Histone deacetylases (HDACs) modulate chromatin structure and transcription, but little is known about their function in mammalian development. HDAC1 was implicated previously in the repression of genes required for cell proliferation and differentiation. Here we show that targeted disruption of both HDAC1 alleles results in embryonic lethality before E10.5 due to severe proliferation defects and retardation in development. HDAC1-deficient embryonic stem cells show reduced proliferation rates, which correlate with decreased cyclin-associated kinase activities and elevated levels of the cyclin-dependent kinase inhibitors p21(WAF1/CIP1) and p27(KIP1). Similarly, expression of p21 and p27 is up-regulated in HDAC1-null embryos. In addition, loss of HDAC1 leads to significantly reduced overall deacetylase activity, hyperacetylation of a subset of histones H3 and H4 and concomitant changes in other histone modifications. The expression of HDAC2 and HDAC3 is induced in HDAC1-deficient cells, but cannot compensate for loss of the enzyme, suggesting a unique function for HDAC1. Our study provides the first evidence that a histone deacetylase is essential for unrestricted cell proliferation by repressing the expression of selective cell cycle inhibitors.

Abstract. Desmin, the muscle specific intermediate filament (IF) protein encoded by a single gene, is expressed in all muscle tissues. In mature striated muscle, desmin IFs surround the Z-discs, interlink them together and integrate the contractile apparatus with the sarcolemma and the nucleus. To investigate the function of desmin in all three muscle types in vivo, we generated desmin null mice through homologous recombination. Surprisingly, desmin null mice are viable and fertile. However, these mice demonstrated a multisystern disorder involving cardiac, skeletal, and smooth muscle. Histological and electron microscopic analysis in both heart and skeletal muscle tissues revealed severe disruption of muscle architecture and degeneration. Structural abnormalities included loss of lateral alignment of myofibrils and abnormal mitochondrial organization. The consequences of these abnormalities were most severe in the heart, which exhibited progressive degeneration and necrosis of the myocardium accompanied by extensive calcification. Abnormalities of smooth muscle included hypoplasia and degeneration. The present data demonstrate the essential role of desmin in the maintenance of myofibril, myofiber, and whole muscle tissue structural and functional integrity, and show that the absence of desmin leads to muscle degeneration.

ABSTRACT. Desmin, the muscle-specific member of the intermediate filament (IF) family, is one of the earliest knownmyogenic markers in both skeletal muscle and heart. Its expression precedes that of all knownmuscle proteins including the members of the MyoDfamily of myogenic helix-loop-helix (mHLH)regulators with the exception of myf5. In mature striated muscle, desmin IFs surround the Z-discs, interlink them together and integrate the contractile apparatus with the sarcolemmaand the nucleus. In vitro studies using both antisense RNAand homologous recombination techniques in embryonic stem (ES) cells demonstrated that desmin plays a crucial role during myogenesis, as inhibition of desmin expression blocked myoblast fusion and myotube formation. Both in C2C12cells and differentiating embryoid bodies, the absence of desmin interferes with the normal myogenic program, as manifested by the inhibition of the mHLH transcription regulators. To investigate the function of desmin in all muscle types in vivo, we generated desmin null mice through homologous recombination. Surprisingly, a considerable numberof these mice are viable and fertile, potentially due to compensation by vimentin, nestin or synemin. However, desmin null mice demonstrate a multisystem disorder involving cardiac, skeletal and smooth muscle, beginning early in their postnatal life. Histological and electron microscopic analysis in both heart and skeletal muscle tissues reveals severe disruption of muscle architecture and degeneration. Structural abnormalities include loss of lateral alignment of myofibrils, perturbation of myofibril anchorage to the sarcolemma, abnormal mitochondrial number and organization, and loss of nuclear shape and positioning. Loose cell adhesion and increased intercellular space are prominent defects. The consequences of these abnormalities are most severe in the heart, which exhibits progressive degeneration and necrosis of the myocardiumaccompaniedby extensive calcification. Abnormalities of smooth muscle included hypoplasia and degeneration. There is a direct correlation between severity of damage and muscle usage, possibly due to increased susceptibility to normal mechanical damage and/or to repair deficiency in the absence of desmin. In conclusion, the studies so far have demonstrated that though desmin is absolutely necessary for muscle differentiation in vitro, muscle development can take place in vivo in the absence of this intermediate filament protein. However, desmin seems to play an essential role in the maintenance of myofibril, myofiber and whole muscle tissue structural and functional integrity.

Abstract. We have determined the complete cDNA sequence of rat plectin from a number of wellcharacterized overlapping lambda gtll clones . The 4,140-residue predicted amino acid sequence (466,481 D) is consistent with a three-domain structural model in which a long central rod domain, having mainly an alpha-helical coiled coil conformation, is flanked by globular NH2-and COOH-terminal domains. The plectin sequence has a number of repeating motifs . The rod domain has five subregions N200-residues long in which there is a strong repeat in the charged amino acids at 10.4 residues that may be involved in association between plectin molecules . The globular COOH-terminal domain has a prominent sixfold tandem repeat, with each repeat having a strongly conserved central region based on nine tandem repeats of a 19-residue motif. The plectin sequence has several marked similarities to that of desmoplakin (Green, K. J., D. A. D. Parry, P M. Steinert, M . L . A. Virata, R. M . Wagner, B. D. Angst, and P LECTIN is an abundant high molecular weight cytomatrix protein found in a wide variety of different cell types. Based mainly on biochemical and immunolocalization studies it has been proposed that plectin plays a role in the cross-linking of intermediate filaments, the interlinking of intermediate filaments with microtubules and microfilaments, and the anchoring of intermediate filaments to the plasma membrane and the nuclear membrane (for reviews see Wiche, 1989;Foisner and Wiche, 1991.). Furthermore, plectin's ability to form networks through self-association may convey structural stability to cytoplasmic areas which are devoid of cytoskeletal filaments. Plectin molecules exist in solution predominantly as dumbbell-shaped ti200-nm-long structures that have a high tendency to aggregate via their globular end domains yielding networklike protein as-1. Abbreviation used in this paper: BP, bullous pemphigoid .