Histone deacetylase 1 (HDAC1) is a nuclear enzyme involved in transcriptional repression. We report here that cytosolic HDAC1 is detected in damaged axons in brains of human patients with Multiple Sclerosis and of mice with cuprizone-induced demyelination, ex vivo models of demyelination and in cultured neurons exposed to glutamate and TNF-α. Nuclear export of HDAC1 is mediated by the interaction with the nuclear receptor CRM-1 and leads to impaired mitochondrial transport. The formation of complexes between exported HDAC1 and members of the kinesin family of motor proteins hinders the interaction with cargo molecules thereby inhibiting mitochondrial movement and inducing localized beadings. This effect is prevented by inhibiting HDAC1 nuclear export with leptomycin B, treating neurons with pharmacological inhibitors of HDAC activity or silencing HDAC1 but not other HDAC isoforms. Together these data identify nuclear export of HDAC1 as a critical event for impaired mitochondrial transport in damaged neurons.

Fast axonal conduction depends on myelin, which is formed by Schwann cells in the PNS. We found that the transcription factor Yin Yang 1 (YY1) is crucial for peripheral myelination. Conditional ablation of Yy1 in the Schwann cell lineage resulted in severe hypomyelination, which occurred independently of altered Schwann cell proliferation or apoptosis. In Yy1 mutant mice, Schwann cells established a 1:1 relationship with axons but were unable to myelinate them. The Schwann cells expressed low levels of myelin proteins and of Egr2 (also called Krox20), which is an important regulator of peripheral myelination. In vitro, Schwann cells that lacked Yy1 did not upregulate Egr2 in response to neuregulin1 and did not express myelin protein zero. This phenotype was rescued by overexpression of Egr2. In addition, neuregulin-induced phosphorylation of YY1 was required for transcriptional activation of Egr2. Thus, YY1 emerges as an important activator of peripheral myelination that links neuregulin signaling with Egr2 expression.

Mammalian circadian rhythms are generated by a negative feedback loop in which PERIOD (PER) proteins accumulate, form a large nuclear complex (PER complex), and bind the transcription factor CLOCK-BMAL1, repressing their own expression. We found that mouse PER complexes include the Mi-2/nucleosome remodelling and deacetylase (NuRD) transcriptional corepressor. Unexpectedly, two NuRD subunits, CHD4 and MTA2, constitutively associate with CLOCK-BMAL1, with CHD4 functioning to promote CLOCK-BMAL1 transcriptional activity. At the onset of negative feedback, the PER complex delivers the remaining complementary NuRD subunits to DNA-bound CLOCK-BMAL1, thereby reconstituting a NuRD corepressor that is important for circadian transcriptional feedback and clock function. The PER complex thus acquires full repressor activity only upon successful targeting of CLOCK-BMAL1. Our results show how specificity is generated in the clock despite its dependence on generic transcriptional regulators and reveal the existence of active communication between the positive and negative limbs of the circadian feedback loop.

Circadian clocks in mammals are built on a negative feedback loop in which the heterodimeric transcription factor circadian locomotor output cycles kaput (CLOCK)-brain, muscle Arnt-like 1 (BMAL1) drives the expression of its own inhibitors, the PERIOD and CRYPTOCHROME proteins. Reactivation of CLOCK-BMAL1 occurs at a specific time several hours after PERIOD and CRYPTOCHROME protein turnover, but the mechanism underlying this process is unknown. We found that mouse BMAL1 complexes include TRAP150 (thyroid hormone receptor-associated protein-150; also known as THRAP3). TRAP150 is a selective coactivator for CLOCK-BMAL1, which oscillates under CLOCK-BMAL1 transcriptional control. TRAP150 promotes CLOCK-BMAL1 binding to target genes and links CLOCK-BMAL1 to the transcriptional machinery at target-gene promoters. Depletion of TRAP150 caused low-amplitude, long-period rhythms, identifying it as a positive clock element. The activity of TRAP150 defines a positive feedback loop within the clock and provides a potential mechanism for timing the reactivation of circadian transcription. C ircadian clocks are endogenous oscillators that drive daily rhythms of physiology and behavior. The mammalian clock, intrinsic to most cells and tissues (1, 2), is built on a conserved negative feedback loop that generates circadian rhythms at the molecular level (3). The core positive element of the clock is the heterodimeric transcription factor circadian locomotor output cycles kaput (CLOCK)-brain, muscle Arnt-like 1 (BMAL1), which drives transcription of Period (Per) and Cryptochrome (Cry) genes from E-box sites (4). PER and CRY proteins, acting as negative elements of the clock, enter the nucleus, associate with CLOCK-BMAL1 (5) at E-box sites (6), and suppress the transcriptional activity of CLOCK-BMAL1 in part by recruiting the SIN3-HDAC histone deacetylase complex (6) and inhibiting transcriptional termination (7). Turnover of PERs and CRYs ends the negative-feedback phase of the cycle (8-11). An interlocked feedback loop involving REV-ERBα and -β (nuclear receptor subfamily 1, group D, members 1 and 2, respectively) contributes to clock function (12)(13)(14).Reactivation of CLOCK-BMAL1 transcription of circadian target genes occurs several hours after the end of negative feedback (15,16), suggesting that the onset of circadian transcription in each cycle by CLOCK-BMAL1 is not simply a passive consequence of the turnover of negative-feedback proteins, but is positively regulated and timed by unknown clock-controlled factors. Evidence that there is active positive regulation of CLOCK-BMAL1 comes from reports showing enhancement of CLOCK-BMAL1 transcriptional activity by chromatin-modifying proteins by CBP/p300 (17), MLL1 (18), and JARID1a (19). Although not previously described, a clock-controlled, rhythmic positive factor for CLOCK-BMAL1 would provide a potential mechanism for precisely setting the onset of transcription each circadian cycle. ResultsTo identify factors associated with CLOCK-BMAL1, we used FLAG antibodies to affinit...

Oligodendrogliopathy, microglial infiltration, and lack of remyelination are detected in the brains of patients with multiple sclerosis and are accompanied by high levels of the transcription factor p53. In this study, we used the cuprizone model of demyelination, characterized by oligodendrogliopathy and microglial infiltration, to define the effect of p53 inhibition. Myelin preservation, decreased microglial recruitment, and gene expression were observed in mice lacking p53 or receiving systemic administration of the p53 inhibitor pifithrin-␣, compared with untreated controls. Decreased levels of lypopolysaccharide-induced gene expression were also observed in vitro, in p53 Ϫ/Ϫ primary microglial cultures or in pifithrin-␣-treated microglial BV2 cells. An additional beneficial effect of lack or inhibition of p53 was observed in Sox2ϩ multipotential progenitors of the subventricular zone that responded with increased proliferation and oligodendrogliogenesis. Based on these results, we propose transient inhibition of p53 as a potential therapeutic target for demyelinating conditions primarily characterized by oligodendrogliopathy.

Previous studies have suggested the existence of a gender bias in repair after demyelination. Here we report the existence of gender dimorphism for the regulation of cell number in the subventricular zone (SVZ), an area that has been studied for its repair potential. The number of Sox2 + multipotential cells in the SVZ of young adult female mice was greater than in age-matched male siblings, but this difference was not evident prior to the surge of sex hormones (i.e., in prepubertal mice). To begin asking whether hormonally derived signals were responsible for these gender-related differences, we analyzed proliferation and survival of cultured male-and female-derived SVZ cells. Estrogen, but not testosterone treatment increased cell proliferation and survival of cultured cells after IFN-γ treatment or after UV irradiation, regardless of the gender of origin. Because apoptosis in UVirradiated SVZ cells correlated with the expression of the proapoptotic molecule p53, we postulated that this molecule could be responsible for the gender dimorphism in the SVZ. In agreement with this prediction, no difference in the SVZ cell number was detected in male and female p53 null mice. Together with previous reports, these results implicate p53 as an important component of the mechanism regulating gender dimorphism in the SVZ. Keywordsestrogen; subventricular zone; testosterone; apoptosis; cell cycle Multiple sclerosis (MS) is an inflammatory demyelinating disorder of the central nervous system (CNS), and in MS patients gender dimorphism is well characterized in the progression, risk, and recovery of the disease. Women are more susceptible than men to contracting the disease (Duquette et al., 1992(Duquette et al., , 1993Hawkins and McDonnell, 1999;Whitacre et al., 1999); they show a greater number of active lesions on MRI (Pozzilli et al., 2003), but they also tend to have a more favorable clinical course of disease than men (Duquette et al., 1992). These results are paralleled by a greater susceptibility of female mice to develop experimental autoimmune encephalomyelitis (EAE; Voskhul and Palaszynski, 2001). In contrast, men are more often affected by progressive forms of MS (Duquette and Girard, 1993), and they tend to show a greater number of hypointense MRI lesions, indicative of greater tissue damage than in females (Pozzilli et al., 2003). Multiple factors can contribute to the explanation of gender dimorphism, including a differential response to sex steroids or intrinsic differences in immune response (Dalal et al., 1997;Wilcoxen et al., 2000;Pelfrey et al., 2002). The reduction in the severity of disease and the decreased relapse rate during pregnancy have further supported the notion that a relationship exists between the hormonal status of the patient and the disease progression (Confavreux et al., 1998;Lorenzi and Ford, 2002;El-Etr et al., 2005). The protective role of estrogens was supported by in vitro studies in cultured rat Schwann cells (Zhu and Glaser, 2008) and rodent (Takao et al., 2004) and human ...

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