KLK5

KLK5 is a trypsin-like serine protease that initiates the epidermal desquamation cascade by cleaving corneodesmosome proteins (DSG1, DSC1, CDSN) at acidic pH and activating pro-KLK7, thereby linking protease activity to controlled stratum corneum shedding (PMID:15140227). Its activity is tightly regulated by the pH-dependent inhibitor LEKTI (encoded by SPINK5), which forms a tight-binding complex with KLK5 at neutral pH but releases active KLK5 at the acidic pH of the outermost skin surface, and by the palmo-plantar-specific inhibitor SPINK9 whose inhibitory potency likewise increases at acidic pH (PMID:17596512, PMID:22505519). Loss of LEKTI in Netherton syndrome results in unrestrained KLK5 activity and compromised skin barrier integrity, and allosteric inhibitory antibodies targeting KLK5 restore barrier function in mouse models of this disease (PMID:15675955, PMID:36516271). Beyond its epidermal role, KLK5 cleaves DPP4 from T cell surfaces, activates betacoronavirus spike proteins at both the S1/S2 and S2' sites enabling viral entry in airway epithelia, and participates in PAR2-dependent signaling in cancer contexts (PMID:29107298, PMID:39163389, PMID:36313631).

1. 2004 High

   Established KLK5 as the initiator of the desquamation protease cascade by showing it directly degrades corneodesmosome substrates and activates pro-KLK7, answering how stratum corneum shedding is enzymatically triggered.

   Evidence In vitro cleavage assays with recombinant KLK5 against DSG1, DSC1, CDSN, and pro-KLK7 at acidic pH

   PMID:15140227

   Open questions at the time

   • Cleavage sites on corneodesmosome proteins not mapped
   • No in vivo validation of the cascade hierarchy
   • Whether KLK5 alone is sufficient for desquamation was untested
2. 2005 High

   Resolved how premature KLK5 activation is prevented in normal epidermis: KLK5/KLK7 and LEKTI are stored in separate lamellar granule compartments and secreted sequentially, with loss of LEKTI in Netherton syndrome causing unchecked proteolysis.

   Evidence Immunoelectron microscopy and confocal microscopy of normal and Netherton syndrome human epidermis

   PMID:15675955

   Open questions at the time

   • Mechanism of sequential secretion not defined
   • Whether other inhibitors partially compensate in Netherton syndrome skin was unknown
3. 2007 High

   Defined the molecular logic of LEKTI-mediated KLK5 inhibition: LEKTI domains D8-D11 form tight-binding complexes with KLK5 at neutral pH but release active protease at acidic pH, explaining how desquamation is spatially restricted to the outermost stratum corneum.

   Evidence Kinetic inhibition assays of recombinant LEKTI fragments against KLK5 across a pH range

   PMID:17596512

   Open questions at the time

   • Structural basis of pH-dependent binding not resolved
   • Relative contribution of individual LEKTI domains in vivo unknown
4. 2012 High

   Identified SPINK9 as a site-specific inhibitor of KLK5 in palmo-plantar skin and demonstrated that reactive-loop residues and His48 protonation govern selectivity and pH-enhanced inhibition.

   Evidence Mutagenesis of SPINK9 reactive-loop residues combined with kinetic assays and molecular modeling

   PMID:22505519

   Open questions at the time

   • No crystal structure of the SPINK9–KLK5 complex
   • In vivo relevance in palmo-plantar barrier not tested
5. 2014 Medium

   Showed KLK5 can suppress breast cancer malignancy by downregulating the mevalonate pathway, reducing RhoA prenylation — an unexpected non-epidermal function linking KLK5 to lipid metabolism and oncogenic signaling.

   Evidence Stable KLK5 expression in MDA-MB-231 cells; metabolic rescue with geranylgeranyl pyrophosphate; in vivo tumor growth assays

   PMID:24158494

   Open questions at the time

   • Direct proteolytic substrate mediating mevalonate pathway suppression unknown
   • Single cell line system
   • Not confirmed in clinical tumor specimens
6. 2017 Medium

   Demonstrated a non-skin proteolytic role for KLK5: it is secreted by CD4+ T cells and sheds DPP4 from the surface of Th17 cells, increasing plasma DPP4 in type 2 diabetes patients.

   Evidence Co-immunoprecipitation, cell-surface shedding assays, and plasma ELISA in type 2 diabetes patient samples

   PMID:29107298

   Open questions at the time

   • Cleavage site on DPP4 not mapped
   • Causal role of KLK5-mediated DPP4 shedding in diabetes pathophysiology not established
   • Single cohort study
7. 2021 Medium

   Placed KLK5 under transcriptional control of STAT3 in keratinocytes: STAT3 represses KLK5 while promoting SPINK5 expression, establishing a single transcription factor that coordinates both the protease and its inhibitor for barrier homeostasis.

   Evidence Keratinocyte-specific STAT3 knockout mice; siRNA knockdown and overexpression in keratinocytes

   PMID:34378233

   Open questions at the time

   • Whether STAT3 binds the KLK5 promoter directly was not shown
   • Downstream barrier phenotype in STAT3-KO skin not fully characterized
8. 2022 High

   Revealed an allosteric inhibition mechanism for KLK5: an antibody Fab binds distal to the active site yet blocks protease activity, and dual KLK5/KLK7 antibody inhibition restores barrier integrity in Netherton syndrome and atopic dermatitis mouse models, validating KLK5 as a therapeutic target.

   Evidence Crystal structure of KLK5–Fab complex; in vivo treatment of Netherton syndrome and atopic dermatitis mouse models

   PMID:36516271

   Open questions at the time

   • Allosteric conformational change not characterized dynamically
   • Human clinical efficacy not yet demonstrated
9. 2022 Medium

   Linked KLK5 to PAR2/AKT signaling in gallbladder carcinoma: IGF2BP3 stabilizes KLK5 mRNA in an m6A-dependent manner, and KLK5 protein activates PAR2/AKT to promote proliferation and migration.

   Evidence RNA immunoprecipitation, MeRIP, dual-luciferase assays, gain/loss-of-function in vitro and in vivo

   PMID:36313631

   Open questions at the time

   • Whether KLK5 directly cleaves PAR2 in this context not shown
   • Single cancer type studied
10. 2024 High

    Established KLK5 as a host airway protease capable of both S1/S2 priming and S2' activation of betacoronavirus spike proteins, demonstrating a single-protease viral activation mechanism and showing that KLK5 inhibition reduces viral replication.

    Evidence In vitro spike cleavage assays; infection of differentiated human bronchial epithelial cells; mouse coronavirus infection model with KLK5 inhibitor (ursolic acid)

    PMID:39163389

    Open questions at the time

    • Ursolic acid is not a specific KLK5 inhibitor — genetic validation in airway cells pending
    • Relative contribution of KLK5 versus TMPRSS2 in vivo not quantified
11. 2025 Medium

    Demonstrated that KLK5 (with KLK7) activates KLK14 to drive PAR-2/RhoA/NF-κB signaling in HPV-dependent cervical carcinogenesis, and that KLK5/KLK7 double knockout ameliorates the malignant phenotype.

    Evidence KLK5/KLK7 double-knockout mice crossed with HPV transgenic model; RNA-seq; RhoA and NF-κB reporter assays; human biopsy analysis

    PMID:40753921

    Open questions at the time

    • Whether KLK5 directly activates pro-KLK14 or acts indirectly not resolved
    • PAR-2 cleavage by KLK14 in this model not biochemically validated
12. 2025 Medium

    Revealed that KLK5 deletion in a CDSN-knockout background paradoxically worsens desquamation and barrier permeability, indicating compensatory protease activity and that KLK5 initiates but does not solely sustain the desquamation cascade.

    Evidence KLK5/CDSN double-knockout mice and shRNA-mediated CDSN knockdown in human epidermal equivalents; ultrastructural and permeability analysis

    PMID:40943523

    Open questions at the time

    • Identity of compensating proteases not determined
    • Mechanism by which KLK5 loss exacerbates barrier dysfunction in CDSN-null background unclear

• The full set of proteases that compensate for KLK5 loss in desquamation, the structural basis of pH-dependent LEKTI release, and whether KLK5 directly cleaves PAR2 in cancer and inflammatory contexts remain unresolved.
• No high-resolution structure of the KLK5–LEKTI complex
• Compensatory proteases in KLK5-null epidermis unidentified
• Direct PAR2 cleavage by KLK5 not biochemically demonstrated