Worm Breeder's Gazette 13(3): 47 (June 1, 1994)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Dominant mutations of the mec-4 gene cause swelling and degeneration of the six mechanosensory neurons. mec-4 encodes an ion channel subunit homologous to the amiloride-sensitive Na+ channel in vertebrates. Several other "degenerin" genes encode similar channel subunits that also can mutate to cause similar cell deaths in different groups of neurons in C. elegans; e.g. deg-1 , deg-2 ,etc. We have characterized the molecular and cellular changes that occur as a result of expression of the toxic mec-4 (d)gene product, thought to generate a channel that does not close efficiently.
mec-4 is expressed about two hours after the touch receptors are born. L1 larvae from the ZB-7 strain that overexpresses mec-4 (d)were staged by DIC, then fixed and embedded for electron microscopy. The stages of cell death of one touch receptor, PVM, were examined in serial thin sections.
The earliest defects are found at the plasma membrane, where numerous tightly wound membrane lamellae form, some invaginating into the cytoplasm, others possibly shedding from the cell. Almost simultaneously, membrane-bound vacuoles of unknown origin begin to enlarge rapidly within the cytoplasm. Within 30 min after onset of mec-4 (d)expression, these vacuoles swell the cell markedly, after which the nuclear membrane also begins to invaginate. Some cytoplasmic organelles begin to deteriorate thereafter. By 60 min, the neuron soma is grossly distended, the nucleus has moved to an eccentric position and broken into fragments, and the cytoplasm has undergone lysis. Throughout this process, membrane bound vacuoles and membranous lamellae continue to grow, until the cell ends as a large ghost containing a few large lamellae. A few membranous lamellae were noted in other cell types apart from the touch cells; these small defects may represent residual background expression of mec-7 due to the high copy number in the ZB-7 strain. No other cells undergo degeneration.
The early stages of degenerin-induced death look rather different than programmed cell death, but are reminiscent of some excitotoxic cell deaths.