Emerging role of synaptic actin-regulatory pathway in the pathophysiology of mood disorders

Knockdown of miRNA-134-5p rescues dendritic deficits by promoting AMPK-mediated mitophagy in a mouse model of depression

Neuronal dendrites and dendritic spines are essential for normal synaptic transmission and may be critically involved in the pathophysiology of various neurological disorders, including depression. Emerging data supports the role of mitochondria in dendritic protrusions in modulating the development and morphological plasticity of spines. Mitophagy, a mitochondria-specific form of autophagy, is the fundamental process of clearing damaged mitochondria to maintain cellular homeostasis. As a brain-specific microRNA, miR-134 is localized to the synaptodendritic compartment of hippocampal neurons and negatively regulates the development of dendritic spines. However, the role of miR-134 in mitophagy related to dendritic deficits in the pathophysiology of depression remains unclear. In this study, we showed that miR-134-5p knockdown abrogated depressive-like behavioral symptoms and corrected aberrant spine morphology in hippocampal neurons of chronic unpredictable mild stress (CUMS) mice. Moreover, knockdown of miR-134-5p triggered autophagy in dendrites, improved mitochondrial impairment, and induced the generation of autophagosomes in the hippocampus of CUMS mice. We further found that AMP-activated protein kinase (AMPK), which mediates the impairment of defective mitochondria via mitophagy, can bind directly to miR-134-5p and is negatively regulated by this miRNA. This study demonstrates that miR-134-5p exerts an enormous effect on dendritic deficits by promoting AMPK-mediated mitophagy and provides a potential new target for antidepressant drug research and development.

Identification of altered protein expression in major depressive disorder and bipolar disorder patients using liquid chromatography-tandem mass spectrometry

Emerging high-throughput proteomic technologies have recently been considered as a powerful means of identifying substrates involved in mood disorders. We performed proteomic profiling using liquid chromatography-tandem mass spectrometry to identify dysregulated proteins in plasma samples of 42 and 45 patients with major depressive disorder (MDD) and bipolar disorder (BD), respectively, in comparison to 51 healthy controls (HCs). Fourteen and six proteins in MDD and BD patients, respectively, were differentially expressed compared to HCs, among which coagulation factor XIII A chain (F13A1), platelet basic protein (PPBP), platelet facor 4 (PF4), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and thymosin beta-4 (TMSB4X) were altered in both disorders. For proteins dysregulated in both, except F13A1, higher fold changes were observed in MDD than in BD patients. These findings may help identify candidate biomarkers of mood disorders and elucidate their underlying pathophysiology and biochemical abnormalities.

Neuronal function and dysfunction of CYFIP2: from actin dynamics to early infantile epileptic encephalopathy

The cytoplasmic FMR1-interacting protein family (CYFIP1 and CYFIP2) are evolutionarily conserved proteins originally identified as binding partners of the fragile X mental retardation protein (FMRP), a messenger RNA (mRNA)-binding protein whose loss causes the fragile X syndrome. Moreover, CYFIP is a key component of the heteropentameric WAVE regulatory complex (WRC), a critical regulator of neuronal actin dynamics. Therefore, CYFIP may play key roles in regulating both mRNA translation and actin polymerization, which are critically involved in proper neuronal development and function. Nevertheless, compared to CYFIP1, neuronal function and dysfunction of CYFIP2 remain largely unknown, possibly due to the relatively less well established association between CYFIP2 and brain disorders. Despite high amino acid sequence homology between CYFIP1 and CYFIP2, several in vitro and animal model studies have suggested that CYFIP2 has some unique neuronal functions distinct from those of CYFIP1. Furthermore, recent whole-exome sequencing studies identified de novo hot spot variants of CYFIP2 in patients with early infantile epileptic encephalopathy (EIEE), clearly implicating CYFIP2 dysfunction in neurological disorders. In this review, we highlight these recent investigations into the neuronal function and dysfunction of CYFIP2, and also discuss several key questions remaining about this intriguing neuronal protein.

Spontaneous seizure and partial lethality of juvenile Shank3-overexpressing mice in C57BL/6 J background

The SH3 and multiple ankyrin repeat domains 3 (SHANK3) gene encodes core scaffolds in neuronal excitatory postsynapses. SHANK3 duplications have been identified in patients with hyperkinetic disorders and early-onset generalized tonic-clonic seizures. Consistently, Shank3 transgenic (TG) mice, which mildly overexpress Shank3 proteins exhibit hyperkinetic behavior and spontaneous seizures. However, the seizure phenotype of Shank3 TG mice has only been investigated in adults of the seizure-sensitive strain FVB/N. Therefore, it remains unknown if spontaneous seizures occur in Shank3 TG mice from the early postnatal stages onward, or even in seizure-resistant strains. Clinically, generalized tonic-clonic seizures are the critical risk factor for epilepsy-associated mortality. However, the potential association between Shank3 overexpression and mortality, at least in mice, has not been investigated in detail. In the present study, we backcrossed Shank3 TG mice in seizure-resistant C57BL/6 J strain and monitored their home-cage activities at 3 weeks of age. Of the 15 Shank3 TG mice monitored, two exhibited spontaneous tonic-clonic seizures, and one died immediately after the seizure event. Based on this observation, we determined the survival rate of the Shank3 TG mice from 3 to 12 weeks of age. We found that approximately 40–45% of the Shank3 TG mice, both males and females, died before reaching 12 weeks of age. Notably, 53% and 70% of the total deaths in male and female Shank3 TG mice, respectively, occurred in the juvenile stages. These results suggest spontaneous seizure and partial lethality of juvenile Shank3 TG mice in seizure-resistant background, further supporting the validity of this model.

Integrative Brain Transcriptome Analysis Reveals Region-Specific and Broad Molecular Changes in Shank3-Overexpressing Mice

Variants of the SH3 and multiple ankyrin repeat domain 3 (SHANK3) gene, encoding excitatory postsynaptic core scaffolding proteins, are causally associated with numerous neurodevelopmental and neuropsychiatric disorders, including autism spectrum disorder (ASD), bipolar disorder, intellectual disability, and schizophrenia (SCZ). Although detailed synaptic changes of various Shank3 mutant mice have been well characterized, broader downstream molecular changes, including direct and indirect changes, remain largely unknown. To address this issue, we performed a transcriptome analysis of the medial prefrontal cortex (mPFC) of adult Shank3-overexpressing transgenic (TG) mice, using an RNA-sequencing approach. We also re-analyzed previously reported RNA-sequencing results of the striatum of adult Shank3 TG mice and of the prefrontal cortex of juvenile Shank3^+/ΔC mice with a 50–70% reduction of Shank3 proteins. We found that several myelin-related genes were significantly downregulated specifically in the mPFC, but not in the striatum or hippocampus, of adult Shank3 TG mice by comparing the differentially expressed genes (DEGs) of the analyses side by side. Moreover, we also found nine common DEGs between the mPFC and striatum of Shank3 TG mice, among which we further characterized ASD- and SCZ-associated G protein-coupled receptor 85 (Gpr85), encoding an orphan Gpr interacting with PSD-95. Unlike the mPFC-specific decrease of myelin-related genes, we found that the mRNA levels of Gpr85 increased in multiple brain regions of adult Shank3 TG mice, whereas the mRNA levels of its family members, Gpr27 and Gpr173, decreased in the cortex and striatum. Intriguingly, in cultured neurons, the mRNA levels of Gpr27, Gpr85, and Gpr173 were modulated by the neuronal activity. Furthermore, exogenously expressed GPR85 was co-localized with PSD-95 and Shank3 in cultured neurons and negatively regulated the number of excitatory synapses, suggesting its potential role in homeostatic regulation of excitatory synapses in Shank3 TG neurons. Finally, we performed a gene set enrichment analysis of the RNA-sequencing results, which suggested that Shank3 could affect the directional expression pattern of numerous ribosome-related genes in a dosage-dependent manner. To sum up, these results reveal previously unidentified brain region-specific and broad molecular changes in Shank3-overexpressing mice, further elucidating the complexity of the molecular pathophysiology of SHANK3-associated brain disorders.

Excitatory and inhibitory synaptic dysfunction in mania: an emerging hypothesis from animal model studies

Bipolar disorder (BD) is a common psychiatric disorder characterized by recurrent mood swings between depression and mania, and is associated with high treatment costs. The existence of manic episodes is the defining feature of BD, during which period, patients experience extreme elevation in activity, energy, and mood, with changes in sleep patterns that together severely impair their ability to function in daily life. Despite some limitations in recapitulating the complex features of human disease, several rodent models of mania have been generated and characterized, which have provided important insights toward understanding its underlying pathogenic mechanisms. Among the mechanisms, neuronal excitatory and inhibitory (E/I) synaptic dysfunction in some brain regions, including the frontal cortex, hippocampus, and striatum, is an emerging hypothesis explaining mania. In this review, we highlight recent studies of rodent manic models having impairments in the E/I synaptic development and function. We also summarize the molecular and functional changes of E/I synapses by some mood stabilizers that may contribute to the therapeutic efficacy of drugs. Furthermore, we discuss potential future directions in the study of this emerging hypothesis to better connect the outcomes of basic research to the treatment of patients with this devastating mental illness.

Studies in rodents offer insights into bipolar disorder that may help understanding and treatment of this common and debilitating condition. Kihoon Han and colleagues at Korea University in Seoul review research using mice and rats to model the episodes of mania in patients with bipolar disorder. The research supports an emerging hypothesis implicating specific problems with nervous transmission in the brain in the onset of mania. The hypothesis suggests that the transmission of signals between particular nerve cells whose normal function is either to excite or to inhibit other nerve cells may be involved. It also indicates regions of the brain most involved in manic episodes. Changes at the affected nerve junctions—called synapses—brought about by mood-stabilizing drugs are examined. The hypothesis suggests new approaches to treatment options for researchers to explore.

Integrative Analysis of Brain Region-specific Shank3 Interactomes for Understanding the Heterogeneity of Neuronal Pathophysiology Related to SHANK3 Mutations

Recent molecular genetic studies have identified 100s of risk genes for various neurodevelopmental and neuropsychiatric disorders. As the number of risk genes increases, it is becoming clear that different mutations of a single gene could cause different types of disorders. One of the best examples of such a gene is SHANK3, which encodes a core scaffold protein of the neuronal excitatory post-synapse. Deletions, duplications, and point mutations of SHANK3 are associated with autism spectrum disorders, intellectual disability, schizophrenia, bipolar disorder, and attention deficit hyperactivity disorder. Nevertheless, how the different mutations of SHANK3 can lead to such phenotypic diversity remains largely unknown. In this study, we investigated whether Shank3 could form protein complexes in a brain region-specific manner, which might contribute to the heterogeneity of neuronal pathophysiology caused by SHANK3 mutations. To test this, we generated a medial prefrontal cortex (mPFC) Shank3 in vivo interactome consisting of 211 proteins, and compared this protein list with a Shank3 interactome previously generated from mixed hippocampal and striatal (HP+STR) tissues. Unexpectedly, we found that only 47 proteins (about 20%) were common between the two interactomes, while 164 and 208 proteins were specifically identified in the mPFC and HP+STR interactomes, respectively. Each of the mPFC- and HP+STR-specific Shank3 interactomes represents a highly interconnected network. Upon comparing the brain region-enriched proteomes, we found that the large difference between the mPFC and HP+STR Shank3 interactomes could not be explained by differential protein expression profiles among the brain regions. Importantly, bioinformatic pathway analysis revealed that the representative biological functions of the mPFC- and HP+STR-specific Shank3 interactomes were different, suggesting that these interactors could mediate the brain region-specific functions of Shank3. Meanwhile, the same analysis on the common Shank3 interactors, including Homer and GKAP/SAPAP proteins, suggested that they could mainly function as scaffolding proteins at the post-synaptic density. Lastly, we found that the mPFC- and HP+STR-specific Shank3 interactomes contained a significant number of proteins associated with neurodevelopmental and neuropsychiatric disorders. These results suggest that Shank3 can form protein complexes in a brain region-specific manner, which might contribute to the pathophysiological and phenotypic diversity of disorders related to SHANK3 mutations.

Bipolar Disorder Associated microRNA, miR-1908-5p, Regulates the Expression of Genes Functioning in Neuronal Glutamatergic Synapses

Bipolar disorder (BD), characterized by recurrent mood swings between depression and mania, is a highly heritable and devastating mental illness with poorly defined pathophysiology. Recent genome-wide molecular genetic studies have identified several protein-coding genes and microRNAs (miRNAs) significantly associated with BD. Notably, some of the proteins expressed from BD-associated genes function in neuronal synapses, suggesting that abnormalities in synaptic function could be one of the key pathogenic mechanisms of BD. In contrast, however, the role of BD-associated miRNAs in disease pathogenesis remains largely unknown, mainly because of a lack of understanding about their target mRNAs and pathways in neurons. To address this problem, in this study, we focused on a recently identified BD-associated but uncharacterized miRNA, miR-1908-5p. We identified and validated its novel target genes including DLGAP4, GRIN1, STX1A, CLSTN1 and GRM4, which all function in neuronal glutamatergic synapses. Moreover, bioinformatic analyses of human brain expression profiles revealed that the expression levels of miR-1908-5p and its synaptic target genes show an inverse-correlation in many brain regions. In our preliminary experiments, the expression of miR-1908-5p was increased after chronic treatment with valproate but not lithium in control human neural progenitor cells. In contrast, it was decreased by valproate in neural progenitor cells derived from dermal fibroblasts of a BD subject. Together, our results provide new insights into the potential role of miR-1908-5p in the pathogenesis of BD and also propose a hypothesis that neuronal synapses could be a key converging pathway of some BD-associated protein-coding genes and miRNAs.

Regulation of the actin cytoskeleton by the Ndel1-Tara complex is critical for cell migration

Nuclear distribution element-like 1 (Ndel1) plays pivotal roles in diverse biological processes and is implicated in the pathogenesis of multiple neurodevelopmental disorders. Ndel1 function by regulating microtubules and intermediate filaments; however, its functional link with the actin cytoskeleton is largely unknown. Here, we show that Ndel1 interacts with TRIO-associated repeat on actin (Tara), an actin-bundling protein, to regulate cell movement. In vitro wound healing and Boyden chamber assays revealed that Ndel1- or Tara-deficient cells were defective in cell migration. Moreover, Tara overexpression induced the accumulation of Ndel1 at the cell periphery and resulted in prominent co-localization with F-actin. This redistribution of Ndel1 was abolished by deletion of the Ndel1-interacting domain of Tara, suggesting that the altered peripheral localization of Ndel1 requires a physical interaction with Tara. Furthermore, co-expression of Ndel1 and Tara in SH-SY5Y cells caused a synergistic increase in F-actin levels and filopodia formation, suggesting that Tara facilitates cell movement by sequestering Ndel1 at peripheral structures to regulate actin remodeling. Thus, we demonstrated that Ndel1 interacts with Tara to regulate cell movement. These findings reveal a novel role of the Ndel1-Tara complex in actin reorganization during cell movement.

Post-transcriptional regulation of SHANK3 expression by microRNAs related to multiple neuropsychiatric disorders

Background

Proper neuronal function requires tight control of gene dosage, and failure of this process underlies the pathogenesis of multiple neuropsychiatric disorders. The SHANK3 gene encoding core scaffolding proteins at glutamatergic postsynapse is a typical dosage-sensitive gene, both deletions and duplications of which are associated with Phelan-McDermid syndrome, autism spectrum disorders, bipolar disorder, intellectual disability, or schizophrenia. However, the regulatory mechanism of SHANK3 expression in neurons itself is poorly understood.

Results

Here we show post-transcriptional regulation of SHANK3 expression by three microRNAs (miRNAs), miR-7, miR-34a, and miR-504. Notably, the expression profiles of these miRNAs were previously shown to be altered in some neuropsychiatric disorders which are also associated with SHANK3 dosage changes. These miRNAs regulated the expression of SHANK3 and other genes encoding actin-related proteins that interact with Shank3, through direct binding sites in the 3′ untranslated region (UTR). Moreover, overexpression or inhibition of miR-7 and miR-504 affected the dendritic spines of the cultured hippocampal neurons in a Shank3-dependent manner. We further characterized miR-504 as it showed the most significant effect on both SHANK3 expression and dendritic spines among the three miRNAs. Lentivirus-mediated overexpression of miR-504, which mimics its reported expression change in postmortem brain tissues of bipolar disorder, decreased endogenous Shank3 protein in cultured hippocampal neurons. We also revealed that miR-504 is expressed in the cortical and hippocampal regions of human and mouse brains.

Conclusions

Our study provides new insight into the miRNA-mediated regulation of SHANK3 expression, and its potential implication in multiple neuropsychiatric disorders associated with altered SHANK3 and miRNA expression profiles.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0165-3) contains supplementary material, which is available to authorized users.