# Direct Rap1-talin interaction in platelets, leukocytes, and endothelial cells

> **NIH NIH P01** · UNIVERSITY OF CALIFORNIA, SAN DIEGO · 2020 · $489,337

## Abstract

Abstract
Rap1 is a central signaling node connecting agonist stimulation to talin recruitment to integrins, a
final common step in integrin activation. In leukocytes, RIAM is a Rap1 effector that mediates
talin-dependent activation; however, in platelets the identity of such Rap1 effectors is obscure
and is a key gap in our understanding. Preliminary data suggest a new evolutionarily-conserved
paradigm that talin contains two Rap1 binding sites that enable it to serve as a Rap1 effector. The
applicant hypothesizes that talin itself is the principal, and perhaps only, Rap1 effector implicated
in platelet integrin activation. Secondly, he suggests that that the talin-Rap1 interaction may
contribute to integrin function even in leukocytes that contain an abundant Rap1 effector, RIAM
and in endothelial cells that contain substantial lamellipodin, a RIAM paralogue. To examine
these ideas: Specific Aim 1 will test the hypothesis that a direct Rap1-talin interaction plays
a central role in platelet integrin activation. Mice in which the megakaryocytes and platelets
express talin with either or both Rap1 binding sites disabled will be analyzed for platelet structure,
function, and formation. Hemostasis and thrombosis will be tested in a variety of assays. A
biomembrane force probe will characterize the role of the Rap1-talin interaction in individual
IIb3 integrin-ligand bonds. Specific Aim 2 will test the hypothesis that direct Rap1-Talin
interaction is important in RIAM-replete cells. In collaboration with Project 2 Integrin
activation, conformation, and topology in leukocytes will be analyzed in cells bearing each of the
three talin alleles described in Aim1. The effect of each mutation in combination with RIAM
deletion or with RIAM over-expression will test the relative roles of these two Rap1 effectors.
Specific Aim 3 will test the hypothesis that the talin-Rap1 interaction is important in
endothelial cells. Mice expressing one of the three mutant talin alleles selectively in
endothelium will be studied for hemorrhage and developmental phenotypes. Effects on integrin
activation, spreading, and cell-cell junctions will be assessed in vitro in primary lung and brain
microvascular endothelial cells. Lamellipodin deletion and over expression will enable an analysis
of the relationship of lamellipodin and Rap1 binding to talin in endothelial cell functions. Together,
achievement of these aims will test the paradigm-shifting hypothesis that talin itself serves as
the major Rap1 effector in platelet function in hemostasis and thrombosis and evaluate the
importance of the Rap1-talin interaction in leukocytes and endothelial cells.

## Key facts

- **NIH application ID:** 9936851
- **Project number:** 1P01HL151433-01
- **Recipient organization:** UNIVERSITY OF CALIFORNIA, SAN DIEGO
- **Principal Investigator:** Mark HOWARD Ginsberg
- **Activity code:** P01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2020
- **Award amount:** $489,337
- **Award type:** 1
- **Project period:** 2020-08-05 → 2025-07-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/9936851

## Citation

> US National Institutes of Health, RePORTER application 9936851, Direct Rap1-talin interaction in platelets, leukocytes, and endothelial cells (1P01HL151433-01). Retrieved via AI Analytics 2026-05-24 from https://api.ai-analytics.org/grant/nih/9936851. Licensed CC0.

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