Characterization of the nuclear transport of a novel leucine-rich acidic nuclear protein-like protein

ANP32e Binds Histone H2A.Z in a Cell Cycle-Dependent Manner and Regulates Its Protein Stability in the Cytoplasm

ANP32e, a chaperone of H2A.Z, is receiving increasing attention because of its association with cancer growth and progression. An unanswered question is whether ANP32e regulates H2A.Z dynamics during the cell cycle; this could have clear implications for the proliferation of cancer cells. We confirmed that ANP32e regulates the growth of human U2OS cancer cells and preferentially interacts with H2A.Z during the G1 phase of the cell cycle. Unexpectedly, ANP32e does not mediate the removal of H2A.Z from chromatin, is not a stable component of the p400 remodeling complex and is not strongly associated with chromatin. Instead, most ANP32e is in the cytoplasm. Here, ANP32e preferentially interacts with H2A.Z in the G1 phase in response to an increase in H2A.Z protein abundance and regulates its protein stability. This G1-specific interaction was also observed in the nucleoplasm but was unrelated to any change in H2A.Z abundance. These results challenge the idea that ANP32e regulates the abundance of H2A.Z in chromatin as part of a chromatin remodeling complex. We propose that ANP32e is a molecular chaperone that maintains the soluble pool of H2A.Z by regulating its protein stability and acting as a buffer in response to cell cycle-dependent changes in H2A.Z abundance.

Karyopherin α deficiency contributes to human preimplantation embryo arrest

Preimplantation embryo arrest (PREMBA) is a common cause of female infertility and recurrent failure of assisted reproductive technology. However, the genetic basis of PREMBA is largely unrevealed. Here, using whole-exome sequencing data from 606 women experiencing PREMBA compared with 2,813 controls, we performed a population and gene-based burden test and identified a candidate gene, karyopherin subunit α7 (KPNA7). In vitro studies showed that identified sequence variants reduced KPNA7 protein levels, impaired KPNA7 capacity for binding to its substrate ribosomal L1 domain-containing protein 1 (RSL1D1), and affected KPNA7 nuclear transport activity. Comparison between humans and mice suggested that mouse KPNA2, rather than mouse KPNA7, acts as an essential karyopherin in embryonic development. Kpna2-/- female mice showed embryo arrest due to zygotic genome activation defects, recapitulating the phenotype of human PREMBA. In addition, female mice with an oocyte-specific knockout of Rsl1d1 recapitulated the phenotype of Kpna2-/- mice, demonstrating the vital role of substrate RSL1D1. Finally, complementary RNA (cRNA) microinjection of human KPNA7, but not mouse Kpna7, was able to rescue the embryo arrest phenotype in Kpna2-/- mice, suggesting mouse KPNA2 might be a homologue of human KPNA7. Our findings uncovered a mechanistic understanding for the pathogenesis of PREMBA, which acts by impairing nuclear protein transport, and provide a diagnostic marker for PREMBA patients.

The acidic protein rich in leucines Anp32b is an immunomodulator of inflammation in mice

ANP32B belongs to a family of evolutionary conserved acidic nuclear phosphoproteins (ANP32A-H). Family members have been described as multifunctional regulatory proteins and proto-oncogenic factors affecting embryonic development, cell proliferation, apoptosis, and gene expression at various levels. Involvement of ANP32B in multiple processes of cellular life is reflected by the previous finding that systemic gene knockout (KO) of Anp32b leads to embryonic lethality in mice. Here, we demonstrate that a conditional KO of Anp32b is well tolerated in adult animals. However, after immune activation splenocytes isolated from Anp32b KO mice showed a strong commitment towards Th17 immune responses. Therefore, we further analyzed the respective animals in vivo using an experimental autoimmune encephalomyelitis (EAE) model. Interestingly, an exacerbated clinical score was observed in the Anp32b KO mice. This was accompanied by the finding that animal-derived T lymphocytes were in a more activated state, and RNA sequencing analyses revealed hyperactivation of several T lymphocyte-associated immune modulatory pathways, attended by significant upregulation of Tfh cell numbers that altogether might explain the observed strong autoreactive processes. Therefore, Anp32b appears to fulfill a role in regulating adequate adaptive immune responses and, hence, may be involved in dysregulation of pathways leading to autoimmune disorders and/or immune deficiencies.

Highly polarized C-terminal transition state of the leucine-rich repeat domain of PP32 is governed by local stability

The leucine-rich repeat domain of PP32 is composed of five β-strand-containing repeats anchored by terminal caps. These repeats differ in sequence but are similar in structure, providing a means to connect topology, sequence, and folding pathway selection. Through kinetic studies of PP32, we find folding to be rate-limited by the formation of an on-pathway intermediate. Destabilizing core substitutions reveal a transition state ensemble that is highly polarized toward the C-terminal repeat and cap. To determine if this nucleus for folding corresponds to the most stable region of PP32, we monitored amide hydrogen exchange by NMR spectroscopy. Indeed, we find the highest protection to be biased toward the C terminus. Sequence manipulations that destabilize the C terminus spread out the transition state toward the middle of the protein. Consistent with results for helical ankyrin repeat proteins, these results suggest that local stabilities determine folding pathways.

Identification of cargo proteins specific for importin-β with importin-α applying a stable isotope labeling by amino acids in cell culture (SILAC)-based in vitro transport system

The human importin (Imp)-β family consists of 21 nucleocytoplasmic transport carrier proteins, which transport thousands of proteins (cargoes) across the nuclear envelope through nuclear pores in specific directions. To understand the nucleocytoplasmic transport in a physiological context, the specificity of cargoes for their cognate carriers should be determined; however, only a limited number of nuclear proteins have been linked to specific carriers. To address this biological question, we recently developed a novel method to identify carrier-specific cargoes. This method includes the following three steps: (i) the cells are labeled by stable isotope labeling by amino acids in cell culture (SILAC); (ii) the labeled cells are permeabilized, and proteins in the unlabeled cell extracts are transported into the nuclei of the permeabilized cells by a particular carrier; and (iii) the proteins in the nuclei are quantitatively identified by LC-MS/MS. The effectiveness of this method was demonstrated by the identification of transportin (Trn)-specific cargoes. Here, we applied this method to identify cargo proteins specific for Imp-β, which is a predominant carrier that exclusively utilizes Imp-α as an adapter for cargo binding. We identified candidate cargoes, which included previously reported and potentially novel Imp-β cargoes. In in vitro binding assays, most of the candidate cargoes bound to Imp-β in one of three binding modes: directly, via Imp-α, or via other cargoes. Thus, our method is effective for identifying a variety of Imp-β cargoes. The identified Imp-β and Trn cargoes were compared, ensuring the carrier specificity of the method and illustrating the complexity of these transport pathways.

Analysis of nucleocytoplasmic trafficking of the HuR ligand APRIL and its influence on CD83 expression

Dendritic cells (DC) are the most potent antigen-presenting cells of the immune system and are able to sensitize even naïve T cells. Mature DC are characterized by expression of CD83, a surface molecule that is proposed to be involved in efficient T cell activation. It has been recently shown that CD83 mRNA is transported from the nucleus to the cytoplasm in a HuR- and CRM1-dependent manner. Therefore we here investigated the impact of two known protein ligands of HuR, pp32 and APRIL, on CD83 expression. Both pp32 (ANP32A) and APRIL (ANP32B) are shuttle proteins, and it has been reported earlier that these HuR ligands can act as adaptors that link HuR and the CRM1-specific nuclear export pathway. By employing RNA interference (RNAi) technology we demonstrate that pp32 is dispensable for CD83 expression, whereas APRIL contributes to the nuclear export and subsequent translation of CD83 mRNA. Furthermore, we have determined the nuclear import signal (NLS) as well as the nuclear export signal (NES) of human APRIL. Moreover, we analyzed the status of phosphorylation of endogenous APRIL and identified threonine 244 to be an as yet unrecognized phosphate acceptor. Finally, we were able to show that phosphorylation of this specific amino acid residue regulates the nuclear export of APRIL. In sum, we report here the signal sequences in APRIL that mediate its intracellular trafficking and provide evidence that this protein ligand of HuR is an important player in the post-transcriptional regulation of CD83 expression by affecting the nucleocytoplasmic translocation of CD83 mRNA.

Anp32e/Cpd1 regulates protein phosphatase 2A activity at synapses during synaptogenesis

Anp32e/Cpd1, a member of the acidic nuclear phosphoprotein (Anp)32 family, is characterized by the presence of an amino terminal domain containing four leucine-rich repeats and a carboxyl-terminal low-compositional complexity acidic region. In previous studies performed to understand the biological role of Anp32e/Cpd1, we showed a predominant presence of Anp32e/Cpd1 in the nucleus. However, when Anp32e/Cpd1 is in the cytoplasm, it co-localizes spatially with protein phosphatase 2A (PP2A) near cell membranes, far from the synapses. In the present work, we show that Anp32e/Cpd1 is also present as a membrane-bound 74/76-kDa protein with a widespread distribution in the brain. We reveal that the expression, synthesis and half-life of this high-molecular-weight form of Anp32e/Cpd1 are spatially and temporally correlated with the cerebellar synaptogenesis period. We demonstrate that synaptic Anp32e/Cpd1 co-localizes, interacts and inhibits PP2A activity, and that phosphorylation of Anp32/Cpd1 is required for the Anp32e-PP2A interaction. Also, subcellular localization was shown with electronic microscopy. Finally, we examine Anp32e/Cpd1 and PP2A distribution in two ataxic mutant models, weaver and staggerer, and show that their co-localization in Purkinje cell dendrites depends on parallel fibre/Purkinje cell contacts. Based on these observations, we propose that Anp32e/Cpd1 mediates synaptogenesis process by modulating PP2A activity.

The Anp32 family of proteins containing leucine-rich repeats

Herein we describe the characteristic features of the Anp32 family represented by the cerebellar leucine-rich repeat protein (Lanp) and the cerebellar developmental-regulated protein 1 (Cpd1). The Anp32 family consists of 32 evolutionarily-conserved proteins and is included within the superfamily of leucine-rich repeat (LRR) proteins characterized by the presence of tandem arrays of a LRR, a structural motif implicated in the mediation of protein-protein interactions. We describe three novel human Anp32 proteins, reveal the evolutionary relationships of the members of the Anp32 family, provide insights into their biochemical and structural properties, and review their macromolecular interactions, substrate specificities, tissue distribution/expression patterns, and physiological and pathological roles. Recent findings indicate a conserved role of members of the Anp32 family during evolution in the modulation of cell signalling and transduction of gene expression to regulate the morphology and dynamics of the cytoskeleton, cell adhesion, neural development or cerebellar morphogenesis.

Generation and characterization of LANP/pp32 null mice

The leucine-rich acidic nuclear protein (LANP) belongs to a family of evolutionarily conserved proteins that are characterized by an amino-terminal domain rich in leucine residues followed by a carboxy-terminal acidic tail. LANP has been implicated in the regulation of a variety of cellular processes including RNA transport, transcription, apoptosis, vesicular trafficking, and intracellular signaling. Abundantly expressed in the developing cerebellum, this protein has also been hypothesized to play a role in cerebellar morphogenesis. LANP has been implicated in disease biology as well, both as a mediator of toxicity in spinocerebellar ataxia type 1 and as a tumor suppressor in cancers of the breast and prostate. To better understand the function of this multifaceted protein, we have generated mice lacking LANP. Surprisingly, these mice are viable and fertile. In addition we could not discern any derangements in any of the major organ systems, including the nervous system, which we have studied in detail. Overall our results point to a functional redundancy of LANP's function, most likely provided by its closely related family members.

Mapmodulin/leucine-rich acidic nuclear protein binds the light chain of microtubule-associated protein 1B and modulates neuritogenesis

We had previously described the leucine-rich acidic nuclear protein (LANP) as a candidate mediator of toxicity in the polyglutamine disease, spinocerebellar ataxia type 1 (SCA1). This was based on the observation that LANP binds ataxin-1, the protein involved in this disease, in a glutamine repeat-dependent manner. Furthermore, LANP is expressed abundantly in purkinje cells, the primary site of ataxin-1 pathology. Here we focused our efforts on understanding the neuronal properties of LANP. In undifferentiated neuronal cells LANP is predominantly a nuclear protein, requiring a bona fide nuclear localization signal to be imported into the nucleus. LANP translocates from the nucleus to the cytoplasm during the process of neuritogenesis, interacts with the light chain of the microtubule-associated protein 1B (MAP1B), and modulates the effects of MAP1B on neurite extension. LANP thus could play a key role in neuronal development and/or neurodegeneration by its interactions with microtubule associated proteins.

Differential expression of CPD1 during postnatal development in the mouse cerebellum

Several regulated mRNAs were detected by applying differential display to the mouse cerebellum during postnatal development. One cDNA fragment, referred to as CPD1 (GenBank U89345), was characterized and cloned. Northern blots showed maximum mRNA expression at postnatal day seven (P7). The mRNA encodes a protein of 260 amino acids. In situ RT-PCR showed that CPD1 is expressed mainly in granule cells and faintly in Purkinje cells. Polyclonal rabbit antibodies and oligobodies (oligonucleotide-based synthetic antibodies) revealed a protein of 34 kDa in Western blots. Immunohistochemistry showed not only marked nuclear staining but also mild cytoplasmic localization. Granule cells undergoing active division (P4) showed very little expression of CPD1 protein, which increases from P7 to P17. CPD1, affinity-purified using a chemically synthesized oligobody inhibits the activity of protein phosphatase PP2A but not protein phosphatase PP1. Differentiated PC12 cells also showed nuclear and cytoplasmic localization. Interestingly, maximal cytoplasmic CPD1/PP2A colocalization was observed near cell membrane regions that are far from growing neurites, and on growing cones. These results suggest that CPD1 might have an important role in cerebellar development.