Activation of Gαq Signaling Enhances Memory Consolidation and Slows Cognitive Decline

Male-specific behavioral and transcriptomic changes in aging C. elegans neurons

Aging is a complex biological process with sexually dimorphic aspects. Although cognitive aging of Caenorhabditis elegans hermaphrodites has been studied, less is known about cognitive decline in males. We found that cognitive aging has both sex-shared and sex-dimorphic characteristics, and we identified neuron-specific age-associated sex-differential targets. In addition to sex-shared neuronal aging genes, males differentially downregulate mitochondrial metabolic genes and upregulate GPCR genes with age, while the X chromosome exhibits increased gene expression in hermaphrodites and altered dosage compensation complex expression with age, indicating possible X chromosome dysregulation that contributes to sexual dimorphism in cognitive aging. Finally, the sex-differentially expressed gene hrg-7, an aspartic-type endopeptidase, regulates male cognitive aging but does not affect hermaphrodites' behaviors. These results suggest that males and hermaphrodites exhibit different age-related neuronal changes. This study will strengthen our understanding of sex-specific vulnerability and resilience and identify pathways to target with treatments that could benefit both sexes.

Behavioral screening of conserved RNA-binding proteins reveals CEY-1/YBX RNA-binding protein dysfunction leads to impairments in memory and cognition

RNA-binding proteins (RBPs) regulate translation and plasticity which are required for memory. RBP dysfunction has been linked to a range of neurological disorders where cognitive impairments are a key symptom. However, of the 2,000 RBPs in the human genome, many are uncharacterized with regards to neurological phenotypes. To address this, we used the model organism C. elegans to assess the role of 20 conserved RBPs in memory. We identified eight previously uncharacterized memory regulators, three of which are in the C. elegans Y-Box (CEY) RBP family. Of these, we determined that cey-1 is the closest ortholog to the mammalian Y-Box (YBX) RBPs. We found that CEY-1 is both necessary in the nervous system for memory ability and sufficient to increase memory. Leveraging human datasets, we found both copy number variation losses and single nucleotide variants in YBX1 and YBX3 in individuals with neurological symptoms. We identified one predicted deleterious YBX3 variant of unknown significance, p.Asn127Tyr, in two individuals with neurological symptoms. Introducing this variant into endogenous cey-1 locus caused memory deficits in the worm. We further generated two humanized worm lines expressing human YBX3 or YBX1 at the cey-1 locus to test evolutionary conservation of YBXs in memory and the potential functional significance of the p.Asn127Tyr variant. Both YBX1/3 can functionally replace cey-1, and introduction of p.Asn127Tyr into the humanized YBX3 locus caused memory deficits. Our study highlights the worm as a model to reveal memory regulators and identifies YBX dysfunction as a potential new source of rare neurological disease.

Adult Single-nucleus Neuronal Transcriptomes of Insulin Signaling Mutants Reveal Regulators of Behavior and Learning

The insulin/insulin-like signaling (IIS) pathway regulates many of C. elegans' adult functions, including learning and memory 1 . While whole-worm and tissue-specific transcriptomic analyses have identified IIS targets 2,3 , a higher-resolution single-cell approach is required to identify changes that confer neuron-specific improvements in the long-lived insulin receptor mutant, daf-2 . To understand how behaviors that are controlled by a small number of neurons change in daf-2 mutants, we used the deep resolution of single-nucleus RNA sequencing to define each neuron type's transcriptome in adult wild-type and daf-2 mutants. First, we found surprising differences between wild-type L4 larval neurons and young adult neurons in chemoreceptor expression, synaptic genes, and learning and memory genes. These Day 1 adult neuron transcriptomes allowed us to identify adult AWC-specific regulators of chemosensory function and to predict neuron-to-neuron peptide/receptor pairs. We then identified gene expression changes that correlate with daf-2's improved cognitive functions, particularly in the AWC sensory neuron that controls learning and associative memory 4 , and used behavioral assays to test their roles in cognitive function. Combining deep single-neuron transcriptomics, genetic manipulation, and behavioral analyses enabled us to identify genes that may function in a single adult neuron to control behavior, including conserved genes that function in learning and memory.

One-Sentence Summary

Single-nucleus sequencing of adult wild-type and daf-2 C. elegans neurons reveals functionally relevant transcriptional changes, including regulators of chemosensation, learning, and memory.

Neuronal activation of Gαq EGL-30/GNAQ late in life rejuvenates cognition across species

Loss of cognitive function with age is devastating. EGL-30/GNAQ and Gαq signaling pathways are highly conserved between C. elegans and mammals, and murine Gnaq is enriched in hippocampal neurons and declines with age. We found that activation of EGL-30 in aged worms triples memory span, and GNAQ gain of function significantly improved memory in aged mice: GNAQ(gf) in hippocampal neurons of 24-month-old mice (equivalent to 70- to 80-year-old humans) rescued age-related impairments in well-being and memory. Single-nucleus RNA sequencing revealed increased expression of genes regulating synaptic function, axon guidance, and memory in GNAQ-treated mice, and worm orthologs of these genes were required for long-term memory extension in worms. These experiments demonstrate that C. elegans is a powerful model to identify mammalian regulators of memory, leading to the identification of a pathway that improves memory in extremely old mice. To our knowledge, this is the oldest age at which an intervention has improved age-related cognitive decline.

Olfactory imprinting enhances associative learning and memory in C. elegans

Learning and memory are fundamental processes for an organism's normal physiological function. Learning can occur at any stage of the organism's physiological development. Imprinted memories formed during the early developmental stage, unlike learning and memory, can last a lifetime. It is not clear whether these two types of memories are interlinked. In this study, we investigated whether imprinted memory influences adult learning and memory in a C. elegans model system. We trained the worms for short-term (STAM) and long-term associated memory (LTAM) towards butanone (BT) after conditioning them for imprinted memory towards isoamyl alcohol (IAA). We observed that these worms had improved learning abilities. However, functional imaging revealed that the worms had a long-term depression in the firing pattern in the AIY interneuron, indicating that there were significant changes in neuronal excitation pattern after imprinting, which could explain the enhanced behavioural alterations in animals after imprinting.

Bacterial diet affects the age-dependent decline of associative learning in Caenorhabditis elegans

The causality and mechanism of dietary effects on brain aging are still unclear due to the long time scales of aging. The nematode Caenorhabditis elegans has contributed to aging research because of its short lifespan and easy genetic manipulation. When fed the standard laboratory diet, Escherichia coli, C. elegans experiences an age-dependent decline in temperature-food associative learning, called thermotaxis. To address if diet affects this decline, we screened 35 lactic acid bacteria as alternative diet and found that animals maintained high thermotaxis ability when fed a clade of Lactobacilli enriched with heterofermentative bacteria. Among them, Lactobacillus reuteri maintained the thermotaxis of aged animals without affecting their lifespan and motility. The effect of Lb. reuteri depends on the DAF-16 transcription factor functioning in neurons. Furthermore, RNA sequencing analysis revealed that differentially expressed genes between aged animals fed different bacteria were enriched with DAF-16 targets. Our results demonstrate that diet can impact brain aging in a daf-16-dependent manner without changing the lifespan.

Uncovering novel regulators of memory using C. elegans genetic and genomic analysis

How organisms learn and encode memory is an outstanding question in neuroscience research. Specifically, how memories are acquired and consolidated at the level of molecular and gene pathways remains unclear. In addition, memory is disrupted in a wide variety of neurological disorders; therefore, discovering molecular regulators of memory may reveal therapeutic targets for these disorders. C. elegans are an excellent model to uncover molecular and genetic regulators of memory. Indeed, the nematode's invariant neuronal lineage, fully mapped genome, and conserved associative behaviors have allowed the development of a breadth of genetic and genomic tools to examine learning and memory. In this mini-review, we discuss novel and exciting genetic and genomic techniques used to examine molecular and genetic underpinnings of memory from the level of the whole-worm to tissue-specific and cell-type specific approaches with high spatiotemporal resolution.

Atmospheric particulate matter impairs cognition by modulating synaptic function via the nose-to-brain route

Atmospheric particulate matter (PM), a ubiquitous air pollutant, is the leading environmental risk factor for mortality worldwide. Experimental and epidemiological studies consistently suggest a strong link between long-term exposure to PM_2.5 (<2.5 μm, fine PM) and cognitive impairment. The neuroinflammatory response is presumed to be one of the main mechanisms of PM_2.5-induced cognitive impairment, possibly leading to synaptic dysfunction. However, the main route and mechanism underlying the cause of cognitive dysfunction and pathogenic alterations in PM_2.5-exposure mice remain poorly understood. Therefore, this study aimed to investigate the main route and mechanism of PM_2.5-induced cognitive impairment. Our results showed that PM_2.5 directly entered the brain following nasal administration, and both the short-term PM_2.5 administration via atomization and nasal drops induced learning and memory impairments and neuronal damage in adult mice. Moreover, astrocytes and microglia were both activated in the two short-term PM_2.5 exposure models, while few changes in the inflammatory response were observed in the peripheral circulatory system. Furthermore, a further transcriptional analysis revealed that short-term PM_2.5 administration led to cognitive impairment mainly by modulating synaptic functions and that although glia were activated, the glia-related pathological pathways were not significantly activated. Notably, following PM_2.5 exposure, PLX3397-induced microglial deletion did not restore the cognitive function of the mice. In conclusion, our results provide evidence that PM_2.5 enters the brain via the nose-to-brain route to impair cognitive function, and short-term exposure to PM_2.5 directly alters synaptic function rather than the neuroinflammatory response to affect cognition.

GNAQ mutations drive port wine birthmark-associated Sturge-Weber syndrome: A review of pathobiology, therapies, and current models

Port-wine birthmarks (PWBs) are caused by somatic, mosaic mutations in the G protein guanine nucleotide binding protein alpha subunit q (GNAQ) and are characterized by the formation of dilated, dysfunctional blood vessels in the dermis, eyes, and/or brain. Cutaneous PWBs can be treated by current dermatologic therapy, like laser intervention, to lighten the lesions and diminish nodules that occur in the lesion. Involvement of the eyes and/or brain can result in serious complications and this variation is termed Sturge-Weber syndrome (SWS). Some of the biggest hurdles preventing development of new therapeutics are unanswered questions regarding disease biology and lack of models for drug screening. In this review, we discuss the current understanding of GNAQ signaling, the standard of care for patients, overlap with other GNAQ-associated or phenotypically similar diseases, as well as deficiencies in current in vivo and in vitro vascular malformation models.

Inhibition of the expression of rgs-3 alleviates propofol-induced decline in learning and memory in Caenorhabditis elegans

BACKGROUND

Exposure to anesthesia leads to extensive neurodegeneration and long-term cognitive deficits in the developing brain. Caenorhabditis elegans also shows persistent behavioral changes during development after exposure to anesthetics. Clinical and rodent studies have confirmed that altered expression of the regulators of G protein signaling (RGS) in the nervous system is a factor contributing to neurodegenerative and psychological diseases. Evidence from preclinical studies has suggested that RGS controls drug-induced plasticity, including morphine tolerance and addiction. This study aimed to observe the effect of propofol exposure in the neurodevelopmental stage on learning and memory in the L4 stage and to study whether this effect is related to changes in rgs-3 expression.

METHODS

Caenorhabditis elegans were exposed to propofol at the L1 stage, and learning and memory abilities were observed at the L4 stage. The expression of rgs-3 and the nuclear distribution of EGL-4 were determined to study the relevant mechanisms. Finally, RNA interference was performed on rgs-3-expressing cells after propofol exposure. Then, we observed their learning and memory abilities.

RESULTS

Propofol time- and dose-dependently impaired the learning capacity. Propofol induced a decline in non-associative and associative long-term memory, rgs-3 upregulation, and a failure of nuclear accumulation of EGL-4/PKG in AWC neurons. Inhibition of rgs-3 could alleviate the propofol-induced changes.

CONCLUSION

Inhibition of the expression of rgs-3 alleviated propofol-induced learning and memory deficits in Caenorhabditis elegans.

The redundancy and diversity between two novel PKC isotypes that regulate learning in Caenorhabditis elegans

The nematode Caenorhabditis elegans learns the concentration of NaCl and moves toward the previously experienced concentration. In this behavior, the history of NaCl concentration change is reflected in the level of diacylglycerol and the activity of protein kinase C, PKC-1, in the gustatory sensory neuron ASER and determines the direction of migration. Here, through a genetic screen, we found that the activation of Gq protein compensates for the behavioral defect of the loss-of-function mutant of pkc-1 We found that Gq activation results in hyperproduction of diacylglycerol in ASER sensory neuron, which leads to recruitment of TPA-1, an nPKC isotype closely related to PKC-1. Unlike the pkc-1 mutants, loss of tpa-1 did not obviously affect migration directions in the conventional learning assay. This difference was suggested to be due to cooperative functions of the C1 and C2-like domains of the nPKC isotypes. Furthermore, we investigated how the compensatory capability of tpa-1 contributes to learning and found that learning was less robust in the context of cognitive decline or environmental perturbation in tpa-1 mutants. These results highlight how two nPKC isotypes contribute to the learning system.

DAF-2c signaling promotes taste avoidance after starvation in Caenorhabditis elegans by controlling distinct phospholipase C isozymes

Previously, we reported that DAF-2c, an axonal insulin receptor isoform in Caenorhabditis elegans, acts in the ASER gustatory neuron to regulate taste avoidance learning, a process in which worms learn to avoid salt concentrations experienced during starvation. Here, we show that secretion of INS-1, an insulin-like peptide, after starvation conditioning is sufficient to drive taste avoidance via DAF-2c signaling. Starvation conditioning enhances the salt-triggered activity of AIA neurons, the main sites of INS-1 release, which potentially promotes feedback signaling to ASER to maintain DAF-2c activity during taste avoidance. Genetic studies suggest that DAF-2c-Akt signaling promotes high-salt avoidance via a decrease in PLCβ activity. On the other hand, the DAF-2c pathway promotes low-salt avoidance via PLCε and putative Akt phosphorylation sites on PLCε are essential for taste avoidance. Our findings imply that animals disperse from the location at which they experience starvation by controlling distinct PLC isozymes via DAF-2c.

Modelling learning in C. elegans chemosensory and locomotive circuitry for T-maze navigation

Recently, a new type of Caenorhabditis elegans associative learning was reported, where nematodes learn to reach a target arm in an empty T‐maze, after they have successfully located reward (food) in the same side arm of a similar, baited, training maze. Here, we present a simplified mathematical model of C. elegans chemosensory and locomotive circuitry that replicates C. elegans navigation in a T‐maze and predicts the underlying mechanisms generating maze learning. Based on known neural circuitry, the model circuit responds to food‐released chemical cues by modulating motor neuron activity that drives simulated locomotion. We show that, through modulation of interneuron activity, such a circuit can mediate maze learning by acquiring a turning bias, even after a single training session. Simulated nematode maze navigation during training conditions in food‐baited mazes and during testing conditions in empty mazes is validated by comparing simulated behaviour with new experimental video data, extracted through the implementation of a custom‐made maze tracking algorithm. Our work provides a mathematical framework for investigating the neural mechanisms underlying this novel learning behaviour in C. elegans. Model results predict neuronal components involved in maze and spatial learning and identify target neurons and potential neural mechanisms for future experimental investigations into this learning behaviour.

Investigating the molecular mechanisms of learning and memory using Caenorhabditis elegans

Learning is an essential biological process for survival since it facilitates behavioural plasticity in response to environmental changes. This process is mediated by a wide variety of genes, mostly expressed in the nervous system. Many studies have extensively explored the molecular and cellular mechanisms underlying learning and memory. This review will focus on the advances gained through the study of the nematode Caenorhabditis elegans. C. elegans provides an excellent system to study learning because of its genetic tractability, in addition to its invariant, compact nervous system (~300 neurons) that is well-characterised at the structural level. Importantly, despite its compact nature, the nematode nervous system possesses a high level of conservation with mammalian systems. These features allow the study of genes within specific sensory-, inter- and motor neurons, facilitating the interrogation of signalling pathways that mediate learning via defined neural circuits. This review will detail how learning and memory can be studied in C. elegans through behavioural paradigms that target distinct sensory modalities. We will also summarise recent studies describing mechanisms through which key molecular and cellular pathways are proposed to affect associative and non-associative forms of learning.

Early-life chronic di(2-ethylhexyl)phthalate exposure worsens age-related long-term associative memory decline associated with insulin/IGF-1 signaling and CRH-1/CREB in Caenorhabditis elegans

The ubiquitous contamination of di(2-ethylhexyl)phthalate (DEHP) in the environment, biota, and food poses potential ecological and human health risks. DEHP exposure can adversely affect learning and memory, yet the underlying mechanisms remain unclear. In this study, Caenorhabditis elegans was used to investigate the effect of early-life DEHP exposure on age-related long-term associative memory (LTAM) decline, as well as the associations with the cAMP-responsive element-binding protein (CREB) transcription factor and insulin/IGF-1 signaling (IIS). We showed that early-life exposure to DEHP reduced LTAM in wild-type worms at day-0 adulthood. Chronic exposure to DEHP from the L1 stage to day-5 adulthood worsened the age-dependent decline of LTAM. Moreover, the effect of DEHP on age-related LTAM requires CRH-1, a homolog of CREB. Mutations in daf-2, the sole receptor of C. elegans IIS, ameliorated the inhibition of LTAM by DEHP, and the effect depended on daf-16. In addition, daf-2 mutation restored the CRH-1 level in DEHP-exposed worms, and the effect required daf-16. Our study suggests that early-life chronic exposure to DEHP worsens age-related LTAM decline and the effect is associated with CRH-1 and IIS in C. elegans. The evolutionary conservation of IIS and CREB implies possible adverse effects by DEHP across species.

CREB mediates the C. elegans dauer polyphenism through direct and cell-autonomous regulation of TGF-β expression

Animals can adapt to dynamic environmental conditions by modulating their developmental programs. Understanding the genetic architecture and molecular mechanisms underlying developmental plasticity in response to changing environments is an important and emerging area of research. Here, we show a novel role of cAMP response element binding protein (CREB)-encoding crh-1 gene in developmental polyphenism of C. elegans. Under conditions that promote normal development in wild-type animals, crh-1 mutants inappropriately form transient pre-dauer (L2d) larvae and express the L2d marker gene. L2d formation in crh-1 mutants is specifically induced by the ascaroside pheromone ascr#5 (asc-ωC3; C3), and crh-1 functions autonomously in the ascr#5-sensing ASI neurons to inhibit L2d formation. Moreover, we find that CRH-1 directly binds upstream of the daf-7 TGF-β locus and promotes its expression in the ASI neurons. Taken together, these results provide new insight into how animals alter their developmental programs in response to environmental changes.

Caenorhabditis elegans learning in a structured maze is a multisensory behavior

We show that C. elegans nematodes learn to associate food with a combination of proprioceptive cues and information on the structure of their surroundings (maze), perceived through mechanosensation. By using the custom-made Worm-Maze platform, we demonstrate that C. elegans young adults can locate food in T-shaped mazes and, following that experience, learn to reach a specific maze arm. C. elegans learning inside the maze is possible after a single training session, it resembles working memory, and it prevails over conflicting environmental cues. We provide evidence that the observed learning is a food-triggered multisensory behavior, which requires mechanosensory and proprioceptive input, and utilizes cues about the structural features of nematodes' environment and their body actions. The CREB-like transcription factor and dopamine signaling are also involved in maze performance. Lastly, we show that the observed aging-driven decline of C. elegans learning ability in the maze can be reversed by starvation.

•

C. elegans can be trained to reach a target arm in a T-shaped maze

•

Learning requires the contribution of tactile and proprioceptive cues

•

C. elegans follow a kind of response learning strategy in the maze environment

•

Learning is short-term and sensitive to distraction

Eleutheroside E Enhances the Long-Term Memory of Radiation-Damaged C. elegans through G-Protein-Coupled Receptor and Neuropeptide Signaling Pathways

Eleutheroside E (EE), a principal active compound of Acanthopanax senticosus, has been shown to have a certain neuromodulation effect. Our previous study indicates that EE protects nerve damage caused by radiation. However, its specific function and underlying mechanism remain unknown. Therefore, the objective of this study is to apply the C. elegans model to illuminate the property and mechanism of EE protecting against nerve damage caused by radiation. Here, we found that EE significantly improved the long-term memory of radiation-damaged C. elegans. Through transcriptome sequencing, the results showed that EE protected radiation-damaged C. elegans mainly through G-protein-coupled receptor and neuropeptide signaling pathways. Further research indicated that EE affected the activity of CREB by cAMP-PKA, G_qα-PLC, and neuropeptide signaling pathways to ultimately improve the long-term memory of radiation-damaged C. elegans. In addition, the activity of G_qα and neuropeptides in AWC neurons and the activity of CREB in AIM neurons might be crucial for EE to function.

Nervous system-wide profiling of presynaptic mRNAs reveals regulators of associative memory

Presynaptic protein synthesis is important in the adult central nervous system; however, the nervous system-wide set of mRNAs localized to presynaptic areas has yet to be identified in any organism. Here we differentially labeled somatic and synaptic compartments in adult C. elegans with fluorescent proteins, and isolated synaptic and somatic regions from the same population of animals. We used this technique to determine the nervous system-wide presynaptic transcriptome by deep sequencing. Analysis of the synaptic transcriptome reveals that synaptic transcripts are predicted to have specialized functions in neurons. Differential expression analysis identified 542 genes enriched in synaptic regions relative to somatic regions, with synaptic functions conserved in higher organisms. We find that mRNAs for pumilio RNA-binding proteins are abundant in synaptic regions, which we confirmed through high-sensitivity in situ hybridization. Presynaptic PUMILIOs regulate associative memory. Our approach enables the identification of new mechanisms that regulate synaptic function and behavior.

A Memory Circuit for Coping with Impending Adversity

Organisms’ capacity to anticipate future conditions is key for survival. Associative memories are instrumental for learning from past experiences, yet little is known about the processes that follow memory retrieval and their potential advantage in preparing for impending developments. Here, using C. elegans nematodes, we demonstrate that odor-evoked retrieval of aversive memories induces rapid and protective stress responses, which increase animal survival prospects when facing imminent adversities. The underlying mechanism relies on two sensory neurons: one is necessary during the learning period, and the other is necessary and sufficient for memory retrieval. Downstream of memory reactivation, serotonin secreted from two head neurons mediates the systemic stress response. Thus, evoking stressful memories, stored within individual sensory neurons, allows animals to anticipate upcoming dire conditions and provides a head start to initiate rapid and protective responses that ultimately increase animal fitness.

•

Reactivation of an aversive memory induces a fast protective stress response

•

The fast response provides the animals with a fitness advantage

•

One neuron is necessary for memory formation

•

Another neuron is necessary and sufficient for memory reactivation

Non-canonical activation of CREB mediates neuroprotection in a Caenorhabditis elegans model of excitotoxic necrosis

Excitotoxicity, caused by exaggerated neuronal stimulation by Glutamate (Glu), is a major cause of neurodegeneration in brain ischemia. While we know that neurodegeneration is triggered by overstimulation of Glu-receptors (GluRs), the subsequent mechanisms that lead to cellular demise remain controversial. Surprisingly, signaling downstream of GluRs can also activate neuroprotective pathways. The strongest evidence involves activation of the transcription factor cAMP response element-binding protein (CREB), widely recognized for its importance in synaptic plasticity. Canonical views describe CREB as a phosphorylation-triggered transcription factor, where transcriptional activation involves CREB phosphorylation and association with CREB-binding protein. However, given CREB's ubiquitous cross-tissue expression, the multitude of cascades leading to CREB phosphorylation, and its ability to regulate thousands of genes, it remains unclear how CREB exerts closely tailored, differential neuroprotective responses in excitotoxicity. A non-canonical, alternative cascade for activation of CREB-mediated transcription involves the CREB co-factor cAMP-regulated transcriptional co-activator (CRTC), and may be independent of CREB phosphorylation. To identify cascades that activate CREB in excitotoxicity we used a Caenorhabditis elegans model of neurodegeneration by excitotoxic necrosis. We demonstrated that CREB's neuroprotective effect was conserved, and seemed most effective in neurons with moderate Glu exposure. We found that factors mediating canonical CREB activation were not involved. Instead, phosphorylation-independent CREB activation in nematode excitotoxic necrosis hinged on CRTC. CREB-mediated transcription that depends on CRTC, but not on CREB phosphorylation, might lead to expression of a specific subset of neuroprotective genes. Elucidating conserved mechanisms of excitotoxicity-specific CREB activation can help us focus on core neuroprotective programs in excitotoxicity. Cover Image for this issue: doi: 10.1111/jnc.14494.

PKCε Inhibits Neuronal Dendritic Spine Development through Dual Phosphorylation of Ephexin5

Protein kinase C (PKC)-dependent mechanisms promote synaptic function in the mature brain. However, the roles of PKC signaling during synapse development remain largely unknown. Investigating each brain-enriched PKC isoform in early neuronal development, we show that PKCε acutely and specifically reduces the number of dendritic spines, sites of eventual synapse formation on developing dendrites. This PKCε-mediated spine suppression is temporally restricted to immature neurons and mediated through the phosphorylation and activation of Ephexin5, a RhoA guanine nucleotide exchange factor (GEF) and inhibitor of hippocampal synapse formation. Our data suggest that PKCε acts as an early developmental inhibitor of dendritic spine formation, in contrast to its emerging pro-synaptic roles in mature brain function. Moreover, we identify a substrate of PKCε, Ephexin5, whose early-elevated expression in developing neurons may in part explain the mechanism by which PKCε plays seemingly opposing roles that depend on neuronal maturity.