Tau reduction with artificial microRNAs modulates neuronal physiology and improves tauopathy phenotypes in mice.

Abnormal tau accumulation is the hallmark of several neurodegenerative diseases, named tauopathies. Strategies aimed at reducing tau in the brain are promising therapeutic interventions, yet more precise therapies would require targeting specific nuclei and neuronal subpopulations affected by disease while avoiding global reduction of physiological tau. Here, we developed artificial microRNAs directed against the human MAPT mRNA to dwindle tau protein by engaging the endogenous RNA interference pathway. In human differentiated neurons in culture, microRNA-mediated tau reduction diminished neuronal firing without affecting neuronal morphology or impairing axonal transport. In the htau mouse model of tauopathy, we locally expressed artificial microRNAs in the prefrontal cortex (PFC), an area particularly vulnerable to initiating tau pathology in this model. Tau knockdown prevented the accumulation of insoluble and hyperphosphorylated tau, modulated firing activity of putative pyramidal neurons, and improved glucose uptake in the PFC. Moreover, such tau reduction prevented cognitive decline in aged htau mice. Our results suggest target engagement of designed tau-microRNAs to effectively reduce tau pathology, providing a proof of concept for a potential therapeutic approach based on local tau knockdown to rescue tauopathy-related phenotypes.

Selective refinement of glutamate and GABA synapses on dorsal raphe 5-HT neurons during postnatal life.

Serotonin (5-hydroxytryptamine, 5-HT) neurons are implicated in the etiology and therapeutics of anxiety and depression. Critical periods of vulnerability during brain development enable maladaptive mechanisms to produce detrimental consequences on adult mood and emotional responses. 5-HT plays a crucial role in these mechanisms; however, little is known about how synaptic inputs and modulatory systems that shape the activity of early 5-HT networks mature during postnatal development. We investigated in mice the postnatal trajectory of glutamate and GABA synaptic inputs to dorsal raphe nucleus (DRN) 5-HT neurons, the main source of forebrain 5-HT. High-resolution quantitative analyses with array tomography and ex vivo electrophysiology indicate that cortical glutamate and subcortical GABA synapses undergo a profound refinement process after the third postnatal week, whereas subcortical glutamate inputs do not. This refinement of DRN inputs is not accompanied by changes in 5-HT1A receptor-mediated inhibition over 5-HT neurons. Our study reveals a precise developmental pattern of synaptic refinement of DRN excitatory and inhibitory afferents, when 5-HT-related inhibitory mechanisms are in place. These findings contribute to the understanding of neurodevelopmental vulnerability to psychiatric disorders. This article has an associated 'The people behind the papers' interview.

Tau mis-splicing correlates with motor impairments and striatal dysfunction in a model of tauopathy.

Tauopathies are neurodegenerative diseases caused by the abnormal metabolism of the microtubule associated protein tau (MAPT), which is highly expressed in neurons and critically involved in microtubule dynamics. In the adult human brain, the alternative splicing of exon 10 in MAPT pre-mRNA produces equal amounts of protein isoforms with either three (3R) or four (4R) microtubule binding domains. Imbalance in the 3R:4R tau ratio is associated with primary tauopathies that develop atypical parkinsonism, such as progressive supranuclear palsy and corticobasal degeneration. Yet, the development of effective therapies for those pathologies is an unmet goal. Here we report motor coordination impairments in the htau mouse model of tauopathy which harbour abnormal 3R:4R tau isoforms content, and in contrast to TauKO mice, are unresponsive to l-DOPA. Preclinical-PET imaging, array tomography and electrophysiological analyses indicated the dorsal striatum as the candidate structure mediating such phenotypes. Indeed, local modulation of tau isoforms by RNA trans-splicing in the striata of adult htau mice, prevented motor coordination deficits and restored basal neuronal firing. Together, these results suggest that abnormal striatal tau isoform content might lead to parkinsonian-like phenotypes and demonstrate a proof of concept that modulation of tau mis-splicing is a plausible disease-modifying therapy for some primary tauopathies.

Serotonin-related rodent models of early-life exposure relevant for neurodevelopmental vulnerability to psychiatric disorders.

Mental disorders including depression and anxiety are continuously rising their prevalence across the globe. Early-life experience of individuals emerges as a main risk factor contributing to the developmental vulnerability to psychiatric disorders. That is, perturbing environmental conditions during neurodevelopmental stages can have detrimental effects on adult mood and emotional responses. However, the possible maladaptive neural mechanisms contributing to such psychopathological phenomenon still remain poorly understood. In this review, we explore preclinical rodent models of developmental vulnerability to psychiatric disorders, focusing on the impact of early-life environmental perturbations on behavioral aspects relevant to stress-related and psychiatric disorders. We limit our analysis to well-established models in which alterations in the serotonin (5-HT) system appear to have a crucial role in the pathophysiological mechanisms. We analyze long-term behavioral outcomes produced by early-life exposures to stress and psychotropic drugs such as the selective 5-HT reuptake inhibitor (SSRI) antidepressants or the anticonvulsant valproic acid (VPA). We perform a comparative analysis, identifying differences and commonalities in the behavioral effects produced in these models. Furthermore, this review discusses recent advances on neurodevelopmental substrates engaged in these behavioral effects, emphasizing the possible existence of maladaptive mechanisms that could be shared by the different models.

Activity-dependent reduction of dopamine D2 receptors during a postnatal critical period of plasticity in rat striatum is not affected by prenatal haloperidol treatment.

Motor activity induced in the Circling Training test (CT) during a postnatal (PN) critical period of plasticity (PN30-37) produces a long-lasting decrease in the number of binding sites and mRNA expression levels of the dopamine D2 receptor (D2R) in rat striatum. Prenatal exposure to the antipsychotic haloperidol also decreases postnatal levels of the striatal D2R in the offspring. We examined whether such fetal exposure to haloperidol could affect the activity-dependent reduction of the D2R system during the critical period. Half of the male offspring exposed to either haloperidol (2.5 mg/kg/day), i.p.) or saline during gestational days 5-18 were subjected to the CT during the critical period, while the remaining represented CT control animals. The adult number of binding sites and mRNA expression levels of the striatal D2R at PN90 were not changed by prenatal haloperidol treatment alone. On the other hand, only pups subjected to the CT during the critical period showed decreases in both studied parameters, regardless the prenatal treatment. These findings indicated that the postnatal reduction of the striatal D2R binding induced prenatally by haloperidol does not affect long-lasting activity-dependent plastic changes on the same receptor system elicited by motor activity in an ontogenetic critical period of plasticity in rat striatum.

Developmental striatal critical period of activity-dependent plasticity is also a window of susceptibility for haloperidol induced adult motor alterations.

Experience-dependent plasticity during critical periods of postnatal (PN) development shapes the adult brain anatomy and function. In rat motor system, there is a critical period of activity-dependent plasticity in the striatum (PN30-37). In this period, motor activity of running in a circular path induced in the Circling Training test (CT), elicits several plasticity changes on striatal synapses. It has been recently proposed that developmental critical periods might represent a unique pharmacological window of vulnerability to induce life-lasting behavioral modifications. In this paper we tested the hypothesis of existence of a pharmacological susceptibility to induce adult alterations on motor behavior during the striatal critical period. Due to its main action on the striatum and developmental motor behavioral effects, we applied the prototypical antipsychotic haloperidol to male rats (i.p. 0.7 or 2.5 mg/kg/day) before, during or after the period of plasticity (PN20-27, PN30-37 or PN40-47 respectively). Then, in the adulthood (PN80), we evaluated induced motor activity in the CT. The results showed that only rats exposed to the D2R blocker during the period PN30-37 increased the CT activity in comparison to control rats. Moreover, only these animals also showed an increase in the spontaneous locomotor activity at the open field test. These behavioral alterations were not accompanied by permanent striatal changes either on the number of D2R binding sites or on its mRNA expression levels. In conclusion, we have shown a pharmacological susceptibility of inducing adult motor behavior alterations by haloperidol during a natural critical period of activity-dependent plasticity (PN30-37) in rat striatum development. These results also emphasize the importance of behavioral screening for pharmacological agents to be used in developmental stages of maturation.

Postnatal haloperidol eliminates the deficit in circling behavior produced by prenatal exposure to the same drug.

Up to 35% of pregnant women take psychotropic drugs at least once during gestation [Austin and Mitchell, 1998]. From concurrent animal and human evidence, it has been proposed that exposure to several psychoactive medications in utero or during lactation increases the risk for permanent brain disorders. Present preventive or therapy practices applied on humans for this type of long-lasting behavioral alterations are mainly based on empirical results. Here, we test an experimental approach designed to counteract a circling performance deficit that appears in Sprague-Dawley rats at puberty on exposure to the dopaminergic blocker haloperidol (HAL) during gestation [J.L. Brusés, J.M. Azcurra, The circling training: A behavioral paradigm for functional teratology testing, in: P.M. Conn (Ed.), Paradigms for the study of behavior, Acad. Press, New York, 1993, pp. 166-179. Method Neurosci. 14]. Gestational exposure to HAL (GD 5-18, 2.5 mg/kg/day ip) induced the expected circling activity decrease in the offspring at the fifth week of life. When prenatal exposure to HAL was continued through lactation (PD5-21, 1.5 mg/kg/day ip), rats otherwise showed a control-like circling performance. No difference was yet found between lactation-only, HAL-exposed pups and saline (SAL)-treated controls (n=8 each group). We further performed saturating (3H)-spiroperidol (SPI) binding assays on striatal P2 membrane fractions 2 months later. The dopamine-type D2-specific binding results suggested that above circling behavior findings could be partially explained by enduring HAL-induced neurochemical changes. The role of critical periods of sensitivity as transient windows for opportunistic therapies for behavioral teratology is discussed.

Different dopamine D1 and D2 receptors expression after motor activity in the striatal critical period.

Circling training during rat striatum postnatal critical period (PN30 to 37 days) induces a life-lasting co-reduction of muscarinic acetylcholine receptors (mAChR) and dopamine D2 receptors (D2R) binding. Here, we evaluated the expression of D1R and D2R under similar experimental conditions. Trained rats showed a decrease of 40% in D2R binding sites (p<0.01) and of 45% in the D2R mRNA expression which involve short (p<0.05) and long (p<0.01) isoforms. In contrast, D1R binding sites nor its mRNA expression levels were affected by training, indicating a differential synaptic refinement during this ontogenetically fixed period.

Delayed and bilateral changes of GAP-43/B-50 phosphorylation after circling training during a critical period in rat striatum.

During the critical period of activity-dependent plasticity in rat striatum (30-37 days after birth) physiological circling behavior induces delayed modifications in GAP-43/B-50 phosphorylation by PKC. Postexercise, ipsi- and contralateral striatum to the circling direction show a similar temporal pattern of GAP-43/B-50 phosphorylation, with an initial decrease followed by a subsequent increase. However, there is a lag between initiation of the phosphorylation response in this asymmetrical task which does not occur when animals are subjected to exercise under conditions of symmetrical motor activity.