Cells dispose of 'garbage' in cyclic waves
Most organisms – animal, plant, fungi – are made up of many cells. Part of their existence involves the programmed death of some cells, a process called apoptosis.
Taking out the "garbage" or getting rid of the dead cells is an intricately choreographed ballet that involves both the cells’ encapsulation and degradation, a process governed by a specific set of molecules that are turned off and on at specific times.
Removing garbage critical
As in a ballet, timing is everything. When these molecules turn on and off is utterly crucial to the success of the garbage disposal and the life of the organism, said researchers at Baylor College of Medicine in the current issue of the journal PLoS Biology.
"Tens of billions of cells die daily in the human body, said Dr. Nan Lu, the first author who recently graduated from BCM with a doctoral degree and is continuing his postdoctoral training at BCM. "Removing them is critical to health."
For example, if genetic material in the nuclei of cells that undergo apoptosis is not degraded in time, it will cause the body’s inflammatory and autoimmune responses to go into overdrive, resulting in a host of health problems.
"These dead or dying cells are swiftly taken up by scavenger cells into membrane-bound cellular compartments called phagosomes, where they are subsequently degraded," said Dr. Zheng Zhou, associate professor of biochemistry and molecular biology at BCM.
Three enzymes play crucial roles
In a roundworm (nematode) called Caenorhabditis elegans or C. elegans, a key signaling molecule in the dead-cell degradation process is PtdIns(3)P (Phosphatidylinositol 3-phosphate). It is produced on the membranes of the phagosomes in two waves while the scavenger cells go about their task. The first wave occurs soon after a phagosome forms and lasts for 10 to 15 minutes. A second, weaker wave occurs 10 minutes later and lasts until the dead cell inside a phagosome is completely digested.
The researchers found that three enzymes play crucial roles in the regulation of PtdIns(3)P levels over time, said Zhou, senior author of the report.
Two phosphoinositide 3-kinases, PIKI-1 and VPS-34, add a phosphate molecule to PtdIns, producing PtdIns(3)P, which lures certain kinds of proteins to associate with phagosomes. Another protein, MTM-1, removes the phosphate molecule from PtdIns(3)P, creating a gap period between the two PtdIns(3)P waves.
"You need both kinases -- PIKI-1 and VPS-34 -- to make enough PtdIns(3)P," said Zhou. PIKI-1 is involved in initiating the production of PtdIns(3)P, and the second kinase VPS-34 keeps PtdIns(3)P production going.
Further studies by the Zhou group show that the coordinated actions of all three enzymes are important for keeping PtdIns(3)P at proper levels during different phases of degradation.
"Balanced action is the key," said Zhou. If PIKI-1 and VPS-34 are both removed, then no PtdIns(3)P is generated and the whole process of degrading the dead cells grinds to a halt. On the other hand, too much of a good thing is also damaging. When MTM-1 is inactivated, too many PtdIns(3)P molecules cause degradation to stop and create an unhealthy condition in the organism.
Zhou and Lu believe that the elegant temporal regulation of PtdIns(3)P on phagosomes, proven in the worm, may be duplicated in the more complex organisms such as mammals.
"Every protein we study here has a close mammalian homologue," said Lu, "Plus, PtdIns(3)P is known as an important molecule for the digestion of invading pathogens caught inside mammalian phagosomes, yet how the PtdIns(3)P level is regulated throughout the digestion process has not been revealed in mammals."
Others who took part in this work include Qian Shen, Timothy R. Mahoney and Ying Wang of BCM and Lukas J. Neukomm, now of the University of Massachusetts Medical School in Worcester.
Funding for this research came from the National Institutes of Health and the March of Dimes Foundation.