You will be presented with the abstract, introduction, and discussion section from a biomedical research article. Your task is to create a summary that a layperson can understand, capturing the essence of the research article. Keep essential scientific terms, but ensure that the language remains clear and the concepts are explained in an uncomplicated manner.

Title: Dynein Regulates Epithelial Polarity and the Apical Localization of stardust A mRNA}
Article:
Abstract:
Intense investigation has identified an elaborate protein network controlling epithelial polarity. Although precise subcellular targeting of apical and basolateral determinants is required for epithelial architecture, little is known about how the individual determinant proteins become localized within the cell. Through a genetic screen for epithelial defects in the Drosophila follicle cells, we have found that the cytoplasmic Dynein motor is an essential regulator of apico–basal polarity. Our data suggest that Dynein acts through the cytoplasmic scaffolding protein Stardust( Sdt) to localize the transmembrane protein Crumbs, in part through the apical targeting of specific sdt mRNA isoforms. We have mapped the sdt mRNA localization signal to an alternatively spliced coding exon. Intriguingly, the presence or absence of this exon corresponds to a developmental switch in sdt mRNA localization in which apical transcripts are only found during early stages of epithelial development, while unlocalized transcripts predominate in mature epithelia. This work represents the first demonstration that Dynein is required for epithelial polarity and suggests that mRNA localization may have a functional role in the regulation of apico–basal organization. Moreover, we introduce a unique mechanism in which alternative splicing of a coding exon is used to control mRNA localization during development.
Introduction:
A polarized architecture increases the level of sophistication at which cells can interact with their neighbors and the environment. Thus, it is not surprising that highly polarized epithelia have become the predominant tissue type among metazoans. Epithelial cells have distinct apical and basolateral membrane domains, each of which contains a unique complement of lipids and proteins to regionalize cellular functions. Intracellular junctions separate the apical from basolateral domains and mediate cell–cell adhesion and selective permeability within the monolayer. Although the mechanisms that specify apico–basal polarity have been a topic of great interest in recent years, there are still many open questions about how this cell architecture is established and maintained. Studies in model organisms and mammalian tissue culture have identified an elaborate protein network regulating epithelial polarity across animal species[1–3]. At the core of this network are three protein complexes, which sequentially localize to and specify one of the two membrane domains and position the adherens junctions at their interface. The Baz complex( Bazooka/aPKC/Par6) acts first in the hierarchy to specify the apical domain[4–7]. The Scrib complex( Scribble/Discs Large) then functions as a basolateral determinant by repressing the apicalizing activity of the Baz complex. Finally the Crb complex( Crumbs/Stardust/DPATJ) is targeted to the apical membrane to antagonize the Scrib complex[6, 7]. Crb is the only transmembrane protein among the known core polarity determinants and is a potent regulator of apical identity. Over-expression of this protein is sufficient to cause an expansion of the apical membrane at the expense of the baso-lateral domain in Drosophila[8]. Crb is stabilized in the plasma membrane by the cytoplasmic scaffolding protein, Stardust/MPP5( Sdt), which binds Crb's cytoplasmic tail through its PDZ( PSD-95/Discs large/ZO-1) domain[9–12]. A third member of the complex, DPATJ, interacts with Sdt via shared L27 domains[12, 13]. The Crb complex is highly conserved both at the structural and functional level between flies and vertebrates[12, 14, 15], and mutations in human CRB1 have been linked to two severe forms of retinal dystrophy[16]. The Crb complex must be precisely localized to the apical domain to exert its effect on epithelial polarity, yet little is known about how the individual proteins within this complex are targeted to this position. Extensive studies in cultured mammalian cells have implicated exocytic vesicle trafficking in polarity regulation[17, 18]. Under this paradigm, vesicles carrying newly synthesized transmembrane proteins are specifically targeted to either the apical or basolateral domain. While direct vesicle trafficking is an appealing potential mechanism for the localization of Crb, it is likely to be of limited use in the apical targeting of Sdt and DPATJ, the cytoplasmic members of this complex. Moreover, the currently recognized members of the Baz and Scrib complexes are entirely comprised of cytoplasmic proteins, which must be specifically localized to either the apical or lateral domains to perform their polarity functions. Uncovering the mechanisms that govern the subcellular targeting of cytoplasmic polarity determinants is critical, therefore, to our understanding of apico–basal polarity regulation. One mechanism that many cell types employ to position proteins within the cytoplasm involves the subcellular localization of their corresponding mRNAs[19–24]. There are several strategies by which mRNAs can become enriched within a given cytoplasmic domain, but the most common strategy involves the assembly of transcripts into large RNA-protein particles that are transported along cytoskeletal elements by molecular motors. Subcellular transport, followed by localized translation, ensures that new proteins will only be produced in the cytoplasmic region where their function is required[25]. This mechanism, which is conserved across such diverse taxa as fungi, plants and animals, has been particularly well studied in the Drosophila oocyte where mRNAs encoding patterning regulators such as oskar, gurken and bicoid are precisely targeted to the three different regions of the oocyte where their protein products are required[23]. mRNA localization has also been observed in the Drosophila blastoderm, where transcripts such as wingless, and those encoding pair-rule genes, are targeted to the apical cytoplasm[26–29]. Here we show that loss of the minus end-directed microtubule motor Dynein in the Drosophila follicle cell epithelium leads to defects in apico–basal polarity. Our data indicate that Dynein is required for the localization of Sdt and suggest a mechanism whereby the Dynein-dependent apical targeting of specific sdt mRNA isoforms enriches Sdt protein in the apical cytoplasm. Surprisingly, the apical localization signal for the sdt mRNA maps to an alternatively spliced coding exon. Selective removal of this exon coincides with a developmental switch between localized and unlocalized isoforms of the transcript, in which apically localized sdt transcripts play an early role in the assembly of the Crb complex, while unlocalized transcripts are sufficient to maintain polarity in mature epithelia. These results suggest that the strategy for the apical targeting of the Crb complex changes during epithelial maturation and introduce a new class of gene products, mRNAs, to the existing paradigm for apico–basal polarity regulation
Discussion:
Dynein's role in MT-based apical transport has been studied in the epithelia of both mammals and flies[5, 28, 45–48], but an explicit link between Dynein and apico–basal polarity has not been found. One cause for this deficiency may be that Dynein is required for a number of essential cellular processes, making it difficult to study this motor under strong loss-of-function conditions. For instance, previous studies of Dynein function in Drosophila embryos have relied on combinations of hypomorphic Dhc alleles or injection of α-Dhc antibodies which only partially block Dynein function[5, 28, 33]. Notably, these manipulations have failed to produce epithelial polarity phenotypes. We have shown that strong loss of Dynein function in the FCs disrupts both molecular and morphological aspects of apico–basal polarity, providing the first direct evidence for the role of this motor in this key cell biological process. It is currently unclear why the FCs tolerate strong loss of Dynein function better than other tissues, but this property provides a unique opportunity to begin dissecting the many roles that Dynein is likely to play in epithelial organization. How does Dynein regulate apico–basal polarity? Since Dynein is a minus-end directed MT motor and minus ends are apically oriented in epithelia, we investigated whether Dynein might ferry components of the Baz and/or the Crb complexes to their sites of action at the apical surface. Two pieces of data indicate that the Baz complex is not the primary Dynein cargo contributing to FC polarity. Although Baz is occasionally relocalized to the lateral FC surface in Dhc clones, most mutant cells display significant amounts of apical Baz. Furthermore, ECs in which the entire epithelium is mutant for baz or aPKC display multilayering at both EC poles([49], unpublished data), a phenotype that is distinct from the posterior multilayering in Dhc mutant epithelia. These observations are consistent with recent studies in the embryonic blastoderm, where it was shown that Dynein does play a role in Baz localization, but that it is not the only means by which this protein is targeted to the apical domain[5]. Together, these data indicate that, while Dynein likely does play a role in the apical targeting of Baz in both the embryo and the FCs, Dynein's major contribution to FC apico–basal polarity corresponds to a different cargo. Our results favor a model in which Dynein works primarily through the Crb complex to influence epithelial polarity. When Dynein function is reduced in the FCs, Crb and Sdt disappear from the apical surface, even in Dhc cells that remain cuboidal. Moreover, the morphology of an egg chamber in which the entire epithelium is mutant for Dhc is quite similar to that seen for crb and sdt mutant egg chambers, as all three display multilayering predominantly in the posterior([31], data not shown). A major challenge comes in deciphering which Crb complex components are specifically transported by Dynein. We have focused on the relationship between Dynein and Sdt because genetic interaction and molecular epistasis experiments indicate that loss of apical Sdt can account for many aspects of the Dhc polarity phenotype. Our finding that the apical localization of sdt transcripts is Dynein-dependent suggests a mechanism by which Dynein could localize Sdt, in part, through the apical targeting and localized translation of its mRNA. sdt transcripts cannot be the only Crb complex component transported by Dynein, however, as the phenotype of sdtEH681 mutant FCs does not recapitulate all aspects of the polarity phenotype of Dhc mutant clones. Interestingly, crb transcripts are also targeted to the apical FC cytoplasm in a Dynein-dependent manner, although at a later stage than sdt( Figure S3). This finding raises the possibility that, in addition to Baz, crb mRNA and/or Crb complex proteins represent other Dynein cargoes required for full epithelial polarization. Future work will be required to more finely dissect Dynein's complex contributions to the apical targeting of the Crb complex. While investigating whether sdt mRNA was likely to be a primary Dynein cargo contributing to apico–basal polarity, we made the surprising discovery that the apical targeting of sdt transcripts is regulated through the alternative splicing of a coding exon, exon 3. We are aware of only two other genes in which transcript localization is regulated in this way[50, 51]. In both instances, however, the signal lies within the 3′UTR, so the splicing event does not affect protein structure. It is curious that alternative splicing of the sdt mRNA localization signal also deletes 433 amino acids from the protein. The role of the amino acids encoded by exon 3 in Sdt function is not yet known. These amino acids are not conserved in vertebrate Sdt homologs. Moreover, Sdt A and Sdt B bind the intracellular domain of Crb with equal efficiency in vitro[10] and our work with the sdtEH681 allele, as well as over-expression studies with the sdt A and sdt B transgenes( Figure S1) indicate that both protein isoforms stabilize Crb in vivo. Although we have not ruled out the possibility that exon 3 regulates both mRNA localization and protein function, together these observations suggest that the splicing of exon 3 may primarily regulate mRNA localization. A potential role for Dynein-dependent apical targeting of sdt mRNA in epithelial polarity is supported by our analysis of the relative contributions of the sdt A and sdt B isoforms. Although the lack of Sdt A in sdtEH681 FC clones reduces Crb at the apical membrane, this deficit leads to relatively mild effects on other aspects of polarity. This apparent discrepancy can be explained by the extrinsic cue for apical identity that is provided to the FCs through their direct contact with the germline[30, 31], which may compensate for reduced Crb complex function in this tissue. However, in the embryo, where no such cue is available, sdtEH681 has a nearly null phenotype, which is rescued much more efficiently by sdt A than sdt B. Overall, these data suggest that the apical targeting of sdt transcripts may represent an important mechanism contributing to apico–basal polarity. Interestingly, the apical targeting of sdt mRNA is developmentally regulated in both embryonic and adult epithelia. Specifically, we have shown that apically localized sdt transcripts are found only during early stages, while unlocalized transcripts predominate at later stages. Why is sdt transcript localization regulated in this way? It is tempting to speculate that apical transcripts are required primarily for the establishment of apico–basal polarity but not its maintenance. In reality, however, the functional distinction between the two phases of sdt mRNA localization is almost certainly more subtle. When apical sdt A transcripts are present, the epithelia are relatively immature; they tend to be proliferative, display incomplete junctional structures and have yet to adopt their final cell morphology. By contrast, when unlocalized sdt B transcripts predominate, the epithelia are more likely to be post-mitotic and highly differentiated. These observations raise the possibility that a concentrated pool of Sdt protein, generated by localized translation of apical transcripts, could function to stabilize Crb primarily during periods when cell polarity is labile, but that this dynamic regulatory mechanism would be dispensable in fully differentiated cells.
Cells within epithelial sheets are highly polarized with distinct apical and basolateral membrane domains . This cellular organization is critical to both epithelial form and function , and a failure to maintain epithelial polarity is often linked to tumor progression . The protein network that establishes and maintains the two membrane domains relies on the precise subcellular localization of its molecular components , but little is known about how these proteins are targeted to their sites of action . We have shown that the localization of the apical determinant protein Stardust depends on the microtubule motor Dynein . While investigating the relationship between Dynein and Stardust , we also made two unexpected observations about stardust mRNA regulation . First , the mechanism by which Dynein localizes Stardust may depend , in part , on the apical targeting of the stardust mRNA . Second , some stardust mRNA is apically localized during early stages of epithelial development , but the selective removal of the apical localization signal leads to the sole production of uniformly localized transcripts in mature epithelial cells . Together , these results introduce roles for Dynein in apico–basal polarity regulation and raise important questions about the role of mRNA localization in the targeting of polarity determinant proteins and epithelial maturation .