|M.Sc Student||Drucker-Gonen Nitzan|
|Subject||Studying the Role of Homothorax in the Development of|
Drosophila Melanogaster Malpighian Tubules
|Department||Department of Medicine||Supervisor||Professor Adi Salzberg|
|Full Thesis text|
The Drosophila melanogaster Malpighian tubules (MT) serve as the functional equivalent of the mammalian kidney. Significant homology exists between these two systems and the MT serve as a useful model system for studying kidney development. The MTs are composed of two pairs of epithelial tubes that bud from the midgut-hindgut boundary during embryogenesis. These buds grow and migrate in a stereotyped manner, the longer ones in the direction of the head of the embryo, the shorter ones toward the posterior end.
Homothorax (Hth) is a homeodomain-containing protein of the Meis family which is known to interact with another homeodomain-containing protein, Extradenticle (Exd). Hth regulates the sub-cellular localization of Exd and is capable of translocating it to the nucleus. Inside the nucleus, Hth and Exd are known to form ternary complexes with Hox proteins, thus affecting their binding specificity.
In this work, we describe the expression pattern of Hth in the developing MTs and show that Hth is expressed only in the distal portion of each tubule. We also show that the main role of Hth within the tubules is to translocate Exd to the nucleus, where they are probably required to define distal cell identity. We hypothesize that the difference between distal and proximal cell identities marks the correct position for the formation of the tubule’s loop. We demonstrate that the loop is leading the migrating tubule. Thus, a failure to form a loop in the correct position might prevent proper migration. This hypothesis is further supported by the observation that in the absence of Hth or Exd, the tubules fail to rearrange and migrate properly.
In order to determine where is Hth expression required for proper development of the MTs, we performed rescue experiments in which the expression of Hth was restored in different tissues in hth mutant background and its ability to rescue the MT phenotype was evaluated. These experiments suggest that Hth is required both autonomously and non-autonomously for MT development. Inside the tubules Hth probably defines distal identity, whereas outside the tubules, in the mesoderm, it affects the migration of the anterior tubules perhaps by regulating the expression of guiding molecules. In an attempt to characterize which pathways control Hth expression in the MTs, we found that the Wg pathway and the master-gene cut work upstream to hth and regulates its expression.