Biological Functions of Shps

Genetic models for murine Shp1 and for Shp2 orthologs in mouse, Drosophila, and C. elegans have been invaluable for defining the biological functions of the Shps. The phe-notypes of Shp-deficient organisms will be described briefly here; more complete descriptions are available in several other reviews.

The motheaten Phenotype

Two naturally occurring point mutations exist in the murine Shp1 gene, each of which causes abnormal splicing of Shp1 transcripts [91,92]. The motheaten (me) allele generates an early frameshift; consequently, me/me mice are protein null. The motheaten viable (mev) allele encodes two aberrant Shp1 proteins; one with a small deletion, the other with a small insertion in the PTP domain. Together, these retain only about 20% of wild-type (WT) Shp1 activity, demonstrating the essential role for PTP activity in Shp1 function.

The phenotypes of me/me and mev/mev mice differ only in severity, with me/me mice succumbing to abnormalities earlier (2-3 weeks) than mev/mev (9-12 weeks) [93-98]. For this reason, we use me to refer generically to Shp1-deficient mice. The me phenotype derives its name from patchy hair loss, which gives the mice a motheaten appearance. The hair loss, in turn, results from sterile dermal abscesses consisting of neu-trophils. Inflammation also is prominent elsewhere, including the joints, liver, and lungs. The latter leads to the early demise of me mice, due to severe interstitial pneumonitis caused by accumulations of alveolar macrophages and neutrophils. The macrophage population in me mice is expanded and exhibits abnormal differentiation, with a dramatic increase in CD5+ monocytoid cells and a decrease in cells expressing tissue/marginal zone macrophage markers [99,100]. Some dendritic cell populations are increased, whereas others are diminished [100]; osteoclast numbers and function are enhanced, leading to osteopenia in me mice [101]. Shp1-deficient mice on either a nude or rag knockout background still develop inflammatory disease [102], and the disease can be reproduced by transplantation of bone marrow cells and prevented by treatment with Mac-1 antibodies [103]. Thus, lymphoid cells are dispensable, and defects in the myeloid lineage are critical, if not sufficient, for development of the me inflammatory syndrome.

Although from the host standpoint, the myeloid defects present the gravest problems, every other hematopoietic lineage is affected by Shp1 deficiency [93-98]. The thymus undergoes premature involution, possibly due to defective homing of a thymic accessory cell [104,105]. Indeed, thy-mocytes and peripheral T cells lacking Shp1 actually exhibit increased mitogenesis in response to T-cell antigen receptor (TCR) stimulation [106,107]. Consistent with enhanced responsiveness, crosses to TCR transgenic mice show that Shp1 deficiency lowers the threshold for thymic selection [108-111]. Normal B (B2 cell) lymphopoiesis is reduced, but there is a marked increase in B1a (CD5+) cells. The remaining B cells appear hyperactivated and produce autoantibodies [93,98]. Proliferation [112,113] and calcium flux [114] in response to B-cell antigen receptor (BCR) stimulation are reportedly enhanced in me lymphocytes, and the response threshold of a transgenic BCR is lowered in me mice [114]. Natural killer (NK) cell activity is decreased [115], but the remaining NK cells show enhanced lytic activity [116]. Motheaten mice are anemic, probably due to chronic hemolysis, although their erythroid progenitors are hyper-responsive to erythropoietin (EPO) [117-119]. Increased numbers of certain mast cell populations also have been reported [120,121]. Because the lymphohematopoietic system is highly interactive, identifying which me abnormalities are primary (i.e., cell autonomous) defects, as opposed to secondary consequences of the myeloid defects, has posed major (and ongoing) challenges. Nevertheless, many of the abnormalities have been ascribed to loss of negative regulation of specific signaling pathways in the absence of Shp1.

Invertebrate Models of Shp2 Deficiency

Csw is a maternal effect mutation affecting the so-called terminal class pathway [13], which is initiated by the RTK Torso and controls embryonic head and tail development [122]. Loss-of-function mutations in csw were found to have a phenotype similar to, although less severe than, torso mutations, which provided the first evidence of a positive (i.e., signal enhancing) function for an Shp2 ortholog [13]. Csw also is a required positive component of the sevenless, breathless (fibroblast growth factor receptor [FGFR]), and Drosophila epidermal growth factor receptor (EGFR; DER) pathways [14,123,124].

Ptp-2 functions in at least two RTK signaling pathways. In vulval development, which is controlled by the EGFR ortholog Let-23, ptp-2 mutation alone has no obvious effect. However, ptp-2 deficiency suppresses the multivulva pheno-type induced by mutation of the negative regulator lin-15. Interestingly, lin-15 mutations cause Let-23 activation even in the absence of the EGFR ligand, Lin-3, implying that Ptp-2 may play an important role in a ligand-independent Let-23 pathway [15]. Ptp-2 also is important for signaling by the FGFR ortholog EGL-15 [125] and has an essential role in an as yet unidentified pathway required for oogenesis [15].

Functions of Vertebrate Shp2

Studies of Xenopus embryogenesis provided initial evidence of a role for Shp2 in vertebrate development [10], Expression of dominant-negative Shp2 disrupts gastrula-tion, causing severe tail truncations reminiscent of, but less severe than, the effects of dominant-negative FGFR. Dominant-negative Shp2 also blocks FGF-induced meso-derm induction and elongation of ectodermal explants. Recently, two activated mutants of Shp2 (similar to those found in Noonan syndrome) were found to induce elongation of ectodermal explants in the absence of exogenous FGF. Activated mutants do not, by themselves, induce meso-dermal gene expression, although they potentiate induction of the Erk pathway by FGF [71].

Targeted mutations of murine Shp2 indicate a key role for Shp2 in mammalian development. Homozyotic deletion of either Exon 2 [126] or Exon 3 [127] results in early embryonic lethality. Exon 3 (Ex3)-/- embryos die between E8.5 and E10.5, with a range of abnormalities consistent with defective gastrulation and mesodermal differentiation [127,128]. These defects resemble the effects of dominantnegative Shp2 (and FGFR) mutants in Xenopus and the effects of vertebrate FGFR mutations [129]. Chimeric analyses using Ex3-/- embryonal stem (ES) cells reveal an essential role for Shp2 in limb development and branchial arch formation, two other pathways controlled by FGFR signaling [130,131]. Studies of hematopoietic differentiation in Ex3-/- ES cells [132] and in chimeric mice [130] indicate a stringent requirement for Shp2 in the earliest progenitors, consistent with a role for Shp2 in Kit (stem cell factor receptor) signaling [133].

The Ex3 mutation generates a truncated Shp2 protein that lacks part of its N-SH2 domain and is expressed at « 25% of WT levels in Ex3-/- cells. Due to the N-SH2 deletion, however, the Ex3 mutant is activated markedly; consequently, Ex3-/- cells actually have increased Shp2 activity [127,132], although the mutant protein is defective at localizing to at least some signaling pathways [134]. This finding raised the possibility that some effects of Ex3 deletion might be neomorphic.

Recent studies of other targeted mutations argue against this possibility. Ex2-/- embryos die earlier (« E6-E6.5) than Ex3-/- embryos. Despite earlier reports (which used antibodies against the N terminus to assess expression), Ex2-/-mice also express an N-terminally truncated protein. However, for reasons that are unclear, this mutant protein is not hyperactivated. More convincingly, a variant Ex2 mutation (Ex2*), in which a strong splice acceptor sequence was introduced into the targeting construct, is, in fact protein null, and Ex2*-/- embryos also die at E6 to E6.5. Total Shp2 deficiency causes defective inner cell mass expansion, due to markedly increased apoptosis (W. Yang and B.G.N., manuscript in preparation). The timing and nature of the lethality of Ex2*-/- embryos are consistent with roles for Shp2 in FGF-4 [135] and/or 01 integrin [136] signaling.

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