Bifidobacteria comprise a significant proportion of the human gut microbiota. Several bifidobacterial strains are currently used as therapeutic interventions, claiming various health benefits by acting as probiotics. However, the precise mechanisms by which they maintain habitation within their host and consequently provide these benefits are not fully understood. Here we show that Bifidobacterium breve UCC2003 produces a cell surface-associated exopolysaccharide (EPS), the biosynthesis of which is directed by either half of a bidirectional gene cluster, thus leading to production of one of two possible EPSs. Alternate transcription of the two opposing halves of this cluster appears to be the result of promoter reorientation. Surface EPS provided stress tolerance and promoted in vivo persistence, but not initial colonization. Marked differences were observed in host immune response: strains producing surface EPS (EPS + ) failed to elicit a strong immune response compared with EPS-deficient variants. Specifically, EPS production was shown to be linked to the evasion of adaptive B-cell responses. Furthermore, presence of EPS + B. breve reduced colonization levels of the gut pathogen Citrobacter rodentium . Our data thus assigns a pivotal and beneficial role for EPS in modulating various aspects of bifidobacterial–host interaction, including the ability of commensal bacteria to remain immunologically silent and in turn provide pathogen protection. This finding enforces the probiotic concept and provides mechanistic insights into health-promoting benefits for both animal and human hosts.

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Protein kinases play key roles in signaling and response to changes in the external environment. The ability of Candida albicans to quickly sense and respond to changes in its environment is key to its survival in the human host. Our guiding hypothesis was that creating and screening a set of protein kinase mutant strains would reveal signaling pathways that mediate stress response in C. albicans. A library of protein kinase mutant strains was created and screened for sensitivity to a variety of stresses. For the majority of stresses tested, stress response was largely conserved between C. albicans, Saccharomyces cerevisiae, and Schizosaccharomyces pombe. However, we identified eight protein kinases whose roles in cell wall regulation (CWR) were not expected from functions of their orthologs in the model fungi Saccharomyces cerevisiae and Schizosaccharomyces pombe. Analysis of the conserved roles of these protein kinases indicates that establishment of cell polarity is critical for CWR. In addition, we found that septins, crucial to budding, are both important for surviving and are mislocalized by cell wall stress. Our study shows an expanded role for protein kinase signaling in C. albicans cell wall integrity. Our studies suggest that in some cases, this expansion represents a greater importance for certain pathways in cell wall biogenesis. In other cases, it appears that signaling pathways have been rewired for a cell wall integrity response.

Highlights d aS impacts lipid homeostasis, triggering excess oleic acid (OA) and diglycerides (DG) d Triglycerides and lipid droplets protect against toxicity by sequestering OA and DG d Stearoyl-CoA desaturase (SCD) inhibition rescues aS toxicity and neuron degeneration d SCD inhibition decreases aS inclusions and increases aS multimerization and solubility

Lipids play a more significant role in Parkinson’s disease and its related brain disorders than is currently recognized, supporting a “lipid cascade”. The 14 kDa protein α-synuclein (αS) is strongly associated with Parkinson’s disease (PD), dementia with Lewy bodies (DLB), other synucleinopathies such as multiple system atrophy, and even certain forms of Alzheimer’s disease. Rigorously deciphering the biochemistry of αS in native systems is the key to developing treatments. αS is highly expressed in the brain, the second most lipid-rich organ, and has been proposed to be a lipid-binding protein that physiologically interacts with phospholipids and fatty acids (FAs). αS-rich cytoplasmic inclusions called Lewy bodies and Lewy neurites are the hallmark lesions of synucleinopathies. Excess αS–membrane interactions may trigger proteinaceous αS aggregation by stimulating its primary nucleation. However, αS may also exert its toxicity prior to or independent of its self-aggregation, e.g., via excessive membrane interactions, which may be promoted by certain lipids and FAs. A complex αS-lipid landscape exists, which comprises both physiological and pathological states of αS. As novel insights about the composition of Lewy lesions occur, new lipid-related PD drug candidates emerge, and genome-wide association studies (GWAS) increasingly validate new hits in lipid-associated pathways, it seems timely to review our current knowledge of lipids in PD and consider the roles for these pathways in synucleinopathies.Fig. 1αS ↔ lipid interplay: aspects of cellular αS homeostasis (blue oval), aspects of lipid homeostasis (green oval), and overlapping aspects.Pathological states are labeled in red. Simplified schematic of both select αS and select lipid species. Several existing publications suggest αS effects on lipids and vice versa, as indicated by arrows. DG diglyceride, ER endoplasmic reticulum, FA fatty acid, LD, lipid droplet, TG triglyceride.

α-Synuclein (αS) forms round cytoplasmic inclusions in Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Evidence suggests a physiological function of αS in vesicle trafficking and release. In contrast to earlier tenets, recent work indicates that αS normally exists in cells in a dynamic equilibrium between monomers and tetramers/multimers. We engineered αS mutants incapable of multimerization, leading to excess monomers at vesicle membranes. By EM, such mutants induced prominent vesicle clustering, leading to round cytoplasmic inclusions. Immunogold labeling revealed abundant αS intimately associated with vesicles of varied size. Fluorescence microscopy with marker proteins showed that the αS-associated vesicles were of diverse endocytic and secretory origin. An αS '3K' mutant (E35K + E46K + E61K) that amplifies the PD/DLB-causing E46K mutation induced αS-rich vesicle clusters resembling the vesicle-rich areas of Lewy bodies, supporting pathogenic relevance. Mechanistically, E46K can increase αS vesicle binding via membrane-induced amphipathic helix formation, and '3K' further enhances this effect. Another engineered αS variant added hydrophobicity to the hydrophobic half of αS helices, thereby stabilizing αS-membrane interactions. Importantly, substituting charged for uncharged residues within the hydrophobic half of the stabilized helix not only reversed the strong membrane interaction of the multimer-abolishing αS variant but also restored multimerization and prevented the aberrant vesicle interactions. Thus, reversible αS amphipathic helix formation and dynamic multimerization regulate a normal function of αS at vesicles, and abrogating multimers has pathogenic consequences.

ABSTRACTCandida albicansis a causative agent of oropharyngeal candidiasis (OPC), a biofilm-like infection of the oral mucosa. Biofilm formation depends upon theC. albicanstranscription factor Bcr1, and previous studies indicate that Bcr1 is required for OPC in a mouse model of infection. Here we have used a nanoString gene expression measurement platform to elucidate the role of Bcr1 in OPC-related gene expression. We chose for assays a panel of 134 genes that represent a range of morphogenetic and cell cycle functions as well as environmental and stress response pathways. We assayed gene expression in whole infected tongue samples. The results sketch a portrait ofC. albicansgene expression in which numerous stress response pathways are activated during OPC. This one set of experiments identifies 64 new genes with significantly altered RNA levels during OPC, thus increasing substantially the number of known genes in this expression class. Thebcr1Δ/Δ mutant had a much more limited gene expression defect during OPC infection than previously reported forin vitrogrowth conditions. Among major functional Bcr1 targets, we observed thatALS3was Bcr1 dependentin vivowhileHWP1was not. We used null mutants and complemented strains to verify that Bcr1 and Hwp1 are required for OPC infection in this model. The role of Als3 is transient and mild, though significant. Our findings suggest that the versatility ofC. albicansas a pathogen may reflect its ability to persist in the face of multiple stresses and underscore that transcriptional circuitry during infection may be distinct from that detailed duringin vitrogrowth.

Microscopy of Lewy bodies in Parkinson’s disease (PD) suggests they are not solely filamentous deposits of α-synuclein (αS) but also contain vesicles and other membranous material. We previously reported the existence of native αS tetramers/multimers and described engineered mutations of the αS KTKEGV repeat motifs that abrogate the multimers. The resultant excess monomers accumulate in lipid membrane-rich inclusions associated with neurotoxicity exceeding that of natural familial PD mutants, such as E46K. Here, we use the αS “3K” (E35K+E46K+E61K) engineered mutation to probe the mechanisms of reported small-molecule modifiers of αS biochemistry and then identify compounds via a medium-throughput automated screen. αS 3K, which forms round, vesicle-rich inclusions in cultured neurons and causes a PD-like, l-DOPA–responsive motor phenotype in transgenic mice, was fused to YFP, and fluorescent inclusions were quantified. Live-cell microscopy revealed the highly dynamic nature of the αS inclusions: for example, their rapid clearance by certain known modulators of αS toxicity, including tacrolimus (FK506), isradipine, nilotinib, nortriptyline, and trifluoperazine. Our automated 3K cellular screen identified inhibitors of stearoyl-CoA desaturase (SCD) that robustly prevent the αS inclusions, reduce αS 3K neurotoxicity, and prevent abnormal phosphorylation and insolubility of αS E46K. SCD inhibition restores the E46K αS multimer:monomer ratio in human neurons, and it actually increases this ratio for overexpressed wild-type αS. In accord, conditioning 3K cells in saturated fatty acids rescued, whereas unsaturated fatty acids worsened, the αS phenotypes. Our cellular screen allows probing the mechanisms of synucleinopathy and refining drug candidates, including SCD inhibitors and other lipid modulators.