Development, Degeneration and Damage Control: How We Check the Quality of Neurons


By Vaishali Talwar | SQ Staff Writer | SQ Online (2013-14)

Language, motor skills, behavior, speech and memory. These highly vital functions are dependent on a group of cells, called neurons. The human body consists of hundreds of billions of neurons spread throughout the body, carrying important signals and information to each and every cell. A malfunction in the development in any one of these important nerve cells can have serious consequences. Imagine having to wear a brace because your spine is abnormally shaped. Imagine having a severely reduced field of vision because of the inability to physically move your eyes from side to side. All these are symptoms of neurodevelopmental disorders.

Researchers at the Yishi Jin Laboratory based in UCSD became interested in this topic of abnormal nerve growth by noticing mutations in a wide variety of organisms such as roundworms, mice and humans The lab, which specializes in research on the topic of neurogenesis, the development of neurons, analyzed how the efficiency of neural development is maintained. Researchers noticed certain mutations in a laboratory roundworm called Caenorhabditis elegans. Some of the worms had uncoordinated movements while others laid less eggs than normal. However, these seemingly random mutations had one thing in common.

“We tagged the mechanosensory neurons of the worms with a green fluorescent protein and we noticed that the fluorescence in some of the worms followed a different path than the fluorescence in most others,” said Dr. Zhiping Wang, the lead author on this research.

By tracking the fluorescence in the mutated worms, the researchers found that important axons were oriented incorrectly and were growing parallel instead of ventrally.

“We discovered that these abnormalities were caused by mutations in the gene known as EBAX-1. Thus EBAX-1, somehow was responsible for maintaining the quality of neurons in the worms body, perhaps even in our body,” Dr. Wang said.

The C elegans worm with its neurons tagged with the green fluorescent protein as seen under the microscope.

The C elegans worm with its neurons tagged with the green fluorescent protein as seen under the microscope. Photo by Dr. Zhiping Wang

Labeled diagram of the laboratory roundworm, C elegans.

The C elegans worm with its neurons tagged with the green fluorescent protein as seen under the microscope.

EBAX-1 – What is it?

EBAX-1 is an important gene that controls the quality of a protein called Robo3. Robo3 acts like a signal receiver on the axon and guides proper axon development in nerves.

If EBAX-1 is operational, it will ensure that Robo3 receives the correct signals and guides the axon correctly. If Robo3 does not carry out the task correctly, EBAX-1 will send the Robo3 to a protein degradation complex, which essentially acts as a trashcan.

Therefore, when the EBAX-1 gene does not function correctly, it causes a cascade of effects that eventually leads to a malfunctioning nerve. Therefore, a mutated EBAX-1 gene could possibly be the cause of symptoms such as abnormal nerve growth, in genetically inherited human diseases.

This shows two types of worms. One has normal neuronal growth and lays a normal amount of eggs, the other has a mutation in the gene EBAX-1 and lays less eggs.

This shows two types of worms. One has normal neuronal growth and lays a normal amount of eggs, the other has a mutation in the gene EBAX-1 and lays less eggs. Photo Dr. Zhiping Wang

This shows two types of worms. One has normal neuronal growth and lays a normal amount of eggs, the other has a mutation in the gene EBAX-1 and lays less eggs.

Discovering The Function of EBAX-1

The function of EBAX-1 was discovered by linking the gene to various proteins and then tracking down the functions of those proteins.

“The most challenging part of the research was to link EBAX-1 to the mutations present in the roundworm,” Dr. Wang said.

The research team at the Yishi Jin lab used mass spectrometry to determine the function of EBAX-1 in C.elegans. Mass spectrometry involves using light to determine the elemental structure of a molecule in order to deduce its chemical structure.

The research team also used protein tags to be able to track the proteins and genes throughout the process of neural development. They first tagged commercial antibodies with a viral tag known as FLAG. These commercial antibodies then recognized the EBAX-1 gene. The EBAX-1 gene in turn, recognized the protein degradation complex as well as a heat shock protein, hsp 90, which is a common protein regulator. By tracking which proteins EBAX-1 responded to, the research team was able to determine the function of this important gene.

With this method, researchers were able to pinpoint Robo3 as one of the most important proteins in axon development by observing the mutations in the roundworm C.elegans. Mutations in EBAX-1 seemed to be the cause of a dysfunctional Robo3, which in turn attributed to the abnormalities found in neurons. Thus EBAX-1 had a definitive role to play in the nerve development of C. elegans, an organism which has a number of genes similar to those found in humans.

The Future

Interestingly, C. elegans is not the only organism to show the EBAX-1 gene and the Robo3 signal receiver. Mice and humans also show homologous, or similar, genes and proteins.

The next step for the Jin Lab is to figure out if EBAX-1 does indeed control neuronal development in mice and how it does so.

“Finding out the exact target of EBAX-1 is very important,” said Dr. Wang.

Increasing research prospects include looking at the same homologous genes in mice and hopefully in the near future, humans. This research has incredible ramifications for neuronal development diseases such as ‘horizontal gaze palsy with progressive scoliosis,’ a disease that causes vision impairment and affects the development of the spine. By specifying the exact target and sequence of EBAX-1, it may be possible to cure this disease.

As Dr. Wang says, “This is just the beginning.”

Dr Zhiping Wang observing the C elegans worm under the microscope. The C elegans has been tagged with a green fluorescent protein (GFP).

Dr Zhiping Wang observing the C elegans worm under the microscope. The C elegans has been tagged with a green fluorescent protein (GFP). Photo by Vaishali Talwar

A sample of C elegans kept under the microscope.

A sample of C elegans kept under the microscope. Photo by Vaishali Talwar

Dr Zhiping Wang observing the C elegans worm under the microscope. The C elegans has been tagged with a green fluorescent protein (GFP).