MicroRNAs in ALS: small pieces to the puzzle

MicroRNAs have emerged as crucial regulators of neuronal function, suggesting that aberrant microRNA expression might contribute to pathologies of the nervous system. In this issue of The EMBO Journal, Emde et al ( 2015 ) report a global decrease in microRNAs as common hallmark of different forms of amyotrophic lateral sclerosis (ALS). Strikingly, enhancing microRNA biogenesis has beneficial effects on the neuromuscular function in mouse models of ALS. Thus, the microRNA pathway represents a promising novel target for therapeutic intervention in neurodegeneration.     

Down-regulation of miR-17 family expression in response to retinoic acid induced neuronal differentiation

Whole-genome microRNA and gene expression analyses were used to monitor changes during retinoic acid induced differentiation of neuroblasts in vitro. Interestingly, the entire miR-17 family was over-represented among the down-regulated miRNA. The implications of these changes are considerable, as target gene prediction suggests that the miR-17 family is involved in the regulation of the mitogen-activated protein kinase (MAPK) signaling pathway, synaptic plasticity and other markers of neuronal differentiation. Significantly, many of the target responses predicted by changes in miRNA expression were supported by the observed changes in gene expression. As expected, markers of neuronal differentiation such as anti-apoptotic protein B-cell lymphoma 2 (BCL2), myocyte enhancer factor-2D (MEF2D) and zipper protein kinase (MAP3K12; aka ZPK/MUK/DLK) were each up-regulated in response to differentiation. The expression of these genes was also reduced in response to miR-17 and miR-20a transfection, and more specifically they were also shown to contain functional miRNA recognition elements for members of the miR-17 family by reporter gene assay. This suggests that the miR-17 family have an integral role in fine-tuning the pathways involved in the regulation of neuronal differentiation.     

The dynamic recruitment of TRBP to neuronal membranes mediates dendritogenesis during development

MicroRNAs are important regulators of local protein synthesis during neuronal development. We investigated the dynamic regulation of microRNA production and found that the majority of the microRNA‐generating complex, consisting of Dicer, TRBP, and PACT, specifically associates with intracellular membranes in developing neurons. Stimulation with brain‐derived neurotrophic factor (BDNF), which promotes dendritogenesis, caused the redistribution of TRBP from the endoplasmic reticulum into the cytoplasm, and its dissociation from Dicer, in a Ca²⁺‐dependent manner. As a result, the processing of a subset of neuronal precursor microRNAs, among them the dendritically localized pre‐miR16, was impaired. Decreased production of miR‐16‐5p, which targeted the BDNF mRNA itself, was rescued by expression of a membrane‐targeted TRBP. Moreover, miR‐16‐5p or membrane‐targeted TRBP expression blocked BDNF‐induced dendritogenesis, demonstrating the importance of neuronal TRBP dynamics for activity‐dependent neuronal development. We propose that neurons employ specialized mechanisms to modulate local gene expression in dendrites, via the dynamic regulation of microRNA biogenesis factors at intracellular membranes of the endoplasmic reticulum, which in turn is crucial for neuronal dendrite complexity and therefore neuronal circuit formation and function.     

14-3-3 proteins in neuronal development and function

The 14-3-3 proteins are small, cytosolic, evolutionaritly conserved proteins expressed abundantly in the nervous system. Although they were discovered more than 30 yr ago, their function in the nervous system has remained enigmatic. Several recent studies have helped to clarify their biological function. Crystallographic investigations have revealed that 14-3-3 proteins exist as dimers and that they contain a specific region for binding to other proteins. The interacting proteins, in turn, contain a 14-3-3 binding motif; proteins that interact with 14-3-3 dimers include PKC and Raf, protein kinases with critical roles in neuronal signaling. These proteins are capable of activating Raf in vitro, and this role has been verified by in vivo studies inDrosophila. Most interestingly, mutations in theDrosophila 14-3-3 genes disrupt neuronal differentiation, synaptic plasticity, and behavioral plasticity, establishing a role for these proteins in the development and function of the nervous system.     

The neural cell adhesion molecule and synaptic plasticity

Highly stereotyped patterns of neuronal connections are laid down during the development of the nervous system via a range of activity independent and activity dependent mechanisms. Whereas the coarse hard-wiring of the nervous system appears to rely on molecular recognition events between the neuron, its pathway, and its target, the establishment of precisely patterned functional circuits is thought to be driven by neuronal activity. In this review we discuss the role that the neuronal cell adhesion molecule (NCAM) plays in morphological plasticity. Recent studies on NCAM and its probable species homologue in Aplysia (apCAM) suggests that an individual CAM can function to both promote synaptic plasticity and maintain the structure of the synapse. In the adult brain, changes between stability and plasticity are likely to underlie dynamic morphological changes in synaptic structures associated with learning and memory. In this review we use NCAM as an example to illustrate mechanisms that can change the function of an individual CAM from a molecule that promotes plasticity to one that does not. We also discuss evidence that NCAM promotes plasticity by activating a conventional signal transduction cascade, rather than by modulating adhesion perse. Finally, we consider the evidence that supports a role for NCAM in learning and memory. © 1995 John Wiley & Sons, Inc.     

New partners and phosphorylation sites of focal adhesion kinase identified by mass spectrometry

The regulation of focal adhesion kinase (FAK) involves phosphorylation and multiple interactions with other signaling proteins. Some of these pathways are relevant for nervous system functions such as branching, axonal guidance, and plasticity. In this study, we screened mouse brain to identify FAK-interactive proteins and phosphorylatable residues as a first step to address the neuronal functions of this kinase. Using mass spectrometry analysis, we identified new phosphorylated sites (Thr 952, Thr 1048, and Ser 1049), which lie in the FAT domain; and putative new partners for FAK, which include cytoskeletal proteins such as drebrin and MAP 6, adhesion regulators such as neurabin-2 and plakophilin 1, and synapse-associated proteins such as SynGAP and a NMDA receptor subunit. Our findings support the participation of brain-localized FAK in neuronal plasticity.     

Regulation of neuronal plasticity in the central nervous system by phosphorylation and dephosphorylation

Neuronal plasticity can be defined as adaptive changes in structure and function of the nervous system, an obvious example of which is the capacity to remember and learn. Long-term potentiation and long-term depression are the experimental models of memory in the central nervous system (CNS), and have been frequently utilized for the analysis of the molecular mechanisms of memory formation. Extensive studies have demonstrated that various kinases and phosphatases regulate neuronal plasticity by phosphorylating and dephosphorylating proteins essential to the basic processes of adaptive changes in the CNS. These proteins include receptors, ion channels, synaptic vesicle proteins, and nuclear proteins. Multifunctional kinases (cAMP-dependent protein kinase, Ca²⁺/phospholipid-dependent protein kinase, and Ca²⁺/calmodulin-dependent protein kinases) and phosphatases (calcineurin, protein phosphatases 1, and 2A) that specifically modulate the phosphorylation status of neuronal-signaling proteins have been shown to be required for neuronal plasticity. In general, kinases are involved in upregulation of the activity of target substrates, and phosphatases downregulate them. Although this rule is applicable in most of the cases studied, there are also a number of exceptions. A variety of regulation mechanisms via phosphorylation and dephosphorylation mediated by multiple kinases and phosphatases are discussed.     

Activity-dependent and hormonal regulation of neurotrophin mRNA levels in brain–implications for neuronal plasticity

The neurotrophins exhibit neurotrophic effects on specific, partially overlapping populations of neurons both in the peripheral and the central nervous system (CNS). In the periphery, they are synthesized by a variety of nonneuronal cells, and their synthesis seems to be independent of the neuronal input. In contrast, in the CNS all neurotrophins are expressed under physiological conditions primarily by neurons. The production of NGF and BDNF is controlled by neuronal activity: up-regulation by glutamate and acetylcholine, down-regulation by gamma-aminobutyric acid. In contrast, NT-3 regulation is independent of neuronal activity, but it is up-regulated by thyroid hormones and BDNF. The latter observation suggests that NT-3 might be controlled indirectly by neuronal activity via BDNF. In peripheral nonneuronal tissues, glucocorticoid hormones down-regulate NGF mRNA levels both in vitro and in vivo. In contrast, in the CNS, neuronal production of NGF is enhanced by glucocorticoids. The rapid regulation of NGF and BDNF by subtle physiological stimuli together with the recent demonstration that the neurotrophin release neurotransmitters such as acetylcholine opens up interesting perspectives for the function of neurotrophins as mediators of neuronal plasticity. 1994 John Wiley & Sons, Inc.     

Regulatory T cell differentiation: cooperation saves the day

MicroRNA are important regulators of CD4 T cell differentiation, altering the balance between the immunogenic and tolerogenic pathways. Studies in mice with microRNA‐deficient T cells have revealed defects in differentiation into the regulatory T cell lineage; however, the individual microRNA responsible have remained elusive. A recent paper in The EMBO Journal uses a systematic screen to find a novel cooperative action between an inducible and a constitutive microRNA in aiding regulatory T cell induction.     

体外培养的神经元网络可塑性

神经元网络是大脑执行高级认知行为的结构基础,研究证明学习记忆及神经退行性疾病与神经元网络可塑性密切相关。因此,揭示调控和改变神经元网络可塑性的机制对理解神经系统信息交互以及疾病治疗具有重大意义。目前,基于微电极阵列（microelectrode array, MEA）培养的神经元网络是体外探究学习和记忆机制的理想模型,同时针对该模型的研究为预防和治疗神经退行性疾病提供了独特的视角。本文综述了基于MEA采集体外培养神经元网络的放电信号来构建功能网络的相关研究,分别从二维神经元网络和三维脑类器官发育,以及开环和闭环电刺激对神经元网络可塑性影响的角度,总结了体外培养神经元网络可塑性的相关研究,最后对该方向的应用前景进行了展望。     

Silencing of the tobacco pollen pectin methylesterase NtPPME1 results in retarded in vivo pollen tube growth

Sperm delivery in flowering plants requires extensive pollen tube growth through the female sporophytic tissues of the pistil. The apical cell wall emerges as a central player in the control of pollen tube growth, since it provides strength to withstand the internal turgor pressure, while imparting sufficient plasticity to allow cell wall extension through the incorporation of new membrane and wall material. Within this scenario, pectin methylesterases (PMEs; EC 3.1.1.11) emerge as crucial regulators in determining the mechanical properties of pectins, the major component of the apical pollen tube wall. We previously identified NtPPME1, a pollen specific PME from Nicotiana tabacum. Here we show that silencing of NtPPME1 results in a mild but significant decrease of in vivo pollen tube growth while the overall PME activity in pollen is not significantly affected. Although the precise mechanisms responsible for the observed phenotype are not known, it seems likely that the cell must maintain a closely regulated level of PME activity in order to maintain the equilibrium between strength and plasticity in the apical cell wall. A relatively minor disturbance of this equilibrium, as caused by NtPPME1 silencing, compromises pollen tube growth.     

The cell adhesion molecule EphA4 is involved in circadian clock functions

Circadian (～24 h) rhythms of cellular network plasticity in the central circadian clock, the suprachiasmatic nucleus (SCN), have been described. The neuronal network in the SCN regulates photic resetting of the circadian clock as well as stability of the circadian system during both entrained and constant conditions. EphA4, a cell adhesion molecule regulating synaptic plasticity by controlling connections of neurons and astrocytes, is expressed in the SCN. To address whether EphA4 plays a role in circadian photoreception and influences the neuronal network of the SCN, we have analyzed circadian wheel‐running behavior of EphA4 knockout (EphA4^?/?) mice under different light conditions and upon photic resetting, as well as their light‐induced protein response in the SCN. EphA4^?/? mice exhibited reduced wheel‐running activity, longer endogenous periods under constant darkness and shorter periods under constant light conditions, suggesting an effect of EphA4 on SCN function. Moreover, EphA4^?/? mice exhibited suppressed phase delays of their wheel‐running activity following a light pulse during the beginning of the subjective night (CT15). Accordingly, light‐induced c‐FOS (FBJ murine osteosarcoma viral oncogene homolog) expression was diminished. Our results suggest a circadian role for EphA4 in the SCN neuronal network, affecting the circadian system and contributing to the circadian response to light.     

Regulation of DNA methylation by ethanol induces tissue plasminogen activator expression in astrocytes

Alcohol exposure affects neuronal plasticity in the adult and developing brain. Astrocytes play a major role in modulating neuronal plasticity and are a target of ethanol. Tissue plasminogen activator (tPA) is involved in modulating neuronal plasticity by degrading the extracellular matrix proteins including fibronectin and laminin and is up‐regulated by ethanol in vivo. In this study we explored the hypothesis that ethanol affects DNA methylation in astrocytes thereby increasing expression and release of tPA. It was found that ethanol increased tPA mRNA levels, an effect mimicked by an inhibitor of DNA methyltransferase (DNMT) activity. Ethanol also increased tPA protein expression and release, and inhibited DNMT activity with a corresponding decrease in DNA methylation levels of the tPA promoter. Furthermore, it was observed that protein levels of DNMT3A, but not DNMT1, were reduced in astrocytes after ethanol exposure. These novel studies show that ethanol inhibits DNA methylation in astrocytes leading to increased tPA expression and release; this effect may be involved in astrocyte‐mediated inhibition of neuronal plasticity by alcohol.