Conditional Knockout of Cav2.1 Disrupts the Accuracy of Spatial Recognition of CA1 Place Cells and Spatial/Contextual Recognition Behavior

Involvement of the Voltage-Gated Calcium Channels L- P/Q- and N-Types in Synapse Elimination During Neuromuscular Junction Development

During the nervous system development, synapses are initially overproduced. In the neuromuscular junction (NMJ) however, competition between several motor nerve terminals and the synapses they made ends with the maturation of only one axon. The competitive signaling between axons is mediated by the differential activity-dependent release of the neurotransmitter ACh, co-transmitters, and neurotrophic factors. A multiple metabotropic receptor-driven downstream balance between PKA and PKC isoforms modulates the phosphorylation of targets involved in transmitter release and nerve terminal stability. Previously, we observed in the weakest endings on the polyinnervated NMJ that M₁ mAChR receptors reduce ACh release through the PKC pathway coupled to an excess of Ca²⁺ inflow through P/Q- N- and L-type voltage-gated calcium channels (VGCC). This signaling would contribute to the elimination of this nerve terminal. Here, we investigate the involvement of the P/Q-, N-, and L-subtype channels in transgenic B6.Cg-Tg (Thy1-YFP)16-Jrs/J mice during synapse elimination. Then, the axon number and postsynaptic receptor cluster morphologic maturation were evaluated. The results show that both L- and P/Q-type VGCC (but not the N-type) are equally involved in synapse elimination. Their normal function favors supernumerary axonal loss by jointly enhancing intracellular calcium [Ca²⁺]_i. The block of these VGCCs or [Ca2+]i  i sequestration results in the same delay of axonal loss as the cPKCβI and nPKCε isoform block or PKA activation. The specific block of the muscle cell's contraction with μ-conotoxin GIIIB also delays synapse maturation, and thus, a retrograde influence from the postsynaptic site regulating the presynaptic CaV1.3 may contribute to the synapse elimination.

The Ventral Midline Thalamus Mediates Hippocampal Spatial Information Processes upon Spatial Cue Changes

The ventral midline thalamus, consisting of the reuniens and rhomboid nuclei (RE/Rh), is a thalamic structure interconnected with the limbic systems including the hippocampus. Recently, many studies have revealed that this structure plays distinctive roles in spatial learning and memory in collaboration with hippocampal functions. However, what aspects of spatial information process are influenced by the RE/Rh is not clearly known. To elucidate the roles of RE/Rh in spatial information processing and its effects on hippocampal activity, specifically with the manipulation of spatial contents, we measured hippocampal-dependent spatial memory performance and hippocampal place cell activities after RE/Rh lesion using male C57BL/6J × 129/SvJae hybrid mice. We found that the lesion altered the behavioral aptitude in recognizing locational changes of an object. Furthermore, CA1 place cells in the lesion group showed different spatial representation patterns in recognizing the environment with cue locational changes compared with the control group. Interestingly, the patterns of CA1 place cells in recognizing the same environment previously visited were not disrupted in the lesion group compared with the control group. These findings demonstrate that the ventral midline thalamus (RE/Rh) is important in recognizing the spatial relationships, especially when spatial rearrangement of cue position was introduced.SIGNIFICANCE STATEMENT The ventral midline thalamic nuclei (reuniens and rhomboid) interact with the hippocampus to influence various cognitive functions requiring spatial memories, yet what aspects of spatial information process are influenced by these nuclei is not clearly known. Here, we reveal that these nuclei play a crucial role in modulating hippocampal properties only with locational rearrangement of cues, not with the familiar arrangement. These nuclei are distinctively involved in cue-dependent spatial information processes of CA1 place cells. In particular, we suggest that these nuclei modulate spatial information processing on discrete components, especially when the spatial cue relationship is modified.

Inhibitory Signaling to Ion Channels in Hippocampal Neurons Is Differentially Regulated by Alternative Macromolecular Complexes of RGS7

The neuromodulatory effects of GABA on pyramidal neurons are mediated by GABA_B receptors (GABA_BRs) that signal via a conserved G-protein-coupled pathway. Two prominent effectors regulated by GABA_BRs include G-protein inwardly rectifying K⁺ (GIRK) and P/Q/N type voltage-gated Ca²⁺ (Ca_V2) ion channels that control excitability and synaptic output of these neurons, respectively. Regulator of G-protein signaling 7 (RGS7) has been shown to control GABA_B effects, yet the specificity of its impacts on effector channels and underlying molecular mechanisms is poorly understood. In this study, we show that hippocampal RGS7 forms two distinct complexes with alternative subunit configuration bound to either membrane protein R7BP (RGS7 binding protein) or orphan receptor GPR158. Quantitative biochemical experiments show that both complexes account for targeting nearly the entire pool of RGS7 to the plasma membrane. We analyzed the effect of genetic elimination in mice of both sexes and overexpression of various components of RGS7 complex by patch-clamp electrophysiology in cultured neurons and brain slices. We report that RGS7 prominently regulates GABA_BR signaling to Ca_V2, in addition to its known involvement in modulating GIRK. Strikingly, only complexes containing R7BP, but not GPR158, accelerated the kinetics of both GIRK and Ca_V2 modulation by GABA_BRs. In contrast, GPR158 overexpression exerted the opposite effect and inhibited RGS7-assisted temporal modulation of GIRK and Ca_V2 by GABA. Collectively, our data reveal mechanisms by which distinctly composed macromolecular complexes modulate the activity of key ion channels that mediate the inhibitory effects of GABA on hippocampal CA1 pyramidal neurons.SIGNIFICANCE STATEMENT This study identifies the contributions of distinct macromolecular complexes containing a major G-protein regulator to controlling key ion channel function in hippocampal neurons with implications for understanding molecular mechanisms underlying synaptic plasticity, learning, and memory.

Effective expression of Drebrin in hippocampus improves cognitive function and alleviates lesions of Alzheimer's disease in APP (swe)/PS1 (ΔE9) mice

AIMS

Alzheimer's disease (AD), a progressive development dementia, is increasingly impacting patients' living conditions worldwide. Despite medical care and funding support, there are still no highly individualized drugs and practical strategies for clinical prevention and treatment. Developmentally regulated brain protein (abbreviated as Drebrin or Dbn, also known as Dbn1 in mouse) exists in neurons, especially in dendrites, and is an actin-binding protein that modulates synaptic morphology and long-term memory. However, the majority of previous studies have focused on its upstream proteins and neglected the impact Drebrin has on behavior and AD in vivo.

METHODS

Here, we tracked the behavioral performances of 4-, 8-, 12-, and 16-month-old AD mice and investigated the expression level of Drebrin in their hippocampi. A Pearson correlation analysis between Drebrin levels and behavioral data was performed. Subsequently, 2-month-old AD mice were injected with rAAV-zsGreen-Dbn1 vector, composing the APP/PS1-Dbn1 group, and sex- and age-matched AD mice were injected with rAAV-tdTomato vector to serve as the control group. All mice were conducted behavioral tests and molecular detection 6 months later.

RESULTS

(i) The expression of Drebrin is decreased in the hippocampus of aged AD mice compared with that of age-matched WT and young adult AD mice; (ii) cognitive ability of APP/PS1 mice decreases with age; (iii) Drebrin protein expression in the hippocampus correlates with behavioral performance in different aged AD mice; (iv) cognitive ability improved significantly in APP/PS1-Dbn1 mice; (v) the expression level of Drebrin in APP/PS1-Dbn1 mouse hippocampus was significantly increased; (vi) the pathological lesion of AD was alleviated in APP/PS1-Dbn1 mice; (vii) the filamentous actin (F-actin) and microtubule-associated protein 2(MAP-2) in APP/PS1-Dbn1 mice were notably more than control mice.

CONCLUSION

In this study, an effective expression of Drebrin improves cognitive abilities and alleviates lesions in an AD mouse model. These results may provide some valid resources for therapy and research of AD.

Down-Regulated Drebrin Aggravates Cognitive Impairments in a Mouse Model of Alzheimer's Disease

The developmentally regulated brain protein drebrin (Dbn) is a functional protein involved with long-term memory formation and is widely distributed in brain neurons, especially in the dendritic spines. A noticeable decline of this protein has been found in the hippocampus and cortex of patients with Alzheimer's disease (AD), yet the relationship between Dbn and AD has not been fully understood. In the present study, we examined how down-regulation of Dbn impacts the progression of AD in experimental animals. Accordingly, we injected Dbn interference vector (rAAV-mDbn1 ShRNA) into the hippocampus of three-month old APP(swe)/PS1(ΔE9) mice (APP/PS1 mice) and then successfully down-regulated Dbn expression in this brain region. Behavioral tests, including the Morris water maze test, the open field test, and the novel object test were conducted when the animals were nine months old. Subsequently, MicroPET/CT imaging to monitor glucose metabolism was done. We then investigated Aβ, GFAP, PSD-95, MAP2, vimentin, Cox43, and Syn1 expressions in the brain of the experimental animals via immunohistochemical or immunofluorescence methods. We found that AD mice with a low expression of Dbn performed poorly in the behavioral tests and showed decreased glucose utilization. In the brains of these animals, we detected a slight increase of Aβ, GFAP and vimentin and a significant decline of PSD-95. Altogether our data warrant further studies to elucidate the effect of Dbn on the development and progression of AD.