Cell Death Pathway That Monitors Spore Morphogenesis

Abstract

The use of quality control mechanisms to stall developmental pathways or completely remove defective cells from a population is a widespread strategy to ensure the integrity of morphogenetic programs. Endospore formation (sporulation) is a well conserved microbial developmental strategy in the Firmicutes phylum wherein a progenitor cell that faces starvation differentiates to form a dormant spore. Despite the conservation of this strategy, it has been unclear what selective pressure maintains the fitness of this developmental program, composed of hundreds of unique genes, during multiple rounds of vegetative growth when sporulation is not required. Recently, a quality control pathway was discovered in Bacillus subtilis which monitors the assembly of the spore envelope and specifically eliminates, through cell lysis, sporulating cells that assemble the envelope incorrectly. Here, we review the use of checkpoints that govern the entry into sporulation in B. subtilis and discuss how the use of regulated cell death pathways during bacterial development may help maintain the fidelity of the sporulation program in the species.

Keywords: SpoIVA; SpoVM; apoptosis; gametogenesis; lamins; nuclear envelope; programmed cell death.

Figure 1. Vegetative growth and sporulation in B. subtilis

Left (green): During the vegetative life cycle, cells divide symmetrically, resulting in two genetically identical and morphologically similar daughter cells. Right (blue): When faced with nutrient starvation, B. subtilis sporulates to produce a dormant spore. To this end, sporulating cells first divide asymmetrically to create two genetically identical but morphologically different cells; the large mother cell (MC) and the smaller forespore (FS). The forespore is then engulfed by the mother cell to create a double-membrane bound organelle-like structure. Next, the proteinaceous spore coat (red) is constructed atop the forespore, after which the peptidoglycan cortex (dashed gray) is built between the double membranes. Once the spore is fully matured, the mother cell lyses and releases the completed spore into the environment. Membranes: yellow; peptidoglycan cell wall: solid gray.

Figure 2. Mutant cells without a checkpoint mechanism accumulate mutations and eventually take over the population

The system was modeled in Python Spyder (v. 2.7) using equations 2–13 as described in the text. The simulation was initiated with 10⁹ cells, half of which were wild type (blue) and the other half consisting of otherwise wild type cells that did not harbor checkpoints to monitor the progress of sporulation (red). Both cell types accumulated mutations in sporulation genes at a rate of .003. A third population, checkpoint-less cells that accumulated mutations in sporulation genes (green) arose from the checkpoint-less population (red) and, in the absence of additional environmental insults, ultimately took over the population.

Figure 3. CmpA mediates a quality control mechanism that monitors proper spore coat assembly

Top: SpoVM (red), SpoIVA (green), and CmpA (purple) localize to the surface of the forespore (orange double membranes). Left: in a single cell shown in the population, SpoIVA misassembles the basement layer of the coat (due, for example, to a mutation in spoIVA). Middle: in the cell harboring a mis-assembled coat, CmpA, the ClpXP adaptor protein, is stabilized and leads to the ClpXP-mediated degradation of SpoIVA. Right, in a cell in which SpoIVA properly assembles and polymerizes, CmpA is degraded. Bottom: Cells that correctly assembled the spore envelope complete the sporulation program to produce dormant spores, whereas cells in which SpoIVA was degraded are removed from the population by lysis. Adapted from (50).