Biomedical Engineering Reference
In-Depth Information
9.1.2.4 Neurodegeneration
Elevated levels of KLK1 and KLK6 have been found in plasma from multiple
sclerosis (MS) patients,
46
while reduced levels of KLK6 were measured in CSF
of Alzheimer disease patients.
47
In animal models of MS, KLK6 is secreted by
infiltrating immune cells at sites of CNS inflammation and is considered to
cleave myelin proteins.
48
Blocking the activity of KLK6 reduces clinical
symptoms and the onset of experimental autoimmune (allergic) encephalo-
myelitis (EAE) in rodents, which is an established experimental model for MS.
48
On the other hand, it has been reported that KLK6 is secreted by oligoden-
drocytes and is necessary for the remyelination process and maintenance of
normal myelination patterns.
49
Consequently, it is speculated that also in the
nervous system, KLKs may exert dual functions, as also described for thrombin.
Although low concentrations of thrombin are neuroprotective, high con-
centrations, which may be caused by an increased blood-brain barrier perme-
ability, exacerbate neuronal cell death.
50
In addition, it has been found that
KLK6 localizes in Lewy bodies,
51
hallmark features of synucleinopathies, such
as Parkinson's disease, and is linked to degradation of a-synuclein in vitro.
51-53
9.1.3 Structure of KLKs
All KLK genes are composed of five coding exons with conserved intron phases
(I-II-I-0). The three catalytic amino acid residues, namely His, Asp, and Ser, are
encoded by exons II, III, and V, respectively (Figure 9.1). All KLK proteins are
synthesized as preproenzymes with lengths varying between 244 and 293 amino
acid residues.
7,8,16
The secretion signal (pre) contains 16-33 residues and is
necessary to direct KLK proteins to the secretory pathway. Activation of KLK
proforms requires removal of the pro-peptide sequence, which is composed of
three to 37 N-terminal residues. The mechanisms of pro-peptide removal
include autoactivation, activation by other KLKs, or crossactivation by other
proteases.
7,8
The length of mature KLK enzymes varies between 223 residues
(for KLK6) and 252 (for KLK13). The three-dimensional structure has been
resolved by X-ray crystallography for KLK1,
54
KLK3,
55
KLK4, 5, and 7,
56
KLK6,
57
and pro-KLK6.
58
Five or six disulfide bonds participate in KLK
protein folding. Removal of the pro-peptide results in a salt bridge formation
between the ammonium group of the N-terminal Ile/Leu
16
(chymotrypsin
numbering is followed throughout this review) and the side chain of Asp,
194
a
prerequisite for their enzymatic activity. In addition, the kallikrein loop at
position 99 (or loop-99) is a characteristic feature for kallikreins, but it is
present only in KLK1, 2, and 3. This loop is located prior to the active site Asp
and is characteristic for kininogenase activity resulting in release of kinin (lys-
bradykinin) from LMWK. Interestingly, however, KLK3 does not cleave
kininogen, while KLK2 has 1000-fold lower activity than KLK1 in vitro.
59
On
the other hand, KLK5 and KLK14 can cleave LMWK in vitro, but they do not
generate kinins,
60,61
and KLK8 can cleave HMWK.
62
