Common obesity is primarily characterized by resistance to the actions of the hormone leptin. Mice deficient in protein tyrosine phosphatase 1B (PTP1B) are resistant to diabetes and diet-induced obesity, prompting us to further define the relationship between PTP1B and leptin in modulating obesity. Leptin-deficient (Lep(ob/ob)) mice lacking PTP1B exhibit an attenuated weight gain, a decrease in adipose tissue, and an increase in resting metabolic rate. Furthermore, PTP1B-deficient mice show an enhanced response toward leptin-mediated weight loss and suppression of feeding. Hypothalami from these mice also display markedly increased leptin-induced Stat3 phosphorylation. Finally, substrate-trapping experiments demonstrate that leptin-activated Jak2, but not Stat3 or the leptin receptor, is a substrate of PTP1B. These results suggest that PTP1B negatively regulates leptin signaling, and provide one mechanism by which it may regulate obesity.

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.

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.

Chondroitin sulfate proteoglycans (CSPGs) represent a major barrier to regenerating axons in the central nervous system (CNS), but the structural diversity of their polysaccharides has hampered efforts to dissect the structure-activity relationships underlying their physiological activity. By taking advantage of our ability to chemically synthesize specific oligosaccharides, we demonstrate that a sugar epitope on CSPGs, chondroitin sulfate-E (CS-E), potently inhibits axon growth. Removal of the CS-E motif significantly attenuates the inhibitory activity of CSPGs on axon growth. Furthermore, CS-E functions as a protein recognition element to engage receptors including the transmembrane protein tyrosine phosphatase PTPσ, thereby triggering downstream pathways that inhibit axon growth. Finally, masking the CS-E motif using a CS-Especific antibody reversed the inhibitory activity of CSPGs and stimulated axon regeneration in vivo. These results demonstrate that a specific sugar epitope within chondroitin sulfate polysaccharides can direct important physiological processes and provide new therapeutic strategies to regenerate axons after CNS injury.