ASSESSING THE ROLE OF CASPASE ACTIVITY AND METACASPASE EXPRESSION ON VIRAL SUSCEPTIBILITY OF THE COCCOLITHOPHORE, EMILIANIA HUXLEYI (HAPTOPHYTA)

Abstract

As part of their strategy to infect the globally important coccolithophore, Emiliania huxleyi (Lohmann) W.W. Hay & H.P. Mohler, Coccolithoviruses trigger and regulate the host's programmed cell death (PCD) machinery during lytic infection. The induction and recruitment of host metacaspases, specialized, ancestral death proteases that facilitate viral lysis, suggests they may be important subcellular determinants to infection. We examined the "basal" levels and patterns of caspase activity and metacaspase expression in exponentially growing resistant and sensitive E. huxleyi strains and linked them with susceptibility to E. huxleyi virus 1 (EhV1). Resistant E. huxleyi strains were consistently characterized by low caspase specific activity and a relatively simple metacaspase expression profile. In contrast, sensitive E. huxleyi strains had markedly elevated caspase specific activity and consistently expressed more diverse metacaspase proteins. Using pooled data sets from triplicate experiments, we observed statistically significant linear correlations between infectivity, caspase activity, and metacaspase expression, with each strain forming distinct clusters, within a gradient in viral susceptibility. At the same time, we observed positive correlations between the expression of a subset of metacaspase proteins and lower susceptibility, suggestive of potential protective roles. Our findings implicate the importance of subtle differences in the basal physiological regulation of the PCD machinery to viral resistance or sensitivity and cell fate.

The mutual interplay between calcification and coccolithovirus infection

“…Strains DHB611 and DHB659 were also calcified but displayed more moderate cellular PIC quotas, while both CCMP374 and CCMP1516 had extremely low quotas (Fig A and Supporting Information Fig. S10), consistent with previous reports that they are naked strains (Bidle et al ., ; Bidle and Kwityn, ). Naked and calcified cells differed in their equivalent spherical diameters (ESDs), measuring ∼3.8 and 4.3 μm respectively.…”

“…). Both naked strains, CCMP374 and CCMP1516, were highly sensitive to EhV86 infection, consistent with previous observations (Schroeder et al ., ; Bidle et al ., ; Vardi et al ., ; Bidle and Kwityn, ; Fulton et al ., ; Kendrick et al ., ) with cell abundance declining within 48–72 h post infection (hpi) concomitant with high levels of virus production (~ 9 × 10 8 viruses ml −1 ). Consequently, these naked strains served as positive controls for sensitivity to infection.…”

Coccolithoviruses: A Review of Cross-Kingdom Genomic Thievery and Metabolic Thuggery

“…As a result of these studies, considerable insight has been gained regarding (1) the EhV intein [18] and the expression profile of EhV genes during infection [19,20,21]; (2) the EhV life cycle, including its utilization of lipid rafts for budding from the host cells [22,23]; (3) host cellular processes in response to infection such as autophagy and the induction of programmed cell death (PCD) pathways [17,24,25,26,27]; (4) the manipulation of fatty acid and lipid metabolism within infected cells, leading to the production of virus-induced lipids crucial for the progression of the infection [10,28,29,30,31]; (5) vector transmission of EhVs in the natural environment via aerosols and zooplankton faecal pellets [32,33]; and (6) the co-occurring diversity of EhVs and their viruses in a range of habitats, including the Atlantic Ocean, Norwegian Fjords and coastal regions of the Black Sea [8,34,35,36,37,38]. Perhaps the most astonishing finding to date was the identification of a de novo sphingolipid biosynthesis pathway in the EhV genome [1,9,39], encoding virus-derived glycosphingolipids (vGSLs) that are crucial regulators of infection [28].…”

Specificity of archaeal caspase activity in the extreme halophile Haloferax volcanii

“…Our observed caspase-like-specific activities are by far the highest ever reported in a unicellular microbe, more notably in an exponentially growing archaeon. Reported caspase-specific (IETDase) activities in diverse unicellular marine phytoplankton, including cyanobacteria, diatoms, coccolithophores, and chlorophytes, are comparatively low for exponentially growing cells [10 s to 100 s of RFUs h -1 mg protein -1 (Segovia et al, 2003;Berman-Frank et al, 2004;Bidle and Falkowski, 2004;Bidle et al, 2007;Bidle and Bender, 2008;Bidle and Kwityn, 2012)], with rates increasing~10-fold when cells have activated autocatalytic cell death pathways. A wider survey of hydrolysis rates in unicellular microbes using other canonical caspase substrates has only been examined with the unicellular chlorophyte, Dunaliella tertiolecta (Segovia et al, 2003), whereby hydrolytic cleavage of various caspase substrates was tested in cell extracts from stressed cells that were in different stages of light deprivation (0-5 days).…”