Physical constraints and biological regulations underlie universal osmoresponses

  1. Yiyang Ye
  2. Qirun Wang
  3. Jie Lin  Is a corresponding author
  1. Center for Quantitative Biology, Academy for Advanced Interdisciplinary Sudies, Peking University, China
  2. Peking-Tsinghua Center for Life Sciences, Academy for Advanced Interdisciplinary Sudies, Peking University, China
12 figures, 2 videos, 4 tables and 2 additional files

Figures

A schematic of the osmoresponse model.

(A) The total cytoplasmic volume includes the free and bound volumes. The free volume sets the internal osmotic pressure Πin=kBTNa/Vf, where Vf is the free volume and Na is the number of osmolyte molecules. The bound volume Vb comprises the dry mass Vbd and bound water Vbw, i.e., Vb=Vbd+Vbw, all proportional to the total protein mass. (B) We model osmoregulation through the change of ribosome translation strategy. When the protein density increases, the fraction of ribosomes translating the osmolyte-producing protein χa is upregulated, leading to the subsequent increase in the mass fraction of the osmolyte-producing protein ϕa. Here, μr denotes the dry-mass growth rate. (C) The cell-wall synthesis process is controlled by the turgor pressure σ, which is proportional to the cell-wall strain ϵ=(VVcw)/Vcw. Here, V is the cytoplasmic volume, and Vcw is the relaxed cell-wall volume.

Steady-state properties under a constant external osmolarity.

(A) Normalized growth rate vs. normalized internal osmotic pressure of different species under various culture media. The experiment data (scatter markers) are fitted by our theoretical prediction Equation 10b. The data of E. coli are from Cayley et al., 1991; Dai et al., 2018; Rojas et al., 2014, the data of B. subtilis is from Rojas et al., 2017, and the data of S. cerevisiae is from our own experiments, where sorbitol is added to increase the external osmolarity. (B) Growth curves of wild-type (WT) cells, mutant cells without osmoregulation (Ha=0), and mutant cells without cell-wall synthesis regulation (Hcw=0). The dotted line indicates the region where plasmolysis occurs for the mutant cells with Hcw=0. (C) Mutant cells without cell-wall synthesis regulation cannot maintain a stable turgor pressure in a hypertonic environment, while WT cells can maintain a constant turgor pressure. The mutant cells reach plasmolysis at a threshold of external osmolarity. In (B) and (C), the parameters for WT cells are chosen as the values for S. pombe, and the mutant values are set such that they have the same growth rate as the WT cells in the reference medium (Appendix 1—table 2).

Figure 3 with 3 supplements
Transient dynamics after a constant osmotic shock.

(A) Numerical simulations of cells undergoing a constant 500 mM hyperosmotic shock. The dotted lines represent the steady-state values for the reference growth medium (green) and the medium after perturbation (yellow). (B) Numerical simulations of cells undergoing a constant 500 mM hypoosmotic shock. The purple circle in the third panel marks the growth rate peak during the supergrowth phase. (C) The dynamics of the internal state of a cell characterized by (ρ~p,ηa). The dotted curve represents the constraint on the steady-state solution ρ~p,ηa=1, and the solid trajectory is from numerical simulations. The triangles indicate the steady-state solution before the perturbation and the steady-state solution after the perturbation for a long enough time. The yellow open circle represents the immediate steady-state solution after applying the hyperosmotic shock. (D) The same analysis as (C) but for a constant 500 mM hypoosmotic shock. (E) The growth rate peak in the supergrowth phase (yellow) and the immediate value of turgor pressure after the hypoosmotic shock 𝜎𝑓 (green) vs. the amplitude of the hypoosmotic shock.

Figure 3—figure supplement 1
Dynamics of other variables under a constant 500 mM hypoosmotic shock.

Simulation of a cell undergoing a constant 500 mM hypoosmotic shock, the same simulation as Figure 3B. Here, we show the time dependence of the internal osmotic pressure,Πin the turgor pressure, σ the free volume fraction f, and the dry-mass growth rate μr. All these quantities relax to their steady-state values (dotted lines) on the timescale set by the doubling time.

Figure 3—video 1
The trajectory in the internal state space for a wide-type cell during a 500 mM hyperosmotic shock.

The video shown here corresponds to the dynamics depicted in Figure 3C.

Figure 3—video 2
The trajectory in the internal state space for a wide-type cell during a 500 mM hypoosmotic shock.

The video shown here corresponds to the dynamics depicted in Figure 3D.

Figure 4 with 2 supplements
Supergrowth phenomena under osmotic oscillation.

(A) Numerical simulations of wild-type (WT) S. pombe undergo 24 cycles of 500 mM osmotic oscillations with a 10 min period. We show a 30 min window average in the third panel of growth rate. (B–D) Quantitative agreement between simulations and experiments for the growth rate peak μsg vs. different oscillation parameters, including (B) amplitude, (C) period length, and (D) number of periods. The red lines in (B, C) are predictions, and the blue line in (D) is fitting from which we infer the values of Hcw and τcw+. Green dots with error bars are experimental data from Knapp et al., 2019. (E) In the case of osmotic oscillation with a single period, the hyperosmotic period persists for 120 min before reverting to the reference medium. The vertical dotted blue line represents the minimal amplitude to induce cytoplasm jamming during the hyperosmotic period. The excess turgor pressure σfσc upon exiting the hyperosmotic period is approximately equal to the recovered turgor pressure δσ during the hyperosmotic period. (F) The growth rate peak μsg at different Hr vs. the amplitude of a single oscillation. Hr=3.031 is the value of the WT S. pombe. Parameters of WT S. pombe are used in this figure unless otherwise mentioned (Table 1).

Figure 4—figure supplement 1
Dynamics of other variables under 24 cycles of 500 mM osmotic oscillation.

Simulation of a wild-type cell undergoing 24 cycles of 500 mM osmotic oscillation with a 10 min period, the same simulation as Figure 4A. We plot the cell-wall synthesis efficiency ηcw and the dry-mass density ρ^d for better comparison with experimental data (Knapp et al., 2019).

Figure 4—figure supplement 2
The non-monotonic relationship between μsg and amplitude.

Growth rate peak vs. oscillation amplitude under different numbers of oscillation periods. The total duration of the oscillation stimulus is fixed.

Appendix 1—figure 1
Fit of Vf/V vs. ΔΠout for S. pombe growing in YE5S media, from which we infer the value of f.

The data is extracted from Molines et al., 2022.

Appendix 1—figure 2
Dynamics of osmoresponse for unwalled cells.

(A) The dynamics in the internal state space after a constant 500 mM hyperosmotic shock. The dotted curve represents ρ~pηa=1, and the solid trajectory is from numerical simulations. The arrows represent the stream flow described by Equation 20 and Equation 21. The inset depicts the separation of timescale into shock periods and adaptation periods. (B) The temporal trajectory of growth rate (upper panel) and protein density (lower panel) corresponding to the 500 mM hyperosmotic shock in (A). The dashed lines represent the steady-state values in the reference medium (green) and the medium after perturbation (yellow). (C) The same analysis as (A) but for a constant 500 mM hypoosmotic shock. (D) The same analysis as (B) but for a constant 500 mM hypoosmotic shock. (E) The trajectory of the internal state during a 500 mM osmotic oscillation with a 10 min period (the purple line). The low inset depicts the separation of timescale into shock periods and adaptation periods. The upper inset shows that the trajectory finally converges to a periodic circle around a dynamic equilibrium point (marked by an orange star) on the red dotted curve ρ~pηa=1. The trajectory for a constant hyperosmotic shock with an amplitude of 222 mM (orange curve) almost coincides with the trajectory for the oscillator shock. (F) The purple curves represent the temporal trajectory of growth rate (upper panel) and protein density (lower panel) corresponding to the 500 mM osmotic oscillation in (E). The orange curves represent the corresponding ones for the constant hyperosmotic shock with an amplitude of 222 mM.

Appendix 1—figure 3
Dynamics of deficient cells after removing a sufficiently long oscillatory stimulus.

(A) An oscillation stimulus is initially applied in the simulation until the cell reaches the periodic steady state (stars). The external osmolarity is then switched to the reference growth medium. The internal state evolves toward the steady state in the reference growth medium. Here, we show the results of an unwalled cell (blue curve) and a walled cell deficient in cell-wall synthesis regulation (purple curve). (B) The time dependence of the various growth rates corresponding to the simulations in (A).

Appendix 1—figure 4
The relationship between the growth rate peak μsg and the turgor pressure at the growth rate peak σsg under different amplitudes of hypoosmotic shock.

The solid lines are the theoretical predictions, Equation A23. The open circles are obtained by approximating the turgor pressure as the immediate value after the shock σf in Equation A23. The gray crosses represent the peak growth rates obtained by direct simulations using different τcw+. As the color goes from light to dark, τcw+ gradually decreases to zero.

Appendix 1—figure 5
The upper panel shows the time dependence of the external osmolarity, and the lower panel shows the trajectory of the turgor pressure under a mild osmotic oscillation.
Appendix 1—figure 6
Trajectory of a wild-type walled cell in the internal state space during a 500 mM osmotic oscillation.

During the oscillation, the equilibrium points move along the dotted curve ρ~pηa=1. The circles indicate the equilibrium points during the first oscillation cycle, while the triangles indicate their positions after a large enough number of oscillations.

Appendix 1—figure 7
Time dependence of the free-volume growth rate μf, the dry-mass growth rate μr, and the overall growth rate μ under 500 mM osmotic oscillations.

Here, we show the results of an intact wild-type (WT) cell (upper panel), a walled cell deficient in cell-wall synthesis regulation (middle panel), and an unwalled cell (lower panel).

Appendix 1—figure 8
Given 24 cycles of osmotic oscillations with a period of 10 min, we vary the parameter Hcw, which dictates the sensitivity of cell-wall synthesis regulation.

A higher growth rate peak μsg is observed as the sensitivity Hcw increases, irrespective of the oscillation amplitude. The dotted line represents the growth rate in the reference growth medium.

Videos

Appendix 1—video 1
The trajectory in the internal state space for an unwalled cell during a 500 mM osmotic oscillation with a 10 min period.

The video shown here corresponds to the dynamics depicted in Appendix 1—figure 2E.

Appendix 1—video 2
The trajectory in the internal state space for a wide-type cell during a 500 mM osmotic oscillation with a 10 min period.

The video shown here corresponds to the dynamics depicted in Appendix 1—figure 6.

Tables

Table 1
Model parameters for different species in their corresponding reference growth media.
E. coliValueReference
σc1 [atm]Rojas and Huang, 2018
α1.68 [ml/g]Deduce from Scott Cayley et al., 2000
MBM
(Cayley et al., 1991)
MOPS+fructose
(Dai et al., 2018)
MOPS+glucose
(Dai et al., 2018)
LB
(Rojas et al., 2014)
krmaxχr0.743 [1/hr]0.776 [1/hr]1.14 [1/hr]2.05 [1/hr]
Πin,c1.54 [Osm]1.49 [Osm]1.61 [Osm]2.18 [Osm]
Hr/(Ha+1)1.681.301.182.72
Bacillus subtilis (LB)ValueReference
σc19 [atm]Whatmore and Reed, 1990
2.52 [1/hr]Fit to Rojas et al., 2017
Hr/(Ha+1)2.18
Πin,c3.09 [Osm]
S. pombe (YE5S)ValueReference
Πout0.2 [Osm]Atilgan et al., 2015
σc10 [atm]Lemière and Chang, 2023
ρ^d0.282 [g/ml]Odermatt et al., 2021
ρp0.104 [g/ml]See Section B of Appendix 1
μ0.35 [1/hr]Knapp et al., 2019
f0.788Fit to Molines et al., 2022 (Section B of Appendix 1)
ϵ0.584Atilgan et al., 2015
α2.60 [ml/g]See Section B of Appendix 1
G17.1 [atm]G=σ/ϵ
Πout,c3.5 [Osm]
kw100 [1/(min atm)]
ρc0.267 [g/ml]Deduce from Molines et al., 2022 (Section B of Appendix 1)
Hr3.03Copied from S. cerevisiae in YPD
Ha0.974Set according to Πin/Πin,c=(ρp/ρc)Ha
krmaxχr0.371 [1/hr]Set according to μr=krmaxχr(1(ρp/ρc)Hr)
kBTkamaxχamax2.25 [(atm ml)/(g min)]Set according to kBTkamaxχamaxηaρp=krmaxχrΠin
τcw0.1 [min]
τcw+12.5 [min]Fit to Knapp et al., 2019
Hcw1.7
S. cerevisiae (YPD)ValueReference
Πout0.26 [Osm]
σc3.1 [atm]Lemière and Chang, 2023
ρ^d0.295 [g/ml]Feijó Delgado et al., 2013
μ0.448 [1/hr]Our experiment
f0.6Miermont et al., 2013
ρp0.155 [g/ml]See Section B of Appendix 1
ρc0.994 [g/ml]See Section B of Appendix 1
α4.29 [ml/g]See Section B of Appendix 1
krmaxχr0.450 [1/hr]Fit to our data
Πin,c3.52 [Osm]
Hr/(Ha+1)2.54
Hr3.03Set according to 1(ρp/ρc)Hr=μ/(krmaxχr)
Appendix 1—table 1
The fraction of protein mass in the total dry mass, mpmd(=mtot,pmtot,d), and the fraction of cytoplasmic protein mass in the total protein mass, mp/mtot,p.
E. coliValueReference
p/mtot,p0.8Cayley et al., 1991
mp/md0.68Cayley et al., 1991
S. cerevisiae and S. pombeValueReference
mp/mtot,p0.65Chong et al., 2015
mp/md0.4Yamada and Sgarbieri, 2005
Appendix 1—table 2
Comparison of the model parameters between defective cells and intact cells.

Variables with are for the reference growth medium.

ParametersS. pombe(WT)S. pombe(No osmoregulation)S. pombe(No CW synthesis regulation)S. pombe(No CW)
Πout0.2 [Osm]0.2 [Osm]0.2 [Osm]0.2 [Osm]+
10 [atm]
σ10 [atm]10 [atm]10 [atm]
ρp0.104 [g/ml]0.104 [g/ml]0.104 [g/ml]0.104 [g/ml]
μ0.35 [1/hr]0.35 [1/hr]0.35 [1/hr]0.35 [1/hr]
f0.7880.7880.7880.788
ϵ0.5840.5840.584
α2.60 [ml/g]2.60 [ml/g]2.60 [ml/g]2.60 [ml/g]
G17.1 [atm]17.1 [atm]17.1 [atm]
Πout,c3.5 [Osm]1.15 [Osm]3.5 [Osm] +
10 [atm]
3.5 [Osm] +
10 [atm]
kw100 [1/(min atm)]100 [1/(min atm)]100 [1/(min atm)]100 [1/(min atm)]
ρc0.267 [g/ml]0.267 [g/ml]0.267 [g/ml]0.267 [g/ml]
σc10 [atm]10 [atm]10 [atm]
Hr3.033.033.033.03
Ha0.97400.9740.974
krmaxχr0.371 [1/hr]0.371 [1/hr]0.371 [1/hr]0.371 [1/hr]
kBTkamaxχamax2.25 [(atm ml)/(g min)]0.894 [(atm ml)/(g min)]2.25 [(atm ml)/(g min)]2.25 [(atm ml)/(g min)]
τcw0.1 [min]0.1 [min]
τcw+12.5 [min]12.5 [min]
Hcw1.71.70
Appendix 1—table 3
A summary of the symbols involved in our model.
SymbolDescription
Vtotal cytoplasmic volume
Vffree volume: cytoplasmic volume occupied by free water
Vbbound volume: cytoplasmic volume occupied by dry mass and bound water
Vbwvolume occupied by bound water
Vbddry volume: volume occupied by dry mass
Vcwrelaxed cell-wall volume
ffree volume fraction
αbound volume per total protein mass
Πincytoplasmic osmotic pressure
Πoutexternal osmotic pressure
Πin,ccritical cytoplasmic osmolarity where cell growth arrests
Πout,ccritical external osmolarity where cell growth arrests
σturgor pressure
Gcell-wall elastic modulus
ϵelastic strain of the cell wall
kwwater permeability of the cell membrane
Nanumber of osmolyte molecules in cytoplasm
mptotal mass of the proteome
mp,amass of the osmolyte-producing protein
mp,rmass of the ribosomal protein
ρpprotein density
ρccritical protein density where cell growth arrests
ρ~pnormalized protein density
ρ~cnormalized critical protein density
ρ¯pnormalization factor in protein density
ϕamass fraction of the osmolyte-producing protein in total proteome
ϕrmass fraction of the ribosomal protein in total proteome
χafraction of ribosome translating osmolyte-producing protein
χrfraction of ribosome translating ribosomal protein
χamaxlargest possible fraction of ribosome translating osmolyte-producing protein
Hasensitivity of osmoregulation to intracellular crowding
Hcwsensitivity of cell-wall synthesis regulation to turgor pressure
τcw±timescale of up(down)-regulation of cell-wall synthesis regulation
kaosmolyte production rate
kamaxmaximum osmolyte production rate
krribosomal protein production rate
krmaxmaximum ribosomal protein production rate
ηaefficiency of osmoregulation
ηcwefficiency of cell-wall synthesis regulation
ηrcrowding factor
μgrowth rate of total volume
μrgrowth rate of ribosomal protein (dry mass)
μfgrowth rate of free volume
μcwgrowth rate of relaxed cell-wall volume
μsgpeak growth rate of total volume during supergrowth phase

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  1. Yiyang Ye
  2. Qirun Wang
  3. Jie Lin
(2025)
Physical constraints and biological regulations underlie universal osmoresponses
eLife 13:RP102858.
https://doi.org/10.7554/eLife.102858.3