A hydraulic instability drives the cell death decision in the nematode germline

Background

Oocytes are large and resourceful- they transmit genetic information to the next generation, and provide cellular material to develop a fertilized zygote into an embryo. During oogenesis some germ cells undergo extensive growth, typically at the expense of others that ultimately undergo apoptosis. This phenomenon, and the associated exchange of material is facilitated by a syncytial structure with a shared cytoplasm. Although apoptosis in the germline can serve as quality control by removing damaged cells, the majority of germ cells that undergo apoptosis under normal conditions seem healthy. Chartier et al explore in their work, how from a seemingly homogeneous population, cells are selected to live or die (1).

Key findings

Transition from a homogeneous to a heterogeneous growth mode.

The authors explore the question on how germ cells are selected to live or die out of a homogeneous population. For this question, they use as a model organism C. elegans. During their maturation, some germ cells grow to become oocytes while others shrink and die by apoptosis. The authors explore a potential relationship between germ cell growth, shrinkage and apoptosis. Following their development by confocal microscopy, they measured individual germ cell volumes, and noted a transition from a homogeneous growth mode, to a heterogeneous one along the gonad. Following this observation, the authors identified the transition zone along 65% of the germline length, and that importantly, proximal to this location physiological apoptosis begins to occur.

Flux of cytoplasm through the rachis along the gonad

The next step consisted on investigating flux of cytoplasm through the rachis along the gonad: An increase in rachis flux implies that germ lines contribute material to the rachis, while a decrease in rachis flux implies that the germ cells receive material from the rachis. Particle imaging velocimetry showed that the flux of cytoplasm through the rachis increases along the distal part of the gonad, peaks at around 60% germline length and decreases thereafter. This suggests germ cells donate material to the rachis prior to 60% germline length, while they receive material from the rachis thereafter. However, germ cells grow prior to 60% germline, despite losing cytoplasm to the rachis, implying they receive material from the outside. Up to 60% gonad length, material uptake is positive and germ cells grow by receiving material from the outside. Material uptake becomes negative beyond 60%, indicating loss of material to the outside.

Pressure differences and transition from homogeneous to heterogeneous mode of germ cell growth

Pressure differences between germ cells and rachis drive cytoplasmic exchange through rachis bridges. The authors thus explored the hydraulics of the gonad. For this, they constructed a one-dimensional physical model that relates pressure profiles to flows of germ cells and rachis cytoplasm, as well as material exchange between germ cells and rachis. Because pressure differences drive cell-to-rachis currents, the pressure difference between cells and rachis turns at the 60% gonad length.

The authors investigated whether the inversion of the pressure difference might explain the transition from homogeneous to heterogeneous mode of germ cell growth. Considering tissue hydraulics, the state of equal germ cell volumes is stable only if the pressure inside germ cells is larger than in the rachis. An instability occurs when the pressure in the rachis is larger than in germ cells: a small difference in germ cell volumes will increase, leading to the growth of the larger germ cell at the expense of the smaller one. The authors conclude this instability is expected to be triggered when the pressure difference between rachis and germ cells becomes negative at 60% gonad length. This hydraulic instability presents a possible mechanism by which germ cells become fated to die.

 An alternative hypothesis: inhibition of apoptosis

The authors explored another hypothesis, whereby unknown molecular signals first induce apoptosis, which subsequently leads to the shrinkage of cells fated to die. For this, they inhibited apoptosis of germ cells by RNAi targeted against caspase ced-3, and evaluated whether the germ cell still shrinks. While in the absence of apoptosis, germ cells are no longer removed, some cells in the proximal region shrink. Moreover, mutant gonads still show a transition from a homogeneous to a heterogeneous mode of growth. This eliminates apoptosis as the cause of germ cell shrinkage and supports the idea that germ cell fate is determined by a hydraulic instability.

Mechanical manipulation of the life and death decision of the gonad

To explore further the idea that the life and death decision of the gonad is a mechanical one, the authors used unidirectional thermoviscous pumping to artificially reduce the volume of individual germ cells to increase their likelihood to undergo apoptosis. This resulted in apoptosis of over 50% of germ cells within 3 hours. Together, this work reveals a robust mechanism of mechanochemical cell fate decision-making in the germline.

What I like about this preprint

I found it an interesting topic, combining in the same question, molecular, cell biology and biophysical aspects. I enjoyed reading the work, and think this opens important questions in the field.

Open questions 

1. Is the decision to grow or shrink random? Or is it influenced to some extent by location, neighbours, etc?
2. Until what point of shrinkage is the fate decision reversible?
3. You explored by molecular methods, influencing apoptosis. How do you link, at a molecular and cell biological level, the initiation of apoptosis with the mechanical aspect?
4. You used C. elegans as a model organism. To what extent do your findings apply to other organisms?
5. Out of curiosity, you described material influx from the cytoplasm and the outside. What are the specific components of these materials, and do they also play a role in cell fate?

References

1. Chartier NT et al, A hydraulic instability drives the cell death decision in the nematode germline, bioRxiv, 2020

 

doi: https://doi.org/10.1242/prelights.22752

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