Preventing polyglutamine-induced activation of c-Jun delays neuronal dysfunction in a mouse model of SCA7 retinopathy

Gene Deregulation and Underlying Mechanisms in Spinocerebellar Ataxias With Polyglutamine Expansion

Polyglutamine spinocerebellar ataxias (polyQ SCAs) include SCA1, SCA2, SCA3, SCA6, SCA7, and SCA17 and constitute a group of adult onset neurodegenerative disorders caused by the expansion of a CAG repeat sequence located within the coding region of specific genes, which translates into polyglutamine tract in the corresponding proteins. PolyQ SCAs are characterized by degeneration of the cerebellum and its associated structures and lead to progressive ataxia and other diverse symptoms. In recent years, gene and epigenetic deregulations have been shown to play a critical role in the pathogenesis of polyQ SCAs. Here, we provide an overview of the functions of wild type and pathogenic polyQ SCA proteins in gene regulation, describe the extent and nature of gene expression changes and their pathological consequences in diseases, and discuss potential avenues to further investigate converging and distinct disease pathways and to develop therapeutic strategies.

Molecular Targets and Therapeutic Strategies in Spinocerebellar Ataxia Type 7

Spinocerebellar ataxia type 7 (SCA7) is a rare autosomal dominant neurodegenerative disorder characterized by progressive neuronal loss in the cerebellum, brainstem, and retina, leading to cerebellar ataxia and blindness as major symptoms. SCA7 is due to the expansion of a CAG triplet repeat that is translated into a polyglutamine tract in ATXN7. Larger SCA7 expansions are associated with earlier onset of symptoms and more severe and rapid disease progression. Here, we summarize the pathological and genetic aspects of SCA7, compile the current knowledge about ATXN7 functions, and then focus on recent advances in understanding the pathogenesis and in developing biomarkers and therapeutic strategies. ATXN7 is a bona fide subunit of the multiprotein SAGA complex, a transcriptional coactivator harboring chromatin remodeling activities, and plays a role in the differentiation of photoreceptors and Purkinje neurons, two highly vulnerable neuronal cell types in SCA7. Polyglutamine expansion in ATXN7 causes its misfolding and intranuclear accumulation, leading to changes in interactions with native partners and/or partners sequestration in insoluble nuclear inclusions. Studies of cellular and animal models of SCA7 have been crucial to unveil pathomechanistic aspects of the disease, including gene deregulation, mitochondrial and metabolic dysfunctions, cell and non-cell autonomous protein toxicity, loss of neuronal identity, and cell death mechanisms. However, a better understanding of the principal molecular mechanisms by which mutant ATXN7 elicits neurotoxicity, and how interconnected pathogenic cascades lead to neurodegeneration is needed for the development of effective therapies. At present, therapeutic strategies using nucleic acid-based molecules to silence mutant ATXN7 gene expression are under development for SCA7.

Mouse models of polyglutamine diseases in therapeutic approaches: review and data table. Part II

Mouse models of human diseases are created both to understand the pathogenesis of the disorders and to find successful therapies for them. This work is the second part in a series of reviews of mouse models of polyglutamine (polyQ) hereditary disorders and focuses on in vivo experimental therapeutic approaches. Like part I of the polyQ mouse model review, this work is supplemented with a table that contains data from experimental studies of therapeutic approaches in polyQ mouse models. The aim of this review was to characterize the benefits and outcomes of various therapeutic strategies in mouse models. We examine whether the therapeutic strategies are specific to a single disease or are applicable to more than one polyQ disorder in mouse models. In addition, we discuss the suitability of mouse models in therapeutic approaches. Although the majority of therapeutic studies were performed in mouse models of Huntington disease, similar strategies were also used in other disease models.

Effect of brain-derived neurotrophic factor on c-jun expression in the rd mouse retina

AIM

To determine the location of c-jun protein, dynamic changes in c-jun mRNA and protein expression, and ultrastructure characteristics in the rd mouse retina, following a single dose of brain-derived neurotrophic factor (BDNF) in a short period of time.

METHODS

A single intravitreal injection of BDNF at two dosages (25µg/L or 50µg/L) was given to the right eye of the rd mouse at age 2 and 3 weeks respectively. Two weeks after injection, the location of c-jun protein in the retina was observed by immunofluorescence detection, c-jun mRNA and protein expression in retinas were detected by quantitative real time polymerase chain reaction (RT-PCR) and western immunoblotting analysis, ultrastructure characteristics of retinas were detected by transmission electron microscope (TEM) observation.

RESULTS

c-jun protein was expressed in the inner nuclear layer (INL) of retina. BDNF at two dosages (25µg/L and 50µg/L) increased c-jun mRNA expression at PN-4 weeks respectively (P(1)=0.019, P(2)=0.021). 50µg/L BDNF increased c-jun protein expression at PN-4 weeks (P =0.000). The retinal ultrastructure was improved.

CONCLUSION

The effects of BDNF exerts on the c-jun expression in the retina are dose-dependent and time-dependent, which may mediate photoreceptor rescue indirectly in the pathological process of retinitis pigmentosa (RP) at early stage.

Genetically engineered mouse models of the trinucleotide-repeat spinocerebellar ataxias

The spinocerebellar ataxias (SCAs) are dominantly inherited disorders that primarily affect coordination of motor function but also frequently involve other brain functions. The models described in this review address mechanisms of trinucleotide-repeat expansions, particularly those relating to polyglutamine expression in the mutant proteins. Modeling chronic late-onset human ataxias in mice is difficult because of their short life-span. While this potential hindrance has been partially overcome by using over-expression of the mutant gene, and/or worsening of the mutation by increasing the length of the trinucleotide repeat expansion, interpretation of results from such models and extrapolation to the human condition should be cautious. Nevertheless, genetically engineered murine models of these diseases have enhanced our understanding of the pathogenesis of many of these conditions. A common theme in many of the polyglutamine-repeat diseases is nuclear localization of mutant protein, with resultant effects on gene regulation. Conditional mutant models and transgenic knock-down therapy have demonstrated the potential for reversibility of disease when production of mutant protein is halted. Several other genetically engineered murine models of SCA also have begun to show utility in the identification and assessment of more classical drug-based therapeutic modalities.

Structure and function relationship study of allium organosulfur compounds on upregulating the pi class of glutathione S-transferase expression

Allium organosulfides are potential chemopreventive compounds due to their effectiveness on the induction of phase II detoxification enzyme expression. In this study, we examined the structure and function relationship among various alk(en)yl sulfides on the expression of the pi class of glutathione S-transferase (GSTP) in rat Clone 9 cells, and what mechanism is involved. Cells were treated with 300 μM dipropyl sulfide (DPS), dipropyl disulfide (DPDS), propyl methyl sulfide (PMS), and propyl methyl disulfide (PMDS) for 48 h. DPDS and PMDS displayed more potency on GSTP protein and mRNA induction than that of DPS and PMS. Next, we compared the effectiveness of DPDS, PMDS, and diallyl disulfide (DADS), which have the same number of sulfur atoms but differ in the side alk(en)yl groups. The maximum increases on protein expression, mRNA level, and enzyme activity were noted in cells treated with DADS, followed by DPDS and PMDS. A reporter assay showed that three disulfides increased GSTP enhancer I (GPE I) activity (P < 0.05) in the order DADS > DPDS ≥ PMDS. Electromobility gel shift assays showed that the DNA binding of GPE I to nuclear proteins reached a maximum at 1 to 3 h after alk(en)yl disulfide treatment. Supershift assay revealed that c-jun bound to GPE I. Silencing of extracellular signal-regulated kinase (ERK) 2 expression inhibited c-jun activation and GSTP induction. Results suggest that both the type of alk(en)yl groups and number of sulfur atoms are determining factors of allium organosulfides on inducing GSTP expression, and it is likely related to the ERK-c-Jun-GPE I pathway.

Transcriptional activation of REST by Sp1 in Huntington's disease models

In Huntington's disease (HD), mutant huntingtin (mHtt) disrupts the normal transcriptional program of disease neurons by altering the function of several gene expression regulators such as Sp1. REST (Repressor Element-1 Silencing Transcription Factor), a key regulator of neuronal differentiation, is also aberrantly activated in HD by a mechanism that remains unclear. Here, we show that the level of REST mRNA is increased in HD mice and in NG108 cells differentiated into neuronal-like cells and expressing a toxic mHtt fragment. Using luciferase reporter gene assay, we delimited the REST promoter regions essential for mHtt-mediated REST upregulation and found that they contain Sp factor binding sites. We provide evidence that Sp1 and Sp3 bind REST promoter and interplay to fine-tune REST transcription. In undifferentiated NG108 cells, Sp1 and Sp3 have antagonistic effect, Sp1 acting as an activator and Sp3 as a repressor. Upon neuronal differentiation, we show that the amount and ratio of Sp1/Sp3 proteins decline, as does REST expression, and that the transcriptional role of Sp3 shifts toward a weak activator. Therefore, our results provide new molecular information to the transcriptional regulation of REST during neuronal differentiation. Importantly, specific knockdown of Sp1 abolishes REST upregulation in NG108 neuronal-like cells expressing mHtt. Our data together with earlier reports suggest that mHtt triggers a pathogenic cascade involving Sp1 activation, which leads to REST upregulation and repression of neuronal genes.

Protection against chronic hypoperfusion-induced retinal neurodegeneration by PARP inhibition via activation of PI-3-kinase Akt pathway and suppression of JNK and p38 MAP kinases

Poly(ADP-ribose) polymerase (PARP) activation is considered as a major regulator of cell death in various pathophysiological conditions, however, no direct information is available about its role in chronic hypoperfusion-induced neuronal death. Here, we provide evidence for the protective effect of PARP inhibition on degenerative retinal damage induced by bilateral common carotid artery occlusion (BCCAO), an adequate chronic hypoperfusion murine model. We found that BCCAO in adult male Wistar rats led to severe degeneration of all retinal layers that was attenuated by a carboxaminobenzimidazol-derivative PARP inhibitor (HO3089) administered unilaterally into the vitreous body immediately following carotid occlusion and then 4 times in a 2-week-period. Normal morphological structure of the retina was preserved and the thickness of the retinal layers was increased in HO3089-treated eyes compared to the BCCAO eyes. For Western blot studies, HO3089 was administered immediately after BCCAO and retinas were removed 4 h later. According to Western blot analysis utilizing phosphorylation-specific primary antibodies, besides activating poly-ADP-ribose (PAR) synthesis, BCCAO induced phosphorylation of c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK). HO3089 inhibited PAR synthesis, and decreased the phosphorylation of these proapoptotic MAPKs. In addition, HO3089 treatment induced phosphorylation, that is activation, of the protective Akt/glycogen synthase kinase (GSK)-3beta and extracellular signal-regulated kinase (ERK1/2) signaling pathways. These data indicate that PARP activation has a major role in mediating chronic hypoperfusion-induced neuronal death, and inhibition of the enzyme prevents the pathological changes both in the morphology and the kinase signaling cascades involved. These results identify PARP inhibition as a possible molecular target in the clinical management of chronic hypoperfusion-induced neurodegenerative diseases including ocular ischemic syndrome.

Induction of chemokines, MCP-1, and KC in the mutant huntingtin expressing neuronal cells because of proteasomal dysfunction

Huntington's disease is a hereditary neurodegenerative disorder caused by an aberrant polyglutamine expansion in the amino terminus of the huntingtin protein. The resultant mutant huntingtin form aggregates in neurons and causes neuronal dysfunction and degeneration in many ways including transcriptional dysregulation. Here, we report that the expression of mutant huntingtin in the mouse neuroblastoma cell results in massive transcriptional induction of several chemokines including monocyte chemoattractant protein-1 (MCP-1) and murine chemokine (KC). The mutant huntingtin expressing cells also exhibit proteasomal dysfunction and down-regulation of NF-kappaB activity in a time-dependent manner and both these phenomena regulate the expression of MCP-1 and KC. The expression of MCP-1 and KC are increased in the mutant huntingtin expressing cells in response to mild proteasome inhibition. However, the expression of MCP-1 and KC and proteasome activity are not altered and inflammation is rarely observed in the brain of 12-week-old Huntington's disease transgenic mice in comparison with their age-matched controls. Our result suggests that the mutant huntingtin-induced proteasomal dysfunction can up-regulate the expression of MCP-1 and KC in the neuronal cells and therefore might trigger the inflammation process.