Is Gravity Genetically Coded

The objective of this research is the analysis of molecular and physiological interactions that generate the proper organism. It relies on the use of mutant mice to delineate the molecular basis of development, including differentiation to their final destination, and the use of physiological methods to understand the meaning of related modifications.

This research program is related to ground-based analyses on the impact of genetic modifications (for example by knock-down approaches) on specific gravity-related systems and functions of the body. So far, these questions preferably have been related to the development of neuronal and sensory systems. It is likely that almost everything from formation and path-finding to specific targets and survival is regulated by sophisticated genetic programs that perform most of their function independently of external (epigenetic) inputs such as altered gravity. It can be therefore assumed that the formation of the most initial connections and synapses will occur normally regardless of the vestibular stimulation. However, following this initial "hard wired" molecular phase, a second phase will presumably concentrate on the fine-tuning of these early connections via the physiological factor activity. So, the task ahead is to unravel the molecular conditions that produce the most striking and most lasting effect of connectivity modifications driven by altered gravity (see Fritzsch 2003 for review).

A step into this type of research was done using the fish model Oryzias. Its otolith-deficient mutants were compared with other strains, and different strains of this species were crossed. One of the most intriguing results was the formation of fish that were less dependent on gravity and therefore less sensitive to microgravity, as shown by less frequent loop swimming during the flight (Ijiri et al. 2003).

Future studies in developmental biology will consider to which extent gravitational forces affect gene expression. In periods of limited access to space, microgravity simulation methods such as clinostats or bioreactors will give insights into the extent of gene expression, and its relationship with the periods of embryonic and postembryonic life. An example of this approach is the study in transgenic zebrafish, which were exposed to simulated microgravity for different durations at various developmental times, in an attempt to determine the susceptibility periods for a large number of developing organs, including the heart, notochord18, eye, somites, and Rohon-Beard neurons'9. The observations revealed changes in gene expression with periods of maximum susceptibilities characteristic for each organ. They also indicated a complete recovery of gene expression, despite of continuous exposure to simulated microgravity (Shimada et al. 2005). These observations support the idea of a self-organized intrinsic normalization of the development during continuous exposure to altered gravity, despite of the often-observed deviations from normal development.

Figure 5-25. Behavioral studies in parabolic flight.

18 Notochord is a rudimentary of embryonic spinal column in the fish.

19 Rohon-Beard neurons are primary mechanosensory neurons that differentiate in the dorsal spinal cord and are found in most lower vertebrates.

0 0

Post a comment