Rapid reversal of translational silencing: Emerging role of microRNA degradation pathways in neuronal plasticity

Widespread posttranscriptional regulation of cotransmission

While neurotransmitter identity was once considered singular and immutable for mature neurons, it is now appreciated that one neuron can release multiple neuroactive substances (cotransmission) whose identities can even change over time. To explore the mechanisms that tune the suite of transmitters a neuron releases, we developed transcriptional and translational reporters for cholinergic, glutamatergic, and GABAergic signaling in Drosophila. We show that many glutamatergic and GABAergic cells also transcribe cholinergic genes, but fail to accumulate cholinergic effector proteins. Suppression of cholinergic signaling involves posttranscriptional regulation of cholinergic transcripts by the microRNA miR-190; chronic loss of miR-190 function allows expression of cholinergic machinery, reducing and fragmenting sleep. Using a "translation-trap" strategy, we show that neurons in these populations have episodes of transient translation of cholinergic proteins, demonstrating that suppression of cotransmission is actively modulated. Posttranscriptional restriction of fast transmitter cotransmission provides a mechanism allowing reversible tuning of neuronal output.

Widespread post-transcriptional regulation of co-transmission

While neurotransmitter identity was once considered singular and immutable for mature neurons, it is now appreciated that one neuron can release multiple neuroactive substances (co-transmission) whose identities can even change over time. To explore the mechanisms that tune the suite of transmitters a neuron releases, we developed transcriptional and translational reporters for cholinergic, glutamatergic, and GABAergic signaling in Drosophila . We show that many glutamatergic and GABAergic cells also transcribe cholinergic genes, but fail to accumulate cholinergic effector proteins. Suppression of cholinergic signaling involves posttranscriptional regulation of cholinergic transcripts by the microRNA miR-190; chronic loss of miR-190 function allows expression of cholinergic machinery, reducing and fragmenting sleep. Using a "translation-trap" strategy we show that neurons in these populations have episodes of transient translation of cholinergic proteins, demonstrating that suppression of co-transmission is actively modulated. Posttranscriptional restriction of fast transmitter co-transmission provides a mechanism allowing reversible tuning of neuronal output.

One-Sentence Summary

Cholinergic co-transmission in large populations of glutamatergic and GABAergic neurons in the Drosophila adult brain is controlled by miR-190.

Deciphering the Role of microRNAs in Large-Artery Stiffness Associated With Aging: Focus on miR-181b

Large artery stiffness (LAS) is a major, independent risk factor underlying cardiovascular disease that increases with aging. The emergence of microRNA signaling as a key regulator of vascular structure and function has stimulated interest in assessing its role in the pathophysiology of LAS. Identification of several microRNAs that display age-associated changes in expression in aorta has focused attention on defining their molecular targets and deciphering their role in age-associated arterial stiffening. Inactivation of the microRNA-degrading enzyme, translin/trax, which reverses the age-dependent decline in miR-181b, confers protection from aging-associated arterial stiffening, suggesting that inhibitors targeting this enzyme may have translational potential. As LAS poses a major public health challenge, we anticipate that future studies based on these advances will yield innovative strategies to combat aging-associated arterial stiffening.

From Circuits to Chromatin: The Emerging Role of Epigenetics in Mental Health

A central goal of neuroscience research is to understand how experiences modify brain circuits to guide future adaptive behavior. In response to environmental stimuli, neural circuit activity engages gene regulatory mechanisms within each cell. This activity-dependent gene expression is governed, in part, by epigenetic processes that can produce persistent changes in both neural circuits and the epigenome itself. The complex interplay between circuit activity and neuronal gene regulation is vital to learning and memory, and, when disrupted, is linked to debilitating psychiatric conditions, such as substance use disorder. To develop clinical treatments, it is paramount to advance our understanding of how neural circuits and the epigenome cooperate to produce behavioral adaptation. Here, we discuss how new genetic tools, used to manipulate neural circuits and chromatin, have enabled the discovery of epigenetic processes that bring about long-lasting changes in behavior relevant to mental health and disease.

Selective role of the translin/trax RNase complex in hippocampal synaptic plasticity

Activity-dependent local protein synthesis is critical for synapse-specific, persistent plasticity. Abnormalities in local protein synthesis have been implicated in psychiatric disorders. We have recently identified the translin/trax microRNA-degrading enzyme as a novel mediator of protein synthesis at activated synapses. Additionally, translin knockout (KO) mice, which lack translin/trax, exhibit some of the behavioral abnormalities found in a mouse model of fragile X syndrome (fragile X mental retardation protein-FMRP-KO mice). Therefore, identifying signaling pathways interacting with translin/trax to support persistent synaptic plasticity is a translationally relevant goal. Here, as a first step to achieve this goal, we have assessed the requirement of translin/trax for multiple hippocampal synaptic plasticity paradigms that rely on distinct molecular mechanisms. We found that mice lacking translin/trax exhibited selective impairment in a form of persistent hippocampal plasticity, which requires postsynaptic protein kinase A (PKA) activity. In contrast, enduring forms of plasticity that are dependent on presynaptic PKA were unaffected. Furthermore, these mice did not display exaggerated metabotropic glutamate receptor-mediated long-term synaptic depression (mGluR-LTD), a hallmark of the FMRP KO mice. On the contrary, translin KO mice exhibited deficits in N-methyl-d-aspartate receptor (NMDAR) dependent LTD, a phenotype not observed in the FMRP knockouts. Taken together, these findings demonstrate that translin/trax mediates long-term synaptic plasticity that is dependent on postsynaptic PKA signaling and suggest that translin/trax and FMRP play distinct roles in hippocampal synaptic plasticity.

Differential Stability of miR-9-5p and miR-9-3p in the Brain Is Determined by Their Unique Cis- and Trans-Acting Elements

microRNAs (miRs) are fundamental regulators of protein coding genes. In the CNS, miR-9 is highly enriched and critical for neuronal development and function. Mature miRs are derived from a duplex precursor, and the -5p strand ("guide") is preferentially incorporated into an RNA-induced silencing complex (RISC) to exert its regulatory functions, while the complementary -3p strand ("passenger") is thought to be rapidly degraded. By contrast, both strands of the miR-9 duplex have unique functions critical for neuronal physiology, yet their respective degradation rates and mechanisms governing degradation are not well understood. Therefore, we determined the degradation kinetics of miR-9-5p and miR-9-3p and investigated the cis and trans elements that affected their stability in the brain. Using a combination of homogeneous neuronal/astrocyte cell models and heterogeneous brain tissue lysate, we demonstrate the novel finding that miR-9-3p was more stable than the miR-9-5p guide strand in all models tested. Moreover, the degradation kinetics of both miR-9-5p and miR-9-3p were brain-region specific, suggesting that each brain region was differentially enriched for specific degradation factors. We also determined that the 3' nucleotides harbor important cis elements required to not only maintain stability, but also to recruit potential protein degradation factors. We used mass spectrometry to assess the miR-9 interacting proteins and found that the -5p and -3p strands were associated with functionally distinct proteins. Overall, these studies revealed unique miR-9-5p and miR-9-3p degradation kinetics in the brain and proposed critical nucleotide sequences and protein partners that could contribute to this differential stability.

MicroRNA-34a Acutely Regulates Synaptic Efficacy in the Adult Dentate Gyrus In Vivo

Activity-dependent synaptic plasticity involves rapid regulation of neuronal protein synthesis on a time-scale of minutes. miRNA function in synaptic plasticity and memory formation has been elucidated by stable experimental manipulation of miRNA expression and activity using transgenic approaches and viral vectors. However, the impact of rapid miRNA modulation on synaptic efficacy is unknown. Here, we examined the effect of acute (12 min), intrahippocampal infusion of a miR-34a antagonist (antimiR) on medial perforant path-evoked synaptic transmission in the dentate gyrus of adult anesthetised rats. AntimiR-34a infusion acutely depressed medial perforant path-evoked field excitatory post-synaptic potentials (fEPSPs). The fEPSP decrease was detected within 9 min of infusion, lasted for hours, and was associated with knockdown of antimiR-34a levels. AntimiR-34a-induced synaptic depression was sequence-specific; no changes were elicited by infusion of scrambled or mismatch control. The rapid modulation suggests that a target, or set of targets, is regulated by miR-34a. Western blot analysis of dentate gyrus lysates revealed enhanced expression of Arc, a known miR-34a target, and four novel predicted targets (Ctip2, PKI-1α, TCF4 and Ube2g1). Remarkably, antimiR-34a had no effect when infused during the maintenance phase of long-term potentiation. We conclude that miR-34a regulates basal synaptic efficacy in the adult dentate gyrus in vivo. To our knowledge, these in vivo findings are the first to demonstrate acute (< 9 min) regulation of synaptic efficacy in the adult brain by a miRNA.

Deletion of the microRNA-degrading nuclease, translin/trax, prevents pathogenic vascular stiffness

Vascular stiffness plays a key role in the pathogenesis of hypertension. Recent studies indicate that the age-associated reduction in miR-181b levels in vascular smooth muscle cells (VSMCs) contributes to increased vascular stiffness. As these findings suggest that inhibiting degradation of miR-181b might prevent vascular stiffening, we have assessed whether the microRNA-degrading translin/trax (TN/TX) complex mediates degradation of miR-181b in the aorta.We found that TN^-/- mice display elevated levels of miR-181b expression in the aorta. Therefore, we tested whether TN deletion prevents vascular stiffening in a mouse model of hypertension, induced by chronic high-salt intake (4%NaCl in drinking water for 3 wk; HSW). TN^-/- mice subjected to HSW stress do not show increased vascular stiffness, as monitored by pulse wave velocity and tensile testing. The protective effect of TN deletion in the HSW paradigm appears to be mediated by its ability to increase miR-181b in the aorta since HSW decreases levels of miR-181b in WT mice, but not in TN KO mice. We demonstrate for the first time that interfering with microRNA degradation can have a beneficial impact on the vascular system and identify the microRNA-degrading TN/TX RNase complex as a potential therapeutic target in combatting vascular stiffness.NEW & NOTEWORTHY While the biogenesis and mechanism of action of mature microRNA are well understood, much less is known about the regulation of microRNA via degradation. Recent studies have identified the protein complex, translin(TN)/trax(TX), as a microRNA-degrading enzyme. Here, we demonstrate that TN/TX is expressed in vascular smooth muscle cells. Additionally, deletion of the TN/TX complex selectively increases aortic miR-181b and prevents increased vascular stiffness caused by ingestion of high-salt water. To our knowledge, this is first report describing the role of a microRNA RNAse in cardiovascular biology or pathobiology.

Trax: A versatile signaling protein plays key roles in synaptic plasticity and DNA repair

Translin-associated protein X (TSNAX), also called trax, was first identified as a protein that interacts with translin. Subsequent studies demonstrated that these proteins form a heteromeric RNase complex that mediates degradation of microRNAs, a pivotal finding that has stimulated interest in understanding the role of translin and trax in cell signaling. Recent studies addressing this question have revealed that trax plays key roles in both synaptic plasticity and DNA repair signaling pathways. In the context of synaptic plasticity, trax works together with its partner protein, translin, to degrade a subset of microRNAs. Activation of the translin/trax RNase complex reverses microRNA-mediated translational silencing to trigger dendritic protein synthesis critical for synaptic plasticity. In the context of DNA repair, trax binds to and activates ATM, a central component of the double-stranded DNA repair process. Thus, these studies focus attention on trax as a critical signaling protein that interacts with multiple partners to impact diverse signaling pathways. To stimulate interest in deciphering the multifaceted role of trax in cell signaling, we summarize the current understanding of trax biology and highlight gaps in our knowledge about this protean protein.

Multiple Pathways Mediate MicroRNA Degradation: Focus on the Translin/Trax RNase Complex

The discovery of the microRNA system has revolutionized our understanding of translational control. Furthermore, growing appreciation of the pivotal role that de novo translation plays in activity-dependent synaptic plasticity has fueled interest among neuroscientists in deciphering how the microRNA system impacts neuronal signaling and the pathophysiology of neuropsychiatric disorders. Although we have a general understanding of how the microRNA system operates, many key questions remain. In particular, the biosynthesis of microRNAs and their role in translational silencing are fairly well understood. However, much less is known about how microRNAs are degraded and silencing is reversed, crucial aspects of microRNA signaling. In contrast to microRNA synthesis which is mediated almost exclusively by a single pathway that culminates in Dicer, recent studies indicate that there are multiple pathways of microRNA degradation that target different subpopulations of microRNAs. While the Lin-28 pathway of microRNA degradation has been investigated extensively, the translin/trax RNase complex has emerged recently as another pathway mediating microRNA degradation. Accordingly, we summarize herein key features of the translin/trax RNase complex as well as important gaps in our understanding of its regulation and function that are the focus of ongoing studies.