SPINK9

SPINK9 (LEKTI-2) is a single Kazal-domain serine protease inhibitor expressed focally in the stratum granulosum and stratum corneum of palmoplantar epidermis, where it functions in the regulation of desquamation through highly selective inhibition of the kallikrein KLK5 (PMID:19194479, PMID:19190773). It binds and inhibits KLK5 (apparent Ki ~65 nM) while sparing KLK7, KLK14, trypsin, plasmin, and thrombin, and fully blocks KLK5-mediated fibrinogen degradation (PMID:19194479, PMID:19190773). Selectivity and inhibitory potency are governed by its reactive loop at residues 48/49, where protonation of His48 at the acidic pH of outer skin slows the enzyme-inhibitor dissociation rate and enhances inhibition, providing a pH-tuned mechanism matched to the epidermal microenvironment (PMID:22505519). Beyond protease inhibition, SPINK9 is constitutively secreted by keratinocytes and, independently of its KLK5-inhibitory activity, transactivates EGFR through ADAM metalloprotease upregulation and purinergic receptor signaling to stimulate keratinocyte migration (PMID:24441102). It additionally acts as an antimicrobial peptide that enters and kills Escherichia coli in a manner dependent on its N-terminal sequence and the KLK-inhibitory motif, with the bacterial chaperone SKP identified as its major interacting partner conferring bacterial resistance (PMID:30468739).

1. 2009 High

   Established SPINK9 as a protease inhibitor and resolved which protease it targets, defining its biochemical function and selectivity.

   Evidence Recombinant inhibition assays against multiple serine proteases with Ki determination, SPR binding, and fibrinogen degradation assay; immunohistochemistry localizing SPINK9 near KLK5 in palmoplantar skin

   PMID:19190773 PMID:19194479

   Open questions at the time

   • KLK8 binding was heterogeneous and no inhibition was confirmed
   • Physiological consequence of KLK5 inhibition on desquamation not demonstrated in vivo
   • No structural model of the inhibitory complex at this stage
2. 2012 High

   Pinpointed the structural determinants of inhibition, showing how reactive-loop residues and pH control specificity and potency toward kallikreins.

   Evidence Site-directed mutagenesis of reactive-loop residues 48/49, inhibition assays, binding kinetics, and molecular modeling of enzyme-inhibitor complexes

   PMID:22505519

   Open questions at the time

   • No experimental crystal structure of the SPINK9-KLK5 complex
   • pH dependence shown biochemically but not validated in tissue context
3. 2014 Medium

   Revealed a protease-inhibition-independent signaling role, expanding SPINK9 function from passive inhibitor to active driver of keratinocyte behavior.

   Evidence Recombinant SPINK9 treatment of keratinocytes with EGFR transactivation assays, ADAM metalloproteinase inhibitor, EGFR-blocking antibody, purinergic antagonists, and migration readouts

   PMID:24441102

   Open questions at the time

   • Single lab; no identification of the specific purinergic receptor or ADAM isoform
   • Direct receptor for SPINK9 not identified
   • In vivo relevance to wound healing or migration not shown
4. 2018 Medium

   Demonstrated a third function as an antimicrobial peptide and identified its bacterial target interaction, linking the KLK-inhibitory motif to bactericidal activity.

   Evidence Characterization of natural N-terminal variants, bacterial killing assays, ultrastructural EM showing cell entry, E. coli interactome pull-down identifying SKP, and Skp-deletion sensitivity assay

   PMID:30468739

   Open questions at the time

   • Single lab; mechanism of intracellular killing after entry not defined
   • Activity tested mainly against E. coli; spectrum across other organisms limited
   • How SKP confers resistance mechanistically unresolved

• Whether SPINK9's three activities (KLK5 inhibition, EGFR transactivation, antimicrobial killing) are integrated in palmoplantar skin physiology or pathology remains unresolved.
• No in vivo or disease-association evidence in the corpus
• No genetic loss-of-function phenotype reported
• Relationship between the secreted signaling pool and the desquamation-regulating pool unknown