Abstract 806: Loss of Smooth Muscle Myosin -SM2 Affects Muscle Physiology and Leads to Postnatal Lethality
Most smooth muscle tissues express both S1 and SM2 isoforms, which are products of alternate splicing of the same gene. Compared to SM1, which has a longer carboxyl terminus (43 aa), SM2 myosin has a unique C-terminus comprised of 9 amino acids. During development SM1 is expressed from early developmental stages, whereas SM2 myosin appears only after birth and SM1/SM2 ratio reaches 1:1 in the adult. To establish the role of SM2 myosin in smooth muscle physiology, we developed a “novel gene targeting strategy” which results in the loss of SM2-myosin but preserves the expression of SM1 myosin in mice. The deletion of SM2-isoform-specific exon results in a complete loss of SM2 myosin, and switches to SM1 myosin. As a result, the total myosin content was not affected. The SM2 KO mice were delivered at the expected Mendelian ratio and were of normal body weight at birth. 1 week after birth SM2 KO mice started to show growth retardation. The body weight of 25 day old SM2 KO mice is significantly less than wild type mice (7.5±0.4, n=5; 17.8±0.6, n=5, respectively). The mice die ~30 – 40 days after birth. Histological studies showed that the smooth muscle layers of SM2 knock out mice in gastrointestinal duct and bladder were thinner than wild type and loss of SM2 myosin caused distention in gastrointestinal system and bladder. Bladder function was significantly compromised and hydronephrosis was observed in SM2 KO mice. To study vascular function, we used thoracic and abdominal aortas for isometric force measurements. Results showed that the contractile response of vessels from SM2 KO mice treated with 60 mM KCl (48.5 ± 24.0 vs 181 ± 29.1 mg) or 3-micromole a-1adrenergic agonist phenylephrine (52 ± 27.0 vs 242 ±32.7 mg) was significantly decreased as compared to WT mice. In conclusion, our data suggest that SM2 myosin isoform is important for maintaining smooth muscle function in aorta and other smooth muscle organs; compensatory increase in SM1 is not sufficient to rescue function.