Russel J Reiter, Department of Cell Systems and Anatomy, UT Health San Antonio, Texas, USA

Sergio Rosales-Corral, Centro de Investigacion Biomedica de Occidente, Instituto Mexicano del Seguro Social, Guadalajara, Mexico

https://doi.org/10.36866/pn.113.43

In 1958, when Aaron Lerner isolated and characterised melatonin from bovine pineal tissue, he hoped it would be a treatment for vitiligo – a disease which causes loss of skin colour. When this was proven not to be the case, Lerner almost abandoned research on the molecule. Since then, however, due to the efforts of a troop of inquisitive scientists, a bewildering array of functions have been ascribed to this bewitching molecule. Melatonin predictably evolved several billion years ago in the earliest life forms, bacteria, for protection against toxic chemically reduced oxygen species. Since its creation, melatonin seems to have been preserved in every living organism, both animal and plant.

During its long evolutionary history, melatonin has insinuated itself into many aspects of organ and organismal physiology to the extent that its “fingerprints” are all over subcellular infrastructure. Indeed, it seems possible that cells cannot maintain a healthy lifestyle if melatonin is not present. As melatonin has been loitering in cells for a very long evolutionary period, it also has taken advantage of the opportunities to “learn” to functionally cooperate with other beneficial molecules. Subsequently, one feature that characterises melatonin’s actions is diversification.

Melatonin: it’s not all in your head

Since melatonin was discovered in the pineal gland, for years it was assumed that this organ was the exclusive source of this serotonin derivative. In the pineal gland of mammals, melatonin is produced in a cyclic manner such that only at night does the gland manufacture significant amounts of this indoleamine. The nocturnal synthesis and release of melatonin is controlled by the master circadian oscillator, the suprachiasmatic nucleus (SCN), whose output is dictated by information it receives about the light-dark environment as detected by highly specialised melanopsin-expressing, intrinsically photoreceptive retinal ganglion cells (ipRGCs) in the eyes.

Unlike many endocrine organs, the pineal gland does not selfishly store any of the product it generates; rather it quickly liberates it into the circulatory and ventricular systems. At night, blood concentrations and especially concentrations in the third ventricular cerebrospinal fluid (CSF) are much higher than daytime values. As a consequence, every cell in an organism that is contacted by blood or CSF is apprised of the light:dark status and can adjust its physiology accordingly, which many do. Within 15 years of Lerner and colleagues (1958) identifying melatonin in the pineal gland, its synthesis was also uncovered in the retina, a neurally-derived tissue. The production of melatonin at this site may not be so extraordinary since the vertebrate eyes and pineal gland have a number of common features. Even in present day vertebrates, remnants of retinal rods and cones are ultrastructurally present in the pineal gland.

Beginning in the 1980s and continuing to the present, high melatonin levels, likely due to its local production, have been identified in a plethora of tissues/cells. Based on these data and immunocytochemical findings related to the intracellular location of a melatonin-forming enzyme, we proposed that melatonin is synthesised in the mitochondria of all cells (Tan et al., 2013). We argued that this is consistent with the Endosymbiotic Theory for the origin of mitochondria. Thus, mitochondria evolved from proteobacteria that were engulfed by early prokaryotic cells for their nutritional value. Eventually, the phagocytosed bacteria, which produce melatonin, established a symbiotic association with the cells that ingested them and they became mitochondria. During this evolutionary process, the evolved organelle retained its melatonin synthesising capability. If valid, every cell in multicellular organisms may produce melatonin. Recent findings which identified melatonin-forming enzymes in extrapineal cells is compatible with this theory (Suofa et al., 2017). Also, melatonin production occurs in the mammalian oocyte mitochondria (He et al., 2016). Considering that mitochondria of all cells are derived from the oocyte, it is likely that the melatonin-forming potential has been retained in all cells [Fig. 1]. Non-pineal cells use the indoleamine as a defence against oxidative stress in the mitochondria where it is generated. If they release it into interstitial fluid, it acts locally in an autocrine or paracrine manner. While melatonin that escapes cyclically from the pineal gland serves as an essential time giver to many tissues, peripherally generated melatonin is unconcerned with providing timing information but rather has many other critical functions, consistent with its multitasking image.

Melatonin: It is not just for vertebrates

As noted above, the ability of present day animals to produce melatonin is a process inherited from their evolutionary ancestors i.e. bacteria; as a result, all species between bacteria and humans probably likewise engage in the production of this beneficial multifunctional molecule. All organisms that have been tested including unicells, insects, arachnids, non-mammalian vertebrates, mammals, etc., have capitalised on melatonin’s favourable actions by synthesising it. The synthetic pathway for melatonin likely did not evolve independently in all these species but rather they acquired this capability during their evolution from more primitive organisms. There are, however, slight variations in the synthetic pathway that the transitional organisms developed possibly because it was more efficient to do so or it afforded them a metabolic advantage. In all vertebrates, tryptophan is a precursor and serotonin is an intermediate metabolite in the trail of melatonin production.

As with mitochondria, the photosynthetic capability of plant cells developed after early eukaryotes engulfed melatonin-synthesising, photosynthetic cyanobacteria for their nutrient value. Over time, the phagocytosed bacteria matured into chloroplasts where they retained their photosynthetic and melatonin-synthesising capabilities. As a consequence, the photosynthetic portions of all green plants produce melatonin at the cellular level in two sites, in their chloroplasts and in mitochondria (Zheng et al., 2017) (Fig. 1). Because they have two organelles to produce melatonin, plant cells generally have higher melatonin concentrations than animal cells.

Melatonin: Physiological diversification

Many scientists and members of the lay public realise that melatonin has sleep-promoting properties, possibly not because of any direct soporific actions but because appropriately timed melatonin administration, via circadian means, “opens the normal sleep gate.” This and other observations document the function of melatonin as a circadian and circannual rhythm modulator. This circadian action becomes readily apparent when the pineal-derived day:night fluctuation in blood melatonin levels is interrupted e.g., in shiftworkers, blind individuals, inhabitants of the International Space Station, or individuals suffering from jet lag or social jet lag. In other words, these individuals have trouble sleeping. This action is exclusively a result of a disturbance in the differential release of melatonin over a 24-hour period.

Prior to the discovery of melatonin, some unidentified product of the pineal gland was often linked to reproductive physiology. After Lerner and coworkers (1958) identified the pineal gland as an origin of melatonin, it was soon found that surgical removal of the gland prevented seasonally related reproductive changes in photoperiod-dependent mammals (Hoffman & Reiter, 1965). This melatonin receptor-mediated action is now known to be achieved at the level of the pars tuberalis and the medial basal hypothalamus and is widely accepted to relate to the circadian actions of melatonin. Other functions of melatonin that depend on receptor-mediated processes include its ability to inhibit some aspects of tumour cell proliferation and metastasis, alterations in serotonin release in the retina and activation of antioxidative enzymes, etc. (Hill et al., 2015; Reiter et al., 2017)

Beyond melatonin’s functions that involve widely distributed membrane melatonin receptors, the indoleamine does not always rely on an interaction with a receptor/binding site. Melatonin is also a direct reactive oxygen species (ROS) scavenger (Reiter et al.,

2017). In this receptor-independent process, ROS, which are often free radicals, are ensnared and neutralised by melatonin. ROS are especially abundantly generated during oxidative phosphorylation (OXPHOS) in mitochondria and during photosynthesis in chloroplasts. Hence, it is highly fortuitous that mitochondria and chloroplasts, which are so involved in the production of partially reduced toxic oxygen species, also produce a highly efficient radical scavenger to detoxify them before they mutilate neighbouring critical macromolecules in the mitochondrial respiratory chain complexes or in the chloroplastic photosynthetic machinery. Oxidative damage to these organelles compromises efficient OXPHOS and photosynthesis, respectively. The functional deterioration to these processes then becomes a violent cycle of more oxidative destruction accompanied by greater ROS production. Other high free radical-generating situations where the direct scavenging actions and the indirect antioxidant functions of melatonin come into play include the following: ionising radiation exposure, obesity, ischaemia/reperfusion injury, cytokine production, toxin and toxic drug exposure, severe inflammation, etc. (Anderson & Maes, 2014; Cipolla-Neto et al., 2014).

In addition to the manufacture of melatonin by mitochondria, these organelles avidly take up endogenously produced or exogenously administered melatonin from blood or other bodily fluids. This process may involve specific active transporters in the cell and mitochondrial membranes allowing the latter to retain much higher concentrations than exist in other subcellular structures (Reiter

et al., 2017). This mitochondrial targeting by melatonin helps to explain its high efficiency as an antioxidant. Related to this, the pharmaceutical industry has been designing mitochondria-targeted antioxidants for years realising they could be useful in modifying the cause of many diseases that have a free radical component. When these synthetic antioxidants were compared with melatonin in reference to their oxidative stress-quenching potential, they were no better, and for some responses less effective, than naturally occurring and non-toxic melatonin.

In plants, several functions have been described for melatonin. Any process that stresses a plant, e.g., excessive heat or cold, drought, increased salinity, etc., induces a rapid rise in the levels of melatonin, which are used to quell the greatly augmented numbers of ROS produced in response to the stress (Reiter et al., 2015). The induction of melatonin synthesis under stressful circumstances may be a feature of animals as well. Besides its synthesis, plants also take up melatonin through their root system. The higher melatonin concentrations in plants compared to those in animals may be especially beneficial as a protection against ROS since plants are sessile and cannot avoid stressful stimuli when they occur. A second identified action of melatonin in plants is as a growth promoter. Incubating seeds in a melatonin-containing solution before germination or spraying it on plants during their early development results in larger and more productive plants. Since melatonin receptors have yet to be identified in plants, these observed actions are presumed to be receptor-independent.

Finally, melatonin aids plants in resisting diseases. The actions described, especially the inducibility of melatonin in stressed plants, could be highly significant during global climate change if plants are genetically engineered to produce elevated levels of this protective indoleamine.

Epilogue

Melatonin is uncommonly effective in quenching renegade radicals. These brigands typically molecularly devastate cells and weaken their defences against the development of pathologies and death. By shielding mitochondria/chloroplasts from the large-scale damage inflicted by ROS, melatonin improves the viability and/or productivity of organisms. Thus, the significance of the pronounced antioxidant potential of melatonin cannot be overstated. There are many other noteworthy actions of melatonin that have justifiably attracted attention. Its ability to adjust the circadian clock, regulate seasonal reproduction, its oncostatic actions and anti-inflammatory effects, and its protection against toxins, all assist melatonin in resisting functional deterioration and diseases of cells. In view of the diversity of these observed functions, it seems likely that they may be merely epiphenomena of more basic actions of this indoleamine, which are yet to be uncovered.

Melatonin’s functional “toolkit” is extraordinarily large. This may relate to its three billion year longevity, an interval during which it developed a working relationship with many other essential molecules all of which provide an advantage to organisms. Viewed in this context, the altruism of melatonin should not be underestimated.

References

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