Isolation and characterization of the 5´-upstream region of the human voltage-gated Ca2+ channel α2δ-1 auxiliary subunit gene: promoter analysis and regulation by transcription factor Sp1

Molecular cloning of the gene promoter encoding the human CaVγ2/Stargazin divergent transcript (CACNG2-DT): characterization and regulation by the cAMP-PKA/CREB signaling pathway

CaVγ2 (Stargazin or TARPγ2) is a protein expressed in various types of neurons whose function was initially associated with a decrease in the functional expression of voltage-gated presynaptic Ca2+ channels (CaV) and which is now known to promote the trafficking of the postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPAR) towards the cell membrane. Alterations in CaVγ2 expression has been associated with several neurological disorders, such as absence epilepsy. However, its regulation at the transcriptional level has not been intensively addressed. It has been reported that the promoter of the Cacng2 gene, encoding the rat CaVγ2, is bidirectional and regulates the transcription of a long non-coding RNA (lncRNA) in the antisense direction. Here, we investigate the proximal promoter region of the human CACNG2 gene in the antisense direction and show that this region includes two functional cAMP response elements that regulate the expression of a lncRNA called CACNG2-DT. The activity of these sites is significantly enhanced by forskolin, an adenylate cyclase activator, and inhibited by H89, a protein kinase A (PKA) antagonist. Therefore, this regulatory mechanism implies the activation of G protein-coupled receptors and downstream phosphorylation. Interestingly, we also found that the expression of CACNG2-DT may increase the levels of the CaVγ2 subunit. Together, these data provide novel information on the organization of the human CACNG2-DT gene promoter, describe modulatory domains and mechanisms that can mediate various regulatory inputs, and provide initial information on the molecular mechanisms that regulate the functional expression of the CaVγ2 protein.

Role of the Ca2+ channel α2δ-1 auxiliary subunit in proliferation and migration of human glioblastoma cells

The overexpression of α2δ-1 is related to the development and degree of malignancy of diverse types of cancer. This protein is an auxiliary subunit of voltage-gated Ca2+ (CaV) channels, whose expression favors the trafficking of the main pore-forming subunit of the channel complex (α1) to the plasma membrane, thereby generating an increase in Ca2+ entry. Interestingly, TLR-4, a protein belonging to the family of toll-like receptors that participate in the inflammatory response and the transcription factor Sp1, have been linked to the progression of glioblastoma multiforme (GBM). Therefore, this report aimed to evaluate the role of the α2δ-1 subunit in the progression of GBM and investigate whether Sp1 regulates its expression after the activation of TLR-4. To this end, the expression of α2δ-1, TLR-4, and Sp1 was assessed in the U87 human glioblastoma cell line, and proliferation and migration assays were conducted using different agonists and antagonists. The actions of α2δ-1 were also investigated using overexpression and knockdown strategies. Initial luciferase assays and Western blot analyses showed that the activation of TLR-4 favors the transcription and expression of α2δ-1, which promoted the proliferation and migration of the U87 cells. Consistent with this, overexpression of α2δ-1, Sp1, and TLR-4 increased cell proliferation and migration, while their knockdown with specific siRNAs abrogated these actions. Our data also suggest that TLR-4-mediated regulation of α2δ-1 expression occurs through the NF-kB signaling pathway. Together, these findings strongly suggest that the activation of TLR-4 increases the expression of α2δ-1 in U87 cells, favoring their proliferative and migratory potential, which might eventually provide a theoretical basis to examine novel biomarkers and molecular targets for the diagnosis and treatment of GBM.

Regulation of the Ca2+ channel α2δ-1 subunit expression by epidermal growth factor via the ERK/ELK-1 signaling pathway

Voltage-gated Ca²⁺ (Ca_V) channels are expressed in endocrine cells where they contribute to hormone secretion. Diverse chemical messengers, including epidermal growth factor (EGF), are known to affect the expression of Ca_V channels. Previous studies have shown that EGF increases Ca²⁺ currents in GH3 pituitary cells by increasing the number of high voltage-activated (HVA) Ca_V channels at the cell membrane, which results in enhanced prolactin (PRL) secretion. However, little is known regarding the mechanisms underlying this regulation. Here, we show that EGF actually increases the expression of the Ca_Vα₂δ-1 subunit, a key molecular component of HVA channels. The analysis of the gene promoter encoding Ca_Vα₂δ-1 (CACNA2D1) revealed binding sites for transcription factors activated by the Ras/Raf/MEK/ERK signaling cascade. Chromatin immunoprecipitation and site-directed mutagenesis showed that ELK-1 is crucial for the transcriptional regulation of CACNA2D1 in response to EGF. Furthermore, we found that EGF increases the membrane expression of Ca_Vα₂δ-1 and that ELK-1 overexpression increases HVA current density, whereas ELK-1 knockdown decreases the functional expression of the channels. Hormone release assays revealed that Ca_Vα₂δ-1 overexpression increases PRL secretion. These results suggest a mechanism for how EGF, by activating the Ras/Raf/MEK/ERK/ELK-1 pathway, may influence the expression of HVA channels and the secretory behavior of pituitary cells.

Transcription Factor Sp1 Regulates the Expression of Calcium Channel α2δ-1 Subunit in Neuropathic Pain

High voltage-activated (HVA) Ca²⁺ (Ca_V) channels are oligomeric complexes formed by an ion-conducting main subunit (Cavα₁) and at least two auxiliary subunits (Cavβ and Ca_Vα₂δ). It has been reported that the expression of Ca_Vα₂δ1 increases in the dorsal root ganglia (DRGs) of animals with mechanical allodynia, and that the transcription factor Sp1 regulates the expression of the auxiliary subunit. Hence, the main aim of this work was to investigate the role of Sp1 as a molecular determinant of the exacerbated expression of Ca_Vα₂δ-1 in the nerve ligation-induced model of mechanical allodynia. Our results show that ligation of L5/L6 spinal nerves (SNL) produced allodynia and increased the expression of Sp1 and Ca_Vα₂δ-1 in the DRGs. Interestingly, intrathecal administration of the Sp1 inhibitor mithramycin A (Mth) prevented allodynia and decreased the expression of Sp1 and Ca_Vα₂δ-1. Likewise, electrophysiological recordings showed that incubation with Mth decreased Ca²⁺ current density in the DRG neurons, acting mostly on HVA channels. These results suggest that L5/L6 SNL produces mechanical allodynia and increases the expression of the transcription factor Sp1 and the subunit Ca_Vα₂δ-1 in the DRGs, while Mth decreases mechanical allodynia and Ca²⁺ currents through HVA channels in sensory neurons by reducing the functional expression of the Ca_Vα₂δ-1 subunit.

Regulation of the voltage-gated Ca2+ channel CaVα2δ-1 subunit expression by the transcription factor Egr-1

It is well known that the Ca_Vα₂δ auxiliary subunit regulates the density of high voltage-activated Ca²⁺ channels in the plasma membrane and that alterations in their functional expression might have implications in the pathophysiology of diverse human diseases such as neuropathic pain. However, little is known concerning the transcriptional regulation of this protein. We previously characterized the promoter of Ca_Vα₂δ, and here we report its regulation by the transcription factor Egr-1. Using the neuroblastoma N1E-115 cells, we found that Egr-1 interacts specifically with its binding site in the promoter, affecting the transcriptional regulation of Ca_Vα₂δ. Overexpression and knockdown analysis of Egr-1 showed significant changes in the transcriptional activity of the Ca_Vα₂δ promoter. Egr-1 also regulated the expression of Ca_Vα₂δ at the level of protein. Also, functional studies showed that Egr-1 knockdown significantly decreases Ca²⁺ currents in dorsal root ganglion (DRG) neurons, while overexpression of the transcription factor increased Ca²⁺ currents in the F11 cell line, a hybrid of DRG and N18TG2 neuroblastoma cells. Studying the effects of Egr-1 on the transcriptional expression of Ca_Vα₂δ could help to understand the regulatory mechanisms of this protein in both health and disease.

Transcriptional regulation of voltage-gated Ca2+ channels

The transcriptional regulation of voltage-gated Ca²⁺ (Ca_V ) channels is an emerging research area that promises to improve our understanding of how many relevant physiological events are shaped in the central nervous system, the skeletal muscle and other tissues. Interestingly, a picture of how transcription of Ca_V channel subunit genes is controlled is evolving with the identification of the promoter regions required for tissue-specific expression and the identification of transcription factors that control their expression. These promoters share several characteristics that include multiple transcriptional start sites, lack of a TATA box and the presence of elements conferring tissue-selective expression. Likewise, changes in Ca_V channel expression occur throughout development, following ischaemia, seizures or chronic drug administration. This review focuses on insights achieved regarding the control of Ca_V channel gene expression. To further understand the complexities of expression and to increase the possibilities of detecting Ca_V channel alterations causing human disease, a deeper knowledge on the structure of the 5' upstream regions of the genes encoding these remarkable proteins will be necessary.

Transcription factor Sp1 regulates T-type Ca(2+) channel CaV 3.1 gene expression

Voltage-gated T-type Ca(2+) (CaV 3) channels mediate a number of physiological events in developing and mature cells, and are implicated in neurological and cardiovascular diseases. In mammals, there are three distinct T-channel genes (CACNA1G, CACNA1H, and CACNA1I) encoding proteins (CaV 3.1-CaV 3.3) that differ in their localization as well as in molecular, biophysical, and pharmacological properties. The CACNA1G is a large gene that contains 38 exons and is localized in chromosome 17q22. Only basic characteristics of the CACNA1G gene promoter region have been investigated classifying it as a TATA-less sequence containing several potential transcription factor-binding motifs. Here, we cloned and characterized a proximal promoter region and initiated the analysis of transcription factors that control CaV 3.1 channel expression using the murine Cacna1g gene as a model. We isolated a ∼1.5 kb 5'-upstream region of Cacna1g and verified its transcriptional activity in the mouse neuroblastoma N1E-115 cell line. In silico analysis revealed that this region possesses a TATA-less minimal promoter that includes two potential transcription start sites and four binding sites for the transcription factor Sp1. The ability of one of these sites to interact with the transcription factor was confirmed by electrophoretic mobility shift assays. Consistent with this, Sp1 over-expression enhanced promoter activity while siRNA-mediated Sp1 silencing significantly decreased the level of CaV 3.1 protein and reduced the amplitude of whole-cell T-type Ca(2+) currents expressed in the N1E-115 cells. These results provide new insights into the molecular mechanisms that control CaV 3.1 channel expression.