MadSci Network: Cell Biology
Query:

Re: Do cells have 'memory'?

Date: Sat Apr 7 19:47:53 2001
Posted By: Alexander Craig, Grad student, Physiology/Neurobiology
Area of science: Cell Biology
ID: 984981563.Cb
Message:

Hi,

There are in fact plenty of examples that cells do have a form of memory but sometimes these phenomena are given other names. I will concentrate on selected examples from development, immunobiology, genetics, and neurobiology.

A) Development
You might have heard about the so called HOX genes. HOX genes are activated during certain stages of develo pment and play a role in determining the identity of cells (and therefore cell groups and more complex structures) along any the 3 major axes of an organism. HOX genes do not code for structural proteins but rather for DNA regulatory proteins (=transcription factors) that determine where and when other genes are active.
One of the most thoroughly researched organisms is the fruit fly Drosophila melanogaster but there are plenty of other examples such as the vertebrate neural tube or the wing bud of birds. Scientists use the catchy phrase of the HOX code to describe the fact that certain groups of HOX genes are expressed only in certain positions along an axis and thereby function to give cells unique combinations of active HOX genes that encode a distinct position and/or identity. The gradient itself is established gradually over time and refined through interactions between different genes and their gene products (either mRNA or more notably proteins, predominantly so called transcription factors that regulate where and when and to what degree a gene is expressed). The 'memory' is in this case, very similar to the brain and its component neurons, created by network-like interactions that fix the expression pattern in any given individual cell. A cell for itself doesn't really have a memory because it is to a degree dependent on information provided by its neighbors. The whole system contributes to maintain the information.

B) The immune system
Certain types of immune cells called memory (B or T) cells store information on certain foreign structures (usually short snippets of protein called peptides) they have encountered. If they are challenged with this very specific piece of information later on they jump-start a rapid and adaptive (meaning the answer to various kinds of pathogens such as bacteria, viruses, etc. is not stereotypical but has an ability to 'learn') immune response. But once again they can't do it all by themselves because other cells have to provide information by presenting antigen through specific receptors on their surfaces, or they have to contribute specific activating factors such as soluble interleukins or various kinds of surface molecules.

What does this system have in common with the brain? It is its ability to learn which also requires 'memory' to compare a current input to a previous one. This memory is stored in one specific cell or cell clone (and more specifically in the pattern of active genes in either the immune cell or the developing organism) but has to be activated (immune system) or established/stabilized (development) by the interaction with other cells.
This is a to a degree similar to what happens in the brain (here, synaptic connections between cells are stabilized) although here the network character of the whole system is much more prominent. This becomes quite evident in pathological situations (stroke, tumors, etc.) where immediate effects such as memory deficits or loss of other brain functions occur due to the loss of neurons. Nonetheless, in many cases the brain is able to compensate for these losses in a long process of remodeling and rebuilding itself, i.e. different connections between neurons are now being used and enhanced.

Finally, I'd like to mention changes in DNA where information seems to be retained physically in the form of certain chemical modifications (methylation) which constitute a sort of memory. Most notably these specific patterns have been described while researching the phenomenon of imprinting. Here, the exclusive expression of one the alleles inherited from father or mother makes the difference. Most of the genes where imprinting plays a role have to do with early embryonic development but also seem to be involved in cancer and other diseases. I hope I could contribute my 5-cents-worth with this answer. The subject is quite intricate and I hope this is a good starting point for further research should it be necessary. In any case, feel free to email me for further explanations.


REFERENCES

Immunobiology



Developmental biology

Neurobiology

Imprinting and DNA methylation


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