p27(Kip1) participates in the regulation of endoreplication in differentiating chick retinal ganglion cells

Targeting cyclin-dependent kinase 4/6 as a therapeutic approach for mucosal melanoma

Mucosal melanoma is a rare but devastating subtype of melanoma which typically has a worse prognosis than other melanoma subtypes. Large-scale next-generation sequencing studies, including our recent research, have also proved that the molecular landscape and potential oncogenic drivers of mucosal melanoma remain distinct from that of cutaneous melanoma. Recently, a number of selective cyclin-dependent kinase 4 (CDK4)/6 inhibitors have been approved for clinical application in breast cancer or entered phase III clinical trial in other solid tumors. Additionally, we have revealed that the dysregulation of cell cycle progression, caused by CDK4 amplification, is a key genetic feature in half of mucosal melanoma and targeting of CDK4 in selected mucosal melanoma patients is a potentially promising direction for precision cancer treatment by using molecular-characterized mucosal melanoma patient-derived-xenograft models. This review summarizes the current literature regarding CDK4/6 dysregulation in mucosal melanoma, preclinical and clinical studies of CDK4/6 inhibitors and potential combinational strategies in treating mucosal melanoma.

Cell Cycle Re-entry in the Nervous System: From Polyploidy to Neurodegeneration

Terminally differentiated cells of the nervous system have long been considered to be in a stable non-cycling state and are often considered to be permanently in G0. Exit from the cell cycle during development is often coincident with the differentiation of neurons, and is critical for neuronal function. But what happens in long lived postmitotic tissues that accumulate cell damage or suffer cell loss during aging? In other contexts, cells that are normally non-dividing or postmitotic can or re-enter the cell cycle and begin replicating their DNA to facilitate cellular growth in response to cell loss. This leads to a state called polyploidy, where cells contain multiple copies of the genome. A growing body of literature from several vertebrate and invertebrate model organisms has shown that polyploidy in the nervous system may be more common than previously appreciated and occurs under normal physiological conditions. Moreover, it has been found that neuronal polyploidization can play a protective role when cells are challenged with DNA damage or oxidative stress. By contrast, work over the last two and a half decades has discovered a link between cell-cycle reentry in neurons and several neurodegenerative conditions. In this context, neuronal cell cycle re-entry is widely considered to be aberrant and deleterious to neuronal health. In this review, we highlight historical and emerging reports of polyploidy in the nervous systems of various vertebrate and invertebrate organisms. We discuss the potential functions of polyploidization in the nervous system, particularly in the context of long-lived cells and age-associated polyploidization. Finally, we attempt to reconcile the seemingly disparate associations of neuronal polyploidy with both neurodegeneration and neuroprotection.

Growth Arrest Triggers Extra-Cell Cycle Regulatory Function in Neurons: Possible Involvement of p27kip1 in Membrane Trafficking as Well as Cytoskeletal Regulation

Cell cycle regulation is essential for the development of multicellular organisms, but many cells in adulthood, including neurons, exit from cell cycle. Although cell cycle-related proteins are suppressed after cell cycle exit in general, recent studies have revealed that growth arrest triggers extra-cell cycle regulatory function (EXCERF) in some cell cycle proteins, such as p27(kip1), p57(kip2), anaphase-promoting complex/cyclosome (APC/C), and cyclin E. While p27 is known to control G1 length and cell cycle exit via inhibition of cyclin-dependent kinase (CDK) activities, p27 acquires additional cytoplasmic functions in growth-arrested neurons. Here, we introduce the EXCERFs of p27 in post-mitotic neurons, mainly focusing on its actin and microtubule regulatory functions. We also show that a small amount of p27 is associated with the Golgi apparatus positive for Rab6, p115, and GM130, but not endosomes positive for Rab5, Rab7, Rab8, Rab11, SNX6, or LAMTOR1. p27 is also colocalized with Dcx, a microtubule-associated protein. Based on these results, we discuss here the possible role of p27 in membrane trafficking and microtubule-dependent transport in post-mitotic cortical neurons. Collectively, we propose that growth arrest leads to two different fates in cell cycle proteins; either suppressing their expression or activating their EXCERFs. The latter group of proteins, including p27, play various roles in neuronal migration, morphological changes and axonal transport, whereas the re-activation of the former group of proteins in post-mitotic neurons primes for cell death.

Role of oxidative stress in Retinitis pigmentosa: new involved pathways by an RNA-Seq analysis

Retinitis pigmentosa (RP) is a very heterogeneous inherited ocular disorder group characterized by progressive retinal disruption. Retinal pigment epithelium (RPE) degeneration, due to oxidative stress which arrests the metabolic support to photoreceptors, represents one of the principal causes of RP. Here, the role of oxidative stress in RP onset and progression was analyzed by a comparative whole transcriptome analysis of human RPE cells, treated with 100 µg/ml of oxLDL and untreated, at different time points. Experiment was thrice repeated and performed on Ion Proton^TM sequencing system. Data analysis, including low quality reads trimming and gene expression quantification, was realized by CLC Genomics Workbench software. The whole analysis highlighted 14 clustered "macro-pathways" and many sub-pathways, classified by selection of 5271 genes showing the highest alteration of expression. Among them, 23 genes were already known to be RP causative ones (15 over-expressed and 8 down-expressed), and their enrichment and intersection analyses highlighted new 77 candidate related genes (49 over-expressed and 28 down-expressed). A final filtering analysis then highlighted 29 proposed candidate genes. This data suggests that many new genes, not yet associated with RP, could influence its etiopathogenesis.

Strand-specific CpG hemimethylation, a novel epigenetic modification functional for genomic imprinting

Imprinted genes are regulated by allele-specific differentially DNA-methylated regions (DMRs). Epigenetic methylation of the CpGs constituting these DMRs is established in the germline, resulting in a 5-methylcytosine-specific pattern that is tightly maintained in somatic tissues. Here, we show a novel epigenetic mark, characterized by strand-specific hemimethylation of contiguous CpG sites affecting the germline DMR of the murine Peg3, but not Snrpn, imprinted domain. This modification is enriched in tetraploid cortical neurons, a cell type where evidence for a small proportion of formylmethylated CpG sites within the Peg3-controlling DMR is also provided. Single nucleotide polymorphism (SNP)-based transcriptional analysis indicated that these epigenetic modifications participate in the maintainance of the monoallelic expression pattern of the Peg3 imprinted gene. Our results unexpectedly demonstrate that the methylation pattern observed in DMRs controlling defined imprinting regions can be modified in somatic cells, resulting in a novel epigenetic modification that gives rise to strand-specific hemimethylated domains functional for genomic imprinting. We anticipate the existence of a novel molecular mechanism regulating the transition from fully methylated CpGs to strand-specific hemimethylated CpGs.