Sunday, August 2, 2009

Single cells, memory, learning

Ever since I read Seth Grant's paper on synaptic evolution last year, which discusses proteomics at the level of synapses and how we have synaptic proteins in common with yeast, I've been thinking about synapses as gateways to information flow, controlled by proteins that are common to multiple life forms. It caught my full attention that we have synaptic proteins in common with yeast.

Last night I read "Microbes exploit groundhog day" in Nature's July issue. Excerpt:
"The proposal that microorganisms can associate a stimulus with an appropriate response to a future environment might seem far-fetched. After all, without cognition, microorganisms rely on simple regulatory networks to sense and respond to their environment. A canonical example of gene-regulation, the response of Escherichia coli to the sugar lactose, illustrates why it seems surprising that such networks can be used to anticipate environmental changes."
The write-up discusses a paper by Mitchell et al., Adaptive prediction of environmental changes by microorganisms (same issue).
"The insight of Mitchell et al., building on previous work, was to realize that the connection between stimulus and response can be offset in time. For example, if a non-lactose sugar consistently follows the availability of lactose, selection might favour the evolution of a regulatory network that directly links the presence of lactose to the expression of the non-lactose-utilization genes. This network would serve to 'prime' cells conferring an advantage by preparing them to use the non-lactose sugar in anticipation of its imminent availability and thereby reducing the lag time characteristic of de novo activation of response genes. Mitchell et al. call this mechanism adaptive anticipatory conditioning."
A clever experimental design was employed to examine the responses of E. coli and baker's yeast, Saccharomyces cerevisiae, to an environment simulating what each organism would ordinarily find in a typical higher intestinal tract (higher in lactose and low in maltose), compared to lower part of the tract (low in lactose and higher in maltose). Mitchell et al. found that "microorganisms can interpret their environment and respond in a way that provides a benefit only in following a future environmental change." A few wrinkles remain, but "one message is clear" -
"The regulatory networks that link environmental stimuli to microbial responses are complex and can evolve rapidly. The potential for microorganisms to offset responses from environments in which those responses are useful provides both a warning and an opportunity for researchers involved in testing the functional significance of links between stimuli and responses."
Possibly related, in some way, somewhere down the road, are these two recent tidbits from New Scientist:

1. Memristor minds: The future of artificial intelligence
by Justin Mullins. It discusses artificial intelligence and a "fourth" ingredient, "memristor" (in addition to resistor, capacitor and inductor):
"Chua had anticipated the idea that memristors might have something to say about how biological organisms learn. While completing his first paper on memristors, he became fascinated by synapses - the gaps between nerve cells in higher organisms across which nerve impulses must pass. In particular, he noticed their complex electrical response to the ebb and flow of potassium and sodium ions across the membranes of each cell, which allow the synapses to alter their response according to the frequency and strength of signals. It looked maddeningly similar to the response a memristor would produce. "I realised then that synapses were memristors," he says. "The ion channel was the missing circuit element I was looking for, and it already existed in nature."

To Chua, this all points to a home truth. Despite years of effort, attempts to build an electronic intelligence that can mimic the awesome power of a brain have seen little success. And that might be simply because we were lacking the crucial electronic components - memristors." - (my bold)

2. Evolution's third replicator: Genes, memes, and now what? by Susan Blackmore. She comments (excerpts):

"We humans have let loose something extraordinary on our planet - a third replicator - the consequences of which are unpredictable and possibly dangerous.

What do I mean by "third replicator"? The first replicator was the gene - the basis of biological evolution. The second was memes - the basis of cultural evolution. I believe that what we are now seeing, in a vast technological explosion, is the birth of a third evolutionary process. We are Earth's Pandoran species, yet we are blissfully oblivious to what we have let out of the box."

"Billions of years ago, free-living bacteria are thought to have become incorporated into living cells as energy-providing mitochondria. Both sides benefited from the deal. Perhaps the same is happening to us now. The growing web of machines we let loose needs us to run the power stations, build the factories that make the computers, and repair things when they go wrong - and will do for some time yet. In return we get entertainment, tedious tasks done for us, facts at the click of a mouse and as much communication as we can ask for. It's a deal we are not likely to turn down."

Additional resources:

1. BrainScience Podcast #51 Dr. Seth Grant on Synapse Evolution

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