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Folk wisdom teaches us that old people tend to be more forgetful than they were in their prime. Sadly, folk wisdom also teaches us that everybody gets old eventually. In more scientific terms, declines in learning and memory capabilities are often seen alongside human aging. It seems reasonable, therefore, to postulate that molecular factors of longevity (some of which were previously found in C.elegans, among other organisms1) are likely to mediate cognitive health, e.g. memory formation, as well.

But how might one go about testing this postulate? Or more specifically, how might one test the effects of aging factors—and the potential treatments targeting such factors—on memory formation within the half-life of graduate students and postdocs? Moreover, a confounding observation in humans is the apparent reduction in cognitive capacity (e.g. short-term memory2) without detectable neuropathology/neurodegeneration in late life3. Therefore, to study aging without becoming terribly aged, and to track the progress of cognitive decline independently of pathological onsets, one ought to seek a model organism. This organism ought to:

  1. provide abundant genomic information,
  2. be a suitable subject for well-developed molecular tools,
  3. experience little neuronal turn-over or age-related neurodegeneration, and
  4. possess naturally short life spans (measured in days, not weeks).

Dr. Coleen Murphy and her colleagues at Princeton chose the tiny, bacteriophagic worm C.elegans (pictured below), which has all four aforementioned benefits1, 4, to investigate the (possibly therapeutic) potentials of known longevity-inducing mutations in memory formation.

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Elegant, truly.

Their 2010 study in PloS Biology, for example, examined two well-known C.elegans longevity regulators, Insulin/IGF-1 Signaling (IIS) and Dietary Restriction (DR).5  The primary molecular factor of IIS are the DAF-2 insulin receptor, which has a human homolog, and its target DAF16/FOXO (a transcription factor). As one might expect, daf-2 mutants with low insulin signaling maintain chemo/thermotaxis abilities better with age. Another gene, aptly named EAT-2, is involved in DR, whereby eat-2 mutants (whose feeding is impaired due to inefficient pharynx movement) live up to 50% longer than wild type.

IIS Insulin/IGF-1 Signaling (impairment leads to longevity) DAF-2 Insulin receptor involved in IIS. STAM Short-term associative memory
DR Dietary restriction(can be caused by eat-2 mutation) EAT-2 Nicotinic acetylcholine receptor involved in DR. LTAM Long-term associative memory
CREB Transcription factor implicated in memory formation Crh-1 CREB gene in C.elegans DAF16/FOXO Downstream transcription factor of DAF-2

A table of jargon.  Every paper should have one. 

While both IIS and DR have been shown to affect cognitive performance in mice6, 7, little is known about their effect on memory formation5. Murphy et al. designed, therefore, positive olfactory associative assays to measure learning and memory in aging wildtype, daf-2, and eat-2 worms. First, after starving the worms briefly, food was given alongside with butanone, which by itself is only a weak chemoattractant. After butanone-accompanied feeding, worms were then segregated into short-term associative memory (STAM) and long-term associative memory (LTAM) training groups, followed by learning efficiency (i.e. post-conditioning response toward butanone) and memory testing.

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Diagram for positive associative olfactory learning and memory assays. (Fig. 1A, Murphy et al.)

In all worms, STAM was demonstrated by elevated response toward butanone for about 2h post-training, and LTAM was observed as the post-assay retention of association between food and butanone after the 16-24h resting periods. LTAM, and not learning/STAM, was affected by disruptions of transcription/protein synthesis, and Murphy et al.’s genetic screening yielded the (nigh legendary) transcription factor CREB as requisite for LTAM. The CREB deletion mutant, crh-1, learned the association as fast as its peers, recalled the information well by worming its way toward butanone with alacrity, but forgot everything by 2h post-training and had to start anew. Murphy et al. posited, then, that learning is molecularly distinct from, but required for, LTAM. Longevity factors that affect learning and STAM, then, may act through pathways unrelated to LTAM, and vice versa.

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Left: CREB mutant’s learning (peak of index at time 0) efficiency and loss of STAM are not significantly different from wildtype. Right: LTAM in CREB mutant is significantly reduced compared to wildtype, as tested by spaced training with 16h rest periods. (Murphy et al., Fig. 3A,B)

During the first week of C.elegans adulthood (ending at the equivalent of, one might imagine, somewhere between mid-life crisis and retirement), Murphy et al. found aging to be accompanied by learning, STAM, and LTAM declines in wildtype. In early adulthood, IIS-impaired daf-2 mutants (whom one’s youthful brain surely remembers from a previous paragraph) were shown to be better at forming STAM and LTAM, with the enhancement dependent on the downstream DAF-16/FOXO being unperturbed. Early adult eat-2 mutants, however, did not show such cognitive enhancement, and even had mild LTAM deficiency.

The two mutants, then, while having similar pro-longevity effects, act independently in maintaining cognitive function. Would this hold true for their effects—if any—on cognitive decline as well? Indeed, as already hinted with the CREB mutant, Murphy et al. observed LTAM impairment on par with wildtype in the aged daf-2 mutants, who still learned better than old wildtype worms did. Aged eat-2 mutants also learned better than their ordinary peers, and proved to be cognitively better preserved even than the daf-2 mutants, as eat-2 mutants had greater LTAM retention than wildtype, despite the slight handicap they suffered as young adults.

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Top: loss of LTAM seen in aged wildtype and daf-2 mutant. Bottom: retention of LTAM seen in eat-2 mutant. (Murphy et al., Fig. 8B,C)

In sum, the requirement of CREB in C.elegans memory could count as an important discovery (although it did not come as a surprise), especially when it turned out that crh-1 gene could be useful as an estimator of LTAM (based on its LTAM-correlating expression level in aged daf-2 and eat-2 mutants). Another achievement, as suggested in this study’s title, was the support provided for the following conjecture: specific types of longevity treatments (such as IIS/DR) could have either positive or negative effects on learning and memory, depending on the time window in which one investigates, even when restricted to relatively well-known pathways in a simple model organism.

So, to speak more fantastically: would one prefer to undergo IIS (i.e. “cut the dessert”), gain the cognitive edge and prosper as an elite, then live a mental invalid’s dreary, forgetful old age? Or would one rather suffer DR (i.e. “cut the meal”), becoming a painfully average run-of-the-miller forever, so as to have a long and mentally active (averagely active, of course) retirement? Choose wisely.

As part of the UCSD Neurosciences Graduate Program Seminar Series, at 4:00pm on Tuesday, January 21, 2014, in the CNCB Large Conference Room, Dr. Coleen Murphy will give a talk on CREB regulation in C.elegans neurons of a long-term memory network. It is advisable to come early and avoid worming one’s way in.

Xi Jiang is a first year student in the UCSD Neurosciences Graduate Program. After a brief tenure as a jellyfish obstetrician, he is now a rotation student under the guidance of Dr. Eric Halgren, studying sleep spindles.

References:

  1. J.F. Morley, R.I. Morimoto. Regulation of longevity in Caenorhabditis elegans by heat shock factor and molecular chaperones. Molecular Biology of the Cell 15: 657-664 (2004)
  2. J.H. Morrison, P.R. Hof. Life and death of neurons in the aging brain. Science 278: 412–419 (1997)
  3. P.A. Boyle, R.S. Wilson, L. Yu, A.M. Barr, W.G. Honer, J.A. Schneider, D.A. Bennett. Much of late life cognitive decline is not due to common neurodegenerative pathologies. Annals of neurology 74: 478-489 (2013)
  4. L.A. Herndon, P.J. Schmeissner, J.M. Dudaronek, P.A. Brown, K.M. Listner, et al. Stochastic and genetic factors influence tissue-specific decline in ageing C.elegans. Nature 419: 808–814 (2002)
  5. A.L. Kauffman, J.M. Ashraf, M.R. Corces-Zimmerman, J.N. Landis, C.T. Murphy. Insulin signaling and dietary restriction differentially influence the decline of learning and memory with age. PLoS Biology 8: e1000372 (2010)
  6. M.A. Akanmu, N.L. Nwabudike, O.R. Ilesanmi. Analgesic, learning and memory and anxiolytic effects of insulin in mice. Behavioral Brain Research 196: 237–241 (2009)
  7. L.W. Means, J.L. Higgins, T.J. Fernandez. Mid-life onset of dietary restriction extends life and prolongs cognitive functioning. Physiology & Behavior 54 503-508 (1993)
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