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Using a combination of static and dynamic laser scattering, we examined the association and dissociation of linear poly(N-isopropylacrylamide) (PNIPAM) chains in dilute aqueous solutions. There exists a hysteresis in the temperature dependence of the average hydrodynamic radius (〈R h 〉), average radius of gyration (〈R g 〉), and apparent weight-average molar mass (M w,app ) in one heating-and-cooling cycle. In the heating process, the chains first undergo intrachain contraction before interchain association to form stable aggregates at temperatures much higher than the lower critical solution temperature (LCST ∼ 32 °C) of PNIPAM in water. In the cooling process before the solution temperature approaches the LCST, M w,app remains a constant and both 〈R g 〉 and 〈R h 〉 increase, but the ratio of 〈R g 〉/〈R h 〉 decreases. In other words, the aggregates undergo an unevenly swelling; namely, the periphery swells more than the center, and there is no chain dissociation. FTIR spectra reveal that as the temperature increases, the adsorption peak area related to the hydrogen bonding 〉CdO‚‚‚H-O-H decreases, but the adsorption peak related to the hydrogen bonding 〉CdO‚‚‚H-N〈 appears when the temperature is higher than the LCST, reflecting the dehydration and the formation of some additional intersegment hydrogen bonds in the collapsed state during the heating. Therefore, the chain contraction is entropy-driven, and the hysteresis can be attributed to these additional hydrogen bonds that act as the "cross-linking" points to make the chain aggregates behave like a "gel". The chain dissociation only occurs when the temperature is much lower than the LCST, at which water becomes a very good solvent for PNIPAM.

In mammals, myocardial cell death due to infarction results in scar formation and little regenerative response. In contrast, zebrafish have a high capacity to regenerate the heart after surgical resection of myocardial tissue. However, whether zebrafish can also regenerate lesions caused by cell death has not been tested. Here, we present a simple method for induction of necrotic lesions in the adult zebrafish heart based on cryoinjury. Despite widespread tissue death and loss of cardiomyocytes caused by these lesions, zebrafish display a robust regenerative response, which results in substantial clearing of the necrotic tissue and little scar formation. The cellular mechanisms underlying regeneration appear to be similar to those activated in response to ventricular resection. In particular, the epicardium activates a developmental gene program, proliferates and covers the lesion. Concomitantly, mature uninjured cardiomyocytes become proliferative and invade the lesion. Our injury model will be a useful tool to study the molecular mechanisms of natural heart regeneration in response to necrotic cell death.

The induction of heat-shock transcription factor (HSF) binding to DNA is accomplished by a heat-induced oligomerization. The transition to the induced state is accompanied by a chromosomal redistribution of HSF to the heat-shock puff sites. Over 150 additional chromosomal sites also accumulate HSF, including developmental loci that are repressed during heat shock. These findings suggest an unforeseen role for HSF as a repressor of normal gene activity during heat stress.

In contrast to mammals, zebrafish regenerate heart injuries via proliferation of cardiomyocytes located near the wound border. To identify regulators of cardiomyocyte proliferation, we used spatially resolved RNA sequencing (tomo-seq) and generated a high-resolution genome-wide atlas of gene expression in the regenerating zebrafish heart. Interestingly, we identified two wound border zones with distinct expression profiles, including the re-expression of embryonic cardiac genes and targets of bone morphogenetic protein (BMP) signaling. Endogenous BMP signaling has been reported to be detrimental to mammalian cardiac repair. In contrast, we find that genetic or chemical inhibition of BMP signaling in zebrafish reduces cardiomyocyte dedifferentiation and proliferation, ultimately compromising myocardial regeneration, while bmp2b overexpression is sufficient to enhance it. Our results provide a resource for further studies on the molecular regulation of cardiac regeneration and reveal intriguing differential cellular responses of cardiomyocytes to a conserved signaling pathway in regenerative versus non-regenerative hearts.

The development of safe, efficient and controllable gene-delivery vectors has become a bottleneck to human gene therapy. Synthetic polymeric vectors, although safer than viral carriers, generally do not possess the required efficacy, apparently due to a lack of functionality to overcome at least one of many intracellular gene-delivery obstacles. Currently, the exact mechanisms of how these polymeric vectors navigate each intracellular obstacle ("slit"), as well as their particular physical/chemical properties that contribute to efficient intracellular trafficking remain largely unknown, making it rather difficult to further improve the efficacy of non-viral polymeric vectors in vitro and in vivo. In this review, we first give a brief overview of synthetic polymeric vectors that have been designed and developed for gene delivery and highlight some promising candidates for clinical applications. Our main focus is on discussing the intracellular trafficking mechanisms of the DNA-polymer complexes ("polyplexes"), with less effort on the DNA-polymer complexation in the extracellular space as well as the in vivo systemic administration of genes in animal models and human clinical trials. In particular, we identified and discussed four critical, YananYUE received her B.S. degree in Polymer Materials and Engineering from Zhejiang University in 2007 and her Ph.D. in Chemistry at the Chinese University of Hong Kong, under the supervision of Professor Chi WU. Her research interests mainly focus on the development of nonviral vectors for molecular medicines, especially the effect of free polycationic chains on the intracellular trafficking of DNApolymer complexes in gene transfection. She is now working in the Department of Chemistry, the University of Chicago, under the supervision of Professor Chuan HE. Chi WU graduated from the Department of Chemical Physics at the University of Science and Technology of China (USTC) in 1982. After obtaining his Ph.D. in 1987 followed by two-year postdoctoral experience under the supervision of Professor Benjamin Chu in the State University of New York at Stony Brook, he moved to BASF (Ludwigshafen, Germany) in 1989; first as an Alexander von Humboldt Fellow for one year with Dr. Dieter Horn and then as the supervisor of laser light-scattering laboratory. In 1992, he resigned from BASF and joined the Department of Chemistry at the Chinese University of Hong Kong (CUHK) as a Lecturer. He underwent a double promotion directly to Reader in 1996; and became a Chair Professor of Chemistry and Honorary Professor of Physics in 1999. Recently, he has been appointed as a Wei Lun Chair Professor of Chemistry since 2010.

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Mammalian cardiomyocytes become post-mitotic shortly after birth. Understanding how this occurs is highly relevant to cardiac regenerative therapy. Yet, how cardiomyocytes achieve and maintain a post-mitotic state is unknown. Here, we show that cardiomyocyte centrosome integrity is lost shortly after birth. This is coupled with relocalization of various centrosome proteins to the nuclear envelope. Consequently, postnatal cardiomyocytes are unable to undergo ciliogenesis and the nuclear envelope adopts the function as cellular microtubule organizing center. Loss of centrosome integrity is associated with, and can promote, cardiomyocyte G0/G1 cell cycle arrest suggesting that centrosome disassembly is developmentally utilized to achieve the post-mitotic state in mammalian cardiomyocytes. Adult cardiomyocytes of zebrafish and newt, which are able to proliferate, maintain centrosome integrity. Collectively, our data provide a novel mechanism underlying the post-mitotic state of mammalian cardiomyocytes as well as a potential explanation for why zebrafish and newts, but not mammals, can regenerate their heart.DOI: http://dx.doi.org/10.7554/eLife.05563.001