Novel secreted isoform of adhesion molecule ICAM-4: potential regulator of membrane-associated ICAM-4 interactions.
Intercellular adhesion molecule-4 (ICAM-4), a newly characterized adhesion molecule, is expressed early in human erythropoiesis and functions as a ligand for binding alpha4beta1 and alphaV integrin-expressing cells. Within the bone marrow, erythroblasts surround central macrophages forming erythroblastic islands. Evidence suggests that these islands are highly specialized subcompartments where cell adhesion events, in concert with cytokines, play critical roles in regulating erythropoiesis and apoptosis. Since erythroblasts express alpha4beta1 and ICAM-4 and macrophages exhibit alphaV, ICAM-4 is an attractive candidate for mediating cellular interactions within erythroblastic islands. To determine whether ICAM-4 binding properties are conserved across species, we first cloned and sequenced the murine homologue. The translated amino acid sequence showed 68% overall identity with human ICAM-4. Using recombinant murine ICAM-4 extracellular domains, we discovered that hematopoietic alpha4beta1- expressing HEL cells and nonhematopoietic alphaV-expressing FLY cells adhered to mouse ICAM-4. Cell adhesion studies showed that FLY and HEL cells bound to mouse and human proteins with similar avidity. These data strongly suggest conservation of integrin-binding properties across species. Importantly, we characterized a novel second splice cDNA that would be predicted to encode an ICAM-4 isoform, lacking the membrane-spanning domain. Erythroblasts express both isoforms of ICAM-4. COS-7 cells transfected with green flourescent protein constructs of prototypic or novel ICAM-4 cDNA showed different cellular localization patterns. Moreover, analysis of tissue culture medium revealed that the novel ICAM-4 cDNA encodes a secreted protein. We postulate that secretion of this newly described isoform, ICAM-4S, may modulate binding of membrane-associated ICAM-4 and could thus play a critical regulatory role in erythroblast molecular attachments.
Lee
Gloria
G
Life Sciences Division, University of California, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
Spring
Frances A
FA
Parsons
Stephen F
SF
Mankelow
Tosti J
TJ
Peters
Luanne L
LL
Koury
Mark J
MJ
Mohandas
Narla
N
Anstee
David J
DJ
Chasis
Joel Anne
JA
D017398:Alternative Splicing; D000595:Amino Acid Sequence; D000818:Animals; D002448:Cell Adhesion; D015815:Cell Adhesion Molecules / Q000737:chemistry / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D002874:Chromosome Mapping; D003001:Cloning, Molecular; D017077:Culture Media, Conditioned / Q000737:chemistry; D018076:DNA, Complementary / Q000235:genetics; D004900:Erythroblasts / Q000166:cytology* / Q000378:metabolism; D020224:Expressed Sequence Tags; D005622:Friend murine leukemia virus; D006801:Humans; D039041:Integrin alpha4beta1 / Q000378:metabolism; D007438:Introns / Q000235:genetics; D051379:Mice; D008969:Molecular Sequence Data; D011485:Protein Binding; D020033:Protein Isoforms / Q000737:chemistry / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D017434:Protein Structure, Tertiary; D011993:Recombinant Fusion Proteins / Q000502:physiology; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013045:Species Specificity
D015815:Cell Adhesion Molecules; D017077:Culture Media, Conditioned; D018076:DNA, Complementary; C548377:ICAM-4 protein, mouse; C095100:ICAM4 protein, human; D039041:Integrin alpha4beta1; D020033:Protein Isoforms; D011993:Recombinant Fusion Proteins