Rhythmic brain circuits built from stem cells

The TV commercial is nearly 20 years old but I remember it vividly: a couple is driving down a street when they suddenly realize the music on their tape deck is in sync with the repetitive activity on the street. From the guy casually dribbling a basketball to people walking along the sidewalk to the delivery people passing packages out of their truck, everything and everyone is moving rhythmically to the beat.

The ending tag line was, “Sometimes things just come together,” which is quite true. Many of our basic daily activities like breathing and walking just come together as a result of repetitive movement. It’s easy to take them for granted but those rhythmic patterns ultimately rely on very intricate, interconnected signals between nerve cells, also called neurons, in the brain and spinal cord.

Circuitoids: a neural network in a lab dish

A CIRM-funded study published yesterday in eLife by Salk Institute scientists reports on a method to mimic these repetitive signals in a lab dish using neurons grown from embryonic stem cells. This novel cell circuitry system gives the researchers a tool for gaining new insights into neurodegenerative diseases, like Parkinson’s and ALS, and may even provide a means to fix neurons damaged by injury or disease.

The researchers changed or specialized mouse embryonic stem cells into neurons that either stimulate nerve signals, called excitatory neurons, or neurons that block nerve signals, called inhibitory neurons. Growing these groups of cells together led to spontaneous rhythmic nerve signals. These clumps of cells containing about 50,000 neurons each were dubbed circuitoids by the team.

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Confocal microscope immunofluorescent image of a spinal cord neural circuit made entirely from stem cells and termed a “circuitoid.” Credit: Salk Institute.

Making neural networks dance to a different beat

A video produced by the Salk Institute (see below), shows some fascinating microscopy visualizations of these circuitoids’ repetitive signals. In the video, team leader Samuel Pfaff explains that changing the ratio of excitatory vs inhibitory neurons had noticeable effects on the rhythm of the nerve impulses:

“What we were able to do is combine different ratios of cell types and study properties of the rhythmicity of the circuitoid. And that rhythmicity could be very tightly control depending on the cellular composition of the neural networks that we were forming. So we could regulate the speed [of the rhythmicity] which is kind of equivalent to how fast you’re walking.”

It’s possible that the actual neural networks in our brains have the flexibility to vary the ratio of the active excitatory to inhibitory neurons as a way to allow adjustments in the body’s repetitive movements. But the complexity of those networks in the human brain are staggering which is why these circuitoids could help:

Samuel Pfaff. (Salk Institute)

Samuel Pfaff. (Salk Institute)

“It’s still very difficult to contemplate how large groups of neurons with literally billions if not trillions of connections take information and process it,” says Pfaff in a press release. “But we think that developing this kind of simple circuitry in a dish will allow us to extract some of the principles of how real brain circuits operate. With that basic information maybe we can begin to understand how things go awry in disease.”

Brain Models Get an Upgrade: 3D Mini-Brains

Every year, companies like Apple, Microsoft and Google work tirelessly to upgrade their computer, software and smartphone technologies to satisfy growing demands for more functionality. Much like these companies, biomedical scientists work tirelessly to improve the research techniques and models they use to understand and treat human disease.

Today, I’ll be talking about a cool stem cell technology that is an upgrade of current models of neurological diseases. It involves growing stem cells in a 3D environment and turning them into miniature organs called organoids that have similar structures and functions compared to real organs. Scientists have developed techniques to create organoids for many different parts of the body including the brain, gut, lungs and kidneys. These tiny 3D models are useful for understanding how organs are formed and how viruses or genetic mutations can affect their development and ability to function.

Brain Models Get an Upgrade

Organoids are especially useful for modeling complex neurological diseases where current animal and 2D cell-based models lack the ability to fully represent the cause, nature and symptoms of a disease. The first cerebral, or brain, organoids were generated in 2013 by Dr. Madeline Lancaster in Austria. These “mini-brains” contained nerve cells and structures found in the cortex, the outermost layer of the human brain.

Since their inception, mini-brains have been studied to understand brain development, test new drugs and dissect diseases like microcephaly – a disease that causes abnormal brain development and is characterized by very small skulls. Mini-brains are still a new technology, and the question of whether these organoids are representative of real human brains in their anatomy and behavior has remained unanswered until now.

Published today in Cell Reports, scientists from the Salk Institute reported that mini-brains are more like human brains compared to 2D cell-based models where brain cells are grown in a single layer on a petri dish. To generate mini-brains, they collaborated with a European team that included the Lancaster lab. They grew human embryonic stem cells in a 3D environment with a cocktail of chemicals that prompted them to develop into brain tissue over a two-month period.

Cross-section of a mini-brain. (Madeline Lancaster/MRC-LMB)

Cross-section of a mini-brain. (Madeline Lancaster/MRC-LMB)

After generating the mini-brains, the next step was to prove that these organoids were an upgrade for modeling brain development. The teams found that the cells and structures formed in the mini-brains were more like human brain tissue at the same stage of early brain development than the 2D models.

Dr. Juergen Knoblich, co-senior author of the new paper and head of the European lab explained in a Salk News Release, “Our work demonstrates the remarkable degree to which human brain development can be recapitulated in a dish in cerebral organoids.”

Are Mini-Brains the Real Thing?

The next question the teams asked was whether mini-brains had similar functions and behaviors to real brains. To answer that question, the scientists turned to epigenetics. This is a fancy word for the study of chemical modifications that influence gene expression without altering the DNA sequence in your genome. The epigenome can be thought of as a set of chemical tags that help coordinate which genes are turned on and which are turned off in a cell. Epigenetics plays important roles in human development and in causing certain diseases.

The Salk team studied the epigenomes of cells in the mini-brains to see whether their patterns were similar to cells found in human brain tissue. Interestingly, they found that the epigenetic patterns in the 3D mini-brains were not like those of real brain tissue at the same developmental stage. Instead they shared a commonality with the 2D brain models and had random epigenetic patterns. While the reason for these results is still unknown, the authors explained that it is common for cells and tissues grown in a lab dish to have these differences.

In a Salk news release, senior author and Salk professor Dr. Joseph Ecker said that even though the current mini-brain models aren’t perfect yet, scientists can still gather valuable information from them in the meantime.

“Our findings show that cerebral organoids as a 3D model of brain function are getting closer to a real brain than 2D models, so perhaps by using the epigenetic pattern as a gauge we can get even closer.”

And while the world eagerly waits for the next release of the iPhone 7, neuroscientists will be eagerly waiting for a new and improved version of mini-brains. Hopefully the next upgrade will produce organoids that behave more like the real thing and can model complex neurological diseases, such as Alzheimer’s, where so many questions remain unanswered.

Stem Cell Stories that caught our eye: a womb with a view, reversing aging and stabilizing stem cells

Here are some stem cell stories that caught our eye this past week. Some are groundbreaking science, others are of personal interest to us, and still others are just fun.

Today we bring you a trifecta of stem cell stories that were partially funded by grants from CIRM.

A womb with a view: using 3D imaging to observe embryo implantation. Scientists have a good understanding of how the beginning stages of pregnancy happen. An egg cell from a woman is fertilized by a sperm cell from a man and the result is a single cell called a zygote. Over the next week, the zygote divides into multiple cells that form the developing embryo. At the end of that period, the embryo hatches out of its protective membrane and begins implanting itself into the lining of the mother’s uterus.

It’s possible to visualize the early stages of embryo development in culture dishes, which has helped scientists understand the biological steps required for an embryo to survive and develop into a healthy fetus. However, something that is not easy to observe is the implantation stage of the embryo in the uterus. This process is complex and involves a restructuring of the uterine wall to accommodate the developing embryo. As you can imagine, replicating these events would be extremely complicated and difficult to do in a culture dish, and current imaging techniques aren’t adequate either.

That’s where new CIRM-funded research from a team at UCSF comes to the rescue. They developed a 3D imaging technology and combined it with a previously developed “tissue clearing” method, which uses chemicals to turn tissues translucent, to provide clear images of the uterine wall during embryo implantation in mice. Their work was published this week in the journal Development.

According to a UCSF news release,

“Using their new approach, the team observed that the uterine lining becomes extensively folded as it approaches its window of receptivity for an embryo to implant. The geometry of the folds in which the incoming embryos dwell is important, the team found, as genetic mutants with defects in implantation have improper patterns of folding.”

Ultimately, the team aims to use their new imaging technology to get an inside scoop on how to prevent or treat pregnancy disorders and also how to improve the outcome of pregnancies by in vitro fertilization.

Senior author on the study, UCSF professor Diana Laird concluded:

“This new view of early pregnancy lets us ask fundamentally new questions about how the embryo finds its home within the uterus and what factors are needed for it to implant successfully. Once we can understand how these processes happen normally, we can also ask why certain genetic mutations cause pregnancies to fail, to study the potential dangers of environmental toxins such as the chemicals in common household products, and even why metabolic disease and obesity appears to compromise implantation.”

If you want to see this womb with a view, check out the video below.

Watch these two videos for more information:

Salk scientists reverse signs of aging in mice. For our next scintillating stem cell story, we’re turning back the clock – the aging clock that is. Scientists from the Salk Institute in La Jolla, reported an interesting method in the journal Cell  that reverses some signs of aging in mice. They found that periodic expression of embryonic stem cell genes in skin cells and mice could reverse some signs of aging.

The Salk team made use of cellular reprogramming tools developed by the Nobel Prize winning scientist Shinya Yamanaka. He found that four genes normally expressed in embryonic stem cells could revert adult cells back to a pluripotent stem cell state – a process called cellular reprogramming. Instead of turning adult cells back into stem cells, the Salk scientists asked whether the Yamanaka factors could instead turn back the clock on older, aging cells – making them healthier without turning them back into stem cells or cancer-forming cells.

The team found that they could rejuvenate skin cells from mice without turning them back into stem cells if they turned on the Yamanaka genes on for a short period of time. These skin cells were taken from mice that had progeria – a disease that causes them to age rapidly. Not only did their skin cells look and act younger after the treatment, but when the scientists used a similar technique to turn on the Yamanaka genes in progeria mice, they saw rejuvenating effects in the mice including a more rapid healing and regeneration of muscle and pancreas tissue.

(Left) impaired muscle repair in aged mice; (right) improved muscle regeneration in aged mice subjected to reprogramming. (Salk Institute)

(Left) impaired muscle repair in aged mice; (right) improved muscle regeneration in aged mice subjected to reprogramming. (Salk Institute)

The senior author on the study, Salk Professor Juan Carlos Izpisua Belmonte, acknowledged in a Salk news release that this is early stage work that focuses on animal models, not humans:

“Obviously, mice are not humans and we know it will be much more complex to rejuvenate a person. But this study shows that aging is a very dynamic and plastic process, and therefore will be more amenable to therapeutic interventions than what we previously thought.”

This story was very popular, which is not surprising as aging research is particularly fascinating to people who want to live longer lives. It was covered by many news outlets including STATnews, Scientific American and Science Magazine. I also recommend reading Paul Knoepfler’s journal club-style blog on the study for an objective take on the findings and implications of the study. Lastly, you can learn more about the science of this work by watching the movie below by the Salk.

Movie:

Stabilizing unstable stem cells. Our final stem cell story is brought to you by scientists from the UCLA Broad Stem Cell Research Center. They found that embryonic stem cells can harbor genetic instabilities that can be passed on to their offspring and cause complications, or even disease, later in life. Their work was published in two separate studies in Cell Stem Cell and Cell Reports.

The science behind the genetic instabilities is too complicated to explain in this blog, so I’ll refer you to the UCLA news release for more details. In brief, the UCLA team found a way to reverse the genetic instability in the stem cells such that the mature cells that they developed into turned out healthy.

As for the future impact of this research, “The research team, led by Kathrin Plath, found a way to correct the instability by resetting the stem cells from a later stage of development to an earlier stage of development. This fundamental discovery could have great impact on the creation of healthy tissues to cure disease.”

Advancements in gene editing make blind rats see light

Gene editing is a rapidly advancing technology that scientists are using to manipulate the genomes of cells with precision and accuracy. Many of these experiments are being conducted on stem cells to genetic mutations in an attempt to find cures for various diseases like cancer, HIV and blindness.

Speaking of blindness, researchers from the Salk Institute reported today that they’ve improved upon the current CRISPR/Cas9 gene editing technology and found a more efficient way to edit the genomes of cells in living animals. They used their technology on blind rats that had a genetic disease called retinitis pigmentosa (RP) and found that the rats were able to see light following the treatment.

The really exciting part about their findings is that their CRISPR technology works well on dividing cells like stem cells and progenitor cells, which is typically how scientists use the CRISPR technology, but it also works on adult cells that do not divide – a feat that hasn’t been accomplished before.

Their results, which were published today in the journal Nature, offer a new tool that scientists can use to target cells that no longer divide in tissues and organs like the eye, brain, pancreas and heart.

According to a Salk news release:

“The new Salk technology is ten times more efficient than other methods at incorporating new DNA into cultures of dividing cells, making it a promising tool for both research and medicine. But, more importantly, the Salk technique represents the first time scientists have managed to insert a new gene into a precise DNA location in adult cells that no longer divide, such as those of the eye, brain, pancreas or heart, offering new possibilities for therapeutic applications in these cells.”

CRISPR gene edited neurons, which are non-dividing brain cells, are shown in green while cell nuclei are shown in blue. (Salk)

CRISPR gene edited neurons, which are non-dividing brain cells, are shown in green while cell nuclei are shown in blue. (Salk)

Salk Professor and senior author on the study, Juan Carlos Izpisua Belmonte, explained the big picture of their findings:

“We are very excited by the technology we discovered because it’s something that could not be done before. For the first time, we now have a technology that allows us to modify the DNA of non-dividing cells, to fix broken genes in the brain, heart and liver. It allows us for the first time to be able to dream of curing diseases that we couldn’t before, which is exciting.”

If you want to learn more about the science behind their new CRISPR gene editing technology, check out the Salk news release and coverage in Genetic Engineering & Biotechnology News. You can also watch this short three minute video about the study made by the Salk Institute.

Salk scientists explain why brain cells are genetically diverse

twin_boys

I’ve always wondered why some sets of genetically identical twins become not so identical later in life. Sometimes they differ in appearance. Other times, one twin is healthy while the other is plagued with a serious disease. These differences can be explained by exposure to different environmental factors over time, but there could also be a genetic explanation involving our brains.

The brain is composed of approximately 100 billion cells called neurons, each with a DNA blueprint that contains instructions that determine the function of these neurons in the brain. Originally it was thought that all cells, including neurons, have the same DNA. But more recently, scientists discovered that the brain is genetically diverse and that neurons within the same brain can have slightly different DNA blueprints, which could give them slightly different functions.

Jumping genes and genetic diversity

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Fred “Rusty” Gage: Photo courtesy Salk Institute

Why and how neurons have differences in their DNA are questions that Salk Institute professor Fred Gage has pursued for more than a decade. In 2005, his lab discovered a mechanism during neural development that causes differences in the DNA of neurons. As a brain stem cell develops into a neuron, long interspersed nuclear elements (L1s), which are small pieces of DNA, copy and paste themselves, seemingly at random, throughout a neuron’s genome.

These elements were originally dubbed “jumping genes” because of their ability to hop around and insert themselves into DNA. It turns out that L1s do more than copy and paste themselves to create changes in DNA, they also can delete chunks of DNA. In a CIRM-funded study published this week in the journal Nature Neuroscience, Gage and colleagues at the Salk Institute reported new insights into L1 activity and how it creates genetic diversity in the brain.

Copy, paste, delete

Gage and his team had clues that L1s can cause DNA deletions in neurons back in 2013. They used a technique called single-cell sequencing to record the sequence of individual neuronal genomes and saw that some of their genomes had large sections of DNA added or missing.

They thought that L1s could be the reason for these insertions and deletions, but didn’t have proof until their current study, which used an improved method to identify areas of the neuronal genome modified by L1s. This method, combined with a computer algorithm that can easily tell the difference between various types of L1 modifications, revealed that areas of the genome with L1s were susceptible to DNA cutting caused by enzymes that home in on the L1 sequences. These breaks in the DNA then cause the observed deletions.

Gage explained their findings in a news release:

“In 2013, we discovered that different neurons within the same brain have various complements of DNA, suggesting that they function slightly differently from each other even within the same person. This recent study reveals a new and surprising form of variation that will help us understand the role of L1s, not only in healthy brains but in those affected by schizophrenia and autism.”

Jennifer Erwin, first author on the study, further elaborated:

“The surprising part was that we thought all L1s could do was insert into new places. But the fact that they’re causing deletions means that they’re affecting the genome in a more significant way,” says Erwin, a staff scientist in Gage’s group.”

Insights into brain disorders

It’s now known that L1s are important for the brain’s genetic diversity, but Gage also believes that L1s could play a role in causing brain disorders like schizophrenia and autism where there is heightened L1 activity in the neurons of these patients. In future work, Gage and his team will study how L1s can cause changes in genes associated with schizophrenia and autism and how these changes can effect brain function and cause disease.

Salk Scientists Unlock New Secrets of Autism Using Human Stem Cells

Autism is a complex neurodevelopmental disorder whose mental, physical, social and emotional symptoms are highly variable from person to person. Because individuals exhibit different combinations and severities of symptoms, the concept of autism spectrum disorder (ASD) is now used to define the range of conditions.

There are many hypotheses for why autism occurs in humans (which some estimates suggest now affects around 3.5 million people in the US). Some of the disorders are thought to be at the cellular level, where nerve cells do not develop normally and organize properly in the brain, and some are thought to be at the molecular level where the building blocks in cells don’t function properly. Scientists have found these clues by using tools such as studying human genetics and animal models, imaging the brains of ASD patients, and looking at the pathology of ASD brains to see what has gone wrong to cause the disease.

Unfortunately, these tools alone are not sufficient to recreate all aspects of ASD. This is where cellular models have stepped in to help. Scientists are now developing human stem cell derived models of ASD to create “autism in a dish” and are finding that the nerve cells in these models show characteristics of these disorders.

Stem cell models of autism and ASD

We’ve reported on some of these studies in previous blogs. A group from UCSD lead by CIRM grantee Alysson Muotri used induced pluripotent stem cells or iPS cells to model non-syndromic autism (where autism is the primary diagnosis). The work has been dubbed the “Tooth Fairy Project” – parents can send in their children’s recently lost baby teeth which contain cells that can be reprogrammed into iPS cells that can then be turned into brain cells that exhibit symptoms of autism. By studying iPS cells from individuals with non-syndromic autism, the team found a mutation in the TRPC6 gene that was linked to abnormal brain cell development and function and is also linked to Rett syndrome – a rare form of autism predominantly seen in females.

Another group from Yale generated “mini-brains” or organoids derived from the iPS cells of ASD patients. They specifically found that ASD mini-brains had an increased number of a type of nerve cell called inhibitory neurons and that blocking the production of a protein called FOXG1 returned these nerve cells back to their normal population count.

Last week, a group from the Salk Institute in collaboration with scientists at UC San Diego published findings about another stem cell model for ASD that offers new clues into the early neurodevelopmental defects seen in ASD patients.  This CIRM-funded study was led by senior author Rusty Gage and was published last week in the Nature journal Molecular Psychiatry.

Unlocking clues to autism using patient stem cells

Gage and his team were fascinated by the fact that as many as 30 percent of people with ASD experience excessive brain growth during early in development. The brains of these patients have more nerve cells than healthy individuals of the same age, and these extra nerve cells fail to organize properly and in some cases form too many nerve connections that impairs their overall function.

To understand what is going wrong in early stages of ASD, Gage generated iPS cells from ASD individuals who experienced abnormal brain growth at an early age (their brains had grown up to 23 percent faster when they were toddlers compared to normal toddlers). They closely studied how these ASD iPS cells developed into brain stem cells and then into nerve cells in a dish and compared their developmental progression to that of healthy iPS cells from normal individuals.

Neurons derived from people with ASD (bottom) form fewer inhibitory connections (red) compared to those derived from healthy individuals (top panel). (Salk Institute)

Neurons derived from people with ASD (bottom) form fewer inhibitory connections (red) compared to those derived from healthy individuals (top panel). (Salk Institute)

They quickly observed a problem with neurogenesis – a term used to describe how brain stem cells multiply and create new nerve cells in the brain. Brain stem cells derived from ASD iPS cells displayed more neurogenesis than normal brain stem cells, and thus were creating an excess amount of nerve cells. The scientists also found that the extra nerve cells failed to form as many synaptic connections with each other, an essential process that allows nerve cells to send signals and form a functional network of communication, and also behaved abnormally and overall had less activity compared to healthy neurons. Interestingly, they saw fewer inhibitory neuron connections in ASD neurons which is contrary to what the Yale study found.

The abnormal activity observed in ASD neurons was partially corrected when they treated the nerve cells with a drug called IGF-1, which is currently being tested in clinical trials as a possible treatment for autism. According to a Salk news release, “the group plans to use the patient cells to investigate the molecular mechanisms behind IGF-1’s effects, in particular probing for changes in gene expression with treatment.”

Will stem cells be the key to understanding autism?

It’s clear that human iPS cell models of ASD are valuable in helping tease apart some of the mechanisms behind this very complicated group of disorders. Gage’s opinion is that:

“This technology allows us to generate views of neuron development that have historically been intractable. We’re excited by the possibility of using stem cell methods to unravel the biology of autism and to possibly screen for new drug treatments for this debilitating disorder.”

However, to me it’s also clear that different autism stem cell models yield different results, but these differences are likely due to which populations the iPS cells are derived from. Creating more cell lines from different ASD subpopulations will surely answer more questions about the developmental differences and differences in brain function seen in adults.

Lastly, one of the co-authors on the study, Carolina Marchetto, made a great point in the Salk news release by acknowledging that their findings are based on studying cells in a dish, not actual patient’s brains. However, Marchetto believes that these cells are useful tools for studying autism:

“It never fails to amaze me when we can see similarities between the characteristics of the cells in the dish and the human disease.”

Rusty Gage and Carolina Marchetto. (Salk Institute)

Rusty Gage and Carolina Marchetto. (Salk Institute)


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An inside look reveals the adult brain prunes its own branches

Did you know that when you’re born, your brain contains around 100 billion nerve cells? This is impressive considering that these nerve cells, also called neurons, are already connected to each other through an intricate, complex neural network that is essential for brain function.

Here’s how the brain does it. During development, neural stem cells produce neurons that navigate their way through the brain. Once at their destination, neurons set up shop and send out long extensions called axons and branched extensions called dendrites that allow them to form what are called synaptic connections through which they can communicate through electrical and chemical signals.

Studies of early brain development revealed that neurons in the developing brain go on overdrive and make more synaptic connections than they need. Between birth and early adulthood, the brain carefully prunes away weak or unnecessary connections, and by your mid-twenties, your brain has eliminated almost half of the synaptic connections you started out with as a baby.

This synaptic pruning process allows the brain to fine-tune its neural network and strengthen the connections between neurons that are important for brain function. It’s similar to how a gardener prunes away excess branches on fruit trees so that the resulting branches can produce healthier and better tasting fruit.

The brain can make new neurons

It was thought that by adulthood, this process of pruning excess connections between neurons was over. However, a new study from the Salk Institute offers visual proof that synaptic pruning occurs during adulthood similarly to how it does during development. The work was published today in the journal Nature Neuroscience, and it was funded in part by CIRM.

Rusty Gage, Salk Institute.

Rusty Gage, Salk Institute.

The study was led by senior author and Salk Professor Rusty Gage. Gage is well known for his earlier work on adult neurogenesis. In the late 90’s, he discovered that the adult brain can in fact make new neurons, a notion that overturned the central dogma that the brain doesn’t contain stem cells and that we’re born with all the neurons we will ever have.

There are two main areas of the adult brain that harbor neural stem cells that can generate new neurons. One area is called the dentate gyrus, which is located in the memory forming area of the brain called the hippocampus. Gage and his team were curious to know whether the new neurons generated from stem cells in the dentate gyrus also experienced the same synaptic overgrowth and pruning that the neurons in the developing brain did.

Pruning the Adult Brain

They developed a special microscope technique that allowed them to visually image the development of new neurons from stem cells in the dentate gyrus of the mouse brain. Every day, they would image the growing neurons and monitor how many dendritic branches they sent out.

Newly generated neurons (green) send out branched dendritic extensions to make connections with other neurons. (Image credit: Salk Institute)

Newly generated neurons (green) send out branched dendritic extensions to make connections with other neurons. (Image credit: Salk Institute)

After observing the neurons for a few weeks, they were amazed to discover that these new neurons behaved similarly to neurons in the developing brain. They sent out dozens of dendritic branches and formed synaptic connections with other neurons, some of which were eventually pruned away over time.

This phenomenon was observed more readily when they made the mice exercise, which stimulated the stem cells in the dentate gyrus to divide and produce more neurons. These exercise-induced neurons robustly sent out dendritic branches only to have them pruned back later.

First author on the paper, Tiago Gonçalves commented on their observations:

“What was really surprising was that the cells that initially grew faster and became bigger were pruned back so that, in the end, they resembled all the other cells.”

Rusty Gage was also surprised by their findings but explained that developing neurons, no matter if they are in the developing or adult brain, have evolved this process in order to establish the best connections.

“We were surprised by the extent of the pruning we saw. The results suggest that there is significant biological pressure to maintain or retain the dendrite tree of these neurons.”

A diagram showing how the adult brain prunes back the dendritic branches of newly developing neurons over time. (Image credit: Salk Institute).

A diagram showing how the adult brain prunes back the dendritic branches of newly developing neurons over time. (Image credit: Salk Institute).

Potential new insights into brain disorders

This study is important because it increases our understanding of how neurons develop in the adult brain. Such knowledge can help scientists gain a better understanding of what goes wrong in brain disorders such as autism, schizophrenia, and epilepsy, where defects in how neurons form synaptic connections or how these connections are pruned are to blame.

Gonçalves also mentioned that this study raises another important question related to the regenerative medicine applications of stem cells for neurological disease.

“This also has big repercussions for regenerative medicine. Could we replace cells in this area of the brain with new stem cells and would they develop in the same way? We don’t know yet.”


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Keeping elderly cells old to understand the aging process

Aging is a key risk factor for many diseases, particularly disorders of the brain like Alzheimer’s or Parkinson’s, which primarily occur in the elderly. So a better understanding of the aging process should provide a better understanding of these neurodegenerative diseases.

The induced pluripotent stem cell (iPSC) technique makes it possible to grow human brain cells, or neurons, in the lab from elderly patient skin samples. Unfortunately, this method has a major pitfall when it comes to aging research: reprogramming skin cells back into the embryonic stem cell-like state of iPSCs strips away many of their old age-related characteristics.

Based on data published last week in Cell Stem Cell, Salk Institute researchers used a different technique called direct reprogramming as a means to keep old cells old. This alternative method sidesteps the need to make iPSCs (which brings cells all the way back to the pluripotent state) and instead converts a skin cell directly into the desired cell type.

First author Jerome Mertens and senior author Rusty Gage (Courtesy of the Salk Institute for Biological Studies).

First author Jerome Mertens and senior author Rusty Gage (Courtesy of the Salk Institute for Biological Studies).

iPSC and direct reprogramming go head-to-head

The study, funded in part by CIRM, relied on skin samples from people ranging in age from newly born to 89 years. The team generated iPSC and iPSC-derived neurons from these samples. They also made so-called induced neurons (iNs) from the skin cells using the direct reprogramming method. Other CIRM grantees have pioneered direct reprogramming of skin into nerve cells (see link below).

Skin cell samples from elderly human donors are directly converted into induced neurons (iNs), shown. (image: Courtesy of the Salk Institute for Biological Studies)

Skin cells from elderly human donors are directly converted into induced neurons (iNs), shown. (Image courtesy of the Salk Institute for Biological Studies).

When comparing skin cells from donors younger than 40 years old versus cells from the over 40 group, the team found several genes had age-dependent activity patterns. Those differences virtually disappeared in the iPSCs and iPSC-derived neurons from the same individuals. However, unlike iPSCs, direct reprogramming of the skin cells to neurons (iNs) hung on to age-dependent differences in gene activity.

Loss of RanBP17 protein a fountain of youth in reverse

A deeper analysis identified one gene called RanBP17 whose activity levels declined with increased age of the donor in both the original skin cells and those directly converted into iNs. But when those same donor skin cells were turned into iPSCs or even iPSC-derived neurons, RanBP17 levels in the older cells were no longer reduced and were on par with RanBP17 levels in the younger cells. In follow up experiments, a reduction in RanBP17 protein led to glitches in the transport of proteins into the cell’s nucleus, which other studies have linked to neurodegenerative diseases as well as the aging process.

fx1

Gene expression patterns of age-related factors like RanBP17 are maintained in induced neurons but not iPSCs. (Mertens et al., 2015)

Altogether, these results encourage researchers to select iNs over iPSC-derived neurons when it comes to faithfully representing the aging process of brain cells. Based on a Salk Institute press release, you can tell that professor Martin Hezter, a contributing author, is excited about future studies with iNs:

By using this powerful approach, we can begin to answer many questions about the physiology and molecular machinery of human nerve cells–not just around healthy aging but pathological aging as well.

 


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Watch Spinal Cord Cells Take a Hike!

magic school busWhat exactly goes on inside the human body? If you asked this question to the children’s book character Ms. Frizzle, she would throw you into her Magic School Bus and take you on a wild ride “Inside the Human Body” to get you up close and personal with the different organs and structures within our bodies.

Ms. Frizzle had a wild imagination, but she was on to something with her crazy adventures. Recently, scientists took a page out of one of Ms. Frizzle children’s books and took their own wild ride to check out what’s going on with the human spinal cord.

In a paper published yesterday in Neuron, scientists from the Salk Institute in San Diego reported that they were able to watch spinal cord cells walk around the spine of mice in real-time. They used a special microscope that could track and record the movement of motor neurons, an important nerve cell that controls the movement of muscles in your body. What they found when they watched these cells was equivalent to a pot of gold at the end of the rainbow.

Check out their stunning movie here:

The scientists not only recorded the activity of these motor neurons, but they identified the other spinal cord cells that these neurons interact and make connections with. One of their most significant findings was a population of spinal cord cells that connected to a subtype of motor neurons to foster important muscle movements like walking.

Understanding how the different cells of the spinal cord work together is very important because it will allow scientists and doctors to figure out better ways to treat patients with spinal cord injuries or neurodegenerative diseases, like ALS, that affect motor neurons.

Senior author Samuel Pfaff commented in a press release on the importance of this study and how easy his team’s technology is to use:

Pfaff_S09

Samuel Pfaff

Using optical methods to be able to watch neuron activity has been a dream over the past decade. Now, it’s one of those rare times when the technology is actually coming together to show you things you hadn’t been able to see before. You don’t need to do any kind of post-image processing to interpret this. These are just raw signals you can see through the eyepiece of a microscope. It’s really a jaw-dropping kind of visualization for a neuroscientist.

While this study doesn’t provide a direct avenue for therapeutic development, it does pave the way for a better understanding of the normal, healthy processes that go on in the human spine. Having more knowledge of “what is right” will help scientists to develop better strategies to fix “what is wrong” in spinal cord injuries and diseases like ALS.


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Harder, Better, Faster, Stronger: Scientists Work to Create Improved Immune System One Cell at a Time

The human immune system is the body’s best defense against invaders. But even our hardy immune systems can sometimes be outpaced by particularly dangerous bacteria, viruses or other pathogens, or even by cancer.

Salk Institute scientists have developed a new cellular reprogramming technique that could one day boost a weakened immune system.

Salk Institute scientists have developed a new cellular reprogramming technique that could one day boost a weakened immune system.

But what if we could give our immune system a boost when it needs it most? Last week scientists at the Salk Institute for Biological Sciences devised a new method of doing just that.

Reporting in the latest issue of the journal Stem Cells, Dr. Juan Carlos Izpisua Belmonte and his team announce a new method of creating—and then transplanting—white blood cells into laboratory mice. This new and improved method could have significant ramifications for how doctors attack the most relentless disease.

The authors achieved this transformation through the reprogramming of skin cells into white blood cells. This process builds on induced pluripotent stem cell, or iPS cell, technology, in which the introduction of a set of genes can effectively turn one cell type into another.

This Nobel prize-winning approach, while revolutionary, is still a many months’ long process. In this study, the Salk team found a way to shorten the cellular ‘reprogramming’ process from several months to just a few weeks.

“The process is quick and safe in mice,” said Izpisua Belmonte in a news release. “It circumvents long-standing obstacles that have plagued the reprogramming of human cells for therapeutic and regenerative purposes.”

Traditional reprogramming methods change one cell type, such as a skin cell, into a different cell type by first taking them back into a stem cell-like, or ‘pluripotent’ state. But here, the research team didn’t take the cells all the way back to pluripotency. Instead, they simply wiped the cell’s memory—and gave it a new one. As first author Dr. Ignacio Sancho-Martinez explained:

“We tell skin cells to forget what they are and become what we tell them to be—in this case, white blood cells. Only two biological molecules are needed to induce such cellular memory loss and to direct a new cell fate.”

This technique, which they dubbed ‘indirect lineage conversion,’ uses the molecule SOX2 to wipe the skin cell’s memory. They then use another molecule called miRNA 125b to reprogram the cell into a white blood cell.

These newly generated cells appear to engraft far better than cells derived from traditional iPS cell technology, opening the door to therapies that more effectively introduce these immune cells into the human body. As Sanchi-Martinez so eloquently stated:

“It is fair to say that the promise of stem cell transplantation is now closer to realization.”