Encoding of contextual fear memory requires de novo proteins in the prelimbic cortex

BACKGROUND

Despite our understanding of the significance of the prefrontal cortex in the consolidation of long-term memories (LTM), its role in the encoding of LTM remains elusive. Here we investigated the role of new protein synthesis in the mouse medial prefrontal cortex (mPFC) in encoding contextual fear memory.

METHODS

Because a change in the association of mRNAs to polyribosomes is an indicator of new protein synthesis, we assessed the changes in polyribosome-associated mRNAs in the mPFC following contextual fear conditioning (CFC) in the mouse. Differential gene expression in mPFC was identified by polyribosome profiling (n = 18). The role of new protein synthesis in mPFC was determined by focal inhibition of protein synthesis (n = 131) and by intra-prelimbic cortex manipulation (n = 56) of Homer 3, a candidate identified from polyribosome profiling.

RESULTS

We identified several mRNAs that are differentially and temporally recruited to polyribosomes in the mPFC following CFC. Inhibition of protein synthesis in the prelimbic (PL), but not in the anterior cingulate cortex (ACC) region of the mPFC immediately after CFC disrupted encoding of contextual fear memory. Intriguingly, inhibition of new protein synthesis in the PL 6 hours after CFC did not impair encoding. Furthermore, expression of Homer 3, an mRNA enriched in polyribosomes following CFC, in the PL constrained encoding of contextual fear memory.

CONCLUSIONS

Our studies identify several molecular substrates of new protein synthesis in the mPFC and establish that encoding of contextual fear memories require new protein synthesis in PL subregion of mPFC.

High-throughput screening for small molecule modulators of motor protein Kinesin

The kinesin superfamily of motor proteins are involved in the active transport of a large number of cargos such as organelles, proteins, and RNAs from the neuronal cell body to distal neuronal processes. Previously, we have shown that kinesin-mediated axonal transport of proteins and RNAs are important for long-term memory storage. Identification of small molecules that can activate or inhibit kinesins is of specific interest due to the significance of kinesin-mediated functions in neuronal health and plasticity. Here, we describe a high-throughput screening assay designed to specifically identify compounds that inhibit or activate adenosine triphosphatase activity of the kinesin 5B of humans. The luminescence-based assay that we developed is highly reproducible and robust. Using this approach, we screened a pharmacologically characterized compound collection and have identified small molecules with either activator or inhibitor-like activity. To further characterize screening hits, we also developed an orthogonal assay based on absorbance and a counter screen assay based on luminescence. Development of such assays is important to help identify small molecules that can be used in potential drug development efforts targeted at modulating the function of kinesin.

Transcriptome analyses of adult mouse brain reveal enrichment of lncRNAs in specific brain regions and neuronal populations

Despite the importance of the long non-coding RNAs (lncRNAs) in regulating biological functions, the expression profiles of lncRNAs in the sub-regions of the mammalian brain and neuronal populations remain largely uncharacterized. By analyzing RNASeq datasets, we demonstrate region specific enrichment of populations of lncRNAs and mRNAs in the mouse hippocampus and pre-frontal cortex (PFC), the two major regions of the brain involved in memory storage and neuropsychiatric disorders. We identified 2759 lncRNAs and 17,859 mRNAs in the hippocampus and 2561 lncRNAs and 17,464 mRNAs expressed in the PFC. The lncRNAs identified correspond to ~14% of the transcriptome of the hippocampus and PFC and ~70% of the lncRNAs annotated in the mouse genome (NCBIM37) and are localized along the chromosomes as varying numbers of clusters. Importantly, we also found that a few of the tested lncRNA-mRNA pairs that share a genomic locus display specific co-expression in a region-specific manner. Furthermore, we find that sub-regions of the brain and specific neuronal populations have characteristic lncRNA expression signatures. These results reveal an unexpected complexity of the lncRNA expression in the mouse brain.

Asymmetric localization of natural antisense RNA of neuropeptide sensorin in Aplysia sensory neurons during aging and activity

Despite the advances in our understanding of transcriptome, regulation and function of its non-coding components continue to be poorly understood. Here we searched for natural antisense transcript for sensorin (NAT-SRN), a neuropeptide expressed in the presynaptic sensory neurons of gill-withdrawal reflex of the marine snail Aplysia californica. Sensorin (SRN) has a key role in learning and long-term memory storage in Aplysia. We have now identified NAT-SRN in the central nervous system (CNS) and have confirmed its expression by northern blotting and fluorescent RNA in situ hybridization. Quantitative analysis of NAT-SRN in micro-dissected cell bodies and processes of sensory neurons suggest that NAT-SRN is present in the distal neuronal processes along with sense transcripts. Importantly, aging is associated with reduction in levels of NAT-SRN in sensory neuron processes. Furthermore, we find that forskolin, an activator of CREB signaling, differentially alters the distribution of SRN and NAT-SRN. These studies reveal novel insights into physiological regulation of natural antisense RNAs.

Aplysia Ganglia preparation for electrophysiological and molecular analyses of single neurons

A major challenge in neurobiology is to understand the molecular underpinnings of neural circuitry that govern a specific behavior. Once the specific molecular mechanisms are identified, new therapeutic strategies can be developed to treat abnormalities in specific behaviors caused by degenerative diseases or aging of the nervous system. The marine snail Aplysia californica is well suited for the investigations of cellular and molecular basis of behavior because neural circuitry underlying a specific behavior could be easily determined and the individual components of the circuitry could be easily manipulated. These advantages of Aplysia have led to several fundamental discoveries of neurobiology of learning and memory. Here we describe a preparation of the Aplysia nervous system for the electrophysiological and molecular analyses of individual neurons. Briefly, ganglion dissected from the nervous system is exposed to protease to remove the ganglion sheath such that neurons are exposed but retain neuronal activity as in the intact animal. This preparation is used to carry out electrophysiological measurements of single or multiple neurons. Importantly, following the recording using a simple methodology, the neurons could be isolated directly from the ganglia for gene expression analysis. These protocols were used to carry out simultaneous electrophysiological recordings from L7 and R15 neurons, study their response to acetylcholine and quantitating expression of CREB1 gene in isolated single L7, L11, R15, and R2 neurons of Aplysia.

Regulation of the apolipoprotein gene cluster by a long noncoding RNA

Apolipoprotein A1 (APOA1) is the major protein component of high-density lipoprotein (HDL) in plasma. We have identified an endogenously expressed long noncoding natural antisense transcript, APOA1-AS, which acts as a negative transcriptional regulator of APOA1 both in vitro and in vivo. Inhibition of APOA1-AS in cultured cells resulted in the increased expression of APOA1 and two neighboring genes in the APO cluster. Chromatin immunoprecipitation (ChIP) analyses of a ∼50 kb chromatin region flanking the APOA1 gene demonstrated that APOA1-AS can modulate distinct histone methylation patterns that mark active and/or inactive gene expression through the recruitment of histone-modifying enzymes. Targeting APOA1-AS with short antisense oligonucleotides also enhanced APOA1 expression in both human and monkey liver cells and induced an increase in hepatic RNA and protein expression in African green monkeys. Furthermore, the results presented here highlight the significant local modulatory effects of long noncoding antisense RNAs and demonstrate the therapeutic potential of manipulating the expression of these transcripts both in vitro and in vivo.

Decreased response to acetylcholine during aging of aplysia neuron R15

How aging affects the communication between neurons is poorly understood. To address this question, we have studied the electrophysiological properties of identified neuron R15 of the marine mollusk Aplysia californica. R15 is a bursting neuron in the abdominal ganglia of the central nervous system and is implicated in reproduction, water balance, and heart function. Exposure to acetylcholine (ACh) causes an increase in R15 burst firing. Whole-cell recordings of R15 in the intact ganglia dissected from mature and old Aplysia showed specific changes in burst firing and properties of action potentials induced by ACh. We found that while there were no significant changes in resting membrane potential and latency in response to ACh, the burst number and burst duration is altered during aging. The action potential waveform analysis showed that unlike mature neurons, the duration of depolarization and the repolarization amplitude and duration did not change in old neurons in response to ACh. Furthermore, single neuron quantitative analysis of acetylcholine receptors (AChRs) suggested alteration of expression of specific AChRs in R15 neurons during aging. These results suggest a defect in cholinergic transmission during aging of the R15 neuron.

Age-associated bidirectional modulation of gene expression in single identified R15 neuron of Aplysia

Background

Despite the advances in our understanding of aging-associated behavioral decline, relatively little is known about how aging affects neural circuits that regulate specific behaviors, particularly the expression of genes in specific neural circuits during aging. We have addressed this by exploring a peptidergic neuron R15, an identified neuron of the marine snail Aplysia californica. R15 is implicated in reproduction and osmoregulation and responds to neurotransmitters such as acetylcholine, serotonin and glutamate and is characterized by its action potential bursts.

Results

We examined changes in gene expression in R15 neurons during aging by microarray analyses of RNAs from two different age groups, mature and old animals. Specifically we find that 1083 ESTs are differentially regulated in mature and old R15 neurons. Bioinformatics analyses of these genes have identified specific biological pathways that are up or downregulated in mature and old neurons. Comparison with human signaling networks using pathway analyses have identified three major networks [(1) cell signaling, cell morphology, and skeletal muscular system development (2) cell death and survival, cellular function maintenance and embryonic development and (3) neurological diseases, developmental and hereditary disorders] altered in old R15 neurons. Furthermore, qPCR analysis of single R15 neurons to quantify expression levels of candidate regulators involved in transcription (CREB1) and translation (S6K) showed that aging is associated with a decrease in expression of these regulators, and similar analysis in three other neurons (L7, L11 and R2) showed that gene expression change during aging could be bidirectional.

Conclusions

We find that aging is associated with bidirectional changes in gene expression. Detailed bioinformatics analyses and human homolog searches have identified specific biological processes and human-relevant signaling pathways in R15 that are affected during aging. Evaluation of gene expression changes in different neurons suggests specific transcriptomic signature of single neurons during aging.

Genomics and proteomics in solving brain complexity

The human brain is extraordinarily complex, composed of billions of neurons and trillions of synaptic connections. Neurons are organized into circuit assemblies that are modulated by specific interneurons and non-neuronal cells, such as glia and astrocytes. Data on human genome sequences predicts that each of these cells in the human brain has the potential of expressing ∼20 000 protein coding genes and tens of thousands of noncoding RNAs. A major challenge in neuroscience is to determine (1) how individual neurons and circuitry utilize this potential during development and maturation of the nervous system, and for higher brain functions such as cognition, and (2) how this potential is altered in neurological and psychiatric disorders. In this review, we will discuss how recent advances in next generation sequencing, proteomics and bioinformatics have transformed our understanding of gene expression and the functions of neural circuitry, memory storage, and disorders of cognition.

Dynamic ensemble view of the conformational landscape of HIV-1 TAR RNA and allosteric recognition

RNA conformational dynamics and the resulting structural heterogeneity play an important role in RNA functions, e.g., recognition. Recognition of HIV-1 TAR RNA has been proposed to occur via a conformational capture mechanism. Here, using ultrafast time-resolved fluorescence spectroscopy, we have probed the complexity of the conformational landscape of HIV-1 TAR RNA and monitored the position-dependent changes in the landscape upon binding of a Tat protein-derived peptide and neomycin B. In the ligand-free state, the TAR RNA samples multiple families of conformations with various degrees of base stacking around the three-nucleotide bulge region. Some subpopulations partially resemble those ligand-bound states, but the coaxially stacked state is below the detection limit. When Tat or neomycin B binds, the bulge region as an ensemble undergoes a conformational transition in a position-dependent manner. Tat and neomycin B induce mutually exclusive changes in the TAR RNA underlying the mechanism of allosteric inhibition at an ensemble level with residue-specific details. Time-resolved anisotropy decay measurements revealed picosecond motions of bases in both ligand-free and ligand-bound states. Mutation of a base pair at the bulge--stem junction has differential effects on the conformational distributions of the bulge bases. A dynamic model of the ensemble view of the conformational landscape for HIV-1 TAR RNA is proposed, and the implication of the general mechanism of RNA recognition and its impact on RNA-based therapeutics are discussed.

Conformational distribution and ultrafast base dynamics of leadzyme

The dynamic nature of ribozymes represents a significant challenge in elucidating their structure-dynamics-function relationship. Here, using femtosecond time-resolved spectroscopy and other biophysical tools, we demonstrate that the active site of leadzyme does not have a unique structure, but rather samples an ensemble of conformations that undergo picosecond structural changes. Various base modifications have a profound context-dependent impact on the catalysis.