Cell News 4/2014
11
Research news
General features of calcineurin
As the diversity of calcineurin targets is quite high (Li et al.,
2011) the phosphatase plays a role in many different cellular
processes. Since the discovery of calcineurin in the late 1970s
this phosphatase was mainly investigated as the target of the
immunosuppressive drugs CsA and FK506 (Siekierka and Sigal,
1992). Therefore the involvement of calcineurin in the immu-
ne system is one of the best-studied functions of the protein.
However, in mammals calcineurin is also involved in different
diseases like Alzheimer and prion disease, hypertrophy of the
heart, diabetes, or different cancer types (Lawrence et al., 2001;
Heit, 2007; Bousette et al., 2010; Liu et al., 2010; Mukherjee et
al., 2010; Wu et al., 2010b).
In lower eukaryotes calcineurin plays a role in various cellular
processes, of which the most important are the regulation of
stress response, cell wall integrity, drug resistance, glucose me-
tabolism, and pathogenesis (Kraus and Heitman, 2003; Stein-
bach et al., 2007; Bastidas et al., 2008; Ruiz et al., 2008; Calvo
et al., 2009). Although most of the research about calcineurin in
lower eukaryotes is restricted to fungi, the phosphatase became
also interesting as a virulence factor and a potential drug tar-
get in parasites, especially in the malarial parasite
Plasmodium
falciparum
(Bell et al., 1994; Dobson et al., 1999; Kumar et al.,
2005; Bastidas et al., 2008; Singh et al., 2013). It turned out
that inhibition of calcineurin with CsA and FK506 results in a
synergistic effect if combined with established antifungal or an-
timalarial drugs. But it has to be taken into account that both,
CsA and FK506, might have severe side effects on the host cells
(Calne et al., 1978; Todo et al., 1987) or on the environment, e.g.
insects (Sartain and Wolfner, 2013). Therefore, researches aim at
finding new drugs, which inhibit more specific downstream tar-
gets of the calcineurin signalling pathway instead of the phos-
phatase itself. For the calcineurin-NFAT signalling in mammals
this approach becomes more and more attractive (reviewed in
(Sieber and Baumgrass, 2009)). For lower eukaryotes, especially
pathogenic fungi, specific structures of fungal calcineurin and/
or Crz1 and its orthologues might be as well promising drug
targets (Juvvadi et al., 2013).
Additionally, the calcineurin signal transduction pathway might
be an attractive target to optimise biotechnological processes
like e.g. ganoderic acid production of the mushroom
Ganoderma
lucidum
(Xu and Zhong, 2012; Xu et al., 2013).
The role of calcineurin during development of higher
eukaryotes
In recent years it has been shown that calcineurin is involved in
a couple of developmental processes in higher eukaryotes (Ta-
ble 1). It has been shown that the impairment of calcineurin
function in mice leads to an increased branching of dendrites
(inhibition of calcineurin (Spires-Jones et al., 2011)) or to a de-
crease in dendritic branching (constitutively active calcineurin
(Wu et al., 2010a)). Inhibition of calcineurin also leads to defects
in axonal outgrowth during mouse embryogenesis (Graef et al.,
2003). The exact cellular events underlying these defects are
not known but some of the defects are mediated by the cal-
cineurin substrate NFAT (Graef et al., 2003; Wu et al., 2010a).
Similar results were obtained with calcineurin B mutant mice.
The mutants die at E10.0 due to a failure to properly pattern the
developing vasculature (Graef et al., 2001). Again, this phenoty-
pe is partially dependent on the downstream NFAT-cascade as
NFAT mutants also die during embryogenesis due to defects in
vessel assembly (Graef et al., 2001). However, mice lacking the
calcineurin A isoforms
α
or
β
are viable and do not show any
severe developmental defects (Zhang et al., 1996; Bueno et al.,
2002) pointing towards redundant functions of the isoforms.
Analyzing the brain of calcineurin B mutants at E9.5 revealed a
size reduction in the mutant brains and inhibition of calcineurin
during differentiation of embryonic stem cells in vitro blocked
neural induction (Cho et al., 2014). This is in accordance with
results showing that during gastrulation cytoplasmatic Ca
2+
-
levels rise in dorsal, but not in ventral, ectoderm and this might
be critical for neural induction (Leclerc et al., 2011). Interestin-
gly, it was shown that NFAT is not responsible for these brain
defects but that calcineurin modulates the bone morphogenic
protein (BMP) pathway (Cho et al., 2014). A study investigating
the role of calcineurin in limiting the synapse density during
development also showed that NFAT is not involved (Elmer et
al., 2013). The authors found that calcineurin acts via another
transcription factor, the myocete enhancer factor 2 (MEF2), to
regulate genes involved in this process (Elmer et al., 2013). This
signalling pathway was previously unknown and underscores
the need to investigate further targets of calcineurin in addition
to the well-established transcription factor NFAT.
Beside defects in brain and vasculature development, impair-
ment of calcineurin results in defects in kidney, bone, heart,
muscle and thymocyte development (Gooch et al., 2004; Neil-
son et al., 2004; Sun et al., 2005; Winslow et al., 2006; Gallo
et al., 2007; Mallinson et al., 2009; Yang et al., 2014). For the
kidney it has been shown that only the calcineurin A-
α
isoform
is required for normal development (Gooch et al., 2004). Mice
with deleted CNA-
α
show defects in the postnatal maturation
of the nephrogenic zone and the glomeruli, which leads to an
impaired kidney function. CNA-
α
is further important during
bone formation (Sun et al., 2005). Whereas overexpression of
this calcineurin A isoform increased the expression of the osteo-
blast differentiation marker Runx-2 combined with a dramatic
enhancement of bone formation, deletion of the isoform leads
to severe osteoporosis. Calcineurin acts thereby via NFAT as
mutants of this transcription factor (overexpression or deletion)
show the same phenotypes (Winslow et al., 2006). Concerning
the development of thymocytes it has been shown that calci-
neurin is important for the discrimination of signals (Neilson et
al., 2004; Gallo et al., 2007). During the developmental process
self-reactive thymocytes have to be eliminated by negative se-
lection whereas cells capable of responding to foreign antigens
must be positively selected (Neilson et al., 2004). Usually signals
for negative selection are very strong whereas signals for posi-
tive selection are weak. Calcineurin now is involved in detec-
tion of the weak signals to induce positive selection (Neilson
