Can astronauts prevent bone loss by eating genetically engineered lettuce?

TeachableMedicalNews article 11272022

Teachable moment in classrooms:

1. cellular basis of life chapter – concept of one gene, one protein
2. skeletal system chapter – actions of osteoblasts and osteoclasts in living bones
3. skeletal system chapter – osteons and trabeculae provide structural strength to bones
4. endocrine system chapter – actions of PTH made by parathyroid gland
5. digestive system chapter – ingested proteins are degraded by several digestive enzymes

The news item:  Recently the following report appeared:

Humans on Mars may feast on gene-edited salad to stop bones breaking

Genetically modified salad could be crucial to keeping the bones of humans on Mars healthy.

The article states that a human gene was added to lettuce, and that the human protein, parathyroid hormone (PTH), will fight the osteoporosis that astronauts develop in space. Astronauts lose 1.5% of bone mass from weight-bearing bones, and the expectation is the PTH made by the lettuce will prevent bone loss on a trip to Mars that may last for over a year.

So, Why Do I Care??  Osteoporosis, as stated in the article, weakens bones, but not only in astronauts. Over 10 million people in the US alone has osteoporosis that makes fractures more likely. PTH is currently administered through injections, so, if we could demonstrate that eating PTH-enriched lettuce, then treatment of osteoporosis may become easier.

Plain English, Please!!! First, let’s talk about what osteoporosis is. In our bones we need structural reinforcement to keep bones from fracturing when we walk, run, jump or fall. The microscopic building units called osteons and trabeculae are the structural elements that provide such reinforcement.  Imagine a tall building or a bridge; columns or pillars are the elements of structural reinforcement in them. A corrosion of those pillars and columns weakens the building, and may cause their collapse. In our bones the cells called osteoclasts erode the osteons and trabeculae, but the cells called osteoblasts repair, and, thus, reverse that erosion. In old age, or with hormonal changes, such as menopause, the osteoblasts slow down, and the osteoclasts erode osteons and trabeculae. Osteoporosis is the condition where the erosion weakens the entire bone, and fractures are more likely.

Second, let’s talk about why weightlessness leads to osteoporosis. When we move around in regular gravity, our bone cells are under pressure caused by the weight of the body. That pressure makes osteocytes to tell osteoblasts to form new bone matrix. When the astronauts are in space, the lack of gravity means that their osteocytes are no longer under pressure, and when the osteoblasts are not instructed to form new bone, then unopposed osteoclast action causes bone loss.

Third, let’s talk about how eating PTH in lettuce may fight osteoporosis. PTH is well known to promote bone degradation when secreted for long periods of time. This makes us wonder why anyone would expect PTH to prevent bone loss.  The answer is that PTH, when administered in short bursts, stimulates osteoblasts, and increases bone formation. It is those bursts of PTH that could stimulate bone building and prevent bone loss in astronauts. It is unclear whether eating PTH has an effect on bone loss, because the PTH is a protein, and it could be degraded in the stomach, before reaching the bones of the astronauts.