Excessive feedback of Cyp26a1 promotes cell non-autonomous loss of retinoic acid signaling

Abstract

Teratogenic levels of retinoic acid (RA) signaling can cause seemingly contradictory phenotypes indicative of both increases and decreases of RA signaling. However, the mechanisms underlying these contradictory phenotypes are not completely understood. Here, we report that using a hyperactive RA receptor to enhance RA signaling in zebrafish embryos leads to defects associated with gain and loss of RA signaling. While the gain-of-function phenotypes arise from an initial increase in RA signaling, using genetic epistasis analysis we found that the loss-of-function phenotypes result from a clearing of embryonic RA that requires a rapid and dramatic increase in cyp26a1 expression. Thus, the sensitivity of cyp26a1 expression to increased RA signaling causes an overcompensation of negative feedback and loss of embryonic RA signaling. Additionally, we used blastula transplantation experiments to test if Cyp26a1, despite its cellular localization, can limit RA exposure to neighboring cells. We find that enhanced Cyp26a1 expression limits RA signaling in the local environment, thus providing the first direct evidence that Cyp26 enzymes can have cell non-autonomous consequences on RA levels within tissues. Therefore, our results provide novel insights into the teratogenic mechanisms of RA signaling and the cellular mechanisms by which Cyp26a1 expression can shape a RA gradient.

Cyp26 Enzymes Facilitate Second Heart Field Progenitor Addition and Maintenance of Ventricular Integrity

“…Nonspecific MO-induced cell death was counteracted with 2 ng p53 MO for all injections. The cyp26a1 mRNA was injected at 500 pg as previously described [79,80]. …”

Alterations in retinoic acid signaling affect the development of the mouse coronary vasculature

“…As a result, altered RA-signaling can induce a series of defects related to looping, outflow tract and chamber formation (Lammer et al, 1985; Lee et al, 1997; Yasui et al, 1997; Kolodzinska et al, 2013). Paradoxically, excess and deficiency of RA can often result in similar defects, due to either compensatory mechanisms or simply because both excess or deficiency of RA ultimately result in impaired development (Frenz et al, 2010; Lee et al, 2012; Rydeen et al, 2015)…”

Lineage-specific duplication of amphioxus retinoic acid degrading enzymes (CYP26) resulted in sub-functionalization of patterning and homeostatic roles

“…In vertebrates, normal development requires a very tightly controlled balance of the total amount of available RA, which is maintained through positive and negative feedback loops associated, respectively, with RA production (chiefly by RALDH1, 2, and 3, for retinaldehyde dehydrogenase 1, 2, and 3) and RA degradation (chiefly by CYP26A1, B1, and C1, for cytochrome P450 subfamily 26A1, B1, and C1) [8–12]. The biological response to endogenous RA, in turn, is mediated by heterodimers of two nuclear receptors, the retinoic acid receptor (RAR) and the retinoid X receptor (RXR), with the expression levels of RAR in particular being tightly linked to the availability of RA [1, 3, 4].…”