The CspC pseudoprotease regulates germination of Clostridioides difficile spores in response to multiple environmental signals

The gastrointestinal pathogen, Clostridioides difficile, initiates infection when its metabolically dormant spore form germinates in the mammalian gut. While most spore-forming bacteria use transmembrane germinant receptors to sense nutrient germinants, C. difficile is thought to use the soluble pseudoprotease, CspC, to detect bile acid germinants. To gain insight into CspC's unique mechanism of action, we solved its crystal structure. Guided by this structure, we identified CspC mutations that confer either hypo- or hyper-sensitivity to bile acid germinant. Surprisingly, hyper-sensitive CspC variants exhibited bile acid-independent germination as well as increased sensitivity to amino acid and/or calcium co-germinants. Since mutations in specific residues altered CspC's responsiveness to these different signals, CspC plays a critical role in regulating C. difficile spore germination in response to multiple environmental signals. Taken together, these studies implicate CspC as being intimately involved in the detection of distinct classes of co-germinants in addition to bile acids and thus raises the possibility that CspC functions as a signaling node rather than a ligand-binding receptor.

Regulation of Clostridium difficile Spore Formation by the SpoIIQ and SpoIIIA Proteins

Sporulation is an ancient developmental process that involves the formation of a highly resistant endospore within a larger mother cell. In the model organism Bacillus subtilis, sporulation-specific sigma factors activate compartment-specific transcriptional programs that drive spore morphogenesis. σ^G activity in the forespore depends on the formation of a secretion complex, known as the “feeding tube,” that bridges the mother cell and forespore and maintains forespore integrity. Even though these channel components are conserved in all spore formers, recent studies in the major nosocomial pathogen Clostridium difficile suggested that these components are dispensable for σ^G activity. In this study, we investigated the requirements of the SpoIIQ and SpoIIIA proteins during C. difficile sporulation. C. difficile spoIIQ, spoIIIA, and spoIIIAH mutants exhibited defects in engulfment, tethering of coat to the forespore, and heat-resistant spore formation, even though they activate σ^G at wildtype levels. Although the spoIIQ, spoIIIA, and spoIIIAH mutants were defective in engulfment, metabolic labeling studies revealed that they nevertheless actively transformed the peptidoglycan at the leading edge of engulfment. In vitro pull-down assays further demonstrated that C. difficile SpoIIQ directly interacts with SpoIIIAH. Interestingly, mutation of the conserved Walker A ATP binding motif, but not the Walker B ATP hydrolysis motif, disrupted SpoIIIAA function during C. difficile spore formation. This finding contrasts with B. subtilis, which requires both Walker A and B motifs for SpoIIIAA function. Taken together, our findings suggest that inhibiting SpoIIQ, SpoIIIAA, or SpoIIIAH function could prevent the formation of infectious C. difficile spores and thus disease transmission.

The bacterial spore-forming pathogen Clostridium difficile is a leading cause of nosocomial infections in the United States and represents a significant threat to healthcare systems around the world. As an obligate anaerobe, C. difficile must form spores in order to survive exit from the gastrointestinal tract. Accordingly, spore formation is essential for C. difficile disease transmission. Since the mechanisms controlling this process remain poorly characterized, we analyzed the importance of highly conserved secretion channel components during C. difficile sporulation. In the model organism Bacillus subtilis, this channel had previously been shown to function as a “feeding tube” that allows the mother cell to nurture the developing forespore and sustain transcription in the forespore. We show here that conserved components of this structure in C. difficile are dispensable for forespore transcription, although they are important for completing forespore engulfment and retaining the protective spore coat around the forespore, in contrast with B. subtilis. The results of our study suggest that targeting these conserved proteins could prevent C. difficile spore formation and thus disease transmission.

Identification of a Novel Lipoprotein Regulator of Clostridium difficile Spore Germination

Clostridium difficile is a Gram-positive spore-forming pathogen and a leading cause of nosocomial diarrhea. C. difficile infections are transmitted when ingested spores germinate in the gastrointestinal tract and transform into vegetative cells. Germination begins when the germinant receptor CspC detects bile salts in the gut. CspC is a subtilisin-like serine pseudoprotease that activates the related CspB serine protease through an unknown mechanism. Activated CspB cleaves the pro-SleC zymogen, which allows the activated SleC cortex hydrolase to degrade the protective cortex layer. While these regulators are essential for C. difficile spores to outgrow and form toxin-secreting vegetative cells, the mechanisms controlling their function have only been partially characterized. In this study, we identify the lipoprotein GerS as a novel regulator of C. difficile spore germination using targeted mutagenesis. A gerS mutant has a severe germination defect and fails to degrade cortex even though it processes SleC at wildtype levels. Using complementation analyses, we demonstrate that GerS secretion, but not lipidation, is necessary for GerS to activate SleC. Importantly, loss of GerS attenuates the virulence of C. difficile in a hamster model of infection. Since GerS appears to be conserved exclusively in related Peptostreptococcaeace family members, our results contribute to a growing body of work indicating that C. difficile has evolved distinct mechanisms for controlling the exit from dormancy relative to B. subtilis and other spore-forming organisms.

The cleavage activities of aphthovirus and cardiovirus 2A proteins

The primary 2A/2B polyprotein cleavage of aphtho-and cardioviruses is mediated by their 2A proteins cleaving C-terminally. Whilst the aphthovirus 2A region is only 16 aa (possibly 18 aa) long, the cardiovirus 2A protein is some 150 aa. We have previously shown that foot-and-mouth disease virus (FMDV) 2A is able to mediate cleavage in an artificial (chloramphenicol acetyltransferase/FMDV 2A/beta-glucuronidase [CAT-2A-GUS]) polyprotein system devoid of any other FMDV sequences with high (approximately 85%), although not complete, cleavage. In this paper we show that insertion of upstream FMDV capsid protein 1 D sequences increases the activity. In addition, we have demonstrated that the cardiovirus Theiler's murine encephalomyelitis virus(TME) 2A protein, when linked to GUS in a single ORF, is able to cleave at its own C terminus with high efficiency--if not completely. The C-terminal 19 aa of TME 2A, together with the N-terminal proline residue of protein 2B, were inserted into the CAT/GUS artificial polyprotein system (in a single ORF). This recombinant [CAT-deltaTME2A-GUS] polyprotein was able to mediate cleavage with high (approximately 85%) efficiency--directly comparable to the activity observed when FMDV 2A was inserted. A similar insertion into [CAT-GUS] of the C-terminal 19 aa of the cardiovirus encephalomyocarditis virus (EMC) 2A, together with the N-terminal proline residue of protein 2B, produced a [CAT-delta EMC2A-GUS] polyprotein which also mediated cleavage at approximately 85%. Analysis of the products of expression of these artificial polyproteins in a prokaryotic translation system did not, apparently, reveal any GUS cleavage product.

Geriatric Assessment and Treatment Units: Providing care for frail seniors

Geriatric assessment and treatment units provide a multidisciplinary approach to managing a targeted group of frail seniors who are hard to care for in traditional systems. They are also important teaching and research centers for geriatrics.This article describes, as an example, the Short Term Assessment and Treatment Unit at Vancouver General Hospital.