Secreting and sensing the same molecule allows cells to achieve versatile social behaviors

Abstract

Cells that secrete and sense the same signaling molecule are ubiquitous. To uncover the functional capabilities of the core "secrete-and-sense" circuit motif shared by these cells, we engineered yeast to secrete and sense the mating pheromone. Perturbing each circuit element revealed parameters that control the degree to which the cell communicated with itself versus with its neighbors. This tunable interplay of self-communication and neighbor communication enables cells to span a diverse repertoire of cellular behaviors. These include a cell being asocial by responding only to itself and social through quorum sensing, and an isogenic population of cells splitting into social and asocial subpopulations. A mathematical model explained these behaviors. The versatility of the secrete-and-sense circuit motif may explain its recurrence across species.

Fig. 1. Synthetic secrete-and-sense circuit motif in yeast

(A) Cells that secrete a signaling molecule without sensing (top), cells that sense a molecule without secreting (middle), and cells that secrete and sense the same signaling molecule (bottom). (B) Examples of ‘secrete-and-sense’ cells in nature: bacteria secrete and sense an autoinducer for sensing a quorum, human pancreatic beta-cells secrete and sense insulin, human T cells secrete and sense the cytokine interleukin-2 to control their proliferation, and the vulva precursor cells in C. elegans secrete and sense the diffusible Delta for specifying their cell-fates. (C) Schematic of self-communication and neighbor-communication between two identical secrete-and-sense cells. (D) Schematic of synthetic secrete-and-sense system: haploid budding yeast (yellow box) engineered to secrete and sense α-factor (orange circle). GFP fluorescence is a read-out of the concentration of α-factor sensed by the cell.

Fig. 2. Varying receptor abundance and secretion rate to tune degrees of self- and neighbor-communication

(A) Secrete-and-sense strain (‘Cell A’) and sense-only strain (‘Cell B’) were cultured together for all experiments in this figure. (B) Each secrete-and-sense strain used a different promoter P_varied to express STE2 but all used the pTET07 to express MFα1. For each secrete-and-sense strain, a matching sense-only strain with the same Ste2 abundance was constructed. (C-D) Equal densities of ‘basic secrete-and-sense strain’ and ‘basic sense-only strain’ were cultured together for two representative doxycycline concentrations: [doxycycline] = 6 μg/ml (‘low secretion rate’) and 30 μg/ml (‘high secretion rate’), and at two total cell densities: (C) low (OD = 0.001) and (D) high (OD = 0.1). Each strain's single-cell GFP fluorescence at various time points are shown. Error bars: SEM, N=3. (E-F) Each secrete-and-sense strain (‘Cell A’) was cultured with its partner sense-only strain (‘Cell B’) (i.e., Ste2 expressed by the same promoter in both strains). 7 such pairs of strains were cultured for five hours in 11 different concentrations of doxycycline (table S1, fig. S8), yielding heat maps with 7×11 pixels for (E) low (OD=0.001) and (F) high (OD=0.1) cell densities. Each pixel represents the difference between the GFP fluorescence of Cell A and of Cell B at the end of the time course (subtracting GFP fluorescence of Cell B from that of Cell A, averaged from three independent experiments).

Fig. 3. Effects of self- and neighbor-communication on positive feedback linking secretion with sensing

(A) Basic secrete-and-sense circuit modified by a positive feedback link (highlighted in blue). (B-C) Representative histograms showing the single-cell GFP fluorescence level of the basic secrete-and-sense strain with the positive feedback link obtained by a flow cytometer. This strain was cultured by itself at two different initial cell densities ((B) low cell density (OD=0.001), (C) high cell density (OD=0.1)) and in two representative concentrations of doxycycline ([doxycycline] = 3 μg/ml: weak positive feedback, and [doxycycline] = 40 μg/ml: strong positive feedback). Blue histograms: beginning of the time-course (0 hour) and red histograms (8 hours into the time course) (full data sets in figs. S11 and S12). Under each panel, the corresponding type of activation behavior is mentioned. (D) Main population-level behavior: Activation of all cells in near unison.

Fig. 4. Effects of self- and neighbor-communication on positive feedback with signal degradation

(A) Basic secrete-and-sense circuit with positive feedback link (blue highlight) and the Bar1 protease (grey). Six different strains of this type were constructed, each with a different constitutive promoter P_varied controlling expression of BAR1. (B-C) An example strain (with pCYC1-BAR1) cultured by itself at two different initial cell densities ((E) low cell density (OD=0.001), (F) high cell density (OD=0.1) and in two representative doxycycline concentrations ([doxycycline]= 6 μg/ml (weak positive feedback), and 20 μg/ml (strong positive feedback)). Representative histograms showing the single-cell GFP fluorescence levels of this strain are plotted at two different time points (blue and red histograms). Under each panel, the corresponding type of activation behavior is mentioned (figs. S14 and S15). (D) Phase diagrams from analyzing each time-course for the seven secrete-and-sense strains, each with different amounts of Bar1 (including none, Fig. 3) and positive feedback strengths at low (OD=0.001) and high (OD=0.1) cell density cultures (summarizes fig. S11, S12, S14, and S15). (E) Main population-level behavior: Bifurcation of an isogenic population into subpopulations of quiescent and maximally secreting cells.

Fig. 5. A simple mathematical model provides intuition

(A-B) A phenomenological model provides qualitative insights underlying the main features of the secrete-and-sense circuit revealed by our experiments (61). (A) Model explains the individual cellular response of a secrete-and-sense cell that self-communicates (red curve) and of a sense-only cell at a low cell density (blue curve) and at a higher cell density (green curve). These curves are analogous to those seen in Fig. 2, C and D. (B) Model summarizes self- and neighbor-communication in a phase diagram representing the ‘degree of sociability’ (defined in Supplementary Text (61)). (C) Summary of the main behavioral classes spanned by the secrete-and-sense circuit motif.