Regulation of differentiation in slime mold cells

在细胞黏菌的发育过程中，细胞聚集形成一个组织，其中两种类型的细胞(前柄和前孢子)分别在前部和后部分化。为了阐明分化的调节机制，我们做了以下研究：1.细胞生长和分化的关系组织中的细胞是否分化成前柄或前孢子细胞与饥饿时的细胞周期阶段有关。确定了从生长到分化的突变点，并检测了在该点上特异表达的基因。鉴定了三种基因(退出1，2，3)。该基因编码cAMP受体，CAR1和Quit2是一种新型的钙结合蛋白(CAF-1)。2.柄细胞诱导因子的机制在发育过程中细胞会分泌一种柄细胞分化因子(STIF)。8-溴-cAMP取代STIF的事实表明STIF是通过蛋白激酶A(PKA)发挥作用的。这种可能性得到了验证，因为用显性的PKA负抑制物转化的细胞不能形成柄细胞。STIF激活茎特异的ecmB基因，对一个无聚集性突变体的分析表明，ERK2基因在细胞的趋化作用中起重要作用。ERK2基因被胞外cAMP通过膜受体CAR1和CAR3激活，最终激活腺苷环化酶(AC)，而不涉及异源G蛋白。AC的激活导致细胞内cAMP的增加，进而使ERK2失活。ERK2的激活是细胞生长所必需的，这是由叶酸引起的，并且强烈依赖G蛋白。

In the development of cellular slime molds, cells aggregate to form a tissue, in which two types of cells (prestalk and prespore) differentiate in the anterior and the posterior parts, respectively.To elucidate regulatory mechanisms of differentiation, we made the following studies.1.Relation of cell growth and differentiationWhether cells differentiate into either prestalk or prespore cells in the tissue is related to the cell cycle phase at the time of starvation.Putative shift (PS) point from growth to differentiation was determined and genes specifically expressed at the point were examined.Three kinds of genes (Quit 1,2,3) were identified.The Quit gene encodes a cAMP receptor, CAR1 and the Quit2 a new type of calcium-binding protein (CAF-1).2.Mechanism of stalk cell-inducing factorCells secrete during development a factor inducing stalk cell differentiation (STIF).The fact that STIF is replaced by 8-bromo-cAMP suggests that STIF acts through protein kinase A (PKA).This possibility was verified, as cells transformed with a dominant negative inhibitor of PKA was incapable of stalk cell formation.STIF activates stalk-specific ecmB gene, but not ecmA gene.The upstream region of ecmB promoter where STTF acts was identified by using cells transformed with various deleted genes.3.Role of MAP-kinase in signal transduction.Analysis of an aggregateless mutant indicated a MAP-kinase, ERK2 gene has an important role in chemotaxis of cells.ERK2 was activated by extracellular cAMP through membrane receptors, CAR1 and CAR3 and eventually activates adenylate cyclase (AC), without involving heterotrimetric G-proteins.Activation of AC results in an increase in cellular cAMP which in turn inactivates ERK2.Activation of ERK2 is essential for cell growth during the vegetative phase, which is brought about by folic acid and strongly depends on the G proteins.