Lysosomal Cystine and Apoptosis
How does lysosomal cystine storage cause the illness we know as cystinosis?
This question has engaged me for as long as I have known about the disease. Lysosomes are small sacks inside the cells of all higher organisms. They contain powerful digestive enzymes that are needed to allow recycling of complex molecules, like proteins. If these digestive enzymes were not contained in protected spaces, they would digest the cells from inside out. But the barrier nature of lysosomes also poses a problem: How can lysosomal cystine accumulation, which is the hallmark of cystinosis, leave the containment area and therefore cause illness?
Not all cells are supposed to live as long as the whole individual - some are lost during fetal life as the infant develops. Others are deliberately destroyed by apoptosis during adulthood when a cell is detected t have undergone malignant change, or is harboring a virus. When we studied apoptosis in cultured cystinosis skin cells, we found that the presence of lysosomal cystine increases the proportion of cells entering apoptosis by two hundred per cent. Over time, this inappropriate commitment to cell death can reduce the number of remaining cells leading to generalized hypocellularity, and the short stature, failure to thrive and, most importantly, the kidney disease we see in nephropathic cystinosis.
This years funding has permitted us to begin to inquire into the actual molecular mechanism whereby lysosomal cystine augments apoptosis. Apoptosis is a complex process with more than 300 proteins involved. Obviously, if cells have an auto-destruct mechanism, they must be certain that it isn’t activated inappropriately; this there are many checks and balances involved. We and others have shown that lysosomes become leaky in apoptosis before the irrevocable step in which mitochondria (the cells energy stations) lose their integrity, leaking a protein called cytochrome C, and signaling the end of that cell. Margaret Park, a graduate student in my lab at Tulane has done elegant histochemical studies showing that a lysosomal marker, cathepsin B, becomes diffused in the cytoplasm before the mitochondria loose cytochrome C.
In the coming year we plan to continue similar studies to work out the mechanism of enhanced apoptosis at the molecular level. The long-term goal of this project is to understand how lysosomal cystine storage in cystinosis causes the lethal nephropathic phenotype. The intermediate goal is to elucidate the mechanism(s) whereby lysosomal cystine storage markedly increases the rate of apoptosis in cultured cells. Our hypothesis is that lysosomal cystine, when released from lysosomes, augments apoptosis by amplifying the apoptotic cascade and increasing lysosome-mitochondrion cross-talk.
We are extremely grateful or the support of the CRF, which has helped us to continue our studies into the causality of disease in cystinosis. Now that the genome project is completed, many workers believe that proteomics and understanding how diseases occur at the molecular level is the next challenge. We plan to take cystinosis to that new frontier.
Jess G. Thoene, M.D.
Director, Hayward Genetics Center
Karen Gore Professor of Pediatrics
Tulane University, School of Medicine
New Orleans, LA
