Receptor Protein Tyrosine Phosphatases (RPTPs) and axon growth

PTPs and neuronal differentiation. Most aspects of cellular growth and differentiation, including neuronal differentiation, are regulated through changes in tyrosine phosphorylation. These changes, in turn, are regulated by the opposing activities of two classes of enzyme: protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Although the study of PTPs has lagged behind that of PTKs, it has become clear in recent years that PTPs are of major importance in regulating neuronal differentiation, particularly axon growth and guidance (Bixby, 2000). PTPs can be grouped into two major structural classes: cytoplasmic PTPs and transmembrane or receptor PTPs (RPTPs). Cytoplasmic PTPs are likely important in numerous aspects of neuronal differentiation (Chen et al., 2002), but the best evidence for roles in axon growth and guidance relates to RPTPs.

Classes of RPTPs. RPTPs have been grouped into structural types on the basis of sequence motifs in their extracellular domains (ECDs; Fig. 1). The ECDs of type IIa RPTPs are similar to Ig superfamily cell adhesion molecules (CAMs) in motif structure, comprising Ig repeats and fibronectin type III repeats. The ECDs of type III RPTPs consist entirely of various numbers of fibronectin type III repeats. The presence of functional motifs from CAMs is consistent with a role of these RPTPs in the regulation of neuronal adhesion and axon growth. Type IIa and type III RPTPs are of particular interest for the study of axon growth and guidance (Bixby, 2000; Johnson and Van Vactor, 2003).




Figure 1. Structural types of RPTPs. The type IIa phosphatases, which have ECDs like those of immunoglobulin superfamily CAMs, include PTP-δ and its relatives in mammals (LAR, PTP- ). The type III RPTPs, the ECDs of which comprise FN type III repeats, include PTPRO. Key: CA-like, carbonic anhydrase-like domain; CS, chondroitin sulfate side chains; FN type III, fibronectin type III domain; Ig, immunoglobulin domain; MAM, meprin/A2/mu domain; PTPase, tyrosine phosphatase catalytic domain.



Type IIa RPTPs and axon growth. Genetic evidence in flies implicates the type IIa RPTPs in axon guidance. Deletion of Type IIa RPTPs leads to defects in the guidance of motor axons, photoreceptor axons, and CNS axons. Interestingly, both ligand and receptor functions of these RPTPs are important in vivo (Schindelholz et al., 2001).

There are 3 type IIa RPTPs in vertebrates, called LAR, PTP-δ , and PTP-σ . Perturbation studies have shown that PTP-σ affects the growth of chick retinal ganglion cell axons on basement membranes (Ledig et al., 1999b), and suggest that this RPTP has a negative role in retinal axon guidance. For PTP-δ in vitro evidence suggests a role in promotion of neurite growth (Wang and Bixby, 1999), and as an attractive guidance cue for growth cones (Sun et al., 2000b). PTP-δ may also promote retinal axon growth in vivo, and LAR has been implicated in peripheral and central nerve regeneration.

The roles of vertebrate type IIa RPTPs in vivo are being explored. Mice null for PTP-σ have clear neurological defects, and some of these appear to be related to axon growth and/or regeneration. Mice in which the LAR gene is deleted exhibit a deficiency in regrowth of sciatic nerve axons following nerve crush. The phenotype of null mutations in PTP-δ is currently unknown, but mice lacking the PTP-δ catalytic domains exhibit a neurological phenotype. Because neurological phenotypes of type IIa knockouts (based on limited observations) are less severe than anticipated, it is possible that the three vertebrate type IIa RPTPs (PTP-σ , PTP-δ , and LAR) exhibit overlapping functions, or can assume such functions in the absence of one member of the family. Important interactions may also exist with Type III RPTPs. As well, some pathfinding defects may be found in these mutants only after intensive and extensive study. The data so far suggest that vertebrate type IIa RPTPs play important roles in axon growth and guidance, but the specific roles played by individual RPTPs and the underlying mechanisms are unclear. This is an active area of investigation in our lab.

Type III RPTPs and axon growth. Drosophila type III RPTPs (PTP99A, PTP10D, PTP52F) can also regulate motor axon guidance as well as guidance of CNS axons across the embryonic midline. Although some interactions between Type III and Type IIa RPTPs appear cooperative in nature, Type III RPTPs can also act in opposition to the Type IIa RPTPs (Schindelholz et al., 2001). An unresolved issue is the relative contributions of receptor functions and potential ligand functions of these RPTPs. PTP10D and PTP99A are widely expressed in the CNS, and it is unclear whether axons are misrouted in mutants because growth cones express these RPTPs or because they come in contact with their (extrinsic) extracellular domains, or both.

Little is known concerning the involvement of type III RPTPs in vertebrate axon growth. A relative of PTP10D has been identified in the nervous systems of chicks and mammals. This RPTP is now known as PTPRO. Expression of PTPRO is strongest in the nervous system, is downregulated late in development, and is selectively on the axons and growth cones of projection neurons (Bodden and Bixby, 1996; Ledig et al., 1999a; Beltran et al., 2003). Our lab has shown that the recombinant extracellular domain of PTPRO is an anti-adhesive, neurite-inhibitory, repulsive guidance cue, in direct contrast to the observations for PTP- (Stepanek et al., 2001). These results could help to explain the functional opposition between type III and type IIa RPTPs seen in fly genetic experiments. The ligand/receptor interactions of type III RPTPs, including PTPRO, are unknown; we are working on this.

Signaling through RPTPs. Genetic studies in the fly suggest a specific role of type IIa RPTPs in axon guidance through regulation of proteins controlling actin polymerization, such as Rac and Ena. Other important interactions of type IIa RPTPs in signal transduction could be with receptor PTKs, or with elements of cadherin- and integrin-based adhesion complexes. Little is known about signaling through type III RPTPs, but CD148/DEP-1 can interact with p120ctn and with the Met RTK.

The relevant ligands for RPTPs are generally not known, though some can bind homophically. PTP-δ is a homophilic CAM (Wang and Bixby, 1999). Though evidence is consistent with a role for homophilic binding in axon growth, no physiological significance has yet been demonstrated for these binding interactions. Heterophilic ligands for Type IIa RPTPs have also been identified. The identities of ligands for type III RPTPs are unclear. The catalytic phosphatase activity of RPTPs is likely to be important in neural functions. However, the extracellular domains of type IIa RPTPs, for example, have the motif structure of Ig superfamily CAMs, which can signal to cells without possessing catalytic activity.

Signaling through RPTPs, like signaling through PTKs, appears to involve homodimerization. Although studies with some RPTPs suggest that PTPase activity is downregulated by ligand binding/dimerization, studies with other RPTPs suggest the opposite. RPTPs also form heterodimers with other RPTPs. Such heterodimeric interactions suggest the possibility of signaling cross-talk between RPTPs. Of course, ligand binding may function not to alter catalytic activity, but to recruit RPTPs into signaling complexes in which their activity can be functionally expressed through increased access to relevant substrates. A major class of relevant substrate is the group of receptor PTKs (Bixby, 2001; Beltran and Bixby, 2003). We are investigating intracellular interactions between PTPRO and the Trk receptor PTKs, as part of an effort to understand how such signaling complexes might operate.

References:



Copyright 2003, Dr. Vance Lemmon and Dr. John Bixby. All Rights Reserved