Selective recruitment of src family kinase Hck by leukocyte integrin αMβ2but not αLβ2or αXβ2

Early-onset pulmonary and cutaneous vasculitis driven by constitutively active SRC-family kinase HCK

BACKGROUND

Inborn errors of immunity (IEI) are genetic disorders characterized by various degrees of immune dysregulation that can manifest as immune deficiency, autoimmunity or autoinflammation. The routine use of next-generation sequencing in the clinic has facilitated the identification of an ever-increasing number of IEI, revealing the roles of immunologically important genes in human pathologies. However, despite this progress, treatment is still extremely challenging.

OBJECTIVE

We report a new monogenic autoinflammatory disorder caused by a de novo activating mutation, p.Tyr515*, in hematopoietic cell kinase (HCK). The disease is characterized by cutaneous vasculitis and chronic pulmonary inflammation that progresses to fibrosis.

METHODS

Whole-exome sequencing, Sanger sequencing, mass spectrometry and western blotting were performed to identify and characterize the pathogenic HCK mutation. Dysregulation of mutant HCK was confirmed ex vivo in primary cells and in vitro in transduced cell lines.

RESULTS

Mutant HCK lacking the C-terminal inhibitory tyrosine Tyr522 exhibited increased kinase activity and enhanced myeloid cell priming, migration and effector functions, such as production of the inflammatory cytokines IL-1β, IL-6, IL-8 and TNFα and production of reactive oxygen species. These aberrant functions were reflected by inflammatory leukocyte infiltration of the lungs and skin. Moreover, an overview of the clinical course of the disease, including therapies, provides evidence for the therapeutic efficacy of the Janus kinase (JAK) 1/2 inhibitor ruxolitinib in inflammatory lung disease.

CONCLUSION

We propose HCK-driven pulmonary and cutaneous vasculitis as a novel autoinflammatory disorder of IEI.

Phagocytic Integrins: Activation and Signaling

Phagocytic integrins are endowed with the ability to engulf and dispose of particles of different natures. Evolutionarily conserved from worms to humans, they are involved in pathogen elimination and apoptotic and tumoral cell clearance. Research in the field of integrin-mediated phagocytosis has shed light on the molecular events controlling integrin activation and their effector functions. However, there are still some aspects of the regulation of the phagocytic process that need to be clarified. Here, we have revised the molecular events controlling phagocytic integrin activation and the downstream signaling driving particle engulfment, and we have focused particularly on α_Mβ₂/CR3, α_Xβ₂/CR4, and a brief mention of α_Vβ₅/α_Vβ₃integrins.

Expression of kindlin-3 in melanoma cells impedes cell migration and metastasis

Kindlins are a small family of 4.1-ezrin-radixin-moesin (FERM)-containing cytoplasmic proteins. Kindlin-3 is expressed in platelets, hematopoietic cells, and endothelial cells. Kindlin-3 promotes integrin activation, clustering and outside-in signaling. Aberrant expression of kindlin-3 was reported in melanoma and breast cancer. Intriguingly, kindlin-3 has been reported to either positively or negatively regulate cancer cell metastasis. In this study, we sought to clarify the expression of kindlin-3 in melanoma cells and its role in melanoma metastasis. Two widely used metastatic mouse and human melanoma cell lines B16-F10 and M10, respectively, were examined and found to lack kindlin-3 mRNA and protein expression. When kindlin-3 was ectopically expressed in these cells, cell migration was markedly reduced. These are attributed to aberrant Rac1 and RhoA activation and overt membrane ruffling. Our data demonstrate for the first time that despite its well established role as a positive regulator of integrin-mediated cell adhesion, aberrant expression of kindlin-3 could lead to imbalanced RhoGTPases signaling that impedes rather than promotes cell migration.

Structure and binding interface of the cytosolic tails of αXβ2 integrin

BACKGROUND

Integrins are signal transducer proteins involved in a number of vital physiological processes including cell adhesion, proliferation and migration. Integrin molecules are hetero-dimers composed of two distinct subunits, α and β. In humans, 18 α and 8 β subunits are combined into 24 different integrin molecules. Each of the subunit comprises a large extracellular domain, a single pass transmembrane segment and a cytosolic tail (CT). The CTs of integrins are vital for bidirectional signal transduction and in maintaining the resting state of the receptors. A large number of intracellular proteins have been found to interact with the CTs of integrins linking integrins to the cytoskeleton.

METHODOLOGY/PRINCIPAL FINDINGS

In this work, we have investigated structure and interactions of CTs of the leukocyte specific integrin αXβ2. We determined the atomic resolution structure of a myristoylated CT of αX in perdeuterated dodecylphosphocholine (DPC) by NMR spectroscopy. Our results reveal that the 35-residue long CT of αX adopts an α-helical conformation for residues F4-N17 at the N-terminal region. The remaining residues located at the C-terminal segment of αX delineate a long loop of irregular conformations. A segment of the loop maintains packing interactions with the helical structure by an extended non-polar surface of the αX CT. Interactions between αX and β2 CTs are demonstrated by (15)N-(1)H HSQC NMR experiments. We find that residues constituting the polar face of the helical conformation of αX are involved in interactions with the N-terminal residues of β2 CT. A docked structure of the CT complex indicates that a network of polar and/or salt-bridge interactions may sustain the heteromeric interactions.

CONCLUSIONS/SIGNIFICANCE

The current study provides important insights into the conservation of interactions and structures among different CTs of integrins.

The leucocyte β2 (CD18) integrins: the structure, functional regulation and signalling properties

Leucocytes are highly motile cells. Their ability to migrate into tissues and organs is dependent on cell adhesion molecules. The integrins are a family of heterodimeric transmembrane cell adhesion molecules that are also signalling receptors. They are involved in many biological processes, including the development of metazoans, immunity, haemostasis, wound healing and cell survival, proliferation and differentiation. The leucocyte-restricted β2 integrins comprise four members, namely αLβ2, αMβ2, αXβ2 and αDβ2, which are required for a functional immune system. In this paper, the structure, functional regulation and signalling properties of these integrins are reviewed.

Structures and interaction analyses of integrin αMβ2 cytoplasmic tails

Integrins are heterodimeric (α and β subunits) signal transducer proteins involved in cell adhesions and migrations. The cytosolic tails of integrins are essential for transmitting bidirectional signaling and also implicated in maintaining the resting states of the receptors. In addition, cytosolic tails of integrins often undergo post-translation modifications like phosphorylation. However, the consequences of phosphorylation on the structures and interactions are not clear. The leukocyte-specific integrin αMβ2 is essential for myeloid cell adhesion, phagocytosis, and degranulation. In this work, we determined solution structures of the myristoylated cytosolic tail of αM and a Ser phosphorylated variant in dodecylphosphocholine micelles by NMR spectroscopy. Furthermore, the interactions between non-phosphorylated and phosphorylated αM tails with β2 tail were investigated by NMR and fluorescence resonance energy transfer (FRET). The three-dimensional structures of the 24-residue cytosolic tail of αM or phosphorylated αM are characterized by an N-terminal amphipathic helix and a loop at the C terminus. The residues at the loop are involved in packing interactions with the hydrophobic face of the helix. 15N-1H heteronuclear single quantum coherence experiments identified residues of αM and β2 tails that may be involved in the formation of a tail-tail heterocomplex. We further examined interactions between myristoylated β2 tail in dodecylphosphocholine micelles with dansylated αM tail peptides by FRET. These studies revealed enhanced interactions between αM or phosphorylated αM tails with β2 tail with Kd values ∼5.2±0.6 and ∼4.4±0.7 μm, respectively. Docked structures of tail-tail complexes delineated that the αM/β2 interface at the cytosolic region could be sustained by a network of polar interactions, ionic interactions, and/or hydrogen bonds.

Signaling networks regulating leukocyte podosome dynamics and function

Podosomes are ventral adhesion structures prominent in cells of the myeloid lineage. A common aspect of these cells is that they are highly motile and must to traverse multiple tissue barriers in order to perform their functions. Recently podosomes have gathered attention from researchers as important cellular structures that can influence cell adhesion, motility and matrix remodeling. Adhesive and soluble ligands act via transmembrane receptors and propagate signals to the leukocyte cytoskeleton via small G proteins of the Rho family, tyrosine kinases and scaffold proteins and are able to induce podosome formation and rearrangements. Manipulation of the signals that regulate podosome formation and dynamics can therefore be a strategy to interfere with leukocyte functions in a multitude of pathological settings, such as infections, atherosclerosis and arthritis. Here, we review the major signaling molecules that act in the formation and regulation of podosomes.

Integrin alphaMbeta2 clustering triggers phosphorylation and activation of protein kinase C delta that regulates transcription factor Foxp1 expression in monocytes

Integrins are type I membrane and heterodimeric (alphabeta) cell adhesion receptors. Intracellular signals triggered by ligand-bound integrins are important for cell growth, differentiation, and migration. Integrin alpha(M)beta(2) plays key roles in myeloid cell adhesion, phagocytosis, and degranulation. In this study, we show that protein kinase C (PKC) delta is involved in alpha(M)beta(2) signaling. In human monocytic U937 cells and peripheral blood monocytes, alpha(M)beta(2) clustering induced PKCdelta translocation to the plasma membrane, followed by Tyr(311) phosphorylation and activation of PKCdelta by the src family kinases Hck and Lyn. Interestingly, alpha(M)beta(2)-induced PKCdelta Tyr(311) phosphorylation was not mediated by the tyrosine kinase Syk, which is a well reported kinase in beta(2) integrin signaling. Analysis of the beta(2) cytoplasmic tail showed that the sequence Asn(727)-Ser(734) is important in alpha(M)beta(2)-induced PKCdelta Tyr(311) phosphorylation. It has been shown that alpha(M)beta(2) clustering regulates the expression the transcription factor Foxp1 that has a role in monocyte differentiation. We show that Foxp1 expression was reduced in monocytes that were allowed to adhere to human microvascular endothelial cells. However, the expression of Foxp1 was not affected in monocytes that were treated with PKCdelta-targeting small interfering RNA, suggesting that PKCdelta regulates Foxp1 expression. These results demonstrate a role of PKCdelta in alpha(M)beta(2)-mediated Foxp1 regulation in monocytes.

Solution structure of a Hck SH3 domain ligand complex reveals novel interaction modes

We studied the interaction of hematopoietic cell kinase SH3 domain (HckSH3) with an artificial 12-residue proline-rich peptide PD1 (HSKYPLPPLPSL) identified as high affinity ligand (K(D)=0.2 muM). PD1 shows an unusual ligand sequence for SH3 binding in type I orientation because it lacks the typical basic anchor residue at position P(-3), but instead has a tyrosine residue at this position. A basic lysine residue, however, is present at position P(-4). The solution structure of the HckSH3:PD1 complex, which is the first HckSH3 complex structure available, clearly reveals that the P(-3) tyrosine residue of PD1 does not take the position of the typical anchor residue but rather forms additional van der Waals interactions with the HckSH3 RT loop. Instead, lysine at position P(-4) of PD1 substitutes the function of the P(-3) anchor residue. This finding expands the well known ligand consensus sequence +xxPpxP by +xxxPpxP. Thus, software tools like iSPOT fail to identify PD1 as a high-affinity HckSH3 ligand so far. In addition, a short antiparallel beta-sheet in the RT loop of HckSH3 is observed upon PD1 binding. The structure of the HckSH3:PD1 complex reveals novel features of SH3 ligand binding and yields new insights into the structural basics of SH3-ligand interactions. Consequences for computational prediction tools adressing SH3-ligand interactions as well as the biological relevance of our findings are discussed.