Are renal ciliopathies (replication) stressed out?Trends in Cell Biology

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Authors
Gisela G. Slaats, Rachel H. Giles
Year
2015
DOI
10.1016/j.tcb.2015.03.005
Subject
Cell Biology

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Are renal ciliopathies (replic al

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TICB-1133; No. of Pages 3with the cell cycle checkpoint proteins ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3related protein (ATR) during activated DDR signaling [4] and depletion of Cep164 increases phosphorylation of

H2AX in healthy, undamaged cells in vitro and in vivo would mimic a loss-of-cilia condition (Figure 1). Ciliation was rescued in Nek8-depleted cells treated with CDK inhibitors, although architectural changes of 3D renal spheroids were not rescued, possibly due to cell migration defects [1]. It is important to note that, while NEK8 depletion results in enhanced CDK activity, the role of other NPHP proteins in

CDK activity remains to be tested [1]. Furthermore, pharmaceutical reduction of CDK activity by inhibitors of CDK requires further investigation to determine its role in ameliorating cell cycle defects associated with depletion of other

NPHP-proteins. Cell cycle progression can also be indirectly 0962-8924/  2015 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tcb.2015.03.005

Corresponding author: Giles, R.H. (r.giles@umcutrecht.nl).

Keywords: cilia; kidney cysts; DNA damage; replication stress; cyclin-dependent kinase; fibrosis; cell cycle.Gisela G. Slaats and Rachel H. Giles

Department of Nephrology and Hypertension, University Medic

Juvenile renal failure is commonly caused by the ciliopathy nephronophthisis (NPHP). Since all NPHP genes regulate cilia function, it has been assumed that NPHP onset is due to cilia loss. However, recent data suggest that DNA damage caused by replication stress, possibly concomitant with or upstream of cilia dysfunction, causes NPHP.

Renal ciliopathies: not just loss of cilia

The leading genetic causes of pediatric as well as adult kidney failure can be traced back to an organelle called the cilium. Cilia loss of function is thought to be the cellular defect responsible for two classes of renal ciliopathies: the common autosomal dominant polycystic kidney disease (ADPKD), which affects adults; and the rare, recessive pediatric/juvenile ciliopathies collectively referred to as

NPHP-related ciliopathies (NPHP-RCs) (Box 1). For example, all 19 NPHP-associated genes reported to date encode gene products known to localize to primary cilia and regulate ciliary structure or function. In addition, many NPHPassociated proteins possess extraciliary functions that potentially contribute to the development of disease, and these functions have only recently been explored.

Recent molecular evidence argues that the nuclear/DNA damage response (DDR) functions of some NPHP proteins may be critical in disease onset or progression. DDR signaling includes mechanisms to detect DNA damage (lesions), signal the presence of damage, arrest the cell cycle, and promote repair. The question is raised of whether nuclear effects of NPHP gene mutations are upstream, downstream, or independent of cilia dysfunction [1–3]. Initial studies of the NPHP genes ZNF423 (NPHP14), and

CEP164 (NPHP15) revealed that they colocalize with the

DDR proteins SC-35 (a splicing factor), checkpoint kinase 1 (CHK1), and Tat-interactive protein 60 (TIP60) in the nucleus [2]. Additionally, morpholino injection of zebrafish embryos targeting znf423 or cep164 induced sensitivity to

DNA damage-causing reagents [2], suggesting that a role for NPHP proteins in DDR signaling may contribute to the pathophysiology of NPHP. Moreover, CEP164 interactsation) stressed out?

Center Utrecht, Utrecht, The Netherlands [5]. Supporting this hypothesis, a Sdccag8 (Nphp10) mouse model showed increased DNA damage signaling, as evidenced by increased phosphorylation of ATM and H2AX in vivo and in vitro and disturbed cell cycle progression [3]. These findings strengthen the link between DNA damage signaling and NPHP but fail to address the molecular mechanism of how enhanced DDR signaling is initiated in NPHP. Is DDR signaling activated by phosphorylation of proteins followed by repair and cell cycle arrest or are DNA breaks actually accumulating?

Replication stress at the root of the stalk

Recent studies with cells depleted for CEP164, SDCCAG8, and NEK8 are beginning to address the relation between the DDR and NPHP onset [1–3,5]. Mutation in one of the less-frequently mutated NPHP proteins, the ciliary kinase

NEK8 (NPHP9), which associates with NPHP and polycystic kidney disease, results in replication stress as evidenced by accumulating DNA damage in S-phase cells and increased replication fork defects in mouse embryonic fibroblasts derived from Nek8 mutant jck mice [1]. Replication stress is the slowing or stalling of the replication fork progression and/or DNA synthesis, which can lead to decreased cell survival and genome stability [1]. Stalled replication forks are unstable and can collapse, leading to double-strand break formation and chromosomal rearrangements. Moreover, immunohistochemistry of kidneys from 3-week-old jck mice shows higher basal levels of phosphorylated H2AX compared with wild type siblings [1], suggesting that these effects are early.

What are the molecular mechanisms linking NPHP proteins to DNA damage? Depletion of NEK8 results in DDR signaling through enhanced cyclin A-associated cyclin-dependent kinase (CDK) activity, leading to replication stress, and activated ATR–CHK1 signaling upon replication stress induction in S phase. Similarly, live-cell imaging of CEP164depleted cells showed delayed S phase progression that could be rescued with wild type but not CEP164 patient alleles [5], suggesting that these variants are causative for cell cycle impairment in patients. Since cells in S phase do not have cilia, an increase of unciliated cells in S phaseTrends in Cell Biology xx (2015) 1–3 1 regulated through ciliary signaling mediated by growth factor receptors and mechanosensation [6]. Therefore,

NPHP-associated DDR signaling could interfere with normal cell cycle progression via both nuclear DNA damageassociated cell cycle control and cilia-associated cell cycle