The Social Psychology of Nerve Cells

The functional organization of the central nervous system depends upon a precise architecture and connectivity of distinct types of neurons. Multiple cell types are present within any brain structure, but the rules governing their positioning, and the molecular mechanisms mediating those rules, have been relatively unexplored.

A new study by UC Santa Barbara researchers demonstrates that a particular neuron, the cholinergic amacrine cell, creates a “personal space” in much the same way that people distance themselves from one another in an elevator. In addition, the study, published in the Proceedings of the National Academy of Sciences, shows that this feature is heritable and identifies a genetic contributor to it, pituitary tumor-transforming gene 1 (Pttg1).

Patrick Keeley, a postdoctoral scholar in Benjamin Reese’s laboratory at UCSB’s Neuroscience Research Institute, has been using the retina as a model system for exploring such principles of developmental neurobiology. The retina is ideal because this portion of the central nervous system lends itself to such spatial analysis. 

“Populations of neurons in the retina are laid out in single strata within this layered structure, lending themselves to accurate quantitation and statistical analysis,” explained Keeley. “Rather than being distributed as regular lattices of nerve cells, populations in the retina appear to abide by a simple rule, that of minimizing proximity to other cells of the same type. We would like to understand how such populations create and maintain such spacing behavior.”

To address this, Keeley and colleagues quantified the regularity in the population of a particular type of amacrine cell in the mouse retina. They did so in 26 genetically distinct strains of mice and found that every strain exhibited this same self-spacing behavior but that some strains did so more efficiently than others. Amacrine cells are retinal interneurons that form connections between other neurons and regulate bipolar cell output.

“The regularity in the patterning of these amacrine cells showed little variation within each strain, while showing conspicuous variation between the strains, indicating a heritable component to this trait,” said Keeley.

“This itself was something of a surprise, given that the patterning in such populations has an apparently stochastic quality to it,” said Reese, a professor in the Department of Psychological and Brain Sciences. Stochastic systems are random and are analyzed, at least in part, using probability theory.

This strain variation in the regularity of this cellular patterning showed a significant linkage to a location in the genome on chromosome 11, where the researchers identified Pttg1, previously unknown to play any role in the retina.

Working in collaboration with colleagues at the University of Tennessee Health Science Center in Memphis, Keeley’s team demonstrated that the expression of this gene varies across the 26 strains of mice and that there was a positive correlation between gene expression and regularity. They then identified a mutation in this gene that itself correlated with expression levels and with regularity. Working with colleagues at Cedars-Sinai Medical Center in Los Angeles, the team also demonstrated directly that this mutation controlled gene expression.   

“Pttg1 has diverse functions, being an oncogene for pituitary tumors, and is known to have regulatory functions orchestrating gene expression elsewhere in the body,” explained Keeley. “Within this class of retinal neurons, it should be regulating the way in which cells integrate signals from their immediate neighbors, translating that information to position the cell farthest from those neighbors.” Future studies should decipher the genetic network controlled by Pttg1 that mediates such nerve-cell spacing.

Brain traffic jams that can disappear in 30 seconds

Motorists in Los Angeles, San Francisco and other gridlocked cities could learn something from the fruit fly.

Scientists have found that cellular blockages, the molecular equivalent to traffic jams, in nerve cells of the insect’s brain can form and dissolve in 30 seconds or less.

The findings, presented in the journal PLOS ONE, could provide scientists much-needed clues to better identify and treat neurodegenerative diseases such as Alzheimer’s and Huntington’s.

“Our research suggests that fixed, permanent blocks may impede the transport of important cellular components and, ultimately, lead to cellular degeneration and death,” says lead researcher Shermali Gunawardena, PhD, an assistant professor of biological sciences in the University at Buffalo’s College of Arts and Sciences. “Conversely, blocks that resolve themselves may be benign.”

She continues: “This is an important distinction that could help researchers decide which kind or type of blocks to focus on when developing drugs and other forms of therapy for some of these debilitating diseases.”

Scientists have long known that many essential cellular components are transported along tracts of nerve cells called neuronal pathways, and that these movements are required for the growth, function and maintenance of neurons. Only recently, however, have they been able to understand the many proteins that help control these movements.

In the UB study, researchers examined isolated nerve cells from fruit fly larvae. Neuronal pathways of these larvae are similar to neuronal pathways in humans.

Traditionally, researchers have identified blockages through still images of dead larvae. These images provide a snapshot only, instead of a depiction of the behavior of the accumulated components over distinct periods of time.

UB researchers altered the approach by analyzing the neuronal pathways of living larvae. Unlike the still images, this method shows how the transport of components changes as neuronal pathways evolve over time.

The researchers found that certain blockages form and dissolve rather quickly. For example, one blockage appeared and disappeared within 29 seconds. Its relatively short life, Gunawardena said, indicates that the blockage is likely benign and not harmful to the cell.

The distinction is significant, she said, because it could allow researchers to focus on permanent blockages that likely halt cellular movement and may pose more serious health risks.

Researchers also looked at how the transport of essential materials over several days contributed to the growth of neurons. If transport was disrupted, growth of the neuron was compromised. As the neuron grew, the movement of some components carrying synaptic proteins increased while other components did not show significant changes.

This suggests that the transport of components in neuronal pathways is linked to the growth and function of the nerve cell.

Taken together, the findings suggest that more research must be conducted to better understand the spatial and temporal characteristics of how essential materials are transported within neurons of living organisms. This, in turn, will provide clues into how defects in this system can lead to neurodegenerative diseases and, perhaps, better ways to identify and treat these ailments.

Researchers See Promise in Transplanted Fetal Stem Cells for Parkinson’s

Researchers at Harvard-affiliated McLean Hospital have found that fetal dopamine cells transplanted into the brains of patients with Parkinson’s disease were able to remain healthy and functional for up to 14 years, a finding that could lead to new and better therapies for the illness.

The discovery, reported in the June 5, 2014 issue of the journal Cell Reports, could pave the way for researchers to begin transplanting dopamine neurons taken from stem cells grown in laboratories, a way to get treatments to many more patients in an easier fashion.

"We have shown in this paper that the transplanted cells connect and live well and do all the required functions of nerve cells for a very long time," said Ole Isacson, MD (DR MED SCI), director of the Neuroregeneration Research Institute at McLean and a professor of neurology and neuroscience at Harvard Medical School.

The researchers looked at the brains of five patients who got fetal cell transplants over a period of 14 years and found that their dopamine transporters (DAT), proteins that pump the neurotransmitter dopamine, and mitochondria, the power plants of cells, were still healthy at the time the patients died, in each case of causes other than Parkinson’s.

The fact that these cells had remained healthy indicated that the transplants had been successful and that the transplanted cells had not been corrupted as some researchers had suggested they likely had been in other studies, said Dr. Isacson, lead author of the paper.

"These findings are critically important for the rational development of stem cell-based dopamine neuronal replacement therapies for Parkinson’s," the paper concluded.

So far, about 25 patients worldwide have been treated with this particular method of transplanting fetal dopamine cells over a period of two decades and most saw their symptoms improve markedly, he said.

Fetal cell transplants can reduce both Parkinson’s symptoms for many years and can reduce the need for dopamine replacement drugs, even though they can take months or years to start working, the paper said.

However, Dr. Isacson said proof had been lacking that the transplanted cells were able to remain healthy — until this study. This is important for research in the transplant field to move ahead, he said.

All of the patients were in the late stages of Parkinson’s disease at the time of their transplants. Parkinson’s is a disease characterized by tremors, rigidity, slowness of movement and poor balance. It is a chronic, progressive disease that results when dopamine-producing nerve cells in a part of the brain die or are impaired.

Dr. Isacson said there was a need to understand how transplanted neurons could survive despite ongoing disease process in the patients’ brains. He said there has been controversy among scientists, some of whom believe that the transplanted cells could be corrupted by toxic proteins associated with the disease process, even at the same time patients seemed to be doing better.

"Everything we saw looked very healthy," he said, referring to the dopamine transporters and mitochondria cells.

He said the method used to transplant the cells into these patients’ brains was different than another method used on about 60 other patients worldwide. In some of those other trials, scientists said the cells might have been damaged as a result of the disease process.

It may have been that the method used on the patients in this study, which injected tiny bits of liquefied dopamine nerve cells into the brain via a thin needle, was superior to the method used in other studies, which transplanted larger chunks of nerve cells using a larger needle, he said. The transplants on the patients in this study were done in Canada.

In this study, the researchers led by Dr. Isacson compared the patients’ own dopamine producing cells with the transplanted ones. “We found very different patterns,” he said.

The difference was seen in the DAT and mitochondria, which were unhealthy around the patients’ own dopamine neurons and healthy around the transplanted ones. “The transplanted cells don’t have the disease,” he said.

"This is very important in the quest for new therapies," he added.

It is very difficult to obtain dopamine nerve cells from fetal tissue, he said. It would be far easier to grow the cells in a laboratory from stem cells, he noted. There have been no stem cell transplants as of yet for Parkinson’s patients.

Crows’ memories are made of this

An important prerequisite for intelligence is a good short-term memory which can store and process the information needed for ongoing processes. This “working memory” is a kind of mental notepad – without it, we could not follow a conversation, do mental arithmetic, or play any simple game.

In the animal kingdom, the group of birds including crows and ravens – the corvids – are known for their intelligence because they have just such a working memory. However, their endbrain – which is highly-developed but has a fundamentally different structure from that of mammals – has no cerebral cortex; and that is the part of the brain which in mammals produces the working memory. How do corvids manage to store important information from moment to moment?

To answer that question, three researchers from the Institute for Neurobiology at Tübingen University taught crows to play a version of the children’s game of “pairs.” Using a computer monitor, Lena Veit, Konstantin Hartmann and Professor Andreas Nieder briefly showed the crows a random image. The crows had to remember it for one second before choosing the same image from a selection of four by tapping the remembered picture with their beaks. In order to choose the correct image, they must have stored it in a working memory – which they appeared to do quite easily.

Simultaneous measurements of electric potentials in the crows’ brains showed that nerve cells in one particular area of the endbrain were responsible for this capacity to remember. Although the image had disappeared from the screen, those cells remained active during the short period of remembering – retaining the information about the image until the crow retrieved it in order to make the right choice. If a crow couldn’t remember and selected a wrong image, those particular endbrain cells were barely activated. Prolonged activation of such cells ensured that important information could be stored and later accessed.

Professor Nieder and his team conclude that cognitive abilities are possible in a range of differently-structured brains. “Clearly, a good working memory – an important characteristic of human beings – can also exist without a layered cerebral cortex. The corvids’ fundamentally differently-structured endbrain shows that evolution has found a number of independent solutions,” says Lena Veit.

There are great benefits in the ability to temporarily store information. “An organism with a good working memory is intelligent; it is released from the compulsion to respond immediately to stimuli,” says Professor Nieder. “The big question is now – how do neural networks in the brain have to be composed in order to actively store and process information?”

Timing is everything: scientists control rapid re-wiring of brain circuits using patterned visual stimulation

In a new study, published in this week’s issue of the journal Science, researchers show for the first time how the brain re-wires and fine-tunes its connections differently depending on the relative timing of sensory stimuli. In most neuroscience textbooks today, there is a widely held model that explains how nerve circuits might refine their connectivity based on patterned firing of brain cells, but it has not previously been directly observed in real time. This “Hebbian Theory”, named after the McGill University psychologist Donald Olding Hebb who first proposed it in 1949 has been summarized as:

“Cells that fire together, wire together. Cells that fire out of sync, lose their link”

In other words, a nerve cell that fires at the same time as its nerve cell neighbors will cooperatively form strong, stable connections onto its partner cells. On the other hand, a nerve cell that fires out of synchrony with its neighbours, will end up destabilizing and withdrawing its connections. “For the first time, we have direct, real-time evidence from watching brain cells in an intact animal to support Hebb’s model, but, we also provide surprising, new details, fundamentally updating the model for the 21^st century,” says Dr. Edward Ruthazer, senior investigator on the study at the Montreal Neurological Institute and Hospital –The Neuro at McGill University and the McGill University Health Centre. 

The study, which used multiphoton laser-scanning microscopy to observe cells in the brains of intact animals, discovered that asynchronous firing, or “firing out of sync” not only caused brain cells to lose their ability to make other cells fire, but unexpectedly, also caused them to dramatically increase their elaboration of new branches in search of better matched partners. “The surprising and entirely unexpected finding is that even though nerve circuit remodeling from asynchronous stimulation actively weakens connections, there is a 60% increase in axon branches that are exploring the environment but these exploratory branches are not long-lived,” said Dr. Ruthazer.

IMAGES of nerves in action in transparent xenopus tadpoles: http://bit.ly/1lNuux0

Dr. Ruthazer’s lab charts the formation of brain circuitry during development in the hopes of better understanding the rules that control healthy brain wiring and of advancing treatments for injuries to the nervous system and therapies for neurodevelopmental disorders such as autism and schizophrenia. Astoundingly, nearly one out of every 100 Canadians suffers from one of these disorders, estimated to cost the Canadian economy over $10 billion annually in addition to inflicting a devastating impact on patients and their families.

In the developing brain, initially imprecise connections between nerve cells are gradually pruned away, leaving connections that are stronger and more specific. This refinement occurs in response to patterned stimulation from the environment. “The way we perceive the world as adults is directly impacted by what we saw when we were younger,” says Dr. Ruthazer.

Dr. Ruthazer’s team studies brain development in Xenopus tadpoles, which have the distinct advantage of being transparent, enabling the team to clearly see the nervous system inside. They have developed a model that allows them to watch nerve cell remodeling in vivo, in real time, and to measure the efficacy of connections between cells. Optic fibers were used to stimulate the eyes of the tadpoles with different light patterns, while imaging and recording nerve cell branch formation.  Asynchronous stimulation involved light flashes presented to each eye at different times, while synchronous stimulation involved simultaneous stimulation of both eyes.

Importantly, Dr. Ruthazer’s group also has begun to identify the molecular mechanisms underlying these changes in the nervous system. They show that the stabilization of the retinal nerve cell branches caused by synchronous firing involves signaling downstream of the synaptic activation of a neurotransmitter receptor called the N-methyl-D-aspartate receptor. In contrast, the enhanced exploratory growth that occurs with asynchronous activity does not appear to require the activation of this receptor.

(Image caption: These are mature nerve cells generated from human cells using enhanced transcription factors. Credit: Fahad Ali)

Functional nerve cells from skin cells

A new method of generating mature nerve cells from skin cells could greatly enhance understanding of neurodegenerative diseases, and could accelerate the development of new drugs and stem cell-based regenerative medicine.

The nerve cells generated by this new method show the same functional characteristics as the mature cells found in the body, making them much better models for the study of age-related diseases such as Parkinson’s and Alzheimer’s, and for the testing of new drugs.

Eventually, the technique could also be used to generate mature nerve cells for transplantation into patients with a range of neurodegenerative diseases.

By studying how nerves form in developing tadpoles, researchers from the University of Cambridge were able to identify ways to speed up the cellular processes by which human nerve cells mature. The findings are reported in the May 27th edition of the journal Development.

Stem cells are our master cells, which can develop into almost any cell type within the body. Within a stem cell, there are mechanisms that tell it when to divide, and when to stop dividing and transform into another cell type, a process known as cell differentiation. Several years ago, researchers determined that a group of proteins known as transcription factors, which are found in many tissues throughout the body, regulate both mechanisms.

More recently, it was found that by adding these proteins to skin cells, they can be reprogrammed to form other cell types, including nerve cells. These cells are known as induced neurons, or iN cells. However, this method generates a low number of cells, and those that are produced are not fully functional, which is a requirement in order to be useful models of disease: for example, cortical neurons for stroke, or motor neurons for motor neuron disease.

In addition, for age-related diseases such as Parkinson’s and Alzheimer’s, both of which affect millions worldwide, mature nerve cells which show the same characteristics as those found in the body are crucial in order to enhance understanding of the disease and ultimately determine the best way to treat it.

"When you reprogramme cells, you’re essentially converting them from one form to another but often the cells you end up with look like they come from embryos rather than looking and acting like more mature adult cells," said Dr Anna Philpott of the Department of Oncology, who led the research. "In order to increase our understanding of diseases like Alzheimer’s, we need to be able to work with cells that look and behave like those you would see in older individuals who have developed the disease, so producing more ‘adult’ cells after reprogramming is really important."

By manipulating the signals which transcription factors send to the cells, Dr Philpott and her collaborators were able to promote cell differentiation and maturation, even in the presence of conflicting signals that were directing the cell to continue dividing.

When cells are dividing, transcription factors are modified by the addition of phosphate molecules, a process known as phosphorylation, but this can limit how well cells can convert to mature nerves. However, by engineering proteins which cannot be modified by phosphate and adding them to human cells, the researchers found they could produce nerve cells that were significantly more mature, and therefore more useful as models for disease such as Alzheimer’s.

Additionally, very similar protein control mechanisms are at work to mature important cells in other tissues such as pancreatic islets, the cell type that fails to function effectively in type 2 diabetes. As well as making more mature nerves, Dr Philpott’s lab is now using similar methods to improve the function of insulin-producing pancreas cells for future therapeutic applications.

"We’ve found that not only do you have to think about how you start the process of cell differentiation in stem cells, but you also have to think about what you need to do to make differentiation complete - we can learn a lot from how cells in developing embryos manage this," said Dr Philpott.