Drosophila and leech models of nervous system development demonstrate that protein tyrosine phosphatase (PTP) receptors regulate developmental neurite outgrowth. Whether PTP receptors regulate neurite outgrowth in adult systems or in regenerative states remains unknown. The leukocyte common antigenrelated (LAR) receptor is known to be present in rodent dorsal root ganglion (DRG) neurons; therefore, the well established model of postcrush sciatic nerve regeneration was used to test the hypothesis that LAR is required for neurite outgrowth in the adult mammalian nervous system. In uninjured sciatic nerves, no differences in nerve morphology and sensory function were detected between wild-type and LAR-deficient littermate transgenic mice. Sciatic nerve crush resulted in increased LAR protein expression in DRG neurons. In addition, nerve injury led to an increase in the proportion of LAR protein isoforms known to have increased binding affinity to neurite-promoting lamininnidogen complexes. Two weeks after nerve crush, morphological analysis of distal nerve segments in LAR-deficient transgenic mice demonstrated significantly decreased densities of myelinated fibers, decreased axonal areas, and increased myelin/axon area ratios compared with littermate controls. Electron microscopy analysis revealed a significant twofold reduction in the density of regenerating unmyelinated fibers in LARϪ/Ϫ nerves distal to the crush site. Sensory testing at the 2 week time point revealed a corresponding 3 mm lag in the proximal-to-distal progression of functioning sensory fibers along the distal nerve segment. These studies introduce PTP receptors as a major new gene family regulating regenerative neurite outgrowth in vivo in the adult mammalian system. Key words: LAR; protein tyrosine phosphatase receptor; PTP; nerve regeneration; sciatic nerve; dorsal root ganglion; neurite outgrowthEvidence that protein tyrosine phosphatase (PTP) receptors regulate neurite outgrowth in vivo has been derived primarily from insect models (for review, see Desai et al

Previous work indicating that nerve growth factor (NGF) protein loops 2 and 4 interact with TrkA receptors raise the possibility that small molecule mimetics corresponding to TrkA-interacting domains that have NGF agonist activity can be developed. We applied our previously developed strategy of dimeric peptidomimetics to address the hypothesis that loop 4 small molecule dimeric mimetics would activate TrkA-related signal transduction and mimic NGF neurotrophic effects in a structure-specific manner. A loop 4 cyclized peptide dimer demonstrated NGF-like neurotrophic activity, whereas peptides with scrambled sequence, added or substituted residues, or cyclized in monomeric form were inactive. Activity was blocked by the TrkA inhibitors K252a and AG879 but not by NGF p75 receptor blocking antibody. Dimeric, but not monomeric, peptides partially blocked NGF activity. This profile was consistent with that of a NGF partial agonist. ERK and AKT phosphorylation was stimulated only by biologically active peptides and was blocked by K252a. The ERK inhibitor U0126 blocked the neurite-but not the survival-promoting activity of both NGF and active peptide. These studies support the proof of concept that small molecule NGF loop 4 mimetics can activate NGF signaling pathways and can mimic death-preventing and neurite-promoting effects of NGF. This finding will guide the rational design of NGF single-domain mimetics and contribute to elucidating NGF signal transduction mechanisms.Nerve growth factor (NGF) 1 acts via TrkA and p75 receptors to regulate neuronal survival, promote neurite outgrowth, and up-regulate certain neuronal functions such as mediation of pain and inflammation (1-5). These actions suggest that NGF agonists or antagonists might be useful in regulating these processes (6 -8). Factors limiting therapeutic applications of the NGF protein include restricted penetration of the central nervous system and the poor medicinal properties characteristic of most proteins (9, 10). The development of small molecule mimetics with favorable chemical properties that function as agonists or antagonists that mimic or inhibit NGF functions in the appropriate biological context will be critical in advancing potential in vivo applications of NGF. Moreover, in settings in which NGF might contribute to neuronal death, pain, or inflammatory mechanisms, NGF antagonists may be particularly relevant. Creation of single domain NGF mimetics will also constitute a powerful approach for linking specific NGF domains with specific patterns of intracellular signal transduction.A NGF mimetic (agonist or antagonist) would be expected to contain structural determinants of one or more NGF active sites that interact with NGF receptors. Multiple techniques have been used to deduce which domains of the NGF protein interact with NGF receptors (11). A peptide mapping approach in which synthetic peptides with sequences corresponding to specific NGF regions were tested for their ability to inhibit NGF activity pointed to residues 29 -35 as a key active ...

Inhibition of protein-tyrosine phosphatases (PTPs) counterbalancing protein-tyrosine kinases (PTKs) offers a strategy for augmenting PTK actions. Conservation of PTP catalytic sites limits development of specific PTP inhibitors. A number of receptor PTPs, including the leukocyte common antigen-related (LAR) receptor and PTP, contain a wedge-shaped helix-loop-helix located near the first catalytic domain. Helix-loop-helix domains in other proteins demonstrate homophilic binding and inhibit function; therefore, we tested the hypothesis that LAR wedge domain peptides would exhibit homophilic binding, bind to LAR, and inhibit LAR function. Fluorescent beads coated with LAR or PTP wedge peptides demonstrated PTP-specific homophilic binding, and LAR wedge peptide-coated beads precipitated LAR protein. Administration of LAR wedge Tat peptide to PC12 cells resulted in increased proliferation, decreased cell death, increased neurite outgrowth, and augmented Trk PTK-mediated responses to nerve growth factor (NGF), a phenotype matching that found in PC12 cells with reduced LAR levels. PTP wedge Tat peptide had no effect on PC12 cells but blocked the PTP-dependent phenotype of neurite outgrowth of retinal ganglion neurons on a PTP substrate, whereas LAR wedge peptide had no effect. The survival-and neurite-promoting effect of the LAR wedge peptide was blocked by the Trk inhibitor K252a, and reciprocal co-immunoprecipitation demonstrated LAR/ TrkA association. The addition of LAR wedge peptide inhibited LAR co-immunoprecipitation with TrkA, augmented NGF-induced activation of TrkA, ERK, and AKT, and in the absence of exogenous NGF, induced activation of TrkA, ERK, and AKT. PTP wedge domain peptides provide a unique PTP inhibition strategy and offer a novel approach for augmenting PTK function.Within intracellular signaling networks, protein-tyrosine kinases (PTKs) 3 can be counterbalanced by protein-tyrosine phosphatases (PTPs) (1-4). Trk-family neurotrophin PTK receptors undergo ligandinduced tyrosine transphosphorylation and downstream activation of mitogen-activated protein kinase and phosphatidylinositol 3-kinase/ AKT signaling pathways (5, 6). A fundamental area of Trk signaling that remains to be investigated is the identification of the PTPs that directly or indirectly associate with Trk receptors and regulate their signaling. The leukocyte common antigen-related (LAR) receptor is a prototypical receptor PTP containing tandem catalytic domains (D1 and D2) in its cytoplasmic region with D1, constituting the primary catalytic site, and D2, conferring regulatory function (7). LAR is expressed by neurons and regulates neuronal survival and neurite outgrowth (2, 8 -14). The physiological ligand(s) for LAR in mammalian systems is unknown, although a LAR ectodomain isoform binds LAR homophilically and promotes neurite outgrowth (15). The LAR enzymatic substrates within neurons remain to be established. Several lines of evidence point to LAR as a candidate PTP modulating Trk phosphorylation. First, LAR and Trk are co-expresse...