Signaling by Cytoplasmic Tyrosine Kinases in Leukocytes

Investigator: Clifford A. Lowell, MD, PhD
Sponsor: NIH National Institute of Allergy and Infectious Disease

Location(s): United States


The Src-family tyrosine kinases play pivotal roles in initiating intracellular signaling from a diverse repertoire of receptors on innate immune leukocytes. The Src-family kinases initiate intracellular signaling from these receptors by phosphorylating specific tyrosine residues within ITAM (Immunoreceptor Tyrosine-based Activation Motifs) domains that are either part of the cytoplasmic domain of the receptor or are located on receptor-associated signaling adapters, such as the FcR? or DAP-12 molecules. Phosphorylation of these ITAM domains leads to recruitment of Syk/Zap70 kinases which in turn initiate downstream signaling, leading to cellular activation. Src-family kinases also phosphorylate inhibitory receptors on specific tyrosines within ITIM (Immunoreceptor Tyrosine-based Inhibitory Motifs) domains that are located within the cytoplasmic regions of each receptor. ITIM phosphorylation leads to recruitment of the tyrosine phosphatase SHP-1, which in turn dephosphorylates both the Src kinases as well as downstream substrates in the signaling cascade. The overall balance between activating and inhibitory signaling determines the cellular response. As a result, the functions of the Src-family kinases are opposed in large part by SHP-1, which serves as a major brake to intracellular signaling in innate cells. Mutation of the SHP-1 gene (Ptpn6) results in severely hyperactive lymphocytes, myeloid cells and platelets. As a result, SHP-1 deficient animals (motheaten (me/me) or motheaten viable (mev/mev)) develop dramatic autoimmunity, inflammation and early mortality due to lung inflammation (~4 weeks for complete SHP-1 loss in me/me mice and ~8-10 weeks in the hypomorphic mev/mev mice). Though these SHP-1 deficient animals have been available for years, the complex interactions and indirect effects between the many hyperactive leukocyte subsets has limited their utility. Thus it remains unclear which specific SHP-1 regulated signaling pathways in each of the different hyperactive leukocyte types is responsible for the different aspects of the autoimmune/inflammatory disease. To address this problem, we have bred the Ptpn6flx/flx (SHP-1flx) mice to a series of Cre expressing animals to achieve lineage-specific deletion of SHP-1.  Immune cells sense pathogens such as bacteria, fungi and viruses through a complex array of cellular receptors. Binding of the pathogen molecules to these receptors activates intracellular enzymes that initiate intracellular signaling pathways that lead to immune cell activation to eliminate the pathogen. Appropriate control of these intracellular signaling pathways is critical to prevent immune cell hyperactivation that can cause damage to host tissues during responses to pathogen invasion. One of the major molecules that downregulates immune cells are the tyrosine phosphatase SHP-1. We have generated a series of mutant mice that lack the SHP-1 protein in defined immune cell types. These conditional SHP-1 mutant animals develop a spectrum of diseases, including severe skin inflammation, autoimmunity or lethal pulmonary inflammation. By studying the specific intracellular signaling pathways that are affected by these different SHP-1 conditional mutations, we will gain a new insight into the regulation of immune cell reactivity. This will lead to better design of therapeutcs to target immune diseases, as well as leukemia and lymphoma.