Patient-Specific Neurons Reveal Vital Clues About Autism

The brains of some people with autism spectrum disorder grow faster than usual early on in life, often before diagnosis. Now new research from scientists at the Salk Institute has used cutting-edge stem cell-based techniques to elucidate those mechanisms that drive excess brain growth, which affects as many as 30 percent of people with autism.

These findings show that it is possible to use stem cell reprogramming technologies to model the earliest stages of complex disorders and to evaluate potential therapeutic drugs. The Salk team, led by Alysson Muotri, discovered that stem cell-derived neurons, derived from stem cells that had been made from cells taken from autism patients, made fewer connections in culture compared to cells from healthy individuals. These same scientists also restored cell-cell communication between these cells by adding a growth factor called IGF-1 (insulin-like growth factor-1). IGF-1 is in the process of being evaluated in clinical trials of autism.

“This technology allows us to generate views of neuron development that have historically been intractable,” said senior investigator Fred H. Gage. “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.”

In the United States alone, autism affects approximately one out of every 68 children. Autistic children have problems communicating, show an inhibited ability to interact with others, and usually engage in repetitive behaviors. Mind you, the symptomatic manifestations in autistic children can vary dramatically in type and severity. Autism, to date, has no known, identified cause.

In 2010, Gage and collaborators recreated features of Rett syndrome (a rare disorder that shares features of autism but is caused by mutations in a single gene; MECP2) in a cell culture system. They extracted skin cells from Rett Syndrome patients and converted those cells into induced pluripotent stem cells (iPSCs). Then Gage and others differentiated those Rett-Syndrome-specific iPSCs into neurons, which they grew in culture. These neurons were then studied in detail in a neuron-specific culture system. “In that study, induced pluripotent stem cells gave us a window into the birth of a neuron that we would not otherwise have,” said Marchetto, the study’s first author. “Seeing features of Rett syndrome in a dish gave us the confidence to next study classical autism.”

In this new study, Gage and others created iPSCs from autism patients whose brains had grown up to 23 percent faster than usual during toddlerhood but had subsequently normalized. These iPSCs were then differentiated into neuron precursor cells (NPCs). Examinations of these NPCs revealed that the NPCs made from iPSCs derived from autism patients proliferated faster than those derived from typically developing individuals. This finding supports a theory advanced by some experts that brain enlargement is caused by disruptions to the cell’s normal cycle of division, according to Marchetto.

In addition, the neurons derived from autism-specific iPSCs behaved abnormally in culture. They fired less often compared with those cells derived from healthy people. The activity of these neurons, however, improved if they were treated with IGF-1. IGF-1 enhances the formation of cell-cell connections between neurons, and the establishment and stabilization of these connections seem to normalize neuronal function.

Muotri and Gage and others plan to use these patient-derived cells to elucidate the molecular mechanisms behind IGF-1’s effects. They will examine changes in gene expression and attempt to correlate them with changes in neuronal function. Although the newly derived cells are far from the patients’ brains, a brain cell by itself may, hopefully, reveal important clues about a person and their brain.

This work was published in the journal Molecular Psychiatry: M. C. Marchetto et al., “Altered proliferation and networks in neural cells derived from idiopathic autistic individuals,” Molecular Psychiatry, 2016; DOI: 10.1038/mp.2016.95.

Directly Reprogramming Skin Cells into White Blood Cells

Scientists from the Salk Institute have, for the first time, directly converted human skin cells into transplantable white blood cells, which are the soldiers of the immune system that fight infections and invaders. This work could prompt the creation of new therapies that introduce new white blood cells into the body that can attack diseased or cancerous cells or augment immune responses for other conditions.

This work, which shows that only a small amount of genetic manipulation could prompt this direct conversion, was published in the journal Stem Cells.

“The process is quick and safe in mice,” says senior author Juan Carlos Izpisua Belmonte, who holds the Salk’s Roger Guillemin Chair. “It circumvents long-standing obstacles that have plagued the reprogramming of human cells for therapeutic and regenerative purposes.”

The problems that Izpisua Belmonte mentions, includes the long time (at least two months) numbingly tedious cell culture work it takes to produce, characterize and differentiate induced pluripotent stem (iPS) cells. Blood cells derived from iPSCs also have other obstacles: they engraft into organs or bone marrow poorly and can cause tumors.

The new method designed by Izpisua Belmonte and his team, however, only takes two weeks, does not produce tumors, and engrafts well.

“We tell skin cells to forget what they are and become what we tell them to be—in this case, white blood cells,” says one of the first authors and Salk researcher Ignacio Sancho-Martinez. “Only two biological molecules are needed to induce such cellular memory loss and to direct a new cell fate.”

This faster reprogramming technique developed by Belmonte’s team utilized a form of reprogramming that does not go through a pluripotency stage. Such techniques are called indirect lineage conversion or direct reprogramming. Belmonte’s group has demonstrated that such approaches can reprogram cells to form the cells that line blood vessels. Thus instead of de-differentiating cells into an embryonic stem cell-type stage, these cells are rewound just enough to instruct them to form the more than 200 cell types that constitute the human body.

Direct reprogramming used in this study uses a molecule called SOX2 to move the cells into a more plastic state. Then, the cells are transfected with a genetic factor called miRNA125b that drives the cells to become white blood cells. Belmonte and his group are presently conducting toxicology studies and cell transplantation proof-of-concept studies in advance of potential preclinical and clinical studies.

“It is fair to say that the promise of stem cell transplantation is now closer to realization,” Sancho-Martinez says.

Study co-authors include investigators from the Center of Regenerative Medicine in Barcelona, Spain, and the Centro de Investigacion Biomedica en Red de Enfermedades Raras in Madrid, Spain.

Scientists Generate “Mini-kidney” Structures from Human Stem Cells

Kidney Disease represents a major and unsolved health issue worldwide. Once damaged by disease, kidneys rarely recover their original level of function, and this highlights the urgent need for better knowledge of kidney development and physiology.

Now, a team of researchers led by scientists at the Salk Institute for Biological Studies has developed a novel platform to study kidney diseases. This new platform should open new avenues for the future application of regenerative medical strategies to restore kidney function.

For the first time, the Salk researchers have generated three-dimensional kidney structures from human stem cells. These findings were reported November 17, 2013 in Nature Cell Biology, and they suggest new ways to study the development and diseases of the kidneys and to discover and test new drugs that target human kidney cells.

Scientists had created precursors of kidney cells using stem cells as recently as this past summer, but the Salk team was the first to coax human stem cells into forming three-dimensional cellular structures similar to those found in our kidneys.

“Attempts to differentiate human stem cells into renal cells have had limited success,” says senior study author Juan Carlos Izpisua Belmonte, a professor in Salk’s Gene Expression Laboratory and holder of the Roger Guillemin Chair. “We have developed a simple and efficient method that allows for the differentiation of human stem cells into well-organized 3D structures of the ureteric bud (UB), which later develops into the collecting duct system.”

The Salk findings demonstrate for the first time that pluripotent stem cells capable of differentiating into the many cells and tissue types that make up the body can be induced to differentiate into those cells found in the ureteric bud, which is an early developmental structure of the kidneys. Furthermore, these same cells can differentiate further into three-dimensional structures in organ cultures. Ureteric bud cells form the early stages of the human urinary and reproductive organs during development and later develop into a conduit for urine drainage from the kidneys. Izpisua Belmonte’s research group accomplished this with both human embryonic stem cells and induced pluripotent stem cells (iPSCs), human cells from the skin that have been reprogrammed into their pluripotent state.

Kidney development

After generating iPSCs that demonstrated pluripotent properties and were able to differentiate into mesoderm, the embryonic germ cell layer from which the kidneys develop, the Salk Institute team used growth factors known to be essential during the natural development of our kidneys to culture both iPSCs and embryonic stem cells.  The combination of signals from these growth factors, molecules that guide the differentiation of stem cells into specific tissues, committed the cells to become progenitors that exhibit clear characteristics of renal cells in only four days.

The researchers then guided these cells to further differentiate into organ structures similar to those found in the ureteric bud by culturing them with kidney cells from mice. This demonstrated that the mouse cells were able to provide the appropriate developmental cues to allow human stem cells to form three-dimensional structures of the kidney.

Izpisua Belmonte’s team also tested their protocol on iPSCs from a patient clinically diagnosed with polycystic kidney disease (PKD), a genetic disorder characterized by multiple, fluid-filled cysts that can lead to decreased kidney function and kidney failure. They found that their methodology could produce kidney structures from patient-derived iPSCs.

Polycystic kidneys
Polycystic kidneys

Because of the many clinical manifestations of the disease, neither gene- nor antibody-based therapies are realistic approaches for treating PKD. The Salk team’s technique might help circumvent this obstacle and provide a reliable platform for pharmaceutical companies and other investigators studying drug-based therapeutics for PKD and other kidney diseases.

“Our differentiation strategies represent the cornerstone of disease modeling and drug discovery studies,” says lead study author Ignacio Sancho-Martinez, a research associate in Izpisua Belmonte’s laboratory. “Our observations will help guide future studies on the precise cellular implications that PKD might play in the context of kidney development.”

Neurons Derived from Cord Blood Cells

A research group at the Salk Institute in San Diego has discovered a new protocol for converting umbilical cord blood cells into neuron-like cells. These new cells could prove valuable for the treatment of a wide variety of neurological conditions, including stroke, traumatic brain injury and spinal cord injury.

Physicians have used umbilical cord blood for more than 20 years to treat many different types of illnesses, including cancer, immune disorders, and blood and metabolic diseases. However, these Salk Institute researchers demonstrated that cord blood (CB) cells can be differentiated into cell types from which brain, spinal and nerve cells arise.

Juan Carlos Izpisua Belmonte, a professor in Salk’s Gene Expression Laboratory, who led the research team, said: “This study shows for the first time the direct conversion of a pure population of human cord blood cells into cells of neuronal lineage by the forced expression of a single transcription factor.”

Izpisua Belmonte’s group used an engineered retrovirus to introduce a gene called Sox2, a transcription factor that acts as a switch inside cells that converts them into neurons. Therefore, by introducing Sox2 into CB cells, and culturing them in the lab, the cells formed colonies that expressed genes normally found in neurons.

Were these cells actual neurons or faux neurons? Cells might make neuron-specific genes, but they do not assemble those gene products into neuron-specific machinery, then they are not neurons. To if such cells are neurons, they should be able to manipulate the electrical charges across their cell membranes. But subjecting cells to electrophysiological tests, they determined that these new cells, which they called induced neuronal-like cells or iNCs, could transmit electrical impulses. This shows that the iNCs were mature and functional neurons. Next, they implanted these Sox2-transformed CB cells to a mouse brain and found that they integrated into the existing mouse neuronal network and were capable of transmitting electrical signals like mature functional neurons.

Mo Li, a scientist in Belmonte’s lab and a co-first author on the paper, said: “We also show that the CB-derived neuronal cells can be expanded under certain conditions and still retain the ability to differentiate into more mature neurons both in the lab and in a mouse brain. Although the cells we developed were not for a specific lineage-for example, motor neurons or mid-brain neurons-we hope to generate clinically relevant neuronal subtypes in the future.”

Scientists can use these cells in the future to model neurological diseases such as autism, schizophrenia, Parkinson’s or Alzheimer’s disease.

CB cells offer several advantages over other types of stem cells. First, they are not embryonic stem cells and are not controversial. They are more plastic, or flexible, than adult stem cells from sources like bone marrow, which may make them easier to convert into specific cell lineages. The collection of CB cells is safe and painless and poses no risk to the donor, and they can be stored in blood banks for later use.

“If our protocol is developed into a clinical application, it could aid in future cell-replacement therapies,” said Li. “You could search all the cord blood banks in the country to look for a suitable match.”