Chronic kidney disease associated heart disease:
Clarification of mechanism and expectations for new therapeutic agents

Approximately 13% (13.3 million people) of the adult Japanese population is estimated to have chronic kidney disease (CKD), of which approximately 340,000 are undergoing dialysis. Of these patients undergoing dialysis, around 30% die of heart disease; however, the underlying risk factors are unknown. The research group of Professor Shigehiro Ohdo and Assistant Professor Yuya Yoshida from the Graduate School of Pharmaceutical Sciences, Kyushu University, in collaboration with Professor Naoya Matsunaga and Professor Satoru Koyanagi and colleagues, clarified how cardiac inflammation and fibrosis during CKD may be exacerbated by circadian clock-induced changes in vitamin A and monocytes. "Previously unrecognized high expression of clock genes in monocytes leads to heart disease," according to Dr. Ohdo. "We are also screening this gene as a potential target for drug discovery, and we hope to connect these efforts to new applications and therapeutic agents." Their findings were published in Nature Communications.

Many organisms have biological clocks, which regulate various functions in their bodies. These functions are rhythmicity controlled by multiple clock genes that function in a sequential manner. For example, sympathetic nerves are activated during the day; parasympathetic nerves, at night; and blood leukocytes double or triple, at night. Molecular clocks are composed of approximately 20 clock genes and function in 24-h cycles. Attempts have been made to use this rhythm for treating diseases.

Professor Ohdo has been studying the association between biological clocks and diseases for 30 years. In 2016, Professor Matsunaga et al. demonstrated that CKD progression was accompanied by an increased expression of the renal profibrotic factor TGF-β1 and altered expression of hepatic clock genes and metabolic enzymatic CYPs, resulting in abnormal retinol metabolism and further deterioration of renal function by accumulated retinol. In brief, through the internal clock mechanism, an inter-organ network unique to the disease state is formed. Both researchers seemed to be concerned about cardiac insufficiency in CKD. They analyzed the mechanisms underlying heart disease deterioration using mouse mutations of the CLOCK gene as clues.

First, in the CKD-model mice, the kidneys became fibrotic, but knocking out CLOCK stopped the occurrence of fibrosis. Simultaneously, the heart failure marker (BNP) in the blood was also significantly reduced. It was also found that the hearts of CKD-model mice were infiltrated with monocytes expressing high protein GPR68 levels, and the rhythmic expression of the GBR68 gene was disrupted in CKD-model mice. The researchers investigated the cause of this abnormality.

They focused on vitamin A (retinol) and found that GPR68 expression also increased when mouse or human monocytes were cultured in high concentrations of vitamin A. Thus, vitamin A accumulation in the body induced GPR68 expression via monocyte clock genes. Thus, knocking out the expression of GPR68 in the monocytes of CKD-model mice suppressed heart failure, even with restricted vitamin A intake. The examination of BNP (heart failure marker) levels in actual patients with CKD showed that they could be classified into three groups: extremely low level, requiring observation, and requiring treatment. Serum GPR68 expression was higher in patients at a high risk. The same was true for retinol levels.

Professor Ohdo reported that "It is not practical to continue a diet without vitamin A for a prolonged period of time. Therefore, I thought that a therapeutic drug targeting GPR68 can be developed. When screening for approved drugs at the Kyushu University Green Pharma Research Institute, some approved drugs were found to have a GPR68 inhibitory effect. However, pharmaceutical companies do not consider drug repositioning as very advantageous; thus, we are also designing new therapeutic agents." "It is difficult to target clock genes because drug treatments necessarily involve some time lags," according to Professor Matsunaga. "Therefore, we are also developing therapeutic devices that directly affect the coordination of clock genes by external electrical stimulation. Within a few years, we wish to establish a venture company."