(Received for publication, November 29, 1995; and in revised form, February 13, 1996)
From the
Nitric oxide (NO) functions as a molecular mediator in numerous processes in cellular development and physiology. Differential expression and regulation of a family of three NO synthase (NOS) gene products help achieve this diversity of action. Previous studies identify post-translational modification and interaction of NOS with specific protein targets as tissue-specific modes of regulation. Here, we show that alternative splicing specifically regulates neuronal NOS (nNOS, type I) in striated muscle. nNOS in skeletal muscle is slightly more massive than nNOS from brain owing to a 102-base pair (34-amino acid) alternatively spliced segment between exons 16 and 17. Following purification, this novel nNOSµ isoform has similar catalytic activity to that of nNOS expressed in cerebellum. nNOSµ appears to function exclusively in differentiated muscle as its expression occurs coincidentally with myotube fusion in culture. An isoform-specific antibody detects nNOSµ protein only in skeletal muscle and heart. This study identifies alternative splicing as a means for tissue-specific regulation of nNOS and reports the first additional protein sequence for a mammalian NOS since the original cloning of the gene family.
Endogenous nitric oxide (NO) ()formation is catalyzed
by a family of NO synthase (NOS) enzymes that directly produce NO from L-arginine and NADPH in a calmodulin-dependent reaction that
stoichiometrically produces citrulline as a
coproduct(1, 2, 3, 4, 5) .
Molecular analyses identify three genetic loci for NOS. The
corresponding protein products have been named according to their
original sites of identification. Hence, neuronal NOS (nNOS or type I)
occurs at highest densities in brain; endothelial NOS (eNOS or type
III) is prominent in endothelial cells; and inducible NOS (iNOS or type
II) expression is dynamically up-regulated in cells following
immunological stimulation.
Endogenous NO was originally identified as the endothelial derived factor responsible for smooth muscle relaxation(6) . More recent studies have identified major functions for NO in the development and physiology of mammalian skeletal and cardiac muscle. NO facilitates fusion of cultured myoblasts (7) and mediates retrograde synaptic signaling in myocyte neuronal co-cultures(8) . In mature skeletal muscle, calcium influx associated with muscle depolarization is linked to NO formation which in turn stimulates guanylyl cyclase(9, 10) . This cascade modulates contractile force. NO is formed in skeletal muscle by nNOS that is localized beneath the sarcolemma of fast twitch muscle fibers. Subcellular localization is mediated by association of nNOS with the dystrophin glycoprotein complex(11, 12) . Disruption of the dystrophin complex in Duchenne muscular dystrophy causes a displacement of nNOS from the sarcolemma to the cytosol. This aberrant localization of nNOS may contribute to disease progression in muscular dystrophy.
Recent studies also indicate a role for NO in cardiac muscle physiology (13) . NOS activity in cardiac myocytes is stimulated by cholinergic agonists acting at inositol phosphate-linked muscarinic receptors. NO formed in this pathway mediates ionotropic and chronotropic depression of heart function in response to cholinergic vagal nerve activity(14, 15) . Constitutive expression of eNOS in cardiac myocytes has been described(16) .
Excitation-contraction coupling in cardiac and skeletal muscle is a complex process that is modulated by overlapping signaling pathways (17, 18) . Enzymes involved in regulation of muscle contraction are often expressed as unique isoforms in skeletal and cardiac muscle. These specialized isoforms may arise from differential gene expression (19) or alternative splicing(20) . Here, we report that nNOSµ is a novel isoform expressed in skeletal muscle. nNOSµ in muscle is slightly larger than nNOS from brain due to alternative splicing that adds a 102-base pair (34-amino acid) insert between exons 16 and 17. nNOSµ displays catalytic activity similar to that of the nNOS. Expression of nNOSµ is induced coincident with myotube fusion in cultured cells. Throughout the body nNOSµ occurs only in skeletal and cardiac muscle.
NOS activity in mouse skeletal muscle largely derives from
the nNOS locus as mutant mice carrying a targeted disruption of nNOS
lack detectable NOS activity in skeletal muscle
homogenates(11) . Western blotting demonstrates that nNOS
migrates as a slightly larger protein in skeletal muscle than in brain
homogenates (Fig. 1A). To determine whether this
difference results from alternative splicing, we conducted RT-PCR
experiments with overlapping pairs of primers that spanned the full
open reading frame of nNOS. Skeletal muscle and brain cDNA were
amplified; bands were resolved by electrophoresis and transferred to
nitrocellulose membranes, and nNOS-specific products were identified by
hybridization with a labeled nNOS cDNA probe. One pair of primers that
amplifies amino acids 690-896 of nNOS (21) consistently
yielded an nNOS PCR product from muscle that was 100 bp larger
than that from brain (Fig. 1B). All other nNOS primer
pairs amplified products of similar size from brain and muscle cDNA
(data not shown). Cloning and sequencing of the unique product
amplified from skeletal muscle indicated that it contains the
appropriate nucleotide sequence of nNOS with the addition of a 102-bp
sequence inserted precisely between exons 16 and 17(24) . This
102-base pair insert appears to reside within the 16th exon of nNOS as
the newly introduced sequence does not correspond to the sequences of
the 5` donor or 3` acceptor sequences in intron 16 of the nNOS gene
previously reported(24) . This insert occurs after amino acid
839 of nNOS (21) and codes for 34 amino acids that do not share
significant sequence homology with NOS or any other cloned gene (Fig. 1C). Adding this novel sequence to nNOS predicts
a 164-kDa protein product in skeletal muscle, consistent with that
observed by Western blotting. We therefore name this muscle-specific
isoform nNOSµ.
Figure 1:
nNOS in skeletal
muscle contains a unique 102-base pair (34-amino acid) insert between
the calmodulin and FMN domains. A, Western blotting shows that
nNOS from skeletal muscle (M) migrates as a slightly larger
protein band than nNOS from brain (B). Solubilized protein
homogenates from brain and skeletal muscle were partially purified by
2`,5`ADP-agarose chromatography, analyzed by 7.5% SDS-PAGE, and probed
with nNOS monoclonal antibody. Molecular weight standards are marked at
the left side of the panel. B and C, RT-PCR analysis
indicates that nNOS mRNA in skeletal muscle contains an additional
102-nucleotide sequence encoding 34 amino acids between Lys-839 and
Ser-840 of nNOS. B, products from brain and muscle RNA were
amplified using primers P1 and P2, transferred to a nitrocellulose
membrane, and the blot was hybridized with a cDNA probe to nNOS. The
product from brain migrates as a 650-bp band whereas that from
muscle runs at
750 bp. C, schematic model showing the
nucleotide and predicted amino acid sequence of the unique region
amplified from skeletal muscle cDNA. Alignment with eNOS and iNOS shows
that the alternatively spliced region corresponds to the domain not
present in iNOS. Abbreviations: CaM, calmodulin; myr,
myristoylation site.
We evaluated nNOSµ protein expression by heterologous cell transfection. Expression constructs encoding nNOS or nNOSµ were transfected into COS cells, and following 3 days incubation, protein homogenates from the cells were analyzed. Western blotting demonstrated that nNOSµ migrates as a 164-kDa band that is slightly larger than nNOS which is 160-kDa. The migration of transfected nNOSµ and nNOS is indistinguishable from that of nNOS proteins purified from skeletal muscle and brain, respectively (Fig. 2A).
Figure 2:
nNOSµ and nNOS have similar catalytic
activity. A, Western blotting shows that nNOS and nNOSµ
expressed in COS cells comigrate with nNOS proteins purified from rat
brain (B) and skeletal muscle (M), respectively. COS
cells were transfected expression vectors encoding nNOS or nNOSµ.
Solubilized protein homogenates from transfected cells or rat tissues
were partially purified by 2`,5`ADP-agarose and analyzed by Western
blotting. B, kinetic constants V and K
are nearly identical for nNOSµ and
nNOS. NOS isoforms were partially purified from COS cells. Assays were
carried out with normalized amount of enzyme in presence of 0.25 mM of free calcium, 0.1 µM calmodulin. C and D, nNOSµ and nNOS show similar EC
for calcium
and calmodulin. NOS activity was measured in the presence of 0.1
µM calmodulin (C) or in the presence of 200
nM of free calcium (D). Data are means of triplicate
determinations that varied by <10%. These experiments were repeated
twice with similar results.
We evaluated catalytic activity of
nNOSµ purified from transfected cells. nNOSµ displayed specific
NOS activity similar to that of nNOS. Kinetic constants V and K
(
3
µM) are essentially identical between nNOSµ and nNOS (Fig. 2B). The alternative splice occurs in the
vicinity of the calmodulin binding domain of nNOS. We therefore
carefully evaluated calcium and calmodulin dependence of the novel
isoform. nNOSµ showed an absolute dependence on calcium and
calmodulin. The EC
for both calcium (
100 nM)
and calmodulin (
10 nM) was essentially identical for
nNOSµ and nNOS (Fig. 2, C and D).
Many specialized proteins involved in regulation of contraction are uniquely expressed in differentiated muscle but not in developing myoblasts(25) . To determined whether nNOSµ represents a regulator of this type, we evaluated expression in cultured myocytes. Primary rat muscle cultures were prepared from postnatal day 3 gastrocnemius. Myoblasts were grown to confluence in media containing 20% serum and were then fused to myotubes in media containing 2% serum. Protein homogenates were collected from myoblasts and myotubes and were evaluated for nNOS expression. Western blotting showed expression of a 160-kDa nNOS protein band in cultured myoblasts that comigrated with nNOS from brain (Fig. 3A). By contrast fused myotube cultures contained both a 164- and a 160-kDa band.
Figure 3:
Expression of nNOS in primary myocyte
culture. A, Western blot shows the appearance of nNOSµ
(164 kDa) only in differentiated myotubes, whereas the nNOS form (160
kDa) is present in both myoblasts and myotubes. Primary cultures of
neonatal rat muscle were grown for 48 h in vitro (myoblasts)
and fused in differentiation media for 24 h (myotubes). Protein
extracts were separated by SDS-PAGE and analyzed by Western blotting. B, nNOSµ mRNA expression is induced during myotube
differentiation. RT-PCR analysis with primers 1 and 2 shows that
amplification from myoblast cDNA generates only a 650-bp product
that corresponds to the nNOS amplified from brain (B).
Amplification from myotube cDNA yields a mixture of this 650-bp product
and a
750-bp product. This larger band comigrates with nNOSµ
amplified from skeletal muscle (M). + represents
amplification from a positive control nNOS cDNA in pBluescript. RNA
from cultured cells or rat tissues were processed by RT-PCR; the
amplified products were resolved by 1% agarose gel electrophoresis,
transferred to nylon membranes, and the blot hybridized with a labeled
nNOS probe.
To determine
whether this 164-kDa protein band in myotubes represents nNOSµ, we
evaluated nNOS mRNA from the cultured cells. RT-PCR analysis with
primers that flank the identified alternative splice were used to
amplify nNOS from myoblast or myotube cDNA samples. RT-PCR from
myoblast cultures selectively amplified an 650-bp band that
comigrated with the band amplified from brain cDNA (Fig. 3B). By contrast amplification of myotube cDNA
generated bands of
650 and
750 bp that comigrated with nNOS
products amplified from brain and muscle cDNA, respectively. Sequencing
revealed that the 750-bp band contained a 102-bp alternative splice
between exons 16 and 17 exactly as found in skeletal muscle cDNA. These
data indicate that a developmental switch from nNOS to nNOSµ occurs
coincident with myotube fusion.
To definitively determine whether the alternative splice identified here is translated in vivo, we generated a polyclonal antiserum to a unique peptide sequence encoded by the 102-bp insert (see ``Experimental Procedures''). We evaluated specificity of this serum by Western blotting. nNOS and nNOSµ from transfected cells and purified nNOS from brain and muscle were resolved by SDS-PAGE, and proteins were transferred to a nylon membrane. The blot was first analyzed with a monoclonal antibody to nNOS that reacted with appropriate 160- or 164-kDa bands in all four lanes (Fig. 4B). We reprobed the same blot with an nNOSµ-specific antiserum that selectively reacted with 164-kDa nNOSµ bands from transfected cells and from skeletal muscle extracts (Fig. 4A). nNOSµ was not detectable in brain.
Figure 4: nNOSµ is uniquely expressed in skeletal muscle and heart. A, Western blotting with an nNOSµ-specific peptide antibody shows that nNOSµ protein occurs in skeletal muscle (M) but not in brain (B). This antibody also selectively recognized nNOSµ expressed in transfected COS cells. B, reprobing the same blot with a general nNOS antibody shows that nNOS proteins were present in all 4 samples. Protein extracts from transfected COS cells or rat tissue homogenates of NOS from tissue homogenate were analyzed by Western blotting. C, Western blotting shows that nNOS protein is differentially expressed in variety of rat tissues (S, spleen; K, kidney; H, heart; L, liver; M, skeletal muscle; B, brain). Reprobing a duplicate blot with nNOSµ-specific antibody shows selective expression of this isoform in skeletal muscle and heart. NOS proteins from 5 mg of solubilized protein from rat tissues were partially purified by 2`,5`ADP-agarose and analyzed by Western blotting. The brain sample was loaded at 10% the level of other tissues to compensate for the higher expression of nNOS in brain.
We used this specific antibody to determine the distribution of nNOSµ in a variety of tissues. NOS proteins were partially purified from brain, skeletal muscle, lung, liver, heart, and kidney and were separated by SDS-PAGE. Western blotting showed that nNOS was most prominently expressed in brain and, in descending order of abundance, occurs in skeletal muscle, spleen, heart, kidney, and liver (Fig. 4C). By contrast, nNOSµ occurred selectively in skeletal muscle and heart.
nNOS was first cloned from a brain cDNA library based on peptides sequenced from the purified cerebellar protein(21) . Subsequent studies have identified putative alternative splicing of nNOS mRNA. Previously identified examples of alternative splicing within the coding region of nNOS involve deletions of specific exons that are predicted to yield inactive proteins(24, 26) . Translation of these alternatively spliced products in intact tissues has not been detected. Alternative splicing of exon 1 of nNOS has been elegantly demonstrated; however, this occurs in the 5`-untranslated domain of the mRNA and does not affect the nNOS protein product(27) . The 34-amino acid insert reported here represents the first novel translated sequence for nNOS identified since the original cloning.
The alternative splice we find in muscle nNOSµ occurs between amino acids 839 and 840 of the neuronal protein. Alignment of the three known mammalian NOS gene products shows that they share >50% sequence identity. A significant gap in their alignment occurs only in the region between amino acids 832 and 875 of nNOS. It is interesting to note that the alternative splice we have identified occurs in this particular domain that is divergent from the other NOS isoforms. This region of NOS may be unique in allowing catalytically active alternatively spliced isoforms. Indeed, nNOSµ displays specific catalytic activity similar to that of nNOS. Although the alternatively spliced insert occurs in proximity to the calmodulin binding site of nNOS (amino acids 726-747), nNOSµ displays regulation by calcium and calmodulin similar to that of nNOS. The 34-amino insert contains 5 serine residues in proximity to basic amino acids consistent with possible sites of protein phosphorylation.
NO functions as a physiological regulator of skeletal muscle contractility(9) . Formation of NO is coupled to sarcolemmal depolarization (10) owing to the association of nNOSµ with the sarcolemmal dystrophin-associated glycoprotein complex(11) . Previous studies show that an N-terminal PDZ/GLGF protein motif is necessary for interaction of skeletal muscle nNOS with the sarcolemma. Disruption of the dystrophin complex in mdx mice and humans with Duchenne muscular dystrophy causes a displacement of nNOS from sarcolemma and accumulation in the cytosol(11) . By contrast membrane association of nNOS in brain does not require dystrophin. The selective expression of nNOSµ in skeletal and cardiac muscles, tissues enriched with dystrophin(28) , may indicate a role for nNOSµ as an effector enzyme for the dystrophin complex.
In cardiac muscle, NO formation is stimulated by parasympathetic nerve activity(13, 15) . NOS inhibitors block the suppression of cardiac muscle contraction that normally occurs following vagal nerve stimulation. Mechanistically, NO appears to inhibit adrenergic increases in L-type calcium currents(14) . Molecular studies by Michel and colleagues (16) have demonstrated eNOS expression in protein homogenates from heart. Immunohistochemical studies identified eNOS in capillary endothelial cells of intact heart and in cultured cardiac myocytes. These investigators did not identify nNOS in heart. By contrast we detect low levels of nNOS by Western blotting of protein extracts from rat heart. Furthermore, we detect the nNOSµ isoform, which is otherwise restricted to skeletal muscle. This likely indicates that nNOS in heart occurs in the cardiomyocytes rather than in neurons or other cell types.