The establishment of a novel high-throughput screening system using RNA-guided genome editing to identify chemicals that suppress aldosterone synthase expression

A CRISPR-based high-throughput screening system identifies bromodomain inhibitors as transcriptional suppressors of CYP11B1

CYP11B1 encodes steroid 11β-hydroxylase, the final rate-limiting enzyme for cortisol biosynthesis in the adrenal cortex. Excessive cortisol production is a hallmark of Cushing's disease (CD). While direct enzymatic inhibitors have been explored, achieving specificity remains a challenge due to the high homology between CYP11B1 and CYP11B2, highlighting transcriptional suppression of CYP11B1 as an alternative therapeutic strategy. To identify transcriptional regulators of CYP11B1, we generated genome-edited H295R adrenal cells carrying a luciferase reporter knocked into the endogenous CYP11B1 locus. Using this reporter cell line, we established a high-throughput screening (HTS) platform and screened a focused chemical library targeting epigenetic-related factors, given the importance of epigenetic mechanisms in gene regulation. Among eight candidate compounds identified, we focused on JQ1, a bromodomain inhibitor. JQ1 significantly suppressed Forskolin-induced CYP11B1 promoter activity and mRNA expression without causing cytotoxicity, suggesting the involvement of epigenetic readers in the transcriptional regulation of steroidogenic genes. Furthermore, the reporter-based HTS platform developed here, when combined with our previously established CYP11B2-luciferase system, may facilitate the identification of compounds that selectively modulate adrenal steroidogenic pathways. These findings provide a foundation for the development of novel transcription-targeted therapies for CD.

Calcineurin inhibitor effects on kidney electrolyte handling and blood pressure: tacrolimus versus voclosporin

BACKGROUND

Calcineurin inhibitors (CNIs) affect kidney electrolyte handling and blood pressure (BP) through an effect on the distal tubule. The second-generation CNI voclosporin causes hypomagnesaemia and hypercalciuria less often than tacrolimus. This suggests different effects on the distal tubule, but this has not yet been investigated experimentally.

METHODS

Rats were treated with voclosporin, tacrolimus or vehicle for 28 days. Dosing was based on a pilot experiment to achieve clinically therapeutic concentrations. Drug effects were assessed by electrolyte handling at day 18 and 28, thiazide testing at day 20, telemetric BP recordings and analysis of messenger RNA (mRNA) and protein levels of distal tubular transporters at day 28.

RESULTS

Compared with vehicle, tacrolimus but not voclosporin significantly increased the fractional excretions of calcium (>4-fold), magnesium and chloride (both 1.5-fold) and caused hypomagnesaemia. Tacrolimus but not voclosporin significantly reduced distal tubular transporters at the mRNA and/or protein level, including the sodium-chloride cotransporter, transient receptor melastatin 6, transient receptor potential vanilloid 5, cyclin M2, sodium-calcium exchanger and calbindin-D28K. Tacrolimus but not voclosporin reduced the mRNA level and urinary excretion of epidermal growth factor. The saluretic response to hydrochlorothiazide at day 20 was similar in the voclosporin and vehicle groups, whereas it was lower in the tacrolimus group. The phosphorylated form of the sodium-chloride cotransporter was significantly higher at day 28 in rats treated with voclosporin than in those treated with tacrolimus. Tacrolimus transiently increased BP, whereas voclosporin caused a gradual but persistent increase in BP that was further characterized by high renin, normal aldosterone and low endothelin-1.

CONCLUSIONS

In contrast to tacrolimus, voclosporin does not cause hypercalciuria and hypomagnesaemia, but similarly causes hypertension. Our data reveal differences between the distal tubular effects of tacrolimus and voclosporin and provide a pathophysiological basis for the clinically observed differences between the two CNIs.

Calcineurin inhibitors and the renin-angiotensin-aldosterone system

Calcineurin inhibitors (CnIs) are effective immunosuppressants with decades of accumulated experience in treating immune disorders and, most notably, solid organ transplantation. While CnIs have significantly increased graft survival and transformed the patient standard of care, their use has been overshadowed by a number of undesired side effects. For instance, CnI-associated nephrotoxicity has been reported since early studies and remains a major therapeutic concern. The occurrence of several ion imbalances alongside hypertension was also noted early on, indicating the involvement of the renin-angiotensin-aldosterone system (RAAS) in CnI-mediated toxicity. However, the literature in this field is crowded with conflicting reports from clinical trials as well as studies using animal and invitro models. With this review, we aim to provide a structured and updated overview of the physiological and pathophysiological evidence supporting the involvement of the classical RAAS in CnI-associated toxicity.

Potassium homeostasis - Physiology and pharmacology in a clinical context

Membrane voltage controls the function of excitable cells and is mainly a consequence of the ratio between the extra- and intracellular potassium concentration. Potassium homeostasis is safeguarded by balancing the extra-/intracellular distribution and systemic elimination of potassium to the dietary potassium intake. These processes adjust the plasma potassium concentration between 3.5 and 4.5 mmol/L. Several genetic and acquired diseases but also pharmacological interventions cause dyskalemias that are associated with increased morbidity and mortality. The thresholds at which serum K+ not only associates but also causes increased mortality are hotly debated. We discuss physiologic, pathophysiologic, and pharmacologic aspects of potassium regulation and provide informative case vignettes. Our aim is to help clinicians, epidemiologists, and pharmacologists to understand the complexity of the potassium homeostasis in health and disease and to initiate appropriate treatment strategies in dyskalemic patients.

Calcineurin regulates aldosterone production via dephosphorylation of NFATC4

The mineralocorticoid aldosterone, secreted by the adrenal zona glomerulosa (ZG), is critical for life, maintaining ion homeostasis and blood pressure. Therapeutic inhibition of protein phosphatase 3 (calcineurin, Cn) results in inappropriately low plasma aldosterone levels despite concomitant hyperkalemia and hyperreninemia. We tested the hypothesis that Cn participates in the signal transduction pathway regulating aldosterone synthesis. Inhibition of Cn with tacrolimus abolished the potassium-stimulated (K+-stimulated) expression of aldosterone synthase, encoded by CYP11B2, in the NCI-H295R human adrenocortical cell line as well as ex vivo in mouse and human adrenal tissue. ZG-specific deletion of the regulatory Cn subunit CnB1 diminished Cyp11b2 expression in vivo and disrupted K+-mediated aldosterone synthesis. Phosphoproteomics analysis identified nuclear factor of activated T cells, cytoplasmic 4 (NFATC4), as a target for Cn-mediated dephosphorylation. Deletion of NFATC4 impaired K+-dependent stimulation of CYP11B2 expression and aldosterone production while expression of a constitutively active form of NFATC4 increased expression of CYP11B2 in NCI-H295R cells. Chromatin immunoprecipitation revealed NFATC4 directly regulated CYP11B2 expression. Thus, Cn controls aldosterone production via the Cn/NFATC4 pathway. Inhibition of Cn/NFATC4 signaling may explain low plasma aldosterone levels and hyperkalemia in patients treated with tacrolimus, and the Cn/NFATC4 pathway may provide novel molecular targets to treat primary aldosteronism.