'Skin-and-bones' mechanism underlying zebrafish fin regeneration discovered by researchers

Zebrafish have the ability to regenerate their complex skeleton fin in just two weeks after being amputated.  This is possible due to clusters of specialized cells called rays.  Rays migrate over reforming bones and escort bones into the right position to form a branched skeleton.  Sonic hedgehog is a protein that interacts with osteoblasts to promote bone pattering during bone regeneration.  In this experiment genetically modified zebrafish were used.  These zebrafish produce a fluorescent protein to help identify skin and bone cells that respond to hedgehog signals.

The research team said that the findings won't lead to regeneration of limbs in humans but could lead to better therapeutic treatments for broken bones.  This article is very interesting because understanding this mechanism could eventually lead to regrowing limbs in humans.  I'm sure that's 

something everybody is excited about.  

https://www.sciencedaily.com/releases/2017/03/170328092431.htm

http://dev.biologists.org/content/144/7/1165

The Mutation That Caused Snakes To Lose Their Limbs

Around 150 million years ago, snakes used to have arms and legs.  Studies have shown that a DNA mutation called ZRS (the Zone of Polarizing Activity Regulatory Sequence) is responsible for altering snake's appendages.  It was very interesting to see how different research teams used different methods to come to the same conclusion.

One study published in the online journal Cell removed the ZRS gene from mice and replaced it with the ZRS gene from snakes.  When the mice developed, the hardly developed limbs.  They concluded that ZRS is vital in limb development.  The idea of how snakes lost their limbs came about when observing snakes that closely related to the earliest snakes on the phylogenetic tree.  These snakes include the boa and the python who show signs of tiny vestigial limb bones hiding in their muscles.  The gene named "sonic hedgehog," which contains the ZRS sequence, is heavily responsible for embryonic development.  It turns out that the ZRS sequence located in this vital region of DNA  is mutated in snakes alone.

An interesting study published in the Current Biology journal found that while adult snakes do not have appendages, young embryonic snakes do.  The mutation does not show until 24 hours after their existence.  While embryonic snakes start off with "limbs", they can never fully develop because of the sonic hedgehog transcription being switched off from the ZRS disabling mutation.  The adult pythons and boas only show signs of their limb bones because the disabling mutation occurs later in embryonic development.

Full Article Here

Snakes Used to Have Legs and Arms...Until These Mutations Happened

Snakes are limbless due to a mutation in a stretch of their DNA called the Zone of Polarizing Activity Regulatory Sequence (ZRS).  This zone contains the information required to make limbs for most vertebrates.  Scientists looked at the genomes of earlier snakes like the python and boa.  They have vestigial legs or bones embedded in the muscles.  Scientists also looked at "advanced" snakes which include the cobra and viper.  These snakes do not have any limb structures at all.  Scientists found that the Sonic Hedgehog gene, which is located in the ZRS and is responsible for embryonic limb development, had mutated in snakes.  To prove that mutations in the ZRS sequence was responsible for limb loss, scientists used CRISPR to eliminate the ZRS stretch of DNA in mice and then replaced it with the stretch from different types of snakes.  This resulted in mice with truncated limbs.  Researchers then looked at the ZRS section of a snake and found that it resulted from a deletion of 17 base pairs.  They fixed the sequence and then inserted it into mice embryos.  The limbs of the mice then grew like normal again.  Complications arose when attempting to insert the fixed sequence into python embryos.  Transcription of sonic hedgehog stops within a day of the egg being laid so the snake never develops legs.  However in the short amount of time that it exists initially, early development of a femur, tibia, and fibula occur.

http://www.livescience.com/56573-mutation-caused-snakes-to-lose-legs.html

http://www.sciencedirect.com/science/article/pii/S0092867416313101

https://www.eurekalert.org/pub_releases/2016-10/cp-hsl101316.php

Sonic Hedgehog Mutations Lead to Limb Loss in Snakes

Martin Cohn ph.D. and ph.D. candidate Francisca Leal of the UF College of Medicine have discovered the exact genetic mutations that lead to the loss of legs in snakes. It took scrupulous  comparison of lizard and snake genomes and careful observation of the development of python embryos as well as the genetics of formation at each step to discern the cause. The creation of limbs in this case is inhibited by three key mutations on the enhancer of the so-called Sonic hedgehog gene (you can’t make this up). These mutations interact together and disrupt the coding for the transcription factor which then does not allow for the gene to be expressed, and thus no legs.

To confirm this sequence was the cause, CRISPR was used to insert bits of DNA from the ZRS (an area in Sonic Hedgehog the python exhibited mutations) from different mammals into mice. These mice had normal limb creation, but in the mice that got the python sequence added, no normal limbs formed (shown in figure above).

Such complete decay of the enhancer is evident in more evolutionary ‘advanced’ snakes like cobras and vipers while some others such as pythons have vestigial nubs where a leg would have formed if it were not for the Sonic hedgehog gene switching off due to the decay of its enhancer. Some scientists have hypothesized that other snakes have re-evolved legs and this discovery shows how it is possible that the snakes are still retaining the genetic machinery to do so. This is especially true in snakes like pythons whose Sonic Hedgehog genes are active for a short while before the enhancer decays. In addition based on Cohn and Leal’s genomic comparisons, they also believe these mutations likely occurred 100 million years ago (the Upper Cretaceous Period).

Links:
http://phys.org/news/2016-10-genetic-snakes-legs.html
http://phys.org/news/2016-10-snakes-lost-blueprint-limbs.html