Kathelijne Szekér

Interactor-guided dephosphorylation by protein phosphatase-1

Protein phosphatase-1 (PP1) is an essential enzyme for every eukaryotic cell and catalyzes more than half of all protein dephosphorylations at serine and threonine residues. The free catalytic subunit of PP1 shows little substrate selectivity but is tightly regulated in vivo by a large variety of structurally unrelated PP1-interacting proteins (PIPs). PIPs form highly specific dimeric or trimeric PP1 holoenzymes by acting as substrates, inhibitors, and/or substrate-specifiers. The surface of PP1 contains many binding sites for short PP1-docking motifs that are combined by PIPs to create a PP1-binding code that is universal, specific, degenerate, nonexclusive, and dynamic. These properties of the PP1-binding code can be used for the rational design of small molecules that disrupt subsets of PP1 holoenzymes and have a therapeutic potential.

The deletion of the phosphatase regulator NIPP1 causes progenitor cell expansion in the adult liver

The Ppp1r8 gene encodes NIPP1, a nuclear interactor of protein phosphatase PP1. The deletion of NIPP1 is embryonic lethal at the gastrulation stage, which has hampered its functional characterization in adult tissues. Here, we describe the effects of a conditional deletion of NIPP1 in mouse liver epithelial cells. Ppp1r8−/− livers developed a ductular reaction, that is, bile‐duct hyperplasia with associated fibrosis. The increased proliferation of biliary epithelial cells was at least partially due to an expansion of the progenitor cell compartment that was independent of liver injury. Gene‐expression analysis confirmed an upregulation of progenitor cell markers in the liver knockout livers but showed no effect on the expression of liver‐injury associated regulators of cholangiocyte differentiation markers. Consistent with an inhibitory effect of NIPP1 on progenitor cell proliferation, Ppp1r8−/− livers displayed an increased sensitivity to diet‐supplemented 3,5‐diethoxycarbonyl‐1,4‐dihydrocollidine, which also causes bile‐duct hyperplasia through progenitor cell expansion. In contrast, the liver knockouts responded normally to injuries (partial hepatectomy, single CCl4 administration) that are restored through proliferation of differentiated parenchymal cells. Our data indicate that NIPP1 does not regulate the proliferation of hepatocytes but is a suppressor of biliary epithelial cell proliferation, including progenitor cells, in the adult liver. Stem Cells 2016;34:2256–2262

Brief Report: The Deletion of the Phosphatase Regulator NIPP1 Causes Progenitor Cell Expansion in the Adult Liver

The Ppp1r8 gene encodes NIPP1, a nuclear interactor of protein phosphatase PP1. The deletion of NIPP1 is embryonic lethal at the gastrulation stage, which has hampered its functional characterization in adult tissues. Here, we describe the effects of a conditional deletion of NIPP1 in mouse liver epithelial cells. Ppp1r8−/− livers developed a ductular reaction, that is, bile‐duct hyperplasia with associated fibrosis. The increased proliferation of biliary epithelial cells was at least partially due to an expansion of the progenitor cell compartment that was independent of liver injury. Gene‐expression analysis confirmed an upregulation of progenitor cell markers in the liver knockout livers but showed no effect on the expression of liver‐injury associated regulators of cholangiocyte differentiation markers. Consistent with an inhibitory effect of NIPP1 on progenitor cell proliferation, Ppp1r8−/− livers displayed an increased sensitivity to diet‐supplemented 3,5‐diethoxycarbonyl‐1,4‐dihydrocollidine, which also causes bile‐duct hyperplasia through progenitor cell expansion. In contrast, the liver knockouts responded normally to injuries (partial hepatectomy, single CCl4 administration) that are restored through proliferation of differentiated parenchymal cells. Our data indicate that NIPP1 does not regulate the proliferation of hepatocytes but is a suppressor of biliary epithelial cell proliferation, including progenitor cells, in the adult liver. Stem Cells 2016;34:2256–2262

The protein phosphatase 1 regulator NIPP1 is essential for mammalian spermatogenesis

NIPP1 is one of the major nuclear interactors of protein phosphatase PP1. The deletion of NIPP1 in mice is early embryonic lethal, which has precluded functional studies in adult tissues. Hence, we have generated an inducible NIPP1 knockout model using a tamoxifen-inducible Cre recombinase transgene. The inactivation of the NIPP1 encoding alleles (Ppp1r8) in adult mice occurred very efficiently in testis and resulted in a gradual loss of germ cells, culminating in a Sertoli-cell only phenotype. Before the overt development of this phenotype Ppp1r8−/− testis showed a decreased proliferation and survival capacity of cells of the spermatogenic lineage. A reduced proliferation was also detected after the tamoxifen-induced removal of NIPP1 from cultured testis slices and isolated germ cells enriched for undifferentiated spermatogonia, hinting at a testis-intrinsic defect. Consistent with the observed phenotype, RNA sequencing identified changes in the transcript levels of cell-cycle and apoptosis regulating genes in NIPP1-depleted testis. We conclude that NIPP1 is essential for mammalian spermatogenesis because it is indispensable for the proliferation and survival of progenitor germ cells, including (un)differentiated spermatogonia.