Cannabinoid receptor signaling regulates liver development and metabolism

Abstract

Endocannabinoid (EC) signaling mediates psychotropic effects and regulates appetite. By contrast, potential roles in organ development and embryonic energy consumption remain unknown. Here, we demonstrate that genetic or chemical inhibition of cannabinoid receptor (Cnr) activity disrupts liver development and metabolic function in zebrafish (Danio rerio), impacting hepatic differentiation, but not endodermal specification: loss of cannabinoid receptor 1 (cnr1) and cnr2 activity leads to smaller livers with fewer hepatocytes, reduced liver-specific gene expression and proliferation. Functional assays reveal abnormal biliary anatomy and lipid handling. Adult cnr2 mutants are susceptible to hepatic steatosis. Metabolomic analysis reveals reduced methionine content in Cnr mutants. Methionine supplementation rescues developmental and metabolic defects in Cnr mutant livers, suggesting a causal relationship between EC signaling, methionine deficiency and impaired liver development. The effect of Cnr on methionine metabolism is regulated by sterol regulatory element-binding transcription factors (Srebfs), as their overexpression rescues Cnr mutant liver phenotypes in a methioninedependent manner. Our work describes a novel developmental role for EC signaling, whereby Cnr-mediated regulation of Srebfs and methionine metabolism impacts liver development and function.

Role of the endocannabinoid system in vertebrates: Emphasis on the zebrafish model

“…4A). These data were consistent with in situ hybridization analysis (A) (B) (C) (Liu et al 2016). Loss of CB1 and CB2 function results in a smaller liver with less hepatocytes and reduced expression of liver-specific genes in zebrafish.…”

“…(B) Mutation of cb1 and cb2 impair liver development and function (Liu et al . ). Loss of CB 1 and CB 2 function results in a smaller liver with less hepatocytes and reduced expression of liver‐specific genes in zebrafish.…”

“…Liu et al (2016) reported the importance of the eCBs in the liver; cb1 and cb2 double mutant fish have impaired liver development and function. Specifically the authors showed that inhibition of CB receptor activity disrupts liver development and metabolic function in zebrafish, impacting hepatic differentiation and liver size due to fewer hepatocytes and reduced liver-specific gene expression and proliferation (Liu et al 2016). In contrast, when cb1 is overexpressed in zebrafish liver, the expression levels of genes involved in the fatty acid production, transport, and storage are consequently increased, resulting in hepatosteatosis (Pai et al 2013).…”

CB 1 and CB 2 Receptor Pharmacology

“…The deficits induced by cannabinoid receptor deficiency were the result of reduced sterol regulatory element-binding transcription factor(s) (Srebf) expression that persists into adulthood (Jeong et al, 2008; Pai et al, 2013). The decreased Srebf led to reduced methionine pathway intermediates, findings that were also observed in livers from CB 1 −/− mice (Liu et al, 2016). This resulted in a generalized pattern of reduced methylation of proteins (Liu et al, 2016), implicating a reduction in S-adenosylmethionine as a methyl donor for nucleic acid, phospholipid, and protein methylation critical for epigenetic regulation.…”

“…The decreased Srebf led to reduced methionine pathway intermediates, findings that were also observed in livers from CB 1 −/− mice (Liu et al, 2016). This resulted in a generalized pattern of reduced methylation of proteins (Liu et al, 2016), implicating a reduction in S-adenosylmethionine as a methyl donor for nucleic acid, phospholipid, and protein methylation critical for epigenetic regulation. The aberrant hepatogenesis could be overcome by overexpression of Srebf1 during development, but not entirely overcome by methionine replacement, suggesting that cannabinoid receptors and Srebf exert additional developmental regulatory functions not involving methylation.…”