The intestinal TORC2 signaling pathway contributes to associative learning in Caenorhabditis elegans

Several types of associative learning are dependent upon the presence or absence of food, and are crucial for the survival of most animals. Target of rapamycin (TOR), a kinase which exists as a component of two complexes, TOR complex 1 (TORC1) and TOR complex 2 (TORC2), is known to act as a nutrient sensor in numerous organisms. However, the in vivo roles of TOR signaling in the nervous system remain largely unclear, partly because its multifunctionality and requirement for survival make it difficult to investigate. Here, using pharmacological inhibitors and genetic analyses, we show that TORC1 and TORC2 contribute to associative learning between salt and food availability in the nematode Caenorhabditis elegans in a process called taste associative learning. Worms migrate to salt concentrations experienced previously during feeding, but they avoid salt concentrations experienced under starvation conditions. Administration of the TOR inhibitor rapamycin causes a behavioral defect after starvation conditioning. Worms lacking either RICT-1 or SINH-1, two TORC2 components, show defects in migration to high salt levels after learning under both fed and starved conditions. We also analyzed the behavioral phenotypes of mutants of the putative TORC1 substrate RSKS-1 (the C. elegans homolog of the mammalian S6 kinase S6K) and the putative TORC2 substrates SGK-1 and PKC-2 (homologs of the serum and glucocorticoid-induced kinase 1, SGK1, and protein kinase C-α, PKC-α, respectively) and found that neuronal RSKS-1 and PKC-2, as well as intestinal SGK-1, are involved in taste associative learning. Our findings shed light on the functions of TOR signaling in behavioral plasticity and provide insight into the mechanisms by which information sensed in the intestine affects the nervous system to modulate food-searching behaviors.

Insulin signaling genes modulate nicotine-induced behavioral responses in Caenorhabditis elegans

Insulin signaling has been suggested to modulate nicotine dependence, but the underlying genetic evidence has been lacking. Here, we used the nematode, Caenorhabditis elegans, to investigate whether genetic alterations in the insulin signaling pathway affect behavioral responses to nicotine. For this, we challenged drug-naive C. elegans with an acute dose of nicotine (100 μmol/l) while recording changes in their locomotion speed. Although nicotine treatment stimulated locomotion speed in wild-type C. elegans, the same treatment reduced locomotion speed in mutants defective in insulin signaling. This phenotype could be suppressed by mutations in daf-16, a gene encoding a FOXO transcription factor that acts downstream of insulin signaling. Our data suggest that insulin signaling genes, daf-2, age-1, pdk-1, akt-1, and akt-2, modulate behavioral responses to nicotine in C. elegans, indicating a genetic link between nicotine behavior and insulin signaling.

Parallel encoding of sensory history and behavioral preference during Caenorhabditis elegans olfactory learning

Sensory experience modifies behavior through both associative and non-associative learning. In Caenorhabditis elegans, pairing odor with food deprivation results in aversive olfactory learning, and pairing odor with food results in appetitive learning. Aversive learning requires nuclear translocation of the cGMP-dependent protein kinase EGL-4 in AWC olfactory neurons and an insulin signal from AIA interneurons. Here we show that the activity of neurons including AIA is acutely required during aversive, but not appetitive, learning. The AIA circuit and AGE-1, an insulin-regulated PI3 kinase, signal to AWC to drive nuclear enrichment of EGL-4 during conditioning. Odor exposure shifts the AWC dynamic range to higher odor concentrations regardless of food pairing or the AIA circuit, whereas AWC coupling to motor circuits is oppositely regulated by aversive and appetitive learning. These results suggest that non-associative sensory adaptation in AWC encodes odor history, while associative behavioral preference is encoded by altered AWC synaptic activity.

Splicing factors control C. elegans behavioural learning in a single neuron by producing DAF-2c receptor

Alternative splicing generates protein diversity essential for neuronal properties. However, the precise mechanisms underlying this process and its relevance to physiological and behavioural functions are poorly understood. To address these issues, we focused on a cassette exon of the Caenorhabditis elegans insulin receptor gene daf-2, whose proper variant expression in the taste receptor neuron ASER is critical for taste-avoidance learning. We show that inclusion of daf-2 exon 11.5 is restricted to specific neuron types, including ASER, and is controlled by a combinatorial action of evolutionarily conserved alternative splicing factors, RBFOX, CELF and PTB families of proteins. Mutations of these factors cause a learning defect, and this defect is relieved by DAF-2c (exon 11.5+) isoform expression only in a single neuron ASER. Our results provide evidence that alternative splicing regulation of a single critical gene in a single critical neuron is essential for learning ability in an organism.

Little is known about the molecular mechanisms regulating neuron-specific alternative splicing. Here, the authors identify a combination of RNA-binding proteins regulating neuron-specific expression of the C. elegans insulin receptor isoform DAF-2c and find disrupting these factors leads to learning deficits.

Neuroprotective effects of the anticancer drug NVP-BEZ235 (dactolisib) on amyloid-β 1-42 induced neurotoxicity and memory impairment

Alzheimer's Disease (AD) is a progressive neurodegenerative disease and the main cause of dementia. Substantial evidences indicate that there is over-activation of the PI3K/Akt/mTOR axis in AD. Therefore, the aim of the present study was to investigate the effects of NVP-BEZ235 (BEZ; dactolisib), a dual PI3K/mTOR inhibitor that is under phase I/II clinical trials for the treatment of some types of cancer, in hippocampal neuronal cultures stimulated with amyloid-β (Aβ) 1-42 and in mice injected with Aβ 1-42 in the hippocampus. In cell cultures, BEZ reduced neuronal death induced by Aβ. BEZ, but not rapamycin, a mTOR inhibitor, or LY294002, a PI3K inhibitor that also inhibits mTOR, reduced the memory impairment induced by Aβ. The effect induced by Aβ was also prevented in PI3Kγ(-/-) mice. Neuronal death and microgliosis induced by Aβ were reduced by BEZ. In addition, the compound increased IL-10 and TNF-α levels in the hippocampus. Finally, BEZ did not change the phosphorylation of Akt and p70s6K, suggesting that the involvement of PI3K and mTOR in the effects induced by BEZ remains controversial. Therefore, BEZ represents a potential strategy to prevent the pathological outcomes induced by Aβ and should be investigated in other models of neurodegenerative conditions.

The C. elegans adult neuronal IIS/FOXO transcriptome reveals adult phenotype regulators

Insulin/IGF-1 signaling (IIS) is a critical regulator of an organism’s most important biological decisions, from growth, development, and metabolism to reproduction and longevity. It primarily does so through the activity of the DAF-16/FOXO transcription factor, whose global targets were identified in C. elegans using whole-worm transcriptional analyses more than a decade ago¹. IIS and FOXO also regulate important neuronal and adult behavioral phenotypes, such as the maintenance of memory² and axon regeneration³ with age, in both mammals⁴ and C. elegans, but the neuron-specific IIS/FOXO targets that regulate these functions are still unknown. By isolating adult C. elegans neurons for transcriptional profiling, we identified both the wild-type and IIS/FOXO adult neuronal transcriptomes for the first time. IIS/FOXO neuron-specific targets are distinct from canonical IIS/FOXO-regulated longevity and metabolism targets, and are required for IIS/daf-2 mutants’ extended memory. We also discovered that the activity of the forkhead transcription factor FKH-9 in neurons is required for daf-2’s ability to regenerate axons with age, and its activity in non-neuronal tissues is required for daf-2’s long lifespan. Together, neuron-specific and canonical IIS/FOXO-regulated targets enable the coordinated extension of neuronal activities, metabolism, and longevity under low insulin-signaling conditions.

Neuronal stress signaling and eIF2α phosphorylation as molecular links between Alzheimer's disease and diabetes

Mounting evidence from clinical, epidemiological, neuropathology and preclinical studies indicates that mechanisms similar to those leading to peripheral metabolic deregulation in metabolic disorders, such as diabetes and obesity, take place in the brains of Alzheimer's disease (AD) patients. These include pro-inflammatory mechanisms, brain metabolic stress and neuronal insulin resistance. From a molecular and cellular perspective, recent progress has been made in unveiling novel pathways that act in an orchestrated way to cause neuronal damage and cognitive decline in AD. These pathways converge to the activation of neuronal stress-related protein kinases and excessive phosphorylation of eukaryotic translation initiation factor 2α (eIF2α-P), which plays a key role in control of protein translation, culminating in synapse dysfunction and memory loss. eIF2α-P signaling thus links multiple neuronal stress pathways to impaired neuronal function and neurodegeneration. Here, we present a critical analysis of recently discovered molecular mechanisms underlying impaired brain insulin signaling and metabolic stress, with emphasis on the role of stress kinase/eIF2α-P signaling as a hub that promotes brain and behavioral impairments in AD. Because very similar mechanisms appear to operate in peripheral metabolic deregulation in T2D and in brain defects in AD, we discuss the concept that targeting defective brain insulin signaling and neuronal stress mechanisms with anti-diabetes agents may be an attractive approach to fight memory decline in AD. We conclude by raising core questions that remain to be addressed toward the development of much needed therapeutic approaches for AD.

A genome-wide screen identifies PAPP-AA-mediated IGFR signaling as a novel regulator of habituation learning

Habituation represents a fundamental form of learning, yet the underlying molecular genetic mechanisms are not well defined. Here we report on a genome-wide genetic screen, coupled with whole-genome sequencing, that identified 14 zebrafish startle habituation mutants including mutants of the vertebrate-specific gene pregnancy-associated plasma protein-aa (pappaa). PAPP-AA encodes an extracellular metalloprotease known to increase IGF bioavailability, thereby enhancing IGF receptor signaling. We find that pappaa is expressed by startle circuit neurons, and expression of wild-type but not a metalloprotease-inactive version of pappaa restores habituation in pappaa mutants. Furthermore, acutely inhibiting IGF1R function in wild-type reduces habituation, while activation of IGF1R downstream effectors in pappaa mutants restores habituation, demonstrating that pappaa promotes learning by acutely and locally increasing IGF bioavailability. In sum, our results define the first functional gene set for habituation learning in a vertebrate and identify PAPPAA-regulated IGF signaling as a novel mechanism regulating habituation learning.

Observing and Quantifying Fluorescent Reporters

Genetically encoded fluorescent reporters take advantage of C. elegans' transparency to allow noninvasive, in vivo observation and recording of physiological processes in intact animals. Here, I discuss the basic microscope components required to observe, image, and measure fluorescent proteins in live animals for students and researchers who work with C. elegans but have limited experience with fluorescence imaging and analysis.

Detection and manipulation of phosphoinositides

Phosphoinositides (PIs) are minor components of cell membranes, but play key roles in cell function. Recent refinements in techniques for their detection, together with imaging methods to study their distribution and changes, have greatly facilitated the study of these lipids. Such methods have been complemented by the parallel development of techniques for the acute manipulation of their levels, which in turn allow bypassing the long-term adaptive changes implicit in genetic perturbations. Collectively, these advancements have helped elucidate the role of PIs in physiology and the impact of the dysfunction of their metabolism in disease. Combining methods for detection and manipulation enables the identification of specific roles played by each of the PIs and may eventually lead to the complete deconstruction of the PI signaling network. Here, we review current techniques used for the study and manipulation of cellular PIs and also discuss advantages and disadvantages associated with the various methods. This article is part of a Special Issue entitled Phosphoinositides.

Subcellular optogenetics - controlling signaling and single-cell behavior

Variation in signaling activity across a cell plays a crucial role in processes such as cell migration. Signaling activity specific to organelles within a cell also likely plays a key role in regulating cellular functions. To understand how such spatially confined signaling within a cell regulates cell behavior, tools that exert experimental control over subcellular signaling activity are required. Here, we discuss the advantages of using optogenetic approaches to achieve this control. We focus on a set of optical triggers that allow subcellular control over signaling through the activation of G-protein-coupled receptors (GPCRs), receptor tyrosine kinases and downstream signaling proteins, as well as those that inhibit endogenous signaling proteins. We also discuss the specific insights with regard to signaling and cell behavior that these subcellular optogenetic approaches can provide.

Zebrafish calsyntenins mediate homophilic adhesion through their amino-terminal cadherin repeats

The calsyntenins are atypical members of the cadherin superfamily that have been implicated in learning in Caenorhabditis elegans and memory formation in humans. As members of the cadherin superfamily, they could mediate cell-cell adhesion, although their adhesive properties have not been investigated. As an initial step in characterizing the calsyntenins, we have cloned clstn1, clstn2 and clstn3 from the zebrafish and determined their expression in the developing zebrafish nervous system. The three genes each have broad, yet distinct, expression patterns in the zebrafish brain. Each of the ectodomains mediates homophilic interactions through two, amino-terminal cadherin repeats. In bead sorting assays, the calsyntenin ectodomains do not exhibit homophilic preferences. These data support the idea that calsyntenins could either act as adhesion molecules or as diffusible, homophilic or heterophilic ligands in the vertebrate nervous system.