Functional redundancy between RAP1 isoforms in murine platelet production and function.

Area of Science:

• Cellular Biology
• Hematology
• Molecular Signaling

Background:

• RAP GTPases regulate cellular adhesion and are abundant in platelets and megakaryocytes.
• Previous studies suggested RAP1B's partial role in platelet integrin activation and a minor role for RAP GTPases in megakaryocyte biology.

Purpose of the Study:

• To investigate the roles of Rap1a and Rap1b isoforms in platelet production and activation.
• To elucidate the specific functions of RAP isoforms in platelet responses to injury and vascular leakage.

Main Methods:

• Generated megakaryocyte-specific knockout mice (mKO) for Rap1a and/or Rap1b.
• Assessed platelet count, proplatelet formation, α-granule secretion, RAC1 activation, thromboxane A2 generation, and integrin activation.
• Evaluated platelet aggregation, spreading, clot retraction, thrombus formation under flow, hemostasis, and vascular integrity.

Main Results:

• Megakaryocyte-specific Rap1a/b deletion (Rap1a/b-mKO) caused macrothrombocytopenia due to impaired proplatelet formation.
• RAP1B deletion reduced α-granule secretion and RAC1 activation; both isoforms contributed to thromboxane A2 generation and integrin activation.
• Combined Rap1a/b deficiency severely impaired platelet aggregation, spreading, clot retraction, and thrombus formation, leading to defective hemostasis.
• Rap1a/b-mKO platelets did not affect vascular integrity during development or inflammation.

Conclusions:

• RAP1 signaling is essential for platelet production via megakaryocyte proplatelet formation.
• RAP1 isoforms play redundant and specific roles in platelet activation, integrin signaling, and aggregation.
• Platelet RAP1 signaling is critical for hemostasis and thrombosis but dispensable for maintaining vascular integrity.