Cyclase-associated protein interacts with actin filament barbed ends to promote depolymerization and formin displacement

Cyclase-associated protein (CAP) has emerged as a central player in cellular actin turnover, but its molecular mechanisms of action are not yet fully understood. Recent studies revealed that the N terminus of CAP interacts with the pointed ends of actin filaments to accelerate depolymerization in conjunction with cofilin. Here, we use in vitro microfluidics-assisted TIRF microscopy to show that the C terminus of CAP promotes depolymerization at the opposite (barbed) ends of actin filaments. In the absence of actin monomers, full-length mouse CAP1 and C-terminal halves of CAP1 (C-CAP1) and CAP2 (C-CAP2) accelerate barbed end depolymerization. Using mutagenesis and structural modeling, we show that these activities are mediated by the WH2 and CARP domains of CAP. In addition, we observe that CAP collaborates with profilin to accelerate barbed end depolymerization and that these effects depend on their direct interaction, providing the first known example of CAP-profilin collaborative effects in regulating actin. In the presence of actin monomers, CAP1 attenuates barbed end growth and promotes formin dissociation. Overall, these findings demonstrate that CAP uses distinct domains and mechanisms to interact with opposite ends of actin filaments and drive turnover. Further, they contribute to the emerging view of actin barbed ends as sites of dynamic molecular regulation, where numerous proteins compete and cooperate with each other to tune polymer dynamics, similar to the rich complexity seen at microtubule ends.

Role of serum CAP1 protein in the diagnosis of patients with first-time acute myocardial infarction

The dysregulation of adenylate cyclase-associated protein 1 (CAP1) is associated with a variety of inflammatory conditions. Here, we aimed to assess the role of serum CAP1 protein in predicting acute myocardial infarction (AMI), and to explore its effect and mechanism in vascular endothelial cells injury. ELISA was utilized to detected CAP1 protein expression in serum from 70 patients with first-time AMI at 0, 6, 12, 24, 48 hours and 7 days of the onset of chest pain. Receiver operating characteristic (ROC) curve analysis was administered to analyze the diagnostic power of CAP1 for AMI. The CCK-8 and 5-BrdU assays were applied to measure cell proliferation and inflammation in a model of oxidized low-density lipoprotein (ox-LDL) induced human umbilical vein endothelial cells (HUVEC). Luciferase reporter gene assay and Western blotting were used to assess the activity of NF-κB pathway. Results showed that serum CAP1 protein expression was upregulated in patients with first-time AMI, its expression was highest at 12 hours of the onset of chest pain. CAP1 protein was positively associated with the levels of cTnI and ox-LDL. CAP1 showed a relatively high diagnostic accuracy in patients with first-time AMI compared with cTnI, and CAP1 combined with cTnI had superior diagnostic value than CAP1 and cTnI alone. The expression of CAP1 protein was increased in supernatants of ox-LDL induced HUVEC in a dose- and time-dependent manner. CAP1 inhibited cell proliferation but promoted inflammation, and induced the activation of NF-κB pathway in vitro. To sum up, increased serum CAP1 expression might serve as a novel diagnostic biomarker for patients with first-time AMI, the mechanism might be related to its induction of NF-κB pathway activation causing abnormal proliferation and inflammation and thus mediating vascular endothelial cell injury.

Regulatory roles of alternative splicing at Ezh2 gene in mouse oocytes

BACKGROUND

Enhancer of zeste homologue 2 (EZH2), the core member of polycomb repressive complex 2 (PRC2), has multiple splicing modes and performs various physiological functions. However, function and mechanism of alternative splicing at Ezh2 exon 3 in reproduction are unknown.

METHODS

We generated Ezh2^Long and Ezh2^Short mouse models with different point mutations at the Ezh2 exon 3 alternative splicing site, and each mutant mouse model expressed either the long or the short isoform of Ezh2. We examined mutant mouse fertility and oocyte development to assess the function of Ezh2 alternative splicing at exon 3 in the reproductive system.

RESULTS

We found that Ezh2^Long female mice had normal fertility. However, Ezh2^Short female mice had significantly decreased fertility and obstructed oogenesis, with compromised mitochondrial function in Ezh2^Short oocytes. Interestingly, increased EZH2 protein abundance and accumulated H3K27me3 were observed in Ezh2^Short oocytes.

CONCLUSIONS

Our results demonstrate that correct Ezh2 alternative splicing at exon 3 is important for mouse oogenesis.

Cofilin regulates actin network homeostasis and microvilli length in mouse oocytes

How multiple actin networks coexist in a common cytoplasm while competing for a shared pool of monomers is still an ongoing question. This is exemplified by meiotic maturation in the mouse oocyte, which relies on the dynamic remodeling of distinct cortical and cytoplasmic F-actin networks. Here, we show that the conserved actin-depolymerizing factor cofilin is activated in a switch-like manner upon meiosis resumption from prophase arrest. Interfering with cofilin activation during maturation resulted in widespread elongation of microvilli, while cytoplasmic F-actin was depleted, leading to defects in spindle migration and polar body extrusion. In contrast, cofilin inactivation in metaphase II-arrested oocytes resulted in a shutdown of F-actin dynamics, along with a dramatic overgrowth of the polarized actin cap. However, inhibition of the Arp2/3 complex to promote actin cap disassembly elicited ectopic microvilli outgrowth in the polarized cortex. These data establish cofilin as a key player in actin network homeostasis in oocytes and reveal that microvilli can act as a sink for monomers upon disassembly of a competing network.

Upregulation of cyclase-associated actin cytoskeleton regulatory protein 2 in epithelial ovarian cancer correlates with aggressive histologic types and worse outcomes

OBJECTIVE

Cyclase-associated actin cytoskeleton regulatory protein 2 (CAP2) regulates actin dynamics to control cell cycles and cell migration. CAP2 overexpression contributes to cancer progression in several tumor types; however, the role of CAP2 expression in ovarian cancer remains unclear. This study aimed to clarify the significance of CAP2 expression in epithelial ovarian tumor.

METHODS

We evaluated CAP2 expression in ovarian cancer cell lines using quantitative real-time polymerase chain reaction, western blotting and immunocytochemistry and examined the effect of CAP2 silencing in migration and proliferation assays. CAP2 immunohistochemistry was conducted using tissue specimens from 432 ovarian carcinoma patients; a further 55 borderline and benign 65 lesions were analyzed. CAP2 expression levels were defined as low, intermediate or high, for correlation analysis with clinicopathological factors.

RESULTS

CAP2 expression was significantly higher in cell lines from Type II ovarian cancer than in those in Type I, and knockdown of CAP2 showed decreased migration and proliferation. Higher levels of CAP2 expression in human tissues were associated with Type II histology, residual lesion, lymph node metastasis, ascites cytology and higher clinical stage. High CAP2 expression levels were observed in 26 (23.4%) of 111 Type II ovarian cancers and in 16 (5.0%) of 321 Type I cancers but not in any borderline or benign lesions. Multivariate analyses showed that CAP2 expression in ovarian cancer is an independent prognostic factor for recurrence-free survival (P = 0.019).

CONCLUSION

CAP2 expression is upregulated in aggressive histologic types of epithelial ovarian cancer and serves as a novel prognostic biomarker for patient survival.

AIP1 and Cofilin control the actin dynamics to modulate the asymmetric division and cytokinesis in mouse oocytes

Actin-interacting protein 1 (AIP1), also known as WD repeat-containing protein 1 (WDR1), is ubiquitous in eukaryotic organisms, and it plays critical roles in the dynamic reorganization of the actin cytoskeleton. However, the biological function and mechanism of AIP1 in mammalian oocyte maturation is still largely unclear. In this study, we demonstrated that AIP1 boosts ADF/Cofilin activity in mouse oocytes. AIP1 is primarily distributed around the spindle region during oocyte maturation, and its depletion impairs meiotic spindle migration and asymmetric division. The knockdown of AIP1 resulted in the gathering of a large number of actin-positive patches around the spindle region. This effect was reduced by human AIP1 (hAIP1) or Cofilin (S3A) expression. AIP1 knockdown also reduced the phosphorylation of Cofilin near the spindle, indicating that AIP1 interacts with ADF/Cofilin-decorated actin filaments and enhances filament disassembly. Moreover, the deletion of AIP1 disrupts Cofilin localization in metaphase I (MI) and induces cytokinesis defects in metaphase II (MII). Taken together, our results provide evidence that AIP1 promotes actin dynamics and cytokinesis via Cofilin in the gametes of female mice.

Mechanism of synergistic actin filament pointed end depolymerization by cyclase-associated protein and cofilin

The ability of cells to generate forces through actin filament turnover was an early adaptation in evolution. While much is known about how actin filaments grow, mechanisms of their disassembly are incompletely understood. The best-characterized actin disassembly factors are the cofilin family proteins, which increase cytoskeletal dynamics by severing actin filaments. However, the mechanism by which severed actin filaments are recycled back to monomeric form has remained enigmatic. We report that cyclase-associated-protein (CAP) works in synergy with cofilin to accelerate actin filament depolymerization by nearly 100-fold. Structural work uncovers the molecular mechanism by which CAP interacts with actin filament pointed end to destabilize the interface between terminal actin subunits, and subsequently recycles the newly-depolymerized actin monomer for the next round of filament assembly. These findings establish CAP as a molecular machine promoting rapid actin filament depolymerization and monomer recycling, and explain why CAP is critical for actin-dependent processes in all eukaryotes.

The cofilin family proteins are actin disassembly factors but the disassembly mechanism is poorly understood. Here authors show that cyclase-associated-protein (CAP) works in synergy with cofilin to accelerate actin filament depolymerization by nearly 100-fold and reveal how CAP destabilizes the interface between terminal actin subunits.

First person – Zhe-Long Jin

First Person is a series of interviews with the first authors of a selection of papers published in Journal of Cell Science, helping early-career researchers promote themselves alongside their papers. Zhe-Long Jin is the first author on ‘CAP1­-mediated actin cycling via ADF/cofilin proteins is essential for asymmetric division in mouse oocytes’, published in Journal of Cell Science. Zhe-Long is a PhD student in the laboratory of Prof. Nam-Hyung Kim at Chungbuk National University, Cheongju, Korea, working on the actin cytoskeleton and actin dynamics during female meiosis.