Tyrosine Phosphatases ε and α Perform Specific and Overlapping Functions in Regulation of Voltage-gated Potassium Channels in Schwann Cells

Abstract

Tyrosine phosphatases (PTPs) and ␣ are closely related and share several molecular functions, such as regulation of Src family kinases and voltage-gated potassium (Kv) channels. Functional interrelationships between PTP and PTP␣ and the mechanisms by which they regulate K ؉ channels and Src were analyzed in vivo in mice lacking either or both PTPs. Lack of either PTP increases Kv channel activity and phosphorylation in Schwann cells, indicating these PTPs inhibit Kv current amplitude in vivo. Open probability and unitary conductance of Kv channels are unchanged, suggesting an effect on channel number or organization. PTP␣ inhibits Kv channels more strongly than PTP; this correlates with constitutive association of PTP␣ with Kv2.1, driven by membranal localization of PTP␣. PTP␣, but not PTP, activates Src in sciatic nerve extracts, suggesting Src deregulation is not responsible exclusively for the observed phenotypes and highlighting an unexpected difference between both PTPs. Developmentally, sciatic nerve myelination is reduced transiently in mice lacking either PTP and more so in mice lacking both PTPs, suggesting both PTPs support myelination but are not fully redundant. We conclude that PTP and PTP␣ differ significantly in their regulation of Kv channels and Src in the system examined and that similarity between PTPs does not necessarily result in full functional redundancy in vivo. INTRODUCTIONReversible phosphorylation of tyrosine residues in proteins is a major mechanism for regulation of protein structure and function. Phosphorylation is regulated by the opposing activities of two superfamilies of enzymes-the protein tyrosine kinases (PTKs) and the protein tyrosine phosphatases (PTPs). More than 100 PTP genes are known in higher organisms, of which 38 belong to the classical, tyrosinespecific PTP family (Alonso et al., 2004;Andersen et al., 2004). The number of tyrosine phosphorylation sites in target proteins is significantly larger than the numbers of known PTPs or PTKs (Manning et al., 2002;Alonso et al., 2004). Each enzyme therefore interacts with several substrates, and a given substrate may be targeted by more than one kinase or phosphatase. Consequently, it is not always clear whether these enzymes fulfill molecular and physiological roles that are entirely independent or that overlap at least in part.Molecular and physiological studies of specific PTPs in recent years indicate that the issue of functional independence versus redundancy is complex. In many cases, specific substrates are currently known to be targeted by an individual PTP; in other cases, however, substrate sharing clearly occurs. A prime example of this is Src, which can be activated by dephosphorylation of its C-terminal inhibitory tyrosine by several PTPs, including PTP1B, SHP1, PTP␣, and PTP (Somani et al., 1997;Ponniah et al., 1999;Su et al., 1999;Bjorge et al., 2000;Gil-Henn and Elson, 2003;Pallen, 2003). Yet, targeting of a specific substrate by several PTPs may not necessarily imply that these PTPs are fully redundant in vi...

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“…Abnormalities in cellular morphology, together with reduced intracellular K ϩ content and increased Ca 2ϩ -activated K ϩ channel activity were also documented in red cells from PTP⑀-deficient mice (21). In addition, at an early post-natal age these mice displayed hypomyelination in the sciatic nerve axons, most likely as a result of hyperphosphorylation of voltage-gated potassium channel Kv2.1 in Schwann cells (22,23).…”

Oxidoreductase regulation of Kv currents in rat ventricle

“…In expressed Shaker channels (Kv1.5, Kv1.4), Src tyrosine kinase binds to the α-subunit and inhibits macroscopic currents [45,46], although rat Kv1.5 does not co-immunoprecipitate with Src as does the human isoform [46]. Finally, inhibition of specific tyrosine phosphatases has been shown to increase currents carried by Kv2.1 channels [47]. Given the diverse isoforms expressed in mammalian cells, it is clear more experimentation is needed to identify the specific tyrosine kinase pathways regulating cardiac Kv channels.…”

Association of Tyrosine Phosphatase Epsilon with Microtubules Inhibits Phosphatase Activity and Is Regulated by the Epidermal Growth Factor Receptor

“…cyt-PTPε is important for proper adhesion of osteoclasts to bone (7) and dephosphorylates and downregulates delayed-rectifier voltage-gated potassium channels in Schwann cells, thereby countering their activation by c-Src and c-Fyn. This effect correlates in vivo with delayed myelination of sciatic nerve axons in young PTPε-deficient mice (33,40,41) and with delayed optic nerve myelination in mice expressing a "substrate-trapping" mutant of RPTPε (29). cyt-PTPε can also downregulate signaling mediated by the mitogen-activated protein kinase (42, 45) and JAK-STAT (37-39) pathways and is required for proper functioning of mouse macrophages (35).…”

“…Immunoprecipitation and elution of FLAG-tagged PTPε expressed in 293 cells were carried out as described previously (41). The purity and amounts of eluted material were determined by gel electrophoresis and silver staining.…”