Healing Damaged Lungs with Stem Cells

Emphysema, bronchitis, asthma and cystic fibrosis are all diseases of the airways and they are the second leading cause of death worldwide. Over 35 million Americans alone suffer from chronic respiratory disease.

Scientists from the Weizmann Institute have now proposed a new direction for treating these diseases that might lead to a new method for alleviating some of their suffering. The study’s findings, which were published in Nature Medicine, show that it might be possible to use fetal stem cells to repair damaged lung tissue.

Particular stem cells normally found in the lungs are highly similar to those in the bone marrow. Organ-specific stem cells tend to be concentrated in special compartments rather than being distributed throughout the tissue. This insight prompted Prof. Yair Reisner of the Weizmann Institute’s Immunology Department to suggest: “That understanding suggested to us that we might be able to apply our knowledge of techniques for transplanting bone marrow stem cells to repairing lung tissue.”

Bone marrow transplants are based on two main principles: the ability of stem cells to navigate through the blood to the appropriate compartment and the prior clearing out of the compartment to make room for the transplanted stem cells. Dr. Reisner and his group thought it might possible to apply these principles to introducing new stem cells into the lungs. However, before they could do this, they needed to find a source of lung stem cells suitable for transplanting.

Reisner and his co-workers used fetal lung tissue from mice and humans (20–22 weeks of gestation for humans, and embryonic day 15–16 or E15–E16 for mice). Cells from these stages have differentiated into lung progenitor cells and are fully capable of lung regeneration. Reisner and his colleagues conducted a series of experiments in which they cleared the lung’s stem cell compartments with a new method developed on their own laboratory, and then were injected these new lung progenitor cells into mouse models of lung damage. The fetal lung stem cells found their way through the blood to the lungs and settled into the proper compartment. By six weeks, these cells were well on their way to differentiating into normal lung tissue. In these mice, their damaged lungs healed, and their breathing improved significantly.

Next, Reisner intends to determine the correct dosage of drugs that are needed to prevent rejection of the transplanted cells, which will be needed following such procedures. “But our real vision, bolstered by this success,” says Reisner, “is to create a bank of lung tissue that will be a resource for embryonic lung stem cells.” This bank could mean that there is a ready source of cells for repairing the damage in those with severe respiratory disease.

Reisner’s work shows that fetal lung progenitors can repopulate lungs and heal them. If Reisner can find a way to generate early lung progenitors from pluripotent stem cells, then such cells can be used to heal damaged lungs.

Reversing Lung Diseases By Directing Stem Cell Differentiation

Lung diseases can scar the respiratory tissues necessary for oxygen exchange. Without proper oxygen exchange, our cells lack the means to make the energy they so desperately need, and they begin to shut down or even die. Lung diseases such as asthma, emphysema, chronic obstructive pulmonary disease and others can permanently diminish lung capacity, life expectancy and activity levels.

Fortunately, a preclinical study in laboratory animals has suggested a new strategy for treating lung diseases. Carla Kim and Joo-Hyeon Lee of the Stem Cell Research Program at Boston Children’s have described a new lung-specific pathway that is activated by lung injury and directs a resident stem cell population in the lung to proliferate and differentiate into lung-specific cell types.

When Kim and Lee enhanced this pathway in mice, they observed increase production of the cells that line the alveolar sacs where gas exchange occurs. Alveolar cells are irreversibly damaged in emphysema and pulmonary fibrosis.

Inhibition of this same pathway increased stem cell-mediated production of airway epithelial cells, which line the passages that conduct air to the alveolar sacs and are damaged in asthma and bronchiolitis obliterans.

For their experiments, Kim and Lee used a novel culture system called a 3D culture system that mimics the milieu of the lung. This culture system showed that a single bronchioalveolar stem cell could differentiate into both alveolar and bronchiolar epithelial cells. By adding a protein called TSP-1 (thrombospondin-1), the stem cells differentiated into alveolar cells.

Next, Kim and Lee utilized a mouse model of pulmonary fibrosis. However, when they cultured the small endothelial cells that line the many small blood vessels in the lung, which naturally produce TSP-1, and directly injected the culture fluid of these cells into the mice, the noticed these injections reverse the lung damage.

When they used lung endothelial cells that do not produce TSP-1 in 3D cultures, lung-specific stem cells produce more airway cells. in mice that were engineered to not express TSP-1, airway repair was enhanced after lung injury.

Lung Stem Cell Repair of Lung Damage

Lee explained his results in this way: “When the lung cells are injured, there seems to be a cross talk between the damaged cells, the lung endothelial cells and the stem cells.”

Kim added: “We think that lung endothelial cells produce a lot of repair factors besides TSP-1. We want to find all these molecules, which could provide additional therapeutic targets.”

Even though this work is preclinical in nature, it represents a remarkable way to address the lung damage that debilitates so many people. Hopefully this work is easily translatable to human patients and clinical trials will be in the future. Before that, more confirmation of the role of TSP-1 is required.

Stem Cells Heal Damaged Cells by Transferring Mitochondria

An Indian team from Delhi, India has identified a protein that increases the transfer of mitochondria from mesenchymal stem cells to lung cells, thus augmenting the healing of lung cells.

Stem cells like mesenchymal stem cells from bone marrow, fat, tendons, liver, skeletal muscle, and so on secrete a host of healing molecules, but they also form bridges to other cells and export their own mitochondria to heal damaged cells. Mitochondria are the structures inside cells that make energy. Damaged cells can have serious energy deficiencies and mitochondrial transfer ameliorates such problems (see Cárdenes N et al, Respiration. 2013;85(4):267-78).

This present work from the laboratory of Anurag Agrawal, who is housed in the Centre of Excellence in Asthma & Lung Disease, at the CSIR‐Institute of Genomics and Integrative Biology in Delhi, India has identified a protein called Miro1 that regulates the transfer of mitochondria to recipient cells.

Mitochondrial transfer has so many distinct benefits that stem cell scientists hope to engineer stem cells to transfer more of their mitochondria to damaged cells, and Miro1 might be a target for such stem cell engineering experiments.

Mitochondrial transfer between stem cells and other cells occurs by means of tunneling nanotubes, which are thread-like structures formed from the plasma membranes of cells that form bridges between different cell types. Under stressful conditions, the number of these nanotubes increases.

In the present study. stem cells engineered to express more Miro1 protein transferred mitochondria more efficiently than control stem cells. When used in mice with damaged lungs and airways, these Miro1-overexpressing cells were therapeutically more effective than control cells.

This study presents the first mechanistic insight into how Mesenchymal Stem Cells (MSC) act as mitochondrial donors during attenuation of lung inflammation and injury. Mitochondrial donation is an essential part of the MSC therapeutic effect in these models and is positively regulated by Miro1 / Rhot1 mitochondrial transport proteins.
This study presents the first mechanistic insight into how Mesenchymal Stem Cells (MSC) act as mitochondrial donors during attenuation of lung inflammation and injury. Mitochondrial donation is an essential part of the MSC therapeutic effect in these models and is positively regulated by Miro1 / Rhot1 mitochondrial transport proteins.

The hope is to use Miro1 manipulations to make better stem cell therapies for human diseases.

To summarize this work:

1. MSCs donate mitochondria to stressed epithelial cells (EC) that have malfunctioning mitochondrial.  Cytoplasmic nanotubular bridges form between the cells and Miro‐1 mediated mitochondrial transfer occurs unidirectionally from MSCs to ECs.

2. Other mesenchymal cells like smooth muscle cells and fibroblasts express Miro1 and can also donate mitochondria to ECs, but with low efficiency. ECs have very low levels of Miro1 and, as a rule, do not donate mitochondria.

3. Enhanced expression of Miro1 in mesenchymal cells increases their mitochondrial donor efficiency.  Conversely, cells lacking Miro1 do not show MSC mediated mitochondrial donation.

4. Miro1‐overexpressing MSCs have enhanced therapeutic effects in three different models of allergic lung inflammation and rat poison-induced lung injury.  Conversely, Miro1‐depleted MSCs lose much of their therapeutic effect.  Miro1 overexpression in MSCs may lead to more effective stem cell therapy.

Modulating Gene Expression to Repair Lungs

According to the American Lung Association and the National Institutes of Health, lung diseases such as asthma and chronic obstructive pulmonary disease (COPD) are on the rise. These are chronic ailments that affect the small airways of the lung. Asthma and COPD involve an injury-repair cycle that leads to the destruction of normal airway structure and function. Presently, drug treatments for COPD only treat the symptoms.

“A healthy lung has some capacity to regenerate itself like the liver,” noted Ed Morrisey, professor of Medicine and Cell and Developmental Biology and the scientific director of the Penn Institute for Regenerative Medicine in the Perelman School of Medicine, University of Pennsylvania. “In COPD, these reparative mechanisms fail.”

Morrisey and his colleagues are examining how epigenetic mechanisms control lung repair and regeneration. Epigenetics consists of chemical modifications to DNA and its supporting proteins that affect gene expression. Previous studies have discovered that smokers with COPD had the most significant decrease in one of the enzymes that controls these modifications, called HDAC2.

“HDAC therapies may be useful for COPD, as well as other airway diseases,” he explained. “The levels of HDAC2 expression and its activity are greatly reduced in COPD patients. We believe that decreased HDAC activity may impair the ability of the lung epithelium to regenerate.”

By using genetic and pharmacological approaches, Morrisey and others showed that the development of progenitor cells in the lung is specifically regulated by the combined function of two highly related HDACs, HDAC/1 and /2. Morrisey and his colleagues published their findings in the prestigious journal Developmental Cell.

By studying how HDAC activity and other epigenetic regulators control lung development and regeneration, they hope to develop new therapies to alleviate the unmet needs of patients with asthma and COPD.

HDAC1/2 deficiency leads to a loss of expression of the an essential transcription factor, a protein called Sox2, which in turn leads to disruption of airway epithelial cell development. This is affected in part by increasing the expression of two genes, Bmp4 and the tumor suppressor Rb1, both of which are inhibitors of cell proliferation including the proteins p16, and p21. This results in decreased epithelial proliferation in lung injury and inhibition of regeneration.

Together, these data support a critical role for HDAC-mediated mechanisms in regulating both development and regeneration of lung tissue. Since HDAC inhibitors and activators are currently in clinical trials for other diseases, including cancer, such compounds could be tested in the future for efficacy in COPD, acute lung injury and other lung diseases that involve defective repair and regeneration, said Morrisey.