Of rings and spines: The multiple facets of Citron proteins in neural development.

The Citron protein was originally identified for its capability to specifically bind the active form of RhoA small GTPase, leading to the simplistic hypothesis that it may work as a RhoA downstream effector in actin remodeling. More than two decades later, a much more complex picture has emerged. In particular, it has become clear that in animals, and especially in mammals, the functions of the Citron gene (CIT) are intimately linked to many aspects of central nervous system (CNS) development and function, although the gene is broadly expressed. More specifically, CIT encodes two main isoforms, Citron-kinase (CIT-K) and Citron-N (CIT-N), characterized by complementary expression pattern and different functions. Moreover, in many of their activities, CIT proteins act more as upstream regulators than as downstream effectors of RhoA. Finally it has been found that, besides working through actin, CIT proteins have many crucial functional interactions with the microtubule cytoskeleton and may directly affect genome stability. In this review, we will summarize these advances and illustrate their actual or potential relevance for CNS diseases, including microcephaly and psychiatric disorders.

Precision Revisited: Targeting Microcephaly Kinases in Brain Tumors.

Glioblastoma multiforme and medulloblastoma are the most frequent high-grade brain tumors in adults and children, respectively. Standard therapies for these cancers are mainly based on surgical resection, radiotherapy, and chemotherapy. However, intrinsic or acquired resistance to treatment occurs almost invariably in the first case, and side effects are unacceptable in the second. Therefore, the development of new, effective drugs is a very important unmet medical need. A critical requirement for developing such agents is to identify druggable targets required for the proliferation or survival of tumor cells, but not of other cell types. Under this perspective, genes mutated in congenital microcephaly represent interesting candidates. Congenital microcephaly comprises a heterogeneous group of disorders in which brain volume is reduced, in the absence or presence of variable syndromic features. Genetic studies have clarified that most microcephaly genes encode ubiquitous proteins involved in mitosis and in maintenance of genomic stability, but the effects of their inactivation are particularly strong in neural progenitors. It is therefore conceivable that the inhibition of the function of these genes may specifically affect the proliferation and survival of brain tumor cells. Microcephaly genes encode for a few kinases, including CITK, PLK4, AKT3, DYRK1A, and TRIO. In this review, we summarize the evidence indicating that the inhibition of these molecules could exert beneficial effects on different aspects of brain cancer treatment.

Correction to: ZIKA virus elicits P53 activation and genotoxic stress in human neural progenitors similar to mutations involved in severe forms of genetic microcephaly.

The authors wish to point out that the name of the first author is appearing incorrectly on Pubmed: it should be El Ghouzzi V (and not Ghouzzi VE). In addition, the words "and p53" appear at the end of the title in the original publication ( https://www.nature.com/articles/cddis2016266 ) and in the previous erratum version ( https://www.nature.com/articles/cddis2016446 ). This is not correct.

Impact of DNA repair and stability defects on cortical development.

Maintenance of genome stability is a crucial cellular function for normal mammalian development and physiology. However, despite the general relevance of this process, genome stability alteration due to genetic or non-genetic conditions has a particularly profound impact on the developing cerebral cortex. In this review, we will analyze the main pathways involved in maintenance of genome stability, the consequences of their alterations with regard to central nervous system development, as well as the possible molecular and cellular basis of this specificity.

Inactivation of Citron Kinase Inhibits Medulloblastoma Progression by Inducing Apoptosis and Cell Senescence.

Medulloblastoma is the most common malignant brain tumor in children. Current treatment for medulloblastoma consists of surgery followed by irradiation of the whole neuraxis and high-dose multiagent chemotherapy, a partially effective strategy associated with highly invalidating side effects. Therefore, identification and validation of novel target molecules capable of contrasting medulloblastoma growth without disturbing brain development is needed. Citron kinase protein (CITK), encoded by primary microcephaly gene MCPH17, is required for normal proliferation and survival of neural progenitors. Constitutive loss of CITK leads to cytokinesis failure, chromosome instability, and apoptosis in the developing brain, but has limited effects on other tissues. On this basis, we hypothesized that CITK could be an effective target for medulloblastoma treatment. In medulloblastoma cell lines DAOY and ONS-76, CITK knockdown increased both cytokinesis failure and DNA damage, impairing proliferation and inducing cell senescence and apoptosis via TP53 or TP73. Similar effects were obtained in the NeuroD-SmoA1 transgenic mouse model, in which CITK deletion increased apoptotic cells and senescence markers such as P21CIP1, P27KIP1, and P16INK4A Most importantly, CITK deletion decreased tumor growth and increased overall survival in these mice, with no apparent side effects. These results suggest that CITK can be a useful molecular target for medulloblastoma treatment.Significance:In vitro and in vivo proof of concept identifies citron kinase protein as a suitable target for medulloblastoma treatment.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/16/4599/F1.large.jpg Cancer Res; 78(16); 4599-612. ©2018 AACR.

Citron kinase-dependent F-actin maintenance at midbody secondary ingression sites mediates abscission.

Abscission is the final step of cytokinesis whereby the intercellular bridge (ICB) linking the two daughter cells is cut. The ICB contains a structure called the midbody, required for the recruitment and organization of the abscission machinery. Final midbody severing is mediated by formation of secondary midbody ingression sites, where the ESCRT III component CHMP4B is recruited to mediate membrane fusion. It is presently unknown how cytoskeletal elements cooperate with CHMP4B to mediate abscission. Here, we show that F-actin is associated with midbody secondary sites and is necessary for abscission. F-actin localization at secondary sites depends on the activity of RhoA and on the abscission regulator citron kinase (CITK). CITK depletion accelerates loss of F-actin proteins at the midbody and subsequent cytokinesis defects are reversed by restoring actin polymerization. Conversely, midbody hyperstabilization produced by overexpression of CITK and ANLN is reversed by actin depolymerization. CITK is required for localization of F-actin and ANLN at the abscission sites, as well as for CHMP4B recruitment. These results indicate that control of actin dynamics downstream of CITK prepares the abscission site for the final cut.

ZIKA virus elicits P53 activation and genotoxic stress in human neural progenitors similar to mutations involved in severe forms of genetic microcephaly.

Epidemiological evidence from the current outbreak of Zika virus (ZIKV) and recent studies in animal models indicate a strong causal link between ZIKV and microcephaly. ZIKV infection induces cell-cycle arrest and apoptosis in proliferating neural progenitors. However, the mechanisms leading to these phenotypes are still largely obscure. In this report, we explored the possible similarities between transcriptional responses induced by ZIKV in human neural progenitors and those elicited by three different genetic mutations leading to severe forms of microcephaly in mice. We found that the strongest similarity between all these conditions is the activation of common P53 downstream genes. In agreement with these observations, we report that ZIKV infection increases total P53 levels and nuclear accumulation, as well as P53 Ser15 phosphorylation, correlated with genotoxic stress and apoptosis induction. Interestingly, increased P53 activation and apoptosis are induced not only in cells expressing high levels of viral antigens but also in cells showing low or undetectable levels of the same proteins. These results indicate that P53 activation is an early and specific event in ZIKV-infected cells, which could result from cell-autonomous and/or non-cell-autonomous mechanisms. Moreover, we highlight a small group of P53 effector proteins that could act as critical mediators, not only in ZIKV-induced microcephaly but also in many genetic microcephaly syndromes.

ASPM and CITK regulate spindle orientation by affecting the dynamics of astral microtubules.

Correct orientation of cell division is considered an important factor for the achievement of normal brain size, as mutations in genes that affect this process are among the leading causes of microcephaly. Abnormal spindle orientation is associated with reduction of the neuronal progenitor symmetric divisions, premature cell cycle exit, and reduced neurogenesis. This mechanism has been involved in microcephaly resulting from mutation of ASPM, the most frequently affected gene in autosomal recessive human primary microcephaly (MCPH), but it is presently unknown how ASPM regulates spindle orientation. In this report, we show that ASPM may control spindle positioning by interacting with citron kinase (CITK), a protein whose loss is also responsible for severe microcephaly in mammals. We show that the absence of CITK leads to abnormal spindle orientation in mammals and insects. In mouse cortical development, this phenotype correlates with increased production of basal progenitors. ASPM is required to recruit CITK at the spindle, and CITK overexpression rescues ASPM phenotype. ASPM and CITK affect the organization of astral microtubules (MT), and low doses of MT-stabilizing drug revert the spindle orientation phenotype produced by their knockdown. Finally, CITK regulates both astral-MT nucleation and stability. Our results provide a functional link between two established microcephaly proteins.