Dynamics of Hippocampal Protein Expression During Long-term Spatial Memory Formation

Proteomic-Based Studies on Memory Formation in Normal and Neurodegenerative Disease-Affected Brains

A critical aspect of cognition is the ability to acquire, consolidate, and evoke memories, which is considerably impaired by neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. These mnemonic processes are dependent on signaling cascades, which involve protein expression and degradation. Recent mass spectrometry (MS)-based proteomics has opened a range of possibilities for the study of memory formation and impairment, making it possible to research protein systems not studied before. However, in the context of synaptic proteome related to learning processes and memory formation, a deeper understanding of the synaptic proteome temporal dynamics after induction of synaptic plasticity and the molecular changes underlying the cognitive deficits seen in neurodegenerative diseases is needed. This review analyzes the applications of proteomics for understanding memory processes in both normal and neurodegenerative conditions. Moreover, the most critical experimental studies have been summarized using the PANTHER overrepresentation test. Finally, limitations associated with investigations of memory studies in physiological and neurodegenerative disorders have also been discussed.

Experience-Induced Remodeling of the Hippocampal Post-synaptic Proteome and Phosphoproteome

The postsynaptic density (PSD) of excitatory synapses contains a highly organized protein network with thousands of proteins and is a key node in the regulation of synaptic plasticity. To gain new mechanistic insight into experience-induced changes in the PSD, we examined the global dynamics of the hippocampal PSD proteome and phosphoproteome in mice following four different types of experience. Mice were trained using an inhibitory avoidance (IA) task and hippocampal PSD fractions were isolated from individual mice to investigate molecular mechanisms underlying experience-dependent remodeling of synapses. We developed a new strategy to identify and quantify the relatively low level of site-specific phosphorylation of PSD proteome from the hippocampus, by using a modified iTRAQ-based TiSH protocol. In the PSD, we identified 3938 proteins and 2761 phosphoproteins in the sequential strategy covering a total of 4968 unique protein groups (at least two peptides including a unique peptide). On the phosphoproteins, we identified a total of 6188 unambiguous phosphosites (75% Collapse

Key Words

• inhibitory avoidance
• learning and memory
• phosphoproteomics
• postsynaptic density

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MESH Headings

• Mice
• Animals
• Proteome/metabolism
• Membrane Proteins/metabolism
• Nerve Tissue Proteins/metabolism
• Hippocampus/metabolism
• Synapses/metabolism
• Peptides/metabolism
• Phosphoproteins/metabolism
• Disks Large Homolog 4 Protein/metabolism

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Grants

• R01 MH112152 NIMH NIH HHS
• R01 NS036715 NINDS NIH HHS

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Affiliation(s)

Seok Heo Solomon H. Snyder Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA; Kavli Neuroscience Discovery Institute, Johns Hopkins University, Baltimore, Maryland, USA
Taewook Kang Department of Biochemistry and Molecular Biology, University of Southern Denmark, Odense, Denmark
Alexei M Bygrave Solomon H. Snyder Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA; Kavli Neuroscience Discovery Institute, Johns Hopkins University, Baltimore, Maryland, USA
Martin R Larsen Department of Biochemistry and Molecular Biology, University of Southern Denmark, Odense, Denmark.
Richard L Huganir Solomon H. Snyder Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA; Kavli Neuroscience Discovery Institute, Johns Hopkins University, Baltimore, Maryland, USA.

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Abl2 Kinase Differentially Regulates iGluRs Current Activity and Synaptic Localization

Abelson non-receptor tyrosine kinases (Abl1 and Abl2) are established cellular signaling proteins, implicated in cytoskeletal reorganization essential for modulation of cell morphology and motility. During development of the central nervous system, Abl kinases play fundamental roles in neurulation and neurite outgrowth, relaying information from axon guidance cues and growth factor receptors to promote cytoskeletal rearrangements. In mature neurons, Abl kinases localize to pre- and postsynaptic compartments and are involved in regulation of synaptic stability and plasticity. Although emerging evidence indicates interchangeability of these isoforms in managing of cellular functions, in healthy adult neurons, Abl1 contribution is less elucidated, while Abl2 is required for optimal synaptic functioning. Our previous study demonstrated compartmentalization of Abl1 to the presynapse and Abl2 to the postsynapse and characterized their modulatory effect on spontaneous excitatory synaptic transmission. Here, we further delineate the role of Abl2 on regulation of the postsynaptic component of miniature excitatory postsynaptic current (mEPSC). Our findings show that both acute and prolonged activation of Abl2, in line with reduction of mEPSC amplitude, also decrease AMPA and NMDA current amplitudes. In contrast with the current-detrimental effect, prolonged Abl2 activity stabilizes spines, particularly contributing to maintenance of active synapses at distal (perhaps apical) segments of dendrites. Hence, we propose that attenuation of ion currents via ionotropic glutamatergic receptors by Abl2 kinase derives from either reduction of the receptor sensitivity for glutamate or is due to alteration of channel gating mechanisms. Abl2 and excitatory postsynapses: Abl2 expression level affects active excitatory synapse density on distal dendrites, while Abl2 activity impacts current density through AMPA and NMDA receptors.

Abelson Kinases Mediate the Depression of Spontaneous Synaptic Activity Induced by Amyloid Beta 1-42 Peptides

Amyloid beta (Aβ) peptides represent one of the most studied etiological factors of Alzheimer's disease. Nevertheless, the effects elicited by different molecular forms of amyloid beta peptides widely vary between the studies, mostly depending on experimental conditions. Despite the enormous amount of accumulated evidences concerning the pathological effects of amyloid beta peptides, the exact identity of the amyloid beta species is still controversial, and even less is clear as regards to the downstream effectors that mediate the devastating impact of these peptides on synapses in the central nervous system. Recent publications indicate that some of the neurotoxic effects of amyloid beta peptides may be mediated via the activation of proteins belonging to the Abelson non-receptor tyrosine kinase (Abl) family, that are known to regulate actin cytoskeleton structure as well as phosphorylate microtubule-associated tau protein, a hallmark of Alzheimer's disease. By performing series of miniature excitatory postsynaptic currents (mEPSC) recordings in cultured hippocampal cells, we demonstrate that activation of Abl kinases by acute application of 42 amino acid-length monomeric amyloid beta (Aβ1-42) peptides reduces spontaneous synaptic release, while this effect can be rescued by pharmacologic inhibition of Abl kinase activity, or by reduction of Abl expression with small interfering RNAs. Our electrophysiological data are further reinforced by a subsequent biochemical analysis, showing enhanced phosphorylation of Abl kinase substrate CT10 Regulator of Kinase-homolog-Like (Crkl) upon treatment of hippocampal neurons with Aβ peptides. Thus, we conclude that Abl kinase activation may be involved in Aβ-induced weakening of synaptic transmission.

De novo proteomic methods for examining the molecular mechanisms underpinning long-term memory

Memory formation is a fundamental function of the nervous system that enables the experience-based adaptation of behaviour. The formation, recall and updating of long-term memory (LTM) requires new protein synthesis through its direct involvement in neuronal processes, such as long-term potentiation (LTP), long-term depression (LTD) and synaptic scaling. We discuss the advantages and limitations of several emerging techniques which enable the tagging of newly synthesised proteins, including stable isotope labelling with amino acids in cell culture (SILAC), puromycin labelling, and non-canonical amino acid (NCAA) labelling. We further present how these methods allow for the identification and visualisation of proteins which are newly synthesised during different stages of memory formation. These emerging techniques will continue to expand our understanding of how memories are formed, consolidated and retrieved.

Quantitative Proteomics Reveals Distinct Molecular Signatures of Different Cerebellum-Dependent Learning Paradigms

The cerebellum improves motor performance by adjusting motor gain appropriately. As de novo protein synthesis is essential for the formation and retention of memories, we hypothesized that motor learning in the opposite direction would induce a distinct pattern of protein expression in the cerebellum. We conducted quantitative proteomic profiling to compare the level of protein expression in the cerebellum at 1 and 24 h after training from mice that underwent different paradigms of cerebellum-dependent oculomotor learning from specific directional changes in motor gain. We quantified a total of 43 proteins that were significantly regulated in each of the three learning paradigms in the cerebellum at 1 and 24 h after learning. In addition, functional enrichment analysis identified protein groups that were differentially enriched or depleted in the cerebellum at 24 h after the three oculomotor learnings, suggesting that distinct biological pathways may be engaged in the formation of three oculomotor memories. Weighted correlation network analysis discovered groups of proteins significantly correlated with oculomotor memory. Finally, four proteins (Snca, Sncb, Cttn, and Stmn1) from the protein group correlated with the learning amount after oculomotor training were validated by Western blot. This study provides a comprehensive and unbiased list of proteins related to three cerebellum-dependent motor learning paradigms, suggesting the distinct nature of protein expression in the cerebellum for each learning paradigm. The proteomics data have been deposited to the ProteomeXchange Consortium with identifiers <PXD008433>.

Cell-specific non-canonical amino acid labelling identifies changes in the de novo proteome during memory formation

The formation of spatial long-term memory (LTM) requires the de novo synthesis of distinct sets of proteins; however, a non-biased examination of the de novo proteome in this process is lacking. Here, we generated a novel mouse strain, which enables cell-type-specific labelling of newly synthesised proteins with non-canonical amino acids (NCAAs) by genetically restricting the expression of the mutant tRNA synthetase, NLL-MetRS, to hippocampal neurons. By combining this labelling technique with an accelerated version of the active place avoidance task and bio-orthogonal non-canonical amino acid tagging (BONCAT) followed by SWATH quantitative mass spectrometry, we identified 156 proteins that were altered in synthesis in hippocampal neurons during spatial memory formation. In addition to observing increased synthesis of known proteins important in memory-related processes, such as glutamate receptor recycling, we also identified altered synthesis of proteins associated with mRNA splicing as a potential mechanism involved in spatial LTM formation.

Cognitive assessment of patients with nonalcoholic fatty liver disease

OBJECTIVE

The aim of this study was to investigate cognitive performance for the first time in participants with nonalcoholic fatty liver disease (NAFLD) using the Montreal Cognitive Assessment (MoCA).

PARTICIPANTS AND METHODS

In total, 70 participants with NAFLD and 73 age-matched and sex-matched healthy participants were enrolled in this prospective cross-sectional study. The diagnosis of NAFLD was made on the basis of abdominal ultrasonography findings. Anthropometric indices were calculated, and routine laboratory analyses were carried out for each participant. All participants provided sociodemographic data and completed the Beck Depression Inventory-II. Cognitive functions were evaluated using the Turkish version of the MoCA, with a cut-off score for mild cognitive impairment of less than 21 points.

RESULTS

The MoCA scores were significantly lower in participants with NAFLD than in the healthy group (P<0.05). In addition, more NAFLD participants than healthy participants presented with deficits in the visuospatial (P<0.05) and executive function domains (P<0.05). In the multivariate model, education level [2.79 (1.12-6.96); P<0.05] and area of residence [5.68 (2.24-14.38); P<0.001] were associated independently with cognitive dysfunction in both the NAFLD and the healthy groups. The MoCA scores were correlated negatively with fibrosis 4 scores in NAFLD participants (r=-0.359; P<0.05). However, hepatosteatosis grade and the presence of metabolic syndrome were not correlated with MoCA scores in the NAFLD group (P>0.05).

CONCLUSION

Our results show that NAFLD patients may have early or subtle cognitive dysfunction, including in the visuospatial and executive function domains, as indexed by scores on the MoCA test. Further targeted psychometric testing will be required to confirm the presence of cognitive impairment in this population.

Age-Dependent Effects of Acute Alcohol Administration in the Hippocampal Phosphoproteome

Alcohol consumption during adolescence is deleterious to the developing brain and leads to persistent deficits in adulthood. Several results provide strong evidence for ethanol-associated alterations in glutamatergic signaling and impaired synaptic plasticity in the hippocampus. Protein phosphorylation is a well-known and well-documented mechanism in memory processes, but information on phosphoprotein alterations in hippocampus after ethanol exposure is limited. This study focuses on age-related changes in the hippocampal phosphoproteome after acute alcohol administration. We have compared the phosphoprotein expression in the hippocampus of adult and adolescent Wistar rats treated with a single dose of ethanol (5 g/kg i.p.), using a proteomic approach including phosphoprotein enrichment by immobilized metal affinity chromatography (IMAC). Our proteomic analysis revealed that 13 proteins were differentially affected by age, ethanol administration, or both. Most of these proteins are involved in neuroprotection and are expressed less in young rats treated with ethanol. We conclude that acute alcohol induces important changes in the expression of phosphoproteins in the hippocampus that could increase the risk of neurodegenerative disorders, especially when the alcohol exposure begins in adolescence.

Status epilepticus triggers long-lasting activation of complement C1q-C3 signaling in the hippocampus that correlates with seizure frequency in experimental epilepsy

Status epilepticus (SE) triggers a myriad of neurological alterations that include unprovoked seizures, temporal lobe epilepsy (TLE), and cognitive deficits. Although SE-induced loss of hippocampal dendritic structures and synaptic remodeling are often associated with this pathophysiology, the underlying mechanisms remain elusive. Recent evidence points to the classical complement pathway as a potential mechanism. Signaling through the complement protein C1q to C3, which is cleaved into smaller biologically active fragments including C3b and iC3b, contributes to the elimination of synaptic structures in the normal developing brain and in models of neurodegenerative disorders. We recently found increased protein levels of C1q and iC3b fragments in human drug-resistant epilepsy. Thus, to identify a potential role for C1q-C3 in SE-induced epilepsy, we performed a temporal analysis of C1q protein levels and C3 cleavage in the hippocampus along with their association to seizures and hippocampal-dependent cognitive functions in a rat model of SE and acquired TLE. We found significant increases in the levels of C1q, C3, and iC3b in the hippocampus at 2-, 3- and 5-weeks after SE relative to controls (p<0.05). In the SE group, greater iC3b levels were significantly correlated with higher seizure frequency (p<0.05). Together, these data support that hyperactivation of the classical complement pathway after SE parallels the progression of epilepsy. Future studies will determine whether C1q-C3 signaling contributes to epileptogenic synaptic remodeling in the hippocampus.

Mass Spectrometry-Based Approaches to Understand the Molecular Basis of Memory

The central nervous system is responsible for an array of cognitive functions such as memory, learning, language, and attention. These processes tend to take place in distinct brain regions; yet, they need to be integrated to give rise to adaptive or meaningful behavior. Since cognitive processes result from underlying cellular and molecular changes, genomics and transcriptomics assays have been applied to human and animal models to understand such events. Nevertheless, genes and RNAs are not the end products of most biological functions. In order to gain further insights toward the understanding of brain processes, the field of proteomics has been of increasing importance in the past years. Advancements in liquid chromatography-tandem mass spectrometry (LC-MS/MS) have enabled the identification and quantification of thousands of proteins with high accuracy and sensitivity, fostering a revolution in the neurosciences. Herein, we review the molecular bases of explicit memory in the hippocampus. We outline the principles of mass spectrometry (MS)-based proteomics, highlighting the use of this analytical tool to study memory formation. In addition, we discuss MS-based targeted approaches as the future of protein analysis.

Cognitive Impairment in Patients with Psoriasis: A Cross-Sectional Study Using the Montreal Cognitive Assessment

BACKGROUND

Psoriasis is a multisystem chronic inflammatory disorder that is thought to be associated with cognitive impairment.

AIMS

We aimed to investigate cognitive performance using the Montreal Cognitive Assessment (MoCA) in patients with psoriasis.

METHODS

In total, 77 patients with psoriasis and 83 age- and sex-matched control subjects were enrolled in this prospective cross-sectional study. Physical and/or histopathological findings were used to diagnose psoriasis vulgaris, and patients with psoriasis were evaluated according to disease characteristics, including duration, severity, onset age, medical treatment, and cosmetic involvement. All participants provided sociodemographic data and completed the Beck Depression Inventory. Cognitive functions were evaluated using the MoCA tool.

RESULTS

The MoCA scores were significantly lower in the psoriasis group than in the control group (p = 0.004). More psoriasis patients than control subjects presented with deficits in visuospatial domain (p = 0.037) and executive functioning (p = 0.010). In the multivariate model, the presence of psoriasis (odds ratio [OR] 3.64; 95 % confidence interval [CI] 1.65-8.02; p = 0.001), education level (3.74; 95 % CI 1.65-8.48; p = 0.002), and area of residence (3.56; 95 % CI 1.61-7.87; p = 0.002) were found to be independently associated with cognitive impairment in patients with psoriasis and control subjects. On the other hand, no correlations were observed between disease characteristics and cognitive impairment in patients with psoriasis vulgaris (p > 0.05).

CONCLUSIONS

The results suggest that psoriasis patients might have early or subtle cognitive impairment, including visuospatial domain and executive functioning.