In 2009, the Nomenclature Committee on Cell Death proposed a set of recommendations
for the definition of distinct cell death morphologies and for the appropriate use
of cell death-related terminology, including ‘apoptosis', ‘necrosis' and ‘mitotic
catastrophe'.
1
Of which, apoptosis and necrosis is always the most concern topic on cell death mode
to researchers. Apoptosis has come to be used synonymously with the phrase ‘programmed
cell death' as it is a cell intrinsic mechanism for suicide that is regulated by a
variety of cellular signaling pathways. During apoptotic death, cells are neatly carved
up by caspases and packaged into apoptotic bodies as a mechanism to avoid immune activation.
In contrast to apoptosis, necrosis has been traditionally thought to be a passive
form of cell death with more similarities to a train wreck than a suicide.
2
Kidney stones are a common and frequently occurring disease, >70% of kidney stone
patients suffer from urolithiasis caused by calcium oxalate (CaOx) stones, but the
mechanism by which kidney stones are formed has not yet been completely clarified.
Especially, the mode of cell death produced by CaOx is confusing to us, by far. Saha
et al.
3
have demonstrated that exposure of cells to CaOx crystals can lead to significant
apoptotic changes, including condensation and margination of nuclear chromatin, DNA
fragmentation, and migration of phosphatidylserine (PS) of the plasma membrane from
inside the cell membrane to the cell surface. However, Schepers et al.
4
have proven that exposure of cells to CaOx crystals results in necrotic cell death
with significant necrotic changes, such as loss of plasma membrane integrity, release
of lactate dehydrogenase, cellular and nuclear swelling, and inflammatory response.
In general, fast-acting metabolic poisons and strong physical stress, such as freezing,
boiling, or shearing, rupture cell membranes and cause rapid cell necrosis. By contrast,
a slow acting form of cell death called apoptosis does not involve membrane damage
and inflammation.
5
Cell death is a complicated and confusing pathological process. Cell apoptosis and
necrosis caused by CaOx crystal exposure may be related to cell types, crystal concentration,
exposure time, and even the unknown physicochemical properties of crystals.
In our new paper in Cell Death Discovery,
6
we comparatively investigated the differences of cell death mode induced by nano-sized
(50 nm) and micron-sized (10 μm) calcium oxalate monohydrate (COM) and dihydrate (COD)
to explore the cell death mechanism. Exposure to nano-/micron-sized COM and COD crystals
triggered both apoptotic and necrotic cell death in renal epithelial cell lines. However,
nano-sized crystals primarily caused apoptotic cell death, leading to cell shrinkage,
PS ectropion, and nuclear shrinkage, whereas micron-sized crystals primarily caused
necrotic cell death, leading to cell swelling and cell membrane and lysosome rupture.
The cell death mechanism induced by nano-/micron-sized COM and COD is summarized in
the schematic in Figure 1. Nano-sized COM and COD crystals are more likely to be internalized
by cells than micron-sized crystals. These internalized nano-sized crystals were transferred
into lysosomes via vesicular transport, and could be degraded in lysosomes to release
calcium and oxalate ions. Internalized nano-sized crystals may increase the lysosomal
membrane permeability in varying degrees. A prevalent assumption is that the reparable
damage of lysosomes can initiate apoptosis, and a sudden massive destruction of lysosomes
leads to necrosis.
7
The nano-sized crystals were evenly distributed on cell surface; they induced mild
injury instead of partial acute injury. After the treatment by nano-sized crystals,
the cells and nuclear shrank, presenting typical apoptosis characteristics. The mitochondria
were seriously injured and the mitochondrial membrane potential was significantly
decreased. Apoptotic cells maintained their plasma membrane integrity, but PS was
translocated on the cell membrane. The internalized crystals could not only be captured
by lysosomes, but also entered into the nucleus through the nuclear pores, leading
to the cleavage of DNA into internucleosomal fragments of 180 bp and multiples, which
is an important characteristic of apoptotic cell death.
For micron-sized COM and COD crystals, their particle number is much less than that
of nano-sized crystals under the same concentration. Micron-sized crystals on cells
presented a nonhomogeneous distribution, which caused uneven injury of the cell membrane
and local strong physical stress, resulting in necrotic cell death. Necrotic cells
released inflammatory factors and led to cell membrane rupture. Cell membrane rupture
can cause an imbalance in cell osmotic pressure and lead to the sudden massive destruction
of lysosomes accompanied with hydrolytic enzyme release, which is an important factor
in necrotic cell death.
8
Meanwhile, sudden massive release of hydrolytic enzyme could lead to the random degradation
of chromatin DNA, resulting in necrotic cell death.
Cell apoptosis induced by nano-sized COM and COD was accompanied with PS ectropion,
whereas necrosis induced by micron-sized crystals was not. This exposed negatively
charged PS acted as a binding site of urine microcrystalline on the cell surface and
increased the adhesion and aggregation of microcrystallines, thereby increasing the
risk of stone formation. Therefore, compared with necrosis, apoptosis may be more
likely to induce stone formation. In general, stone formers tend to excrete urine
that is more supersaturated than that of non-stone formers. The median size of initial
formed crystals is inversely related to relative supersaturation,
9
therefore, the initially formed urinary crystallites in stone formers would be smaller
than that in healthy controls. Thus, the higher supersaturated urine in stone formers
should be more likely to induce apoptotic cell death, which will increase the adhesion
and aggregation of microcrystallines and more easily lead to stone formation.
Besides, crystal shape, crystal structure, and even other physical and chemical properties
may also affect the mode of cell death, but the relevant studies were very limited.
Braydich-Stolle et al.
10
reported that crystal structure of TiO2 could mediate the cell death mode in mouse
keratinocyte cells. The anatase TiO2 nanoparticles induced cell necrosis, while the
rutile TiO2 nanoparticles initiated apoptosis through the formation of ROS. However,
in our present study,
6
the mode of cell death produced by the same-sized COM and COD crystals has no obvious
difference. Therefore, cell death is a complicated pathological process, the detailed
effect of physicochemical properties of crystals in cell death mode still calls for
further research.