Scientists Use Stem Cells to Grow Three-Dimensional Mini Lungs.

In research done in several laboratories, lung tissue was derived from flat cell culture systems or by growing cells on scaffolds made from donated organs.

Now in a new study published in the online journal eLife, a multi-institution team has defined a culture system for generating self-organizing human lung organoids, which are three-dimensional structures that mimic the structure and complexity of human lungs.

“These mini lungs can mimic the responses of real tissues and will be a good model to study how organs form, change with disease, and how they might respond to new drugs,” said study senior author Jason R. Spence, Ph.D., an assistant professor of internal medicine and cell and developmental biology at the University of Michigan Medical School.

Spence and his colleagues successfully grew structures that resembled both the large airways or bronchi and small lung sacs, known as alveoli.

These mini lung structures were developed in a cell culture system. Therefore, they lack several components of the human lung, including blood vessels, which are a critical component of gas exchange during breathing.

Despite that, these cultured organoids can serve as a unique research model system for researchers as they grind out basic science ideas that are turned into clinical innovations. These three-dimensional mini-lungs should be an excellent complement to research in liver laboratory animals.

Traditionally, the behavior of cells has been investigated in the laboratory in two-dimensional culture systems where cells are grown in thin layers on cell-culture dishes. Most cells in the body, however, exist in a three-dimensional environment as part of complex tissues and organs. Tissue engineered have been trying to re-create these environments in the laboratory by successfully generating small version of particular organs known as organoids, which serve as models of the stomach, brain, liver and human intestine. The advantage of growing three-dimensional structures of lung tissue, according to Dr. Spence, is that the organization of organoids bears greater similarity to the human lung.

To make these lung organoids, researchers at the U-M’s Spence Lab and colleagues from the University of California, San Francisco; Cincinnati Children’s Hospital Medical Center; Seattle Children’s Hospital and University of Washington, Seattle manipulated several of the cell signaling pathways that control the formation of organs.

First, stem cells were induced to form a type of tissue called endoderm, which is found in early embryos and gives rise to the lung, liver and several other internal organs. Second, the group activated two important development pathways (FGF and WNT signaling ) that are stimulate endoderm to form three-dimensional tissue. By inhibiting two other key development pathways at the same time (BMP and TGFβ signaling), the endoderm became tissue that resembles the early lung found in embryos.

In the laboratory, this early culture-derived lung-like tissue spontaneously formed three-dimensional spherical structures as it developed. Afterwards, they had to expand these structures and develop them into lung tissue. In order to do this, Spence and his colleagues and collaborators exposed the cells to additional proteins involved in lung development (FGF and Hedgehog).

After all this manipulation, the resulting lung organoids survived in the laboratory for over 100 days.

“We expected different cells types to form, but their organization into structures resembling human airways was a very exciting result,” said author Briana Dye, a graduate student in the U-M Department of Cell and Developmental Biology.

While this type of experiment is remarkable, this is only the beginning of lung tissue engineering.  These mini-lungs  will hopefully serve and new model systems for drug testing and researching genetic diseases that affect the lungs, such as cystic fibrosis, sarcoidosis, or inherited forms of emphysema.  It will be a while before scientists can make replacement lungs for human patients, but these experiments by Spence and others are a remarkable start.

Synthetic Matrices that Induce Stem Cell-Mediated Bone Formation

Biomimetic matrices resemble living structures even though they are made from synthetic materials. Researchers in the laboratory of Shyni Varghese at the UC San Diego Jacobs School of Engineering have used calcium phosphate to direct mesenchymal stem cells to form bone. In doing so, Varghese and his colleagues have identified a surprising pathway from biomaterials to bone.

Varghese and his colleagues think that their work may point out new targets for treating bone defects, such as major fractures, and bone metabolic disorders such as osteoporosis.

The first goal of this research was to use materials to build something that looked like bone. This way, stem cells harvested from bone marrow (the squishy stuff inside our bones) could sense the presence of bone and differentiate into osteoblasts, the cells in our bodies that build bone.

“We knew for years that calcium phosphate-based materials promote osteogenic differentiation of stem cells, but none of use knew why.” said Varghese. “As engineers, we want to build something that is reproducible and consistent, so we need to know how building factors contribute to this end.”

Varghese and co-workers discovered that phosphate ions dissolved from calcium phosphate-based materials and these stray phosphate ions are taken up by the stem cells and used for the production of adenosine triphosphate or ATP. ATP is the energy currency of the cell, and it is the way cells store energy in a form that is readily usable for powering other reactions.

In stem cells, the generation of ATP eventually increases the intracellular concentration of the ATP breakdown product adenosine, and adenosine signals to stem cells to differentiate into osteoblasts and make bone.

Varghese said that she was surprised that “the biomaterials were connected to metabolic pathways. And we didn’t know how these metabolic pathways could influence stem cells,” and their commitment to bone formation.

These results also explain another clinical observation. Plastic surgeons have been using fat-based stem cells for eyelid lifts, breast augmentation, and other types of reconstructive surgeries. In once case, a plastic surgeon injected a dermal filler that contained calcium hydroxyapatite with the fat-based stem cells into a woman’s eyelid to provide an eye lift. However, the stem cells formed bone, and the poor lady’s lid painfully clicked every time she blinked and she had to have surgery to remove the ectopic bone. These results from Varghese’s laboratory explains why these fat-based stem cells formed bone in this case, and great care should be taken to never use such fillers in fat-based transplantation procedures.

Special Brain Cell Helps New Neurons Survive

A specialized type of brain cell that down-regulates stem cell activity seems to encourage the survival of stem cell progeny, according to new research from the laboratory of Hongjun Song, professor of neurology and director of Johns Hopkins Medicine’s Institute for Cell Engineering’s Stem Cell Program.

Uncovering the precise mechanism by which these cells regulate the life and death of neurons is a central to understanding neurodegenerative diseases, aging, and Alzheimer’s disease, since the activity of these cells is linked to these conditions.

“We’ve identified a critical mechanism for keeping newborn neurons alive,” said Song.  “Not only can this help us understand the underlying causes of some diseases, it may also be a step toward overcoming barriers to therapeutic cell transplantation.”

Song collaborated with Guo-li Ming and the members of his research group. Ming is a professor of neurology at the Institute for Cell Engineering.  Song’s team first reported last year that special brain cells called “parvalbumin-expressing interneurons” signal to nearby stem cells not to divide.  They means by which the parvalbumin-expressing interneurons (PEIs) signal to nearby stem cells is by releasing a neurotransmitter called “gamma-aminobutyric acid” (GABA).  In this present study, Ming and Song examined how GABA from surrounding PEIs affects nearby neurons produced by stem cells.

arvalbumin-expressing interneurons
parvalbumin-expressing interneurons

Many of these newborn neurons naturally die soon after they are born.  According to Song, if the new cells survive, these neurons will migrate to a permanent home in the brain and forge connections called synapses with other cells.

To determine whether GABA is a factor in the survival of newborn neurons and their behavior, Song’s team tagged neurons in mouse brains with a fluorescent protein and watched their response to GABA.

“We didn’t expect these immature neurons to form synapses, so we were surprised to see that they had built synapses from surrounding interneurons and that GABA was getting to them that way,” Song said.

In an earlier study, this research team had found that GABA was getting to the synapse-less stem cells by a less direct route – it was drifting across the spaces between cells.

To confirm the finding, the team engineered the interneurons to be stimulated or suppressed by light.  When stimulated by light, the cells activated nearby neurons.  Then they used this light stimulation procedure in live mice, they found that when the specialized neurons were stimulated and gave off more GABA, the newborn neurons survived in greater numbers than otherwise.  This was the opposite of the response of the neural stem cells, which become dormant when given GABA.

Song interpreted these data in the following manner: “This appears to be a very efficient system for tuning the brain’s response to its environment.  When you have a high level of brain activity, you need more newborn neurons, and when you don’t have high activity, you don’t need newborn neurons, but you need to prepare yourself by keeping the stem cells active.  It’s all regulated by the same signal.”

According to Song, the PEIs behave abnormally in neurodegenerative diseases such as Alzheimer’s disease and mental illnesses such as schizophrenia.

“Now we want to see what the role of these interneurons is in the newborn neurons’ next steps” migrating to the right place and integrating into the existing circuitry.  That may be the key to their role in disease,” said Song.  His team is also interested in using the GABA signal to keep transplanted cells alive without affecting other brain processes as a side effect.

See Song J, Sun J, Moss J, Wen Z, Sun GJ, Hsu D, Zhong C, Davoudi H, Christian KM, Toni N, Ming GL, Song H. Parvalbumin interneurons mediate neuronal circuitry-neurogenesis coupling in the adult hippocampus. Nat Neurosci. 2013 Dec;16(12):1728-30. doi: 10.1038/nn.3572. Epub 2013 Nov 10.