Titin is at present the largest known protein (M(r) 3000 kDa) and its expression is restricted to vertebrate striated muscle. Single molecules span from M‐ to Z‐lines and therefore over 1 micron. We have isolated cDNAs encoding five distant titin A‐band epitopes, extended their sequences and determined 30 kb (1000 kDa) of the primary structure of titin. Sequences near the M‐line encode a kinase domain and are closely related to the C‐terminus of twitchin from Caenorhabditis elegans. This suggests that the function of this region in the titin/twitchin family is conserved throughout the animal kingdom. All other A‐band sequences consist of 100 amino acid (aa) repeats predicting immunoglobulin‐C2 and fibronectin type III globular domains. These domains are arranged into highly ordered 11 domain super‐repeat patterns likely to match the myosin helix repeat in the thick filament. Expressed titin fragments bind to the LMM part of myosin and C‐protein. Binding strength increases with the number of domains involved, indicating a cumulative effect of multiple binding sites for myosin along the titin molecule. We conclude that A‐band titin is likely to be involved in the ordered assembly of the vertebrate thick filament.

Partial amino acid sequence was obtained from the massive myofibrillar protein nebulin. This consists of repeating motifs of about 35 residues and super‐repeats of 7 × 35 = 245 residues. The repeat‐motifs are likely to be largely α‐helical and to interact with both actin and tropomyosin in thin filaments. Nebulin from different species was found to vary in size in proportion to filament length. The data are consistent with the proposal that nebulin acts as a protein‐ruler to regulate precise thin filament assembly.

Tissue homeostasis requires an effective, limited wound-healing response to injury. In chronic disease, failure to regenerate parenchymal tissue leads to the replacement of lost cellular mass with a fibrotic matrix. The mechanisms that dictate the balance of cell regeneration and fibrogenesis are not well understood 1 . Here we report that fibrogenic hepatic stellate cells (HSCs) in the liver are negative regulators of hepatocyte regeneration. This negative regulatory function requires stimulation of the 5-hydroxytryptamine 2B receptor (5-HT 2B ) on HSCs by serotonin, which activates expression of transforming growth factor β1 (TGF-β1), a powerful suppressor of hepatocyte proliferation, through signaling by mitogen-activated protein kinase 1 (ERK) and the transcription factor JunD. Selective antagonism of 5-HT 2B enhanced hepatocyte growth in models of acute and chronic liver injury. We also observed similar effects in mice lacking 5-HT 2B or JunD or upon selective depletion of HSCs in wild-type mice. Antagonism of 5-HT 2B attenuated fibrogenesis and improved liver function in disease models in which fibrosis was pre-established and progressive. Pharmacological targeting of 5-HT 2B is clinically safe in humans and may be therapeutic in chronic liver disease.

Thick and thin filaments in asynchronous flight muscle overlap nearly completely and thick filaments are attached to the Z‐disc by connecting filaments. We have raised antibodies against a fraction of Lethocerus flight muscle myofibrils containing Z‐discs and associated filaments and also against a low ionic strength extract of myofibrils. Monoclonal antibodies were obtained to proteins of 800 kd (p800), 700 kd (p700), 400 kd (p400) and alpha‐actinin. The positions of the proteins in Lethocerus flight and leg myofibrils were determined by immunofluorescence and electron microscopy. p800 is in connecting filaments of flight myofibrils and in A‐bands of leg myofibrils. p700 is in Z‐discs of flight myofibrils and an immunologically related protein, p500, is in leg muscle Z‐discs. p400 is in M‐lines of both flight and leg myofibrils. Preliminary DNA sequencing shows that p800 is related to vertebrate titin and nematode twitchin. Molecules of p800 could extend from the Z‐disc a short way along thick filaments, forming a mechanical link between the two structures. All three high molecular weight proteins probably stabilize the structure of the myofibril.

Z‐discs of insect flight muscle contain a large protein of 500–700 kDa. Monoclonal antibodies label an epitope in the molecule at the Z‐disc in Drosophila and Lethocerus (waterbug). A partial cDNA of 1.6 kb from the Drosophila gene has been cloned and sequenced. The corresponding amino acid sequence has a modular structure composed of four conserved repeats of 95 amino acids homologous to immunoglobulin C2 domains (called class II domains in muscle proteins), separated by less conserved linker sequences of 35 amino acids. An expressed class II domain with flanking linker sequences binds to actin and alpha‐actinin but not to myosin. Single molecules of the protein would be large enough to span the Z‐disc. We suggest that the protein acts as scaffolding in the Z‐disc and we call the protein kettin. The Ca2+ activated protease, calpain, disrupts the Z‐disc of striated muscle, releasing alpha‐actinin intact. Calpain digests kettin to a series of peptides of between 30 and 170 kDa which are released from the myofibril. Digestion of kettin may cause disintegration of the Z‐disc and alpha‐actinin release which lead to disassembly of the myofibril.