Category Archives: Biology

More trees than we thought on Earth

Agriculture, Biology, Earth Science

Do you know how many trees there are on Earth?

The previous estimate in 2008 was 400 billion, or 61 trees for every person.

Here, a new study used satellite imagery and tree counts on the ground to calculate there are 3 trillion trees on Earth.

This is 7 times the previous estimate and equates to 422 trees for every person.

The distribution around the world is determined by climate and human activity.

Highest tree density is in northern boreal and tundra forests (e.g. Scandinavia, North America), as well as tropical areas, which contain 43% of the planet’s trees.

But it’s not all good news.  15 billion trees are being cut down each year and we have lost nearly half of the World’s trees since farming began around 12,000 years ago.

 

Do you want more information?

Background

Trees are essential components of many diverse ecosystems around the world.

They provide shelter, carbon sequestration, oxygen production, food, water quality control and protection against erosion, amongst other benefits.

However, their usefulness for construction timber and fires (cooking, warmth), as well as competition for farming land, means they have been cleared by humans for thousands of years.

Knowing how many trees there are in the world and where they are would help policy makers manage the world’s forests against competing pressures.

A previous estimate of 400 billion trees in 2008 was thrown into doubt when a later study found 390 billion trees in the Amazon basin alone.

So how many trees are there in the world?

Materials and Methods

This study used 429,775 ground-source measurements, as well as satellite imagery, of tree density from every continent on Earth (except Antarctica) to generate a global map of forest trees.  This provided a global tree density map at 1 km2 resolution.  Trees were defined as plants with woody stems larger than 10 cm in diameter at breast height.

Results

This study estimates there are 3.04 trillion trees in the world, equating to 422 trees for each of the 7.2 billion people on Earth.

This is 7 times the previous estimate of 400 billion trees (in 2008), and is also more than the number of stars in the Milky Way galaxy.

1.39 trillion trees (46%) are in tropical and subtropical regions, 0.74 trillion (24%) in boreal regions and 0.61 trillion (22%) in temperate regions.

Tree density generally increases with temperature, with moist, warm conditions optimal for tree growth.

A negative relationship between tree density and moisture availability was discovered.  It is most likely due to competition for productive land between forests and farming.

Current global forest clearing rates are estimated to be 15.3 billion trees or 192,000 km2 cleared each year, with the highest being in tropical regions.

Since the onset of human civilisation and farming (around 12,000), it is estimated that Earth’s tree numbers have fallen by 45.8%.

Discussion

It is hoped this survey of tree numbers will help policy makers manage the forests better, especially against powerful competing interests like agriculture and timber industries.

Article

Mapping tree density at a global scale

Crowther et al., 2015 Nature 525: 201-5

Keywords

Science, Earth, World, planet, biology, ecosystem, tree, forest, rainforest, plant, agriculture, timber, wood, tropical, boreal

Subject

ST1-8ES, ACSSU019, ST1-9ES, ACSSU032, ST1-11LW, ACSSU211, ST2-8ES, ACSSU075, ST2-11LW, ACSSU073, SC4-13ES, ACSSU116, SC5-13ES, ACSSU189

Bats send ‘jamming’ signals

Biology, Physics

Did you know that bats can send ‘jamming’ signals to stop other bats catching food?

Bats emit high-pitched soundwaves to locate insect prey (echolocation).

Mexican free-tailed bats form very large colonies, therefore there is lots of competition for food.

A new type of call was discovered called sinFM (30-60 kHz) that jammed the feeding call (echolocation) of another bat.

It caused the hunting bat to miss its insect prey in mid-air (80% less likely to catch it), leaving it for the jamming bat to catch and eat.

These bats have evolved a way to jam echolocation signals of competitors when hunting, reducing their chances of catching prey and increasing their own.

 

Do you want more information?

Background

Bats emit high-pitched soundwaves to locate prey (mostly insects).

The soundwaves bounce off the target and are detected by the bat (echolocation).

The closer the bat is to the prey, the faster the soundwaves bounce back.

Echolocation allows bats to be dominant predators at night.

Mexican free-tailed bats are a common variety in America and form very large colonies. Some caves contain ~1 million bats.

Therefore, there is lots of competition for food.

They have a vocal repertoire of at least 15 communication/social calls.

Here, the authors discovered a brand new call.

Materials and Methods

Scientists recorded the hunting behaviour of Mexican free-tailed bats in Arizona and New Mexico at night (nocturnal animals) using microphones, video and infrared cameras. They also used speakers to play back particular recorded sounds to the bats. Finally, they used computer programs to reconstruct 3D flight paths of the bats.

Results

A new type of call was discovered called sinFM that jammed the feeding call (echolocation) of another bat.

It caused the hunting bat to miss its insect prey in mid-air (80% less likely to catch it), leaving it for the jamming bat to catch and eat.

sinFM soundwaves have a frequency of 30-60 kHz.

When recorded and played back through a speaker, it jammed the hunting calls of bats.

sinFM only jammed hunting calls when they overlapped. It didn’t work if played just before/after or a different noise was emitted.

Discussion

Competition for food is high in large bat colonies.

The Mexican free-tailed bat has evolved a way to jam the echolocation signals of competitors when hunting, reducing their chances of catching prey, while increasing their own.

This is the first discovery of jamming echolocation signals in animals.

Article

Bats jamming bats: Food competition through sonar interference

Corcoran and Conner, 2014 Science 346:745-7

Keywords

Bats, echolocation, sonar, frequency, Hz, sound, wave, soundwave, ultrasonic, prey, food, insect, evolution, jamming, signal

Subject

Science, biology, physics, ST1-6PW, ACSSU020, ST2-11LW, ACSSU073, SC5-10PW, ACSSU182, SC5-15LW, ACSSU185

The best thing about Angelina Jolie’s boobs

Biology, Psychology

In May 2013, famous US actress Angelina Jolie announced she had inherited a mutated BRCA1 gene (family history of breast cancer).

She decided to have a preventative double mastectomy.

In the months following Jolie’s announcement, referrals to 21 cancer clinics and genetic testing facilities in the UK increased 2.5-fold.

In Australia, referrals tripled and remained high for 6 months.

Therefore, high profile celebrities can have a global and long-lasting effect on health care.

A similar effect was observed in Australia in 2005 after Kylie Minogue revealed she had breast cancer, with screening increasing by 40%.

 

Do you want more information?

Background

Women have a 1 in 8 chance of developing breast cancer in their lives.

Their risk greatly increases if they inherit a mutated BRCA1 or 2 gene.

Early detection is important, because breast tumours are most effectively treated at early stages using drugs (e.g. tamoxifen), or by surgery before the cancer spreads to other parts of the body (metastasis).

Women with a family history of breast cancer can have genetic tests to see if they inherited mutated BRCA1/2 genes.

If so, they may choose to undergo risk-reducing mastectomy (RRM, remove their breasts).

In May 2013, famous US actress Angelina Jolie announced she had inherited a mutated BRCA1 gene (family history of breast cancer).

She decided to have a preventative double mastectomy.

What effect did her decision have on other women around the world?

Materials and Methods

This study collated referral data from 21 cancer clinics and genetic testing facilities in the UK in the months following Jolie’s announcement.

Results

In the months following Jolie’s announcement, referrals to 21 cancer clinics and genetic testing facilities in the UK increased 2.5-fold.

Referrals remained high (2-fold) 5 months later, indicating a long-lasting effect.

In Australia, referrals tripled and remained high for 6 months.

During this period, there was no change in referrals for colorectal cancer (control).

There is no evidence of an increase in inappropriate referrals (i.e. they were genuine).

Discussion

High profile celebrities can have a global and long-lasting effect on health care.

A similar effect was observed in Australia in 2005 after Kylie Minogue revealed she had breast cancer, with screening increasing by 40%.

Article

The Angelina Jolie effect: how high celebrity profile can have a major impact on provision of cancer related services

Evans et al., 2014 Breast Cancer Research 16:442

Keywords

Cancer, carcinogenesis, tumour, breast, genetic, testing, screening, gene, mutant, mutation, BRCA1, BRCA2, BRCA, mastectomy, Jolie, Minogue

Subject

Science, Biology, SC4-15LW, ACSHE119, ACSHE134, SC5-15LW, ACSSU185

Brain bit different between altruists and psychopaths

Biology, Psychology

Did you know the anatomy of the brain is different between altruists and psychopaths?

Altruists improve the welfare of others at the expense of their own.

Are they weirdos or superheros? What makes them do good deeds for other people they have never met?

This study measured the activity and size of different brain regions of altruistic kidney donors using magnetic resonance imaging (MRI).

They found their amygdalas are ~8% larger than normal people. This region is important for memory, decision-making and emotional reactions.

Also, altruists were better at recognising fear in others.

This is opposite to psychopaths, who have smaller amydalas and are less able to recognise or respond to fear in others.

Altruism may seem counterintuitive to evolution, since it benefits the success of others. However, it may simply be the extreme end of our innate tendency to care for our offspring, which does fit with evolution.

 

Do you want more information?

Background

Altruists are people that improve the welfare of others at the expense of their own.

An extreme form of altruistic behaviour is donating a kidney to an unknown recipient.

Little is known about the genetic or neural basis for altruistic behaviour.

Materials and Methods

19 altruists and 20 controls were subjected to fMRI (functional magnetic resonance imaging – measures activity of different brain regions), as well as anatomical MRI to measure size. While in the machine, they were shown pictures of people with facial expressions of fear, anger, neutral and other emotions.

NOTE: Altruistic kidney donors are very rare, so finding 19 that are willing to participate in a MRI study is a good achievement.

Results

Altruists displayed increased activity of a brain region called the amygdala when viewing fearful faces, compared to controls.

In fact, their amygdalas are ~8% larger than in normal people.

Altruists were better than controls at recognising fear, possibly due to larger and/or better functioning amygdalas.

This was specific for fear, since they were no different to controls for other emotions.

Discussion

The amygdala is located near the middle of the brain and is important for memory, decision-making and emotional reactions.

This study shows it is larger and has better function in altruists.

This is opposite to psychopaths, who have smaller amydalas and are less able to recognise or respond to fear in others.

This suggests there is a continuum of amygdala size and function, with altruists and psychopaths at opposite ends.

Altruists that can easily recognise distress in others may be highly motivated to help them.

Fear is a particularly intense/urgent emotion (can involve life or death situations) and induces infantile appearances (wide eyes and high brows) that elicit caring responses.

Altruism may seem counterintuitive to evolution/natural selection, since it benefits the success/survival of others at the expense of one’s self. However, altruism may simply be the extreme end of our innate tendency to care for our own offspring, which does fit with evolution/natural selection.

Future Directions

Could it one day be possible to develop treatments that increase the size of the amygdala (e.g. cognitive therapy, stem cells) to increase altruistic behaviour in people? Perhaps even in psychopaths? Perhaps as an adjunct or alternative approach to traditional prison or death sentences?

Article

Neural and cognitive characteristics of extraordinary altruists

Marsh et al., 2013 Proc. Natl. Acad. Sci. USA 110:6925-30

Keywords

Altruism, altruistic, psychopath, brain, neuron, neural, amygdala, MRI, fMRI, imaging, emotion, survival, fear, behaviour, evolution, natural, selection

Subject

Science, Neuroscience, Psychology, SC4-14LW, ACSSU150, SC5-15LW, ACSSU185

Penis size DOES matter!

Biology, Psychology

Did you know that women really do prefer big penises?

Scientists from Canberra and Melbourne presented women with computer-generated images of men to determine which features they prefer most.

They found that women prefer tall men with wide shoulders and large penises (wide shoulders > height = large penis).

Big penises were especially important for tall men. Possibly because the same sized penis appears smaller on a taller man, or short men are unattractive, irrespective of their penis size.

Tall women were especially attracted to tall men.

Obese women were especially attracted to large penises.

When women observed a body shape they were particularly attracted to, they leered at the image for longer.

These results support the hypothesis that female mate choice could have contributed to the evolution/selection of the relatively large male penis.

 

Do you want more information?

Background

Previous studies have shown that women are more attracted to tall men, broad shoulders and symmetrical faces.

This paper now suggests that they may also be attracted to the size of their penis.

Until recently, humans walked around naked.

Also, the human penis is relatively large compared to other species.

Several cultures use sheaths and codpieces to draw attention to the penis and large penises are often portrayed in modern media as being desirable.

Therefore, it is possible that men evolved large penises to impress women, increasing their likelihood of reproducing (positive selection).

Materials and Methods

Scientists from Canberra and Melbourne presented 105 women (average age 26) with life size computer-generated projections of men with different shoulder widths, heights and penis sizes (flaccid). They ranked their relative attractiveness using a scale of 1-7. Tests were performed alone and anonymously.

Results

Women preferred tall men, wide shoulders and large penises.

Big penises were especially important for tall men. This may be because the same sized penis appears smaller on a taller man, or short men are unattractive, irrespective of their penis size.

The attractiveness of height and penis size were approximately equal.

When women found a body shape they were particularly attracted to, they looked at it longer before scoring it (leered at it).

Tall women were especially attracted to tall men.

Obese women (high BMI) were especially attracted to large penises.

Discussion

Women find wide shoulders, height and large penises attractive. (Wide shoulders > height = large penis).

Women may perceive a relationship between large penis size and higher rates of vaginal orgasm.

These results support the hypothesis that female mate choice could have contributed to the evolution/selection of the relatively large male penis.

Article

Penis size interacts with body shape and height to influence male attractiveness

Mautz et al., 2013 Proc. Natl. Acad. Sci. USA. 110:6925-30

Keywords

Man, men, woman, women, human, body, size, shape, shoulder, tall, penis, reproduce, reproduction, mating, mate, evolution, natural, selection, computer-generated image, CGI

Subject

Science, Biology, SC4-14LW, ACSSU150, SC5-15LW, ACSSU184

Obese women’s brains can’t say no to food

Biology

Did you know the brains of obese women have trouble saying no to food?

Despite numerous educational messages about diets and lifestyle, people still eat too much. Why?

This study from Yale University performed behavioural tests (brain/decision tests) to measure people’s reactions to food and money rewards.

They showed that obese women have trouble saying no to food, but not money, indicating the impairment is specific to food.

In contrast, obese men did not have difficulty saying no to either food or money.

It’s possible a vicious cycle might arise in women, whereby obesity causes brain dysfunction, making it difficult for them to say no to food. This promotes further eating/obesity, more brain dysfunction, and so on.

 

Do you want more information?

Background

There are record levels of obesity in Western countries and the incidence is still rising.

It’s predominantly caused by modern lifestyles involving high calorie diets and a lack of exercise.

Too much energy in, not enough out, with the excess stored as fat.

Obesity is the major cause of Type 2 diabetes (also at record levels).

Diabetes (too much sugar in the blood) causes damage to blood vessels, increasing the risk of heart attack, stroke, blindness and limb amputation.

High calorie foods are highly appealing because our brains reward us for eating them. In ancient times when food was scarce, this rewarding feeling provided motivation for finding and eating food. Now that food is plentiful, it promotes excess/unnecessary eating and obesity.

Despite the numerous educational messages about diets and lifestyle, people still eat too much. Why?

Materials and Methods

This study from Yale University performed behavioural tests in which participants had to associate coloured cards with either a food or money reward (psychology, learning experiment).

Results

Obese women had difficulty inhibiting food rewards (trouble saying no to food).

This was proportional to their BMI (body mass index, measure of obesity).

However, they were able to inhibit money rewards as normal. Therefore, the impairment was specific to food.

In contrast, obese men did not have difficulty inhibiting either food or money rewards. The reason for their obesity is not yet clear.

Discussion

Obesity and diabetes are associated with impaired brain function (learning and memory) and even neurodegenerative conditions later in life (e.g. Alzheimer’s Disease).

Therefore, a vicious cycle might arise, whereby obesity causes brain dysfunction, making it difficult for people to say no to food. This promotes further eating/obesity, more brain dysfunction, and so on.

Targeting the brain and its control of eating/appetite might be a good strategy for treating obesity, using either drugs and/or cognitive therapy/psychology.

Article

Impaired associative learning with food rewards in obese women

Zhang et al., 2014 Current Biology 24:1731-6

Keywords

Obese, obesity, diabetes, fat, cognitive, cognition, eating, food, lifestyle, exercise, learning, memory, reward, calorie, BMI, body mass index

Subject

Science, Biology, Neuroscience, SC4-14LW, ACSSU150, SC5-14LW, ACSSU175

Houses get germs from humans

Biology

Did you know that humans contaminate a house, more than the other way round?

US scientists compared bacteria from home surfaces with the families that lived there using genomic sequencing.

They found bacteria from home surfaces matched the family living there, with most of the bacteria coming from human skin.

In blinded tests, floor samples easily predicted the family living there.

When people moved house, the bacteria went with them, indicating that household bacteria come from its human inhabitants.

Also, if a family member was absent for a few days, the microbial communities in the house rapidly changed to reflect that.

This study shows that humans influence (contaminate) the microbiome of a house more than the other way round.

Forensics might soon be able to match bacteria from a crime scene to a suspect.

 

Do you want more information?

Background

There are 10 times as many bacteria on a human body than human cells. – Bacteria (prokaryotes) are much smaller than human cells (eukaryotes).

They line all body surfaces, including skin, digestive tract and genitals.

Bacteria also make up about half the mass of poo.

There are up to 10,000 different species of bacteria on humans and these populations are different for each human (‘microbial fingerprint’).

Skin-to-surface contact can transfer millions of bacteria per event.

Materials and Methods

US researchers measured bacteria associated with seven families and their homes. 1625 samples were taken from hands, noses, feet, floors, door knobs, light switches, kitchen benches and pets. The amount and type of bacteria was identified by genomic sequencing.

Results

Microbial samples from home surfaces matched the family living there.

Most of the bacteria come from human skin.

In blinded tests, floor samples easily predicted the family living there.

Three families moved house during the test period and the microbial communities in both houses were similar, indicating that household bacteria come from its human inhabitants.

If a family member was absent for a few days, the microbial communities changed to reflect this. This indicates the microbiome changes rapidly.

In one house, a couple in a relationship shared with a housemate. The couple had a more similar microbial signature with each other than with the housemate, indicating frequent sharing of bacteria (e.g. kissing).

The hand has a particularly high microbiome turnover rate, probably due to hand washing that resets the communities frequently.

Discussion

This study shows humans influence (contaminate) the microbiome of a house more than the other way round.

Forensics might soon be able to match bacteria from a crime scene to a suspect.

Article

Longitudinal analysis of microbial interactions between humans and the indoor environment

Lax et al., 2014 Science 345:1048-52

Keywords

Bacteria, germs, microbiome, microbial, prokaryote, eukaryote, skin, contaminate, forensic, human, family, house, home

Subject

Science, Biology, Microbiology, ST1-11LW, ACSSU211, SC4-15LW, ACSSU112

Eight-legged super-mum

Biology

Did you know a mummy octopus guarded her eggs for 4.5 years?

A deep-sea robotic submarine found her clinging to a rock 1.4km deep off the Californian coast with a clutch of 160 eggs.

The submarine returned 18 times over the next 4.5 years and found her still protecting and oxygenating her eggs.

Over time, she shrunk in size and withered away because she didn’t eat.

Even when the scientists offered her food with the robotic arm of the submarine, she refused it.

On their final visit (53 months), the eggs had hatched and the mother went away to die.

The long brooding period is a combination of the low temperature of the deep sea (3°C) slowing down the hatchlings development, plus the selective (evolutionary) advantage of producing highly-developed hatchlings (improves their survival).

The low temp probably also helped the mother survive for so long without eating (slowed down her metabolism).

 

Do you want more information?

Background

Gestation/brooding periods differ between animals, from 2 weeks for some possums to 20 months for elephants and 48 months for the alpine salamander.

Graneledone boreopacifica is a common octopus found in the Pacific and Atlantic oceans. Its head is about 9cm long.

Materials and Methods

American marine biologists used a deep-sea robotic submarine to search a canyon 1.4km deep in the Pacific Ocean off the coast of California.

Results

The submarine found a female octopus moving slowly across the ocean floor towards a clump of rocks, 1.4 km deep.

When they returned 38 days later, she was clinging to a rock around a clutch of ~160 eggs (recognised it was her from scars on her legs).

The submarine returned 18 times over 4.5 years to find her protecting her eggs and flushing them with fresh water to oxygenate them.

It was definitely the same batch of eggs because they grew gradually and consistently from 1.5cm to 3.3cm in length.

After 40 months, baby octopuses could be seen inside the eggs.

As time went on, the mother developed pale, wrinkly skin, cloudy eyes and she shrunk in size (withered away).

She probably didn’t eat during the entire brooding period.

Even when the scientists offered her food with the robotic arm of the submarine, she refused it.

On the final visit (53 months), the eggs had hatched and the mother had gone away to die.

Discussion

This behaviour is typical of octopuses. They spend the final quarter of their lives brooding over their eggs (not eating) and then die.

However in this case, it smashes the previous octopus brooding record of 14 months, and is now the longest brooding period of any animal.

The long brooding period is a combination of slow development of the baby octopuses at the low temperature of the deep sea (3°C), which slows down their metabolism/development, and also the selective (evolutionary) advantage of producing highly-developed hatchlings (improves their survival).

The low temperature and slow metabolism probably also helped the mother survive for so long without eating.

Article

Deep-sea octopus (Graneledone boreopacifica) conducts the longest-known egg-brooding period of any animal

Robison et al., 2014 PLoS One 9:e103437

Keywords

Octopus, cephalopod, mollusc, ocean, Pacific, California, canyon, egg, brood, gestation, mother, hatchling, baby, offspring, reproduction, submarine, marine, starvation, sea

Subject

Science, Biology, Zoology, ST1-10LW, ACSSU030, ST1-11LW, ACSSU211, ST2-10LW, ACSSU072, ST3-11LW, ACSSU094, SC4-14LW, ACSSU150, SC5-14LW, ACSSU175

Dirty handshakes

Biology

Did you know some doctors from Los Angeles proposed banning handshakes in hospitals to reduce transmission of bacteria and viruses?

Handshakes transfer twice as many bacteria as a ‘high-five’, and 10 times more than a fist bump.

The presence of sputum makes it even worse.

There is a strong precedence for this. The Hungarian doctor Semmelweiss in the 1840’s noticed that mortality rates of mothers was 3 times higher in doctors wards than midwife wards. He suggested doctors wash their hands in between patients, which dramatically reduced mortality rates.

Banning handshakes might be difficult to implement, since it is deeply entrenched in many cultures.

However, as the authors say, “it would be a mistake to dismiss out of hand (pardon the pun) such a promising, intuitive and affordable ban.”

 

Do you want more information?

Background

Doctors from Los Angeles have proposed banning handshaking in hospital wards to reduce transmission of pathogens (bacteria, viruses).

This is especially important in hospitals where there are vulnerable patients (immune-compromised) or increased frequencies of antibiotic-resistant bacteria (e.g. MRSA, golden staph).

The hands a health care workers become contaminated after working with infected patients, which can be transmited to other patients.

Other studies have shown handshakes transfer twice as many bacteria as a ‘high-five’, and 10 times more than a fist bump.

The presence of sputum makes it even worse.

Compliance with antibacterial handwashes is only ~40% amongst health workers in the USA.

Results

They propose signs saying “Handshake-free zone: to protect your health and the health of those around you, please refrain from shaking hands while on these premises.”

They propose alternatives, such as waving, putting your hand over your heart or bowing.

It might be difficult to implement, since the handshake is deeply entrenched in many cultures (since at least ancient Greek times).

The open palm is a common symbol of honesty and trust.

Discussion

There is a strong precedence. The Hungarian doctor Semmelweiss was working in a Vienna maternity clinic in the 1840’s when he proposed that hospital staff should wash their hands in between patients. He noticed that the mortality rate of mothers was 3 times higher in the doctors wards than the midwife wards. He suggested the doctors wash their hands in chlorinated lime water in between patients. Although initially rejected by offended doctors, it was eventually adopted and dramatically reduced mortality rates.

Also, as the authors point out, “it would be a mistake to dismiss out of hand (pardon the pun) such a promising, intuitive and affordable ban.”

Article

Banning the handshake from the health care setting

Sklansky et al., 2014 J. Am. Med. Assoc. 311:2477-8

Keywords

Pathogen, bacteria, virus, infection, transmission, contagious, doctor, hospital, ward, maternity, Semmelweiss, hand, wash, immune, handshake

Subject

Science, Biology, Microbiology, ST1-11LW, ACSSU211, ST2-10LW, ACSSU072, ST3-11LW, ACSSU094, SC4-15LW, ACSSU112

Too many synapses and spines in Autistic brains

Biology

Did you know that children with Autism have too many brain connections?

It is normal for children’s brains to have too many synapses (connections between neurons).

The unimportant ones are naturally removed during development to improve the signal-to-noise ratio (improve efficiency of communication between neurons in the brain).

However, this doesn’t happen properly in the brains of Autistic people.

They have a net increase in synapse numbers. These extra ‘unimportant’ synapses interfere with the ‘important’ ones, leading to cognitive and communication difficulties.

Synapse removal (pruning) is mediated by autophagy (eating itself).

However, in Autism, autophagy is reduced because the activity of a protein called mTOR is too high.

A drug called rapamycin already exists that could be used to reduce mTOR activity and synapse numbers in Autistic people, although it has really nasty side effects, so more research is needed for better treatments.

 

Do you want more information?

Background

There are ~100 billion neurons in the brain and each can form up to 10,000 synapses (connections) with other neurons.

Therefore, up to 1,000,000,000,000,000 synapses (1 quadrillion).

Neurons have many neurites that extend away from their cell bodies to communicate with other neurons. Like branches from a tree trunk.

A single axon sends information, while many dendrites receive info.

Dotted along these neurites are tiny synapses: the point at which neurons meet to communicate with each other.

These can be thought of like two fists facing each other with a small gap in between. An electrical signal in the first neuron (left arm, axon) causes its pre-synapse (left fist) to release chemicals called neurotransmitters into the gap. These bind to receptors on the post-synapse (right fist), initiating an electrical signal in the second neuron (right arm, dendrite). Thus, transfer of information from one neuron to another.

Post-synapses are on tiny spines that line the surface of the dendrite, a bit like the spines/needles on a cactus.

Children’s brains have excess spines and synapses. But as we age, the spines/synapses we don’t need are removed (pruned). This helps reduce ‘noise’ in the brain, or interference from unimportant signals.

But this doesn’t happen properly in the brains of people with Autism.

Materials and Methods

This team of American scientists counted the number of spines/synapses in the brains of 26 Autistic people and 22 healthy controls after they died of unrelated causes. They ranged from 2-19 years old. The authors confirmed their results using a genetically-modified mouse that has a gene deleted from its genome, causing overproduction of spines/synapses and Autistic-like behaviours.

Results

In healthy children, spine numbers decrease 41% by adulthood.

For Autistic children, spine numbers only decrease by 16%.

Therefore, an abnormally high amount of spines in Autistic brains.

The production of spines is not altered. Instead, removal (pruning) of spines is reduced, causing a net increase in spine numbers.

Spine pruning is mediated by a process called autophagy (eats itself). A protein called mTOR is important for autophagy. Its activity is abnormally high in Autistic people, reducing the amount of autophagy and reducing the pruning of spines (hence net increase).

Genetically-modified mice that have abnormally high mTOR levels display defective autophagy and spine pruning, leading to increased spine numbers. These mice display behaviours resembling Autism in humans.

Mice were treated with a drug called rapamycin that reduced the abnormally high mTOR activity and this successful restored normal pruning of spines and alleviated the behavioural symptoms, suggesting that rapamycin might be a treatment for Autism.

Discussion

Children are born with too many spines/synapses in our brains, but the unimportant ones are naturally removed to improve the signal-to-noise ratio and efficiency of our brains. However, this does not happen properly in Autistic people, leading to too many spines/synapses and impaired cognition and communication skills.

Future Directions

Rapamycin is already used in clinics and hospitals to suppress the immune system when people receive organ transplants. This means it could quickly be adapted to treat Autism as well (called re-purposing). However, it has nasty side effects (e.g. diarrhoea, wound-healing problems), so further research is required to develop a better treatment.

Article

Loss of mTOR-dependent macroautophagy causes Autistic-like synaptic pruning deficits

Tang et al., 2014 Neuron 83:1-13

Further Reading

Enhanced synapse remodelling as a common phenotype in mouse models of Austism

Isshiki et al., 2014 Nature Communications 5:4742

This paper shows that in genetically-modified mouse models of Autism, there is an increase in excitatory synapses, but no change in inhibitory synapses. Therefore, a net increase in excitation in the brain (increased signal-to-noise and interference).

Keywords

Neuron, brain, synapse, spine, axon, dendrite, dendritic, neurotransmitter, neurotransmission, pruning, Autism, Autistic, autophagy, mouse, mice, genetically, modified

Subject

Science, Biology, Neuroscience, ST3-10LW, ACSSU043, SC4-14LW, ACSSU150, SC5-14LW, ACSSU175