A potential new biological marker of the biological age

between the nature of the brain damage in the aging process (in the case of dementia or even in outwardly healthy elderly people) and the brain damage caused by prion diseases of humans and animals (Sobel, 1970; Fraser and McBride, 1985; Masters et al., 1985; Gajdusek, 1988). In both cases, lesions are characterized by the two fundamental processes—the loss of neurons and an active proliferation of glial cells— gliosis (Finch, 2002; Kim and de Vellis, 2005; Deng et al., 2006; Bessis et al., 2007). In case of prion diseases, neuronal cell death was originally coupled with the action of an infectious agent, while gliosis was always considered only as a reparative response of glial cells to the destruction of neurons (Brown, 1997; Ironside and Bell, 1997; Finch, 2002). However, an infectious agent is absent in the aging process, so the reason for the primary neuronal cell death is also absent. These arguments led us to a basic hypothesis about the primary role of gliosis in the development of the aging process, which, as we supposed, results in the death of neurons. We assumed that the distribution of glial elements in the confined space could easily disturb the nutrition of the neuron (Zuev, 2001). Indeed, the interaction between a neuron and a brain capillary is not direct and is realized through an intermediary, the astrocyte, rather than through its (neuron) close proximity to the wall of the capillary (Magistretti et al., 1999). It is also known that this liaison is not sufficiently strong (Cotrina and Nedergaard, 2002). Therefore, all of this allows us to suppose that an active reproduction of the surrounding glial cells may be quite sufficient to break this fragile relationship between an astrocyte and a neuron, on the one hand, and an astrocyte and a capillary, on the other hand, which will eventually lead a neuronal cell to “starve to death.” It is possible that in order to trigger the process of active cell proliferation in the course of aging, a specific factor (“the aging factor”), which will stimulate this active proliferation of glial cells, needs to be accumulated in the brain tissue. In order to find this factor of aging, we conducted the experiments described below. The first series of experiments were aimed at determining the stimulating effect of the brain extracts obtained from young and old mice on the proliferation of glial cells in primary dissociated and transplantable cultures. Extracts of the brain from young mice had a weak (less than 2-fold) stimulatory effect on the proliferation of glial cells. In contrast, the brain extracts obtained from aged mice had a pronounced 3 to 4-fold stimulatory effect (Zuev et al., 2000). The factor which stimulates the proliferation of glial cells in culture was detected in the brain tissue of mice starting from 10 months of age. At the same time, a similar cytoproliferative factor was also found in the blood serum of old mice; however, it was never detectable in the blood serum of young animals (Zuev et al., 2005). Nevertheless, it remained unclear whether the factor of a detectable increase in the proliferation of glial cells was the reason for the neuronal loss or the consequence of this process and, therefore, the process of aging itself. To address this question, we carried out experiments aimed at the artificial aging of young mice. Four groups of animals were studied. Group No. 1 included 10 young 1.5 month-old mice which were daily (10 injections for 2 weeks) intraperitoneally injected with 0.5 ml of a sterile purified brain extract obtained from the young 1.5 month-old mice (first control). Group No. 2 consisted of 10 naturally aged 2-year-old mice (second control). Group No. 3 included 10 young 1.5-month old mice which were daily (10 injections for 2 weeks) intraperitoneally injected with 0.5 ml of a sterile purified brain extract obtained from the aged 2-year-old mice. Group number 4 consisted of 6 young 1.5-month-old mice which daily (10 injections for 2 weeks) received intraperitoneal injections of 0.5 ml of a blood serum diluted 1: 5, which had been obtained from the aged 2-year-old mice (Table 1). Notably, 3 months after the beginning of the experiment, i.e., when the animals reached 4.5 months of age, the mice from group No. 3 were found to exhibit some clinical manifestations of a premature aging, such as sluggish movements and a slow reaction to food; their hair coat looked dull and less dense with some signs of gray on the ends of hairs. Histological samples of the same regions of the cerebral cortex were prepared after removal of the animals from the experiment. A computer analyzer singled out the total area of neurons on the standard sections of each preparation, with the rest of the area being occupied by the glia (Avtandilov, 2002). As seen from Table 1, the number of neurons proved to be almost 2-fold smaller and gliosis was expressed to a greater extent in the young animals from group No. 3, which had received 10 injections of the brain extract from old mice, as compared with the naturally aged mice from group No. 2. Finally, the findings of this study indicate that the factor stimulating the proliferation of glial cells is detected, in addition to the brain tissue, in the bloodstream of aging mammals. This was supported by the results obtained in the study of the young mice from group No.