Membrane-Type 5 Matrix Metalloproteinase (MT5-MMP): Background and Proposed Roles in Normal Physiology and Disease.

Membrane-type 5 matrix metalloproteinase (MT5-MMP/MMP-24) contains 645 amino acids with a predicted molecular mass of 73.2 kDa [ 1 ]. As a member of the MT-MMP family, it shares a common domain structure consisting of a signal peptide, a pro-domain, a catalytic domain, a linker, a hemopexin-like domain, a stalk region, a transmembrane domain, and a short cytoplasmic domain ( Figure 1 ) [ 2 , 3 ]. The overall folds of the catalytic and hemopexin-like domains are predicted to be similar to other MMP family members ( Figure 1 ). The zinc-containing catalytic domain catalyzes peptide bond hydrolysis, while the hemopexin-like domain facilitates the binding of macromolecular substrates. Several mechanisms by which MMPs catalyze peptide hydrolysis have been proposed [ 4 , 5 , 6 , 7 ]. The linker typically plays a key role in MMP processing of macromolecular substrates, as the favorable positioning of the catalytic domain in proximity to the hemopexin-like domain is dependent on the flexibility of the linker [ 8 , 9 ]. While the role of the MT5-MMP cytoplasmic domain is relatively unexplored, the MT1-MMP cytoplasmic domain has been shown to modulate cellular activities via posttranslational modifications of the domain [ 10 , 11 ]. Initially reported in the brain, kidney, pancreas, and lung and during embryonic development [ 1 , 12 , 13 ], its expression is most prominent in the neuronal cells of both central and peripheral nervous systems and in mast cells [ 14 , 15 , 16 , 17 , 18 , 19 ]. It is also expressed in calcitonin-gene-related peptide-positive (CGRP + ) dorsal root ganglion neurons [ 14 , 18 ] and is elevated in the adult hippocampus, olfactory bulb, and cerebellum [ 20 ]. ProMT5-MMP has been shown to be activated by furin in neurons [ 17 ] and by propeptide convertase subtilisin/kexin type 6 (PCSK6) in mouse neuroblastoma (N2A) cells stably expressing human Swedish mutant APP695 (N2A APP ) [ 21 ]. PCSK6 recognizes the Arg-Arg-Arg-Asn-Lys-Arg sequence within the propeptide [ 21 ], and furin-mediated activation occurs in the trans-Golgi network [ 22 ]. Unlike MT1-MMP, MT5-MMP internalization is independent of the dynamin-regulated endocytic pathway [ 23 ]. Once internalized, it colocalizes with the early endosomal marker RUN and FYVE domain containing 1 (RUFY1), indicating delivery to the early endosomes after internalization [ 23 ]. Mint-3 regulates the retrieval of internalized MT5-MMP to the plasma membrane by binding to its carboxyl end motif Glu-Trp-Val [ 23 ]. Both the phosphotyrosine-binding (PTB) and PDZ domains of Mint-3 contribute to this regulation, which occurs in the trans-Golgi network [ 23 ]. Interestingly, while low levels of Mint-3 support MT5-MMP return to the cell surface, higher Mint-3 levels inhibit it in both HEK293 cells and Neuro2A neuronal cells—an effect that appears to be non-specific [ 23 ]. MT5-MMP is active at the cell surface and intracellularly [ 12 , 24 ]. MT5-MMP was also found to be shed in active form from Madin–Darby canine kidney (MDCK) cells and human breast cancer MCF-7 and T47D cells [ 12 ]. MT5-MMP binds α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor binding protein (ABP) and glutamate receptor interacting protein (GRIP), where those interactions may localize MT5-MMP and contribute to synaptic remodeling [ 17 ]. MT5-MMP was originally identified from rat brain when screening for the interacting proteins of the C -terminus of ABPs (amino acids 295–822) [ 17 , 23 , 25 ]. In the rat, expression of MT5-MMP is first detected in the embryonic-day-16 brain, increases at embryonic day 20, and peaks at postnatal day 0 throughout the brain parenchyma [ 20 ]. Postnatal expression decreases to adult levels by day 60 [ 20 ]. During development the enzyme expression is detected in the central nervous system and the peripheral nervous system, including the trigeminal ganglion and dorsal root ganglia [ 20 ], and has also been detected in the thymus and aorta [ 20 ]. MT5-MMP has been proposed to contribute to synaptic plasticity [ 20 ] and to the development of dermal neuro-immune synapses [ 18 ]. MT5-MMP participates in axodendritic development and remodeling after nerve injury [ 14 , 15 , 19 ] and is essential for neuronal cell migration and neurite formation [ 14 , 26 ]. It is found in axonal growth cones and developing dendritic spines [ 14 ]. In its absence, neurons exhibit increased spontaneous depolarizations and are resistant to interleukin-1β (IL-1β)-induced hyperexcitability [ 27 ]. Neural stem cell quiescence is due to N-cadherin-mediated anchorage to ependymocytes [ 28 ]. MT5-MMP sheds the N-cadherin ectodomain and is necessary for the activation of B-cells [ 28 ]. Elevated levels of the enzyme were found to colocalize with N-cadherin in glial fibrillary acidic protein-positive (GFAP + ) B1 and ependymal cells [ 28 ].

MT5-MMP was initially described as an activator of proMMP-2 [ 1 , 12 ] in a tissue inhibitor of metalloproteinase 2 (TIMP-2)-dependent manner, in similar fashion to MT1-MMP [ 29 ]. It was not found to activate proMMP-9 [ 1 ]. Known substrates include gelatin [ 12 , 29 ], laminin-1 [ 29 ], fibronectin [ 29 ], N-cadherin (CDH2) [ 17 ], E-cadherin (CDH1) [ 17 ], KiSS-1 [ 25 ], the KiSS-1-derived decapeptide metastin [ 25 ], Nogo-66 receptor 1 (NgR1, RTN4R) [ 30 ], amyloid precursor protein (APP) [ 31 , 32 ], crystallin αB [ 33 ], and myelin basic protein (MBP) [ 34 ]. The data indicated partial cleavage of laminin, although it was reported that laminin was resistant to MT5-MMP digestion [ 29 ]. MT5-MMP can digest MBP, but is the least efficient when compared to other MT-MMPs [ 34 ]. The enzyme degrades itself rapidly at 37 °C [ 29 ]. MT5-MMP cleaves chondroitin sulfate proteoglycans (CSPGs) and dermatan sulfate proteoglycans (DSPGs) well and fibronectin more slowly [ 29 ]. Ectodomain cleavage of N-cadherin generates a ~35 kDa membrane-bound C -terminal fragment (CTF1), which is further cleaved by γ-secretase or the proteasome [ 17 , 28 ]. MT5-MMP mediates peripheral thermal nociception and inflammatory hyperalgesia by shedding N-cadherin [ 18 ]. Although MT5-MMP was not specifically identified, cleavage of E-cadherin is carried out by a membrane-bound metalloproteinase [ 35 ], occurring at Pro 700 -Val 701 , which produces a 38 kDa membrane-bound fragment [ 36 ]. The enzyme has been reported to bind to CD44 [ 37 ] but it does not cleave it [ 38 , 39 ]. While MT5-MMP was reported to not cleave type I collagen [ 29 ], we observed that MT5-MMP could cleave fluorogenic triple-helical peptide 15 (fTHP-15), whose sequence is derived from types I-III collagen ( Figure 2 ) [ 40 ]. Interestingly, fTHP-15 is cleaved by MT5-MMP at the Gly~Leu bond, which is the same bond cleaved by MT5-MMP in KISS-1 and metastin (Gly 118 ~Leu 119 ) [ 25 ].

There are only a few inhibitors described for MT5-MMP. MT5-MMP is inhibited by the broad-spectrum MMP inhibitors GM6001 [ 17 , 33 ] and BB94 [ 29 ], as well as the chelator ethylenediaminetetraacetic acid (EDTA) [ 29 ]. Consistent with the general behavior of MT-MMPs, MT5-MMP is inhibited by TIMP-2, but not TIMP-1 [ 1 , 29 ]. MT1-MMP fragment antigen-binding (Fab) antibodies have been tested against MT5-MMP [ 41 ]. Six inhibited MT5-MMP in the 54–74% range at 100 nM Fab concentration [ 41 ]. Designing selective inhibitors for MT5-MMP is especially challenging due to the common features of MMP active sites [ 42 ]. Based on its hydrolysis of a triple-helical substrate ( Figure 2 ), we examined the potential inhibition of MT5-MMP by our previously described triple-helical peptide transition state analog inhibitor α1 (I-III)GlyΨ{PO 2 H-CH 2 }Leu THPI [ 43 , 44 ]. We found that MT5-MMP was inhibited by the THPI with an IC 50 value of 1.8 μM ( Figure 3 ). This result opens the possibility of selective MT5-MMP inhibition by combining transition-state analogs and MT5-MMP substrate sequences.

Members of the MMP family have long been established as contributors to cancer initiation, growth, and metastasis [ 45 , 46 ]. Amongst the MT-MMPs, MT1-MMP has been deemed an essential contributor to tumor invasion [ 47 , 48 , 49 ]. Thus, several studies have examined a potential role for MT5-MMP in cancer. MT5-MMP expression has been detected in breast cancer cell lines [ 50 ] and breast cancer tissue [ 51 ], with mRNA expression increased in tumor tissue compared to normal breast tissue [ 52 ]. Its expression correlates with tumor grade [ 52 ]. Roughly 20% of breast cancers lack functional repressor element 1 silencing transcription factor (REST), and these tumors are more aggressive, resulting in poorer prognoses [ 53 , 54 ]. REST directly regulates MT5-MMP expression, and the knockdown of REST leads to the upregulation of MT5-MMP (~45-fold) [ 54 ]. REST binds to the RE1 site in the first intron of MT5-MMP [ 54 ]. These studies focused on MT5-MMP expression but did not explore a mechanistic role for the enzyme in breast cancer. Extracellular matrix stiffness occurs in the tumor microenvironment [ 55 , 56 ] (Paszek 2005 & Levental 2009). This stiffness can modulate tumor behaviors, including promoting invasion and proliferation. It has been proposed that binding to rigid substrates results in increased expression of MT5-MMP and slowing of the progress of breast, lung, and renal cancers [ 57 ]. Increased substrate stiffness activates the Yes-associated protein (YAP)-TEA domain (TEAD), which then promotes the expression of the enzyme via a TEAD recognition sequence upstream of the transcription start site [ 57 ]. This may explain why some studies show that higher MT5-MMP expression correlates with better survival [ 57 ]. The upregulation of MT5-MMP in non-small-cell lung cancer (NSCLC) is associated with worse progression-free survival [ 58 ]. The transcription repressor Capicua (CIC), when inactivated, results in ETV4 upregulating MT5-MMP [ 58 ]. MT5-MMP was localized to the leading edge of primary NSCLC tumors [ 58 ]. In mouse models MT5-MMP was found to promote tumor cell circulatory extravasation and stable lung colonization, as well as metastasis [ 58 ]. MT5-MMP also promotes the invasion of ovarian cancer cells [ 59 ]. As with the breast cancer studies described above, the NSCLC and ovarian cancer studies did not investigate a mechanism for MT5-MMP action. MT5-MMP was overexpressed in gastric cancer compared with peritumoral normal tissue [ 60 ] and with chronic superficial gastritis [ 61 ], where it was suggested as a prognostic molecular marker [ 61 ]. In similar fashion to NSCLC, the inactivation of CIC in gastric cancer results in ETV4 upregulating MT5-MMP [ 58 ]. The gastric cancer studies examined MT5-MMP expression only. In brain tumors, including astrocytomas and glioblastomas, MT5-MMP expression is high [ 1 , 13 ], though its functional role remains unclear [ 62 ]. An evaluation of epidermal growth factor receptor (EGFR) signaling via EGF stimulation of glioma cell lines resulted in only a slight increase (1.3- to 1.6-fold) in MT5-MMP mRNA levels [ 63 ]. Overall, correlations between increased MT5-MMP expression and cancer progression have been documented, but no specific role for MT5-MMP has been identified. Thus, at present MT5-MMP could serve as a cancer biomarker. It appears that further substrate identification will be needed in order to evaluate MT5-MMP activity in cancer invasion and metastasis.

In Alzheimer’s disease (AD), the extracellular accumulation of abnormally folded amyloid beta (Aβ) peptides forms amyloid plaques. The Ab peptides are generated in a two-stage process, where APP is first cleaved by β-site APP cleaving enzyme 1 (BACE1) to produce sAPPβ and CTFβ/C99, followed by γ-secretase cleavage to produce Aβ42 and the amyloid precursor protein intracellular domain (AICD) ( Figure 4 ) [ 64 , 65 , 66 ]. Alternatively, the cleavage of APP in the middle of the Aβ domain by α-secretase (mediated by a disintegrin and metalloprotease 10/17 (ADAM10 and ADAM17)) precludes Aβ generation ( Figure 4 ) [ 64 , 65 , 66 ]. MT5-MMP may have opposing effects in AD versus normal physiology [ 27 ]. The enzyme is found in post-mortem AD brains around senile plaques in dystrophic neurites, and its expression is pro-amyloidogenic [ 32 , 66 , 67 ]. PCSK6 exacerbates AD pathogenesis by promoting MT5-MMP maturation [ 21 ]. Elevated enzyme protein levels are found in the 5xFAD mouse model of AD [ 68 ]. Identified as the η-secretase involved in APP processing, MT5-MMP cleaves at residues 504–505 (Val-Leu-Ala-Asn 504 -Met 505 -Ile-Ser-Glu-Pro-Arg) (APP695 numbering) [ 32 ], contributing directly to AD pathology [ 32 , 67 ]. This cleavage generates a C -terminal membrane-bound APP fragment, CTF-η [ 32 , 67 ], which accumulates in distrophic neurites close to amyloid plaques [ 42 , 66 , 69 , 70 ]. CTF-η is localized in Golgi, endosomes, and extracellular vesicles and contributes to Aβ production [ 69 ]. Processing of CTF-η by ADAM10/17 and BACE1 releases Aη-α (108 residues) and Aη-β (92 residues), respectively ( Figure 5 ) [ 32 , 42 , 69 , 70 ], which are synaptotoxic [ 32 , 66 , 71 ]. Among AD risk loci is ADAM17 [ 72 ]. The processing of APP by η-secretase and the presence of Aη peptides have been observed in patient cerebrospinal fluid (CSF) [ 32 , 71 ]. Aη-β has been found to impair hippocampal long-term potentiation and inhibit neuronal activity [ 71 ], which is a crucial process for learning and memory. MT5-MMP η-secretase activity can be considered pro-amyloidogenic because CTF-η can be processed by β-secretase and γ-secretase to yield Aβ [ 69 ]. A non-amyloidogenic route is also possible following MT5-MMP action on APP as CTF-η can be degraded by proteasomal and autophagic pathways [ 69 ]. Soluble sAPP fragment of 95 kDa (sAPP95, sAPPη), another product of η-secretase/MT5-MMP activity, binds GABA B R1a and impedes presynaptic release [ 66 , 67 , 73 ]. When β-secretase activity is pharmacologically or genetically inhibited, Aη peptide levels are increased [ 32 ]. We found that a β-secretase inhibitor increased the activity of MT5-MMP [ 24 ]. Overall, inhibiting BACE1 may result in the accumulation of alternative APP fragments such as Aη-α [ 64 ]. Reduced levels of Aη-α were observed in the brains of MT5-MMP knockout mice [ 32 ]. Crossing MT5-MMP-deficient mice with the 5xFAD AD mouse model produced bigenic mice that had reduced Aβ plaque deposition and reduced soluble Aβ and soluble APP C -terminal fragments (CTFs) within the brain and improved performance on a behavioral learning task [ 67 , 68 , 74 ]. These effects were maintained even after 16 months. It was proposed that MT5-MMP may influence the intracellular trafficking of APP and/or its targeting to subcellular compartments where the C-terminal fragment of APP (C99) is degraded, thus accounting for the lower Aβ40 and Aβ42 levels when MT5-MMP was absent. The levels of IL-1β decreased by 30% in the bigenic mice, which is indicative of a dampened inflammatory response due to the decreased Aβ burden in the brain [ 67 ]. MT5-MMP knockout in 5xFAD mice prevented dysfunctions of the frontal cortex (including learning and memory deficits) observed in 5xFAD mice [ 74 ]. 5xFAD/MT5-MMP −/− mice had reduced Aβ assembles (soluble, oligomeric, and fibrillary) and C99 compared with 5xFAD mice [ 74 ]. More specifically, soluble Aβ38 was reduced 83%, Aβ40 84%, and Aβ42 90% [ 74 ]. Significant decreases in sAPPα and sAPPβ were also observed in 5xFAD/MT5-MMP −/− mice compared with 5xFAD mice [ 74 ]. Astrocyte activity and tumor necrosis factor alpha (TNF-α) levels were also reduced [ 74 ]. MT5-MMP localized to early endosomes and increased the content of APP and Aβ40 [ 74 ]. It is interesting to note that MT5-MMP deficiency had no impact on microglial reactivity in the frontal cortex in contrast to the hippocampus [ 67 , 74 ]. An examination of neuronal cultures revealed that the absence of MT5-MMP impaired the IL-1β-mediated induction in inflammatory genes in cells from 5xFAD mice compared with cells from 5xFAD mice where MT5-MMP was not deleted [ 27 ]. MT5-MMP can activate proinflammatory pathways [ 75 ]. Its deficiency decreased C83 and C99 levels (which are derived from CTF-η; see Figure 5 ); these fragments are cleared in the absence of MT5-MMP [ 27 ]. The deletion of the MT5-MMP C -terminal domain reduced its ability to process APP and release sAPP95 [ 76 ]. This included the deletion of the intracellular domain, transmembrane domain + intracellular domain, or hemopexin-like domain [ 76 ]. The C -terminal domain of MT5-MMP directly interacted with CTFβ/C99 [ 76 ], and thus when this domain was deleted CTFβ/C99 could be degraded by the proteasome, preventing Aβ accumulation [ 76 ]. MT5-MMP may regulate APP cellular (endo-lysosomal) trafficking as both active and inactive enzymes increased extracellular levels of Aβ40 [ 76 ]. MT5-MMP increased the content of APP/Aβ in early endosomes [ 76 ]. Its non-catalytic domains contributed to the MT5-MMP-catalyzed production of sAPP95 [ 76 ]. It was proposed that an APP/Mint3/MT5-MMP trimolecular complex could promote APP/C99 trafficking to endosomes [ 76 ]. Similar C -terminal interactions were not found in MT1-MMP [ 76 ], and thus this trafficking may be unique for MT5-MMP. It has been hypothesized that the role of MT5-MMP in AD is multifaceted [ 75 ]. MT5-MMP directly acts on APP, but it could also traffic APP to endosomal/lysosomal compartments and enhance proinflammatory responses induced by IL-1β or TNF-α due to N-cadherin processing [ 75 ]. The endosome has been reported as the main site for the MT5-MMP processing of APP in transient transfection studies [ 69 , 74 ]. MT5-MMP could facilitate the β-secretase processing of APP in endosomes [ 66 ]. In addition to endosomes [ 66 ] the Golgi has recently re-emerged as a major location for APP processing and Aβ production [ 77 ]. MT5-MMP can locate to the trans-Golgi network to further recycle to the membrane even though it lacks a di-leucine motif in its C -terminal tail [ 23 ]. We stably co-expressed APP751 and MT5-MMP-GFP in CHO cells and found that the enzyme localized in cytosolic subcellular granules [ 24 ]. Consistent with our observation that MT5-MMP is found in the Golgi apparatus, CTF-η has been found to colocalize with TGN-46, a marker for the Golgi apparatus and trans-Golgi network [ 69 ]. CTF-η fragments can ultimately be transported via exosomes [ 69 ], while Aβ peptides can be transported outside the cell by HSP47 [ 78 ]. The C -terminus of MT5-MMP was utilized as a bait to screen the human brain cDNA library [ 24 ]. MT5-MMP was found to directly bind to transmembrane protein 199 (TMEM199), NEDD4-binding protein 2-like 1 (N4BP2L1), thioredoxin related transmembrane protein 4 isoform X3 (TMX3), bridging integrator 1 (BIN1), RUFY4, high temperature requirement protein A1 (HTRA1), and endosome/lysosome-associated apoptosis and autophagy regulator (also called estrogen induced gene 121; transmembrane protein KIAA1324) (EIG121) [ 24 ]. The binding of N4BP2L1, TMX3, BIN1, TMEM199, or EIG121 to MT5-MMP increased Aη-α and Aη-β production [ 24 ]. The association of several of these binding partners can be linked to MT5-MMP localization to the Golgi apparatus, based on the interaction between lysosomes and the Golgi apparatus [ 79 ]. The transmembrane nature of MT5-MMP suggests the incorporation of the enzyme into endosomes and lysosomes during vacuole formation. The protein encoded by TMEM199 has been observed to localize to the endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC) and coat protein complex I (COPI) [ 80 ]. As the loss of TMEM199 results in the over-acidification of the endo-lysosomal compartments [ 81 ], the processing of APP by the MT5-MMP•TMEM199 complex may be related to the accumulation of CTFβ and Aβ in poorly acidified autolysosomes [ 82 ]. EIG121 is a transmembrane protein localized in the endo-lysosomal compartments [ 83 ], and thus MT5-MMP’s association with EIG121 may occur in the endosomes and lysosomes. MT5-MMP has been shown to be associated with early endosomes [ 76 ], while BIN1 contributes to early-endosome size deregulation, which is an early pathophysiological hallmark of AD pathology [ 84 ]. Thus, the association between BIN1 and MT5-MMP may occur in early endosomes. BIN1 has been identified as an AD susceptibility gene [ 84 ]. RUFY4 promotes the coupling of endo-lysosomes to dynein–dynactin for retrograde transport along microtubules [ 85 ], and thus the association of MT5-MMP with RUFY4 could occur in the endosomes and lysosomes. HTRA1 is a secreted serine protease that degrades various fragments of the amyloid precursor protein and colocalizes with β-amyloid deposits in human brain samples [ 86 ]. One cannot speculate where HTRA1 or TMX3 are associated with MT5-MMP due to the relative lack of information pertaining to their function [ 24 ]. TMX3 (PDIA13) is an endoplasmic reticulum oxidoreductase/disulfide isomerase [ 87 , 88 ], while N4BP2L1 has been identified as a highly significant differentially expressed gene in AD [ 89 ]. It is not readily apparent how most of the MT5-MMP binding partners enhance proteolytic activity. TIMP-3 is increased in AD patient brains and APP transgenic mice [ 90 ]. TIMP-3 inhibits ADAM10 and ADAM17 and thus routes APP processing away from α-secretase and instead enhances β-secretase activity and endocytosis [ 90 ]. This would deter the production of Aη-α. TIMP-3 plasma and cerebrospinal fluid levels are lower in AD patients compared with non-AD patients [ 91 ]. It has been suggested that TIMP-3 could aggregate in the brain in AD [ 91 ].

MT5-MMP is elevated 2 and 7 days after traumatic brain injury (TBI), while N-cadherin protein decreases [ 92 ]. The two proteins localize within reactive astrocytes, which produce MT5-MMP during reactive synaptogenesis [ 92 ]. As N-cadherin links and stabilizes presynaptic terminals with postsynaptic structures, this remodeling activity may be crucial in post-TBI synapse reorganization [ 92 ]. Given the increase in proMMP-2 in TBI, it is possible that MT5-MMP may activate MMP-2, resulting in matrix turnover [ 92 ]. MT5-MMP is expressed in the epithelial tissue in the normal cornea [ 93 ]. P. aeruginosa infection induces its expression in the substantia propria [ 93 ]. The levels of enzyme were found to increase 7 days after infection, possibly due to the infiltration of macrophages [ 93 ]. While it was noted that MT5-MMP hydrolyzes a number of extracellular matrix components (such as fibronectin and proteoglycans) and hydrolysis of these components may contribute to cornea damage, no specific mechanism was explored. Kidney MT5-MMP expression was localized to the epithelial cells of the proximal and distal convoluted tubules in the cortex, the collecting duct, and the loop of Henle in the medulla [ 13 ]. MT5-MMP mRNA and protein are increased in the diabetic kidney [ 13 ]. MMP-2 activity was also increased in the diabetic kidney [ 13 ]. Tubular epithelial cells were found to be associated with tubular atrophy [ 13 ]. It was proposed that during diabetic nephropathy, MT5-MMP and MMP-2 remodeled the basement membrane, resulting in tubular epithelial cell detachment, apoptosis of epithelial cells, and tubular atrophy. MT5-MMP expression is found in normal human endometria and is elevated in endometriosis [ 26 ]. More specifically, MT5-MMP showed an 8.4-fold higher expression in endometriotic lesions in comparison with endometrium [ 26 ]. MT5-MMP protein was detected in luminal epithelial cells [ 26 ]. In the evaluation of the roles of MMPs in the premature rupture of membranes (PROM) during labor and delivery, MT5-MMP was ultimately not considered a contributor [ 94 ].

MT5-MMP upregulation has been observed in a variety of disease states, but the majority of studies are correlative without proposed mechanisms of action. The most detailed studies of MT5-MMP define the role of the enzyme in AD. In the case of AD, the presence of the η-secretase and β-secretase pathways may be the reason for failed clinical trials when targeting Aβ reduction only [ 95 ]. Approaches to decrease the activity of BACE1 and limit the activity of MT5-MMP will shift the pathway from pro-amyloidogenic processing to the non-amyloidogenic one. The development of selective MT5-MMP inhibitors could also be utilized to better define the enzyme’s role in a variety of cancers. An additional approach for regulating MT5-MMP could be targeting activators of the enzyme, such as furin or PCSK6. As noted, MT5-MMP knockout mice are viable and have no overt abnormalities, suggesting that MT5-MMP could be a therapeutic target in AD and cancer [ 18 , 42 , 75 ]. Overall, the substrate profile of MT5-MMP has not been well defined, and selective inhibitors of MT5-MMP have not been described. These advances will be needed for further consideration of MT5-MMP as a therapeutic target.