Mechanical feedback coordinates cell wall expansion and assembly in yeast mating morphogenesis

Abstract

The shaping of individual cells requires a tight coordination of cell mechanics and growth. However, it is unclear how information about the mechanical state of the wall is relayed to the molecular processes building it, thereby enabling the coordination of cell wall expansion and assembly during morphogenesis. Combining theoretical and experimental approaches, we show that a mechanical feedback coordinating cell wall assembly and expansion is essential to sustain mating projection growth in budding yeast (Saccharomyces cerevisiae). Our theoretical results indicate that the mechanical feedback provided by the Cell Wall Integrity pathway, with cell wall stress sensors Wsc1 and Mid2 increasingly activating membrane-localized cell wall synthases Fks1/2 upon faster cell wall expansion, stabilizes mating projection growth without affecting cell shape. Experimental perturbation of the osmotic pressure and cell wall mechanics, as well as compromising the mechanical feedback through genetic deletion of the stress sensors, leads to cellular phenotypes that support the theoretical predictions. Our results indicate that while the existence of mechanical feedback is essential to stabilize mating projection growth, the shape and size of the cell are insensitive to the feedback.

Fig 1. Schematic diagrams of relevant events and quantities in mating projection growth.

(A) Transmitted light image of a S. cerevisiae cell growing a mating projection in the presence of α-factor. Scale bar, 2μm. (B-D) Sketch of molecular events leading to the delivery and activation of cell wall synthases Fks1/2 at the apex. See main text and Table 1 for definitions of parameters. (E) Geometry of the system and definition of the relevant variables. (F) Sketch depicting the increasing cell wall viscosity and decreasing cell wall assembly away from the apex. The inset depicts local normal force balance at the cell wall. All variables are defined in the main text.

Fig 2. Effect of mechanical feedback strength and turgor pressure on cell viability.

The strength of mechanical feedback, Γ, is experimentally varied by deleting MID2 and WSC1. The dimensionless parameter (Pρ_wλ_X)/(12μ₀m_wρ₀k_p) is varied by changing the osmolarity of the external medium through dilution of the yeast growth media, YPD, in deionized H₂O, effectively increasing turgor pressure P in cells. Cell lysis was measured using the PI staining viability assay (Methods). (A) Percent of lysed cells in the absence of α-factor for WT, mid2Δ and wsc1Δ mutants, as well as the mid2 Δwsc1Δ double mutant. (B) Percent of WT lysed cells when grown in the presence of α-factor in YPD medium with decreasing osmolarity. (C) Percent of mid2Δ wsc1Δ lysed cells when grown both in the presence and absence of α-factor in YPD, in osmotically supported conditions (YPD + 1M sorbitol), as well as in hypo-osmotic conditions (100% H₂O). (D) Percent of mid2Δ wsc1Δ lysed cells when grown in the presence of α-factor and osmotically supported media (YPD + sorbitol), diluted for decreasing osmolarities. (E) Percent of lysed cells in mid2Δ and wsc1Δ mutants, as well as the mid2Δwsc1Δ double mutant, when grown in the presence of α-factor in YPD. (F) Theoretically predicted dynamical regimes for varying values of the mechanical feedback strength Γ and the ratio (Pρ_wλ_X)/(12μ₀m_wρ₀k_p). Decreasing osmolarity experimentally, corresponds to increasing P and, therefore, moving along horizontal lines in the positive direction. Addition of zymolyase, a cell wall degrading enzyme, corresponds to decreasing the cell wall viscosity, moving also along horizontal lines in the positive direction. (G) Images (DIC, PI staining and merge) showing the moments before and after the piercing of the cell wall at the tip of a mating projection and subsequent cell lysis of a mid2Δ cell after the addition of zymolyase, for video see S1 Video. Scale bar, 2μm. (H) Temporal increase in the fraction of pierced mating projections for both mid2Δ (squares) and WT (circles) cells after addition of zymolyase.

Fig 3. Steady-state stable solutions for mating projection growth: Projection shape and cell wall expansion.

(A,C,E,G) Mating projection shape (A), as well as the spatial profiles of the cell wall expansion rate ${\dot{ϵ}}_{\mathrm{s}}+{\dot{ϵ}}_{\varphi }$ (C), curvature κ_s (E) and cell wall thickness h (G), for different values of the mechanical feedback strength Γ and the ratio λ_X/λ_m = (Pρ_wλ_X)/(12μ₀m_wρ₀k_p). All insets show a different scaling of each magnitude, with the arclength normalized by the projection radius R and each quantity normalized by its value at the projection tip (s = 0), with the exception of the wall thickness h(s) and the shape r(s), which are normalized by the limiting values far away from the apical region, H and R respectively. The color code indicates the different parameter values, shown as dots of the same color in the parameter space right to each panel. Increasing orange and blue tones of the dots corresponds to decreasing Γ and increasing (Pρ_wλ_X)/(12μ₀m_wρ₀k_p), respectively (arrows in Fig 2E). (B,D,F,H) The variation of the apical value of each magnitude, namely ${\dot{ϵ}}_0\equiv {\dot{ϵ}}_{\mathrm{s}}(s=0)={\dot{ϵ}}_{\varphi }(s=0)$ (D) and κ₀ ≡ κ_s(s = 0) (F), is shown for the different values of the parameters for which stable states exist. The variation of the projection radius and wall thickness away from the apical region, R (B) and H (H) respectively, are shown as a function of the parameters as well.

Fig 4. Steady-state stable solutions for mating projection growth: Cell wall assembly via Fks1/2.

Cell wall assembly via Fks1/2. (A,C,E,G) Total Fks1/2 density ρ_A + ρ_I (A), fraction of active Fks1/2, ρ_A/(ρ_A + ρ_I) (C), active Fks1/2 density (E) and inactive Fks1/2 density (E), for different values of the mechanical feedback strength Γ and the ratio λ_X/λ_m = (Pρ_wλ_X)/(12μ₀m_wρ₀k_p). All insets show a different scaling of each magnitude, with the arclength normalized by the projection radius R and each quantity normalized by its value at the projection tip (s = 0). The color code indicates the different parameter values, shown as dots of the same color in the parameter space right to each panel. Increasing orange and blue tones of the dots corresponds to decreasing Γ and increasing (Pρ_wλ_X)/(12μ₀m_wρ₀k_p), respectively (arrows in Fig 2E). (B,D,F,H) The variation of the apical value of each magnitude, namely $({\rho }_A^0+{\rho }_I^0)/{\rho }_0$ (B), ${\rho }_A^0/({\rho }_A^0+{\rho }_I^0)$ (D), ${\rho }_A^0/{\rho }_0$ (F) and ${\rho }_I^0/{\rho }_0$ (H), is shown for the different values of the parameters for which stable states exist.

Fig 5. Control of mating projection size.

(A) Diagram of a growing mating projection showing the mating projection radius R and length scale of the secretion region (green), λ_X. (B) Theoretically predicted dependence of the projection radius R with the length scale of the secretion region, λ_X, and the strength of mechanical feedback, Γ. (C-D) Confocal images of WT (C) and spa2Δ (D) mutant cells growing mating projections. The cell wall is labeled with calcofluor (white) and the exocytosis profile is defined by Sec3-GFP (green). Scale bar, 1 μm. (E) Measured average cell radius, R, and exocytosis length scale, λ_X, for mid2Δ and wsc1Δ mutants in both WT and spa2Δ backgrounds (mid2Δ, N = 6; wsc1Δ, N = 9; spa2Δ mid2Δ, N = 7; spa2Δ wsc1Δ, N = 6), as well as for WT (N = 7) and spa2Δ (N = 6) cells. Mean and standard deviation are shown.