|Home | About | Journals | Submit | Contact Us | Français|
Cdc42 specifies polarity in various biological processes. Kesavan et al. (2009) now demonstrate that Cdc42 also regulates epithelial cell polarity in the mouse pancreas, where it is required for tubule initiation and maintenance. Furthermore, the polarization of epithelial tissue influences the differentiation of pancreatic progenitor cells, linking cell polarity to cell specification.
Tissue architecture provides an environment for cells to produce and respond to signals. In order to develop into functional organs, cells expressing ligands and cells expressing receptors have to be located at the right place at the right time to establish proper signaling cascades. A recent study showed that establishment of proper epithelial polarity is essential for epithelial specification and organogenesis in the mouse mammary gland (McCaffrey and Macara, 2009). The authors showed that mammary progenitor cells lacking Par3, a component of the Par3/Par6/aPKC polarity complex, failed to differentiate into either myoepithelia or luminal epithelia. In addition, the Par3-depleted mammary epithelia had abnormal ductal structures, suggesting a link between tissue organization, or polarity, and the specification of cell fates. A new study from Kesavan et al. (2009) in this issue of Cell now examines the role of the polarity regulator Cdc42 in the formation of tubules in the mouse pancreas. Kesavan and colleagues report that Cdc42 initiates and maintains the apical lumens that ultimately combine to form tubules, and that these epithelial structures instruct pancreatic progenitor cell differentiation and specification. These findings establish a link between cell polarization and tissue differentiation during organ development. Together, the two studies open up a promising research direction for investigating how cellular polarity affects tissue architecture and regulates cell destinies during development.
Cdc42, a Rho family small GTPase, plays essential roles in a variety of biological processes, such as formation of the cytoskeleton, vesicle trafficking, and establishment of apical polarity. Kesavan et al. examined the role of Cdc42 in the polarization of epithelial cells and in lumen formation in the developing pancreas. The authors first show that tubular structures in the mouse pancreas in vivo are generated through fusion of existing micro-lumens. Initially, clusters of polarized cells, with apical surfaces facing the lumen, form micro-lumens. As more cells become polarized, additional luminal structures form, ultimately fusing into a tubular network (Figure 1). More specifically, the authors demonstrate that atypical protein kinase C (aPKC), a downstream effector of Cdc42, is required for the coalescence of micro-lumens into tubular structures. These tubular structures mature to form a single-layer of polarized epithelium inside the mouse pancreas. Tissue-specific ablation of Cdc42 results in a fragmented pancreatic epithelia and large cellular aggregates that lack tubules (Figure 1). Using this in vivo model, the authors show that Cdc42 is required for establishing micro-lumens early during development of the pancreas and subsequently for maintaining the apical polarity of pancreatic tubules.
As pancreatic cells lacking Cdc42 form large aggregates with no lumen, the authors wondered whether this altered tissue structure affected cell specification, namely the lineage commitment of multipotent pancreatic progenitor cells. Based on genome-wide transcription factor expression analysis, previous work had identified a multipotent compartment at the tip of pancreatic branches containing progenitors that differentiate into exocrine, endocrine and duct cells at different stages of organogenesis (Zhou et al., 2007). So, without the correct branching structure in the Cdc42-ablated pancreas, would these progenitor cells differentiate properly? The authors report that ablating Cdc42 in the mouse pancreas not only disrupted the epithelial structure, but also randomized the distribution of progenitors in the pancreas, increased the relative percentage of undifferentiated cells and led to an increase in the number of progenitor cells differentiating into acinar cells at the expense of endocrine commitment. The authors reason that the increase in differentiation into the acinar lineage is due to misorganization of the extracellular matrix, and the failure of Cdc42-ablated cells to commit to the endocrine lineage may be due to disrupted Notch signaling in the mutant pancreas. Together, these results demonstrated that epithelial structure engineered under the guidance of Cdc42 is required for pancreatic progenitors differentiation.
One striking result in the Kesavan et al. (2009) study is that a drug called Y27632, a Rho kinase (ROCK) inhibitor, restored tube formation in pancreatic epithelia lacking Cdc42. ROCK is a negative regulator of the Par3/Par6/aPKC polarity complex, and Par3 phosphorylation by ROCK prevents its interaction with the complex. Inhibition of ROCK may rescue the tubulation defects in the Cdc42-ablated mouse pancreas by allowing activation of the Par3/Par6/aPKC complex, and thus restoring correct polarity to the epithelia. However, a recent study suggested that Y27632 not only inhibits ROCK but also efficiently inhibits aPKC activity (Atwood and Prehoda, 2009), which points to another interpretation. The rescue effect seen in Cdc42 mutants treated with Y27632 might be a combined effect of inhibiting both ROCK and aPKC activity. If so, why would the inhibition of ROCK, upstream of aPKC, and the inhibition of aPKC, downstream of ROCK, rescue the Cdc42 mutant phenotype? Further experiments, especially determining the activity status of aPKC, will hopefully solve this puzzle.
In summary, Kesavan et al (2009) show how tubules form in the mouse pancreas and how Cdc42 controls epithelial polarization, which underlies this process. Most strikingly, Cdc42 controls the formation of the extracellular matrix, and the resultant microenvironment then determines cell fate. These results raise some questions about the function of Cdc42 and its regulation. Since the discovery of Cdc42 in 1990, numerous studies have shown that this “master polarity” protein plays essential roles in a variety of biological processes. The activity of Cdc42 depends on its nucleotide binding state: the GTP-bound Cdc42 is active whereas the GDP-bound form is inactive. Families of GTPase-activating proteins (GAPs), guanine nucleotide exchange factors (GEFs) and guanine nucleotide dissociation inhibitors have been shown to regulate the activity of Cdc42. It is clear that these regulators do not affect just one small GTPase. Growing evidence suggests that GTPases can influence the activity of each other. For example, FilGAP is regulated by Rho to control Rac activity (Ohta et al., 2006). In another case, Cool-2/a-Pix, a GEF, is a target of Cdc42 and activates Rac (Baird et al., 2005). It seems likely, then, that Cdc42 signaling is also highly connected to other small GTPase signaling pathways. Furthermore, the function of Cdc42 is regulated not only by its activity status, but also by its localization in the cell. Recent studies have suggested, for instance, that apical recruitment of active Cdc42 via phosphatidylinositol 4.5-bisphosphate controls lumen formation in epithelial tissues (Martin-Belmonte et al., 2007). Thus, a pressing research direction for the field of polarity is the plasticity of Cdc42, which includes the spatial-temporal activation of Cdc42 activity, activation-inactivation oscillations, and functional interactions with other small GTPases.