brain evolution

More on Alzheimer's genes

Alzheimer's the evolutionary cost of better brains?

A recent genetics paper reports on evidence that changes in six genes involved in human brain development occurred around 50,000 to 200,000 years ago. These mutations may have helped increase the connectivity of our neurons, making us smarter. But these same genes are also implicated in Alzheimer's. Researchers speculate that the disorder is thus connected to our increased intelligence — the price we pay for having better brains. This is not inconsistent with a previous suggestion that the myelin ("white matter") sheathing our brain wiring was the key evolutionary change in making us unique, and that this myelin sheathing may also be the cause of our unique vulnerability to neurological disorders.

The study examined the genomes of 90 people with African, Asian, or European ancestry.

http://www.scientificamerican.com/article/alzheimer-s-origins-tied-to-rise-of-human-intelligence/

http://biorxiv.org/content/early/2015/05/26/018929

Genetics overlap found between Alzheimer's disease and cardiovascular risk factors

Data from genome-wide association studies of more than 200,000 individuals has revealed a genetic overlap between Alzheimer's disease and two significant cardiovascular disease risk factors: high levels of inflammatory C-reactive protein (CRP) and plasma lipids. The two identified genes (HS3ST1 and ECHDC3, on chromosomes 4 and 10) were not previously associated with Alzheimer's risk. However, the association of high plasma lipid levels and inflammation with Alzheimer's risk is supported by previous research.

The findings support the idea that inflammation and high blood lipids play a role in dementia risk, and may offer therapeutic targets.

http://www.eurekalert.org/pub_releases/2015-04/uoc--gof041615.php

How genetic changes lead to familial Alzheimer's disease

Variants in the presenilin-1 gene are the most common cause of inherited, early-onset Alzheimer's. Because presenilin is a component of gamma secretase, which cuts up amyloid precursor protein into Abeta40 and Abeta42 (the protein found in plaques), it's been thought that these presenilin-1 variants increase the activity of gamma secretase. However, attempts to stop Alzheimer's by using drugs to block gamma-secretase have so far been fruitless (indeed, counter-productive). Now a new mouse study has explained why: it appears that the presenilin-1 variants may in fact decrease, rather than increase, the activity of gamma-secretase. This suggests that the presenilin-1 variants are acting on other causes of Alzheimer's, and also suggests the possibility that restoring gamma-secretase, rather than blocking it, may be a more effective therapeutic strategy.

Mice genetically engineered for Alzheimer's are usually given dispositions for excessive amyloid plaques. However, it's becoming clear that Alzheimer's is more complex than a single cause. This may explain the signal failure of mouse models to provide treatments that work on humans. This research provides a different mouse model, which may help in the development of treatments.

http://www.eurekalert.org/pub_releases/2015-03/nion-srh031115.php

Mining big data yields new Alzheimer's gene

Analysis of brain scans from the ENIGMA Consortium and genetic information from The Mouse Brain Library has revealed a new gene for Alzheimer's risk. The gene MGST3 regulates the size of the hippocampus.

The finding confirms the importance of hippocampal volume for maintaining memory and cognition, and supports the idea that “cognitive reserve” helps prevent age-related cognitive decline and dementia.

http://www.eurekalert.org/pub_releases/2014-10/uom-mbd100914.php

Gene involved in waste removal increases risk of Alzheimer's & other neurodegenerative disorders

Previous research has pointed to the gene TREM2 as a genetic risk factor for Alzheimer's disease. A recent study explains why variants in this gene might be associated with neurodegenerative disorders such as Alzheimer's, Parkinson's, ALS, and frontotemporal dementia.

It appears that the gene is involved in the microglia — the “cleaners” of the brain. Variants in the gene affect the recognition of waste products left behind by dead cells, reducing the amount of debris that the microglia can cope with.

The finding may point to a way of slowing the progression of these neurodegenerative diseases even when the disease is well established.

http://www.eurekalert.org/pub_releases/2014-07/lm-ndg070314.php

http://www.eurekalert.org/pub_releases/2014-07/uadb-lbp070314.php

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Why bigger brains developed

December, 2010

More support for the theory that bigger brains were a response to living in social groups comes from a wide-ranging comparison of 511 mammalian species, but a comparison of wasp brains over time points to the importance of parasitism.

A comparison of the brain and body size of over 500 species of living and fossilised mammals has found that the brains of monkeys grew the most over 60 million years, followed by horses, dolphins, camels and dogs. Those with relatively bigger brains tend to live in stable social groups. The brains of more solitary mammals, such as cats, deer and rhino, grew much more slowly during the same period.

On the other hand, a new study comparing wasp brains over time has revealed that the mushroom bodies (neural clusters responsible for processing and remembering smells and sights) of parasitic wasps are consistently larger and more complex than those of nonparasitic wasps, which represent the very oldest form of wasp.

Previously, findings that social insects tend to have larger mushroom bodies than solitary ones have lead researchers to believe that the transition from solitary to social living was behind the larger brain regions. These new findings suggest that it is parasitism (which evolved 90 million years before social insects appear) that is behind the growth in size. That may be because well-developed mushroom bodies help parasitic wasps better locate hosts for their larvae.

Of course, this doesn’t rule out the possibility that sociality lead to another boost in size and complexity, and indeed the researchers suggest that these neurological developments may have been a crucial precursor for central place foraging. This behavior is widespread in this group of insects (the Aculeata), requires extensive spatial learning, and may have contributed to the various developments of social behavior. A comparison of the brains of social worker bees and those of parasitic wasps would be helpful.

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Big brains attributed to mother's care

October, 2010

Comparison of marsupial and placental mammal brains reveals that maternal investment is a critical factor in evolving a large brain, and primates benefit from two approaches.

Analysis of the brain sizes of 197 marsupial and 457 placental mammals has found that marsupial mammals (e.g. kangaroos, possums), had relative brain sizes that are at least as big as placental mammals. Previous belief that marsupials have relatively smaller brains appears to be produced by the inclusion of the one outlier group — primates. In both placental and marsupial groups, big brains were correlated to length of maternal care (i.e. lactation). Basal metabolic rate (the energy an animal expends at rest), although correlated with brain size in placental mammals, did not correlate with marsupial brain size. Because brain tissue uses so much energy, it has been assumed that a high metabolism was a prerequisite for a big brain.

The new findings indicate that maternal investment is a more critical factor than metabolic rate. It may also be that primates have been especially advantaged by combining both methods of increasing brain size: fast growth in the womb with the help of the mother’s high metabolic rate (placental method), and slower but lengthy growth after birth with the help of extended lactation (marsupial method).

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[1895] Weisbecker, V., & Goswami A.
(2010).  Brain size, life history, and metabolism at the marsupial/placental dichotomy.
Proceedings of the National Academy of Sciences. 107(37), 16216 - 16221.

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Our brains not as unique as we thought

July, 2010

New technology shows that the structure of the mammalian brain is not as special as we thought it was -- an area of the chicken brain shows the same structure.

For a long time, it has been assumed that mammals have different (better!) brains than other animals — partly because of the highly convoluted neocortex. Specifically, the mammalian neocortex features layers of cells (lamination) connected by radially arrayed columns of other cells, forming functional modules characterized by neuronal types and specific connections. Early studies of homologous regions in nonmammalian brains found no similar arrangement. Now new technology has revealed that a part of the chicken brain that handles auditory information is also composed of laminated layers of cells linked by narrow, radial columns of different types of cells with extensive interconnections that form microcircuits that are virtually identical to those found in the mammalian cortex. The finding suggests that the distinct structure of the mammalian neocortex has evolved from circuitry dating back at least 300 million years. The findings also indicate that mammalian and bird brains are more alike than we thought.

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[1637] Wang, Y., Brzozowska-Prechtl A., & Karten H. J.
(2010).  Laminar and columnar auditory cortex in avian brain.
Proceedings of the National Academy of Sciences. 107(28), 12676 - 12681.

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Support for the social brain hypothesis from bees

March, 2010

The first comparison of the brain sizes of social and non-social individuals of the same species provides more support for the social brain hypothesis (we evolved our big brains to deal with social groups).

The first comparison of the brain sizes of social and non-social individuals of the same species provides more support for the social brain hypothesis (we evolved our big brains to deal with social groups). The tropical sweat bee species, Megalopta genalis, have two sorts of queen: solitary ones, who themselves go out from the nest to forage for food, or social ones — who stay at home and sends out her daughters. Although even the social queens don't have bigger brains overall, the area associated with learning and memory (the mushroom body) was more developed in the social queens than in the solitary bees (and also the social daughters — suggesting dominance is also a factor).

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