Datta A, Ganesan K, Natarajan K
Molecular Biology Laboratory, School of Life Sciences, Jawaharlal Nehru University, New Delhi, India.
Adv Microb Physiol. 1989;30:53-88. doi: 10.1016/s0065-2911(08)60110-1.
Candida albicans is an opportunistic pathogen of human beings and other mammals. Two other features, besides its pathogenicity, have made it a popular organism of study. It exists in different cellular forms and can change from one form to another, depending on growth conditions. Thus, it is being used as a model system to study cellular differentiation. It can also heritably and reversibly switch its cellular and colony morphologies. The yeast is diploid and lacks a sexual cycle. Thus, it has not been possible to apply the powerful methods of genetic analysis to understand morphogenesis or pathogenesis. Few clinical isolates are haploid, but they do not form hyphae and are not yet well characterized. Recombinant DNA techniques are increasingly being applied to C. albicans to solve many of the unanswered questions of morphogenesis and pathogenesis. Genetic transformation and gene-disruption techniques were recently developed for the yeast. Thus it is possible to study the role of any cloned gene through directed mutagenesis. However, the difficulty is to clone the putative genes involved in morphogenesis or pathogenesis. Candida albicans exists in four different cellular forms, namely blastospores, pseudohyphae, hyphae and chlamydospores. Blastospore-to-hypha conversion is well studied. A variety of conditions can induce this transition. It is not clear how cells sense such varied conditions and respond appropriately. In other systems where differentiation is well understood, regulatory genes which control differentiation have been uncovered. These genes cause differential expression of other genes, and ultimately differentiated phenotypes. Thus, it is likely that differential gene expression is involved in the bud-to-hypha transition in C. albicans. Certain proteins are expressed exclusively on the cell surface of hyphae. It should be possible to clone genes coding for these proteins. A study of the expression of these genes might allow us to identify the regulatory gene which determines differentiation. Another approach to understanding morphogenesis is to study how the difference in the shape of buds and hyphae is generated. This difference appears to be due to the differential activity of apical and general growth zones, which determine growth of the cell wall. Activity of these growth zones is apparently determined by actin localization. It remains a possibility that conditions which induce hyphae formation may directly affect actin localization or cell-wall growth zones and cause differences in cell shape. Candida albicans can also heritably switch its cellular phenotype. This has come to light from a study of colony-morphology switching. Some strains can switch their colony morphology, both heritably and reversibly.(ABSTRACT TRUNCATED AT 400 WORDS)
白色念珠菌是人类和其他哺乳动物的一种机会致病菌。除了其致病性外，还有另外两个特征使其成为一种广受欢迎的研究生物体。它以不同的细胞形式存在，并且可以根据生长条件从一种形式转变为另一种形式。因此，它正被用作研究细胞分化的模型系统。它还可以遗传且可逆地改变其细胞和菌落形态。这种酵母是二倍体，缺乏有性周期。因此，无法应用强大的遗传分析方法来理解形态发生或发病机制。很少有临床分离株是单倍体，而且它们不形成菌丝，尚未得到充分表征。重组DNA技术越来越多地应用于白色念珠菌，以解决许多关于形态发生和发病机制的未解决问题。最近为这种酵母开发了遗传转化和基因破坏技术。因此，通过定向诱变研究任何克隆基因的作用成为可能。然而，困难在于克隆参与形态发生或发病机制的假定基因。白色念珠菌以四种不同的细胞形式存在，即芽生孢子、假菌丝、菌丝和厚垣孢子。芽生孢子到菌丝的转变已得到充分研究。多种条件可诱导这种转变。尚不清楚细胞如何感知如此多样的条件并做出适当反应。在其他对分化理解良好的系统中，已经发现了控制分化的调节基因。这些基因导致其他基因的差异表达，并最终导致分化表型。因此，差异基因表达很可能参与白色念珠菌从芽到菌丝的转变。某些蛋白质仅在菌丝的细胞表面表达。应该有可能克隆编码这些蛋白质的基因。对这些基因表达的研究可能使我们能够鉴定决定分化的调节基因。另一种理解形态发生的方法是研究芽和菌丝形状差异是如何产生的。这种差异似乎是由于顶端和一般生长区的不同活性所致，这些区域决定了细胞壁的生长。这些生长区的活性显然由肌动蛋白定位决定。诱导菌丝形成的条件可能直接影响肌动蛋白定位或细胞壁生长区并导致细胞形状差异，这种可能性仍然存在。白色念珠菌还可以遗传地改变其细胞表型。这一点已从对菌落形态转换的研究中显现出来。一些菌株可以遗传且可逆地改变其菌落形态。（摘要截于400字）
