This thesis is dedicated to my grandparents

A second important protein group implicated in regulation of mineralization are the Glaproteins
(Figure 5). Named after the glutamic acid (gla) residues which they contain, they all
have a similar metabolic processing comprising transcription and translation to an inactive
protein, followed by post-translational modification. This post-translational modification consists of
the carboxylation of one or more gla-residues by a gamma-carboxylase enzyme.
The γ-carboxylase (GGCX) is one of two enzymes within the vitamin K-cycle, together
with the vitamin K epoxide reductase (VKOR; Figure 5). For these enzymatic processes, vitamin
K acts as an essential co-factor, which needs recycling due to the limited amount of vitamin K in
the diet. Several vitamin K-dependent (or Gla-) proteins have been discovered, which undergo
processing via this metabolic cycle. These include coagulation factors II (prothrombin), VII, IX
and X, protein S and protein C but also several inhibitors of calcification, such as matrix gla
protein (MGP) and osteocalcin (OC), and four integral membrane proteins of unknown function.
The carboxylase enzyme itself is negatively regulated by a protein called calumenin.
Figure 5
The vitamin K cycle. Reduced vitamin K acts as a co-factor of the γ-carboxylase GGCX, activating hypofunctional
proteins. The vitamin K, which becomes oxidized during this modification, is reduced by VKOR.
F2-7-9-10: vitamin K dependent clotting factors; PROC: protein C; PROS: protein S; PROZ: protein Z; MGP: matrix gla
protein; BGLAP: bone gla protein (or osteocalcin); GAS6: growth arrest-specific gene 6; CALU: calumenin.
MGP is expressed in vascular smooth muscle cells and chondrocytes but not in
osteoblasts, whereas OC is expressed in osteoblasts, odontoblasts and terminally differentiated
chondrocytes. Whereas OC-deficient mice show no alteration in ECM mineralization, Mgp-/- mice
show extensive vascular and cartilaginous mineralization, leading to early death due to arterial
rupture [55, 56]. The human correlate is Keutel syndrome, an autosomal dominant disorder
characterized by extensive cartilage calcifications which, in contrast to the animal model, is
however not lethal [57]. The presence of MGP has been demonstrated in association with the
ECM and specifically the elastic laminae in the human arterial vessel wall [58]. In areas of
vascular calcification, colocalization of MGP and the elastic laminae is lost and MGP is found at
the borders of vascular mineralization. It has been shown that absence of MGP promotes
atherosclerosis and plays an important role in ectopic calcification seen in terminal renal
insufficiency patients in need of dialysis [59, 60].
15