Cell Biology Commons^™

Development Of An Injectable Chitosan-Based Bioink For Enhanced Bone Fracture Healing Via Msc-Derived Osteogenesis, Zeina H. Adel, Jana A. Hassan, Hala H. Muhammed, Haneen M. Elsaeid, Raghad M. Nasr, Ahmed M. Deghidy

BAU Journal - Science and Technology

Worldwide, bone fractures are a major public health concern and, there is an urgent requirement to create new treatments to accelerate the rate of the bone healing process. This research mainly relies on creating a hydrogel that satisfies the bone marrow mesenchymal stem cells (BM-MSCs) and makes it a suitable medium to differentiate into osteogenic cells. We combine the BM-MSCs with a chitosan polymer-based hydrogel, infused with PVA to enhance bone healing, this unique mixture forms " Bioink ". We induced a closed fracture model in the rat's right humerus, followed by injection of a single dosage of bioink. On …

Regulation And Function Of Mec-2 Alternative Splicing In Single Cells, Canyon Calovich-Benne

Biological Sciences Theses and Dissertations

One mechanism for generating diversity at the single cell level is selective expression of a unique set of transcription factors (TFs) and RNA binding proteins (RBPs). These regulatory factors contribute to functions of cells by controlling cell fate, turning genes on and off, and processing of transcripts either co- or post-transcriptionally. Many genes, especially neuronal genes, have multiple different splicing events, promoters, and/or polyadenylation sites. Post-transcriptional coordination is in the infancy of being studied and the functional consequences of this coupling at the single cell level has yet to be explored. Here we show, mec-2 coordination of alternative transcription start …

Microwell Fabrication For Impedance And Ac Measurements Of Isolated Ventricular Cardiomyocytes, Fallon Wenck

Multidisciplinary Studies Theses and Dissertations

Electrophysiological behavior of human hearts have been extensively studied for hundreds of years. Even with elucidation of not just biological function, but also time and frequency-dependent current flow, the sheer intricacy of in vivo cardiac tissue function remains somewhat enigmatic. All while heart disease has been the leading cause of death in the United States for over a century, with its rates of occurrence still increasing annually [1]. Thus, new research techniques and tools to quantify cardiac function are still being pursued at smaller and smaller scales to decrease research/innovation time, cost, and diagnostic ambiguity. In fact, it remains incredibly …

P85Α Degradation Mediated By Ubiquitin E3 Ligase Fbxo21 Offers Therapeutic Potential In Leukemia, Kasidy K. Weber

Theses & Dissertations

Acute myeloid leukemia (AML) is a complex and heterogeneous disease characterized by the clonal expansion of myeloid blasts in the bone marrow. Despite significant therapeutic advances over the years, the prognosis for AML patients remains dismal, with high relapse rates and poor overall survival. The ubiquitin-proteasome system (UPS) plays a critical role in maintaining cellular homeostasis by regulating the degradation of proteins involved in essential processes such as cell cycle control, DNA repair, apoptosis, and various signaling pathways. Given this vital function, targeting ubiquitin E3 ligases within the UPS presents a promising strategy for developing more effective and targeted therapies …

Enhanced Glucose Production In Norepinephrine And Palmitate Stimulated Hepatocytes Following Endurance Training, Ken D. Sumida, Vera M. Lordan, Casey M. Donovan

Health Sciences and Kinesiology Faculty Articles

Enhanced hepatic gluconeogenesis plays an important role in exercise glucose homeostasis when hepatic glycogen stores are depleted. Livers from trained animals demonstrate greater rates of gluconeogenesis in the presence of elevated substrate with and without hormonal stimulation. Training has been reported to have a particularly profound impact on norepinephrine-stimulated gluconeogenesis, but this was only demonstrated in the presence of other gluconeogenic hormones. Here we reexamine the impact of endurance training on norepinephrine-stimulated gluconeogenesis in the absence of any other hormones. Isolated hepatocytes from trained and untrained rats were incubated in 6 mM lactate with various concentrations of norepinephrine (0 nM–20 …

Investigating Tumor Growth And Regulation In Supratentorial Ependymomas; The Impact Of Dlk1 And Egr1 Knockout, Om Sinojia

Honors Scholar Theses

Ependymomas (EPNs) are primary brain tumors that often arise from radial glial cells lining the ventricular system. Supratentorial ependymomas (ST-EPNs) are particularly aggressive, and understanding the molecular factors driving their growth is critical for developing targeted therapies. This study investigates the roles of DLK1 and EGR1, key regulators in cellular differentiation and tumorigenesis, in the development of ST-EPNs. We utilized genetically engineered mouse models to induce postnatal knockouts of DLK1 and EGR1 and evaluated tumor growth using histological and imaging techniques. Tumor area was quantified across multiple brain sections from both male and female mice. DLK1 knockout brains exhibited consistent …

Testing Vivo-Morpholino Mediated Gene Knockdown Of Stat6, Spi1b, And Hnf4Α In Adult Threespine Stickleback, Stella Dipippo

Honors Scholar Theses

The fibrotic immune response to the cestode parasite, Schistocephalus solidus, varies among threespine stickleback populations. Fibrosis suppresses parasite growth but has costly effects on the fish. Previous genomic studies in stickleback have suggested potential roles of Spi1b, STAT6, and HNF4α in modulating fibrosis severity, but there is presently no evidence for this in-vivo. We used antisense vivo-morpholino oligonucleotides (MOs) to attempt to knockdown the expression of each gene in the liver, spleen, and intestine of adult stickleback. Vivo-MOs have been proven effective in other species of teleost fish, as well as in mice and humans, but …

Early Host Defense Against Virus Infections, Søren R. Paludan, Thomas Pradeu, Andreas Pichlmair, K. Brad Wray, Jacob Giehm Mikkelsen, David Olagnier, Trine H. Mogensen

Presidential Fellows Articles and Research

Early host defense eliminates many viruses before infections are established while clearing others so they remain subclinical or cause only mild disease. The field of immunology has been shaped by broad concepts, including the pattern recognition theory that currently dominates innate immunology. Focusing on early host responses to virus infections, we analyze the literature to build a working hypothesis for the principles that govern the early line of cellular antiviral defense. Aiming to ultimately arrive at a criteria-based theory with strong explanatory power, we propose that both controlling infection and limiting inflammation are key drivers for the early cellular antiviral …

A Conjugate Of An Egfr-Binding Peptide And Doxorubicin Shows Selective Toxicity To Triple-Negative Breast Cancer Cells, Phi-Phung Than, Shih-Jing Yao, Emad Althagafi, Kamaljit Kaur

Pharmacy Faculty Articles and Research

Selective targeting of cancer cells via overexpressed cell-surface receptors is a promising strategy to enhance chemotherapy efficacy and minimize off-target side effects. In this study, we designed peptide 31 (YHWYGYTPERVI) to target the overexpressed epidermal growth factor receptor (EGFR) in triple-negative breast cancer (TNBC) cells. Peptide 31 is internalized by TNBC cells through EGFR-mediated endocytosis and shares sequence and structural similarities with human EGF (hEGF), a natural EGFR ligand. Unlike hEGF, peptide 31 does not induce cell migration in TNBC cells. A novel conjugate of peptide 31 with doxorubicin (Dox) retains selectivity for TNBC cells and exhibits significant toxicity comparable …

Studies Of Media To Maximize The Expression Of Bacterial Metalloprotease Genes As Potential Therapeutics For Human Alzheimer’S Disease, Uday Kiran Chigurupati

Honors Capstones

This study investigates the potential of bacterial metalloprotease genes as therapeutic agents for Alzheimer's Disease (AD). The research focuses on Pseudomonas 20ei1 (P. 20ei1), a bacterial species isolated from lagoon water, which contains pitrilysin genes that share similarities with human insulysin. Six pqqL-related genes were identified in P. 20ei1 for this study. The research methodology involved bioinformatic studies, bacterial cell culture in various media, RNA extraction, purification, and cDNA synthesis. Quantitative PCR analysis was performed to analyze gene expression levels in different cDNA samples. The bacteria grown in the medium with insoluble phosphate has shown higher expression of selected genes …

Exploration In Cell Growth: Rac3 And Its Effect On Ewing Sarcomas Tumorigenic Properties, Allie Woods, Samuel De La Cruz, Catherine Garcia, Terry Jo Shackleford

Cell and Molecular Methods

Multiple experiments were conducted to measure the effectiveness of certain genes on cell growth. In the exploration of this process, RAC3 acted as the target gene and was applied to both SOX18 overexpressing and normal expressing cells (control). RAC3 is a gene that typically controls the intercellular signaling pathway of a cell. This means it helps with the movement of cells and allows for migration.The purpose of these experiments were to determine how the gene RAC3 would work when introduced to carcinogenic cells, specifically Ewing Sarcoma. Typically, RAC3 acts as a messenger for the cell, but when in conjunction with …

The Effect Of Knockdown Of Gene Kit On Sox18 Gene Expression Through The Jak2/Stat3 Signaling Pathway Within An Ewing Sarcoma Cell, Samantha Aragon, Cristina Herrera, Felicity Rodriguez, Riley Smith, Terry Jo Shackleford

Cell and Molecular Methods

Ewing sarcoma is an aggressive cancer that affects children & adolescents. Common treatments include surgery, radiation, and chemotherapy, yet individuals have a poor survival rate and often develop side effects from treatment, including fertility issues, secondary cancer, heart problems, and growth abnormalities.1SOX18 is a gene identified to aid in cell proliferation and cell differentiation, leading to the development of several cancers, such as gastric, bladder, and lung cancer.2,3 SOX18 resides in the JAK2/STAT3 pathway as a regulator, along with another gene named KIT that instigates the formation of the cell signaling pathway.4KIT is a transmembrane tyrosine kinase, which instigates cell …

The Effects Of Ewing Sarcoma Through The Overexpression And Knockdown Of Transcription Factors Sox18 And Stat1, Victoria Castillo, Melany Cervantes, Nicole Robles, Terry Jo Shackleford

Cell and Molecular Methods

SOX18, a transcription factor with a DNA-binding HMG domain, plays a critical role in regulatory processes linked to cancer progression. Exhibiting oncogenic properties, SOX18 has been linked to various cancers, where it promotes tumor growth by enhancing cell invasion, uncontrolled proliferation, and resistance to apoptosis through dysregulated signaling pathways. STAT1, a protein crucial for immune system regulation, is prominently expressed in immune-associated tissues like the lymph nodes, and bone marrow, suggesting it could play a role in tumor growth. Ewing Sarcoma (EWS), an aggressive cancer that targets bone and soft tissue, carries a high mortality rate of 90% without treatment …

Single-Molecule Studies Reveal The Off-Pathway Early Paused State Intermediates As A Target Of Streptolydigin Inhibition Of Rna Polymerase And Its Dramatic Enhancement By Gre Factors, Anatolii Arseniev, Mikhail Panfilov, Georgii Pobegalov, Alina Potyseva, Polina Pavlinova, Maria Yakunina, Jookyung Lee, Sergei Borukhov, Konstantin Severinov, Mikhail Khodorkovskii

Rowan-Virtua School of Osteopathic Medicine Departmental Research

Antibiotic streptolydigin (Stl) inhibits bacterial transcription by blocking the trigger loop folding in the active center of RNA polymerase (RNAP), which is essential for catalysis. We use acoustic force spectroscopy to characterize the dynamics of transcription elongation in ternary elongation complexes (ECs) of RNAP in the presence of Stl at a single-molecule level. We found that Stl induces long-lived stochastic pauses while the instantaneous velocity of transcription between the pauses is unaffected. Stl enhances the short-lived pauses associated with an off-pathway early paused state intermediates of the RNAP nucleotide addition cycle. Unexpectedly, we found that transcript cleavage factors GreA and …

Sk609, A Novel Dopamine D3 Receptor Agonist And Norepinephrine Transporter Blocker With Putative Pro-Cognitive Actions, Does Not Induce Psychostimulant-Like Increases In Risky Choice During Probabilistic Discounting, Christopher P. Knapp, Brooke Fallon, Sandhya Kortagere, Barry D. Waterhouse, Stan B Floresco, Rachel L Navarra

Rowan-Virtua School of Osteopathic Medicine Departmental Research

RATIONALE: Psychostimulants, such as amphetamine (AMPH) and methylphenidate (MPH), non-selectively elevate extracellular concentrations of the catecholamine neurotransmitters, dopamine (DA) and norepinephrine (NE), and are common pharmacological strategies used to improve prefrontal cortex (PFC)-dependent cognitive dysfunction. However, this approach can be problematic given AMPH has been shown to increase preference for risky choices in a rodent assay of risk/reward decision making. SK609 is a novel NE reuptake blocker that selectively activates DA D3 receptors without affinity for the DA transporter. SK609 has been shown to improve cognitive performance without increasing psychostimulant-like spontaneous locomotor activity, suggesting SK609 may benefit neurocognitive function without …

Glutamate Receptors At The Zebrafish Neuromuscular Junction, Trenton Randall

Honors College Theses

Glutamate is the main excitatory neurotransmitter in the CNS, but at the neuromuscular junction, that role is taken by acetylcholine. However, during embryonic development, non-cholinergic receptors, including those for glutamate, are also present at the NMJ, which are not maintained unless used. These receptors can still form functional circuits, as evidenced by a process called neurotransmitter respecification, in which a neuron changes which neurotransmitter it releases, and is accompanied by the upregulation of the corresponding receptor by the muscle. This process has been observed in frogs and in rats. We predict that zebrafish also exhibit this neurotransmitter plasticity. To assess …

Elucidating The Mechanism Of Interleukin-17a Signaling In The Pathogenesis Of West Nile Virus, Farzana Nazneen

Dissertations

West Nile Virus (WNV), a neurotropic flavivirus transmitted by mosquitoes, is a leading cause of viral encephalitis in the United States and remains a significant public health concern. Unfortunately, no vaccine or therapeutic intervention is available against WNV infection. Previous studies, including ours, demonstrated that interleukin-17A (IL-17A) signaling increases the cytotoxic potential of CD8⁺ T cells to facilitate WNV and parasite clearance; however, the molecular mechanism is not understood. IL-17 receptor C (IL-17RC) is an obligatory co-receptor with IL-17 receptor A (IL-17RA) for signaling induced by IL-17A, IL-17A/F, and IL-17F. In this study, we found that IL-17RC deficient ( …

The Role Of Eor-1/Plzf In Promoting Soma Compartment Specific Death In A Non-Canonical Cell Death Program In C. Elegans, Nathan Rather

2024 Fall Honors Capstone Projects - Archive

Programmed cell death (PCD) is an essential evolutionarily- conserved and genetically encoded program for normal development and homeostasis. A previous study identified a CED-3 dependent but mechanistically non-apoptotic and highly stereotyped elimination program called Compartmentalized Cell Elimination (CCE) in the C. elegans tail-spike epithelial cell (TSC). Here the transcription factor EOR-1/PLZF is found to promote CCE. The loss of which results in a persisting non-engulfed soma with enlarged nuclei in the first larval stage. This study finds that EOR-1 and its established partners function downstream of CED-3 Caspase protease activity, a role distinct from other PCD contexts where EOR-1 has …

Ecological And Pathological Applications Of The Heat Shock Response, Taylor Rae Stanley

Theses and Dissertations

The heat shock response (HSR) is a cytoprotective stress response pathway that regulates cellular proteostasis. The HSR is an evolutionarily conserved pathway that is essential for normal cellular functioning. Here, we explore the broad ecological and pathological impacts of the HSR. In an ecological context, we perform gene level analysis of the transcriptomes of two closely related sunfish. We found that the more invasive bluegill sunfish has gene expansions in two HSR gene families, the HSP70 family and the HSP90 family compared to the redear sunfish. These gene expansions were also observed in several other teleost fish species and were …

Quantitative Analysis Of Protective Effect Of Resveratrol Against Nanoplastic-Induced Oxidative Stress, Bryan Gustafson

All Graduate Reports and Creative Projects, Fall 2023 to Present

Nanoplastics and microplastics are a growing ecological and health concern due to their widespread prevalence. The effects of these microscopic particles in biological systems primarily relate to their potential to produce reactive oxygen species and interactive effects with other cytotoxic agents. Of particular interest in this work is the effect on mammalian intestinal cells, due to the regular unintentional ingestion of micro- and nanoplastics in the normal western diet. In this study, Caco-2 cells were cultured and exposed to polystyrene particles in vitro as a simple model of the human digestive endothelium.

Resveratrol, a naturally occurring polyphenolic compound, has been …

Lack Of Inflammatory Response In Mouse Embryonic Cells To Listeria Infection, Damilola Oyebode

Master's Theses

Approximately 1 in 5 pregnancies in the United States results in miscarriage, with a significant percentage of these losses attributed to failed implantation due to immunological challenges encountered by the early embryo. Recent research has shown that key cell types within mouse blastocysts, including embryonic stem cells (ESCs) and trophoblast stem cells (TSCs), exhibit remarkable resilience against the cytotoxic effects associated with viral infections and embryotoxic cytokines, such as TNFα and IFNγ, both of which are known to hinder blastocyst development. However, the immune properties of the preimplantation stage embryo, specifically the blastocyst, remain poorly understood. Using mouse cell model, …

Apoa2 Increases Cholesterol Efflux Capacity To Plasma Hdl By Displacing The C-Terminus Of Resident Apoa1, Snigdha Sarkar, Jamie Morris, Youngki You, Hannah Sexmith, Scott E. Street, Stephanie M. Thibert, Isaac K. Attah, Chelsea M. Hutchinson Bunch, Irina V. Novikova, James E. Evans, Amy S. Shah, Scott M. Gordon, Jere P. Segrest, Karin E. Bornfeldt, Tomas Vaisar, Jay W. Heinecke, W. Sean Davidson, John T. Melchior

Saha Cardiovascular Research Center Faculty Publications

Abstract The ability of high-density lipoprotein (HDL) to promote cellular cholesterol efflux is a more robust predictor of cardiovascular disease protection than HDL-cholesterol levels in plasma. Previously, we found that lipidated HDL containing both apolipoprotein A-I (APOA1) and A-II (APOA2) promotes cholesterol efflux via the ATP-binding cassette transporter (ABCA1). In the current study, we directly added purified, lipid-free APOA2 to human plasma and found a dose-dependent increase in whole plasma cholesterol efflux capacity. APOA2 likewise increased the cholesterol efflux capacity of isolated HDL with the maximum effect occurring when equal masses of APOA1 and APOA2 coexisted on the particles. Follow-up …

Untargeted Lipidomics Reveals Novel Hdl Metabotypes And Lipid-Clinical Correlates, Peer W. F. Karmaus, Scott M. Gordon, Marcus Y. Chen, Alison A. Motsinger-Reif, Rodney W. Snyder, Timothy R. Fennell, Suramya Waidyanatha, Reshan A. Fernando, Alan T. Remaley, Michael B. Fessler

Saha Cardiovascular Research Center Faculty Publications

Plasma high-density lipoprotein (HDL), originally studied for its role in lipid transport, is now appreciated to have wide-ranging biological functions that become defective during disease. While >200 lipids have collectively been detected in HDL, published HDL lipidomic analyses in different diseases have commonly been targeted to prespecified subsets of lipids. Here, we report the results of untargeted lipidomic analysis of HDL isolated from 101 subjects referred for computed tomographic coronary imaging for whom multiple additional clinical and lipoprotein metadata were measured. Unsupervised clustering of the total HDL lipidome revealed that the subjects fell into one of two discrete groups, herein …

Bactericidal Mechanism Of Peracetic Acid Against Salmonella Mediated By Reactive Oxygen Species, Mohammed Khalid Wheib Al-Doury

Graduate Theses and Dissertations

Peracetic acid (PAA) is extensively used as a bactericidal agent in poultry processing to reduce foodborne pathogens, including Salmonella enterica serotype Typhimurium (S. Typhimurium). Despite its widespread application, the exact mechanisms underlying its bactericidal effects, particularly the role of reactive oxygen species (ROS), remain incompletely understood. This dissertation examines the role of ROS in PAA-mediated bacterial killing and explores both genetic and metabolic factors that influence bacterial susceptibility to PAA. Findings from these studies have implications for enhancing the efficacy of PAA, thereby improving food safety practices in the industry.

The first study investigates ROS as a central component …

Differential Responses Of Human Neutrophils To Legionella Species., Hannah E. Hanford

Electronic Theses and Dissertations

Legionella species are Gram-negative facultative intracellular bacterial pathogens commonly found in freshwater or soil environments where they survive and replicate within their natural protozoan hosts. Humans serve as an accidental host for Legionella species after inhalation of contaminated aerosolized water droplets or soil particles enabling the bacteria to enter the lungs, infect alveolar macrophages, and result in severe pneumonia. Evasion of degradation and proliferation within alveolar macrophages by Legionella species and manifestation of “Legionnaires’ disease” is dependent on the Dot/Icm type IV secretion system (T4SS). The T4SS apparatus is a needle-like channel for translocation of over ~350 effector proteins, depending …

Investigation Of Reproductive Inhibition During Chronic Heat Stress In C. Elegans, Macy Makenna Nielander

Theses and Dissertations

Nearly all organisms experience heterogeneity in environmental conditions that can negatively impact fitness. A common adaptive strategy to reduce the impact of this stress is called facultative diapause, where development and reproduction are temporarily arrested to promote survival. Facultative diapause is characterized by three major criteria, the arrest must be (i) induced by an unfavorable environmental condition, (ii) reversible, and (iii) driven by genetically regulated mechanisms. The aim of this thesis was to determine whether chronic heat-induced inhibition of egg laying in Caenorhabditis elegans represents a novel form of facultative diapause. It was found that the inhibition of egg-laying is …

Integrated Transcriptomics And Proteomics Study Of Starvation Selected Drosophila Melanogaster, Aavash Adhikari

UNLV Theses, Dissertations, Professional Papers, and Capstones

Drosophila melanogaster, selected for starvation resistance exhibits phenotypic traits such as increased body size, slower development, reduced fecundity and obesity like characteristics including higher lipid reserves and cardiac dysfunction. While prior studies have explored the metabolic and biochemical changes in these flies, the underlying genetic and molecular mechanisms remain unclear. This study aims to investigate the gene expression and protein abundance in starvation-selected Drosophila through RNAseq, proteomics, and integrating both datasets using the MixOmics package in R. I found differential gene expression in the fat body at the third instar larval stage, particularly in pathways related to lipid metabolism, calcium …

Opa1 And Disease-Causing Mutants Perturb Mitochondrial Nucleoid Distribution, J. Macuada, I. Molina-Riquelme, G. Vidal, N. Pérez-Bravo, C. Vásquez-Trincado, G. Aedo, D. Lagos, P. Yu-Wai-Man, R. Horvath, T. J. Rudge, B. Cartes-Saavedra, V. Eisner

Department of Pathology, Anatomy, and Cell Biology Faculty Papers

Optic atrophy protein 1 (OPA1) mediates inner mitochondrial membrane (IMM) fusion and cristae organization. Mutations in OPA1 cause autosomal dominant optic atrophy (ADOA), a leading cause of blindness. Cells from ADOA patients show impaired mitochondrial fusion, cristae structure, bioenergetic function, and mitochondrial DNA (mtDNA) integrity. The mtDNA encodes electron transport chain subunits and is packaged into nucleoids spread within the mitochondrial population. Nucleoids interact with the IMM, and their distribution is tightly linked to mitochondrial fusion and cristae shaping. Yet, little is known about the physio-pathological relevance of nucleoid distribution. We studied the effect of OPA1 and ADOA-associated mutants on …

Effects Of Sex, Cadmium, And High-Fat Diet On Accumulation Of Essential Metals In Mouse Kidneys., Justin Y. Xu, Adam Gaweda, Michelle Barati, Micheal Merchant, Jun Cai, Lu Cai, Madhavi J. Rane

The Cardinal Edge

Effects of cadmium and obesity on accumulation of essential metals in mouse kidney were evaluated. Starting in utero and continuing until 24 weeks postweaning, male/female mice were exposed to cadmium (0, 0.5, or 5 ppm) and were fed on Normal Diet (ND)/High-Fat Diet (HFD) post weaning. ICP-MS analysis was performed to evaluate cadmium and essential metals absorbed in the kidneys of aforementioned mice. Metals data obtained was analyzed using MANOVA with multiple comparisons (Bonferroni correction with p

Anti-Aging Properties Of Cosmetics, Anna Lipinski

The Cardinal Edge

It is well established that external factors, specifically UV exposure, are among the highest contributors to the biological aging of the skin. Prolonged exposure may disrupt skin cell homeostasis through shifts in proteostasis, accumulation of reactive oxidative species, premature cellular senescence, and inflammatory pathways. Cosmetic companies have responded to this by producing products marketed as "youthful" and "anti-aging" by including specific compounds into formulas to combat skin aging. This review discusses the specific mechanisms of anti-aging formulas, including polyphenols as senotherapeutic agents and L-ascorbic acid as an anti-inflammatory compound, in resisting the biological aging of the skin.