Jonathan B. Hurov

Atypical PKC phosphorylates PAR-1 kinases to regulate localization and activity

The establishment and maintenance of cellular polarity are essential biological processes that must be maintained throughout the lifetime of eukaryotic organisms. The Par-1 protein kinases are key polarity determinants that have been conserved throughout evolution. Par-1 directs anterior-posterior asymmetry in the one-cell C. elegans embryo and the Drosophila oocyte. In mammalian cells, Par-1 may regulate epithelial cell polarity. Relevant substrates of Par-1 in these pathways are just being identified, but it is not yet known how Par-1 itself is regulated. Here, we demonstrate that human Par-1b (hPar-1b) interacts with and is negatively regulated by atypical PKC. hPar-1b is phosphorylated by aPKC on threonine 595, a residue conserved in Par-1 orthologs in mammals, worms, and flies. The equivalent site in hPar-1a, T564, is phosphorylated in vivo and by aPKC in vitro. Importantly, phosphorylation of hPar-1b on T595 negatively regulates the kinase activity and plasma membrane localization of hPar-1b in vivo. This study establishes a novel functional link between two central determinants of cellular polarity, aPKC and Par-1, and suggests a model by which aPKC may regulate Par-1 in polarized cells.

Absence of Apparent Phenotype in Mice Lacking Cdc25C Protein Phosphatase

ABSTRACT The Cdc25 family of protein phosphatases positively regulate the cell division cycle by activating cyclin-dependent protein kinases. In humans and rodents, three Cdc25 family members denoted Cdc25A, -B, and -C have been identified. The murine forms of Cdc25 exhibit distinct patterns of expression both during development and in adult mouse tissues. In order to determine unique contributions made by the Cdc25C protein phosphatase to embryonic and adult cell cycles, mice lacking Cdc25C were generated. We report thatCdc25C−/− mice are viable and do not display any obvious abnormalities. Among adult tissues in whichCdc25C is detected, its transcripts are most abundant in testis, followed by thymus, ovary, spleen, and intestine. Mice lackingCdc25C were fertile, indicating that Cdc25Cdoes not contribute an essential function during spermatogenesis or oogenesis in the mouse. T- and B-cell development was also found to be normal in Cdc25C −/− mice, andCdc25C −/− mouse splenic T and B cells exhibited normal proliferative responses in vitro. Finally, the phosphorylation status of Cdc2, the timing of entry into mitosis, and the cellular response to DNA damage were unperturbed in mouse embryo fibroblasts lacking Cdc25C. These findings indicate thatCdc25A and/or Cdc25B may compensate for loss ofCdc25C in the mouse.

Immune System Dysfunction and Autoimmune Disease in Mice Lacking Emk (Par-1) Protein Kinase

ABSTRACT Emk is a serine/threonine protein kinase implicated in regulating polarity, cell cycle progression, and microtubule dynamics. To delineate the role of Emk in development and adult tissues, mice lacking Emk were generated by targeted gene disruption.Emk−/− mice displayed growth retardation and immune cell dysfunction. Although B- and T-cell development were normal, CD4+T cells lacking Emk exhibited a marked upregulation of the memory marker CD44/pgp-1 and produced more gamma interferon and interleukin-4 on stimulation through the T-cell receptor in vitro. In addition, B-cell responses to T-cell-dependent and -independent antigen challenge were altered in vivo. AsEmk −/− animals aged, they developed splenomegaly, lymphadenopathy, membranoproliferative glomerulonephritis, and lymphocytic infiltrates in the lungs, parotid glands and kidneys. Taken together, these results demonstrate that the Emk protein kinase is essential for maintaining immune system homeostasis and that loss of Emk may contribute to autoimmune disease in mammals.

Loss of the Par-1b/MARK2 polarity kinase leads to increased metabolic rate, decreased adiposity, and insulin hypersensitivity in vivo

Obesity is a major factor central to the development of insulin resistance and type 2 diabetes. The identification and characterization of genes involved in regulation of adiposity, insulin sensitivity, and glucose uptake are key to the design and development of new drug therapies for this disease. In this study, we show that the polarity kinase Par-1b/MARK2 is required for regulating glucose metabolism in vivo. Mice null for Par-1b were lean, insulin hypersensitive, resistant to high-fat diet-induced weight gain, and hypermetabolic. 18F-FDG microPET and hyperinsulinemic–euglycemic clamp analyses demonstrated increased glucose uptake into white and brown adipose tissue, but not into skeletal muscle of Par-1b null mice relative to wild-type controls. Taken together, these data indicate that Par-1b is a regulator of glucose metabolism and adiposity in the whole animal and may be a valuable drug target for the treatment of both type 2 diabetes and obesity.

The Par-1/MARK family of protein kinases: From polarity to metabolism

The Par-1 protein kinases are conserved from yeast to man and belong to a subfamily of kinases that includes the energy sensor and metabolic regulator, AMPK. Par-1 is regulated by LKB1 and atypical PKC and has been shown in multiple organisms and cell types to be critical for regulation of cellular polarity. Recent studies using knockout mice have revealed several surprising physiological functions for Par-1b/MARK2/EMK1. Our recent study shows that Par-1b regulates metabolic rate, adiposity and insulin sensitivity. This is the first study to implicate these kinases in metabolic functions akin to those previously defined for AMPK. Conversely, another series of recent publications now implicate AMPK in regulation of polarity. Here we discuss the metabolic phenotype seen in Par-1b deficient mice and the synthesis of several findings that link Par-1 and AMPK to a degree that has not been previously appreciated.

Loss of Par-1a/MARK3/C-TAK1 kinase leads to reduced adiposity, resistance to hepatic steatosis, and defective gluconeogenesis

ABSTRACT Par-1 is an evolutionarily conserved protein kinase required for polarity in worms, flies, frogs, and mammals. The mammalian Par-1 family consists of four members. Knockout studies of mice implicate Par-1b/MARK2/EMK in regulating fertility, immune homeostasis, learning, and memory as well as adiposity, insulin hypersensitivity, and glucose metabolism. Here, we report phenotypes of mice null for a second family member (Par-1a/MARK3/C-TAK1) that exhibit increased energy expenditure, reduced adiposity with unaltered glucose handling, and normal insulin sensitivity. Knockout mice were protected against high-fat diet-induced obesity and displayed attenuated weight gain, complete resistance to hepatic steatosis, and improved glucose handling with decreased insulin secretion. Overnight starvation led to complete hepatic glycogen depletion, associated hypoketotic hypoglycemia, increased hepatocellular autophagy, and increased glycogen synthase levels in Par-1a−/− but not in control or Par-1b−/− mice. The intercrossing of Par-1a−/− with Par-1b−/− mice revealed that at least one of the four alleles is necessary for embryonic survival. The severity of phenotypes followed a rank order, whereby the loss of one Par-1b allele in Par-1a−/− mice conveyed milder phenotypes than the loss of one Par-1a allele in Par-1b−/− mice. Thus, although Par-1a and Par-1b can compensate for one another during embryogenesis, their individual disruption gives rise to distinct metabolic phenotypes in adult mice.