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Toniolo, Chiara (2015) The role of CpsABCD in Streptococcus agalactiae capsule biosynthesis. [Ph.D. thesis]

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Abstract (english)

Streptococcus agalactiae or group B Streptococcus (GBS) is a Gram-positive bacterium asymptomatically colonizing 15-35% of women in the gastrointestinal and urogenital tracts. During delivery, neonates born to mothers who carry GBS can be infected themselves and develop severe diseases such as sepsis, pneumonia and meningitis. Pre-partum screenings and prophylactic treatment with antibiotics have reduced the incidence of neonatal GBS disease to 0.04% in USA. But still, in the western world, S. agalactiae represents the major cause of bacterial meningitis in newborns and half of the infected suffer long-term neurodevelopmental defects. Moreover, GBS has also emerged as a pathogen in other patient populations such as the elderly, pregnant women, diabetics and individuals who are immunocompromised. Vaccines based on the capsule polysaccharide (CPS) of this pathogen are currently under development.
The CPS is the main virulence factor of GBS, preventing complement deposition and opsonophagocytosis. The production of a CPS is ubiquitous in bacteria, and the Wzy pathway constitutes one of the prototypical mechanisms to produce these structures. This pathway has been characterized in detail in S. pneumoniae. Briefly, the repeating units of sugars composing the CPS are synthesized inside the cell by a group of glycosyltransferases. The repeating units are then flipped outside the membrane and incorporated into the growing polysaccharide chain by a polymerase. Lastly, the polysaccharide is attached to the cell wall peptidoglycan to create the CPS layer surrounding the bacterium. All the enzymes involved in this process are encoded in a single operon.
The aim of this work is to investigate the role of the CpsABCD proteins encoded in the cps operon of GBS. These proteins are highly conserved in all GBS serotypes, as well as in some other related bacteria, but they are not involved in the synthesis of the basic repeating units of sugars. CpsA is reported to be a transcriptional regulator and/or an enzyme attaching the CPS to the cell wall. CpsBCD homologous proteins in S. pneumoniae constitute a putative phosphoregulatory system, but their role in GBS capsule biosynthesis is unclear. To investigate the role of these proteins we developed twelve knockout and functional GBS mutant strains and we examined them for CPS quantity, size, and attachment to the cell surface, as well as CpsD phosphorylation. Moreover, we used a bacterial two hybrid assay to investigate interdependencies between these proteins.
We observed that in GBS CpsB, C and D constitute a phosphoregulatory system where the CpsD autokinase phosphorylates its C-terminal tyrosines in a CpsC-dependent manner. These Tyr residues are also the target of the cognate CpsB phosphatase. Analysis of cps operon transcription by qRT-PCR on the mutant strains suggested that CpsABCD are not involved in transcriptional regulation of this operon. Furthermore, all the mutant strains retained the capability to produce a CPS, confirming that these proteins are not involved in the synthesis of polysaccharides, however, differences in CPS length and attachment to the cell wall were observed. In particular, we observed that the CpsC extracellular domain appeared necessary for the production of high molecular weight polysaccharides and that the LytR domain of CpsA is required for the attachment of the CPS to the bacterial cell surface. Protein-protein interactions between CpsD and CpsC and between CpsA and CpsC were observed.
These results allowed us to propose tentative roles for the proteins and their interdependencies. We propose a model where these proteins are fine-tuning the steps terminating the CPS biosynthesis, i.e. the balance between polymerization and attachment to the cell wall. In said model, CpsA competes with the CPS polymerase and attaches the CPS to the cell wall. This interplay depends on the cyclic phosphorylation of the CpsCD complex which modulates the activity of CpsA balancing the two competing activities.
Ultimately, to investigate how differences in CPS length, amount and localization impact on S. agalactiae ability to interact with cells, an in vitro adhesion-invasion assay, using lung epithelial cells have been tested. Our results showed that strains with CPS length different from the wild type were defective in associations to cells. Moreover, strains lacking the capsule or producing very little CPS were more efficient in invading cells irrespective of the CPS length.

Abstract (italian)

Streptococcus agalactiae, anche detto streptococco gruppo B (GBS), è un batterio Gram-positivo comunemente identificato come colonizzatore asintomatico del tratto gastrointestinale e urogenitale nel 15-35% delle donne. Durante il parto, GBS può essere trasmesso dalla madre colonizzata al neonato, il quale può sviluppare sepsi, polmoniti o meningiti. La diffusione di screening e trattamenti profilattici pre-parto ha significativamente ridotto l’incidenza delle malattie neonatali causate da GBS. Negli Stati Uniti, ad esempio, l’incidenza media di queste infezioni è dello 0.04%. Nel mondo occidentale, tuttavia, S. agalactiae rappresenta ancora la prima causa di meningiti batteriche nei neonati e la metà degli infetti soffre di difetti nello sviluppo neurologico a lungo termine. Patologie causate da GBS sono riscontrate anche in altri tipi di pazienti, quali gli anziani, le donne gravide, i diabetici e gli immunodepressi. Alcuni vaccini contro GBS sono attualmente in fase di sviluppo e sono basati sul polisaccarde capsulare (CPS) di S. agalactiae.
Il CPS è il maggior fattore di virulenza di GBS ed è in grado di inibire la deposizione del complemento sulla superficie del patogeno e l’opsonofagocitosi. La presenza di una capsula polisaccaridica che riveste la superficie batterica è una caratteristica comune a molti batteri e il pathway Wzy e uno dei tipici meccanismi usati per produrre i polisaccaridi che compongono questa struttura. Questo processo è stato descritto in dettaglio per S. pneumoniae. La produzione del CPS inizia all’interno della cellula con la sintesi delle unità saccaridiche da parte di una serie di glicosiltrasferasi. Successivamente le unità sono trasferite sul lato esterno della membrana batterica dove una polimerasi incorpora le unità ripetute al polisaccaride nascente. Il polisaccaride viene infine attaccato al peptidoglicano della parete cellulare e crea lo strato della capsula. Tutti i geni che codificano gli enzimi responsabili di questo processo si trovano in un unico operone.
Lo scopo di questa tesi è di investigare il ruolo delle proteine CpsABCD codificate dall’operone cps. Queste proteine sono conservate in tutti i sierotipi di GBS e in altri batteri, ma non sono direttamente coinvolte nella biosintesi delle unità saccaridiche che compongono il CPS. In letteratura, CpsA è descritto come un regolatore trascrizionale o un enzima che attacca il CPS alla parete cellulare. Le proteine omologhe di CpsBCD di S. pneumoniae compongono un sistema di fosforegolazione la cui funzione nell’ambito della biosintesi del CPS non è stata chiarita. Per studiare il ruolo di CpsABCD in GBS abbiamo sviluppato dodici mutanti in cui i geni cpsABCD sono stati deleti o mutati. Per ognuno di questi mutanti sono stati caratterizzati la quantità, la dimensione e la localizzazione cellulare del CPS prodotto, e lo stato di fosforilazione di CpsD. Inoltre mediante l’uso del bacterial two hybrid assay sono state analizzate le interazioni tra alcune di queste proteine.
I risultati ottenuti hanno dimostrato che, anche in GBS, CpsB, C e D compongono un sistema di fosforegolazione in cui CpsD è l’autochinasi e CpsB è la fosfatasi. CpsD fosforila le tirosine che si trovano al suo C-terminale e la sua attività è dipendente dalla presenza della coda C-terminale di CpsC. Le tirosine di CpsD sono a loro volta defosforilate da CpsB. La trascrizione dell’operone cps è stata analizzata mediante qRT-PCR in tutti i mutanti e i risultati hanno mostrato che le proteine CpsABCD non sono coinvolte nella regolazione trascrizionale dell’operone. Inoltre, l’osservazione che i mutanti mantengono la capacità di produrre il CPS, conferma che queste proteine non partecipano alla sintesi del polisaccaride. Tuttavia, per alcuni mutanti sono state osservate differenze nella lunghezza del CPS e nella sua localizzazione. I dati ottenuti suggeriscono che il dominio extracellulare di CpsC è necessario per la produzione di polisaccaridi ad alto peso molecolare e il dominio LytR di CpsA è responsabile del trasferimento del CPS alla parete cellulare. Infine, lo studio delle interazioni tra proteine ha dimostrato che CpsC interagisce con CpsD e CpsA.
Queste evidenze sperimentali hanno permesso di suggerire delle possibili funzioni per CpsABCD. Queste proteine sono principalmente coinvolte nella regolazione dei due processi che terminano la biosintesi del CPS: la polimerizzazione e il trasferimento del polisaccaride alla parete cellulare. Nel modello che proponiamo, l’elongazione del CPS da parte della polimerasi viene interrotta dall’azione di CpsA che trasferisce il polisaccaride neosintetizzato alla parete cellulare. L’azione di CpsA viene modulata dallo stato di fosforilazione del complesso CpsCD che è quindi responsabile del bilanciamento dei due processi di elongazione e trasferimento del CPS alla parete cellulare.
Infine, abbiamo studiato l’impatto delle differenze fenotipiche del CPS sulla capacità di GBS di interagire con le cellule umane. A questo scopo è stato impiegato un saggio di adesione-invasione in vitro usando cellule epiteliali polmonari e alcuni dei mutanti sviluppati. I risultati ottenuti hanno mostrato che i mutanti aventi un CPS di lunghezza diversa dal ceppo wild type presentano difetti di adesione alle cellule. Inoltre i ceppi privi di CPS o aventi una bassa quantità di CPS sono in grado di invadere le cellule più efficacemente, indipendentemente dalla lunghezza del polisaccaride prodotto.

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EPrint type:Ph.D. thesis
Tutor:Cesare, Montecucco
Ph.D. course:Ciclo 27 > scuole 27 > BIOSCIENZE E BIOTECNOLOGIE > BIOLOGIA CELLULARE
Data di deposito della tesi:31 January 2015
Anno di Pubblicazione:31 January 2015
Key Words:Streptococcus agalactiae, capsular polysaccharide, capsule biosynthesis
Settori scientifico-disciplinari MIUR:Area 05 - Scienze biologiche > BIO/10 Biochimica
Area 05 - Scienze biologiche > BIO/11 Biologia molecolare
Area 05 - Scienze biologiche > BIO/19 Microbiologia generale
Struttura di riferimento:Centri > Centro Interdipartimentale di servizi A. Vallisneri
Dipartimenti > Dipartimento di Biologia
Codice ID:7854
Depositato il:10 Nov 2015 11:51
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