BackgroundCircadian rhythms govern a large array of physiological and metabolic functions. To achieve plasticity in circadian regulation, proteins constituting the molecular clock machinery undergo various post-translational modifications (PTMs), which influence their activity and intracellular localization. The core clock protein BMAL1 undergoes several PTMs. Here we report that the Akt-GSK3β signaling pathway regulates BMAL1 protein stability and activity.Principal FindingsGSK3β phosphorylates BMAL1 specifically on Ser 17 and Thr 21 and primes it for ubiquitylation. In the absence of GSK3β-mediated phosphorylation, BMAL1 becomes stabilized and BMAL1 dependent circadian gene expression is dampened. Dopamine D2 receptor mediated signaling, known to control the Akt-GSK3β pathway, influences BMAL1 stability and in vivo circadian gene expression in striatal neurons.ConclusionsThese findings uncover a previously unknown mechanism of circadian clock control. The GSK3β kinase phosphorylates BMAL1, an event that controls the stability of the protein and the amplitude of circadian oscillation. BMAL1 phosphorylation appears to be an important regulatory step in maintaining the robustness of the circadian clock.

Dysfunctions of dopaminergic homeostasis leading to either low or high dopamine (DA) levels are causally linked to Parkinson's disease, schizophrenia, and addiction. Major sites of DA synthesis are the mesencephalic neurons originating in the substantia nigra and ventral tegmental area; these structures send major projections to the dorsal striatum (DSt) and nucleus accumbens (NAcc), respectively. DA finely tunes its own synthesis and release by activating DA D2 receptors (D2R). To date, this critical D2R-dependent function was thought to be solely due to activation of D2Rs on dopaminergic neurons (D2 autoreceptors); instead, using site-specific D2R knock-out mice, we uncover that D2 heteroreceptors located on non-DAergic medium spiny neurons participate in the control of DA levels. This D2 heteroreceptor-mediated mechanism is more efficient in the DSt than in NAcc, indicating that D2R signaling differentially regulates mesolimbic-versus nigrostriatal-mediated functions. This study reveals previously unappreciated control of DA signaling, shedding new light on region-specific regulation of DA-mediated effects.

Glutathione (GSH) is a key antioxidant that plays an important neuroprotective role in the brain. Decreased GSH levels are associated with neurodegenerative diseases such as Parkinson’s disease and Alzheimer’s disease. Here we show that a diurnal fluctuation of GSH levels is correlated with neuroprotective activity against oxidative stress in dopaminergic cells. In addition, we found that the cysteine transporter excitatory amino acid carrier 1 (EAAC1), which is involved in neuronal GSH synthesis, is negatively regulated by the microRNA miR-96-5p, which exhibits a diurnal rhythm. Blocking miR-96-5p by intracerebroventricular administration of an inhibitor increased the level of EAAC1 as well as that of GSH and had a neuroprotective effect against oxidative stress in the mouse substantia nigra. Our results suggest that the diurnal rhythm of miR-96-5p may play a role in neuroprotection by regulating neuronal GSH levels via EAAC1.

Mood disorders are considered to be associated with altered circadian rhythms, but the correlation between them has remained obscure. The mood stabilizer, lithium, is an inhibitor of glycogen synthase kinase-3β (GSK-3β), which is a modulator of the circadian clock system. Here, we show that chronic restraint (CR) stress diminishes behavioral activity and rhythmicity in mice. CR stress elevated GSK-3β phosphorylation and blunted the rhythmic expression of PERIOD2 (PER2) in the brain. Moreover, lithium, when administered to the stress-imposed mice, reduced GSK-3β phosphorylation and restored PER2 expression in the suprachiasmatic nucleus in a nighttime-specific manner. These data suggest that CR stress altered the circadian behavioral rhythm through a change in circadian gene expression of PER2 and GSK-3β phosphorylation in the suprachiasmatic nucleus.

Oligodendrocytes are the glial cells responsible for myelin formation. Myelination occurs during the first postnatal weeks and, in rodents, is completed during the third week after birth. Myelin ensures the fast conduction of the nerve impulse; in the adult, myelin proteins have an inhibitory role on axon growth and regeneration after injury. During brain development, oligodendrocytes precursors originating in multiple locations along the antero-posterior axis actively proliferate and migrate to colonize the whole brain. Whether the initial interactions between oligodendrocytes and neurons might play a functional role before the onset of myelination is still not completely elucidated. In this article, we addressed this question by transgenically targeted ablation of proliferating oligodendrocytes during cerebellum development. Interestingly, we show that depletion of oligodendrocytes at postnatal day 1 (P1) profoundly affects the establishment of cerebellar circuitries. We observed an impressive deregulation in the expression of molecules involved in axon growth, guidance and synaptic plasticity. These effects were accompanied by an outstanding increase of neurofilament staining observed 4 hours after the beginning of the ablation protocol, likely dependent from sprouting of cerebellar fibers. Oligodendrocyte ablation modifies localization and function of ionotropic glutamate receptors in Purkinje neurons. These results show a novel oligodendrocyte function expressed during early postnatal brain development, where these cells participate in the formation of cerebellar circuitries, and influence its development.

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The number of patients with neurodegenerative diseases (NDs) is increasing, along with the growing number of older adults. This escalation threatens to create a medical and social crisis. NDs include a large spectrum of heterogeneous and multifactorial pathologies, such as amyotrophic lateral sclerosis, frontotemporal dementia, Alzheimer’s disease, Parkinson’s disease, Huntington’s disease and multiple system atrophy, and the formation of inclusion bodies resulting from protein misfolding and aggregation is a hallmark of these disorders. The proteinaceous components of the pathological inclusions include several RNA-binding proteins (RBPs), which play important roles in splicing, stability, transcription and translation. In addition, RBPs were shown to play a critical role in regulating miRNA biogenesis and metabolism. The dysfunction of both RBPs and miRNAs is often observed in several NDs. Thus, the data about the interplay among RBPs and miRNAs and their cooperation in brain functions would be important to know for better understanding NDs and the development of effective therapeutics. In this review, we focused on the connection between miRNAs, RBPs and neurodegenerative diseases.

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