Rapid alterations of cell cycle control proteins in human T lymphocytes in microgravity

Gravity has been a constant force throughout evolutionary history on Earth. Thus, it is one of the fundamental biological questions, if and how life on Earth requires and responds to gravity at the functional cellular and molecular level. In unicellular organisms, such as Paramecium and Loxodes, gravity can be perceived rapidly by gravireceptors, which are gravi-sensitive ion channels in the cell membrane or statocyst-like organelles [1]. In mammalian cells, rapid gravi-responsive elements are unknown.The sensitivity of human cells exposed to reduced gravity has already been suspected for cells of the immune system since the first Apollo missions, where more than half of the astronauts suffered from bacterial or viral infections [2]. In one instance, an astronaut was infected with an opportunistic pathogen, Pseudomonas aeruginosa, which rarely causes disease in people with functional immune systems. In crew members of Skylab and Soyuz, a reduced reactivity of blood lymphoid cells has also been observed [3,4]. Recent studies found a subclinical re-activation of the varicella zoster virus (VZV) in astronauts [5,6]. This virus becomes latent in the nervous system after primary infection, but is frequently reactivated in immune suppressed individuals, such as after organ transplantation, or patients suffering from cancer or AIDS. Because of the obvious and severe effects on the human immune system, serious concerns arose whether spaceflight-associated immune system weakening ultimately precludes the expansion of human presence beyond Earth's orbit [7].In an extension of this fundamental question, it is important to ask if the molecular and cellular structure of human life on Earth may require gravity for regular function and survival, and if therefore gravity-dependent mechanisms will keep us dependent on the gravity field of Earth. Indeed, about one decade later, a pioneering discovery from Cogoli et al. at the first Spacelab-Missions in the year 1983, where isolated h