SLA Biomedical & Life Sciences Division Blog

Prion proteins: How studies of yeast help us understand the assembly line manufacturing of beta-folding amyloid plaques strongly associated with Alzheimer’s Disease in humans.

Simpler  does not always mean “easier to understand: ”

Going down in size from our kind of complex cells to prions, thus far, the simplest & strangest biological agents of them all

 

As the scale of  matter that is considered to be alive, or at least, active in living systems and able to reproduce itself,  has gotten smaller over time, the newly discovered  and ever smaller units  have fewer and fewer components.

 

The questions arises: How small and how simple can they get?

 

For eukaryotes (that is the kind of cell we’re mostly made of ….as are most animals and many plants) you need a cell membrane or cell wall that contains the cytoplasm, and a variety of specialized organelles within it, most notably a nucleus within that cell, that has its own membrane,  and that contains multiple, separate  strands of nucleic acids, like DNA or RNA, and usually both.

 

All these  are necessary for the eukaryotic cells to function, through the production and metabolism of proteins, fats, and carbohydrates,  and especially are required for these  cells to reproduce themselves.

 

It  took us about 200 years after the microscopic discovery of eukaryotic cells to get much of a basic understanding of how they functioned, with the DNA/RNA/ protein synthesis  part  becoming clear only in the late 1950s.

 

For prokaryotes (and this includes most bacteria and archaea ) there are few if any organelles, and there really is no nucleus. The bacterial and archaeal cell contains only  a single loop of DNA, which can make RNA,  and together these nucleic acids  make the proteins, lipids and carbohydrates necessary for the unit as a whole to function. 

 

And, surprisingly,   this much simpler arrangement still also carries  all the DNA information necessary for this simpler  organism to reproduce.

 

We figured how bacteria really work only a little while later than we did the eukaryotes, because it was apparent early on that bacteria, in particular, could cause diseases.

 

Furthermore, some bacteria, particularly E-coli, turned out to be great tools for understanding genetics and the work of DNA.  Their very simplicity made them ideal for experimental purposes. A check of WorldCat discloses literally hundreds of books and dissertations on E. coli even now. (See Glass, 1982; Vaillancourt, 2003, & Zimmer, 2008, cited below by way of example.)

 

Viruses are even more stripped down than bacteria and archaea. They can be described pretty much as nucleic acids protected by a protein coat. No membranes. No cytoplasm. No organelles. No nucleus. No carbohydrates. Very rarely, some have lipids as a secondary  protective coat, but most have none.

 

The overwhelming majority of viruses then, are just DNA or RNA and their protective protein coat. That coat  also enables them latch onto and pierce the membranes of  bacteria, archaea, or eukaryotes.

 

Once inside the cell, viruses  hijack the cellular machinery of their unwitting hosts in order to function and reproduce through simply replicating their own  DNA or RNA and those proteins coats, instead of what is needed by the cell.

 

It took only about a decade longer to figure to figure this out, largely because electron microscopy was vastly improved by this time and the need to make anti-viral vaccines and drugs had accelerated research.

 

And curiously the nature of a disease that infected a then  important cash crop, called the Tobacco Mosaic Virus, proved almost as important. TMV had whole books written about it, ( see Scholthof, Shaw & Zaitlain, 1999, cited below by way of example) , and took on a life of its own as a model organism.

 

In any case, for the bulk of the twentieth century, it was thought that viruses were about as small and simple as biology could get.

 

But while there were early inklings that there could be something smaller that mattered to biology, it was not really until  1982 that someone could isolate that something smaller and give it a name.

 

That someone was Stanley Prusiner, and that name was “prion. “ Ultimately this got Prusiner the 1992 Nobel Prize, and prions too, have gotten their own whole books.

 

Prions  are even simpler than viruses.  They have no DNA or RNA. They have only misfolded proteins,  yet somehow they manage to reproduce themselves.

 

This would seem to be a kind of magic trick except for the fact that prions seems to be the cause of obscure but invariably fatal human neurological diseases like Creutzfeld-Jacob,  as well that of important agricultural diseases like scrapie in sheep and goats and mad cow disease.

 

This latter obvious affects  cows, and sometimes humans who eat the neurological parts of  cows (largely portions of their brains, spinal cords, or muscle tissue with extensive innervation.)  

 

 Ironically, while these agricultural diseases had been well known but not well understood back into the 1700s, the fact that human infections could be caused by what were later shown by workers like Prusiner   to be prions,  had actually won the Nobel Prize for another scientist Carleton Gajdusek much earlier,  back in 1976.

 

Gajdusek noted that certain South Sea island cannibals came down with the bizarre and fatal illness of kuru, through the ritualized eating of the brains of deceased relatives who had been previously  infected with this mysterious agent we now know as prions.

 

But many  questions remained. How do prions form and reproduce themselves? Are all proteins capable on some level of turning into prions?  Do prions turn into something else. Are they involved in other diseases?

 

While the mad cow disease panic of the late 1990s caused quite an impetus to prion research, perhaps no greater reinforcement has come to pass since the discovery that those misfolded prion proteins  are very similar to  another disease which seems to involved misfolded proteins: Alzheimer’s.

 

Alzheimer’s Disease involves , within the brain, tangles of fibrils or sheets of proteins that have today become best known by their structural classification as beta-folded amyloid proteins.

 

 Have we found a simple model system to understand better the conditions that turn proteins into prions?

 

Just as we have used smaller and simpler biological systems to better understand general biological phenomena, much as we used the bacterium E.  coli to study general genetics or TMV to  study  general virology, it turns out that the simple microorganisms, the yeasts,  have an abundance of proteins, some of which seem to have the  tendency to become prionogenic.

 

 In other words, they tend to form disordered beta- sheets, that remarkably tend to be passed on between different generations of yeast.

 

The type of yeast used had a relatively simple form of reproduction, called budding,

 

In budding,  daughter yeast cells grow in interior dents within the parent yeast membrane, and when the daughter compartment  gets large enough, it breaks off without damaging the parent yeast cell, and assume an independent existence.

 

And fortunately for the yeast, and for the researchers who needed a stable, non-infective source of prions and a model system to study their manufacture, these beta-sheets do not seem to harm the yeast, and in fact, have some beneficial effect in terms of the yeast’s own structure.

 

Over time, a variety of clearly prionogenic proteins ( among those most commonly mentioned are those labeled as Sup35p, Ure2p, and Rnq1p ) have popped up in these yeasts, and yet,  few common factors could be detected among them. 

 

 

Among those few facts was that each of these prionogenic proteins resisted a chemical treatment called denaturing, which, while primarily a lab procedure, is indicative of extra  survival capability.  

 

In addition, all these prionogenic proteins had structural sequences that were rich in either glutamines or asparagines.

 

Building on some earlier pioneering work on genomic and proteomic screening, as well as some studies of protein structures using magnetic resonance imaging,  Alberti et al. (2009, cited below) developed a systematic screening procedure for sorting out which yeast proteins are prionogenic, and then analyzed these to find commonalities. Fowler & Kennedy (2009, cited below) provide us with a very helpful summary of how this process worked

 

Albert et al.  used a 3-way screening procedure   involving 100 different candidate proteins.

 

Candidate prionogenic proteins had to show that they formed aggregates in living yeast that were supplied with all the usual protein precursors in culture that would enable them easily to manufacture proteins, (and therefore beta sheets) ,  in  abundance . 69 out of 100 candidates passed this test.

 

Candidate prionogenic proteins had to form aggregates in  relatively protein deprived in-vitro tests.  Despite the lack of a healthy protein assembly line in operation, 50 of the 69 remaining  candidates passed this test, scavenging enough material to form aggregates in three days.

 

These successful prionogenic proteins all shared that highly indicative denaturing resistance factor.

 

Finally candidate prionogenic proteins had to work well  with another protein called Hsp104p, which was important to the reproductive budding process, so that enough of the prionogenic proteins reliably transferred into each bud, so as to perpetuate the prions and their resulting beta sheets in succeeding generations of yeast.

 

In other words, the prions had the capacity to assemble themselves not only  in parents  but also in their daughters on a continuous basis.

 

19 candidate prionogenic proteins passed this final test.

 

What was most surprising about this group of finalists ?  The fact that they were resistant to denaturing was reassuring but not the most unexpected finding.

 

That was that protein sequences rich in asparagines were dramatically more likely to be prionogenic than were those rich in glutamine.

 

This may seem like a small triumph, but it is a triumph of figuring out what raw materials seem to work best in a model assembly line from which we wish to extrapolate the more complicated and still mysterious  beta-sheet amyloid-making processes going on in our aging brains.

 

 

Tony Stankus [email protected]  Life Sciences Librarian & Professor

University of Arkansas Libraries MULN 223 East

365 North McIlroy Avenue North

Fayetteville AR 72701-4002

Voice: 4790409-0021

Fax: 479-575-4592

 

Alberti, A. Halfman, R., King, O., Kapila, A, & Lundquist, S. A systematic survey identifies prions and illuminates sequence features of prionogenic proteins. Cell,  137, 146-158.

Bahmanyar, S., Higgins, G.A., Goldgaber, D., Lewis, D.A., Morrison, J.H., Wilson, MC., Shankar, S.K. & Gajdusek, D.C.  Localization of amyloid-beta protein messenger RNA in brains from patients with Alzheimer’s Disease. Science, 237 (4810), 77-80.

Chae, Y. K., Cho, K. S., & Chun, W. (2002). A prionogenic peptide derived from Sup35 can force the whole GST fusion protein to show amyloid characteristics. Protein and Peptide Letters, 9(4), 315-321.

Chae, Y. K., Lee, K., & Kim, Y. (2004). NMR studies of the prionogenic peptide derived from Sup35 protein. Protein and Peptide Letters, 11(1), 23-28.

Fowler, D.M. & Kelly, J.W. (2009. Aggregating knowledge about prions and amyloid. Cell, 137, 20-22.

Gajdusek, D. Carleton. (2008). Kuru and its contribution to medicine. Philosophical Transactions of the Royal Society of London B – Biological Sciences, 363 (1510), 3697-3700.

Glass, R.E. (1982). Gene Function: E. Coli and its Heritable Elements. Berkeley, CA: University of California Press.

Klitzman, R. 1998. The trembling mountain: A personal account of kuru, cannibals, and mad cow disease. NY: Plenum Trade Books.

Prusiner, S. B., ed. ((2004). Prion Biology and Diseases.  Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press, 2004.

Scholthof, K.B.G., Shaw, J.G. & Zaitlin, M. eds. (1999) Tobacco Mosaic Birus: One Hundred Years of Contributions to Virology. St. Paul, MN: American Phytopathological Society Press.

Tanaka, M., & Weissman, J. S. (2006). An efficient protein transformation protocol for introducing prions into yeast. Methods in Enzymology, 412, 185-200.

Vaillancourt, P.E. ed. (2003). E. Coli Gene Expression Protocols. Totawa, NJ: Humana Press, 2003.

Volkov, K. V., Aksenova, A. Y., Soom, M. J., Osipov, K. V., Svitin, A. V., Kurischko, C., et al. (2002). Novel non-Mendelian determinant involved in the control of translation accuracy in Saccharomyces cerevisiae. Genetics, 160(1), 25-36.

Yam, P. (2003). The Patholgical Protein: Mad Cow, Chronic Wasting, & Other Deadly Prion Diseases. NY: Copernicus Books, 2003. (A popularization).

Zimmer, C. (2008). Microcosm:  E. Coli And The New Science Of Life. NY: Pantheon Books.

The Evolutionary Psychology of Women’s Preferences for Differing Types of Men at Different Times of Their Reproductive Cycles & Men’s Responses To Them

A very recent paper in Biology Letters (Durante & Li cited below) reports that the level of certain female hormones, in this case, estradiol (American spelling) appears to influence the propensity of women to mate “opportunistically.”

That last phrase, “mate opportunistically,”  is becoming an established term in a field with increasing scientific validity called evolutionary psychology.

Despite the use of the term “psychology,” the overall field has considerable participation from researchers in clinical medicine ( particularly those in endocrinology, medical genetics,  and obstetrics/gynecology), and by basic scientists in the fields of sensory physiology and physical anthropology.

It should be stated outright that the vast bulk of studies deal with heterosexual attractions and relationships, and moreover, the researchers neither make value judgments on personal choices among consenting  adults nor do they condone or advocate behavior that is unlawful or known to be detrimental to children.

Women’s Preferences in Male Mates According to Contemporary Studies in Evolutionary Psychology

Study and research in evolutionary psychology involves many different topical subdivisions, but this blog will focus on what the field itself calls “reproductive strategies.”

A core question is: What do women, consciously or unconsciously,  look for in mates, and when do they do that looking most intensely?

Evolutionary psychology basically posits that despite whatever overt and intellectually well-though-out  statements given by even  highly educated women upon being interviewed, of what  they seek in male sexual partners (at least in economically developed Western societies in North & South America,  Europe, Australia,  New Zealand and the more prosperous parts of Asia), underlying biological urges that have been encoded as offspring survival genes govern their reproductive behavior and  probably actually drive who and what arouses them and when.

Hormones & Assessments of Attractiveness

There are a plethora of studies (many of them cited below) that show a very strong connection between hormone levels in the sexual behaviors of both women and men.

Probably the most  striking finding is that each gender is a pretty good if unconscious estimator of the times of the month when women of reproductive age seem to be most interested in sexual activity.

The levels of certain hormones, particularly estradiol and somewhat surprisingly testosterone, in women make those women feel more attractive and sexually aroused, not only  in their own judgment of themselves, but among their fellow women’s judgment of them, and especially in how the aroused women  estimate that men will be  attracted to them, as compared to mens’ attraction at those times when their hormone levels are not so high.

This persistent finding has been confirmed through a wide variety of means of measuring both the hormone levels of women being studied, and interviewing the same women about their own desirability and attractiveness , and interviewing and videotaping women making desirability or attractiveness  judgments about other women, and men making judgments about the same women.  There is a surprising degree of concordance among all these groups.

Given that most women do not carry a hormone meter to estimate the levels in themselves, in other women, or even in men, and therefore cannot be consciously, cognitively certain of them, how do they unconsciously project and self-rate their relative attractiveness and arousability?

Frankly, they do so by looking at their own faces and bodies, which turns out to be the same way other women and men unconsciously gauge them.   Ratings of  the facial attractiveness  of women at, or near, peak monthly fertility are actually higher than ratings of the same faces at other phases of the month. In addition, their own unconscious scanning of desirable males kicks into high gear, and their preferences for differing types of male shifts, often dramatically, if only temporarily.

Short-Term Sex Partners Versus Long-Term Mates

Most of these studies and arguments concerning sexual behavior from the viewpoint of   evolutionary psychology and the strong effects of hormones and acquiring good genes for their children, would be of perhaps  academic interest only, if they did not have, in fact, real life consequences and strong explanatory power for actual behaviors.

There is ample medical evidence through the analysis of  DNA records of children and parents in economically advanced countries, ironically often undertaken for reasons apart from legal determinations of paternity, most commonly for testing for inherited diseases, that the apparent father, that is, the man who thinks, acts, and financially and emotionally supports a child,  as if he is surely the parent of that particular  child,  is not in fact, the biological father.

This excludes children who are adopted or children who are the result of known prior relationships.

A meta-analysis of 17 studies by Bellis et al (cited below) indicates that the range is somewhere between eight-tenths of one percent and thirty percent, but is  overall around three children out of 100, with variations understandably based on local culture, economic and social mores.

Evolutionary  psychology tends to explain this surreptitious   mating on the woman’s part by a short-term shift of interest in, and  visceral need for,  really “masculine” appearing men, who figuratively (and according to some studies physiologically) exude the air of dominance over other men, even if this is not the woman’s  overtly stated preference, or even their unconscious preference for most of the rest of the month.

For the sake of imagery, women who feel themselves to be at the highest level of sexual  attractiveness  and  willingness to have sex are always going to pick a Tony Soprano (as opposed to say, a Tony Stankus) because he is seen as the ultimate alpha male.

Evolutionary psychologists suggests that this goes back to females seeking the very strongest male among a clan to father their children so that the resulting children would be more likely to be strong owing to the father’s genes for size, vigor and strength.

Under the strong man-good genes strategy, both mother and children would ideally be better protected, and better provided with resources, because potentially, the putative father becomes emotionally invested in them to boot, and in particular defend them against any aggressive males who intend them harm.

Men reciprocate this attraction by unconsciously or otherwise seeking out women who appear to be more confident about their own sexual attractiveness. They are primed for this partly by their own raised levels of testosterone, and the presence of attractive women raises the boldness of their  attempts to seek opportunities to have sex with these women, at least in part because the women’s   mere presence, and the women’s both conscious and unconscious behavior towards those men, increases the men’s  testosterone levels.

Videotaped studies show that women in such situations will flirt more often and more aggressively, even if they are unaware that they are doing so.

Pheromones, Attractiveness & Arousability

One area of ongoing controversy is whether or not men (or women) give off sexually exciting pheromones that  unconsciously stimultate women to take a higher interest in having sex with them.

The preponderance of evidence say yes, but this has not yet resulted in after-shaves or colognes for men (or women)  that claim to have pheromones that demonstrably accomplish this mission, despite advertising claims to the contrary.

One of the very most intriguing studies shows that when a male experimenter conducts a surreptitious test (meaning that no odor can be detected at the conscious level)  of male-derived  sexually attractive odorants among heterosexual women,  results will indicate an increase in the mood or state of sexual arousal of the women subjects, as opposed to having a female experimenter perform the same experiment with the same tiny dosage of pheromone.

The most important finding here  is not that the sexually arousing odorant is somehow fake or ineffective,  because theoretically  it should work the same despite the gender of the experimenter, but rather that the presence of the male potentiates the susceptibility of the women to the odorant’s arousing effect.

Ironically, the source of the male pheromones that appear to result in the greatest arousal on the part of women in serious scientific tests,  comes  from the armpit sweat of men, although armpit-sweat-smelling aftershaves and colognes seem not likely to find a successful market, for some strange reason.

Of course, pheromones and the primitive drive of having the alpha-male in the room be your sex partner in order to have optimal progeny are by no means the only factors that govern women’s attractiveness for men of a given type at times of their peak fertility.

Well educated women  in economically  advanced countries also prefer men who are bosses in their company or organization, as opposed to being mere staff employees. They prefer taller men as opposed to shorter men.  They prefer more educated men as opposed to less  educated men, and in particular, they prefer wealthier men to men or lesser means.

Evolutionary psychologists explain these preferences in the context of their being more modern proxies for evidence of dominance and  virility.  Unconsciously,  these men appear to have the right genes to pass on to have kids who turn out to be more competitive in the modern world.

 But this does not always lead to instant switching of partners in today's world.

 Studies in social psychology show that highly attractive higher-status females are more likely to abandon a financially or otherwise underpeforming male partner before  regularly having sex with a more successful male that she has since identified as more to her liking using criteria that grow more stringent with her own rise in power and self-esteem. The higher the achievement of the female, the more likely it seems that serial monogamy, rather than surreptitious flings will result.

 

Is There A Time When the Slow & Steady Nice Guy Wins Out In Ratings of Attractiveness by Females?

Do women’s stated or unconscious preferences for men of a given type change when they are not at peak fertility?

The preponderance of evidence suggests that they do.

Curiously, their preference for men in their ratings of facial attractiveness indicates that when they are not at the most  fertile peak, and in fact when they are at their highest level of progesterone, and essentially feeling the most ordinary and normal (as it were) they drop their preference for pictures of tough-guy, hypermasculine men, and go for images of men that have been computer “feminized.”

The evolutionary psychology of this phenomenon is that such men would be better long-term nurturers,  even if they were not the actual fathers.

Unfortunately for such men, some studies are showing that hypermasculine men who also show an interest to attractive and fertile females  in fathering their babies (which does not necessarily mean that they are equally interested nurturing them in the long run) are more likely to be successful in having sex with these females,  resulting in what evolutionary psychologists call an increasing number of cases of  “paternal discrepancy”.

In other words, the mild-mannered steady guy has much more risk of raising some alpha male’s child unawares, because this evolutionary combination yields the best of both worlds for the mother and the child, because the short-term acquisition of the strongman genes  by the mother and the long-term partnering in the  good nurturing of the child, are both more reliably obtainable through this subterfuge.

Of course, this has the propensity of mild-mannered men raising alpha-male sons who are more likely to sense the discordant paternity, and ironically the sons may end up having  the evolutionary psychology that makes them identify and behave like their biological fathers, in dominating mild-mannered males, perhaps including their own fathers.

Men Are So Obvious in What Attracts Them! (But They Often Agree With Women in Ratings of Attractiveness)

To the surprise of some, men and women have fundamentally the same preferences and propensities in rating women’s faces as attractive. Symmetry of facial features and a certain proportionality among them is highly valued.

Curiously men and women also tend to agree, in part, on what constitutes an attractive body.

Both rate those  women more highly who have a ratio in which the waist is significantly narrower than the hips, even though the absolute measurements may be larger or smaller in different cultures. In other words, you could have a twenty four inch waist, or a thirty inch waist, and the key is that your hips ought to be proportionally larger, up to a point.

This almost universal preference low waist-to-hip ratio seems to fly in the face of arguments that would suggest that women with large amounts of body fat around their stomach, would have ample stores of energy that would ensure better the survival of the children that she would nurse, particularly  in times of food scarcity.

Therefore you would think that evolutionary theory would predict that  these fat women would seem to be preferred partners for virile males who seek to have many healthy offspring.

But except for isolated cultures, obese women are not rated very highly in scales of attractiveness by either men or women.

For what it is worth, the same is increasingly true on ratings the sexual attractiveness of obese men.

Only smokers now do worse than fat people of either of either gender in polling.

However, evolutionary psychology now  suggests that modern medical thinking on abdominal obesity as posing a multitude of serious health risks for the woman, and therefore burdens on the male and on the children, seems to have already been encoded genetically over the course of human evolution.

In other words, a chronically sick female is optimal for neither the male nor the offspring, and genes that lead to that adverse body type will be ultimately selected against.

What is the dominant physical trait that men in Western cultures seek after an acceptable waist-to-hip ratio?

Well, it’s  breast size, of course. 

Apart from East Asia, which has an outright revulsion to the notion of large breasts as being attractive features (Barbie dolls have to be modified in order to sell successfully there), large, and prominently projecting  breasts are the premier runaway (if not fashion model runway) successes as male attractants.

Two  studies are particularly illuminating, if not surprising. In a pair of serious scientific studies by Gueguen (cited below), videotapes of women in which they wore the same outfits, but had bras that allowed them progressively to increase their bust size, experienced more men “hitting on them” in night clubs and actually had more male (but not female) car drivers pick them up when they were hitchhiking,  when they  displayed the larger breasts versus the smaller ones.

(Hitchhiking in Europe is a far safer, more common, and even socially promoted practice,  particularly for teens and the 20 year old confederate who was used in the study.)    

Evolutionary psychology suggests that this is owing to large breasts being a good advertisement for healthy nutritional status, optimal reproductive potential,  and overall good health, a finding, that while unpopular in some quarters, I’m sure, actually appears true in modern medicine and recent studies of physical anthropology.  (see Jasienska et al, cited below).

But in any case, evolutionary psychology is not as much destiny as unconscious reproductive propensity, and it is scarcely infallible. If it were,  would not everyone who is reading this blog with eyeglasses or contact lenses not be here, owing to genes for bad eyesight? 

Tony Stankus [email protected] Life Sciences Librarian & Professor

University of Arkansas Libraries MULN 223E

365 North McIlroy Avenue

Fayetteville, AR 72701-4002

Voice: 479-409-0021

Fax: 479-575-4592

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