Effect of Kallikrein 4 Loss on Enamel Mineralization

Abstract

Enamel formation depends on a triad of tissue-specific matrix proteins (amelogenin, ameloblastin, and enamelin) to help initiate and stabilize progressively elongating, thin mineral ribbons of hydroxyapatite formed during an appositional growth phase. Subsequently, these proteins are eradicated to facilitate lateral expansion of the hydroxyapatite crystallites. The purpose of this study was to investigate changes in enamel mineralization occurring in mice unable to produce kallikrein 4 (Klk4), a proteinase associated with terminal extracellular degradation of matrix proteins during the maturation stage. Mice lacking functional matrix metalloproteinase 20 (Mmp20), a proteinase associated with early cleavage of matrix proteins during the secretory stage, were also analyzed as a frame of reference. The results indicated that mice lacking Klk4 produce enamel that is normal in thickness and overall organization in terms of layers and rod/ inter-rod structure, but there is a developmental defect in enamel rods where they first form near the dentinoenamel junction. Mineralization is normal up to early maturation after which the enamel both retains and gains additional proteins and is unable to mature beyond 85% mineral by weight. The outmost enamel is hard, but inner regions are soft and contain much more protein than normal. The rate of mineral acquisition overall is lower by 25%. Mice lacking functional Mmp20 produce enamel that is thin and structurally abnormal. Relatively high amounts of protein remain throughout maturation, but the enamel is able to change from 67 to 75% mineral by weight during maturation. These findings reaffirm the importance of secreted proteinases to enamel mineral acquisition.

Fam83h null mice support a neomorphic mechanism for human ADHCAI

“…2011). Soft tissue was removed from the left and right hemimandibles of 7‐week‐old mice, and cross‐sectioned at 1 mm increments from the basal (growing) end of the incisors, and imaged by bSEM.…”

Maturation stage enamel malformations in Amtn and Klk4 null mice

“…Amtn and Klk4 both appear to be functionally specific for dental enamel formation and are pseudogenized in species that have lost the ability to make teeth or dental enamel during evolution [21,15,22]. The Klk4 −/− mouse display severe maturation stage enamel malformations in the absence of systemic abnormalities [23,24] and defects in KLK4 cause an autosomal recessive, non-syndromic form of amelogenesis imperfecta (AI) [25–27]. Mutations in AMTN have not yet been shown to cause AI, but recently the Amtn knockout mouse was reported to exhibit a subtle enamel phenotype that primarily affects the outer and surface enamel layers [28].…”

“…In the absence of KLK4, enamel mineralization is normal up to early maturation after which the enamel retains proteins [38] and is unable to mature beyond 85% mineral by weight. The outer enamel is hard, but the inner and middle enamel is soft and contains much more protein than normal [24]. The enamel fails after eruption, as it tends to fracture in the inner enamel near the dentin surface [23,39].…”

“…The purpose of this study was to obtain a more detailed understanding of the enamel defects in the newly engineered Amtn knock-out/ lac Z knock-in mouse model [28] and to combine it with the well-defined Klk4 knock-out/ lac Z knock-in mouse model [23,24] to determine if AMTN likely participates along with KLK4 to facilitate the removal of enamel proteins or if AMTN acts independently or complementarily to KLK4 to enhance enamel mineralization deposition.…”

Evolution of Klk4 and enamel maturation in eutherians

“…Enamel proteins are retained in the Klk4 null enamel layer even as the teeth erupt into function (Yamakoshi et al, 2011). Klk4 null enamel is progressively less mineralized with depth (Hu et al, 2011; Smith et al, 2011) and tends to fracture near the dentino-enamel junction at the base of the enamel rods (Simmer et al, 2011a). The ability to remove protein from the matrix is not lost in Klk4 null mice, just impaired.…”