Functional characterization of the activity-regulated Syt10 promoter

Discussion

Discussion 6.1.1 Identification and functional characterization of the Syt10 promoter

The bioinformatic analysis of the Syt10 gene carried out in the present study revealed characteristics commonly associated with a core promoter as well as evolutionary conserved potential regulatory elements. By using Luciferase assays these regions were examined in different primary neurons and a neuronal cell line (NG108-15) regarding their promoter activity. Thereby, activating and inhibitory elements within the Syt10 gene were identified, which are summarized in figure 6.1.

Figure 6.1 Graphic summary of activating and repressing regions of the Syt10 promoter. Measurements of basal luciferase activity revealed putative activating (+) or repressing (-) regulatory elements in the Syt10 gene.

Two interesting regulatory regions were uncovered. Firstly, the evolutionary conserved part of the first Syt10 intron (RR-IN) exhibited a lower basal promoter activity than the core promoter. Therefore, this region represents a strongly inhibiting promoter control motif. Intriguingly, the corresponding human intron region was predicted as a heterochromatin-withstanding region (UCSC), referring to a part of the Syt10 gene that has low transcriptional activity. The second region comprising 1036 bp to 306 bp upstream of the start codon strongly enhanced the activity of the core promoter only in hippocampal neurons. This part of the Syt10 gene might represent a region that could mediate the hippocampus-specific induction of Syt10 following SE (Babity et al., 1997; this thesis). Cell-type specific promoters are well established and are used as tools to generate transgenic animals and for gene therapy (reviewed by Boulaire et al., 2009 and Walther & Stein, 1996). The identification of a Syt10 promoter fragment that exhibits hippocampus specific activity could be used to express target cDNAs only in this brain region e.g. by viral gene transfer.

Discussion putative regulatory regions (RR-5’ and RR-3’) were identified. However, these regions did not display a significant increase in promoter activity compared to the core promoter indicating that they do not contain elements that positively affect the basal Syt10 promoter activity. In addition, highly conserved regions (RR1-3) were subcloned and analyzed residing in the fragment 4713 bp upstream of the start ATG and were combined with the core promoter. Interestingly, together with the core promoter, these regions revealed a higher promoter activity than the full -4713 bp fragment. This finding suggests that there might be a repressing region located between 1036 bp and 3675 bp upstream of the start ATG. However, only RR-2 exhibited a higher promoter activity specifically in hippocampal neurons.

Taken together, these results indicate that in accordance with low basal expression levels of Syt10 large negative regulatory regions could be identified. Furthermore, these findings suggest that the identified positive regulatory elements can be activated by relevant stimuli.

6.1.2 Activity-regulated transcription factors mediating Syt10 gene expression The promoters of the activity-regulated genes BDNF and Arc/Arg3.1 have been extensively studied and transcription factors regulating their activity have been identified (Kawashima et al., 2009; Kim et al., 2010; Lyons et al., 2012; Pintchovski et al., 2009; Pruunsild et al., 2011). However, to date the promoter of Syt10 and its regulatory mechanisms have not been resolved yet.

Using different algorithms, several binding sites for transcription factors were predicted in the Syt10 promoter region that were also important for the activity-dependent regulation of the BDNF and/or the Arc promoter. Luciferase assays in NG108 cells showed that some of these transcription factors did not induce the Syt10 promoter (MEF2A, 2C) whereas others (AP1-factors, USF1/2 and NPAS4) strongly increased the promoter activity of Syt10. Functionally relevant transcription factors should also affect endogenous Syt10 expression levels. To test, which of the transcription factors that have putative binding sites, also impact Syt10 expression in vivo, rat hippocampal neurons were transfected with transcription factors and Syt10 expression levels were analyzed using quantitative real-time PCR. NPAS4, USF2 and cFos augmented endogenous Syt10 expression levels. Further analysis revealed that USF2 and cFos were not sufficient to activate the characterized Syt10 promoter fragments in luciferase measurements of primary hippocampal neurons. This

Discussion discrepancy suggests that the promoter fragments might not contain the respective functional binding sites for USF2 and cFos. Whereas in contrast, NPAS4 was the only transcription factor that increased both, Syt10 endogenous expression and Syt10 promoter activity as examined by luciferase assays in NG108-15 cells and neurons. Although the observed level of up-regulation of Syt10 mRNA expression after NPAS4 transfection in neurons is relatively weak (approx. 2-fold), it has to be taken into account that the transfection efficiency of primary neurons is not very high (less than 30 %). Therefore, the actual increase in transfected neurons is higher. As NPAS4 was up-regulated in the hippocampus following PTZ-induced seizure activity (Flood et al., 2004), after kainate (Ooe et al., 2009a) and pilocarpine injection (this thesis), it is a putative factor to induce Syt10 expression in response to SE.

As NPAS4 is upstream of multiple transcription factors (Lin et al., 2008) it remained unclear if the observed effect on the Syt10 promoter is direct or indirect. Therefore, as a first step, the responsive NPAS4 binding sites were identified. Due to the finding that the Syt10 promoter contains several potential sites in the longest fragment, deletion reporter plasmids were generated. Deletion constructs carrying binding sites 1, 2 and 3 (constructs I, II, -306) led to a strong increase of the Syt10 promoter that was not found in the region between the conserved elements (constructs III, IV and -1036), whereas the two longest fragments (constructs V and -4713) were strongly activated by NPAS4. Thus, unknown factors in the region between the conserved elements repress the activation by NPAS4 binding to the core promoter. However, if the binding sites in construct V and -4713 are also present, the repression is overruled resulting in a stronger activation.

Taken together, these results indicate that for the activity of the Syt10 promoter, NPAS4 binding sites 1, 2 and 5 and/or 6, 7 are most important. To test, whether this effect was due to direct binding of NPAS4 to the Syt10 promoter, an in vitro binding assay could be carried out.

6.1.3 The Syt10 promoter is controlled by heterodimerization of bHLH-transcription factors

Discussion 2004; Pongratz et al., 1998). An interesting candidate for the formation of a functional heterodimer with NPAS4 is the transcription factor Arnt2.

Luciferase assays of rat hippocampal neurons transfected with Arnt2 alone did not result in a stimulation of the Syt10 promoter. Moreover, co-transfection of Arnt2 with NPAS4 rather caused a decrease than a positive synergistic effect on Syt10 promoter activity. It could be further investigated if Arnt2 acts as a heterodimer with NPAS4 to reverse the activating function of NPAS4. Thereby, this heterodimer could be involved in the regulation of Syt10 gene expression.

The mammalian homologue of the Drosophila clock gene and bHLH-PAS transcription factor Period1 (Per1) is expressed in the SCN, where Syt10 is expressed as well at high levels (Tei et al., 1997; Husse et al., 2011). Given that epileptic seizures induce Per1 and Syt10 expression levels, respectively, (Babity et al., 1997; Eun et al., 2011), it was hypothesized that this transcription factor has an influence on Syt10 promoter activity alone or as a heterodimer with NPAS4.

Luciferase assays of rat hippocampal neurons transfected with Per1 revealed that the Syt10 promoter was indeed activated by Per1. Furthermore, co-transfection of NPAS4 and Per1 resulted in a synergistic effect.

Besides in the SCN, Per1 is also expressed in the hippocampus. In addition, multiple reports about Per1 rhythmicity in the hippocampus stated that it is expressed in an oscillatory manner (Balsalobre et al., 2000; Gilhooley et al., 2011; Golini et al., 2012;

Jilg et al., 2010; Reick et al., 2001). Taken together, these results suggest a role for NPAS4 and Per1 in regulating Syt10 transcription in the hippocampus. Given that both transcription factors were altered in response to seizures and that Per1 is expressed in a circadian rhythm in both, hippocampus and SCN, there is evidence that these transcription factors play a role in induced Syt10 expression, conceivably in an oscillatory manner.