Titin and diaphragm dysfunction in chronic obstructive pulmonary disease.
BACKGROUND
Recently, we have shown that Ca2+-activated force generation in diaphragm single fibers is impaired in patients with mild to moderate chronic obstructive pulmonary disease (COPD). For optimal active-force generation, the passive elasticity provided by titin is indispensable.


OBJECTIVE
In the present study, we determined the passive-tension-length relations of single fibers of patients with mild to moderate COPD, hypothesizing that passive-elastic properties of diaphragm fibers are compromised.


METHODS
Passive-tension-length relations were determined in diaphragm fibers from patients with and without COPD (predicted mean FEV1, 76 and 102%, respectively). In diaphragm homogenates titin expression was studied at the protein level by gel electrophoresis and at the transcript level by using a novel titin exon microarray.


RESULTS
Diaphragm fibers from patients with COPD generate less passive tension on stretch. Titin content in the diaphragm did not differ between patients with and without COPD. However, titin exon transcript studies revealed up-regulation of seven exons, which code for spring elements in the elastic segment rich in proline, glutamate, valine, and lysine. Immunofluorescence analysis indicated elevated protein expression of the up-regulated splice variant in the COPD diaphragm. Simulation studies on titin molecules including the amino acids encoded by the seven up-regulated exons predicted reduced passive-tension generation on molecule stretch.


CONCLUSIONS
Passive-tension generation of diaphragm single fibers is reduced in patients with COPD. Our results suggest that alternative splicing of the titin gene, resulting in increased length of the elastic segment rich in proline, glutamate, valine, and lysine, is involved. Interestingly, these changes occur already in patients with mild to moderate COPD.
Ottenheijm
Coen A C
CA
Department of Pulmonary Diseases, 454 Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands.
Heunks
Leo M A
LM
Hafmans
Theo
T
van der Ven
Peter F M
PF
Benoist
Caroline
C
Zhou
Honghui
H
Labeit
Siegfried
S
Granzier
Henk L
HL
Dekhuijzen
P N Richard
PN
D000328:Adult; D064211:Connectin; D003964:Diaphragm / Q000378:metabolism / Q000503:physiopathology*; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D008297:Male; D009119:Muscle Contraction; D009124:Muscle Proteins / Q000096:biosynthesis / Q000502:physiology*; D011494:Protein Kinases / Q000096:biosynthesis / Q000502:physiology*; D029424:Pulmonary Disease, Chronic Obstructive / Q000503:physiopathology*
D064211:Connectin; D009124:Muscle Proteins; C577285:TTN protein, human; D011494:Protein Kinases