Trends in Molecular Medicine
Hereditary neutropenia: dogs explain human neutrophil elastase mutations
Section snippets
The biology of the ELA2 gene product, NE
ELA2 is transcribed only in pro-myelocytic and pro-monocytic progenitors in the bone marrow, but the protein, if not the transcript, persists in the cells through terminal differentiation to neutrophils and monocytes [21]. ELA2 encodes a 267-residue protein that is post-translationally processed at both termini. The N-terminus contains a 27-residue pre sequence cleaved by a signal peptidase. The protease cathepsin C, also known as dipeptidyl peptidase I (DPPI), cleaves a remaining two residue
AP3B1 mutations as the cause of canine CN
Canine CN [27] is also known as gray Collie syndrome, because it arose in Collies and the dogs have a diluted coat color. The human and canine forms of the disease differ in a number of ways: human CN lacks pigmentary abnormalities, canine disease demonstrates autosomal-recessive transmission, in dogs, neutrophil counts cycle every two weeks rather than three weeks (Figure 1) and all blood cells cycle.
Using direct DNA sequencing and genetic linkage studies, canine ELA2 mutations were excluded
Interaction between the gene products responsible for human and canine CN
Given the similarities between canine and human CN, an obvious question is whether NE interacts with AP3? Specifically, does NE bind to μ3a or β3a, the subunits responsible for recognizing cargo proteins? A yeast two-hybrid assay established for testing adaptor protein subunit and cargo protein interactions [39], indicates that μ3a interacts with NE via a tyrosine-based recognition signal upstream of the C-terminal tail, but only after the C-terminal tail is removed [26]. Because the C-terminus
NE mutations aligning with predicted transmembrane regions
Previously, NE was recognized as a soluble protein, and its crystal structure supports its behavior as a textbook serine protease [40]. However, would computerized algorithms identifying transmembrane domains detect their presence in human NE? Surprisingly, most programs predict two transmembrane domains [26], each of which is bracketed by disulfide bonds (Figure 2a). Strikingly, when the mutations responsible for hereditary neutropenia are superimposed on the predicted transmembrane domains, a
Pathogenic targets of NE
What are the substrates of mislocalized NE? Any answer to this question must ultimately account for the periodicity observed in CN. Many groups have proposed that oscillations suggest a feedback circuit in which mature neutrophils inhibit progenitor cells [60]. Inhibition of progenitors leads to the loss of successive cohorts of maturing cells, eventually depleting the generation of cells producing the inhibitory signal, thus allowing the pattern to repeat. It is, therefore, expected that the
Concluding remarks
NE has a significant role in myelopoiesis, and its previously underappreciated localization in cellular compartments other than granules, with attendant implications for NE as a membrane protein, opens a search for identifying important regulatory targets. This has far-reaching implications for serine protease biochemistry, subcellular trafficking, hematopoiesis and the pathogenesis of neutropenia and leukemia. Outstanding questions remain (Box 3) and future studies will aim to answer them.
Acknowledgements
We thank David Ehlert for assistance preparing figures, the estate of Dr Lee Ford for access to her notebooks, and Dr Richard Swank for mouse and Ginny Cuneo for dog photographs. Supported by grants (to M.H.) from the NIH (DK55820, DK58161) and Burroughs-Wellcome Fund (SATR-1002189).
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2013, Journal of Biological ChemistryCitation Excerpt :However, the tyrosine-based signal in human ELA2, which interacts with AP-3 (43), is not conserved in canine ELA2 (GenBankTM accession number AF494190). This suggests that a different signal motif, such as a dileucine signal that is recognized by AP-3 (44), is used to target ELA2 to lysosomes in dogs. Moreover, clathrin-mediated endocytosis of bovine AE1 uses a noncanonical YXXXΦ motif instead of the YXXΦ signal (34), and this is another example of variability in the sequences of motifs used for cargo recognition.