Student Theses and Dissertations

Date of Award

2007

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

RU Laboratory

Shaham Laboratory

Abstract

The most intensively studied form of programmed cell death (PCD) is apoptosis,

which is characterized by stereotypical morphological features including

chromatin compaction and by a requirement for the activity of caspase proteases,

which are controlled by conserved gene pathways. Although non-apoptotic,

caspase-independent programmed cell death pathways have been postulated,

there is little evidence to convincingly prove their existence, and few insights

regarding their molecular basis or possible

in vivo functions. To investigate this

question, we have studied the developmentally regulated PCD of the

Caenorhabditis elegans

linker cell.

We have carried out transmission electron microscopy studies of dying

linker cells, which revealed non-apoptotic features, including nuclear crenellation

in the absence of chromatin condensation, swelling of mitochondria and

endoplasmic reticulum, and accumulation of cytoplasmic single- and multilayered

membrane-bound structures. Similar morphological changes occur

during the normal developmental death of some vertebrate neurons in the spinal

cord and ciliary ganglia, suggesting that this is a highly conserved cell death

program.

Our genetic studies demonstrate that linker cell death is a non-apoptotic

programmed cell death. This cell death is independent of the

ced-3 caspase,

other

C. elegans caspase homologs, and can occur even when a broad-spectrum

caspase inhibitor is expressed. We have found that the engulfment of the linker

cell is independent of the known

C. elegans engulfment genes. We tested and

found no evidence for the involvement of autophagic, necrotic, or Wallerian

degeneration genes in linker cell death.

By ablating cells neighboring the linker cell, and by examining mutants in

which the linker cell is abnormally positioned, we demonstrated that the linker cell

employs a cell-autonomous program to promote its demise. Using a candidate

gene approach, we showed that linker cell death is controlled by the microRNA

let-7

and by the zinc finger transcription factor lin-29, both components of the

main developmental timing pathway in the animal.

Conducting a genome-wide RNAi screen, we have identified new

candidate regulators of linker cell death. Characterization of these genes may

uncover the molecular mechanism driving this new type of programmed cell

death in

C. elegans.

Comments

A Thesis Presented to the Faculty of

The Rockefeller University

in Partial Fulfillment of the Requirements for

the Degree of Doctor of Philosophy

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