Programmed cell death or apoptosis is a genetically controlled process with important roles during development of multicellular organisms. Molecular genetic studies have revealed that the basic principles of regulation and execution of apoptosis are conserved (Cashio et al., 2005
). One common feature in the cell death program is the activation of caspases, a highly specialized class of cell death proteases. These enzymes are generally divided into two distinct classes: initiator and effector caspases. Upon activation, initiator caspases activate the downstream effector caspases via proteolytic processing, and activated effector caspases cleave key cellular substrates to promote apoptosis. In Drosophila
, the pro-apoptotic genes head involution defective
are both necessary and sufficient for the induction of apoptosis through activation of the initiator caspase Dronc and the effector caspases DrICE and Dcp-1 (Cashio et al., 2005
). In addition to an essential role of caspases for apoptosis, recent findings have demonstrated that caspases also have important functions in non-apoptotic processes (Kuranaga and Miura, 2007
; Lamkanfi et al., 2007
). One such non-apoptotic process is the induction of compensatory proliferation in apoptotic tissue.
Coordination of cell death and cell proliferation is critical for the maintenance of tissue homeostasis. Excessive cell loss in a developing tissue can be compensated for by additional divisions of the remaining cells. For example, in the Drosophila
wing imaginal disc irradiation-induced cell death is followed by compensatory cell proliferation which results in an adult wing of nearly normal size (Haynie and Bryant, 1977
; James and Bryant, 1981
). Further genetic manipulation using toxins to induce ectopic cell death in wing discs showed that the cells adjacent to the apoptotic cells undergo compensatory proliferation (Milan et al., 1997
). This suggests that apoptotic cells induce compensatory proliferation of neighboring cells through secretion of mitogens.
More recently, in developing wing discs, in which apoptosis was induced by expression of the pro-apoptotic gene hid
, loss of the caspase inhibitor DIAP1 or by X-ray treatment, the accumulation of two major mitogens, Decapentaplegic (Dpp) and Wingless (Wg), has been observed in dying cells (Huh et al., 2004
; Perez-Garijo et al., 2004
; Ryoo et al., 2004
). Key for this finding was the simultaneous expression of the caspase inhibitor P35 (see also ). Under these conditions, the dying cells were kept alive (‘undead’), allowing accumulation of Dpp and Wg. This accumulation appears to be dependent on the initiator caspase Dronc, because it cannot be blocked by expression of P35 which inhibits effector caspases, but not Dronc (Hawkins et al., 2000
; Meier et al., 2000
; Yoo et al., 2002
; Yu et al., 2002
). In addition, the Drosophila
homolog of the tumor suppressor p53, Dp53, has been implicated downstream of Dronc for compensatory proliferation (Wells et al., 2006
) (see also ).
Effector caspases DrICE and Dcp-1 are required for GMR-hid-induced compensatory proliferation
Notably, these studies on mechanisms of compensatory proliferation were carried out in developing larval wing imaginal discs in Drosophila
. Cells in wing discs proliferate extensively during larval stages, and the majority of these cells do not differentiate before they reach pupal development (Garcia-Bellido and Merriam, 1971
; Milan et al., 1996
). Hence, the mechanisms of compensatory proliferation have so far only been investigated in situations where most cells are proliferating. Interestingly, we have recently observed apoptosis-induced compensatory proliferation in differentiating eye tissue of third instar larvae (Srivastava et al., 2007
). However, it is unclear whether this form of compensatory proliferation is controlled by a similar mechanism as reported for larval proliferating wing discs.
Unlike wing discs, cellular differentiation in eye discs starts during mid-third instar larval stage. An indentation known as the morphogenetic furrow (MF) forms at the posterior edge of the eye disc and sweeps across to the anterior of the eye disc. Anterior to the MF, cells are in an uncommitted state and keep proliferating. However, cells immediately before and in the MF arrest in the G1 phase of the cell cycle for an extended period. A subset of these G1-arrested cells starts differentiating into 5 of the final 8 photoreceptor neurons (R8, R2, R5, R3, R4) per ommatidium, while the remaining cells synchronously re-enter S phase and form the second mitotic wave (SMW) posterior to the MF (Wolff and Ready, 1993
) (see also ). After completion of mitosis in the SMW, all cells arrest in G1 indefinitely (Baker and Yu, 2001
). From this pool of undifferentiated cells generated in the SMW, the remaining photoreceptors (R1, R6, R7), non-neuronal cone cells, and pigment cells are recruited into ommatidia (Wolff and Ready, 1993
GMR-hid-induced compensatory proliferation occurs in undifferentiated cells and requires a short-range signal
Here, by taking advantage of the different developmental states of the third instar larval eye disc, we show that different mechanisms control apoptosis-induced compensatory proliferation in proliferating and differentiating eye tissue. As shown in the proliferating wing disc, Dpp and Wg are expressed in response to apoptotic activity in proliferating eye tissue. In contrast, compensatory proliferation in differentiating eye tissue requires Hh signaling. We also show that the effector caspases DrICE and Dcp-1 are required for compensatory proliferation in differentiating tissue, in contrast to proliferating tissue which requires the initiator caspase Dronc. Thus, depending on the developmental context, different caspases trigger distinct forms of compensatory proliferation in an apparently non-apoptotic function.