Smith-Magenis syndrome results in disruption of CLOCK gene transcription and reveals an integral role for RAI1 in the maintenance of circadian rhythmicity.

Haploinsufficiency of RAI1 results in Smith-Magenis syndrome (SMS), a disorder characterized by intellectual disability, multiple congenital anomalies, obesity, neurobehavioral abnormalities, and a disrupted circadian sleep-wake pattern. An inverted melatonin rhythm (i.e., melatonin peaks during the day instead of at night) and associated sleep-phase disturbances in individuals with SMS, as well as a short-period circadian rhythm in mice with a chromosomal deletion of Rai1, support SMS as a circadian-rhythm-dysfunction disorder. However, the molecular cause of the circadian defect in SMS has not been described. The circadian oscillator temporally orchestrates metabolism, physiology, and behavior largely through transcriptional modulation. Data support RAI1 as a transcriptional regulator, but the genes it might regulate are largely unknown. Investigation into the role that RAI1 plays in the regulation of gene transcription and circadian maintenance revealed that RAI1 regulates the transcription of circadian locomotor output cycles kaput (CLOCK), a key component of the mammalian circadian oscillator that transcriptionally regulates many critical circadian genes. Data further show that haploinsufficiency of RAI1 and Rai1 in SMS fibroblasts and the mouse hypothalamus, respectively, results in the transcriptional dysregulation of the circadian clock and causes altered expression and regulation of multiple circadian genes, including PER2, PER3, CRY1, BMAL1, and others. These data suggest that heterozygous mutation of RAI1 and Rai1 leads to a disrupted circadian rhythm and thus results in an abnormal sleep-wake cycle, which can contribute to an abnormal feeding pattern and dependent cognitive performance. Finally, we conclude that RAI1 is a positive transcriptional regulator of CLOCK, pinpointing a novel and important role for this gene in the circadian oscillator.

The renal H+, K+-ATPases as therapeutic targets.

The kidney is an important regulatory organ responsible for maintaining constant blood volume and composition despite wide variations in the intake of food and water. Throughout the nephron, the functional unit of the kidney, there is a wide variety of proteins that function to add additional waste products and to recover needed materials from the lumen filtrate. The collecting duct of the nephron is the primary renal location for the H+, K+-ATPases, a group of ion pumps that function in both acid/base balance and potassium homeostasis. This review summarizes the present understanding of the structure and functions for the different subtypes of the H+, K+-ATPases under specific physiologic conditions. The obstacles in determining the pharmacologic properties of the different subtypes are considered and future directions for the inhibition and/or stimulation of the H+, K+-ATPases are evaluated.

Characterization of the rabbit HKalpha2 gene promoter.

The HKalpha2 gene directs synthesis of the HKalpha2 subunit of the H(+), K(+)-ATPase. In the kidney and colon, the gene is highly expressed and is thought to play a role in potassium (K(+)) conservation. The rabbit has been an important experimental system for physiological studies of ion transport in the kidney, so the rabbit HKalpha2 gene has been cloned and characterized. The genomic clones and the previously reported HKalpha2a and HKalpha2c subunit cDNAs provided a means to address several issues regarding the structure and expression of the HKalpha2 gene. First, the genomic organization established that the rabbit HKalpha2 gene was unambiguously homologous to the mouse HKalpha2 gene and the human ATP1AL1 gene. Second, the mapping of the transcription start site for the alternate transcript, HKalpha2c, confirmed that it was an authentic rabbit transcript. Finally, isolation of DNA from the 5' end of the HKalpha2 gene enabled us to initiate studies on its regulation in the rabbit cortical collecting duct. The promoter and two putative negative regulatory regions were identified and the effect of cell confluency on gene expression was studied.

Molecular regulation of the HKalpha2 subunit of the H+,K(+)-ATPases.

H+, K(+)-Adenosine triphosphatases (H+, K(+)-ATPase) located in the mammalian kidney participate in maintaining ion and acid/base balance. The molecular mechanisms by which physiological conditions lead to an upregulation of H+, K(+)-ATPase activity are poorly understood. However, studies in recent years have provided tangible progress towards understanding the role of the renal H+, K(+)-ATPases. The cloning of cDNAs from the kidneys of several mammalian species has provided evidence for the presence of several H+, K(+)-ATPase isoforms. The different H+, K(+)-ATPases may make distinct contributions to ion and acid/base balance. Studies of mRNA, protein and H+, K(+)ATPase activity levels in the kidney under a variety of physiological conditions have revealed that the HKalpha2 (colonic) subunit of the H+, K(+)ATPase is highly regulated. The pump is responsive to potassium, sodium and probably hormones. Recent developments have focused on defining the HKa2 gene promoter. Analysis of the 5' end of the HKalpha2 gene from human, rat and rabbit has identified conserved elements that may serve as core promoter and regulatory elements. This review will summarize recent data related to the molecular regulation of the HKalpha2 subunit of the H+, K(+)-ATPase.