Aging is a Side Effect of the Ontogenesis Program of Multicellular Organisms

The review presents a brief outline of the current state of the main theoretical approaches to the aging problem. The works of authors, supporting the theory of “accumulation of errors” and theories stating the presence of a hypothetical “aging program” in all multicellular organisms are reviewed. The role of apoptosis and its connection with phenoptosis, as well as the theory of “hyperfunction” are analyzed. Our own approach to this problem is presented, in which aging is explained by the redistribution of limited resources between the two main aims of the organism: its self-sufficiency, based on the function of the housekeeping genes (HG) group, and functional specialization, provided by the integrative genes (IntG) group. Agreeing with the inseparable connection between aging and the ontogenesis program, the main role in the aging mechanisms is assigned to the redistribution of resources from the HG self-sufficiency genes to the IntGs necessary for the operation of all specialized functions of the organism as a whole. The growing imbalance between HGs and IntGs with age, suggests that switching of cellular resources in favor of IntGs is a side effect of ontogenesis program implementation and the main reason for aging, inherent in the nature of genome functioning under conditions of highly integrated multicellularity. The hypothesis of functional subdivision of the genome also points to the leading role of slow-dividing and postmitotic cells, as the most sensitive to reduction of repair levels, for triggering and realization of the aging process.


INTRODUCTION. THE MAIN DIRECTIONS OF AGING THEORIES
At present, there is no single, generally accepted theory of aging, just as there is no generally accepted definition of the phenomenon. There are two main groups of scientific hypotheses explaining the essence and causes of aging in multicellular organisms. The first group is the "error accumulation" theory based on the stochastic approach to this phenomenon. The second group unites researchers who believe that all multicellular organisms have an inherent "aging program" connected in a certain way with individual development. Many supporters of the "error accumulation" theory tend to believe that various processes related to excessive production of free rad-icals play the main role in cellular DNA damage [1]. This recognizes the inherent inability of cells to fully protect themselves from harmful inf luences. A number of experimental findings suggest this conclusion [2]. Thus, the increase in the number of non-repairable damage to cellular DNA in aging cells as compared to young cells allows the authors to conclude that this is the cause of aging [3]. Proponents of the "error accumulation" theory justifiably point out that the main damages accumulate in those cells whose replacement is difficult or impossible. However, just pointing out this fact is obviously not enough to understand the processes occurring in them, which testify to the deficit of reparative activity appearing with age. The fact that such changes are recorded

MINI-REVIEW
in organisms after their maturation simply points to the connection of this phenomenon with ontogenesis.
The second group of hypotheses assumes the presence of a genetic "aging program" in all multicellular organisms. This approach is justified by the evolutionary necessity of aging as a mechanism of generational change [4,5]. Developing the ideas of August Weismann [6], V. P. Skulachev formulated the theory of rapid phenoptosis (sepsis, carcinogenesis, infarction, etc.) and slow phenoptosis (organism aging), giving an important role in them to the mechanism of apoptosis [7]. The studies on the role of apoptosis in the aging cells of the body remain relevant today, although, as we can assume, this phenomenon is not a cause, but a consequence of aging processes in postmitotic cells. In our opinion, the management of apoptosis alone cannot lead to a significant breakthrough in the longevity extension. However, the very attempt to draw attention to the genetic mechanisms associated with aging deserves close attention. The search for the program responsible for ontogenesis is ongoing.
Here the works of A. M. Olovnikov [8] should be noted that present an interesting hypothesis of the developmental program of multicellular organisms, although it has no experimental confirmation so far. The search for the developmental program is continued in the field of epigenetics, where, starting from S. Horvath [9], the studies of the genome methylation processes continue [10][11][12]. It is also recognized that epigenetic processes directly ref lect the work of the ontogenetic program [13]. The data on genome methylation can indicate biological age accurately enough; however, as a statistical indicator, they are not directly connected with the known mechanisms of aging, ref lecting only the fact of genome regulation in the course of ontogenesis. From the evolutionary point of view, in multicellular organisms, the internal mechanisms that contribute to the survival of a species are constantly improving, ignoring its individual representatives. Thus, the presence of mutations may bring harm to an individual, but the presence of a separate mechanism for their acquisition multiplies the adaptive plasticity of the species, as suggested by A. M. Olovnikov [14]. As we will see further, ontogenesis is a program for successful attainment of maturity, and aging is a convenient, from the species' point of view, side effect. A similar opinion is expressed by the author of the hyperfunction theory M. V. Blagosklonny [15,16]. Developing V. M. Dilman's idea [17] of the age-related increase in the hypothalamic sensitivity threshold to the regulatory inf luence of peripheral hormones, the author of the hyperfunction theory agrees with the statement that there are only the developmental programs, not the aging ones. Continuous operation of such a program inevitably leads to the depletion of resources and age-related diseases [18,19]. Thus, hyperfunctioning is a consequence of the fact that the developmental program has no natural switch. Following the author's logic, it is necessary to assume the existence of some permanent stimulus enhancing the work of all genes of the body, which does not find experimental confirmation. Moreover, according to recent data, the level of metabolism measured in a large group of people of different ages remains unchanged, which cannot provide a continuous enhancement of functions. On the contrary, it remains relatively constant and does not increase between 20 and 60 years of age [20].

ONTOGENESIS PROGRAM AND FUNCTIONAL PARTS OF THE GENOME
It makes sense to distinguish between the phenomenon of aging itself and its mechanisms and causes, which is the subject of many articles [21,22]. In our work, we adhere to the definition of the phenomenon "aging" itself given by Gavrilov and Gavrilova [23] as a process occurring in any biological object due to insufficient recovery of damaged or spent resources. In the case of a multicellular organism, its aging is caused by a decrease in the resources required by its cells for their repair and tissue regeneration [24,25]. In other words, aging of a multicellular organism begins at the moment when its repair begins to be incomplete. The reasons for such a phenomenon are well explained in terms of the functional subdivision of the metazoan cell genome presented in our publications [26,27]. Let us recall the basic principles of our hypothesis.
From our point of view, a multicellular organism is a mode of existence of a separate cell colony based on the system-forming principle of highly specialized multicellularity. In the course of individual development (ontogenesis), an independent system of a new level of complexity is formed. Such a collective cell symbiont, providing all specialized functions, is necessary for the existence of the colony itself. We argue that during ontogenesis (ref lecting phylogeny or evolutionary origin), any multicellular organism is built on the basis of not all but only a part of its genome, which has changed in the course of evolution. The other part of the genome remains virtually unchanged, constantly ensuring the viability of the cells. These are genes similar to the majority of genes in unicellular organisms. Thus, as a prerequisite for the development of an organism, the DNA of all its cells contains two functionally independent parts. One of them is the most conservative part of the genome, providing for the internal needs of any cell, or housekeeping genes (HG). A group of genes with this name was defined quite a long time ago, and one of the main criteria of this group was a relatively constant level of activity of its genes [28,29]. Another functional part of the cellular genome are the genes providing the integrative function, or genes responsible for all specialized structures produced by the cells during differentiation and creating the organism as an integrated whole (IntG).
Organismal development and maturation in Metazoa are under the control of the ontogenesis program. We understand the ontogenesis program as a strictly defined sequence of IntG expression with the assignment of an advantage in the consumption of cellular resources in order to form the organism and reach its maximum development by the time of sexual maturation. Changing the role of genes during ontogenesis was put forward by G. Williams [30], who proposed the principle of antagonistic pleiotropy, which remains relevant today [31,32]. From our point of view, pleiotropic properties are possessed not by individual genes acquired in the course of evolution, but by their entire group, which creates an organism as a separate biological system and represents the integrative part (IntG) of the cellular genome. We consider the phenomenon of pleiotropy of the IntG group as the presence of their unilateral advantage in expression and resources. For the organism, the pleiotropy creates a situation when the role of the IntG group in the genome goes from "a necessary symbiont to a parasite" by increasing the "system tax" on the HG group by IntGs, after the organism reaches the fertility stage. In our views we proceed from two basic statements: resource capabilities of any cells are always limited; the main goal of the ontogenesis program is to achieve the maximum competitive advantage by the time the organism reaches the fertile state, since natural selection is aimed exactly at this stage of metazoan development [13,34,35]. The advantage obtained at the moment critical for the continuation of the species and paid for by the suppression of the autonomous and regenerative potential in the future is the IntG pleiotropy. It is the constant growth of integrative functional costs that determines the main mechanism of aging inherent in the very nature of genome functioning under conditions of highly integrated multicellularity. Both for individual cells and for the organism as a whole, there is a zone of functional optimum. This is the state, in which the programmed level of development is accompanied by the lowest metabolic expenditures for its maintenance. Both for individual cells and for the organism as a whole, this occurs at different times. The organism as an integral system of specialized functions always reaches its functional optimum later than its individual cells. Such inconsistency naturally leads to "pushing" the cells of the organism out of the optimum zone, which results in a similar situation for the organism as a whole. Let us repeat that here we are talking about the system-forming principle of highly specialized multicellularity, on which it is built and which determines the course of multicellular organism development. Let us emphasize again, that the difference in approaches between the "hyperfunction theory" and our theoretical model lies in the fact, that for us, the leading role in the aging processes is performed not by direct and continuous gain of functions in the organism, but by the redistribution of resources towards the IntG part of the genome, which increases after the organism reaches fertility and is accompanied by the reduction of repair below the necessary level.

POSTMITOTIC CELLS AND THEIR ROLE IN THE AGING PROCESS
In the works devoted to aging, a lot of attention is paid to cells and tissues with high mitotic index (MI). These properties allow such cells to retain a high regenerative potential and make them a target for experiments aimed at rejuvenation [12,[34][35][36][37]. From our point of view, only inf luencing the epigenetic mechanisms of cells with high MI is clearly insufficient to achieve true rejuvenation of the organism. On the other hand, the organs and tissues composed of postmitotic cells and cells with low MI constitute the bulk of the organism. There are many works devoted to the aging processes in such cells, the inf luence of old cells on the surrounding tissues, and the immune system response to them [38][39][40][41][42]. The self-preservation of this group of cells is based on the processes of intracellular repair. These are the processes in which the balance of activity between HGs and IntGs plays a crucial role, allowing the organism to reach the peak of its development at the expense of highly differentiated tissues by the period of fertility. The continued advantage of IntGs in resource consumption shifts the balance of consumption in their direction, which is especially significant for postmitotic cells and cells with low MI. In our opinion, it is this group of cells constituting the basis of body mass that plays the leading role and triggers the entire cascade of changes accompanying aging. The increase in the number of senescent cells with a deficit of reparative capabilities occurring with age leads to the accumulation of cytotoxic products and related metabolic disorders in them and in their environment. As a result, tissues with high MI are subjected to the increasing metabolic pressure from the mass of slowly dividing and postmitotic cells. For actively dividing cells of constantly renewing tissues, like bone marrow, intestinal epithelium and mucous membranes, and skin growth layer, such toxic exposure leads to a gradual reduction of their stem cell number and a decrease in the rate of regeneration. The contribution to aging processes and the negative effect of old cells on the organism have been shown in a number of articles [39][40][41][42][43][44]. All this makes us take a new look at those cells, whose lifespan is comparable with the lifespan of the organism as a whole. It becomes clear that attempts to succeed in organism rejuvenation based only on regeneration processes do not seem realistic. The very fact of a long time interval between divisions, comparable with the life span, or its absence in postmitotic cells makes them especially sensitive to the intracellular balance between the HG and IntG groups, leading to the redistribution of cell resources in favor of IntGs. Let us emphasize once again that a decrease in the HG production also naturally leads to a decrease in the ability of cells to repair themselves, triggering an "incomplete repair" situation. It is the point when the whole cascade of aging processes is triggered and it is the situation when aging becomes a side effect of the organism development program [45]. Such a situation makes it possible to consider the ratio of activity of the two genome groups in such cells as their "functional clock".

CONCLUSION
Summarizing the discussion and analysis of the currently available theoretical approaches to the phenomenon of aging and its causes, we can state that the true causes of biological aging are still unclear. At present, there is a growing scientific interest in this problem both in already established and in new directions. The theoretical model of aging that we have presented can become one of the new directions in the study of aging. Below we will present the first results confirming it, as well as the direction of our future work. The approach in which we consider not individual genes, but ontogenetically determined groups of genes is fundamental for us. The existence of such groups is confirmed not only by the difference between them, but also by the dynamics of their methylation with age. The foundation for this is the data we obtained in our recently published work [46].
This article presents the result of statistical comparative analysis of the methylation level of the human genome in different age groups. The results showed reliable differences between our suggested groups (HG and IntG) in the level of absolute methylation values, more pronounced in the promoters of the studied gene groups during the entire observation period. Thus, the total level of methylation in the promoters of IntGs was substantially higher than in the HG group with confidence p < 0,0001. The level of methylation of gene bodies showed the same results, but with a slightly smaller difference. The dynamics of a significant decrease in the level of methylation of both promoters and gene bodies in the IntG functional group after puberty was also shown. In the HG group, this level remained constant. The increasing imbalance in the methylation levels between the HGs and IntGs suggests that this shift in the IntG group ref lects a side effect of the ontogenesis program implementation. Considering that the methylation level of different genes is a statistical indicator that has no direct connection to the known mechanisms of aging, it sufficiently ref lects the very fact of genome regulation during ontogenesis. Mathematical analysis, including cluster analysis of "big data" of the whole-genome sequencing, provides an opportunity to obtain direct data on the gene activity. This is the direction in which our work is currently carried out. According to the already available results, we can assert that the functional genome groups identified by us are statistically significant in terms of both mRNA production and their age dynamics. This allows us to confirm the conclusion that all genes providing specialized functions of the organism are evolutionarily late acquisitions for cells and are united by common properties. This is what allows us to direct further our research towards the finding of those specific features in the functional groups of genes we have identified, which provide their different behavior during ontogenesis. These differences, or the "seal of evolution", are likely to be found in the evolutionarily younger group of specialized IntGs. Identification of such features will make it possible to understand the mechanisms and variants of the relationships between the HG and IntG groups.
In conclusion, let us note once again that the decrease in both reparative and regenerative capabilities caused by the increasing imbalance between the HGs and IntGs and occurring in all multicellular organisms over time is not only a side effect of the ontogenesis program, but also the very cause of aging.
Ethics declarations. The author declares no conf lict of interest. This article does not contain any studies involving human participants or animals performed by the author.
Acknowledgments. The author expresses his sincere gratitude to A. M. Olovnikov for his invitation to contribute to the jubilee collection and for his advice in writing this review.
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