Spatial coordination of actin polymerization and ILK-Akt2 activity during endothelial cell migration

Autotaxin signaling via lysophosphatidic acid receptors contributes to vascular endothelial growth factor-induced endothelial cell migration

Important roles for vascular endothelial growth factor (VEGF) and autotaxin (ATX) have been established for embryonic vasculogenesis and cancer progression. We examined whether these two angiogenic factors cooperate in regulation of endothelial cell migratory responses. VEGF stimulated expression of ATX and LPA1, a receptor for the ATX enzymatic product lysophosphatidic acid (LPA), in human umbilical vein endothelial cells. Knockdown of ATX expression significantly decreased mRNA levels for the receptors LPA1, LPA2, S1P1, S1P2, S1P3, and VEGFR2 and abolished cell migration to lysophosphatidylcholine, LPA, recombinant ATX, and VEGF. Migration to sphingosylphosphorylcholine and sphinogosine-1-phosphate was also reduced in ATX knockdown cells, whereas migration to serum remained unchanged. Furthermore, ATX knockdown decreased Akt2 mRNA levels, whereas LPA treatment strongly stimulated Akt2 expression. We propose that VEGF stimulates LPA production by inducing ATX expression. VEGF also increases LPA1 signaling, which in turn increases Akt2 expression. Akt2 is strongly associated with cancer progression, cellular migration, and promotion of epithelial-mesenchymal transition. These data show a role for ATX in maintaining expression of receptors required for VEGF and lysophospholipids to accelerate angiogenesis. Because VEGF and ATX are upregulated in many cancers, the regulatory mechanism proposed in these studies could apply to cancer-related angiogenesis and cancer progression. These data further suggest that ATX could be a prognostic factor or a target for therapeutic intervention in several cancers.

The pseudoactive site of ILK is essential for its binding to alpha-Parvin and localization to focal adhesions

Integrin-linked kinase (ILK) plays a pivotal role in connecting transmembrane receptor integrin to the actin cytoskeleton and thereby regulating diverse cell-adhesion-dependent processes. The kinase domain (KD) of ILK is indispensable for its function, but the underlying molecular basis remains enigmatic. Here we present the crystal structure of the ILK KD bound to its cytoskeletal regulator, the C-terminal calponin homology domain of alpha-parvin. While maintaining a canonical kinase fold, the ILK KD displays a striking pseudoactive site conformation. We show that rather than performing the kinase function, this conformation specifically recognizes alpha-parvin for promoting effective assembly of ILK into focal adhesions. The alpha-parvin-bound ILK KD can simultaneously engage integrin beta cytoplasmic tails. These results thus define ILK as a distinct pseudokinase that mechanically couples integrin and alpha-parvin for mediating cell adhesion. They also highlight functional diversity of the kinase fold and its "active" site in mediating many biological processes.

The ILK/PINCH/parvin complex: the kinase is dead, long live the pseudokinase!

Dynamic interactions of cells with their environment regulate multiple aspects of tissue morphogenesis and function. Integrins are the major class of cell surface receptors that recognize and bind extracellular matrix proteins, resulting in the engagement and organization of the cytoskeleton as well as activation of signalling pathways to regulate cell behaviour and morphogenetic processes. The ternary complex of integrin-linked kinase (ILK), PINCH, and parvin (IPP complex), which was identified more than a decade ago, interacts with the cytoplasmic tail of beta integrins and couples them to the actin cytoskeleton. In addition, ILK has been shown to act as a serine/threonine kinase and to directly activate several signalling pathways downstream of integrins. However, the kinase activity of ILK and the precise functions of the IPP complex have remained elusive and controversial. This review focuses on the recent advances made towards understanding the specialized roles this complex and its individual components have acquired during evolution.

Downregulation of thymosin beta4 in neural progenitor grafts promotes spinal cord regeneration

Thymosin beta4 (Tbeta4) is an actin-binding peptide whose expression in developing brain correlates with migration and neurite extension of neurons. Here, we studied the effects of the downregulation of Tbeta4 expression on growth and differentiation of murine neural progenitor cells (NPCs), using an antisense lentiviral vector. In differentiation-promoting medium, we found twice the number of neurons derived from the Tbeta4-antisense-transduced NPCs, which showed enhanced neurite outgrowth accompanied by increased expression of the adhesion complex N-cadherin-beta-catenin and increased ERK activation. Importantly, when the Tbeta4-antisense-transduced NPCs were transplanted in vivo into a mouse model of spinal cord injury, they promoted a significantly greater functional recovery. Locomotory recovery correlated with increased expression of the regeneration-promoting cell adhesion molecule L1 by the grafted Tbeta4-antisense-transduced NPCs. This resulted in an increased number of regenerating axons and in sprouting of serotonergic fibers surrounding and contacting the Tbeta4-antisense-transduced NPCs grafted into the lesion site. In conclusion, our data identify a new role for Tbeta4 in neuronal differentiation of NPCs by regulating fate determination and process outgrowth. Moreover, NPCs with reduced Tbeta4 levels generate an L1-enriched environment in the lesioned spinal cord that favors growth and sprouting of spared host axons and enhances the endogenous tissue-repair processes.

Both actin and polyproline interactions of profilin-1 are required for migration, invasion and capillary morphogenesis of vascular endothelial cells

The objective of the present study was to evaluate how different ligand interactions of profilin-1 (Pfn1), an actin-binding protein that is upregulated during capillary morphogenesis of vascular endothelial cells (VEC), contribute to migration and capillary forming ability of VEC. We adopted a knockdown-knockin experimental system to stably express either fully functional form or mutants of Pfn1 that are impaired in binding to two of its major ligands, actin (H119E mutant) and proteins containing polyproline domains (H133S mutant), in a human dermal microvascular cell line (HmVEC) against near-null endogenous Pfn1 background. We found that silencing endogenous Pfn1 expression in HmVEC leads to slower random migration, reduced velocity of membrane protrusion and a significant impairment in matrigel-induced cord formation. Only re-expression of fully functional but not any of the two ligand-binding deficient mutants of Pfn1 rescues the above defects. We further show that loss of Pfn1 expression in VEC inhibits three-dimensional capillary morphogenesis, MMP2 secretion and ECM invasion. VEC invasion through ECM is also inhibited when actin and polyproline interactions of Pfn1 are disrupted. Together, these experimental data demonstrate that Pfn1 regulates VEC migration, invasion and capillary morphogenesis through its interaction with both actin and proline-rich ligands.