Figure 1.
Overview of the retinoid metabolism in vertebrates
. A. Major pathways in retinoid metabolism. In the intestine, retinoids and carotenoids taken in the diet (from animal and vegetable sources, respectively) are mainly converted to REs, which are incorporated into chylomicrons and taken to the liver trough the lymph and general circulation. In the liver the REs are taken up by hepatocytes and hydrolyzed to ROL which is bound to RBP for transport to storage or target cells. ROL could be stored as REs in liver stellate cells. When required, the ROL is bound to RBP and mobilized from hepatocytes or stellate cells to circulation, where is bound to TTR to avoid glomerular filtration in the kidney and to ensure delivery to target cells. Once in the target cell, free ROL is oxidized to RAL and then to RA, which enters the nucleus and activates RXRs and/or RARs, regulating gene transcription. For a complete description of retinoid metabolism see references [2] and [3].B: Main biochemical routes involved in the retinoid system. (9cRA), 9-cis-retinoic acid; (ADHs) alcohol dehydrogenases (medium-chain dehydrogenases/reductases family); (ALDHs) aldehyde dehydrogenases; (ARAT) acyl-CoA:retinol acyltransferase; (atRA), all-trans-retinoic acid; (BCO I), □, □-carotene-15,15′-monooxygenase; (BCO II), □, □-carotene 9′,10′-dioxygenase; (LRAT), lecithin:retinol acyltransferase; (RA), retinoic acid; (RAL), retinaldehyde; (RARs), retinoic acid receptors; (RBP), retinol binding protein; (REHs), retinyl ester hydrolases; (REs), retinyl esters; (ROL), retinol; (RXRs), retinoid X receptors; (SDRs) short-chain dehydrogenases/reductases; (TTR), transthyretin.
Figure 2.
Photograph of an Osilinus lineatus individual (undetermined gender) taken at Homem de Leme, Porto, Portugal.
Figure 3.
Retinoids in the digestive gland-gonad complex of Osilinus lineatus.
Free retinol, retinyl palmitate and total retinol (free + esterified) levels in digestive gland-gonad complexes and relative mass of the digestive gland-gonad complex in males of Osilinus lineatus in different sexual maturation stages. Values are shown as mean±SEM (n = 4–8). Different letters indicate significant differences among groups (P<0.05, one-way ANOVA followed by the Tukey post-hoc test). na: not available.
Table 1.
Partition of retinol (ROL) among free ROL, retinyl palmitate (RP) or other esters in digestive gland-gonad complexes of male O. lineatus (n = 4–8) in different sexual maturation stages.
Figure 4.
Distribution of retinoids within digestive gland-gonad complex in male Osilinus lineatus.
Free retinol, retinyl palmitate and total (free + esterified) retinol distribution within mature (stage IV-V) male O. lineatus digestive gland-gonad complex. Values represent mean±SEM (n = 3). Asterisks indicate significant differences with respect to levels in gonad (P<0.05, Student’s t-test).
Figure 5.
Nonpolar retinoids in Osilinus lineatus after intramuscular injections with retinol or retinaldehyde.
Free retinol, retinyl palmitate and total (free + esterified) retinol levels in digestive gland-gonad complex of O. lineatus 48 hours after injection with DMSO (control), retinol (4 µg/g body mass) or retinal (4 µg/g body mass). Values represent mean±SEM (n = 5–7). Asterisks indicate significant differences from respective control group (P<0.05, in males: one-way ANOVA followed by the Tukey post-hoc test; in females: one-sample t-test).
Figure 6.
Polar retinoids in Osilinus lineatus after intramuscular injections with retinol or retinaldehyde.
Retinoic acid isomers and total retinoic acid (sum of isomers) in digestive gland-gonad complex of O. lineatus after injection with DMSO (control), retinol (4 µg/g body mass), or retinal (4 µg/g body mass). Values represent mean±SEM (n = 3). Asterisks indicate significant differences from respective control group (P<0.05, one-way ANOVA followed by the Tukey post-hoc test).
Figure 7.
Distribution of retinyl ester storage capacity and canonical/molluskan retinoid pathways
. (A) Phylogenetic distribution of the confirmed presence of retinyl ester storage capacity in different bilaterian groups. The inference of RE in Lophotrocozoa is derived from Osilinus lineatus. (B) Canonical metabolic pathways from retinyl esters (REs) to retinoic acid (RA). Black arrows indicate the metabolic transformations that are known to occur in amniotes. Green arrows indicate the steps that are functional in the mollusk Osilinus lineatus. The red arrow indicates the metabolic transformations that remain to be demonstrated in O. lineatus. (ROL) retinol; (RAL) retinaldehyde.