Monday, May 28, 2012

Biomedicines Blue Gold

Horseshoe crabs, Limulus polyphemus, evolved 450 million years ago.  They most commonly found in the soft, sandy, muddy bottom of the east coast.  Although they are commonly called crabs, they are more closely related to arachnids.  The population of horseshoe crabs along the Atlantic coast of the United States is listed as threatened.  While not listed as endangered yet, the population numbers have been consistently dropping year after year.  This is due to human predation and climate change.  The main prey of the horseshoe crab is shellfish and the fishermen along New England coast are concerned with the amount of shellfish the horseshoe crabs eat in respect to their business.  Another threat to horseshoe crabs is climate change.  As water temperature and sea level rises it has been correlated to have negative effects on the distribution and reproduction rate of horseshoe crabs.  Horseshoe crab eggs are a very important part of the food web.  Around only 10% of the eggs laid hatch.  This is because many of the shore birds eat the eggs, and the effects of a declining horseshoe crab population would scale through all the tropic levels.

Not only do horseshoe crabs provide ecosystem benefits, but also have uses for biomedical research.  In the 1960’s Dr. Frederik Bang discovered that when he injected bacteria into the horseshoe crab large amounts of clotting occurred.  It was later discovered by Dr. Jack Levin that the clotting was due to the presence of endotoxins produced by the bacteria.  Horseshoe crabs’ blood is a hemolymph based off of copper which makes it blue.  Since horseshoe crabs have evolved so long ago, they have a very primitive immune system.  This primitive immune system evolved to be sensitive to endotoxins.  This endotoxin was believed to have been expressed in cyanobacteria millions of years ago.  Horseshoe crab blood is used to test the purity of drugs and implants.  In the presence of bacteria or endotoxins the blood coagulates and localizes around the area of impurities.  This is because horseshoe crabs have an open circulatory system and wall off the infected areas to stop the infection from spreading.

Biomedical companies use a technique called Limulus Amebocyte Lysate (LAL) which uses the horseshoe crab hemolymph to detect the presence of gram negative microbes.  Laboratories use LAL tests on human injectables such as insulin.  These medications cannot exceed the test limits set by the FDA.  LAL tests are also used on artificial implants such as kidneys and heart valves.  LAL test provide a quick and sure way to determine if the medication or device is sterile.

The medical products are placed in to a reconstituted test tube of LAL.  If the LAL coagulates then the batch of product is contaminated and must be thrown away.  A wide array of biomedical products that people use every day is tested in this manner.  Every shot you have received has past this test to make sure you would not get sick from contaminated medications.

To produce LAL horseshoe crabs must be caught and bleed.  A sterile needle is inserted into the animal and the blood is collected.  It is then centrifuged to separate the amoebocytes from the liquid plasma and then freeze dried for distribution.  This process, while not perfect, has a 10% mortality rate for the horseshoe crabs.  It has been estimated that a quart of LAL is worth $15,000.  The high cost and need is leading researchers to isolate and clone the toxin detecting gene.  If this is possible, LAL can be produced without harming the horseshoe crabs.

The value that these horseshoe crabs provide to the biomedical community is invaluable.  But so is the value of the shellfish to the fishermen of the New England coast.  Horseshoe crabs are most commonly used as bait by these fishermen and are now being regulated on the amount and size of horseshoe crabs they are able to collect.  Horseshoe crabs are used to bait traps for eels and conch, which is an extremely profitable business itself.  This single species is being fought for by three different groups; the fisheries, biomedical, and environmentalists.  All three of these groups what something different from this single species.

Now with the threat of bioterrorism, the LAL tests are used even more.  All vaccines and other injectable medications are thoroughly tested for contamination intentional or not.  With these new demands the horseshoe crab biomedical field needs a larger supply of LAL.  The current policies and processes that the biomedical companies use still kill 10% of the horseshoe crabs used but could be much higher once placed back into the wild.  Current research is looking for alternative ways to produce LAL due to the increasing demand and decreasing population of horseshoe crabs.  Only now are researchers tagging horseshoe crabs and tracking their locations to find effective ways to manage their populations.

But not only do the three groups, fisheries, biomedical, and environmentalists have a hand in the decline of the horseshoe crab populations.  After doing some genetic research, evidence suggests that climate affects horseshoe crab distribution and interbreeding.  Genetic diversity is the basis of a stable and sustainable population.  A study done in 2005 determined that male horseshoe crabs migrated but female horseshoe crabs did not.  This could lead to local extinctions if the males of one area all left.  This study also provides information that could lead to new conservation methods.  Restriction on the number of females captured could help keep populations present in all areas, as well as the relocation of males from different areas to keep the genetic diversity high between all populations.  As more research is done and the more we learn about horseshoe crabs, the better we will be able to keep them around and available for all those who depend on them.

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