Critical Questions In Cell Biology

The following and other intriguing questions were asked when automated and crew-tended space laboratories became available tree decades ago:

a. Are unicellular organisms, from bacteria to mammalian blood cells, sensitive to gravitational forces? If yes, how and why?

b. Are the effects direct or indirect?

c. Which are the structures that perceive gravity?

d. What are the medical and physiological implications for humans in space?

e. Can microgravity become a useful tool for biotechnological processes of commercial importance?

We shall keep in mind that gravitational forces are ubiquitous in our Universe with the exception of very few special points, for example the Lagrange point where Earth's and Moon's gravity vectors have opposite directions and compensate each other. What we call microgravity', or weightlessness, or 0 g, are in fact free-fall conditions analogous to those we would experience in an elevator that precipitates to the ground when the cable breaks. A satellite circling the Earth is permanently falling back to it. However, the satellite's speed is such that the fall is endless and just nearby the Earth.

Another aspect to consider is that evolution of life on Earth and probably in other regions of the Universe has always been conditioned by exposure to gravitational accelerations. On Earth, the gravitational force acting on a body depends on its mass and corresponds to the product of such mass by Earth's gravitational acceleration (1 g = 9.81 m/s2). Consequently, all living beings have developed their physiological and biological functions to work optimally at 1 g. Therefore, we have a skeleton to sustain our body against gravity, a heart to pump the blood from the lower to the upper extremities, muscles to move the mass of our body up and down, forward and backward, and a vestibular system to control our posture.

But what happens to a single cell? Did Mother Nature decide that a cell should also adapt to gravity and therefore develop its own gravity-perception mechanism? How can such questions be addressed? Which aspects shall be investigated first? A few theoretical aspects outlined in the next Section should help to choose the adequate experimental approaches.

2.1 Theoretical Considerations

First we have to consider whether single cells possess structures that can sense gravity as a signal to be transduced into a biological response. Let's call these structures gravity receptors. The best example is the plant gravitropism transmitted by the pressure of dense organelles, the statoliths on the cell membrane, of the statocyte in the root (see Chapter 6, Section 2.1). Statocytes are "professional" gravisensing cells designed by evolution to drive the growth of the plant perpendicular to the Earth's surface. There are also unicellular organisms, like the protozoan Loxodes, having specific gravity receptors, the Miiller's bodies (see below). Therefore, it is reasonable to distinguish between "professional" and "amateur" gravisensitive cells. While in the former a gravity-dependence is clearly identified (e.g., the plant statocytes with their statoliths), in the latter it is difficult to find cellular structures that may interact with gravity (e.g., in lymphocytes or fibroblasts). In addition, we must distinguish between single cells in culture (in vitro) and

6 The word microgravity was invented in 1977 by the "Founding Fathers" of ELGRA, the European Low Gravity Association. Microgravity does not mean 10"6 g as the term micro may suggest but rather the fact that real 0-g conditions are not existing. In fact, the mass of the satellite itself generates a gravitational force on the objects on board.

cells as constituents of a multicellular organism (in vivo). In the last case, cells can be either part of an organ or a tissue, or may circulate in a fluid as blood cells do.

Organelles

Diameter (fini)

Density (g/ml)

Nuclei

5-10

1.4

Mitochondria

1-2

1.1

Ribosomes

0.02

1.6

Lvsosomes

1-2

1.1

Peroxisomes

1.06-1.23

Table 4-01. Density of various cell organelles

Table 4-01. Density of various cell organelles

In principle, any mass is subject to the force of gravity and consequently can be regarded as a gravity receptor (as indeed the statolith is). Table 4-01 indicates that the density of certain organelles can be significantly higher than one, which is the approximate density of cytoplasm. Consequently, at 1 g the organelles will apply a certain pressure on the filaments of the cytoskeleton. Such pressure disappears at 0 g with possible effects on the interactions between the players of the signal transduction chains that are embedded in the cytoskeleton (Figures 4-04, 4-05, and 4-06).

Endoplasmic reticulum

Microtubute membrane -Cell cortex

Mitochondrion

Microfilament

Figure 4-05. The organelles within a cell are embedded in and interact with the cytoskeleton.

Endoplasmic reticulum

Microtubute membrane -Cell cortex

Mitochondrion

Microfilament

Figure 4-05. The organelles within a cell are embedded in and interact with the cytoskeleton.

Crucial is the identification of direct gravitational effects at the cellular level. Direct effects are those caused by the interaction of the force of gravity with cellular structures and organelles or by its absence, respectively. Indirect effects are those caused by changes in the cell microenvironment under altered gravitational conditions. Indirect effects may be due to the absence of convection and sedimentation at 0 g that causes a change of the distribution of nutrients and of waste products around the cells.

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