Myosin II regulates actin rearrangement-related structural synaptic plasticity during conditioned taste aversion memory extinction

Effect of empagliflozin on cytoskeletal repair in the hippocampus of obese mice

OBJECTIVE

We aimed to investigate the effect of empagliflozin on hippocampal phosphorylated protein levels in obese mice.

MATERIALS AND METHODS

Sixteen obese mice successfully modeled on high-fat diet were randomly divided into high-fat feeding group (group H) and empagliflozin group (group H + empagliflozin, group E), eight mice in each group, and eight C57BL/6J male normal mice were selected as the control group (normal control, group C). Group E was treated with empagliflozin 10 mg/kg/d for 12 weeks, while mice in groups H and C were treated with equal amounts of saline. The spatial learning memory ability of the mice was determined by the Morris water maze experiment. Further, their body weights and serological indices were measured. Finally, total proteins were extracted from hippocampal tissues for functional analysis by the phosphorylated proteomics method.

RESULTS

The results showed that escape latency was prolonged, retention time in the target quadrant was shortened, and the number of loop penetrations was reduced in the obese mice induced by a high-calorie diet compared with normal controls, whereas escape latency was shortened, retention time in the target quadrant was increased, and the number of loop penetrations was increased after empagliflozin treatment. Phosphoproteomics in the high-fat/control (H/C), empagliflozin/high-fat (E/H), and E/C groups showed 844, 1,552, and 1,512 differentially significant phosphorylation sites, respectively. The proteins corresponding to these differentially phosphorylated sites were mainly involved in neurodegenerative pathways and actin cytoskeleton regulation. Notably, myosin heavy chain 10 (MYH10), p21 protein-activated kinase 4 (PAK4), phosphatidylinositol 3 -phosphate 5-kinase (PIKfyve), and other differentially phosphorylated proteins were involved in actin cytoskeleton regulation.

CONCLUSION

We concluded that empagliflozin protects cognitive functions by inducing serine phosphorylation in MYH10, PAK4, and PIKfyve in the hippocampal tissue of obese mice.

Brain-derived neurotrophic factor regulates LYN kinase-mediated myosin light chain kinase activation to modulate nonmuscle myosin II activity in hippocampal neurons

Myosins belong to a large superfamily of actin-dependent molecular motors. Nonmuscle myosin II (NM II) is involved in the morphology and function of neurons, but little is known about how NM II activity is regulated. Brain-derived neurotrophic factor (BDNF) is a prevalent neurotrophic factor in the brain that encourages growth and differentiation of neurons and synapses. In this study, we report that BDNF upregulates the phosphorylation of myosin regulatory light chain (MLC2), to increases the activity of NM II. The role of BDNF on modulating the phosphorylation of MLC2 was validated by using Western blotting in primary cultured hippocampal neurons. This result was confirmed by injecting BDNF into the dorsal hippocampus of mice and detecting the phosphorylation level of MLC2 by Western blotting. We further perform coimmunoprecipitation assay to confirm that this process depends on the activation of the LYN kinase through binding with tyrosine kinase receptor B, the receptor of BDNF, in a kinase activity-dependent manner. LYN kinase subsequently phosphorylates MLCK, further promoting the phosphorylation of MLC2. Taken together, our results suggest a new molecular mechanism by which BDNF regulates MLC2 activity, which provides a new perspective for further understanding the functional regulation of NM II in the nervous system.

Synaptosomal Actin Dynamics in the Developmental Visual Cortex Regulate Behavioral Visual Acuity in Rats

Purpose

Synaptosomal actin dynamics are essential for synaptic structural stability. Whether actin dynamics are involved in structural and functional synaptic plasticity within the primary visual cortex (V1) or behavioral visual acuity in rats has still not been thoroughly investigated.

Methods

Synaptosome preparation and western blot analysis were used to analyze synaptosomal actin dynamics. Transmission electron microscopy was used to detect synaptic density and mitochondrial area alterations. A visual water maze task was applied to assess behavioral visual acuity. Microinjection of the actin polymerization inhibitor or stabilizer detected the effect of actin dynamics on visual function.

Results

Actin dynamics, the mitochondrial area, and synaptic density within the area of V1 are increased during the critical period for the development of binocularity. Microinjection of the actin polymerization inhibitor cytochalasin D into the V1 decreased the mitochondrial area, synaptic density, and behavioral visual acuity. Long-term monocular deprivation reduced actin dynamics, the mitochondrial area, and synaptic density within the V1 contralateral to the deprived eye compared with those ipsilateral to the deprived eye and impaired visual acuity in the amblyopic eye. In addition, the mitochondrial area, synaptic density, and behavioral visual acuity were improved by stabilization of actin polymerization by jasplakinolide microinjection.

Conclusions

During the critical period of visual development of binocularity, synaptosomal actin dynamics regulate synaptic structure and function and play roles in behavioral visual acuity in rats.

Taste Processing: Insights from Animal Models

Taste processing is an adaptive mechanism involving complex physiological, motivational and cognitive processes. Animal models have provided relevant data about the neuroanatomical and neurobiological components of taste processing. From these models, two important domains of taste responses are described in this review. The first part focuses on the neuroanatomical and neurophysiological bases of olfactory and taste processing. The second part describes the biological and behavioral characteristics of taste learning, with an emphasis on conditioned taste aversion as a key process for the survival and health of many species, including humans.

Fear conditioning downregulates miR-138 expression in the hippocampus to facilitate the formation of fear memory

Fear memory is important for the survival of animals and is associated with certain anxiety disorders, such as posttraumatic stress disorder. A thorough understanding of the molecular mechanisms of fear memory, especially associative fear memory, is imperative. MicroRNA-138 (miR-138) is a widely distributed microRNA in the brain and is locally enriched at synaptic sites. The role of miR-138 in the formation of fear memory is still largely unknown. In this study, a contextual fear conditioning (CFC) paradigm, bioinformatic methods, a luciferase assay, real-time PCR and western blot were used to evaluate the detailed effects of miR-138 on fear memory. We found that miR-138 transiently decreased in the dorsal hippocampus (DH) after CFC training. Upregulation or downregulation of miR-138 in the DH with miR-138 agomir or antagomir treatment significantly impaired or enhanced the formation of CFC memory, respectively. Moreover, the effects of miR-138 in the DH on the formation of CFC memory were achieved by changing the expression of the downstream target gene calpain 1 (Capn1). Taken together, both the in-vitro evidence and the in-vivo evidence presented in this study support the involvement of miR-138 in CFC memory formation, at least partly via the regulation of Capn1-mediated synaptic plasticity changes. Therapeutic use of miR-138/Capn1 is promising as an alternative option in the treatment of fear memory-related anxiety disorders.

The Insula and Taste Learning

The sense of taste is a key component of the sensory machinery, enabling the evaluation of both the safety as well as forming associations regarding the nutritional value of ingestible substances. Indicative of the salience of the modality, taste conditioning can be achieved in rodents upon a single pairing of a tastant with a chemical stimulus inducing malaise. This robust associative learning paradigm has been heavily linked with activity within the insular cortex (IC), among other regions, such as the amygdala and medial prefrontal cortex. A number of studies have demonstrated taste memory formation to be dependent on protein synthesis at the IC and to correlate with the induction of signaling cascades involved in synaptic plasticity. Taste learning has been shown to require the differential involvement of dopaminergic GABAergic, glutamatergic, muscarinic neurotransmission across an extended taste learning circuit. The subsequent activation of downstream protein kinases (ERK, CaMKII), transcription factors (CREB, Elk-1) and immediate early genes (c-fos, Arc), has been implicated in the regulation of the different phases of taste learning. This review discusses the relevant neurotransmission, molecular signaling pathways and genetic markers involved in novel and aversive taste learning, with a particular focus on the IC. Imaging and other studies in humans have implicated the IC in the pathophysiology of a number of cognitive disorders. We conclude that the IC participates in circuit-wide computations that modulate the interception and encoding of sensory information, as well as the formation of subjective internal representations that control the expression of motivated behaviors.

Different patterns of changes between actin dynamics and synaptic density in the rat's primary visual cortex during a special period of visual development

In our previous study, we found that the normalized levels of the synaptosomal filament actin (F-actin) to monomeric global actin (G-actin) ratio in the primary visual cortex (V1) of rats was significantly lower on postnatal day (P) 45 compared with P30, however, the synaptic density in the monocular area of primary visual cortex (V1M) maintained a stable high level from P30 to P45. The mechanisms underlying the different patterned of change in synaptic density and actin rearrangements from P30 to P45 are unclear. During visual development, there is a synaptic pruning process in the binocular segment of primates' visual cortex (V1B) and we suppose the pruning activity may contribute to the decreased synaptosomal F-actin to G-actin ratio. To address this issue, first, samples were derived from the region of V1B for TEM analysis but no significant difference was demonstrated between the P30 and P45 groups. In addition, the expression of PSD-95 detected by immunobloting in the synaptosomes of V1 at P30 and P45 also showed no significant difference. Combined with the previous results of actin dynamics in the V1 and synaptic density in the V1M, we conclude that the synaptic density and actin dynamics in the rats' primary visual cortex are inter-related but not absolutely identical. This study suggests actin cytoskeleton not only provides the structural basis but also regulates a various array of cellular activities underlying synaptic function. Besides, it highlights a further research of synaptic pruning.

Memory disrupting effects of nonmuscle myosin II inhibition depend on the class of abused drug and brain region

Depolymerizing actin in the amygdala through nonmuscle myosin II inhibition (NMIIi) produces a selective, lasting, and retrieval-independent disruption of the storage of methamphetamine-associated memories. Here we report a similar disruption of memories associated with amphetamine, but not cocaine or morphine, by NMIIi. Reconsolidation appeared to be disrupted with cocaine. Unlike in the amygdala, methamphetamine-associated memory storage was not disrupted by NMIIi in the hippocampus, nucleus accumbens, or orbitofrontal cortex. NMIIi in the hippocampus did appear to disrupt reconsolidation. Identification of the unique mechanisms responsible for NMII-mediated, amygdala-dependent disruption of memory storage associated with the amphetamine class may enable induction of retrieval-independent vulnerability to other pathological memories.