Yeast Mating: Putting Some Fizz into Fungal Sex?

Publisher’s version / Version de l'éditeur:

Current Biology, 19, 6, pp. R258-R260, 2009-01-01

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

NRC Publications Archive

Archives des publications du CNRC

This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.

For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

https://doi.org/10.1016/j.cub.2009.01.043

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Yeast Mating: Putting Some Fizz into Fungal Sex?

Whiteway, Malcolm

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

NRC Publications Record / Notice d'Archives des publications de CNRC:

https://nrc-publications.canada.ca/eng/view/object/?id=2759c1d5-6e55-442c-b657-e20ee9c6bf0f

https://publications-cnrc.canada.ca/fra/voir/objet/?id=2759c1d5-6e55-442c-b657-e20ee9c6bf0f

Yeast Mating: Putting Some Fizz into

Fungal Sex?

The mating-competent form of the fungal pathogen Candida albicans is not stable in vitro at the temperature found in its mammalian host where mating occurs. Recent evidence that high levels of CO2can stabilize the mating-competent state provides a solution to this puzzle.

Malcolm Whiteway

The fungal pathogen Candida albicans has recently been found to contain the genetic capacity to undergo mating. While this was initially surprising, given the length of time that the organism was comfortably classified as asexual, recent analysis of the population structure of C. albicans is compatible with this diploid organism undergoing at least a low level of genetic exchange

[1]. An important insight that facilitated re-evaluation of the sex life of the pathogen was the observation from the genome project that the standard strain being sequenced contained a locus with similarity to MAT, the cell-type-determining locus of the well-studied model yeast

Saccharomyces cerevisiae. The structure of the C. albicans locus, termed MTL (for mating type-like) showed that each homologous chromosome 5 contained a distinct allele of this locus i.e. MTLa or MTLa; this arrangement, on the basis of the S. cerevisiae model, would be expected to confer sterility[2]. Subsequent developments, including the creation of mating-type

homozygous versions of the typically MTLa/MTLa heterozygous diploid organism[3,4], and the recognition that a previously identified and somewhat enigmatic epigenetic cell state termed ‘opaque’ was the mating-competent form of C. albicans[5], have permitted C. albicans mating under laboratory conditions.

Current standard laboratory routes to building mating-competent C. albicans strains involve the

manipulation of strains to contain only MTLa or MTLa information, either by molecular ablation of one or the other allele by transformation with

recombinant DNA, or by obtaining homozygosis of chromosome 5 through selection for growth on plates with sorbose as the sole carbon source

[3,4]. This latter strategy works because of the presence of

dosage-sensitive sorbose utilization suppressors on the MTL-containing chromosome. After confirming the loss of the MTLa or MTLa information, the strains are spread at room temperature on plates containing the vital dye phloxin B to visualize the infrequent, brightly pink staining sectors that may represent the appearance of opaque state cells. The frequency of opaque cell formation can be increased by introducing extra copies of Wor1p, a transcription factor that is the master regulator of the switching process[6]. Identified opaque strains are then purified, carefully maintained because they frequently revert back to the white state (this reversion occurs quantitatively at mammalian body temperature), and finally mixed with similarly derived strains of the opposite mating type to allow the formation of tetraploid mating products. Thus, the formation of mating-competent strains of C. albicans, while conceptually straightforward, is a time-consuming and multifaceted process in the laboratory (Figure 1).

Intriguingly, these details associated with the creation of mating-competent strains of C. albicans in the laboratory raised significant questions with respect to the relevance of mating to the biology of the organism in the wild. If Candida cells used the strategies applied by Candida researchers to reach a mating-competent state, it was hard to anticipate the process actually ever taking place. Both mating-type homozygosis and switching to the opaque state are relatively rare events; the generation of opaque cells of opposite mating types in relatively close physical proximity would be ‘rare squared’. When you add in the inability of the opaque state to be maintained at 37_{C, the temperature of the}

mammalian host of the pathogen, C. albicans appears to have chosen a mating system perversely incompatible with the fundamental characteristics of its natural state.

Yet the complex systems for the production and response to cell-specific pheromones are intact, population structures give evidence for genetic exchange, and recombinant tetraploid strains can be detected in hosts infected with genetically marked diploids, so the mating machinery is clearly active.

One obvious solution to this

conundrum is that C. albicans cells use routes to mating competence that are distinct from those currently chosen by Candida researchers. Recent work from the Soll lab, published in a recent issue of Current Biology[7], sheds important light on the key problem of the instability of the mating-competent opaque state at 37_{C. Following up on}

an old observation the lab had made about enhanced opaque formation in parafilm-wrapped plates, they were able to show that CO2, provided in concentrations from 5 to 20%, was able to dramatically induce and stabilize opaque cell formation even at elevated temperatures. Because levels of CO2 are found to be high in the niches colonized by C. albicans in the host, this observation provides a tidy solution to the biological paradox of mating occurring at temperatures apparently incompatible with the mating-competent state (Figure 1). This observation also has practical

applications for Candida labs; studies on mating should be facilitated by a CO2incubator. Previous strategies of stabilizing the opaque state by anaerobic conditions[8,9]were not so easy to apply to routine strain culture, and the recent work suggests that it is the presence of CO2rather than the absence of O2that ultimately provides stabilization of proliferating opaque cells[7].

So how does the presence of CO2 serve to stabilize the epigenetic state compatible with mating? Not

surprisingly, CO2-mediated switching involves Wor1p, the central

transcription regulator controlling the opaque state, as well as the DNA-binding protein Czf1p, and thus apparently requires the standard opaque-state regulatory circuit[7]. More interestingly, adenylyl cyclase and Ras1p may have an involvement in the process. Previous work[10]had implicated CO2in the regulation of hyphal development in C. albicans and had provided evidence that the CO2signal was directly sensed by

Current BiologyVol 19 No 6 R258

adenylyl cyclase. The characteristics of the opaque connection to CO2were similar; adenylyl cyclase and Ras1 were important for the efficient production of the opaque state at low CO2concentrations but were not required at CO2concentrations above 5%. What this means at a mechanistic level is currently unclear, and thus establishing the ‘target’ of CO2will be an important next step.

Clearly, this answer to the opaque-cell stability question only serves to eliminate one of the problems inherent in defining mating as a relevant component of the natural biology of C. albicans (Figure 1). Homozygosis of

MTL is a requirement for entering the opaque state, since expression of both MTLa and MTLa information represses expression of the WOR1 gene encoding the opaque-state master regulator. Yet events that generate loss of heterozygosity (LOH), such as chromosome loss or mitotic recombination, are typically infrequent. Perhaps regulatory processes, rather than structural loss of MTLa or MTLa information, can provide a solution, as evidence has been presented that environmental conditions, such as the presence of hemoglobin, can influence expression of the MTL locus[11]. And beyond the mechanistic details of

being able to mate, just how do you go about finding a suitable partner? Human colonization by C. albicans is most likely initiated at birth, and populations of the pathogen are predominantly but not exclusively clonal within the human host[12]. So why even bother going through the complication of mating if your most likely partner is your clone? Finally, if mating is occurring in nature, where are the tetraploids? Laboratory manipulation of C. albicans mating products has identified a chromosome loss process that reduces tetraploids to diploid or near-diploid status[13], but it remains to be seen if that is the

α

/α

a/a

α

/α

a/a

a/a α/α

a/α

Gastrointestinal tract? Skin?

(Perhaps facilitated by biofilm) High CO2 Loss of heterozygosity? Repression of MTL expression? 3% of clinical isolates are homozygous Genetically engineered or sorbose selected Air 25o_C low nutrient low frequency CO2 25o_{C or 37}o_C low nutrient high frequency Test tube or plate controlled environment

Tetraploid products?

Parasexual or true meiosis?

Diploid cells

Tetraploid progeny

Random loss of chromosomes

Aneuploid/diploid cells Heterozygous

non-mating competent form

Loss of heterozygosity White to opaque switching Mating Ploidy reduction Natural Laboratory Current Biology

Figure 1. A comparison of natural and laboratory mating in C. albicans.

To be able to mate, C. albicans must enter the opaque cell state, shown in red. This process involves first becoming homozygous for MTLa or MTLa information. In the lab this requires selection or genetic engineering; in the mammalian host this may involve various routes to loss of heterozygosity (about 3% of clinical strains are MTL homozygotes) or regulated suppression of one or the other MTL allele. The second step is to activate the Wor1p-mediated epigenetic feed-forward loop. This process was inefficient in the lab, but Huang et al.[7]established that 5% or higher CO2concentrations enhance and stabilize the process. It is also likely that high CO2levels in the host support the opaque

state, which is not otherwise stable at body temperature. Mating-competent diploids conjugate to form tetraploids: in the lab, this typically occurs on plates, but in the host it was previously proposed that the lower temperature of the skin could be permissive of the opaque state. However, Huang et al.[7]have now suggested that the candidate niches are regions high in CO2. Tetraploid cells can be induced to return to the

diploid state through chromosome loss, defining the mating process in the lab as part of a parasexual cycle. This form of ploidy reduction could also occur in the mammalian host, although conditions permitting a true meiosis may yet be identified.

Dispatch R259

natural route of ploidy reduction after mating in a host. And if it is, why stop at diploidy? Thus, sorting out the link between the laboratory-generated mating and the process occurring in the mammalian host promises to continue to provide interesting insights into the biology of this important human pathogen.

References

1. Tavanti, A., Gow, N.A., Maiden, M.C., Odds, F.C., and Shaw, D.J. (2004). Genetic evidence for recombination in Candida albicans based on haplotype analysis. Fungal Genet. Biol. 41, 553–562.

2. Hull, C.M., and Johnson, A.D. (1999). Identification of a mating type-like locus in the asexual pathogenic yeast Candida albicans. Science 285, 1271–1275.

3. Hull, C.M., Raisner, R.M., and Johnson, A.D. (2000). Evidence for mating of the ‘‘asexual’’ yeast Candida albicans in a mammalian host. Science 289, 307–310.

4. Magee, B.B., and Magee, P.T. (2000). Induction of mating in Candida albicans by construction

of MTLa and MTLalpha strains. Science 289, 310–313.

5. Miller, M.G., and Johnson, A.D. (2002). White-opaque switching in Candida albicans is controlled by mating-type locus homeodomain proteins and allows efficient mating. Cell 110, 293–302.

6. Zordan, R.E., Miller, M.G., Galgoczy, D.J., Tuch, B.B., and Johnson, A.D. (2007). Interlocking transcriptional feedback loops control white-opaque switching in Candida albicans. PLoS Biol. 5, e256.

7. Huang, G., Srikantha, T., Sahni, N., Yi, S., and Soll, D.R. (2009). CO2regulates

white-to-opaque switching in Candida albicans. Curr. Biol. 19, 330–334.

8. Ramirez-Zavala, B., Reuss, O., Park, Y.N., Ohlsen, K., and Morschhauser, J. (2008). Environmental induction of white-opaque switching in Candida albicans. PLoS Pathog. 4, e1000089.

9. Dumitru, R., Navarathna, D.H., Semighini, C.P., Elowsky, C.G., Dumitru, R.V., Dignard, D., Whiteway, M., Atkin, A.L., and Nickerson, K.W. (2007). In vivo and in vitro anaerobic mating in Candida albicans. Eukaryot. Cell 6, 465–472.

10. Klengel, T., Liang, W.J., Chaloupka, J., Ruoff, C., Schroppel, K., Naglik, J.R., Eckert, S.E., Mogensen, E.G., Haynes, K.,

Tuite, M.F., et al. (2005). Fungal adenylyl cyclase integrates CO2 sensing with cAMP signaling and virulence. Curr. Biol. 15, 2021–2026.

11. Pendrak, M.L., Yan, S.S., and Roberts, D.D. (2004). Hemoglobin regulates expression of an activator of mating-type locus alpha genes in Candida albicans. Eukaryot. Cell 3, 764–775.

12. Odds, F.C. (1987). Candida infections: an overview. Crit. Rev. Microbiol. 15, 1–5. 13. Forche, A., Alby, K., Schaefer, D.,

Johnson, A.D., Berman, J., and Bennett, R.J. (2008). The parasexual cycle in Candida albicans provides an alternative pathway to meiosis for the formation of recombinant strains. PLoS Biol. 6, e110.

Genetics Group, Biotechnology Research Institute, National Research Council of Canada, 6100 Royalmount Ave., Montreal, Quebec, Canada H4P 2R2 and Department of Biology, McGill University, Montreal, Quebec, Canada H3A 1B1.

E-mail:malcolm.whiteway@cnrc-nrc.gc.ca

DOI: 10.1016/j.cub.2009.01.043

Dyslexia: Bridging the Gap between

Hearing and Reading

Recent work with dyslexic subjects provides the first empirical evidence linking changes in the brain networks subserving phonological processing to deficits in the matching of speech sounds to their appropriate visual representations. Mark T. Wallace

Although most children rapidly develop a strong facility to read the printed and written word, a surprisingly large number fail to acquire good reading skills, even after intensive instruction. When these reading difficulties are seen in the presence of normal or above-normal intelligence, and when there are measurable deficits in phonological processing — the ability to store, retrieve and manipulate speech sounds — the child (or adult) is typically diagnosed with dyslexia, a term first coined in the late 19th century. The most common form of dyslexia is seen in a developmental context as children fail to meet certain benchmark measures of ‘normal’ reading ability. Although there appear to be cultural and orthography-related differences in its prevalence, some estimates suggest that the incidence of developmental dyslexia may be as high as 10% in the general population[1]. Not surprisingly given this high prevalence, the monetary and societal impacts of reading disabilities are

staggering. The work by Blau et al.[2]

reported in this issue of Current Biology provides important new insights into the neural bases of developmental dyslexia, by showing changes in brain activation patterns in dyslexic readers that are associated with the matching of speech sounds with their appropriate visual

representations (letters). Such letter–speech matching must be both rapid and accurate for the emergence of fluent reading abilities.

Although its diagnosis is still considered to be controversial in some domains, there is a growing consensus that dyslexia has a neurobiological basis, with strong evidence that there is a genetic component to the disability

[3]. Numerous theories abound as to the physiological processes and neural systems that are affected in dyslexia, with several of the more prominent models focusing on alterations in rapid auditory processing[4–6], disturbances in the magnocellular visual pathway[7,8], and cerebellar dysfunction[9]. As alluded to above, however, the best-established

changes (and model) are centered on deficits in phonological encoding and decoding and the networks that support these processes[10–13]. But despite the presence of a strong linkage between disrupted

phonological abilities and poor reading skills in dyslexia, there has remained a fundamental gap in our

understanding of how problems in encoding speech sounds ultimately translate into reading difficulties. A key step in this process must be the rapid and accurate matching of the

component speech sounds (phonemes) with their appropriate written representations (graphemes). Despite the intuitive nature of this multisensory transformation process, there is little empirical evidence that relates across these domains and specifically bridges speech processing and reading.

Advances in non-invasive neuroimaging methods, particularly functional magnetic resonance imaging (fMRI), have made this problem more tractable by allowing a view into the neural correlates of reading and phonological processes

[1,14–17]. With fMRI, changes in the blood oxygenation level dependent (BOLD) signal, an indirect measure of neural activity, can be measured while the participant is presented with certain stimuli and/or engaged in a specific task. Differences in the BOLD signal can then be compared between two

Current BiologyVol 19 No 6 R260