Septin 7 (SEPT7) has been described to be essential for successful completion of cytokinesis in mouse fibroblasts, and Sept7-deficiency in fibroblasts constitutively results in multinucleated cells which stop proliferation. Using Sept7flox/floxfibroblasts we generated a cellular system, where the cytokinetic defects of Cre-mediated deletion of the Sept7 gene can be rescued by ectopically expressed doxycycline-inducible wild type SEPT7. Using this system, we analyzed the ability of SEPT7-mutants with alterations in their GTPase domain-dependent dimerization to prevent multinucleation and rescue proliferation. Although biochemical analysis of the mutants demonstrates differences in homo- and/or hetero-polymerization, in GTP-binding and/or GTPase activities, all analyzed mutants were able to rescue the cytokinesis phenotype of Sept7flox/floxfibroblasts associated with Cre-mediated deletion of endogenous Sept7. These findings indicate that the ability of septins to assemble into well-defined SEPT7-dimerization dependent native filaments is dispensable for cytokinesis in fibroblasts and opens the way to search for other mechanisms of the involvement of SEPT7 in cytokinesis.

Septins are conserved cytoskeletal proteins with unique filament forming capabilities and roles in cytokinesis and cell morphogenesis. Septins undergo hetero-oligomerization and assemble into higher order structures including filaments, rings, and cages. Hetero- and homotypic interactions of septin isoforms involve alternating GTPase (G)-domain interfaces and those mediated by N- and C-terminal extensions. While most septins bind GTP, display weak GTP-hydrolysis activity and incorporate guanine nucleotides in their interaction interfaces, studies using GTPase-inactivating mutations have failed to conclusively establish a crucial role for GTPase activity in mediating septin functions. In this mini-review, we will critically assess the role of GTP-binding and -hydrolysis on septin assembly and function. The relevance of G-domain activity will also be discussed in the context of human septin mutations as well as the development of specific small-molecules targeting septin polymerization. As structural determinants of septin oligomer interfaces, G-domains are attractive targets for ligand-based inhibition of septin assembly. Whether such an intervention can predictably alter septin function is a major question for future research.

BackgroundFmr1 has been known to be a crucial contributor in neurodevelopment. Events such as alternative splicing in its coding region and the use of different trancription start and end sites in its non-coding regions result in the production of a range of mRNA transcripts. Structural and functional characterization of some of these transcripts have been performed, but several of them remain uncharacterized. Differences in the temporal and spatial expession of these transcripts may play important roles in regulating the development of the fetal and adult brain structures and thus, their physiological functions. New informationThe extensive set of experiments suggested in this study plan exploit mice brain, neuronal cultures and in vitro studies. The proposed study for the identification and full characterization of the known and novel transcripts of Fmr1 is an important initial step towards the assignment of the specific known roles and the determination of the so-far unknown roles of Fmr1 at the various stages of neurodevelopment. This systematic study helps in categorizing the transcripts that can produce stable proteins and further understand their cellular localization and cellular functions individually and in concert with each other. Similarly, identification of the non-coding transcripts helps us in exploring their ‡ © Abbey M. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. roles in the regulatory processes, if any, which might also impact the expression of the coding transcripts in the neuronal cells.