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From the Laboratory of Serge Przedborski, MD, PhD Role of Endoplasmic Reticulum Stress in ALS Neurodegeneration
PCD is an active form of cell death that is highly regulated by a complex network of different molecules. A more comprehensive review on the subject can be found in published references. This program of cell suicide proceeds in a very strict sequential manner, respecting a highly hierarchical molecular organization. Mounting evidence has indicated that the caspase protease family plays a central role in the implementation of PCD in vertebrates (Kaufmann and Hengartner, 2001). These enzymes are constitutively (as normal constituents) expressed in healthy cells, where they are synthesized as precursor proteins (procaspases). Caspases are activated upon processing of procaspases into many different pieces (approximately 20 kDa (p20) and 10 kDa (p10) mature fragments, in addition to the N-terminal prodomain) called the caspase family. This family is broadly divided into two groups: initiator caspases (caspases-8, -9, and -12) and effector caspases (caspases-3, -6, and –7). Initiator caspases undergo autoprocessing for activation in response to PCD stimuli. Active initiator caspases in turn process precursors of the effector caspases responsible for dismantling cellular structures. Recent studies have suggested the existence of a novel PCD pathway in which caspase-12 functions as the initiator of cell death in response to a stress in the endoplasmic reticulum (ER). ER in lay definition is the factory for protein production in cells. Because of the potential significance of this new molecular pathway both from a cell death mechanism and therapeutic standpoints, my laboratory has embarked in the assessment of ER stress in both human ALS and in its well-validated experimental model provided by transgenic rodents expressing the mutant form of superoxide dismutase-1 (SOD-1). To ascertain the role of ER stress in ALS motor neuron degeneration Dr. Hitoshi Kikuchi in our laboratory decided to determine the status of the three main ER pathways causing stress in transgenic mutant SOD-1 mice. While these investigations are still ongoing, he has already accumulated insightful information regarding the ER stress machinery in ALS. The vast majority of proteins in our cells exist, not as long straight strings, but as folded cords. It is now well recognize that this complex folding is essential for the proteins to exert their normal function. Not only will misfolded proteins not function properly, but will also not be well tolerated by the cell. Consequently, unless a cell gets rid of misfolded proteins, it will die. One key function of the ER is to detect the abnormal accumulation of misfolded proteins in the cell and trigger the necessary mechanisms to get rid of them via what is called the Unfolded Protein Response (UPR). In support of ER stress in ALS, Dr. H. Kikuchi found mild UPR in spinal cords of affected transgenic mutant SOD1 mice, evidenced by the increased amounts of two proteins called Grp94 and Grp78/BiP. Indeed, Grp94 and Grp78/BiP are known to be the two main proteins responsible in our cells for carrying on the UPR. If a cell is the site of accumulation of misfolded proteins, the ER will attempt to protect the cell from further accumulation of toxic proteins by blocking their production. This is achieved by the ER through activation of a specific pathway that is regulated by a protein called PERK. In contrast to the UPR, this second ER-dependent protective mechanism did not appear to be recruited in this ALS model. A third component is the ER-associated protein degradation, in which the ER will attempt to protect the cell from further accumulation of toxic proteins by stimulating the protein degradation by a complex cellular machinery called the ubiquitin-proteasome system. This aspect of the ER is still under investigation. In response to ER stress, several cell death mechanisms are also recruited which, ultimately lead to the cell demise in case of the ER-protective mechanisms cited above have failed to protect the cell enough against toxic misfolded proteins. These two killers are CHOP and caspase-12. Dr. H. Kikuchi found that both of these death proteins are activated in the ALS mice. Their activation is specifically observed in the spinal cord of these animals and become stronger as the disease progresses. Dr. H. Kikuchi has now obtained mutant mice deficient in CHOP and caspase-12 and is in the process of crossing these mutant animals with the transgenic mutant SOD1 mice. Although these crossbreeding experiments are extremely time-consuming, they should demonstrate whether or not CHOP and caspase-12 alterations are, as expected, important to the mechanism of motor neuron death in ALS. In parallel of these ER stress investigations in transgenic mutant SOD1 mice, Dr. H. Kikuchi has also studied post-mortem samples from ALS patients for evidence of ER stress. So far, his results indicate that all ER stress alterations found in the mice were also found in human ALS tissues, thus supporting the relevance of the rodent data to elucidate the neurodegenerative process in human ALS. Until the next update, my laboratory staff and I send you and your families a Happy New Year. Serge Przedborski
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For the past decade, my laboratory at Columbia University has devoted
a significant part of its research effort to the study of the mechanisms
responsible for the demise of motor neurons in amyotrophic lateral sclerosis
(ALS). Investigations performed in several laboratories, including ours
have led to the conclusion that programmed cell death (PCD) was instrumental
in ALS neurodegeneration. 
