An Examination of the Putative Role of Melatonin in Exosome Biogenesis

Abstract

During the last two decades, melatonin has been found to have pleiotropic effects via different mechanisms on its target cells. Data are abundant for some aspects of the signaling pathways within cells while other casual mechanisms have not been adequately addressed. From an evolutionary perspective, eukaryotic cells are equipped with a set of interrelated endomembrane systems consisting of intracellular organelles and secretory vesicles. Of these, exosomes are touted as cargo-laden secretory vesicles that originate from the endosomal multivesicular machinery which participate in a mutual cross-talk at different cellular interfaces. It has been documented that cells transfer various biomolecules and genetic elements through exosomes to sites remote from the original cell in a paracrine manner. Findings related to the molecular mechanisms between melatonin and exosomal biogenesis and cargo sorting are the subject of the current review. The clarification of the interplay between melatonin and exosome biogenesis and cargo sorting at the molecular level will help to define a cell's secretion capacity. This review precisely addresses the role and potential significance of melatonin in determining the efflux capacity of cells via the exosomal pathway. Certain cells, for example, stem cells actively increase exosome efflux in response to melatonin treatment which accelerates tissue regeneration after transplantation into the injured sites.

Keywords: cross talk; exosome biogenesis; interplay signaling pathways; melatonin; paracrine activity.

FIGURE 1

Neural control of melatonin synthesis in the pineal gland (A). Melatonin, a well-known sleep-promoting molecule, is produced in the pineal gland as summarized in this figure. The secretion of melatonin is tightly controlled by suprachiasmatic and paraventricular nuclei in response to the light: dark cycle and the circadian clock mechanisms of the suprachiasmatic nuclei. The peak secretion of melatonin occurs at night time and reaches minimum levels at day. The synthesis of melatonin is a result of the enzymatic conversion of tryptophan to melatonin via the intermediate, molecule, serotonin (B).

FIGURE 2

The multi-step intracellular maturation of exosomes is illustrated. For the initial internalization via endocytosis, GTPases including Rab5 and Rab21 play a significant role in the formation of early endosomes. The formation of late endosomes is done via engaging other important factors such as ceramide, ALIX, Rab7, 35, tetraspanins, and ESCRT complex. Due to the activity of other Rab types (Rab9 and 24), later endosomes are directed to lysosomes. Alternatively, late endosomes become MVBs. The invagination of the MVBs membrane forms numerous ILVs. Upon the release of ILVs to the ECM, they are thereafter referred to as exosomes. Trans-Golgi apparatus further supports an alternative way to form MVBs by the activity of certain GTPases such as Rab9. The fusion of later MVBs to the cell membrane is mediated by different effectors including Rab7, 11, 27, 25, 37, SNAREs, and other elements. ILVs, intraluminal vesicles; MVBs, multivesicular bodies; ESCRT, endosomal sorting complex required for transport; SNARE, soluble N-ethylmaleimide-sensitive factor attachment protein receptors.

FIGURE 3

Melatonin signaling pathways are summarized in this figure. Both MT1 and MT2 belong to the GPCRs family of receptors. Melatonin also crosses the plasma membrane via passive diffusion. Upon the attachment of melatonin to MT1 and MT2, different effectors including PKC, PLCβ, and PKA are recruited as second messengers to trigger downstream signaling pathways. For subsequent events, the melatonin-MT1 complex linked α_q, α_i, β, and γ subunits to activate IP3 and intracellular accumulation of Ca⁺² after its release from the endoplasmic reticulum. Ca⁺² phosphorylates both PKC and ERK1/2. MT1 receptor also acts through membrane adenyl cyclase, which blocks the phosphorylation of CREB by activation of cAMP and PKC. Melatonin binding to MT2 activates the adenyl cyclase and leads to CREB inactivation. Melatonin promotes conformational changes in MT2 and activation of α_i subunit leads to stimulation of PKG via guanylate cyclase. Also, MT2 engages PKC and ErK1/2 complexes. Melatonin crosses the plasma membrane via passive diffusion and transporters and activates both mitochondrial MT1 and 2 thereby reducing the escape of cytochrome C into the cytosol. Other possible melatonin receptors include cytosolic quinone reductase 2 which activates nuclear RORα/RZR. Mel, melatonin; cAMP, cyclic adenosine monophosphate; cGMP, cyclic guanosine monophosphate; MT, melatonin receptor; PKC, protein kinase C; PKA, protein kinase A; CREB, cAMP response element-binding protein; IP3, inositol trisphosphate; and PDK, pyruvate dehydrogenase kinase.

FIGURE 4

Several aspects of melatonin’s effect on exosome biogenesis and secretion are still unknown. Melatonin can enter the cells via membrane-bound receptors and passive diffusion. It is logical to state that melatonin can be secret via the exosomes to the extracellular niche. Whether melatonin directly or indirectly influences exosome biogenesis, trafficking and abscission need further investigation.

FIGURE 5

The multiple possible means of molecular cross-talk between melatonin and exosome synthesis machinery are summarized here. Irrespective of melatonin how melatonin enters cells, it activates PI3K/Akt complex and inhibits GSK-3β, leading to fusion of secretory vesicles with plasma membrane and exocytosis. However, the underlying mechanisms have not been fully established. Changes in plasma membrane fluidity by an alteration of fatty acid composition (unsaturated/saturate fatty acid ratio) facilitate the exosome release. Also, melatonin promotes specific GTPases including Rab7 and 11 which accelerate the fusion of late MVBs with the plasma membrane. Upon binding of melatonin to MT1, the melatonin-MT1 complex is transferred to the early endosomes via vacuolar sorting machinery such as Rab5. The activation of Rab7 and 11 promotes endosomal secretion and recycles MT1 to the plasma membrane. Further activation of Rab7 directs endosomes with MT1 toward lysosomes which regulate the innate cell response to melatonin. How Rab 7 determines the fate of endosomes with melatonin (secretion or enzymatic digestion) needs further explorations. The interplay between melatonin and autophagy signaling pathway also influences the activity of exosome molecular machinery. Melatonin stimulates autophagic response directly via the activation of ATG4, 5, 7, 10, 12, and 16 and increases the LC3II/I ratio. These features promote the formation of autophagolysosomes and autophagic exocytosis. Melatonin enhances the autophagic machinery including LC3, ATG3, 5, 12, and 16L on the late endosome membrane that facilitates the fusion of these elements with the plasma membrane. Melatonin also increases the intersection of autophagic vacuoles and exosomes into compartments named amphisomes which have multiple fates. Amphisomes can fuse with lysosomes via the activation of P62. The activity of Rab8 and 27a initiates the autophagic release of amphisomes to the ECM. GSK-3, glycogen synthase kinase 3; ATG, autophagy-related protein; P62, sequestosome 1; LC3, microtubule-associated protein 1A/1B-light chain 3; and PI3K/Akt, Phosphatidylinositol-3-kinase/Protein kinase B.

FIGURE 6

Bioinformatics Analysis of Gene Expression Profiles of Wnt and melatonin signaling pathways with exosome biogenesis using NetworkAnalyst version 3. Data revealed shared Pax2/TLE4 interaction between Wnt, melatonin signaling pathway with exosome biogenesis.