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The Journal of Neuroscience, July 1, 2000, 20(13):4885-4889
A Temperature-Sensitive Paralytic Mutant Defines a Primary
Synaptic Calcium Channel in Drosophila
Fumiko
Kawasaki,
Ryan
Felling, and
Richard W.
Ordway
Department of Biology, The Pennsylvania State University,
University Park, Pennsylvania 16802
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ABSTRACT |
Neurotransmission at chemical synapses involves regulated
exocytosis of neurotransmitter from the presynaptic terminal.
Neurotransmitter release is thought to be triggered by calcium influx
through specific classes of voltage-gated calcium channels. Here we
report genetic and functional analysis implicating a specific calcium
channel gene product in neurotransmitter release. We have isolated a
temperature-sensitive paralytic allele of the Drosophila
calcium channel 1 subunit gene, cacophony
(cac). This mutant, referred to as
cacTS2, allows functional analysis of
synaptic transmission after acute perturbation of a specific 1
subunit. Electrophysiological analysis at neuromuscular synapses
revealed that neurotransmitter release in
cacTS2 is markedly reduced at
elevated temperatures, indicating that cac encodes a
primary 1 subunit functioning in synaptic transmission. These
observations further define the molecular basis of voltage-gated calcium entry at synapses and provide a new starting point for further
genetic analysis of synaptic mechanisms.
Key words:
calcium channel; neurotransmitter release; synaptic
transmission; cacophony; Drosophila; temperature-sensitive
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INTRODUCTION |
A number of studies have implicated
specific classes of voltage-gated calcium channels in neurotransmitter
release (for review, see Wheeler et al., 1995 ; Catterall, 1998 ). These
channels are composed of multiple subunits, including 1, the primary
structural subunit, as well as 2 , , and subunits. 1
subunits of the A and B classes have been localized to synaptic
terminals (Robitaille et al., 1990 ; Westenbroek et al., 1995 ), and
heterologous expression shows that their pharmacology resembles that of
calcium channels involved in neurotransmitter release. Furthermore,
these 1 subunits contain a defined synaptic protein interaction
(SYNPRINT) domain that interacts directly with the neurotransmitter
release apparatus and may participate in coupling calcium influx to
fast synaptic vesicle fusion (Mochida et al., 1996 ; Rettig et al.,
1997 ; Sheng et al., 1998 ). Our current understanding of calcium channel
function in neurotransmitter release will be further enhanced by
complementary genetic analysis defining the in vivo
functions and interactions of specific calcium channel gene products.
Here we report analysis of synaptic function in a temperature-sensitive
(TS) calcium channel mutant of Drosophila.
To define the physiological roles of specific proteins in synaptic
transmission, we have focused on Drosophila mutants
exhibiting rapid TS paralysis. These mutants typically develop and
function normally at permissive temperature and can be shifted to
restrictive temperature to examine the acute functional consequences of
perturbing a specific gene product. This approach was pioneered almost
30 years ago (Suzuki et al., 1971 ; Grigliatti et al., 1973 ; Siddiqi and
Benzer, 1976 ), and one of the mutants recovered in these early screens
was comatose (comt). Our previous work has shown
that the comt gene product, a homolog of the
N-ethylmaleimide-sensitive fusion protein (Ordway et
al., 1994 ; Pallanck et al., 1995 ), functions in priming synaptic
vesicles for fast, calcium-triggered fusion (Kawasaki et al., 1998 ;
Kawasaki and Ordway, 1999a ,b ). To broaden our analysis to other
gene products functioning in synaptic vesicle trafficking, we performed
a genetic screen to identify mutations exhibiting functional
interactions with comt (Dellinger et al., 2000 ). One
enhancer of comt was determined to be a TS allele of cacophony (cac) and has been designated
cacTS2. cac encodes a homolog
of voltage-activated calcium channel 1 subunits implicated in
neurotransmitter release (Smith et al., 1996 ).
The cac locus was first identified in a screen for mutants
exhibiting altered courtship song (von Schilcher, 1976 , 1977 ) and was
subsequently found to be allelic to the nightblind A
(nbA) locus (Heisenberg and Götz, 1975 ; Smith et al.,
1998 ). A synaptic function for cac-encoded calcium channels
was suggested by electroretinogram recordings from
cac (nbA) mutants (Heisenberg and Götz,
1975 ; Smith et al., 1998 ), by the similarity between cac and
1 subunits previously implicated in synaptic transmission (Smith et
al., 1996 ), and by the genetic interaction of
cacTS2 with comt
(Dellinger et al., 2000 ). Here we report functional analysis
demonstrating that cac encodes a primary 1 subunit
functioning in neurotransmitter release.
Parts of this work have been reported previously in abstract form
(Dellinger et al., 1999 ; Kawasaki et al., 1999 ).
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MATERIALS AND METHODS |
Drosophila lines. Previously isolated cac
lines were generously provided by Jeffrey C. Hall (Brandeis University,
Waltham, MA). The deficiency line Df(1)KA10 was obtained
from the Bloomington Stock Center (Indiana University, Bloomington,
IN). The left and right limits of this deficiency are 11A01 and
11A07-08, respectively. Wild-type flies were Canton S. All lines used
for electrophysiological recording were maintained at 20°C.
Synaptic electrophysiology. For experiments at dorsal
longitudinal flight muscle (DLM) neuromuscular synapses, dissection, temperature control, motor axon stimulation, and two-electrode voltage-clamp recordings of synaptic currents were performed as described previously (Kawasaki et al., 1998 ).
Recordings of intracoxal lateral levator muscle (ICLM) synaptic
currents were obtained as follows. A fly was anesthetized with
CO2, mounted ventral side up over a hole in an
air tube, and secured with wax (Tackiwax Boekel Industries,
Feasterville, PA). Air was delivered to the tracheal system using an
aquarium pump. The preparation was submerged in recording solution
maintained at 20°C. A tungsten knife was used to remove the ventral
thoracic cuticle and the coxal muscles, thereby exposing the thoracic
ganglion of the CNS. To expose the ICLMs of the first pair of
legs, the overlying cuticle was removed from each coxa. The first leg
nerve exiting the thoracic ganglion was cut and pulled into a suction electrode for stimulation.
ICLM synaptic currents were recorded by discontinuous single electrode
voltage clamp using an AxoClamp-2B amplifier (Axon Instruments, Foster
City, CA). Deviations from the command potential did not exceed 5 mV.
Glass recording microelectrodes were filled with 3 M KCl.
The recording solution consisted of (in mM): 129 NaCl, 2 KCl, 4.0 MgCl2, 1.0 CaCl2,
5 HEPES, and 36 sucrose. The pH was adjusted to 7.0 using NaOH.
Stimulation of the first leg nerve was performed using a suction
electrode driven by an S-900/S-910 stimulator (Dagan Instruments,
Minneapolis, MN). Temperature control was achieved with a TC-202
temperature controller and PDMI microincubator (Medical Systems
Corporation, Greenvale, NY).
All recordings at 33 or 36°C were obtained after 5-20 min of
exposure to the recording temperature. Recordings at 38°C were obtained after 1-2 min at this temperature.
Data acquisition and analysis. Data were acquired on-line
using a Power Macintosh computer (Apple Computers, Cupertino, CA), Pulse software (Heka Electronik, Lambrecht, Germany), and an ITC-16 laboratory interface (Instrutech Corporation, Great Neck, NY). Data
were low-pass filtered at 5 or 10 kHz and acquired at 30 kHz.
Measurement of synaptic currents was performed using cursor measurements in the data analysis software package IGOR (Wavemetrics, Lake Oswego, OR). Microsoft (Seattle, WA) Excel was used for data tabulation, graphing, and statistical analysis. Data are reported as
the mean ± SEM. By the use of an unpaired Student's t
test, statistical significance was assigned to comparisons with
p 0.05
Sequence analysis. The cac 1 subunit sequence
(U55776) was aligned to human 1A (U76666), rat 1B (M92905), and rat 1C (M67516) sequences by the Clustal method, using the MegAlign
feature of the Lasergene sequence analysis software package (DNAStar
Inc., Madison, WI). BLAST sequence similarity searches of the
Drosophila genome were performed using the National Center for Biotechnology Information (NCBI) Drosophila genome
resources (http://www.ncbi.nlm.nih.gov/PMGifs/Genomes/7227.html). Query nucleotide and protein sequences included those corresponding to the
SYNPRINT domain of the above vertebrate 1A and 1B genes [defined
as residues 718-963 in 1B (Sheng et al., 1998 )], as well as the
analogous domain of 1C [residues 753-893 (Wiser et al.,
1999 )].
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RESULTS |
A temperature-sensitive synaptic phenotype in
cacTS2
Electrophysiological analysis was performed in wild-type and
cacTS2 flies to investigate whether
cac-encoded l subunits function in synaptic transmission.
Two-electrode voltage-clamp techniques were used to record synaptic
currents at DLM neuromuscular synapses. This technique prevents
activation of postsynaptic voltage-gated ion channels and thus records
the current passing through ligand-gated neurotransmitter receptor
channels. The cacTS2 mutant showed a
conditional and reversible electrophysiological phenotype (Fig.
1). At 20°C,
cacTS2 exhibited wild-type synaptic
currents. In contrast, exposure to elevated temperatures produced a
marked reduction in the amplitude of the synaptic current with respect
to wild type. This phenotype was reversible, showing full recovery upon
return to 20°C. The extent of the synaptic current reduction was
dependent on temperature (Fig. 2). With
respect to wild type, the current amplitude was reduced to 61.5 ± 10.4% (n = 5) at 33°C, 30.1 ± 5.2%
(n = 8) at 36°C, and 16.6 ± 3.2%
(n = 4) at 38°C.

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Figure 1.
cacTS2 exhibits
a conditional and reversible reduction in the synaptic current. DLM
synaptic currents evoked by stimulation of the DLM motor axon were
recorded at 20 and 36°C, and at 20°C after exposure to 36°C
(20°C recovery). The 20°C recovery traces in wild
type and cacTS2 were recorded after 1 and 10 min at 20°C, respectively. Axon stimulation is marked by the
arrow. In each case, the 36°C and 20°C recovery
traces were obtained from the same preparation.
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Figure 2.
The extent of synaptic current reduction in
cacTS2 is dependent on temperature.
A, Peak amplitude measurements of DLM EPSCs from
cacTS2 are shown as a mean percentage
of wild-type currents. Error bars indicate SEM, and values
significantly different from wild type are marked by an
asterisk. B, Sample recordings from wild
type and cacTS2 at 38°C.
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The above results indicate that cac-encoded 1 subunits
function in synaptic transmission. Although our previous behavioral analysis shows that the cacTS2,
cacS, and l(1)L13
mutations are allelic (Dellinger et al., 2000 ), we performed similar
tests using electrophysiological analysis to confirm that the observed
synaptic phenotype also maps to the cac locus. The recessive
nature of cacTS2 was confirmed in
recordings from heterozygous
cacTS2/+ flies, which exhibit
wild-type synaptic currents at restrictive temperature (Fig.
3A). In flies heterozygous for
cacTS2 and a deficiency that removes
cac, the cacTS2 phenotype
was observed (Fig. 3B). Finally, complementation tests were
performed with other known cac alleles. A previously
identified cac lethal mutation, l(1)L13, failed
to complement the cacTS2
electrophysiological phenotype (Fig. 3C), indicating that
this phenotype maps to the cac locus. This was further
confirmed in recordings at
cacTS2/cacS
synapses as described below.

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Figure 3.
The synaptic current reduction observed in
cacTS2 mutants maps to the
cac locus. DLM synaptic current recordings in
cacTS2/+ indicate a recessive
cacTS2 synaptic phenotype
(A). Recordings from
cacTS2/Df(1)KA10
(B) and cacTS2/l(1)L13
(C) map the observed synaptic phenotype to the
cac locus.
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The above results show that a TS mutation in cac results in
a conditional reduction in the synaptic current. We considered it
likely that the cacTS2 phenotype was
presynaptic on the basis that vertebrate homologs of cac
have been implicated in neurotransmitter release and that our
voltage-clamp analysis should prevent postsynaptic calcium channels
from contributing to the recorded currents. To address this point
directly, experiments were performed at neuromuscular synapses of the
ICLM. This preparation is well suited for recording miniature EPSPs
(mEPSPs). Because the amplitude of these mEPSPs reflects the
postsynaptic response to a single quantum of neurotransmitter, mEPSPs
may be used to monitor postsynaptic sensitivity to neurotransmitter. Voltage-clamp analysis of evoked ICLM synaptic currents confirmed that
cacTS2 exhibits a conditional
reduction in the synaptic current as observed at DLM synapses (Fig.
4A). Despite a marked
reduction in the evoked synaptic current at 36°C,
cacTS2 synapses exhibited wild-type
mEPSP amplitudes under the same conditions (Fig. 4B).
Thus, we conclude that the cacTS2
phenotype is presynaptic, resulting from a conditional reduction in
neurotransmitter release. In light of the striking reduction of the DLM
synaptic current observed in cacTS2
at 38°C (Fig. 2), it appears that cac encodes the primary
calcium channel 1 subunit responsible for neurotransmitter release
at these synapses.

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Figure 4.
Recordings of spontaneous and evoked
neurotransmitter release at ICLM synapses confirm a presynaptic role
for cac-encoded 1 subunits in neurotransmitter
release. A, Recordings of evoked synaptic currents from
ICLM neuromuscular synapses in wild type (WT) and
cacTS2. Similar results were obtained
in three experiments for each genotype. B, Histograms of
wild-type and cacTS2 mEPSP amplitudes
(mEPP Amplitude). The solid line
represents a Gaussian fit to the data. Insets shown
representative mEPSP recordings. Similar results were obtained in three
experiments from each genotype.
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An activity-dependent and temperature-independent synaptic
phenotype in cacS
Consistent with the cacTS2
phenotype, recordings from the original cac allele,
cacS, also showed reduced synaptic
currents. However, unlike cacTS2,
cacS exhibited a strictly
activity-dependent decrease in the synaptic current and no clear
dependence on temperature (Fig. 5).
Stimulation (1 Hz) at either 20°C or 36°C produced a
wild-type synaptic current in response to the first stimulus, followed
by reduced amplitude currents in response to subsequent stimuli. The
distinct activity dependence of the
cacS phenotype is illustrated by
comparing cacS,
cacTS2, and wild-type synaptic
currents during 1 Hz stimulation at 36°C (Fig.
5B).

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Figure 5.
cacS exhibits an
activity-dependent reduction in the synaptic current. A,
Sample DLM synaptic currents from
cacS in response to 1 Hz stimulation
at 20 and 36°C. In each case, the first 50 traces were superimposed.
Note the activity-dependent reduction in the synaptic current at both
temperatures. B, Peak amplitude measurements of DLM
EPSCs from wild type (WT),
cacS, and
cacTS2 are plotted as a function of
time during 1 Hz stimulation trains at 36°C. Each
point represents the mean ± SEM for four
experiments in cacS and
cacTS2 and seven experiments in wild
type.
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As was the case for cacTS2, the
cacS phenotype was observed in flies
heterozygous for cacS and
Df(1)KA10 (data not shown). In addition, recordings obtained from
cacS/cacTS2
flies were informative. cacTS2
complemented the cacS phenotype at
20°C but failed to complement at 36°C (Fig.
6). These results further define the
conditional phenotype of cacTS2 (see
Discussion).

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Figure 6.
Heteroallelic interactions of
cacTS2 and
cacS. DLM synaptic current recordings
at
cacTS2/cacS
synapses. The cut DLM motor axon was stimulated at 1 Hz, and in each
case, the first 50 traces were superimposed. Similar results were
obtained in four experiments.
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DISCUSSION |
The availability of cacTS2, a
calcium channel mutant exhibiting rapid temperature-sensitive
paralysis, has provided a unique opportunity to examine the
physiological role of a specific calcium channel gene product in
synaptic transmission. Here we report genetic and functional analysis
in two cac alleles,
cacTS2 and
cacS, indicating that cac
encodes a primary calcium channel 1 subunit functioning in
neurotransmitter release.
Behavior
The present results provide further functional characterization of
the cac locus, first identified in a screen for mutations affecting the male courtship song (von Schilcher, 1976 , 1977 ). The
courtship song is produced by a patterned beating of the wings, and
this pattern, as well as the wing-beat amplitude, are altered in
cacS mutants (von Schilcher, 1976 ,
1977 ; Smith et al., 1998 ). The results presented here suggest that
impairment of neurotransmitter release at central synapses may
contribute to altered song patterning in
cacS. Given that
cac-encoded 1 subunits function at flight muscle neuromuscular synapses, peripheral synaptic defects may influence the
song phenotype as well.
cacS also exhibits motor defects at
elevated temperatures, including a lack of coordinated movement,
spinning behavior, and ultimately paralysis after long exposures
(Peixoto and Hall, 1998 ). Although the basis of this
temperature-dependent behavior in
cacS remains unclear, spontaneous
neural activity generally increases at elevated temperatures
(cf. Kawasaki and Ordway, 1999a ) and thus may produce a
more severe activity-dependent reduction in synaptic transmission.
The cacS synaptic phenotype
A distinctly activity-dependent and temperature-independent
phenotype was observed at cacS
synapses. The activity-dependent reduction in the synaptic current is
of interest in light of the molecular lesion identified in cacS. The mutation maps to the sixth
transmembrane segment of the third repeat (IIIS6), converting a highly
conserved phenylalanine to an isoleucine (Smith et al., 1998 ). Because
S6 segments have been implicated in fast inactivation of calcium (Zhang
et al., 1994 ; Hering et al., 1998 ), sodium (Rojas et al., 1991 ; McPhee et al., 1995 ; Cannon, 1996 ), and potassium (Hoshi et al., 1991 ) channels, these results raise the possibility that altered inactivation contributes to the activity dependence of the
cacS phenotype. Heterologous
expression of wild-type and mutant forms of the cac gene,
followed by analysis of the resulting calcium currents, is expected to
further address this issue. Similarly, molecular characterization of
cacTS2 is expected to reveal the
structural basis of the TS phenotype in this mutant.
Heteroallelic interactions of
cacTS2 and
cacS
In
cacTS2/cacS
flies, cacTS2 complements the
activity-dependent synaptic phenotype of
cacS at 20°C. However, shifting
the temperature to 36°C does not produce a reduction in the synaptic
current as observed in cacTS2 but
rather reveals the activity-dependent current reduction characteristic of cacS channels. Thus, it appears
that the population of 1 subunits mediating fusion of a synaptic
vesicle may contain a mixture of cacTS2 and
cacS subunits and that each
subpopulation can support a wild-type level of neurotransmitter
release. These observations suggest substantial redundancy in calcium
channel function at sites of synaptic vesicle fusion.
cac-encoded 1 subunits lack homology to defined
SYNPRINT domains
The cac amino acid sequence is most closely related to
the vertebrate 1A and 1B subunits (Smith et al., 1996 ), which
contain a SYNPRINT domain within the second intracellular loop.
However, alignments of these sequences (see Materials and Methods)
indicate that this SYNPRINT domain is absent from the cac
1 subunit, producing a gap at this position in the cac
sequence. Similar results were obtained in alignments of cac
with the rat 1C subunit, which contains a different synaptic protein
binding domain at this position (Wiser et al., 1999 ). The above results
are consistent with BLAST queries of the Drosophila genome
(Adams et al., 2000 ) using either of the above synaptic protein binding
sequences (see Materials and Methods), which reveal no homologous
domains. Given the primary role of the cac gene product in
neurotransmitter release, the absence of a conserved synaptic protein
binding sequence suggests either a novel interaction domain or an
alternative mechanism for fast coupling of calcium influx to synaptic
vesicle fusion.
The importance of synaptic calcium channels is reflected by the
in vivo consequences of their genetic disruption. In
addition to the synaptic and behavioral phenotypes observed in
cac mutants, mutations in homologous 1 subunit genes give
rise to several genetic disorders in humans and mice (Hess, 1996 ;
Miller, 1997 ). Thus, synaptic calcium channels play important roles in
behavior, synaptic transmission, and human disease. The results
presented here further define the molecular mechanisms of voltage-gated calcium entry at synapses and a provide a new starting point for genetic analysis of synaptic mechanisms in this model system (Brooks et
al., 2000 ).
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FOOTNOTES |
Received Jan. 21, 2000; revised April 12, 2000; accepted April 24, 2000.
This work was supported by National Science Foundation Grant
IBN-9514485. We gratefully acknowledge several members of the lab,
Bonnie Dellinger, Missy Hazen, and Kamal Tilakaratne for participating
in aspects of this work. We thank Jeffrey C. Hall (Brandeis University)
for providing several cac stocks and for stimulating
discussions. Deficiency stocks were obtained from the Bloomington Stock Center.
Correspondence should be addressed to Richard W. Ordway, Department of
Biology, 208 Mueller Laboratory, Penn State University, University
Park, PA 16802. E-mail: rwo4{at}psu.edu.
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