After we sense a threat, our brain center incharge of responding goes into gear, setting off a chain of biochemical reactions leading to the release of cortisol – a main stress hormone – from the adrenal glands.
Dr. Gil Levkowitz and his team in the Molecular Cell Biology Department have now revealed
a new kind of ON-OFF switch in the brain for regulating the production of a main biochemical signal from the brain that stimulates cortisol release in the body. This finding, which appeared in
Neuron, may be relevant to research into a number of stress-related neurological disorders.
This signal is corticotropin releasing hormone (CRH), which is manufactured and stored in special neurons in the hypothalamus. By the time the CRH-containing neurons have depleted their supply of the hormone, they are already receiving the directive to produce more.
The research – on zebrafish – was performed in Levkowitz’s lab and spearheaded by Dr. Liat Amir-Zilberstein together with Drs. Janna Blechman, Adriana Reuveny and Natalia Borodovsky, as well as Maayan Tahor. The team found that a protein called Otp is involved in several stages of CRH production. As well as directly activating the genes encoding CRH, it also regulates the production of two different receptors on the neurons’ surface for receiving and relaying CRH production signals – in effect, ON and OFF switches.
The researchers discovered that both receptors are encoded in a single gene. To get two receptors for the price of one, Otp regulates a gene-editing process known as alternative splicing, in which some of the elements in the sequence encoded in a gene can be “cut and pasted” to make slightly different “sentences.” In this case, it generates two variants of a receptor called PAC1: The short version produces the ON receptor; the long version, containing an extra sequence, encodes the OFF receptor. The researchers found that as the threat passed and the supply of CRH was replenished, the ratio between the two types of PAC1 receptor on the neurons’ surface gradually changed from more ON to mostly OFF.
Faulty switching mechanisms may play a role in a number of stress-related disorders. The action of the PAC1 receptor has recently been implicated in post-traumatic stress disorder, as well as in schizophrenia and depression. Malfunctions in alternative splicing have also been associated with epilepsy, mental retardation, bipolar disorder and autism.
Dr. Gil Levkowitz's research is supported by the estate of Lore Lennon; the Kirk Center for Childhood Cancer and Immunological Disorders; the Dekker Foundation; and the Irwin Green Alzheimer's Research Fund. Dr. Levkowitz is the incumbent of the Tauro Career Development Chair in Biomedical Research.
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Proof of Principle
Though effective, these methods can be very expensive and time-consuming. A new protein expression system using whole insect larvae offers a potential alternative, cost-effective method of producing large quantities of human proteins. Before such methods can be employed, however, one question must be answered: Are the proteins generated comparable in structure and function to those produced by the traditional techniques?
Using the state-of-the-art facilities at the Israel Structural Proteomics Center (ISPC) at the Weizmann Institute, of which Sussman is Director, and combining various functional analysis, crystallization and data collection methods, Greenblatt was able to crystallize the human carboxylesterase 1 enzyme isolated from the larvae and confirm that its structure – and function – was identical to previous examples of this enzyme isolated from cultured insect cells.
Sussman: “So far, we have tested only the human carboxylesterase 1 enzyme; but if this new system proves to work for other proteins as well, it will provide an invaluable tool for researchers and industry, alike.”
A page on this study can be seen in Proteopedia at: http://www.proteopedia.org/w/Journal:Acta_Cryst_F:1