Improvement of depressive behaviors by nefiracetam is associated with activation of CaM kinases in olfactory bulbectomized mice

Empagliflozin-activated AMPK elicits neuroprotective properties in reserpine-induced depression via regulating dynamics of hippocampal autophagy/inflammation and PKCζ-mediated neurogenesis

RATIONALE

Major depression has been an area of extensive research during the last decades, for it represents a leading cause of disability and suicide. The stark rise of depression rates influenced by life stressors, economic threats, pandemic era, and resistance to classical treatments, has made the disorder rather challenging. Adult hippocampal neurogenesis and plasticity are particularly sensitive to the dynamic interplay between autophagy and inflammation. In fact, the intricate balance between the two processes contributes to neuronal homeostasis and survival.

OBJECTIVES

Having demonstrated promising potentials in AMPK activation, a major metabolic sensor and autophagy regulator, empagliflozin (Empa) was investigated for possible antidepressant properties in the reserpine rat model of depression.

RESULTS

While the reserpine protocol elicited behavioral, biochemical, and histopathological changes relevant to depression, Empa outstandingly hindered these pathological perturbations. Importantly, hippocampal autophagic response markedly declined with reserpine which disrupted the AMPK/mTOR/Beclin1/LC3B machinery and, conversely, neuro-inflammation prevailed under the influence of the NLRP3 inflammasome together with oxidative/nitrative stress. Consequently, AMPK-mediated neurotrophins secretion obviously deteriorated through PKCζ/NF-κB/BDNF/CREB signal restriction. Empa restored hippocampal monoamines and autophagy/inflammation balance, driven by AMPK activation. By promoting the atypical PKCζ phosphorylation (Thr403) which subsequently phosphorylates NF-κB at Ser311, AMPK successfully reinforced BDNF/CREB signal and hippocampal neuroplasticity. The latter finding was supported by hippocampal CA3 toluidine blue staining to reveal intact neurons.

CONCLUSION

The current study highlights an interesting role for Empa as a regulator of autophagic and inflammatory responses in the pathology of depression. The study also pinpoints an unusual contribution for NF-κB in neurotrophins secretion via AMPK/PKCζ/NF-κB/BDNF/CREB signal transduction. Accordingly, Empa can have special benefits in diabetic patients with depressive symptoms.

LIMITATIONS

The influence of p-NF-κB (Ser311) on NLRP3 inflammasome assembly and activation has not been investigated, which can represent an interesting point for further research.

The Calcium/Calmodulin-Dependent Kinases II and IV as Therapeutic Targets in Neurodegenerative and Neuropsychiatric Disorders

CaMKII and CaMKIV are calcium/calmodulin-dependent kinases playing a rudimentary role in many regulatory processes in the organism. These kinases attract increasing interest due to their involvement primarily in memory and plasticity and various cellular functions. Although CaMKII and CaMKIV are mostly recognized as the important cogs in a memory machine, little is known about their effect on mood and role in neuropsychiatric diseases etiology. Here, we aimed to review the structure and functions of CaMKII and CaMKIV, as well as how these kinases modulate the animals’ behavior to promote antidepressant-like, anxiolytic-like, and procognitive effects. The review will help in the understanding of the roles of the above kinases in the selected neurodegenerative and neuropsychiatric disorders, and this knowledge can be used in future drug design.

Mechanisms underpinning AMP-activated protein kinase-related effects on behavior and hippocampal neurogenesis in an animal model of depression

Adenosine monophosphate-activated protein kinase (AMPK) is critical for whole-body energy metabolism regulation. Recent studies have suggested that physical exercise ameliorates depressive-like behaviors via AMPK activation; however, the underlying mechanism is unclear. Here, we examined the effects and underlying mechanisms of AMPK activation on depressive-like behavior in olfactory bulbectomized (OBX) mice. We treated OBX mice with the AMPK activator, 5-aminoimidazole-4-carboxamide-1-β-d-ribonucleotide (AICAR) on the 7th or 14th day after bilateral bulbectomy and evaluated depressive-like behavior using the tail-suspension test (TST) and forced swimming test (FST) on the 21st day. The expression of phosphorylated AMPK, protein kinase C ζ (PKCζ), nuclear factor-kappa B (NF-κB), brain-derived neurotrophic factor (BDNF), and cAMP response element-binding protein (CREB) in the hippocampus was assessed by western blotting. Hippocampal neurogenesis and localization of AMPK and phosphorylated NF-κB were examined by immunohistochemistry. Chronic AICAR treatment suppressed the prolonged immobility of OBX mice in the TST and FST, and increased the levels of phosphorylated AMPK, PKCζ, NF-κB, CREB, and BDNF. Hippocampal neurogenesis in OBX mice was promoted by chronic AICAR treatment. Co-administration of AICAR with the PKCζ inhibitor or the neurotrophic tyrosine kinase receptor type 2 (TrkB) antagonist, ANA-12, inhibited these effects. Phosphorylated AMPK was detected in mature and immature hippocampal neurons and microglia, while phosphorylated NF-κB was detected only in neurons in AICAR-treated OBX mice. These data indicate that AMPK activation produces anti-depressant effects, which are mediated by elevated hippocampal neurogenesis potentially via PKCζ/NF-κB/BDNF/TrkB/CREB signaling in neurons.

T-type calcium channel enhancer SAK3 produces anti-depressant-like effects by promoting adult hippocampal neurogenesis in olfactory bulbectomized mice

T-type calcium channels are involved in the pathophysiology of epilepsy, pain, and sleep. Recently, we developed a novel spiroimidazopyridine compound, SAK3 (ethyl 8'-methyl-2',4-dioxo-2-(piperidin-1-yl)-2'H-spiro[cyclopentane-1,3'-imidazo[1,2-a]pyridine]-2-ene-3-carboxylate), which enhances T-type calcium channel currents and improves memory deficits in olfactory bulbectomized (OBX) mice. Here, we demonstrated the anti-depressant effects of SAK3 in OBX mice. Chronic SAK3 administration (0.5 or 1.0 mg/kg, p.o.) improved depressive-like behaviors in OBX mice. The impaired adult neurogenesis in the hippocampal dentate gyrus (DG) that occurred 4 weeks after OBX administration was significantly restored by chronic SAK3 administration (0.5 or 1.0 mg/kg, p.o.). Additionally, SAK3 (0.5 mg/kg, p.o.) promoted the proliferation and survival of newborn cells in the naïve DG. Moreover, SAK3 administration (0.5 mg/kg, p.o.) antagonized the reduction of calcium/calmodulin-dependent protein kinase II (CaMKII) and CaMKIV phosphorylation levels, thereby rescuing the decreased levels of cAMP response element-binding protein (CREB)/brain derived neurotrophic factor (BDNF) signaling in the OBX DG. The effects of SAK3 were completely blocked by the T-type calcium channel selective blocker NNC 55-0396 (12.5 mg/kg, i.p.). Altogether, these results suggest that SAK3 improves depressive-like behaviors by promoting adult neurogenesis via T-type calcium channel stimulation in the hippocampus.

Combined Memantine and Donepezil Treatment Improves Behavioral and Psychological Symptoms of Dementia-Like Behaviors in Olfactory Bulbectomized Mice

Memantine, an uncompetitive N-methyl-D-aspartate receptor antagonist, and the cholinesterase inhibitor, donepezil, are approved in most countries for treating moderate-to-severe Alzheimer's disease (AD). These drugs have different molecular targets; thus, it is expected that the effects of combined treatment would be synergistic. Some reports do show memantine/donepezil synergy in ameliorating cognition in AD model animals, but their combined effects on behavioral and psychological symptoms of dementia (BPSD)-like behaviors have not been addressed. Here, we investigate combined memantine/donepezil effects on cognitive impairment and BPSD-like behaviors in olfactory bulbectomized (OBX) mice. Interestingly, combined administration synergistically improved both depressive-like behaviors and impaired social interaction in OBX mice, whereas only weak synergistic effects on cognitive performance were seen. To address mechanisms underlying these effects, we used in vivo microdialysis study and observed impaired nicotine-induced serotonin (5-HT) release in OBX mouse hippocampus. Combined memantine/donepezil administration, but not single administration of either, significantly antagonized the decrease in nicotine-induced 5-HT release seen in OBX mouse hippocampus. Furthermore, decreased autophosphorylation of calcium/calmodulin dependent protein kinase II (CaMKII) was rescued in hippocampal CA1 and dentate gyrus of OBX mice by combined memantine/donepezil administration. These results suggest that improvement of BPSD-like behaviors by the co-administration of both drugs is in part mediated by enhanced 5-HT release and CaMKII activity in OBX mouse hippocampus.

The olfactory bulbectomized rat as a model of depression: The hippocampal pathway

In rodents, the removal of the olfactory bulbs (OBs), i.e. olfactory bulbectomy (OBX), results in numerous alterations in neurotransmitter, endocrine and immune systems, as well as behavioral changes, similar to those observed in depressed patients. Because the behavioral deficits induced in OBX animals are reversed after repeated administration of antidepressants, this is a model often used to test the effectiveness of putative antidepressant agents. Recent evidence suggests that OBX results in the dysfunction of various cellular processes within the hippocampus, including decreases in dentate gyrus neurogenesis, disruption in long-term potentiation in CA1 and CA3 subregions and neuronal atrophy in the CA1 subregion, along with downstream markers, all of which are consistent with abnormal neuronal activity in the hippocampus of clinically depressed populations. Moreover, repeated administration of novel natural and synthetic antidepressant compounds can improve certain aspects of depression-like behavior and hippocampal function. In an effort to bring together the existing literature, this review will focus on the mechanisms by which proposed pharmaceuticals impact hippocampal-dependent processes and behavior.

Effects of 4-hydroxyisoleucine from Fenugreek Seeds on Depression-like Behavior in Socially Isolated Olfactory Bulbectomized Rats

Context:

Antidepressant-like effects of (2S, 3R, 4S)-4-hydroxyisoleucine (4-HI), a major amino acid from fenugreek seeds, has been reported in the animal model of acute depression.

Aims:

To evaluate effects of subacute administration of 4-HI in animal model of stress-induced depression namely socially isolated olfactory bulbectomized rats.

Materials and Methods:

Bilateral olfactory bulbectomy (OBX) were induced in 30 Sprague-Dawley rats. After recovery period of 14 days, rats were randomized into five groups of 6 rats each and stressed with social isolation (individual housing). The rats were orally treated with either vehicle (OBX-Iso), positive control, fluoxetine (30 mg/kg) or 4-HI (10, 30, 100 mg/kg) once a day from day 14 onward. Separate group of rats with social isolation but without OBX (Sham-Iso) was also maintained. The behavioral depression and anxiety related parameters using open field test (OFT), sucrose intake test, novelty suppressed feeding (NSF) and forced swim test (FST), and neurochemical estimation (brain monoamines viz., serotonin and nor-adrenaline, serotonin turnover, and serum cortisol) were performed.

Statistical Analysis Used:

Data was analyzed by either two-way ANOVA (OFT and FST) or one-way ANOVA (sucrose intake test, NSF, and neurochemical estimation) followed by Dunnett's multiple comparisons test. Differences were considered significant at P < 0.05.

Results:

The significant and dose-dependent protection from behavioral and neurochemical changes were observed in 4-HI co-administrated OBX-Iso rats.

Conclusion:

4-HI demonstrated the antidepressant and antianxiety effects in socially isolated stress-induced OBX rats with possible involvement of multiple stress relieving mechanisms.

HIGHLIGHTS OF PAPER

In this study, the subacute pretreatment of 4-HI showed strong and dose-dependent prevention of isolation stress related behavioral and neurochemical responses in olfactory bulbectomized rats. The prevention of hyperactive HPA axis in OBX-Iso stress-induced rats can be envisaged as probable mechanism of antidepressant and antianxiety effects of 4-HI.

SUMMARY

Effect of 4-hydroxyisoleucine (4-HI) in olfactory bulbectomized and socially isolated (Iso) rats was evaluated

4-HI showed significant and dose-dependent antidepressant effects during novelty suppressed feeding (NSF) and forced swim test (FST)

4-HI showed significant and dose-dependent antianxiety effects during OFT (open field test) and sucrose intake test

4-HI showed protection from OBX-Iso stress-induced brain monoamines, serotonin turnover, and serum cortisol level elevation.

Abbreviations used: SSRI: Selective Serotonin Reuptake Inhibitor; 4-HI: (2S, 3R, 4S)-4-hydroxyisoleucine; OBX: Olfactory bulbectomy; CPCSEA: Committee for the Purpose of Control and Supervision of Experiments on Animals; OFT: Open Field Test; NSF: Novelty Suppressed Feeding; FST: Forced Swimming Test; 5HT: 5-Hydroxytryptamine; 5-HIAA: 5-Hydroxyindoleacetic Acid; NA: Nor-adrenaline; and HPA: Hypothalamic-Pituitary Adrenal.

Pathogenesis of depression: Insights from human and rodent studies

Major depressive disorder (MDD) will affect one out of every five people in their lifetime and is the leading cause of disability worldwide. Nevertheless, mechanisms associated with the pathogenesis of MDD have yet to be completely understood and current treatments remain ineffective in a large subset of patients. In this review, we summarize the most recent discoveries and insights for which parallel findings have been obtained in human depressed subjects and rodent models of mood disorders in order to examine the potential etiology of depression. These mechanisms range from synaptic plasticity mechanisms to epigenetics and the immune system where there is strong evidence to support a functional role in the development of specific depression symptomology. Ultimately we conclude by discussing how novel therapeutic strategies targeting central and peripheral processes might ultimately aid in the development of effective new treatments for MDD and related stress disorders.

SKF83959 produces antidepressant effects in a chronic social defeat stress model of depression through BDNF-TrkB pathway

BACKGROUND

SKF83959 stimulates the phospholipase Cβ/inositol phosphate 3 pathway, resulting in the activation of Ca(2+)/calmodulin-dependent kinase IIα, which affects the synthesis of brain-derived neurotrophic factor, a neurotrophic factor critical for the pathophysiology of depression. Previous reports showed that SKF83959 elicited antidepressant activity in the forced swim test and tail suspension test as a novel triple reuptake inhibitor. However, there are no studies showing the effects of SKF83959 in a chronic stress model of depression and the role of phospholipase C/inositol phosphate 3/calmodulin-dependent kinase IIα/brain-derived neurotrophic factor pathway in SKF83959-mediated antidepressant effects.

METHODS

In this study, SKF83959 was firstly investigated in the chronic social defeat stress model of depression. The changes in hippocampal neurogenesis, dendrite spine density, and brain-derived neurotrophic factor signaling pathway after chronic social defeat stress and SKF83959 treatment were then investigated. Pharmacological inhibitors and small interfering RNA/short hairpin RNA methods were further used to explore the antidepressive mechanisms of SKF83959.

RESULTS

We found that SKF83959 produced antidepressant effects in the chronic social defeat stress model and also restored the chronic social defeat stress-induced decrease in hippocampal brain-derived neurotrophic factor signaling pathway, dendritic spine density, and neurogenesis. By using various inhibitors and siRNA/shRNA methods, we further demonstrated that the hippocampal dopamine D5 receptor, phospholipase C/inositol phosphate 3/ calmodulin-dependent kinase IIα pathway, and brain-derived neurotrophic factor system are all necessary for the SKF83959 effects.

CONCLUSION

These results suggest that SKF83959 can be developed as a novel antidepressant and produces antidepressant effects via the hippocampal D5/ phospholipase C/inositol phosphate 3/calmodulin-dependent kinase IIα/brain-derived neurotrophic factor pathway.

Effects of chronic mild food restriction on behavior and the hypothalamic malonyl-CoA signaling pathway

Depression induces anorexia, leading to suppressed feeding behaviors and energy intake. Previously, we revealed that chronic social defeat induced a mild suppression of feeding in rats with elevated levels of hypothalamic malonyl-CoA which regulates feeding. Therefore, we attempted to elucidate the effects of chronic mild food restriction on behavior and on hypothalamic malonyl-CoA. The chronic mild food restricted rats were fed a restricted diet approximately 80% to 90% amount of diet compared to the control for 5 weeks. Ratios of restriction were adjusted with feed consumption in the chronic social defeat stressed rats. Chronic mild food restricted rats exhibited a suppression of body weight gain similar to that of the chronic social defeat stressed rats. Also these rats showed increased time spent in the center area of an open field (OF), prolonged immobility time in forced swim, increased phosphorylation of hypothalamic adenosine monophosphate-activated protein kinase (AMPK) and acetyl-CoA carboxylase and a decreased concentration of hypothalamic malonyl-CoA. Weight of the adrenal glands, locomotion in an OF, mitogen-activated protein kinase cascade and calcium/calmodulin-dependent protein kinases II in the hippocampus were not affected by chronic mild food restriction. Our findings suggest that chronic mild food restriction activates AMPK following a decreased hypothalamic malonyl-CoA.

Bacopa monnieri ameliorates memory deficits in olfactory bulbectomized mice: possible involvement of glutamatergic and cholinergic systems

This study investigated the effects of alcoholic extract of Bacopa monnieri (L.) Wettst. (BM) on cognitive deficits using olfactory bulbectomized (OBX) mice and the underlying molecular mechanisms of its action. OBX mice were treated daily with BM (50 mg/kg, p.o.) or a reference drug, tacrine (2.5 mg/kg, i.p.), 1 week before and continuously 3 days after OBX. Cognitive performance of the animals was analyzed by the novel object recognition test, modified Y maze test, and fear conditioning test. Brain tissues of OBX animals were used for neurochemical and immunohistochemical studies. OBX impaired non-spatial short-term memory, spatial working memory, and long-term fair memory. BM administration ameliorated these memory disturbances. The effect of BM on short-term memory deficits was abolished by a muscarinic receptor antagonist, scopolamine. OBX downregulated phosphorylation of synaptic plasticity-related signaling proteins: NR1 subunit of N-methyl-D-aspartate receptor, glutamate receptor 1 (GluR1), and calmodulin-dependent kinase II but not cyclic AMP-responsive element binding protein (CREB), and reduced brain-derived neurotrophic factor (BDNF) mRNA in the hippocampus. OBX also reduced choline acetyltransferase in the hippocampus and cholinergic neurons in the medial septum, and enlarged the size of lateral ventricle. BM administration reversed these OBX-induced neurochemical and histological alterations, except the decrease of GluR1 phosphorylation, and enhanced CREB phosphorylation. Moreover, BM treatment inhibited ex vivo activity of acetylcholinesterase in the brain. These results indicate that BM treatment ameliorates OBX-induced cognition dysfunction via a mechanism involving enhancement of synaptic plasticity-related signaling and BDNF transcription and protection of cholinergic systems from OBX-induced neuronal damage.

Social isolation differentially affects anxiety and depressive-like responses of bulbectomized mice

Social isolation in rodents may interfere in their behavioural responses on paradigms used to test anxiety- and depressive-like states. Herein we study the influence of social isolation upon the behavioural responses of olfactory bulbectomized mice (OBX). In the open-field test (OFT), social isolation enhanced OBX-induced hyperactivity and exploratory behaviour. However, OBX-induced anxiety in the OFT (central activity) was less apparent after isolation, due to the increased level of anxiety showed by the sham-isolated counterparts. In the novelty-suppressed feeding (NSF), isolation derived in an increased latency to feeding of both OBX and sham mice. The isolation did not affect the response of OBX mice and sham mice in the forced-swimming test (FST). Interestingly, OBX animals exhibited an increased immobility time during the FST, though a dramatic decrease in the climbing scores. Finally, OBX-induced anhedonia in the sucrose intake test was not affected by housing conditions. Our findings demonstrate that social isolation influences the performance of OBX mice in some behavioural paradigms, thus facilitating the characterization of depressive-like states, and by contrast, hindering anxiety-related behaviours. This fact should be taken into account in order to minimize economical and time-consuming efforts when assessing potential antidepressant and anxiolytic drugs.

Impaired structural hippocampal plasticity is associated with emotional and memory deficits in the olfactory bulbectomized rat

Disturbances in olfactory circuitry have been associated with depression in humans. The olfactory bulbectomized (OBX lesion) has been largely used as a model of depression-like behavior in the rat. However, quantitative neuronal rearrangements in key brain regions in this animal model have not been evaluated yet. Accordingly, we investigated changes in hippocampal plasticity as well as behavioral deficits in this animal model. OBX-induced behavioral deficits were studied in a battery of tests, namely the open field test (OFT), forced swim test (FST), and spatial memory disturbances in the Morris water maze (MWM). To characterize the neuronal remodeling, neuroanatomical rearrangements were investigated in the CA1 hippocampus and piriform cortex (PirC), brain regions receiving inputs from the olfactory bulbs and associated with emotional or olfactory processes. Additionally, cell proliferation and survival of newborn cells in the adult dentate gyrus (DG) of the hippocampus were also determined. OBX induced hyperlocomotion and enhanced rearing and grooming in the OFT, increased immobility in the FST as well as required a longer time to find the hidden platform in the MWM. OBX also induced dendritic atrophy in the hippocampus and PirC. In addition, cell proliferation was decreased while the survival remained unchanged in the DG of these animals. These various features are also observed in depressed subjects, adding further support to the validity and usefulness of this model to evaluate potential novel antidepressants.

Fluoxetine modulates hippocampal cell signaling pathways implicated in neuroplasticity in olfactory bulbectomized mice

The olfactory bulbectomy (OB) animal model of depression is a well-established model that is capable of detecting antidepressant activity following chronic drug therapy, and the surgery results in behavioral and biochemical changes that are reminiscent of various symptoms of depression. In the present study, we investigated the degree to which 14 days of p.o. administration of the classic antidepressant fluoxetine (10mg/kg) were able to reverse OB-induced changes in behavior (namely, hyperactivity in the open-field test and reduced motivational and self-care behaviors in the splash test) and in the activation of hippocampal cell signaling pathways that are thought to be involved in synaptic plasticity. OB caused significant increases in ERK1 and CREB (Ser(133)) phosphorylation and in the expression of BDNF immunocontent, all of which were prevented by fluoxetine administration. Moreover, fluoxetine administration also caused a significant decrease in ERK2 phosphorylation in mice that had undergone OB. Neither Akt nor GSK-3β phosphorylation was altered in any experimental condition. In conclusion, the present study shows that OB can induce significant behavioral changes that are accompanied by the activation of hippocampal signaling pathways, namely the ERK1/CREB/BDNF pathway, which is involved in the synaptic plasticity. Conversely, fluoxetine prevented these OB-induced behavioral changes and avoided the activation of ERK1/CREB/BDNF in the hippocampus. Taken together, our results extend the data from the existing literature regarding OB-induced behavioral and neurochemical changes, and suggest a possible underlying mechanism that can account for the antidepressant effect of fluoxetine in this model.

Aberrant calcium/calmodulin-dependent protein kinase II (CaMKII) activity is associated with abnormal dendritic spine morphology in the ATRX mutant mouse brain

In humans, mutations in the gene encoding ATRX, a chromatin remodeling protein of the sucrose-nonfermenting 2 family, cause several mental retardation disorders, including α-thalassemia X-linked mental retardation syndrome. We generated ATRX mutant mice lacking exon 2 (ATRX(ΔE2) mice), a mutation that mimics exon 2 mutations seen in human patients and associated with milder forms of retardation. ATRX(ΔE2) mice exhibited abnormal dendritic spine formation in the medial prefrontal cortex (mPFC). Consistent with other mouse models of mental retardation, ATRX(ΔE2) mice exhibited longer and thinner dendritic spines compared with wild-type mice without changes in spine number. Interestingly, aberrant increased calcium/calmodulin-dependent protein kinase II (CaMKII) activity was observed in the mPFC of ATRX(ΔE2) mice. Increased CaMKII autophosphorylation and activity were associated with increased phosphorylation of the Rac1-guanine nucleotide exchange factors (GEFs) T-cell lymphoma invasion and metastasis 1 (Tiam1) and kalirin-7, known substrates of CaMKII. We confirmed increased phosphorylation of p21-activated kinases (PAKs) in mPFC extracts. Furthermore, reduced protein expression and activity of protein phosphatase 1 (PP1) was evident in the mPFC of ATRX(ΔE2) mice. In cultured cortical neurons, PP1 inhibition by okadaic acid increased CaMKII-dependent Tiam1 and kalirin-7 phosphorylation. Together, our data strongly suggest that aberrant CaMKII activation likely mediates abnormal spine formation in the mPFC. Such morphological changes plus elevated Rac1-GEF/PAK signaling seen in ATRX(ΔE2) mice may contribute to mental retardation syndromes seen in human patients.

Neuronal Abelson helper integration site-1 (Ahi1) deficiency in mice alters TrkB signaling with a depressive phenotype

Recent studies suggest that the human Abelson helper integration site-1 (AHI1) gene on chromosome 6 is associated with susceptibility to schizophrenia and autism, two common neuropsychological disorders with depression symptoms. Mouse Ahi1 protein is abundant in the hypothalamus and amygdala, which are important brain regions for controlling emotion. However, the neuronal function of Ahi1 remains unclear. With the Cre-loxP system, we created a mouse model that selectively reduces Ahi1 expression in neuronal cells. Mice with neuronal Ahi1 deficiency show reduced TrkB level in the brain and depressive phenotypes, which can be alleviated by antidepressant drugs or by overexpression of TrkB in the amygdala. Ahi1 deficiency promotes the degradation of endocytic TrkB and reduces TrkB signaling in neuronal cells. Our findings suggest that impaired endocytic sorting and increased degradation of TrkB can induce depression and that this impaired pathway may serve as a previously uncharacterized therapeutic target for depression.

Ca2+/calmodulin-dependent protein kinase IV-mediated LIM kinase activation is critical for calcium signal-induced neurite outgrowth

Actin cytoskeletal remodeling is essential for neurite outgrowth. LIM kinase 1 (LIMK1) regulates actin cytoskeletal remodeling by phosphorylating and inactivating cofilin, an actin filament-disassembling factor. In this study, we investigated the role of LIMK1 in calcium signal-induced neurite outgrowth. The calcium ionophore ionomycin induced LIMK1 activation and cofilin phosphorylation in Neuro-2a neuroblastoma cells. Knockdown of LIMK1 or expression of a kinase-dead mutant of LIMK1 suppressed ionomycin-induced cofilin phosphorylation and neurite outgrowth in Neuro-2a cells. Ionomycin-induced cofilin phosphorylation and neurite outgrowth were also blocked by KN-93, an inhibitor of Ca(2+)/calmodulin-dependent protein kinases (CaMKs), and STO-609, an inhibitor of CaMK kinase. An active form of CaMKIV but not CaMKI enhanced Thr-508 phosphorylation of LIMK1 and increased the kinase activity of LIMK1. Moreover, the active form of CaMKIV induced cofilin phosphorylation and neurite outgrowth, and a dominant negative form of CaMKIV suppressed ionomycin-induced neurite outgrowth. Taken together, our results suggest that LIMK1-mediated cofilin phosphorylation is critical for ionomycin-induced neurite outgrowth and that CaMKIV mediates ionomycin-induced LIMK1 activation.