Regional synapse gain and loss accompany memory formation in larval zebrafish

Defining the structural and functional changes in the nervous system underlying learning and memory represents a major challenge for modern neuroscience. Although changes in neuronal activity following memory formation have been studied [B. F. Grewe et al., Nature 543, 670–675 (2017); M. T. Rogan, U. V. Stäubli, J. E. LeDoux, Nature 390, 604–607 (1997)], the underlying structural changes at the synapse level remain poorly understood. Here, we capture synaptic changes in the midlarval zebrafish brain that occur during associative memory formation by imaging excitatory synapses labeled with recombinant probes using selective plane illumination microscopy. Imaging the same subjects before and after classical conditioning at single-synapse resolution provides an unbiased mapping of synaptic changes accompanying memory formation. In control animals and animals that failed to learn the task, there were no significant changes in the spatial patterns of synapses in the pallium, which contains the equivalent of the mammalian amygdala and is essential for associative learning in teleost fish [M. Portavella, J. P. Vargas, B. Torres, C. Salas, Brain Res. Bull. 57, 397–399 (2002)]. In zebrafish that formed memories, we saw a dramatic increase in the number of synapses in the ventrolateral pallium, which contains neurons active during memory formation and retrieval. Concurrently, synapse loss predominated in the dorsomedial pallium. Surprisingly, we did not observe significant changes in the intensity of synaptic labeling, a proxy for synaptic strength, with memory formation in any region of the pallium. Our results suggest that memory formation due to classical conditioning is associated with reciprocal changes in synapse numbers in the pallium.

Localized Synaptic Potentiation is Necessary and Sufficient for Context Fear Memory

The nature and distribution of the synaptic changes that underlie memory are not well understood. We examined the synaptic plasticity behind context fear learning and found that conditioning produced potentiation of excitatory synapses specifically onto the basolateral amygdala neurons activated during learning. This synaptic potentiation lasted at least 7 days, and its disruption impaired memory recall. High frequency optogenetic stimulation of the CS and US-activated ensembles or biochemical induction of synaptic potentiation in US-responsive neurons alone was sufficient to produce a context fear association without prior associative training. These results suggest that plasticity of CS inputs onto US-responsive amygdala neurons is a necessary and sufficient step in forming context fear associations, and that context discrimination is determined by the CS-specific amygdala inputs activated during retrieval.

The tetraspanin TSPAN5 regulates AMPARs exocytosis by interacting with the AP-4 complex

Intracellular trafficking of AMPA receptors is a tightly regulated process which involves several adaptor proteins, and is crucial for the activity of excitatory synapses in both basal conditions and during synaptic plasticity. We found that, in rat hippocampal neurons, an intracellular pool of the tetraspanin TSPAN5 specifically promotes exocytosis of newly synthesised GluA2-containing AMPA receptors without affecting their internalisation. TSPAN5 mediates this function by interacting with AP-4 and Stargazin and possibly using recycling endosomes as a delivery route. This work highlights TSPAN5 as a new adaptor regulating AMPA receptor trafficking. In addition, it provides a possible mechanism for the intellectual disability symptoms that occur in AP-4 deficiency syndrome.

Essential regulatory functions of CaMKII T286 phosphorylation in LTP and two distinct forms of LTD

The Ca2+/calmodulin-dependent protein kinase II (CaMKII) mediates both long-term potentiation and depression (LTP and LTD) of excitatory synapses, two opposing forms of synaptic plasticity induced by strong versus weak stimulation of NMDA-type glutamate receptors (NMDARs). NMDAR-dependent LTD is prevalent in juvenile hippocampus, but in mature hippocampus, LTD is still readily induced by stimulating metabotropic glutamate receptors (mGluRs). Here we show that mGluR-dependent LTD also requires CaMKII and its T286 autophosphorylation that induces Ca2+-independent autonomous kinase activity. This autophosphorylation (i) accelerated CaMKII movement to excitatory synapses after LTP stimuli and (ii) was required for the movement to inhibitory synapses after NMDAR-LTD stimuli. Similar to NMDAR-LTD, the mGluR-LTD stimuli did not induce any CaMKII movement to excitatory synapses. However, in contrast to NMDAR-LTD, the mGluR-LTD did not involve CaMKII movement to inhibitory synapses and did not require additional T305/306 autophosphorylation. Taken together, even though CaMKII T286 autophosphorylation has a longstanding prominent role in LTP, it is also required for both major forms of LTD in hippocampal neurons, albeit with differential requirements for the heterosynaptic communication of excitatory signals to inhibitory synapses.

Endocannabinoid-Mediated Control of Neural Circuit Excitability and Epileptic Seizures

Research on endocannabinoid signaling has greatly advanced our understanding of how the excitability of neural circuits is controlled in health and disease. In general, endocannabinoid signaling at excitatory synapses suppresses excitability by inhibiting glutamate release, while that at inhibitory synapses promotes excitability by inhibiting GABA release, although there are some exceptions in genetically epileptic animal models. In the epileptic brain, the physiological distributions of endocannabinoid signaling molecules are disrupted during epileptogenesis, contributing to the occurrence of spontaneous seizures. However, it is still unknown how endocannabinoid signaling changes during seizures and how the redistribution of endocannabinoid signaling molecules proceeds during epileptogenesis. Recent development of cannabinoid sensors has enabled us to investigate endocannabinoid signaling in much greater spatial and temporal details than before. Application of cannabinoid sensors to epilepsy research has elucidated activity-dependent changes in endocannabinoid signaling during seizures. Furthermore, recent endocannabinoid research has paved the way for the clinical use of cannabidiol for the treatment of refractory epilepsy, such as Dravet syndrome, Lennox-Gastaut syndrome and tuberous sclerosis complex. Cannabidiol significantly reduces seizures and is considered to have comparable tolerability to conventional antiepileptic drugs. In this article, we introduce recent advances in research on the roles of endocannabinoid signaling in epileptic seizures and discuss future directions.

Emergence of synaptic organization and computation in dendrites

Single neurons in the brain exhibit astounding computational capabilities, which gradually emerge throughout development and enable them to become integrated into complex neural circuits. These capabilities derive in part from the precise arrangement of synaptic inputs on the neurons’ dendrites. While the full computational benefits of this arrangement are still unknown, a picture emerges in which synapses organize according to their functional properties across multiple spatial scales. In particular, on the local scale (tens of microns), excitatory synaptic inputs tend to form clusters according to their functional similarity, whereas on the scale of individual dendrites or the entire tree, synaptic inputs exhibit dendritic maps where excitatory synapse function varies smoothly with location on the tree. The development of this organization is supported by inhibitory synapses, which are carefully interleaved with excitatory synapses and can flexibly modulate activity and plasticity of excitatory synapses. Here, we summarize recent experimental and theoretical research on the developmental emergence of this synaptic organization and its impact on neural computations.

Information encoded in volumes and areas of dendritic spines is nearly maximal across mammalian brains

Long-term information associated with neuronal memory resides in dendritic spines. However, spines can have a limited size due to metabolic and neuroanatomical constraints, which should effectively limit the amount of encoded information in excitatory synapses. This study investigates how much information can be stored in the sizes of dendritic spines, and whether is it optimal in any sense? It is shown here, using empirical data for several mammalian brains across different regions and physiological conditions, that dendritic spines nearly maximize entropy contained in their volumes and surface areas for a given mean size. This result is essentially independent of the type of a fitting distribution to size data, as both short- and heavy-tailed distributions yield similar nearly 100 % information efficiency in the majority of cases, although heavy-tailed distributions slightly better fit the data. On average, the highest information is contained in spine volume, and the lowest in spine length or spine head diameter. Depending on a species and brain region, a typical spine can encode between 6.1 and 10.8 bits of information in its volume, and 3.1-8.1 bits in its surface area. Our results suggest a universality of entropy maximization in spine volumes and areas, which can be a new principle of memory storing in synapses.

Developmental resilience of synaptome architecture

Neurodevelopmental disorders of genetic origin delay the acquisition of normal abilities and cause disabling phenotypes. Spontaneous attenuation and even complete amelioration of symptoms in early childhood and adolescence occur in many disorders, suggesting that brain circuits possess an intrinsic capacity to repair themselves. We examined the molecular composition of almost a trillion excitatory synapses on a brain-wide scale between birth and adulthood in mice carrying a mutation in the homeobox transcription factor Pax6, a neurodevelopmental disorder model. Pax6 haploinsufficiency had no impact on total synapse number at any age. By contrast, the postnatal expansion of synapse diversity and acquisition of normal synaptome architecture were delayed in all brain regions, interfering with network and cognitive functions. Specific excitatory synapse types and subtypes were affected in two key developmental age-windows. These phenotypes were reversed within 2-3 weeks of onset, restoring synaptome architecture to its normal developmental trajectory. Synapse subtypes with high rates of protein turnover mediated these events. These results show synaptome remodelling confers resilience to neurodevelopmental disorders.

AUTS2 Gene: Keys to Understanding the Pathogenesis of Neurodevelopmental Disorders

Neurodevelopmental disorders (NDDs), including autism spectrum disorders (ASD) and intellectual disability (ID), are a large group of neuropsychiatric illnesses that occur during early brain development, resulting in a broad spectrum of syndromes affecting cognition, sociability, and sensory and motor functions. Despite progress in the discovery of various genetic risk factors thanks to the development of novel genomics technologies, the precise pathological mechanisms underlying the onset of NDDs remain elusive owing to the profound genetic and phenotypic heterogeneity of these conditions. Autism susceptibility candidate 2 (AUTS2) has emerged as a crucial gene associated with a wide range of neuropsychological disorders, such as ASD, ID, schizophrenia, and epilepsy. AUTS2 has been shown to be involved in multiple neurodevelopmental processes; in cell nuclei, it acts as a key transcriptional regulator in neurodevelopment, whereas in the cytoplasm, it participates in cerebral corticogenesis, including neuronal migration and neuritogenesis, through the control of cytoskeletal rearrangements. Postnatally, AUTS2 regulates the number of excitatory synapses to maintain the balance between excitation and inhibition in neural circuits. In this review, we summarize the knowledge regarding AUTS2, including its molecular and cellular functions in neurodevelopment, its genetics, and its role in behaviors.

A brain atlas of synapse protein lifetime across the mouse lifespan.

Protein turnover is required for synapse maintenance and remodelling and may impact memory duration. We quantified the lifetime of postsynaptic protein PSD95 in individual excitatory synapses across the mouse brain and lifespan, generating the Protein Lifetime Synaptome Atlas. Excitatory synapses have a wide range of protein lifetimes that may extend from a few hours to several months, with distinct spatial distributions in dendrites, neuron types and brain regions. Short protein lifetime (SPL) synapses are enriched in developing animals and in regions controlling innate behaviors, whereas long protein lifetime (LPL) synapses accumulate during development, are enriched in the cortex and CA1 where memories are stored, and are preferentially preserved in old age. The protein lifetime synaptome architecture is disrupted in an autism model, with synapse protein lifetime increased throughout the brain. These findings add a further layer to synapse diversity in the brain and enrich prevailing concepts in behavior, development, ageing and brain repair.