IQGAP1 then captures microtubules via interaction with CLIP-170, a protein that is localized at the plus ends of microtubles. Citation14 During mammalian cell polarization and migration, Rac1 marks localized sites of the cortical actin cytoskeleton to which the Rac1 effector IQGAP1 is targeted. Citation5 Spastin promotes branching in both the axonal and dendritic compartments by providing short microtubules for invasion at putative branch-points. Knot upregulates expression of the AAA (ATPases Associated with diverse cellular Activities) family microtubule severing protein Spastin. Citation15 Knot mediates microtubule-based dendrite arbor outgrowth and could function at this microtubule invasion step. Citation13 Could an interaction between Knot and Rac1 be important for the formation of some class IV branches?Ī key step in branch and neurite outgrowth is microtubule invasion along an F-actin core. It has previously been shown that Rac1 is required for class IV neuron dendrite branching. On the other hand, when Rac1 was co-expressed with Knot, together they caused a large increase in dendrite branch formation. Citation5 As a normal function of Cut is promoting filopodia formation, it seems likely that an interaction with Rac1 is part of this process ( Fig. Citation12 When Rac1 was co-expressed with Cut, it enhanced the ability of Cut to promote filopodia formation. In addition to having dissimilar effects on the dendrite cytoskeleton, we have also found that Knot and Cut interact differently with the Rho family small GTPase Rac1, an important regulator of F-actin polymerization. Citation10, Citation11 Hence Knot mediates dendrite outgrowth positive for the microtubule cytoskeleton. On the other hand, Knot promotes extensions of the dendrite arbor entirely positive for the Drosophila MAP1B homologue Futsch, which co-localizes with microtubules. Citation5 We found that Cut promotes outgrowth that is microtubule deficient and filamentous (F)-actin rich. Citation9 We recently demonstrated that a transcription factor code consisting of class IV-specific expression of the zinc finger, helix-loop-helix protein Knot and an ‘intermediate’ level of the homeodomian protein Cut, defines the characteristic dendrite arbor shape of this neuron class. There are four classes of da neurons (I–IV), each with characteristic dendrite arbor morphology ( Fig. ![]() Hence one possibility is that the function of these transcription factors is to modulate basic dendritic developmental programs common to all neuron types in a class-specific manner. Citation2– Citation8 Mature dendrite arbor shape is the outcome of a succession of complex developmental processes. ![]() Citation1 Given the essential role of the dendrite arbor for correct neuron function, an important question in developmental neuroscience is that of how the characteristic arbor shapes of different neuron classes are established.Ī series of recent studies using the dendritic arborization (da) sensory neurons of the Drosophila larva has shown that neuron class-specific dendrite arbor shape is controlled by class-specific codes of transcription factors acting in post-mitotic, differentiating neurons. ![]() Dendrites are the chief site of signal input into a neuron, and the elaborate shape of dendrite arbors influences their ability to process these separate signals into meaningful computation.
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