Renal tumour suppressor function of the Birt-Hogg-Dubé syndrome gene product folliculin.

BACKGROUND: Renal cell carcinoma (RCC) comprises five major molecular and histological subtypes. The Birt-Hogg-Dubé (BHD) syndrome is a hereditary human cancer syndrome that predisposes affected individuals to develop renal carcinoma of nearly all subtypes, in addition to benign fibrofolliculomas, and pulmonary and renal cysts. BHD is caused by loss-of-function mutations in the folliculin (FLCN) protein. The molecular function of FLCN is still largely unknown; opposite and conflicting evidence of the role of FLCN in mammalian target of rapamycin signalling/phosphorylated ribosomal protein S6 (p-S6) activation had recently been reported. RESULTS AND METHODS: Here, the expression pattern of murine Flcn was described, and it was observed that homozygous disruption of Flcn results in embryonic lethality early during development. Importantly, heterozygous animals manifest early preneoplastic kidney lesions, devoid of Flcn expression, that progress towards malignancy, including cystopapillary adenomas. A bona fide tumour suppressor activity of FLCN was confirmed by nude mouse xenograft assays of two human RCC cell lines with either diminished or re-expressed FLCN. It was observed that loss of FLCN expression leads to context-dependent effects on S6 activation. Indeed, solid tumours and normal kidneys show decreased p-S6 upon diminished FLCN expression. Conversely, p-S6 is found to be elevated or absent in FLCN-negative renal cysts. CONCLUSION: In accordance with clinical data showing distinct renal malignancies arising in BHD patients, in this study FLCN is shown as a general tumour suppressor in the kidney.

Plumbing in the embryo: developmental defects of the urinary tracts.

Kidney and urinary tract malformations are among the most frequent developmental defects identified in newborns. Ranging from asymptomatic to neonatal lethal, these malformations represent an important clinical challenge. Recent progress in understanding the developmental origin of urinary tract defects in the mouse and other animal models suggests a new framework for the interpretation of these defects in humans. Gene inactivation studies in mice provided invaluable information on the formation of the Wolffian duct, a central component of embryonic renal development, on ureter and kidney induction as well as on distal ureter maturation. All three developmental processes are crucial for normal urinary tract morphogenesis. A failure to complete these developmental steps is responsible for a spectrum of kidney and urinary tract malformations including renal agenesis, renal dysplasia, vesicoureteral reflux, hydroureter, hydronephrosis and ureterocele. Surprisingly, distal ureter maturation, the process by which the ureter is displaced from the Wolffian duct to its final position within the bladder wall, has only recently been characterized at the morphological level. Anomalies in this process are emerging as a major source of urinary tract developmental defects. This review is aimed at bridging the current knowledge on the morphological and molecular events identified in the mouse, together with clinical observations of urinary tract malformation in humans.

Developmental expression and function analysis of protein tyrosine phosphatase receptor type D in oligodendrocyte myelination.

Receptor protein tyrosine phosphatases (RPTPs) are extensively expressed in the central nervous system (CNS), and have distinct spatial and temporal patterns in different cell types during development. Previous studies have demonstrated possible roles for RPTPs in axon outgrowth, guidance, and synaptogenesis. In the present study, our results revealed that protein tyrosine phosphatase, receptor type D (PTPRD) was initially expressed in mature neurons in embryonic CNS, and later in oligodendroglial cells at postnatal stages when oligodendrocytes undergo active axonal myelination process. In PTPRD mutants, oligodendrocyte differentiation was normal and a transient myelination delay occurred at early postnatal stages, indicating the contribution of PTPRD to the initiation of axonal myelination. Our results also showed that the remyelination process was not affected in the absence of PTPRD function after a cuprizone-induced demyelination in adult animals.

The protein tyrosine phosphatase PRL-2 interacts with the magnesium transporter CNNM3 to promote oncogenesis.

The three PRL (phosphatases of regenerating liver) protein tyrosine phosphatases (PRL-1, -2 and -3) have been identified as key contributors to metastasis in several human cancers, yet the molecular basis of their pro-oncogenic property is unclear. Among the subfamily of PRL phosphatases, overexpression of PRL-2 in breast cancer cells has been shown to promote tumor growth by a mechanism that remains to be uncovered. Here we show that PRL-2 regulates intracellular magnesium levels by forming a functional heterodimer with the magnesium transporter CNNM3. We further reveal that CNNM3 is not a phosphorylated substrate of PRL-2, and that the interaction occurs through a loop unique to the CBS pair domains of CNNM3 that exists only in organisms having PRL orthologs. Supporting the role of PRL-2 in cellular magnesium transport is the observation that PRL-2 knockdown results in a substantial decrease of cellular magnesium influx. Furthermore, in PRL-2 knockout mice, serum magnesium levels were significantly elevated as compared with control animals, indicating a pivotal role for PRL-2 in regulating cellular magnesium homeostasis. Although the expression levels of CNNM3 remained unchanged after magnesium depletion of various cancer cell lines, the interaction between endogenous PRL-2 and CNNM3 was markedly increased. Importantly, xenograft tumor assays with CNNM3 and a mutant form that does not associate with PRL-2 confirm that CNNM3 is itself pro-oncogenic, and that the PRL-2/CNNM3 association is important for conferring transforming activities. This finding is further confirmed from data in human breast cancer tissues showing that CNNM3 levels correlate positively with both PRL-2 expression and the tumor proliferative index. In summary, we demonstrate that oncogenic PRL-2 controls tumor growth by modulating intracellular magnesium levels through binding with the CNNM3 magnesium transporter.

Receptor protein tyrosine phosphatase sigma inhibits axonal regeneration and the rate of axon extension.

Transgenic mice lacking receptor protein tyrosine phophatase-sigma (RPTPsigma), a type IIa receptor protein tyrosine phosphatase, exhibit severe neural developmental deficits. Continued expression of RPTPsigma in the adult suggests that it plays a functional role in the mature nervous system. To determine if RPTPsigma might influence axonal regeneration, the time course of regeneration following facial nerve crush in wild-type and RPTPsigma (-/-) mice was compared. Mice lacking RPTPsigma exhibited an accelerated rate of functional recovery. Immunocytochemical examination of wild-type neurons in cell culture showed RPTPsigma protein in the growth cone. To determine if RPTPsigma affects the ability of a neuron to extend an axon, the rate of axon growth in neuronal cultures derived from wild-type and RPTPsigma (-/-) embryonic mice was compared. RPTPsigma did not affect the rate of axon initiation, but the rate of axon extension is enhanced in neurons obtained from RPTPsigma (-/-) mice. These findings indicate that RPTPsigma slows axon growth via a mechanism intrinsic to the neuron and identify a role for RPTPsigma regulating axonal regeneration by motoneurons.

Impaired learning with enhanced hippocampal long-term potentiation in PTPdelta-deficient mice.

Protein tyrosine phosphatase delta (PTPdelta) is a receptor-type PTP expressed in the specialized regions of the brain including the hippocampal CA2 and CA3, B lymphocytes and thymic medulla. To elucidate the physiological roles of PTPdelta, PTPdelta-deficient mice were produced by gene targeting. It was found that PTPdelta-deficient mice were semi-lethal due to insufficient food intake. They also exhibited learning impairment in the Morris water maze, reinforced T-maze and radial arm maze tasks. Interestingly, although the histology of the hippocampus appeared normal, the magnitudes of long-term potentiation (LTP) induced at hippocampal CA1 and CA3 synapses were significantly enhanced in PTPdelta-deficient mice, with augmented paired-pulse facilitation in the CA1 region. Thus, it was shown that PTPdelta plays important roles in regulating hippocampal LTP and learning processes, and that hippocampal LTP does not necessarily positively correlate with spatial learning ability. To our knowledge, this is the first report of a specific PTP involved in the regulation of synaptic plasticity or in the processes regulating learning and memory.