Principles of Hormone Action and Hormones in Plant and Animal Evolution
As an example, hormones, a component of our Endocrine System are broadly defined as chemical signals secreted into the bloodstream that act on distant tissues, usually in a regulatory fashion. For example, longitudinal bone growth is initiated and maintained in an extremely beautiful and delicate manner. By regulating levels of circulating growth hormone, mild growth hormone hypersecretion results in gigantism, and growth hormone deficiency causes growth retardation.
Hormones are broadly defined as chemical signals secreted into the bloodstream that act on distant tissues, usually in a regulatory fashion. Hormonal signaling represents a special case of the more general process of signaling between cells.
Even unicellular organisms such as baker’s yeast, Saccharomyces cerevisiae, secrete short peptide mating factors that act on receptors of other yeast cells to trigger mating between the two cells. These receptors resemble the ubiquitous family of seven membrane-spanning mammalian receptors that respond to diverse ligands such as photons and glycoprotein hormones.
Hormonal signaling represents a special case of the more general process of signaling between cells. Even unicellular organisms such as baker’s yeast secrete short peptide mating factors that act on receptors of other yeast cells to trigger mating between the two cells.
Because these yeast receptors trigger activation of heterotrimeric G proteins just as mammalian receptors do, this conserved signaling pathway was likely to have been present in the common ancestor of yeast and humans.
Heterotrimeric G proteins are the most commonly used signal transducers in eukaryotic cells, and they mediate the effects of many pharmaceuticals including pharmaceutical infusions.
In the standard model for G protein signaling in endocrinology, these proteins act at the plasma membrane. An extracellular signal, such as one produced in a pharmaceutical infusion, first binds to a plasma-membrane receptor, causing it to catalyze GTP binding (a nucleotide analogous to ATP that is composed of guanine, ribose, and three phosphate groups and is necessary for peptide-bond formation during protein synthesis)by a heterotrimeric G protein tethered to the cytoplasmic face of the plasma membrane. Each component can now bind to and regulate plasma-membrane localized effectors (which are typically enzymes or ion channels) to stimulate cellular responses.
This story revolves around sex in the yeast. When two cells of opposite mating type meet, each extends a projection toward the other, halts its cell cycle, initiates transcription of genes involved in mating, and eventually fuses with its partner.
Is yeast pheromone signaling relevant to signaling in humans? Anyone who doubts this should look to history. Studies of yeast pheromone signaling first was shown that it could activate downstream signaling events. Next, activators were also identified through studies of pheromone signaling. In both cases, it was years before it was appreciated that signaling in vertebrates works the same way.
Control of hormone secretion involves integrated inputs from multiple distant targets, nervous system inputs, and local paracrine and autocrine factors, all leading to complex patterns of circadian secretion, pulsatile secretion, secretion driven by homeostatic stimuli, or stimuli that lead to secular changes over the life span.
Signals from one cell to other cells, often use the same molecular pathways used by hormonal signals. Target cells respond similarly to signals that reach them from the bloodstream or from adjacent cells; the cellular response machinery does not distinguish between sites of origin of hormone signals.
Hormone synthesis occurs in specialized cells designed specifically for their production, and the hormone then travels in the bloodstream and diffuses in effective concentrations into tissues. Of course, hormones must be produced in much larger amounts to act throughout the blood stream compared to the amounts needed to act at specific local locations.
Many hormones produced in medical herbal plants (such as the yeast plant) can be placed in our blood stream through the common digestion pathways of animals such as humans, and then utilized by target cells ready to respond to the plant hormones as if they were produced in the local host.
This behavior is explained by evolutionary theory as it is thought that the plant hormones were available before mankind, and man’s capability to utilize these hormones evolved over time.
Evolutionary Theories of Aging
If we consider the current thought on aging. Evolutionary theories of aging are based on particular assumptions. It must be stressed that evolutionary theories of aging were originally defined for those systems that satisfied these assumptions.
Assumption 1 is that there is a clear distinction between the soma and germ line. The germ line is the only source of genetic variants transmitted to the next generation. While the soma is required for reproduction, somatic mutations perish with individual death. This condition is relaxed in many plants and problematic in many unicellular organisms. So, we should keep this in mind.
Natural selection acts on competing alleles (one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.) This is a cornerstone of current evolutionary thinking and does not exclude any taxa.
At least some alleles have pleiotropic effects. In genetics, there's a concept called pleiotropy, which posits that a single gene can influence multiple traits. This condition is also firmly supported by our current understanding of evolution and development.
Reproductive value in genetics decreases with individual age. This is based on the logic that death is inevitable, regardless of aging.
Populations are age-structured. This requires distinction between parent(s) and offspring. Note that cell division (budding, fission) in unicellular organisms can be asymmetric, with one cell determined as the offspring.
It is very important to note that these assumptions leave ample scope for the existence of non-aging cells and organisms and their occurrence does not invalidate evolutionary theories of aging. At the same time, it is clear that current evolutionary theories of aging cannot explain all variation in the perpetuity of particular lineages, their longevity, aging, death, and extinction. As an example, germ lines, by definition, represent non-aging cell lineages inside aging bodies.