A constitutively expressed fluorescent ubiquitination-based cell-cycle indicator (FUCCI) in axolotls for studying tissue regeneration.

Regulation of cell cycle progression is essential for cell proliferation during regeneration following injury. After appendage amputation, the axolotl (Ambystoma mexicanum) regenerates missing structures through an accumulation of proliferating cells known as the blastema. To study cell division during blastema growth, we generated a transgenic line of axolotls that ubiquitously expresses a bicistronic version of the fluorescent ubiquitination-based cell-cycle indicator (FUCCI). We demonstrate near-ubiquitous FUCCI expression in developing and adult tissues, and validate these expression patterns with DNA synthesis and mitosis phase markers. We demonstrate the utility of FUCCI for live and whole-mount imaging, showing the predominantly local contribution of cells during limb and tail regeneration. We also show that spinal cord amputation results in increased proliferation at least 5 mm from the site of injury. Finally, we use multimodal staining to provide cell type information for cycling cells by combining fluorescence in situ hybridization, EdU click-chemistry and immunohistochemistry on a single FUCCI tissue section. This new line of animals will be useful for studying cell cycle dynamics using in situ endpoint assays and in vivo imaging in developing and regenerating animals.

Making a new limb out of old cells: exploring endogenous cell reprogramming and its role during limb regeneration.

Cellular reprogramming is characterized by the induced dedifferentiation of mature cells into a more plastic and potent state. This process can occur through artificial reprogramming manipulations in the laboratory such as nuclear reprogramming and induced pluripotent stem cell (iPSC) generation, and endogenously in vivo during amphibian limb regeneration. In amphibians such as the Mexican axolotl, a regeneration permissive environment is formed by nerve-dependent signaling in the wounded limb tissue. When exposed to these signals, limb connective tissue cells dedifferentiate into a limb progenitor-like state. This state allows the cells to acquire new pattern information, a property called positional plasticity. Here, we review our current understanding of endogenous reprogramming and why it is important for successful regeneration. We will also explore how naturally induced dedifferentiation and plasticity were leveraged to study how the missing pattern is established in the regenerating limb tissue.

Characterizing the regenerative capacity and growth patterns of the Texas blind salamander (Eurycea rathbuni).

BACKGROUND: Regeneration of complex patterned structures is well described among, although limited to a small sampling of, amphibians. This limitation impedes our understanding of the full range of regenerative competencies within this class of vertebrates, according to phylogeny, developmental life stage, and age. To broaden the phylogenetic breath of this research, we characterized the regenerative capacity of the Texas blind salamander (Eurycea rathbuni), a protected salamander native to the Edwards Aquifer of San Marcos, Texas and colonized by the San Marcos Aquatic Resource Center. As field observations suggested regenerative abilities in this population, the forelimb stump of a live captured female was amputated in the hopes of restoring the structure, and thus locomotion in the animal. Tails were clipped from two males to additionally document tail regeneration. RESULTS: We show that the Texas blind salamander exhibits robust limb and tail regeneration, like all other studied Plethodontidae. Regeneration in this species is associated with wound epithelium formation, blastema formation, and subsequent patterning and differentiation of the regenerate. CONCLUSIONS: The study has shown that the Texas blind salamander is a valuable model to study regenerative processes, and that therapeutic surgeries offer a valuable means to help maintain and conserve this vulnerable species.

Optical and Infrared Spectroelectrochemical Studies of CN-Substituted Bipyridyl Complexes of Ruthenium(II).

Ruthenium(II) polypyridyl complexes [Ru(CN-Me-bpy)x(bpy)3-x]2+ (CN-Me-bpy = 4,4'-dicyano-5,5'-dimethyl-2,2'-bipyridine, bpy = 2,2'-bipyridine, and x = 1-3, abbreviated as 12+, 22+, and 32+) undergo four (12+) or five (22+ and 32+) successive one-electron reduction steps between -1.3 and -2.75 V versus ferrocenium/ferrocene (Fc+/Fc) in tetrahydrofuran. The CN-Me-bpy ligands are reduced first, with successive one-electron reductions in 22+ and 32+ being separated by 150-210 mV; reduction of the unsubstituted bpy ligand in 12+ and 22+ occurs only when all CN-Me-bpy ligands have been converted to their radical anions. Absorption spectra of the first three reduction products of each complex were measured across the UV, visible, near-IR (NIR), and mid-IR regions and interpreted with the help of density functional theory calculations. Reduction of the CN-Me-bpy ligand shifts the ν(C≡N) IR band by ca. -45 cm-1, enhances its intensity ∼35 times, and splits the symmetrical and antisymmetrical modes. Semireduced complexes containing two and three CN-derivatized ligands 2+, 3+, and 30 show distinct ν(C≡N) features due to the presence of both CN-Me-bpy and CN-Me-bpy•-, confirming that each reduction is localized on a single ligand. NIR spectra of 10, 1-, and 2- exhibit a prominent band attributable to the CN-Me-bpy•- moiety between 6000 and 7500 cm-1, whereas bpy•--based absorption occurs between 4500 and 6000 cm-1; complexes 2+, 3+, and 30 also exhibit a band at ca. 3300 cm-1 due to a CN-Me-bpy•- → CN-Me-bpy interligand charge-transfer transition. In the UV-vis region, the decrease of π → π* intraligand bands of the neutral ligands and the emergence of the corresponding bands of the radical anions are most diagnostic. The first reduction product of 12+ is spectroscopically similar to the lowest triplet metal-to-ligand charge-transfer excited state, which shows pronounced NIR absorption, and its ν(C≡N) IR band is shifted by -38 cm-1 and 5-7-fold-enhanced relative to the ground state.

FGF, BMP, and RA signaling are sufficient for the induction of complete limb regeneration from non-regenerating wounds on Ambystoma mexicanum limbs.

Some organisms, such as the Mexican axolotl, have the capacity to regenerate complicated biological structures throughout their lives. Which molecular pathways are sufficient to induce a complete endogenous regenerative response in injured tissue is an important question that remains unanswered. Using a gain-of-function regeneration assay, known as the Accessory Limb Model (ALM), we and others have begun to identify the molecular underpinnings of the three essential requirements for limb regeneration; wounding, neurotrophic signaling, and the induction of pattern from cells that retain positional memory. We have previously shown that treatment of Mexican axolotls with exogenous retinoic acid (RA) is sufficient to induce the formation of complete limb structures from blastemas that were generated by deviating a nerve bundle into an anterior-located wound site on the limb. Here we show that these ectopic structures are capable of regenerating and inducing new pattern to form when grafted into new anterior-located wounds. We additionally found that the expression of Alx4 decreases, and Shh expression increases in these anterior located blastemas, but not in the mature anterior tissues, supporting the hypothesis that RA treatment posteriorizes blastema tissue. Based on these and previous observations, we used the ALM assay to test the hypothesis that a complete regenerative response can be generated by treating anterior-located superficial limb wounds with a specific combination of growth factors at defined developmental stages. Our data shows that limb wounds that are first treated with a combination of FGF-2, FGF-8, and BMP-2, followed by RA treatment of the resultant mid-bud stage blastema, will result in the generation of limbs with complete proximal/distal and anterior/posterior limb axes. Thus, the minimal signaling requirements from the nerve and a positional disparity are achieved with the application of this specific combination of signaling molecules.

Advancements to the Axolotl Model for Regeneration and Aging.

Loss of regenerative capacity is a normal part of aging. However, some organisms, such as the Mexican axolotl, retain striking regenerative capacity throughout their lives. Moreover, the development of age-related diseases is rare in this organism. In this review, we will explore how axolotls are used as a model system to study regenerative processes, the exciting new technological advancements now available for this model, and how we can apply the lessons we learn from studying regeneration in the axolotl to understand, and potentially treat, age-related decline in humans.

Quantifying Triplet State Formation in Zinc Dipyrrin Complexes.

Photocatalysis is a promising method to harness solar energy and use it to form fuels and other high-value chemicals, but most sensitizers used in photocatalytic reactions are complexes of rare and expensive metals such as ruthenium and iridium. Zinc dipyrromethene complexes have potential to be a more earth-abundant alternative, but their photophysical properties are largely unexplored. In this study, triplet state formation was quantified in two zinc dipyrromethene complexes, with and without heavy atoms, by transient absorption spectroscopy. Without heavy atoms, the triplet quantum yield was 16% in toluene and 27% in THF. With the addition of heavy I atoms, the triplet quantum yield increased to 62-63% and was insensitive to solvent polarity. The fact that in the absence of heavy atoms the triplet yield is affected by solvent polarity and in the presence of heavy atoms it is not suggests that triplet formation occurs through different pathways in the two complexes. These triplet yields meet or exceed those of successful organic photosensitizers, illustrating the potential for zinc dipyrromethene complexes as photosensitizers.

Enhancing the Visible-Light Absorption and Excited-State Properties of Cu(I) MLCT Excited States.

A computationally inspired Cu(I) metal-to-ligand charge transfer (MLCT) chromophore, [Cu(sbmpep)2]+ (sbmpep = 2,9-di(sec-butyl)-3,8-dimethyl-4,7-di(phenylethynyl)-1,10-phenanthroline), was synthesized in seven total steps, prepared from either dichloro- or dibromophenanthroline precursors. Complete synthesis, structural characterization, and electrochemistry, in addition to static and dynamic photophysical properties of [Cu(sbmpep)2]+, are reported on all relevant time scales. UV-Vis absorption spectroscopy revealed significant increases in oscillator strength along with a concomitant bathochromic shift in the MLCT absorption bands with respect to structurally related model complexes (ε = 16 500 M-1 cm-1 at 491 nm). Strong red photoluminescence (Φ = 2.7%, λmax = 687 nm) was observed from [Cu(sbmpep)2]+, which featured an average excited-state lifetime of 1.4 µs in deaerated dichloromethane. Cyclic and differential pulse voltammetry revealed ∼300 mV positive shifts in the measured one-electron reversible reduction and oxidation waves in relation to a Cu(I) model complex possessing identical structural elements without the π-conjugated 4,7-substituents. The excited-state redox potential of [Cu(sbmpep)2]+ was estimated to be -1.36 V, a notably powerful reductant for driving photoredox chemistry. The combination of conventional and ultrafast transient  absorption and luminescence spectroscopy successfully map the excited-state dynamics of [Cu(sbmpep)2]+ from initial photoexcitation to the formation of the lowest-energy MLCT excited state and ultimately its relaxation to the ground state. This newly conceived molecule appears poised for photosensitization reactions involving energy and electron-transfer processes relevant to photochemical upconversion, photoredox catalysis, and solar fuels photochemistry.

Vibrational Relaxation and Redistribution Dynamics in Ruthenium(II) Polypyridyl-Based Charge-Transfer Excited States: A Combined Ultrafast Electronic and Infrared Absorption Study.

Ultrafast time-resolved electronic and infrared absorption measurements have been carried out on a series of Ru(II) polypyridyl complexes in an effort to delineate the dynamics of vibrational relaxation in this class of charge transfer chromophores. Time-dependent density functional theory calculations performed on compounds of the form [Ru(CN-Me-bpy) x(bpy)3-x]2+ ( x = 1-3 for compounds 1-3, respectively, where CN-Me-bpy is 4,4'-dicyano-5,5'-dimethyl-2,2'-bipyridine and bpy is 2,2'-bipyridine) reveal features in their charge-transfer absorption envelopes that allow for selective excitation of the Ru(II)-(CN-Me-bpy) moiety, the lowest-energy MLCT state(s) in each compound of the series. Changes in band shape and amplitude of the time-resolved differential electronic absorption data are ascribed to vibrational cooling in the CN-Me-bpy-localized 3MLCT state with a time constant of 8 ± 3 ps in all three compounds. This conclusion was corroborated by picosecond time-resolved infrared absorption measurements; sharpening of the CN stretch in the 3MLCT excited state was observed with a time constant of 3.0 ± 1.5 ps in all three members of the series. Electronic absorption data acquired at higher temporal resolution revealed spectral modulation over the first 2 ps occurring with a time constant of τ = 170 ± 50 fs, in compound 1; corresponding effects are significantly attenuated in compound 2 and virtually absent in compound 3. We assign this feature to intramolecular vibrational redistribution (IVR) within the 3MLCT state and represents a rare example of this process being identified from time-resolved electronic absorption data for this important class of chromophores.

Regenerative Models for the Integration and Regeneration of Head Skeletal Tissues.

Disease of, or trauma to, the human jaw account for thousands of reconstructive surgeries performed every year. One of the most popular and successful treatment options in this context involves the transplantation of bone tissue from a different anatomical region into the affected jaw. Although, this method has been largely successful, the integration of the new bone into the existing bone is often imperfect, and the integration of the host soft tissues with the transplanted bone can be inconsistent, resulting in impaired function. Unlike humans, several vertebrate species, including fish and amphibians, demonstrate remarkable regenerative capabilities in response to jaw injury. Therefore, with the objective of identifying biological targets to promote and engineer improved outcomes in the context of jaw reconstructive surgery, we explore, compare and contrast the natural mechanisms of endogenous jaw and limb repair and regeneration in regenerative model organisms. We focus on the role of different cell types as they contribute to the regenerating structure; how mature cells acquire plasticity in vivo; the role of positional information in pattern formation and tissue integration, and limitations to endogenous regenerative and repair mechanisms.

Cartilage and bone cells do not participate in skeletal regeneration in Ambystoma mexicanum limbs.

The Mexican Axolotl is one of the few tetrapod species that is capable of regenerating complete skeletal elements in injured adult limbs. Whether the skeleton (bone and cartilage) plays a role in the patterning and contribution to the skeletal regenerate is currently unresolved. We tested the induction of pattern formation, the effect on cell proliferation, and contributions of skeletal tissues (cartilage, bone, and periosteum) to the regenerating axolotl limb. We found that bone tissue grafts from transgenic donors expressing GFP fail to induce pattern formation and do not contribute to the newly regenerated skeleton. Periosteum tissue grafts, on the other hand, have both of these activities. These observations reveal that skeletal tissue does not contribute to the regeneration of skeletal elements; rather, these structures are patterned by and derived from cells of non-skeletal connective tissue origin.

Cuprous Phenanthroline MLCT Chromophore Featuring Synthetically Tailored Photophysics.

In the interest of expanding the inventory of available long lifetime, photochemically robust, and strongly reducing Cu(I) MLCT sensitizers, we present detailed structural, photophysical, and electrochemical characterization of [Cu(dipp)2]+, dipp = 2,9-diisopropyl-1,10-phenanthroline, and its sterically encumbered tetramethyl analogue [Cu(diptmp)2]+, diptmp = 2,9-diisopropyl-3,4,7,8-tetramethyl-1,10-phenanthroline. The achiral isopropyl substituents enable similar steric bulk effects to the previously investigated sec-butyl substituents while eliminating the complex NMR structural analyses associated with the presence of two chiral centers in the latter. The photophysical properties of [Cu(diptmp)2]+ are impressive, possessing a 2.3 µs lifetime in deaerated CH2Cl2 and a photoluminescence quantum yield of 4.7%, which were slightly attenuated in coordinating tetrahydrofuran (THF) solutions. Nanosecond transient absorption spectroscopy results matched the transient photoluminescence kinetics enabling complete characterization of MLCT excited-state decay in these molecules. The calculated excited-state potential for the Cu2+/Cu+* couple (E = -1.74 V vs Fc+/0) indicated that [Cu(diptmp)2]+* is a strong photoreductant potentially useful for myriad applications. Ultrafast transient absorption measurements performed in THF solutions are also reported, yielding the relative time scales for both the pseudo-Jahn-Teller distortion (0.4-0.8 ps in [Cu(dipp)2]+ and 0.12-0.5 ps in [Cu(diptmp)2]+) and singlet-triplet intersystem crossing (6.4-10.1 ps for [Cu(dipp)2]+ and 3.5-5.4 ps for [Cu(diptmp)2]+) within these molecules. The disparity in the time scales of pseudo-Jahn-Teller distortion and intersystem crossing between two complexes with different anticipated excited-state geometries suggests that strongly impeded structural distortion in the MLCT excited state (i.e., [Cu(diptmp)2]+) enables more rapid surface crossings in the initial deactivation dynamics.

Positional plasticity in regenerating Amybstoma mexicanum limbs is associated with cell proliferation and pathways of cellular differentiation.

BACKGROUND: The endogenous ability to dedifferentiate, re-pattern, and re-differentiate adult cells to repair or replace damaged or missing structures is exclusive to only a few tetrapod species. The Mexican axolotl is one example of these species, having the capacity to regenerate multiple adult structures including their limbs by generating a group of progenitor cells, known as the blastema, which acquire pattern and differentiate into the missing tissues. The formation of a limb regenerate is dependent on cells in the connective tissues that retain memory of their original position in the limb, and use this information to generate the pattern of the missing structure. Observations from recent and historic studies suggest that blastema cells vary in their potential to pattern distal structures during the regeneration process; some cells are plastic and can be reprogrammed to obtain new positional information while others are stable. Our previous studies showed that positional information has temporal and spatial components of variation; early bud (EB) and apical late bud (LB) blastema cells are plastic while basal-LB cells are stable. To identify the potential cellular and molecular basis of this variation, we compared these three cell populations using histological and transcriptional approaches. RESULTS: Histologically, the basal-LB sample showed greater tissue organization than the EB and apical-LB samples. We also observed that cell proliferation was more abundant in EB and apical-LB tissue when compared to basal-LB and mature stump tissue. Lastly, we found that genes associated with cellular differentiation were expressed more highly in the basal-LB samples. CONCLUSIONS: Our results characterize histological and transcriptional differences between EB and apical-LB tissue compared to basal-LB tissue. Combined with our results from a previous study, we hypothesize that the stability of positional information is associated with tissue organization, cell proliferation, and pathways of cellular differentiation.

Efficient Visible to Near-UV Photochemical Upconversion Sensitized by a Long Lifetime Cu(I) MLCT Complex.

The current investigation compares the photochemical upconversion sensitization properties of two long lifetime Cu(I) metal-to-ligand charge transfer (MLCT) chromophores to 3 distinct anthryl-based triplet acceptors. The sensitizers [Cu(dsbtmp)2](PF6) (1, dsbtmp = 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline) and [Cu(dsbp)2](PF6) (2, dsbp = 2,9-di(sec-butyl-1,10-phenanthroline) were selectively excited in the presence of anthracene, 9,10-diphenylanthracene (DPA), and 9,10-dimethylanthracene (DMA) in degassed dichloromethane solutions. In all instances, triplet energy transfer was observed from selective excitation of the Cu(I) MLCT chromophore to each respective anthryl species. The bimolecular triplet-triplet energy transfer quenching rate constants were extracted from dynamic Stern-Volmer analyses in each case, yielding values below the diffusion limit in dichloromethane. However, the Stern-Volmer quenching constants (KSV's) were sizable enough (up to ∼2300 M(-1) with 1 as a sensitizer) to support efficient photochemical upconversion. As such, visible to near-UV photochemical upconversion was observed in every instance, along with the anticipated quadratic-to-linear incident light power dependence when pumping at 488 nm. The latter verified that it is indeed sensitized triplet-triplet annihilation responsible for the generation of the anthryl-based singlet fluorescence. Photochemical upconversion quantum efficiencies were evaluated using a relative actinometric method as both a function of incident light power density as well as anthryl acceptor/annihilator concentration. When 1 was used as the sensitizer, upconversion quantum yields as large as 9.2% and 17.8% were observed for DMA and DPA, respectively. Finally, the combination of 1 with DMA was shown to be quite robust, showing no obvious signs of decomposition during 12 h of continuous 488 nm photolysis.

Transient absorption dynamics of sterically congested Cu(I) MLCT excited states.

Subpicosecond through supra-nanosecond transient absorption dynamics of the homoleptic Cu(I) metal-to-ligand charge transfer (MLCT) photosensitizers including the benchmark [Cu(dmp)2](+) (dmp =2,9-dimethyl-1,10-phenanthroline) chromophore, as well as [Cu(dsbp)2](+) (dsbp =2,9-di(sec-butyl)-1,10-phenanthroline and [Cu(dsbtmp)2](+) (dsbtmp =2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline) were investigated in dichloromethane and tetrahydrofuran solutions. Visible and near-IR spectroelectrochemical measurements of the singly reduced [Cu(dsbp)2](+) and [Cu(dsbtmp)2](+) species were determined in tetrahydrofuran, allowing for the identification of redox-specific phenanthroline-based radical anion spectroscopic signatures prevalent in the respective transient absorption experiments. This study utilized four different excitation wavelengths (418, 470, 500, and 530 nm) to elucidate dynamics on ultrafast times scales spanning probe wavelengths ranging from the UV to the near-IR (350 to 1450 nm). With the current time resolution of ∼150 fs, initial excited state decay in all three compounds was found to be independent of excitation wavelength. Not surprisingly, there was little to no observed influence of solvent in the initial stages of excited state decay in any of these molecules including [Cu(dmp)2](+), consistent with results from previous investigators. The combined experimental data revealed two ranges of time constants observed on short time scales in all three MLCT chromophores and both components lengthen as a function of structure in the following manner: [Cu(dsbtmp)2](+) < [Cu(dsbp)2](+) < [Cu(dmp)2](+). The molecule with the most inhibited potential for distortion, [Cu(dsbtmp)2](+), possessed the fastest ultrafast dynamics as well as the longest excited state lifetimes in both solvents. These results are consistent with a small degree of excited state distortion, rapid intersystem crossing, and weak vibronic coupling to the ground state. The concomitant systematic variation in both initial time constants, assigned to pseudo-Jahn-Teller distortion and intersystem crossing, suggest that both processes are intimately coupled in all molecules in the series. The variability in these time scales illustrate that strongly impeded structural distortion in Cu(I) MLCT excited state enables more rapid surface crossings in the initial deactivation dynamics.

Triplet state formation in homo- and heterometallic diketopyrrolopyrrole chromophores.

The synthesis, structural characterization, and excited-state dynamics of series of diketopyrrolopyrrole (DPP) bridged homodinuclear Ir(III) and heterodinuclear Ir(III)/Pt(II) complexes is described. Steady-state and time-resolved photoluminescence along with transient absorption measurements were used to probe the nature of the emissive and long-lived excited states. Upon excitation into the (1)DPP ligand-localized excited state in the presence of coordinated Ir(III) or Pt(II) metal centers, the intersystem crossing is enhanced, leading to a quenching of the (1)DPP fluorescence and the formation of the long-lived (τ ≈ 30-40 µs) (3)DPP excited state in all instances.

Excited state equilibrium induced lifetime extension in a dinuclear platinum(II) complex.

Covalently linking two square planar platinum(II) centers using two pyrazolate bridging ligands allows the filled dz(2) orbitals on each Pt center to overlap, producing a Pt-Pt σ interaction and new low energy dσ* → π* metal-metal-to-ligand charge transfer (MMLCT) transitions terminating on an appropriate π-acceptor ligand such as 2-phenylpyridine (ppy). In an effort to extend the lifetime of the associated MMLCT excited state, we decided to append piperidinyl naphthalimide (PNI) chromophores to the 2-phenylpyridine charge transfer ligands. This structural modification introduces low-lying PNI-based triplet states serving as long-lived triplet population reservoirs, thermally capable of repopulating the charge transfer state at room temperature (RT), thereby extending its excited state lifetime. Specifically, [Pt(PNI-ppy)(µ-Ph2pz)]2 (1), where PNI-ppy is N-(2-phenylpyridine)-4-(1-piperidinyl)naphthalene-1,8-dicarboximide and Ph2pz is 3,5-diphenylpyrazolate, was synthesized and structurally characterized. The static and dynamic photophysical behavior of 1 was directly compared to the MMLCT complex [Pt(ppy)(µ-Ph2pz)]2 (2), lacking the PNI substituents, as well as the naked PNI-ppy ligand 3, intended to independently model the MMLCT and NI excited state properties, respectively. Ultimately, experimental evidence for the presence of both the (3)PNI and (3)MMLCT excited states in 1 were revealed at RT in nanosecond transient absorbance and time-resolved photoluminescence spectroscopy, respectively. Temperature-dependent transient absorption spectroscopy permitted the extraction of an energy gap of 1740 cm(-1) between the MMLCT and PNI triplet states in 1 along with the time constants associated with the interconversions between the various excited states resident on this complex chromophore, ultimately decaying back to the ground state with a time constant of 65 µs at RT.