A Molecular Switch that Determines Stem Cell Or Neuron

A University of California, San Diego School of Medicine research team has provided new information about a well-known protein that provides the switch for cells to become neurons. This protein is part of a regulatory circuit that can push an immature neural cell to become a functional neuron.

Postdoctoral fellow Chih-Hong Lou and his colleagues worked with principal investigator Miles F. Wilkinson, who is a professor in the Department of Reproductive Medicine, and is also a member of the UC San Diego Institute for Genomic Medicine. These data were published in the February 13 online issue of the journal Cell Reports. These data may also elucidate a still poorly understood process – neuron specification – and might significantly accelerate the development of new therapies for specific neurological disorders, such as autism and schizophrenia.

Wilkinson, Lou and others discovered that the conversion of immature cells to neurons is controlled by a protein called UPF1. UPF1 works in a pathway called the “nonsense-mediated RNA decay” or NMD pathway. The NMD pathway provides a quality control mechanism that eliminates faulty messenger RNA (mRNA) molecules.

mRNA molecules are synthesized from DNA in the nucleus of cells and are exported to the cytoplasm where they are translated by ribosomes into protein. All proteins are encoded by stretches of DNA known as genes and the synthesis of an RNA copy of this stretch of DNA is called transcription. After the transcription of a messenger RNA molecule, is goes to the cytoplasm and is used as the template for the synthesis of a specific protein. Occasionally, mistakes are made in the transcription of mRNAs, and such aberrant mRNAs will either be translated into junk protein, or are so damaged that they cannot be recognized by ribosomes. Such junk mRNAs will gum up the protein synthesis machinery, but cells have the NMD pathway that degrades junk mRNAs to prevent the collapse of the protein synthesis machinery.

UPF1 mechanism

A second function for the NMD pathway is to degrade a specific group of normal mRNAs to prevent the production of particular proteins. This NMD function is physiologically important, but until now it had not been clear why it is important.

Wilkinson and others have discovered that UPF1, in combination with a particular class of microRNAs, acts as a molecular switch to determine when immature (non-functional) neural cells take the plunge and differentiate into non-dividing (functional) neurons. In particular, UPF1 directs the degradation of a specific mRNA that encodes for a protein in the TGF-beta signaling pathway, which promotes neural differentiation. The destruction of this mRNA prevents the proper functioning of the TGF-beta signaling pathway and neural differentiation fails to occur. Therefore, Wilkinson, Lou and co-workers identified, for the first time, a molecular pathway in which NMD drives a normal biological response.

NMD also promotes the decay of mRNAs that encode proliferation inhibitors, which Wilkinson said might explain why NMD stimulates the proliferative state characteristic of stem cells. There are many potential clinical ramifications for these findings,” Wilkinson said. “One is that by promoting the stem-like state, NMD may be useful for reprogramming differentiated cells into stem cells more efficiently.

Wilkinson continued: “Another implication follows from the finding that NMD is vital to the normal development of the brain in diverse species, including humans. Humans with deficiencies in NMD have intellectual disability and often also have schizophrenia and autism. Therapies to enhance NMD in affected individuals could be useful in restoring the correct balance of stem cells and differentiated neurons and thereby help restore normal brain function.”

Co-authors on this paper include Ada Shao, Eleen Y. Shum, Josh L. Espinoza and Rachid Karam, from the UCSD Department of Reproductive Medicine; and Lulu Huang, from Isis Pharmaceuticals.

Funding for this research came, in part, from National Institutes of Health (grant GM-58595) and the California Institute for Regenerative Medicine.

Possible Treatment for Brain Disorders

Tuberous Sclerosis is a rare genetic disease that causes the growth of tumors in the brain and other vital organs and may also lead to other conditions such as autism, epilepsy, and cognitive impairment; all of which result from the abnormal generation of neurons.

Tuberous sclerosis is also called tuberous sclerosis complex or TSC, and it is a rare genetic disease that affects multiple organ systems. TSC causes the growth of benign tumors in the brain and on other vital organs such as the kidneys, heart, eyes, lungs, and skin. TSC typically affects the central nervous system and results in a combination of symptoms including seizures, developmental delay, behavioral problems, skin abnormalities, and kidney disease.

TSC affects as many as 25,000 to 40,000 individuals in the United States and about 1 to 2 million individuals worldwide. The estimated prevalence of this disease is one in 6,000 newborns, and it occurs in all races and ethnic groups, and in both genders.

TSC derives its name from the characteristic tuber or potato-like nodules in the brain. These growths calcify with age and become hard or sclerotic.

Many TSC patients show evidence of the disorder in the first year of life. However, clinical features can be subtle initially, and many signs and symptoms take years to develop. As a result, TSC can be unrecognized or misdiagnosed for years.

TSC is caused by defects, or mutations, on two genes-TSC1 and TSC2. Only one of the genes needs to be affected for TSC to be present. The TSC1 gene, discovered in 1997, is on chromosome 9 and produces a protein called Hamartin. The TSC2 gene, discovered in 1993, is on chromosome 16 and produces the protein Tuberin. These proteins combine to form a complex that suppresses cell growth by preventing activation of a master control protein called mTOR. Loss of regulation of mTOR occurs in cells lacking either Hamartin or Tuberin, and this leads to abnormal differentiation and development, and to the generation of enlarged cells, as are seen in TSC brain lesions.

Since Tuberous Sclerosis affect stem cell activity, scientists at Clemson University are examining how neurons are formed from neural stem cells and this research is vital to providing a treatment to Tuberous Sclerosis, which affects how neurons are formed in the brain.

David M Feliciano, assistant professor of biological sciences at Clemson University, said: “Current medicine is directed at inhibiting the mammalian target of rapamycin (mTOR), a common feature within these tumors that have abnormally high activity. However, current treatments have severe side effects, like due to mTOR’s many functions and playing an important role in cell survival, growth and migration.”

mTOR pathway

Feliciano continued: “Neural stem cells generate the primary communicating cells of the brain called neurons through the process of neurogenesis, yet how this is orchestrated is unknown.”

Neural stem cells lie at the very heart of brain development and repair, and alterations in the ability of these cells to self-renew and differentiate can have profound consequences for brain function at any stage of life, according to researchers.

In order to further elucidate the regulation of neurogenesis, Feliciano and his team delivered small pieces of DNA into the neural stem cells of the new-born mice. The team used electroporation to introduce the DNA into the mouse cells, and these small pieces of DNA allowed Feliciano’s team to express and control specific components of the mTOR pathway.

By using these tools, Feliciano and others showed that Increasing the activity of the mTOR pathway cause the neural stem cells to make more neurons at the expense of self-renewal. Increasing mTOR activity caused upregulation of 4E-BP2. 4E-BP2, also known as Eukaryotic translation initiation factor 4E-binding protein 2, binds to a component of the protein synthesis machinery and inhibits its function. Mice that lack functional EIF4EBP2 exhibit autism-like symptoms, including poor social interaction, altered communication and repetitive behaviors.

This work suggests that 4E-BP2 might be a new target for the treatment of TSC and that targeting this protein might cause fewer side effects than targeting mTOR. Future experiments hope to identify those proteins that are made due to the activation of this pathway in neural tissues.