Isolation and functional characterization of the 5'-upstream region of mouse P/Q-type Ca2+ channel alpha1A subunit gene

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.

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.

LIM homeobox 8 (Lhx8) is a key regulator of the cholinergic neuronal function via a tropomyosin receptor kinase A (TrkA)-mediated positive feedback loop

Basal forebrain cholinergic neurons play an important role in cognitive functions such as learning and memory, and they are affected in several neurodegenerative diseases, including Alzheimer disease and Down syndrome. Despite their functional importance, the molecular mechanisms of functional maturation and maintenance of these cholinergic neurons after the differentiation stage have not been fully elucidated. This study demonstrates that the LIM homeobox 8 (Lhx8) transcription factor regulates cholinergic function in rat septal cholinergic neurons in primary cultures from E18.5 embryos and in the adult brain. Lhx8 expression modulated tropomyosin receptor kinase A (TrkA) expression in septal cholinergic neurons in vitro and in vivo, resulting in regulated acetylcholine release as an index of cholinergic function. In addition, Lhx8 expression and function were regulated by nerve growth factor (NGF), and the effect of NGF was potentiated by Lhx8-induced TrkA expression. Together, our findings suggest that positive feedback regulation between Lhx8, TrkA, and NGF is an important regulatory mechanism for cholinergic functions of the septum.

Identification of the REST regulon reveals extensive transposable element-mediated binding site duplication

The genome-wide mapping of gene-regulatory motifs remains a major goal that will facilitate the modelling of gene-regulatory networks and their evolution. The repressor element 1 is a long, conserved transcription factor-binding site which recruits the transcriptional repressor REST to numerous neuron-specific target genes. REST plays important roles in multiple biological processes and disease states. To map RE1 sites and target genes, we created a position specific scoring matrix representing the RE1 and used it to search the human and mouse genomes. We identified 1301 and 997 RE1s inhuman and mouse genomes, respectively, of which >40% are novel. By employing an ontological analysis we show that REST target genes are significantly enriched in a number of functional classes. Taking the novel REST target gene CACNA1A as an experimental model, we show that it can be regulated by multiple RE1s of different binding affinities, which are only partially conserved between human and mouse. A novel BLAST methodology indicated that many RE1s belong to closely related families. Most of these sequences are associated with transposable elements, leading us to propose that transposon-mediated duplication and insertion of RE1s has led to the acquisition of novel target genes by REST during evolution.

Expression pattern of voltage-dependent calcium channel alpha1 and beta subunits in adrenal gland of N-type Ca2+ channel alpha1B subunit gene-deficient mice

The Ca2+ channel alpha1B subunit is a pore-forming component capable of generating N-type Ca2+ channel activity. Although N-type Ca2+ channel plays a role in a variety of neuronal functions, alpha1B-deficient mice exhibit normal life span without apparent abnormalities of behavior, histology or plasma norepinephrine level, presumably owing to compensation by some other Ca2+ channel alpha1 or beta subunit. In this study, we studied the levels of alpha1A, alpha1C, alpha1D, C1E, beta1, beta2, beta3 and beta4 mRNAs in adrenal gland of alpha1B-deficient mice. The alpha1A mRNA in homozygous mice was expressed at higher level than in wild or heterozygous mice, but no difference in the expression levels of alpha1c, alpha1D, alpha1E, beta1, beta2, beta3 and beta4 was found among wild, heterozygous and homozygous mice. The protein level of alpha1A in homozygous mice was also expressed at higher level than in wild or heterozygous mice. To examine whether increased expression is induced by cis-regulatory element within 5'-upstream region of alpha1A gene, we examined lacZ expression in alpha1B-deficient x alpha1A6.3-lacZ mice (carrying a 6.3-kb 5'-upstream fragment of alpha1A gene fused to E. coli lacZ reporter gene), which express lacZ in medullar chromaffin cells, but not in cortex. The levels of lacZ expression in homozygous alpha1B-deficient x alpha1A6.3-lacZ mice were higher than in wild or heterozygous mice. Therefore, a possible explanation of the normal behavior and plasma norepinephrine level of alpha1B-deficient mice is that compensation by alpha1A subunit occurs and that 6.3-kb 5'-upstream region of alpha1A gene contains enhancer cis-element(s) for compensation in adrenal medulla chromaffin cells.

Regulation of alpha1G T-type calcium channel gene (CACNA1G) expression during neuronal differentiation

Down-regulation of T-type Ca channel current and mRNA occurs following differentiation of Y79 retinoblastoma cells. To understand how the decrease in expression is linked to cell differentiation, we examined transcriptional regulation of the Cav3.1 Ca channel gene, CACNA1G. We identified two putative promoters (A and B) in 1.3 kb of cloned genomic DNA. Reverse transcriptase-polymerase chain reaction and 5' rapid amplification of cDNA ends-polymerase chain reaction analyses demonstrated that two transcripts with different 5' untranslated regions are generated by different transcription start sites, with promoter A favoured in undifferentiated cells and promoter B favoured in differentiated cells. Functional analyses of the promoter sequence revealed that both promoters are active. Enhancer and repressor sequences were identified upstream of promoter A and B, respectively. These results suggest that the down-regulation of alpha1G mRNA in differentiated Y79 cells is mediated primarily by decreased activity of promoter A, which could occur in conjunction with repression of the activity of promoter B. The decrease in T-type Ca channel expression in Y79 cells may be an essential signal affecting phenotypic maturation and expression of other ion channel subtypes in the differentiated cells.

Charge movement and transcription regulation of L-type calcium channel alpha(1S) in skeletal muscle cells

Several factors, such as Ca(2+), trophic factors and ageing, regulate dihydropyridine-sensitive receptor (DHPR) alpha(1) subunit expression. However, basic mechanisms of DHPR alpha(1S) expression are unknown. To better understand the regulatory elements that control transcription, the 1.2 kb 5'-flanking region fragment immediately upstream of the mouse L-type Ca(2+) channel or DHPR alpha(1S) gene was isolated and sequenced. Luciferase reporter constructs driven by different promoter regions of mouse DHPR alpha(1S) gene were used for transient transfection assays in muscle C2C12 cells. In these preparations we found that three regions corresponding to CREB, GATA-2 and SOX-5 consensus sequence within the 5'-flanking region of the DHPR alpha(1S) gene are important for DHPR alpha(1S) gene transcription. Antisense oligonucleotides against CREB, GATA-2 and SOX-5 significantly reduced charge movement in C2C12 cells. Charge movement was recorded in the whole-cell configuration of the patch clamp technique. Results from cells transfected with antisense (AS) and sense (S) oligonucleotides and nontransfected cells were compared. Charge movement experiments were fitted to a Boltzmann equation. Maximum charge movement (Q(max)) (nC microF(-1), mean +/- S.E.M.) for S- and AS-CREB was 70.3 +/- 2.9 and 52.8 +/- 3.3, respectively (P < 0.05). The same parameter for S- and AS-GATA-2 was 71.3 +/- 3.9 and 48.2 +/- 2.3, respectively (P < 0.05) and for S- and AS-SOX-5 was 70.4 +/- 4.2 and 45.1 +/- 3.2, respectively (P < 0.05). Values recorded in cells transfected with sense S-CREB, S-GATA-2 and S-SOX-5 oligonucleotides were not significantly different from those recorded in nontransfected cells. This study demonstrates that the transcription factors CREB, GATA-2 and SOX-5 play a significant role in the expression of the skeletal muscle DHPR or L-type Ca(2+) channel alpha(1S).

Novel Cav2.1 splice variants isolated from Purkinje cells do not generate P-type Ca2+ current

The alpha(1)2.1 (alpha(1A)) subunits of P-type and Q-type Ca(2+) channels are encoded by a single gene, Cacna1a. Although these channels differ in the inactivation kinetics and sensitivity to omega-agatoxin IVA, the mechanism underlying these differences remains to be clarified. Alternative splicings of the Cacna1a transcript have been postulated to contribute to the respective properties, however, the splice variants responsible for P-type Ca(2+) channels have not been identified. To explore P-type-specific splice variants, we aimed at cloning alpha(1)2.1 from isolated mouse Purkinje cells using single-cell reverse transcription-PCR, because in Purkinje cells P-type currents dominate over the whole currents (>95%) with Q-type currents undetected. As a result, two novel splice variants were cloned. Compared with the previously cloned mouse alpha(1)2.1, two novel variants had additional 48 amino acids at the amino termini, six single amino acid changes, and splicing variations at the exon 46/47 boundary, which produced different carboxyl termini. Furthermore, one variant had one RNA editing site. However, electrophysiological and pharmacological studies indicated that these variants did not generate P-type current in cultured cells. These results suggest that P-type-specific splice variants may exist but that post-translational processing or modification by uncharacterized interacting proteins is also required for generating the P-type current.

Expression analysis of the 5'-upstream region of mouse P/Q-type Ca(2+) channel alpha( lA) subunit gene fused to Escherichia coli lacZ reporter gene in the spinal cord using transgenic mice

The P/Q-type Ca(2+) channel alpha(1A) subunit is expressed in spinal cord including ventral motor neurons and interneurons and dorsal horn. To identify the transcriptional mechanisms of the mouse alpha(IA) subunit gene in spinal cord, transgenic mice carrying a 0.5, 1.5, 3.0 or 6.3-kb 5'-upstream region fused to the Escherichia coli lacZ reporter gene were examined. Transgenic mice carrying the 3.0-kb region expressed the reporter gene in dorsal horn and interneurons of ventral horn, although those with the 0.5-kb, 1.5-kb or 6.3-kb region did not. No transgenic mice expressed the reporter gene in motor neurons of ventral horn. These results suggest that in spinal cord, the expression mechanisms of the alpha(1A) subunit gene are complex, involving both positive and negative cis-regulatory elements, and the 6.3-kb 5'-upstream region alone is not sufficient for the expression.

Expression analysis of Escherichia coli lacZ reporter gene in transgenic mice

To define a gene expression mechanism, it is often advantageous to use a reporter gene and transgenic mouse. The lacZ reporter gene is particularly useful for studies of the cis-regulatory element for tissue-specific expression in transgenic mice because of the ease of the enzyme assay and visualization on sections. In this report, we describe our method for examining the cis-regulatory element in transgenic mice, including choice of the lacZ gene, generation of transgenic mice, and analysis of beta-galactosidase activity.

Neuron-specific expression of reporter gene in transgenic mice carrying the 5'-upstream region of mouse P/Q-type Ca2+ channel alpha 1A subunit gene fused to E. coli lacZ reporter gene

To dissect the molecular mechanisms underlying the neuron-specific expression of the P/Q type calcium channel alpha 1A subunit gene, transgenic mice carrying a 0.5-kb, 1.5-kb, 3.0-kb or 6.3-kb 5'-upstream region of the gene fused to Escherichia coli lacZ reporter gene were produced. In transgenic mice carrying the 1.5-kb, 3.0-kb or 6.3-kb 5'-upstream region, the reporter gene was exclusively expressed in the nervous system, although those with the 0.5-kb 5'-upstream region failed to show reporter expression. Histological examinations showed that the three 5'-upstream regions induced distinct expression patterns of the reporter gene in the CNS and adrenal medulla. The 1.5-kb 5'-upstream region drove reporter gene expression in the olfactory bulb, dorsal cortex and hippocampus, while the regulatory element for the expression in the amygdaloid nucleus, septum, habenula medial nucleus, choroid plexus, substantia nigra, inferior colliculus, pontine nucleus and cerebellum was located in the 5'-upstream sequence between 1.5 kb and 6.3 kb. In the cerebellum, the expression of the reporter gene was induced by the 3.0-kb region in granule cells, whereas it was induced by the 6.3-kb region in Purkinje cells. The expression of the reporter gene in chromaffin cells in the adrenal medulla was induced only by the 6.3-kb 5'-upstream region. These results suggest that the expression of the mouse P/Q-type Ca2+ channel alpha 1A subunit gene is regulated in a complex fashion by both positive and negative cis-regulatory elements.