Mechanostasis in apoptosis and medicine.

Mechanostasis describes a complex and dynamic process where cells maintain equilibrium in response to mechanical forces. Normal physiological loading modes and magnitudes contribute to cell proliferation, tissue growth, differentiation and development. However, cell responses to abnormal forces include compensatory apoptotic mechanisms that may contribute to the development of tissue disease and pathological conditions. Mechanotransduction mechanisms tightly regulate the cell response through discrete signaling pathways. Here, we provide an overview of links between pro- and anti-apoptotic signaling and mechanotransduction signaling pathways, and identify potential clinical applications for treatments of disease by exploiting mechanically-linked apoptotic pathways.

Spatially varying properties of the vocal ligament contribute to its eigenfrequency response.

The vocal ligament is known to have nonlinear variation in geometry, yet this is rarely considered in empirical or computational studies. This paper investigates the effects of a nonlinear variation of the anterior-to-posterior geometry and the corresponding spatial variation in elastic modulus on the fundamental frequency of vibration for the vocal ligament. Uniaxial tensile tests were performed on a vocal ligament specimen dissected from an excised 60-year-old male larynx. Digital image correlation (DIC) was used to obtain the spatial deformation field for the entire ligament specimen. DIC results revealed that the tensile deformation was very heterogeneous, with the least amount of deformation occurring in the region of smallest cross-sectional area. The elastic modulus was calculated locally and was found to be approximately 10 times higher at the midpoint of the vocal ligament than in the anterior and posterior macula flavae regions. Based on the spatially varying material properties obtained, finite element models (isotropic and transversely isotropic) were created to investigate how the effects of varying cross-section, heterogeneous stiffness, and anisotropy could affect the fundamental frequency of vibration. It was found that the spatial cross-section variation and the spatially varying anisotropy (i.e. modulus ratio) are significant to predictions of the vibration characteristics. Fundamental frequencies predicted with a finite element model are discussed in view of rotatory inertia and contribution of transverse shear deformation.

On the regulation and diversity of restriction in Escherichia coli.

The effect of UV irradiation on restriction mediated by four endogenous restriction systems of E. coli K-12 was investigated using a uniform testing method. Restriction by all four systems was reduced when treated cells were separately challenged with lambda phage carrying modification patterns that elicit restriction by each system. The response of each system was genetically and physiologically distinct.

Response to UV damage by four Escherichia coli K-12 restriction systems.

To understand the role of restriction in regulating gene flow in bacterial populations, we would like to understand the regulation of restriction enzyme activity. Several antirestriction (restriction alleviation) systems are known that reduce the activity of type I restriction enzymes like EcoKI in vivo. Most of these do not act on type II or type III enzymes, but little information is available for the unclassified modification-dependent systems, of which there are three in E. coli K-12. Of particular interest are two physiological controls on type I enzymes: EcoKI restriction is reduced 2 to 3 orders of magnitude following DNA damage, and a similar effect is seen constitutively in Dam- cells. We used the behavior of EcoKI as a control for testing the response to UV treatment of the three endogenous modification-dependent restriction systems of K-12, McrA, McrBC, and Mrr. Two of these were also tested for response to Dam status. We find that all four resident restriction systems show reduced activity following UV treatment, but not in a unified fashion; each response was genetically and physiologically distinct. Possible mechanisms are discussed.

Roles of selection and recombination in the evolution of type I restriction-modification systems in enterobacteria.

Restriction-modification systems can protect bacteria against viral infection. Sequences of the hsdM gene, encoding one of the three subunits of type I restriction-modification systems, have been determined for four strains of enterobacteria. Comparison with the known sequences of EcoK and EcoR124 indicates that all are homologous, though they fall into three families (exemplified by EcoK, EcoA, and EcoR124), the first two of which are apparently allelic. The extent of amino acid sequence identity between EcoK and EcoA is so low that the genes encoding them might be better termed pseudoalleles; this almost certainly reflects genetic exchange among highly divergent species. Within the EcoK family the ratio of intra- to interspecific divergence is very high. The extent of divergence between the genes from Escherichia coli K-12 and Salmonella typhimurium LT2 is similar to that for other genes with the same level of codon usage bias. In contrast, intraspecific divergence (between E. coli strains B and K-12) is extremely high and may reflect the action of frequency-dependent selection mediated by bacteriophages. There is also evidence of lateral transfer of a short sequence between E. coli and S. typhimurium.

Mutations that confer de novo activity upon a maintenance methyltransferase.

DNA methyltransferases are not only sequence specific in their action, but they also differentiate between the alternative methylation states of a target site. Some methyltransferases are equally active on either unmethylated or hemimethylated DNA and consequently function as de novo methyltransferases. Others are specific for hemimethylated target sequences, consistent with the postulated role of a maintenance methyltransferase in perpetuating a pattern of DNA modification. The molecular basis for the difference between de novo and maintenance methyltransferase activity is unknown, yet fundamental to cellular activities that are affected by different methylation states of the genome. The methyltransferase activity of the type I restriction and modification system, EcoK, is the only known prokaryotic methyltransferase shown to be specific for hemimethylated target sequences. We have isolated mutants of Escherichia coli K-12 which are able to modify unmethylated target sequences efficiently in a manner indicative of de novo methyltransferase activity. Consistent with this change in specificity, some mutations shift the balance between DNA restriction and modification as if both activities now compete at unmethylated targets. Two genes encode the methyltransferase and all the mutations are loosely clustered within one of them.

A novel activity in Escherichia coli K-12 that directs restriction of DNA modified at CG dinucleotides.

The restriction systems McrA and McrB of Escherichia coli K-12 are known to attack DNA containing modified cytosine. In strains lacking both activities, however, we observed that DNA methylated at CG dinucleotides (as is mammalian DNA) was still significantly restricted. We show that this substantial barrier to the acceptance of 5-methylcytosine-containing DNA is attributable to a hitherto unknown activity of the Mrr restriction system. Strikingly, the multiple systems used by this gut inhabitant to determine the fate of invading DNA will all limit genetic exchange with its mammalian host(s), reinforcing the idea that one role of DNA methylation is to serve as a "molecular passport" (E. A. Raleigh, R. Trimarchi, and H. Revel, Genetics 122:279-296, 1989).

Primary cerebellopontine progressive multifocal leukoencephalopathy diagnosed premortem by cerebellar biopsy.

A subacute progressive cerebellar brainstem syndrome developed in a patient with systemic lupus erythematosus in remission. Cerebellar biopsy documented the diagnosis of progressive multifocal leukoencephalopathy (PML). Data from this patient and 10 others in the literature emphasize the need to consider this diagnosis when ataxia develops in any patient with underlying malignancy, chronic infection, or other disease that involves immunological incompetence. Although the ataxic form of PML is not of nosological relevance, early diagnosis may eventually have therapeutic importance.