Alternative Functions of Cell Cycle-Related and DNA Repair Proteins in Post-mitotic Neurons

Cell Cycle and Senescence Regulation by Podocyte Histone Deacetylase 1 and 2

SIGNIFICANCE STATEMENT

The loss of integrity of the glomerular filtration barrier results in proteinuria that is often attributed to podocyte loss. Yet how damaged podocytes are lost remains unknown. Germline loss of murine podocyte-associated Hdac1 and Hdac2 ( Hdac1/2 ) results in proteinuria and collapsing glomerulopathy due to sustained double-stranded DNA damage. Hdac1/2 deletion induces loss of podocyte quiescence, cell cycle entry, arrest in G1, and podocyte senescence, observed both in vivo and in vitro . Through the senescence secretory associated phenotype, podocytes secrete proteins that contribute to their detachment. These results solidify the role of HDACs in cell cycle regulation and senescence, providing important clues in our understanding of how podocytes are lost following injury.

BACKGROUND

Intact expression of podocyte histone deacetylases (HDAC) during development is essential for maintaining a normal glomerular filtration barrier because of its role in modulating DNA damage and preventing premature senescence.

METHODS

Germline podocyte-specific Hdac1 and 2 ( Hdac1 / 2 ) double-knockout mice were generated to examine the importance of these enzymes during development.

RESULTS

Podocyte-specific loss of Hdac1 / 2 in mice resulted in severe proteinuria, kidney failure, and collapsing glomerulopathy. Hdac1 / 2 -deprived podocytes exhibited classic characteristics of senescence, such as senescence-associated β-galactosidase activity and lipofuscin aggregates. In addition, DNA damage, likely caused by epigenetic alterations such as open chromatin conformation, not only resulted in podocyte cell-cycle entry as shown in vivo by Ki67 expression and by FUCCI-2aR mice, but also in p21-mediated cell-cycle arrest. Through the senescence secretory associated phenotype, the damaged podocytes secreted proinflammatory cytokines, growth factors, and matrix metalloproteinases, resulting in subsequent podocyte detachment and loss, evidenced by senescent podocytes in urine.

CONCLUSIONS

Hdac1 / 2 plays an essential role during development. Loss of these genes in double knockout mice leads to sustained DNA damage and podocyte senescence and loss.

Cdk5 and aberrant cell cycle activation at the core of neurodegeneration

Neurodegenerative diseases are caused by the progressive loss of specific neurons. The exact mechanisms of action of these diseases are unknown, and many studies have focused on pathways related to abnormal accumulation and processing of proteins, mitochondrial dysfunction, and oxidative stress leading to apoptotic death. However, a growing body of evidence indicates that aberrant cell cycle re-entry plays a major role in the pathogenesis of neurodegeneration. The activation of the cell cycle in mature neurons could be promoted by several signaling mechanisms, including c-Jun N-terminal kinases, p38 mitogen-activated protein kinases, and mitogen-activated protein kinase/extracellular signal-regulated kinase cascades; post-translational modifications such as Tau-phosphorylation; and DNA damage response. In all these events, implicated Cdk5, a proline-directed serine/threonine protein kinase, seems to be responsible for several cellular processes in neurons including axon growth, neurotransmission, synaptic plasticity, neuronal migration, and maintenance of neuronal survival. However, under pathological conditions, Cdk5 dysregulation may lead to cell cycle re-entry in post-mitotic neurons. Thus, Cdk5 hyperactivation, by its physiologic activator p25, hyper-phosphorylates downstream substrates related to neurodegenerative diseases. This review summarizes factors such as oxidative stress, DNA damage response, signaling pathway disturbance, and Ubiquitin proteasome malfunction contributing to cell cycle re-entry in post-mitotic neurons. It also describes how all these factors are linked to a greater or lesser extent with Cdk5. Thus, it offers a global vision of the function of cell cycle-related proteins in mature neurons with a focus on Cdk5 and how this protein contributes to the development of Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease by cell cycle activation.

Cis- and trans-resveratrol have opposite effects on histone serine-ADP-ribosylation and tyrosine induced neurodegeneration

Serum tyrosine levels increase during aging, neurocognitive, metabolic, and cardiovascular disorders. However, calorie restriction (CR) and sleep lower serum tyrosine levels. We previously showed that tyrosine inhibits tyrosyl-tRNA synthetase (TyrRS)-mediated activation of poly-ADP-ribose polymerase 1 (PARP1). Here, we show that histone serine-ADP-ribosylation is decreased in Alzheimer's Disease (AD) brains, and increased tyrosine levels deplete TyrRS and cause neuronal DNA damage. However, dopamine and brain-derived neurotrophic factor (BDNF) increase TyrRS and histone serine-ADP-ribosylation. Furthermore, cis-resveratrol (cis-RSV) that binds to TyrRS mimicking a 'tyrosine-free' conformation increases TyrRS, facilitates histone serine-ADP-ribosylation-dependent DNA repair, and provides neuroprotection in a TyrRS-dependent manner. Conversely, trans-RSV that binds to TyrRS mimicking a 'tyrosine-like' conformation decreases TyrRS, inhibits serine-ADP-ribosylation-dependent DNA repair, and induces neurodegeneration in rat cortical neurons. Our findings suggest that age-associated increase in serum tyrosine levels may effect neurocognitive and metabolic disorders and offer a plausible explanation for divergent results obtained in clinical trials using resveratrol.

p27, The Cell Cycle and Alzheimer´s Disease

The cell cycle consists of successive events that lead to the generation of new cells. The cell cycle is regulated by different cyclins, cyclin-dependent kinases (CDKs) and their inhibitors, such as p27^Kip1. At the nuclear level, p27^Kip1 has the ability to control the evolution of different phases of the cell cycle and oppose cell cycle progression by binding to CDKs. In the cytoplasm, diverse functions have been described for p27^Kip1, including microtubule remodeling, axonal transport and phagocytosis. In Alzheimer’s disease (AD), alterations to cycle events and a purported increase in neurogenesis have been described in the early disease process before significant pathological changes could be detected. However, most neurons cannot progress to complete their cell division and undergo apoptotic cell death. Increased levels of both the p27^Kip1 levels and phosphorylation status have been described in AD. Increased levels of Aβ42, tau hyperphosphorylation or even altered insulin signals could lead to alterations in p27^Kip1 post-transcriptional modifications, causing a disbalance between the levels and functions of p27^Kip1 in the cytoplasm and nucleus, thus inducing an aberrant cell cycle re-entry and alteration of extra cell cycle functions. Further studies are needed to completely understand the role of p27^Kip1 in AD and the therapeutic opportunities associated with the modulation of this target.