These activities are independent
of ephrin binding and / or kinase activity and their mechanism is not well understood but in some cases depends on Eph receptor phosphorylation on serine / threonine residues (red circle).
In the 1990s, scientists found the first of those signaling systems, a family
of ephrin proteins that are present in graded fashion in the brain and are involved in axon guidance.
Not exact matches
Recently, however,
ephrins and Eph receptors have also been found in extracellular vesicles / exosomes — small droplets
of fat released by cells, used as transport vehicles, signal transmitters or for eliminating cell components.
When an
ephrin meets the Eph receptor
of another cell, they join to form an
ephrin - Eph complex.
One way this communication happens is through the
ephrin / Eph - receptor system, which is able to guide cell migration and the growth
of neuronal extensions.
Ephrins and Eph receptors have also been found in the exosomes
of cancer cells.
They revealed that many
of these exosomes contained
ephrins and Ephs, and decoded the cellular mechanism by which they were packed into the exosomes.
«This has thrown up the interesting question
of what business Ephs and
ephrins have in exosomes,» says Klein.
Eph receptors and their binding partners, the
ephrins, are found on the surface
of almost all cell types.
«This is why it's so fundamentally important to understand how cells use this system to communicate,» says Rüdiger Klein, whose Department at the Max Planck Institute
of Neurobiology is studying
ephrins and Eph receptors.
However, Rüdiger Klein and his team at the Max Planck Institute
of Neurobiology have now shown that cells can also pack and release active
ephrins and Eph receptors through extracellular vesicles.
Eph receptor and
ephrin ligand signalling has been implicated in vascular development and in in vivo models
of angiogenesis where bi-directional signalling mediates juxtracrine cell — cell contact, cell adhesion to extracellular matrix and cell migration30, 31,32.
Ephrin - B reverse signaling is required for formation
of strictly contralateral auditory brainstem pathways.
Studies by ours and other groups have shown that a number
of EphA2 and EphA3 mutations inactivate Eph receptor canonical signaling by disrupting
ephrin binding or kinase activity, consistent with a role
of canonical signaling in tumor suppression.
This knowledge is useful for the development
of disease treatments based on modulating Eph receptor /
ephrin activities.
This leads to bidirectional signals emanating from Eph receptor -
ephrin complexes positioned at sites
of cell - cell contact.
Binding to
ephrin ligands on the surface
of neighboring cells induces canonical signaling involving receptor clustering, autophosphorylation on tyrosine residues, and kinase activity - dependent downstream signaling.
(A) Eph receptor -
ephrin binding at cell - cell contact sites results in the dimerization / clustering
of Eph receptor -
ephrin complexes, and initiation
of canonical signals through the receptor cytoplasmic domain.
We discovered several Eph receptors and
ephrins, and research in our laboratory is dedicated to the characterization
of Eph receptor signal transduction mechanisms and biological functions using biochemical, mass spectrometry, molecular biology and cell biology approaches in conjunction with animal models.
We also found that the EphB4 receptor expressed on the surface
of breast cancer cells can promote tumor xenograft growth by enhancing blood vessel formation through interactions with its preferred ligand,
ephrin - B2, present in tumor endothelial cells.
We found that canonical signaling by the EphB4 receptor is low in breast cancer cells and that
ephrin - induced stimulation
of EphB4 kinase activity inhibits breast cancer cell malignancy in culture and tumor growth in vivo (Figure 1A) through inhibition
of the CRK proto - oncogene.
Receptor tyrosine kinases
of the Eph family and their ligands, the
ephrins, represent an important cell communication system that controls a vast array physiological and disease processes.
Most
of the peptides are antagonists, but the peptides targeting EphA2 are agonists that activate receptor signaling and endocytosis similarly to the natural
ephrin ligands.
This is unlike the natural
ephrin ligands, each
of which promiscuously binds to multiple Eph receptors.
We have identified tyrosine and serine / threonine phosphorylation sites
of Eph receptors and
ephrins using mass spectrometry and investigated the signaling role
of these phosphorylation sites.
Understanding the effects
of Eph receptor mutations in cancer cells will help shed light on the role
of the Eph receptor /
ephrin system in cancer cell transformation, malignant progression and drug resistance.
More recently, we elucidated an additional mechanism
of tumor suppression mediated by canonical
ephrin - induced EphA2 signaling (Figure 1A), which leads to inhibition
of the AKT - mTORC1 oncogenic pathway through interplay
of EphA2 with a phosphatase that dephosphorylates the AKT serine / threonine kinase.
Peptides can target the
ephrin - binding pocket
of Eph receptors with high affinity and specificity, affecting receptor function.