Disease Spotlight: Idiopathic Pulmonary Fibrosis

Estimated to occur in as many as 18 per 100,000 persons per year, the disease I’m highlighting today is more common in men than in women and is rare in anyone under 50. In fact, the average age at diagnosis is sixty-five. Shockingly, the average time between the diagnosis of this disease and the death of the patient is only 2-5 years. Scientists are on the case to better understand this disease, figure out methods of treatment, and extend the quality and length of life for individuals with Idiopathic Pulmonary Fibrosis (IPF). Let me tell you a bit about what they have figured out.

In Broad Strokes:

The structure and cells of our lungs

Our lungs are the entry point for life-giving oxygen to enter the bloodstream. From there the oxygen is sent off to be used by the rest of the body. Air enters our nose and mouth, and then goes through the trachea, or windpipe, before entering the lungs. From the trachea, several smaller tubes branch off, and from these, even smaller tubes branch off. This continues until the tubes are microscopic. These tiny tubes end in a structure called an “alveolus”, which in Latin means “little cavity.” 

Your lungs don't need a dentist. Lungs are supposed to have little cavities!

Actually, unlike in the teeth, these little cavities are necessary for the function of the lung – this is where the magic happens, actually. The lining of these little cavities are very thin – only two cells thick – and this allows the transfer of gasses from the atmospheric air to your bloodstream though the process of diffusion. I’m not going to go into how breathing actually occurs, for that I’ll send you on to a neat video by Armando Hasudungan (Mechanism of Breathing).

A typical pair of human lungs contains 700 million alveoli (that’s the plural of alveolus), which provides about 70 meters squared of surface area for the process of diffusion to occur (that’s as big as a tennis court)! As you might expect, in order to use all that area for diffusion, each alveolus is wrapped in a fine mesh of small blood vessels called capillaries. Weaving between the capillaries and helping to support them are structural fibers of two main types. The first, collagen, is more rigid and provides support, while the second, elastin, permits the expansion and contraction of the walls of the alveoli during breathing. The extracellular matrix (ECM) is also present. The ECM is a collection of extracellular molecules that provide structural and biochemical support to nearby cells.

Making up the alveoli themselves are three different cell types. The first is called, boringly enough, “Type I alveolar cells.” These cells are large, but thin and flat. They make up 95% of the surface area of the alveolus, forming most of its structure. Because of this, Type I alveolar cells are where gases diffuse across from the atmosphere into the bloodstream, and vice versa.

Gas diffusion of oxygen (O2) across Type I Alveolar cells into the blood, and of carbon dioxide (CO2) from the blood out of the lungs(above) is very different from bomb diffusion (below). However, both diffusion processes are necessary for a healthy life.

The second type of alveolar cell is called, (surprise!), “Type II alveolar cells.” Despite their name, Type II alveolar cells are not second-rate in importance to the lung. No, in fact they have several important functions. For instance, Type II alveolar cells are responsible for replenishing the supply of Type I alveolar cells. If Type I cells die, due to normal cellular lifespan or due to injury, it is the job of the Type II alveolar cells to proliferate and produce new cells destined to become Type I cells and continue the good work of the lung.

The second big job of Type II alveolar cells is to produce something called “pulmonary surfactant.” Pulmonary surfactant is a film of fatty substances, (90% lipids and 10% protein), which is very important to the function of the lung. This surfactant is necessary for lowering the surface tension within the lung. In fact, without surfactant the alveoli actually collapse, which obviously makes them less able to perform their jobs. If you’re interested in the science behind how surfactant works in the lung, there’s a good video by Strong Medicine (Surface Tension and Surfactant).  Type II alveolar cells continually make and secrete this pulmonary surfactant onto the side of the alveolus open to the atmosphere, which helps us all breathe easy.

The final cell type in the alveolus is the “macrophage,” a name which roughly translates as “big eater.” Unlike the other two cell types, this cell does not make up part of the structure of the alveolus, instead it moves about on the atmospheric side of the little cavity, patrolling. Like the border control in the southern part of the United States, these macrophages look for anything foreign trying to enter and make a new home for itself. Unlike the border control however, when these macrophages come across a foreign entity, like a bacterium, they surround it, and essentially ingest it. This traps the bacterium inside the macrophage so it can no longer harm the homeland, and then the bacterium is taken to a specialized organelle within the macrophage and destroyed. Hey, when you’ve got the largest border open to atmospheric oxygen in the body - and that border is only two thin cells thick - you’ve got to mean business.

Successful generals since the Greeks and Romans have followed the adage "Divide and Conquer."
However, macrophages have been just as successful with the slightly more barbaric "Surround and Devour."

Idiopathic Pulmonary Fibrosis

Now that we’ve discussed the structural and cellular makeup of the alveoli within the lung, we can talk about the disease in the spotlight today, Idiopathic Pulmonary Fibrosis. First, let’s break down the jargon in the name: “Idiopathic” simply means “personal or unique disease progression.” In essence, the disease may be started by various triggers, and it may have a faster or slower progression, all depending on the particular patient. “Pulmonary” is just the medical way of saying “of the lung.”

“Fibrosis” is a process that commonly occurs in the body at sites of injury. In a nutshell, the injury causes signals to go out from the injured cells, saying “Ouch, come fix me!” And that starts off a whole chain of events, one of which is the activation of cells called “Fibroblasts.” These cells come roaring in, try to repair the wound by making and secreting molecules found in ECM, and also by calling in immune cells.  This is great in the short term, and if the wound can be quickly healed, the fibroblasts receive signals that their job is over, and the body gets back to business. However, if signals get mixed up somehow and these fibroblasts are not turned off, they can cause a lot of trouble.

For an everyday comparison, imagine if your house was on fire. The firefighters arrive and you’re so happy to see them – “save our home!” They get out their fire hose and spray water at the luckily small fire. It’s put out quickly and most of the house is saved. You can call the insurance company in the morning and go on as before, for the most part. However, if these firefighters get too overzealous and keep spraying and spraying and spraying, the fire will still be put out but now your house is flooded. All that extra water leads to all sorts of trouble in the long run, like mold, structural integrity issues, not to mention the damage to your personal items. The point is, too much of a good thing can be damaging. Fibrosis is the result of too much of a good thing.

The fire needed to be put out. The firefighter did just that. But if the firefighter is too over-zealous, the house will be ruined -- not from fire damage but from water damage. So it is in fibrosis. In the fibroblasts' frenzy to heal a wound, they create more trouble than if the process had been ended at the appropriate time.

In the lungs of Idiopathic Pulmonary Fibrosis (or IPF) patients, guess what we see?  Fibroblasts. These fibroblasts are so numerous they can be seen clumped together in groups called Fibroblast Foci. Their activities lead to the walls of the alveoli – once so thin and useful gas diffusion – to become thick and stiff. This makes parts of the lung look a bit like a honeycomb, and so is called “honeycombing.” Here is a video showing different slides of a lung with IPF, which might give you a better picture of what IPF looks like (IPF-UTP by UAMS Pathophysiology). The fibroblast foci and honeycombing can exist side by side with completely normal-appearing lung tissue.

What causes IPF? An exact answer for this question is still not 100% clear. Several conditions and activities have been associated with a higher risk for developing IPF, including smoking, wood-dust and stone/sand inhalation, as well as farming and rearing livestock. In general, it’s believed that IPF is a consequence of repetitive or ongoing local micro-injuries to the lung, especially in aging alveolar cells. There may be a genetic susceptibility to IPF as well.

Here is what is known about the progression of IPF: The type II alveolar cells, which are responsible for the regeneration of the alveolus, fail to regenerate the alveolus for one reason or another. This failure signals to fibroblasts that they must come and help repair a wound and this begins a whole cascade of trouble. IPF is rarely diagnosed in patients under 50 years of age. The reason for this may be that before that age, type II alveolar cells are still able to proliferate and regenerate the alveolus. Unfortunately, the cells in our body have a limited amount of times they can proliferate, based on the length of the protective pieces at the ends of DNA called telomeres. In general, the telomeres become shorter with each proliferation cycle and when they are too short, the cell will no longer proliferate. You see, if the DNA is not protected, any future cells made from that cell will lose important messages within the DNA and those future cells could be detrimental to the body. So, a commonly seen occurrence in IPF tissue is that type II alveolar cells cannot proliferate-- either because of age, or because they are stressed or even dead-- leading to no new type I alveolar cells.

The type II alveolar cells in IPF can also fail to make enough surfactant. This leads to instability within the lung and alveolar collapse. This in turn leads to mechanical stress, alveolar injury, and release of signals which cause more trouble. Finally, the collapsed alveolar cannot be reopened, due mostly to fibrotic tissue. If you listen to the lungs of someone with IPF as they breathe in, it sound like Velcro as it is pulled apart. Those “velcro crackles” are believed to be the sound of collapsed alveoli trying to pop back open. This video is mercifully silent, but it shows clearly what it looks like when alveoli collapse and try to reopen. What you’ll see if you click the link is that the alveoli on the top are not collapsed, while the alveoli in the bottom left corner of the screen try to open and then collapse with each breath. (Alveolar collapse of the rat lung by Rudolf Hellmuth).

As mentioned previously, the lung contains elastic fibers which allow the lungs to expand and fill with air. However, in IPF lungs the amount of stiff collagen fibers increases, leading to mechanical stress with each breath. As support for this, the honeycombing and other lesions in IPF lungs are found predominantly near the bottom of the lung, where the mechanical strain would be greatest. This part of the lung has the largest mechanical strain because it is here that the largest volume changes that happen with each breath.

Another proposed aspect of IPF disease progression relates to the fibroblast foci. These foci have been shown to become interconnected within the lung by bridges of connective tissue. It has been hypothesized that when this occurs, the overall stiffness of the lung can be suddenly and dramatically increased. This might explain why IPF patients have short survival rates after diagnosis. The diagnosis may be made after a long “sub-clinical phase,” when the lungs of the patient was able to compensate for the IPF and so function okay for the most part. However, eventually the lungs will function poorly enough for the patient to go to the doctor, and it could be the moment that prompts the patient to see the doctor is around the time the separate fibroblast foci begin to form connections with other foci, leading to mechanical stress and alveolar collapse resulting in a phase of rapid deterioration of lung function.

Stressed, old, and injured Type II Alveolar cells are unable to do their normal job of producing surfactant and creating Type I Alveolar cells. Without the proper function of Type II Alveolar cells, fibroblasts become activated, collagen production increases, lungs tissue becomes more stiff, leading to alveolar collapse and to Idiopathic Pulmonary Fibrosis. Several stressors have been identified which hurt type II alveolar cells, including short telomers, mutations in surfactant-related proteins, viral infection, cigarette smoke, mechanical stress and a cytokine calle TGF-beta1.

In the details:

As mentioned, doctors and scientists have been working hard to understand the processes at work that cause IPF. Let’s begin a more detailed look at IPF by first going over data relating to Type II alveolar cells.

From animal models of IPF it is known that if an injury does not affect Type II alveolar cell function, the injury is repaired without fibrotic remodeling. However, the failure of Type II cells to regenerate Type I cells leads to an increase in collagen deposition after wounding. In fact, dead Type II alveolar cells are commonly and prominently seen in IPF histology.

What impairs the function of Type II cells or leads to their death?

First, type II alveolar cells may enter senescence. The term senescence means that the cell has reached the end of its proliferating days, and can no longer replicate its DNA. Usually this is because the protective ends of the DNA, called telomeres are too short to protect the DNA any longer. The genes TERT and TERC, which are needed for maintaining the proper length of telomeres, are linked to IPF cases.

Second, mutations in genes relating to surfactant production are known to hurt type II alveolar cells. These mutations lead to improper folding of the protein which, because one of the roles of the Type II cell is to make massive amounts of surfactant, leads to an accumulation of misfolded proteins and the activation of the endoplasmic reticulum (ER) stress response. This response will eventually result in the death of the cell if not resolved. Known mutations to cause fibrosis in the lung include:

·         Mutations in the surfactant protein C gene (called SFTPC).

·         Mutations in the surfactant protein A (also called SPA or SFTPA2).

·         Mutations in ABCA3, a transporter protein produced by Type II alveolar cells and needed for the proper transportation of surfactant protein also leads to Type II alveolar cell ER stress and death.

·         Lysosomal storage disease can also cause ER stress. For instance, patients with Hermansky-Pudlak syndrome develop a form of fibrosis indistinguishable from that of IPF and they also show an increased accumulation of surfactant proteins and lipids.

·         Mutations in surfactant-related proteins also causes surfactant dysfunction in addition to ER stress, which can cause hydrodynamic stress resulting in shear stress at the outer cell membrane of type II alveolar cells, which can lead to type II alveolar cell dysfunction and death.

Cigarette smoking can cause oxidative stress and this is known to disrupt protein folding, leading to the ER stress response. Smoking can also cause telomere shortening which may lead to the senescence of Type II alveolar cells.

Finally, viral infection can also harm type II alveolar cells. Virus DNA and protein have been found in the lung tissue of IPF patients, and may cause injury to the alveolar epithelium. Viral infections can also overwhelm protein synthesis in the ER and might aggravate ER stress.  In fact, respiratory infections frequently precede the appearance of IPF and also speed its clinical course. However, viral infection alone is unlikely to cause IPF, and instead likely serves as an added stress to push the disease progression forward.

Mechanical stress and alveolar collapse

The highest mechanical stress within the lung is predicted/modeled to occur in the same areas where IPF honeycombing occurs (the basal and subpleural regions). These lesions progress upward as the disease continues on. A mutation in MUC5B, a surfactant protein, may not be able to decrease surface tension, leading to increased mechanical stress on the lung. Mutations in this gene may also make the patient more susceptible desmoplakin, is linked to increased injury of alveolar cells to mechanical stress.

Additionally, the stiffness of lung tissue also correlates with the activation of a cellular signaling pathway which begins with TGF-β1. Surrounding the alveoli is a web of proteins, collectively called the extracellular matrix (ECM), which helps support the cells. However, the ECM can also be used to signal to the cell. Bound up within the ECM is a protein called TGF-β1 and it is bound to the collagen fibrils. Following injury, alveolar epithelial cells contract their cytoskeleton, which applies physical force on the TGF-β1 complex in the ECM, resulting in the activation of TGF-β1. If added directly to a mouse lung, active TGF-β1 can lead to alveolar collapse. This is apparently due to active TGF-β1 causing a loss of polarization of Type II alveolar cells and surfactant dysfunction.

A more well-studied effect of active TGF-β1 is its ability to cause the activation of fibroblasts, which then increase fibrosis and the stiffness of the lung. This starts a process known as a “feed-forward loop” where the active TGF-β1 increases stiffness, which leads to more active TGF-β1 being released resulting in even more lung stiffness.  A mutation in a gene which regulates TGF-β1 signaling, called TOLLIP, affects IPF disease progression. TOLLIP is also involved in regulating innate immune responses, and so may also affect viral susceptibility of the lung as well.

A number of other signaling pathways have been investigated in relation to IPF, but that is another post for another day. I hope you have learned a bit about the cells within our lungs and what can go wrong when even just one cell type cannot continue its proper function. Ours is a cellular life, and cell biologists and doctors the world over are trying to find ways to combat this sort of cellular dysfunction.

References:

Knudsen, Ruppert, and Ochs. “Tissue Remodeling in Pumonary Fibrosis.” Cell Tissue Res (2017) 367:607-626. http://dx.doi.org/10.1007/S00441-016-2543-2.

Lopez-Rodriguez, et al. “Surfact dysfunction during over-expression of TGF-beta1 precedes profibrotic lung remodeling in vivo. Am J Physiol Lung Cell Mol Physiol. 2016. http://dx.doi.org/10.1152/ajplung.00065.2016.

Richeldi, Collard, and Jones. “Idiopathic Pulmonary Fibrosis.” The Lancet. 2017. http://dx.doi.org/10.1016/S0140-6736(17)30866-8.

Cong, Hubmayr, Li and Zhao. “Plasma membrane wounding and repair in pulmonary diseases.” Am J Physiol Lun Cell Mol Physiol (2017) 312: L371-L391. http://dx.doi.org/10.1152/ajplung.00486.2016.