Two NEMO-like Ubiquitin-Binding Domains in CEP55 Differently Regulate Cytokinesis

Biallelic missense CEP55 variants cause prenatal MARCH syndrome

CEP55 encodes centrosomal protein 55 kDa, which plays a crucial role in mitosis, particularly cytokinesis. Biallelic CEP55 variants cause MARCH syndrome (multinucleated neurons, anhydramnios, renal dysplasia, cerebellar hypoplasia and hydranencephaly). Here, we describe a Japanese family with two affected siblings harboring novel compound heterozygous CEP55 variants, NM_001127182: c.[1357 C > T];[1358 G > A] p.[(Arg453Cys)];[(Arg453His)]. Both presented clinically with typical lethal MARCH syndrome. Although a combination of missense and nonsense variants has been reported previously, this is the first report of biallelic missense CEP55 variants. These variants biallelically affected the same amino acid, Arg453, in the last 40 amino acids of CEP55. These residues are functionally important for CEP55 localization to the midbody during cell division, and may be associated with severe clinical outcomes. More cases of pathogenic CEP55 variants are needed to establish the genotype-phenotype correlation.

High-Content RNAi Phenotypic Screening Unveils the Involvement of Human Ubiquitin-Related Enzymes in Late Cytokinesis

CEP55 is a central regulator of late cytokinesis and is overexpressed in numerous cancers. Its post-translationally controlled recruitment to the midbody is crucial to the structural coordination of the abscission sequence. Our recent evidence that CEP55 contains two ubiquitin-binding domains was the first structural and functional link between ubiquitin signaling and ESCRT-mediated severing of the intercellular bridge. So far, high-content screens focusing on cytokinesis have used multinucleation as the endpoint readout. Here, we report an automated image-based detection method of intercellular bridges, which we applied to further our understanding of late cytokinetic signaling by performing an RNAi screen of ubiquitin ligases and deubiquitinases. A secondary validation confirmed four candidate genes, i.e., LNX2, NEURL, UCHL1 and RNF157, whose downregulation variably affects interconnected phenotypes related to CEP55 and its UBDs, as follows: decreased recruitment of CEP55 to the midbody, increased number of midbody remnants per cell, and increased frequency of intercellular bridges or multinucleation events. This brings into question the Notch-dependent or independent contributions of LNX2 and NEURL proteins to late cytokinesis. Similarly, the role of UCHL1 in autophagy could link its function with the fate of midbody remnants. Beyond the biological interest, this high-content screening approach could also be used to isolate anticancer drugs that act by impairing cytokinesis and CEP55 functions.

Global Proteomics Analysis of Bone Marrow: Establishing Talin-1 and Centrosomal Protein of 55 kDa as Potential Molecular Signatures for Myelodysplastic Syndromes

Myelodysplastic syndrome (MDS) is a hematological disorder characterized by abnormal stem cell differentiation and a high risk of acute myeloid leukemia transformation. Treatment options for MDS are still limited, making the identification of molecular signatures for MDS progression a vital task. Thus, we evaluated the proteome of bone marrow plasma from patients (n = 28) diagnosed with MDS with ring sideroblasts (MDS-RS) and MDS with blasts in the bone marrow (MDS-EB) using label-free mass spectrometry. This strategy allowed the identification of 1,194 proteins in the bone marrow plasma samples. Polyubiquitin-C (UBC), moesin (MSN), and Talin-1 (TLN1) showed the highest abundances in MDS-EB, and centrosomal protein of 55 kDa (CEP55) showed the highest relative abundance in the bone marrow plasma of MDS-RS patients. In a follow-up, in the second phase of the study, expressions of UBC, MSN, TLN1, and CEP55 genes were evaluated in bone marrow mononuclear cells from 45 patients by using qPCR. This second cohort included only seven patients from the first study. CEP55, MSN, and UBC expressions were similar in mononuclear cells from MDS-RS and MDS-EB individuals. However, TLN1 gene expression was greater in mononuclear cells from MDS-RS (p = 0.049) as compared to MDS-EB patients. Irrespective of the MDS subtype, CEP55 expression was higher (p = 0.045) in MDS patients with abnormal karyotypes, while MSN, UBC, and TALIN1 transcripts were similar in MDS with normal vs. abnormal karyotypes. In conclusion, proteomic and gene expression approaches brought evidence of altered TLN1 and CEP55 expressions in cellular and non-cellular bone marrow compartments of patients with low-risk (MDS-RS) and high-risk (MDS-EB) MDSs and with normal vs. abnormal karyotypes. As MDS is characterized by disrupted apoptosis and chromosomal alterations, leading to mitotic slippage, TLN1 and CEP55 represent potential markers for MDS prognosis and/or targeted therapy.

Diagnosing and mitigating method-based avidity artifacts that confound polyubiquitin-binding assays

Polyubiquitination is a complex form of posttranslational modification responsible for the control of numerous cellular processes. Many ubiquitin-binding proteins recognize distinct polyubiquitin chain types, and these associations help drive ubiquitin-signaling pathways. There is considerable interest in understanding the specificity of ubiquitin-binding proteins; however, because of the multivalent nature of polyubiquitin, affinity measurements of these interactions that rely on affixing ubiquitin-binding proteins to a surface can display artifactual, method-dependent avidity, or "bridging." This artifact, which is distinct from biologically relevant, avid interactions with polyubiquitin, is commonplace in such polyubiquitin-binding measurements and can lead to dramatic overestimations of binding affinities for particular chain types, and thus, incorrect conclusions about specificity. Here, we use surface-based measurements of ubiquitin binding in three model systems to illustrate bridging and lay out practical ways of identifying and mitigating it. Specifically, we describe a simple fitting model that enables researchers to diagnose the severity of bridging artifacts, determine whether they can be minimized, and more accurately evaluate polyubiquitin-binding specificity.

Loss of Coiled-Coil Protein Cep55 Impairs Neural Stem Cell Abscission and Results in p53-Dependent Apoptosis in Developing Cortex

To build the brain, embryonic neural stem cells (NSCs) tightly regulate their cell divisions, undergoing a polarized form of cytokinesis that is poorly understood. Cytokinetic abscission is mediated by the midbody to sever the daughter cells at the apical membrane. In cell lines, the coiled-coil protein Cep55 was reported to be required for abscission. Mutations of Cep55 in humans cause a variety of cortical malformations. However, its role in the specialized divisions of NSCs is unclear. Here, we elucidate the roles of Cep55 in abscission and brain development. KO of Cep55 in mice causes abscission defects in neural and non-neural cell types, and postnatal lethality. The brain is disproportionately affected, with severe microcephaly at birth. Quantitative analyses of abscission in fixed and live cortical NSCs show that Cep55 acts to increase the speed and success rate of abscission, by facilitating ESCRT recruitment and timely microtubule disassembly. However, most NSCs complete abscission successfully in the absence of Cep55 Those that fail show a tissue-specific response: binucleate NSCs and neurons elevate p53, but binucleate fibroblasts do not. This leads to massive apoptosis in the brain, but not other tissues. Double KO of both p53 and Cep55 blocks apoptosis but only partially rescues Cep55 ^-/- brain size. This may be because of the persistent NSC cell division defects and p53-independent premature cell cycle exit. This work adds to emerging evidence that abscission regulation and error tolerance vary by cell type and are especially crucial in neural stem cells as they build the brain.SIGNIFICANCE STATEMENT During brain growth, embryonic neural stem cells (NSCs) must divide many times. In the last step of cell division, the daughter cell severs its connection to the mother stem cell, a process called abscission. The protein Cep55 is thought to be essential for recruiting proteins to the mother-daughter cell connection to complete abscission. We find that Cep55 mutants have very small brains with disturbed structure, but almost normal size bodies. NSC abscission can occur, but it is slower than normal, and failures are increased. Furthermore, NSCs that do fail abscission activate a signal for programmed cell death, whereas non-neural cells do not. Blocking this signal only partly restores brain growth, showing that regulation of abscission is crucial for brain development.

Centrosomal protein 55: A new paradigm in tumorigenesis

Centrosomal Protein 55 (Cep55), also known as c10orf3 and FLJ10540, was initially discovered as a major player in abscission, the final stage of cytokinesis. Subsequent studies have described its role in regulating the PI3K/AKT pathway, increasing cancer cell stemness, and promoting tumor formation. Clinically, Cep55 has been found to be overexpressed in many cancer types. Cep55 overexpression has been notably associated with tumor stage, tumor aggressiveness, poor prognosis, and metastasis. The present review discusses the role of Cep55 as a crucial biomarker and model in tumorigenesis.