Essential diurnal Rac1 activation during retinal phagocytosis requires alpha v beta 5 integrin but not tyrosine kinases focal adhesion kinase or Mer tyrosine kinase

Diurnal phagocytosis of shed photoreceptor outer-segment particles by retinal pigment epithelial (RPE) cells belongs to a group of conserved clearance mechanisms employing alpha v integrins upstream of tyrosine kinases and Rho GTPases. In this study, we tested the interdependence of the tyrosine kin...

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Bibliographic Details
Published in:Molecular biology of the cell 2012-03, Vol.23 (6), p.1104-1114
Main Authors: Mao, Yingyu, Finnemann, Silvia C
Format: Article
Language:English
Subjects:
Actin
Cdc42 protein
Cell culture
Epidermal growth factor
Focal adhesion kinase
Guanosinetriphosphatase
Integrins
Milk
Phagocytes
Phagocytosis
Photoreceptors
Pigments
Protein-tyrosine kinase
Protein-tyrosine kinase receptors
Rac1 protein
Retina
RhoA protein
Signal transduction
Toxin B
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Summary:Diurnal phagocytosis of shed photoreceptor outer-segment particles by retinal pigment epithelial (RPE) cells belongs to a group of conserved clearance mechanisms employing alpha v integrins upstream of tyrosine kinases and Rho GTPases. In this study, we tested the interdependence of the tyrosine kinases focal adhesion kinase (FAK) and Mer tyrosine kinase (MerTK) and Rho GTPases during engulfment. RPE cells activated and redistributed Rac1, but not RhoA or Cdc42, during phagocytosis. Toxin B, overexpression of dominant-negative Rac1, or decreasing Rac1 expression prevented particle engulfment. Fluorescence microscopy showed that Rac1 inhibition had no obvious effect on F-actin arrangement in resting RPE but prevented recruitment of F-actin to surface-bound phagocytic particles. Quantification of active GTP-Rac1 in wild-type and mutant RPE in culture and in vivo revealed that Rac1 activation during phagocytosis requires alpha v beta 5 integrin and its ligand milk fat globule EGF factor-8 (MFG-E8) but not the receptor tyrosine kinase MerTK. Abolishing tyrosine kinase signaling downstream of alpha v beta 5 toward MerTK by inhibiting FAK specifically or tyrosine kinases generally neither prevented Rac1 activation nor F-actin recruitment during phagocytosis. Likewise, inhibiting Rac1 had no effect on FAK or MerTK activation. We conclude that MerTK activation via FAK and F-actin recruitment via Rac1 both require MFG-E8-ligated alpha v beta 5 integrin. Both pathways are independently activated and required for clearance phagocytosis.
ISSN:1059-1524
DOI:10.1091/mbc.E11-10-0840