its resolution as signaling molecules. Genetic studies have identified key factors that can increase the risk of cardiovascular diseases and metabolic syndrome through their effects on lipogenesis. Lipid scientists have explored how these signaling pathways affect lipid metabolism in the liver, adipose tissue, and macrophages by utilizing a variety of techniques in both humans and animal models, including novel lipidomics approaches and molecular dynamics models. Dissecting out these lipid pathways can help identify mechanisms that can be targeted to prevent or treat cardiometabolic conditions. Continued investigation of the multitude of functions mediated by bioactive lipids may reveal additional components of these pathways that can provide a greater understanding of metabolic homeostasis.

Extending human healthspan and longevity: a symposium report.

For many years, it was believed that the aging process was inevitable and that age-related diseases could not be prevented or reversed. The geroscience hypothesis, however, posits that aging is, in fact, malleable and, by targeting the hallmarks of biological aging, it is indeed possible to alleviate age-related diseases and dysfunction and extend longevity. This field of geroscience thus aims to prevent the development of multiple disorders with age, thereby extending healthspan, with the reduction of morbidity toward the end of life. Experts in the field have made remarkable advancements in understanding the mechanisms underlying biological aging and identified ways to target aging pathways using both novel agents and repurposed therapies. While geroscience researchers currently face significant barriers in bringing therapies through clinical development, proof-of-concept studies, as well as early-stage clinical trials, are underway to assess the feasibility of drug evaluation and lay a regulatory foundation for future FDA approvals in the future.

cGMP-dependent protein kinase type II knockout mice exhibit working memory impairments, decreased repetitive behavior, and increased anxiety-like traits.

Neuronal activity regulates AMPA receptor trafficking, a process that mediates changes in synaptic strength, a key component of learning and memory. This form of plasticity may be induced by stimulation of the NMDA receptor which, among its activities, increases cyclic guanosine monophosphate (cGMP) through the nitric oxide synthase pathway. cGMP-dependent protein kinase type II (cGKII) is ultimately activated via this mechanism and AMPA receptor subunit GluA1 is phosphorylated at serine 845. This phosphorylation contributes to the delivery of GluA1 to the synapse, a step that increases synaptic strength. Previous studies have shown that cGKII-deficient mice display striking spatial learning deficits in the Morris Water Maze compared to wild-type littermates as well as lowered GluA1 phosphorylation in the postsynaptic density of the prefrontal cortex (Serulle et al., 2007; Wincott et al., 2013). In the current study, we show that cGKII knockout mice exhibit impaired working memory as determined using the prefrontal cortex-dependent Radial Arm Maze (RAM). Additionally, we report reduced repetitive behavior in the Marble Burying task (MB), and heightened anxiety-like traits in the Novelty Suppressed Feeding Test (NSFT). These data suggest that cGKII may play a role in the integration of information that conveys both anxiety-provoking stimuli as well as the spatial and environmental cues that facilitate functional memory processes and appropriate behavioral response.

Spatial memory deficits and motor coordination facilitation in cGMP-dependent protein kinase type II-deficient mice.

Activity-dependent trafficking of AMPA receptors to synapses regulates synaptic strength. Activation of the NMDA receptor induces several second messenger pathways that contribute to receptor trafficking-dependent plasticity, including the NO pathway, which elevates cGMP. In turn, cGMP activates the cGMP-dependent protein kinase type II (cGKII), which phosphorylates the AMPA receptor subunit GluA1 at serine 845, a critical step facilitating synaptic delivery in the mechanism of activity-dependent synaptic potentiation. Since cGKII is expressed in the striatum, amygdala, cerebral cortex, and hippocampus, it has been proposed that mice lacking cGKII may present phenotypic differences compared to their wild-type littermates in emotion-dependent tasks, learning and memory, and drug reward salience. Previous studies have shown that cGKII KO mice ingest higher amounts of ethanol as well as exhibit elevated anxiety levels compared to wild-type (WT) littermates. Here, we show that cGKII KO mice are significantly deficient in spatial learning while exhibiting facilitated motor coordination, demonstrating a clear dependence of memory-based tasks on cGKII. We also show diminished GluA1 phosphorylation in the postsynaptic density (PSD) of cGKII KO prefrontal cortex while in hippocampal PSD fractions, phosphorylation was not significantly altered. These data suggest that the role of cGKII may be more robust in particular brain regions, thereby impacting complex behaviors dependent on these regions differently.

Metformin activates an atypical PKC-CBP pathway to promote neurogenesis and enhance spatial memory formation.

VIDEO ABSTRACT: Although endogenous recruitment of adult neural stem cells has been proposed as a therapeutic strategy, clinical approaches for achieving this are lacking. Here, we show that metformin, a widely used drug, promotes neurogenesis and enhances spatial memory formation. Specifically, we show that an atypical PKC-CBP pathway is essential for the normal genesis of neurons from neural precursors and that metformin activates this pathway to promote rodent and human neurogenesis in culture. Metformin also enhances neurogenesis in the adult mouse brain in a CBP-dependent fashion, and in so doing enhances spatial reversal learning in the water maze. Thus, metformin, by activating an aPKC-CBP pathway, recruits neural stem cells and enhances neural function, thereby providing a candidate pharmacological approach for nervous system therapy.

Stimulation of entorhinal cortex promotes adult neurogenesis and facilitates spatial memory.

Deep brain stimulation (DBS) is an established therapeutic modality for the treatment of movement disorders and an emerging therapeutic approach for the treatment of disorders of mood and thought. For example, recently we have shown that DBS of the fornix may ameliorate cognitive decline associated with dementia. However, like other applications of DBS, the mechanisms mediating these clinical effects are unknown. As DBS modulates neurophysiological activity in targeted brain regions, DBS might influence cognitive function via activity-dependent regulation of hippocampal neurogenesis. Using stimulation parameters analogous to clinical high-frequency DBS, here we addressed this question in mice. We found that acute stimulation of the entorhinal cortex (EC) transiently promoted proliferation in the dentate gyrus (DG). Cells generated as a consequence of stimulation differentiated into neurons, survived for at least several weeks, and acquired normal dentate granule cell (DGC) morphology. Importantly, stimulation-induced promotion of neurogenesis was limited to the DG and not associated with changes in apoptotic cell death. Using immunohistochemical approaches, we found that, once sufficiently mature, these stimulation-induced neurons integrated into hippocampal circuits supporting water-maze memory. Finally, formation of water-maze memory was facilitated 6 weeks (but not 1 week) after bilateral stimulation of the EC. The delay-dependent nature of these effects matches the maturation-dependent integration of adult-generated DGCs into dentate circuits supporting water-maze memory. Furthermore, because the beneficial effects of EC stimulation were prevented by blocking neurogenesis, this suggests a causal relationship between stimulation-induced promotion of adult neurogenesis and enhanced spatial memory.

Memory for the order of events in specific sequences: contributions of the hippocampus and medial prefrontal cortex.

Episodic memory involves remembering the incidental order of a series of events that comprise a specific experience. Current models of temporal organization in episodic memory have demonstrated that animals can make memory judgments about the order of serially presented events; however, in these protocols, the animals can judge items based on their relative recency. Thus, it remains unclear as to whether animals use the specific order of items in forming memories of distinct sequences. To resolve this important issue in memory representation, we presented mice repeatedly with two widely separated odor sequences and then tested their natural exploratory preference between pairs of odors selected from within or between sequences. Intact animals preferred to investigate odors that occurred earlier within each sequence, indicating they did remember the order of events within each distinct sequence. In contrast, intact animals did not discriminate between pairs of odors from different sequences. These findings indicate that preferences were not guided by relative recency, which would be expected to support graded discrimination between widely separated events. Furthermore, damage to either the hippocampus or the medial prefrontal cortex eliminated order preference within sequences. Despite the deficit in order memory, control recognition tests showed that normal mice and mice with hippocampal or medial prefrontal damage could correctly identify previously experienced odors compared with novel odors. These findings provide strong evidence that animals form representations of the order of events within specific experiences and that the hippocampus and prefrontal cortex are essential to order memory.

Prefrontal cortex: role in acquisition of overlapping associations and transitive inference.

"Transitive inference" refers to the ability to judge from memory the relationships between indirectly related items that compose a hierarchically organized series, and this capacity is considered a fundamental feature of relational memory. Here we explored the role of the prefrontal cortex in transitive inference by examining the performance of mice with selective damage to the medial prefrontal cortex. Damage to the infralimbic and prelimbic regions resulted in significant impairment in the acquisition of a series of overlapping odor discrimination problems, such that animals with prefrontal lesions required twice as many trials to learn compared to sham-operated controls. Following eventually successful acquisition, animals with medial prefrontal lesions were severely impaired on a transitive inference probe test, whereas they performed as well as controls on a test that involved a nontransitive judgment from a novel odor pairing. These results suggest that the prefrontal cortex is part of an integral hippocampal-cortical network essential for relational memory organization.

Distinct contributions of the hippocampus and medial prefrontal cortex to the "what-where-when" components of episodic-like memory in mice.

There is a current controversy regarding whether non-human animals have a capacity for episodic memory, defined by the ability to remember what happened and where and when it occurred. It is also unclear which brain structures support the "what," "where," and "when" aspects of episodic memory. Here we addressed these issues by testing episodic memory in mice, using an object recognition task that has previously been employed to assess the "what," "where," and "when" components of recognition memory. Whereas intact mice remembered which objects they had explored, as well as when and where they were experienced, mice with damage to the hippocampus were impaired on all three components of the task. In contrast, animals with medial prefrontal cortical lesions were selectively impaired on the "where" component of the task, but had intact memory for "what" and "when." These results are consistent with the hypothesis that the hippocampus integrates the "what," "where," and "when" features of unique experiences, whereas the prefrontal cortex makes a more selective contribution to retrieving source information about where events occurred.

The hippocampus contributes to memory expression during transitive inference in mice.

There is substantial evidence that the hippocampus plays a role in transitive inference, the capacity to link overlapping memories and subsequently make novel judgments between elements of those memories that are only indirectly related. However, it is unclear whether the hippocampus is involved primarily during the original acquisition of the overlapping memories, or additionally during the flexible expression of those memories during transitive judgments. Here, we demonstrated that selective hippocampal damage produced after acquisition of the overlapping memories resulted in a severe impairment in subsequent transitive inference judgments, indicating that the hippocampus does play an important role beyond the initial learning phase. Furthermore, this study extends to mice a role for the hippocampus in transitive inference, as previously observed in other species.

Vasopressin 1b receptor knock-out impairs memory for temporal order.

Mice lacking a functional vasopressin 1b receptor (Avpr1b) display decreased levels of aggression and social memory. Here, we used Avpr1b-knock-out (Avpr1b(-/-)) mice to examine whether an abnormality of this receptor results in specific cognitive deficits in the domain of hippocampal function. Avpr1b(-/-) mice were deficient in sociability and in detecting social novelty, extending previous findings of impairment in social recognition in these mutants. Avpr1b(-/-) mice could recognize previously explored objects and remember where they were experienced, but they were impaired in remembering the temporal order of presentation of those objects. Consistent with this finding, Avpr1b(-/-) mice were also impaired on an object-odor paired associate task that involved a temporal discontiguity between the associated elements. Finally, Avpr1b(-/-) mice performed normally in learning a set of overlapping odor discriminations and could infer relationships among odors that were only indirectly associated (i.e., transitive inference), indicating intact relational memory. The Avpr1b is expressed at much higher levels than any other part of the brain in the pyramidal cells of hippocampal CA2 area, a subfield of the hippocampus that has physiological and genetic properties that distinguish it from subfields CA1 and CA3. The combined results suggest that the Avpr1b, perhaps in CA2, may play a highly specific role in social behavior and episodic memory. Because schizophrenia and bipolar disorder are associated with a unique pathology in CA2 and impairments in both social behavior and episodic memory, this animal model could provide insights into the etiology of these disorders.

One-trial associative odor learning in neonatal mice.

Behavior genetics studies in mice demand efficient training protocols for rapid phenotypic screening. However, the capacity of neonatal mice to form and retain associative memories has been difficult to study due to their limited sensorimotor capacities. The present study describes a method for robust, naturalistic associative learning in neonatal mice as young as 3 days old. After removal of the dam from the home cage for 2 h, preweanling CD-1 mice of ages 3, 5, and 10 days postnatal were conditioned to associate an arbitrary odorant with the suckling and milk delivery that ensued upon her return to the home cage. After a second maternal deprivation, neonates were tested on their acquired preference for that odorant. Neonates exhibited a learned preference for the conditioned odorant over a novel control odorant. No learning was observed without deprivation, that is, when the dam was removed only briefly for scenting. One-trial learning sufficed to show clear preferences for the conditioned odorant, although repeated training (three sessions over 8 days) significantly increased the expression of preference. The development of neonatal associative learning protocols requiring minimal human intervention is important for the behavioral phenotyping of mutant and transgenic strains, particularly those modeling developmental disorders.