Hydrotropic function of ATP in the crystalline lens

Aging of the eye: Lessons from cataracts and age-related macular degeneration

Aging is the greatest risk factor for chronic human diseases, including many eye diseases. Geroscience aims to understand the effects of the aging process on these diseases, including the genetic, molecular, and cellular mechanisms that underlie the increased risk of disease over the lifetime. Understanding of the aging eye increases general knowledge of the cellular physiology impacted by aging processes at various biological extremes. Two major diseases, age-related cataract and age-related macular degeneration (AMD) are caused by dysfunction of the lens and retina, respectively. Lens transparency and light refraction are mediated by lens fiber cells lacking nuclei and other organelles, which provides a unique opportunity to study a single aging hallmark, i.e., loss of proteostasis, within an environment of limited metabolism. In AMD, local dysfunction of the photoreceptors/retinal pigmented epithelium/Bruch's membrane/choriocapillaris complex in the macula leads to the loss of photoreceptors and eventually loss of central vision, and is driven by nearly all the hallmarks of aging and shares features with Alzheimer's disease, Parkinson's disease, cardiovascular disease, and diabetes. The aging eye can function as a model for studying basic mechanisms of aging and, vice versa, well-defined hallmarks of aging can be used as tools to understand age-related eye disease.

Phospholipid analyses of rabbit ocular surface tissues

The tissues of the integument covering the ocular surface comprise a mucus membrane functioning as a protective physical barrier and has the ability to mount a defensive inflammatory response. Since lipid metabolism has a role in both of these functions, we studied normal membrane phospholipids (PL) of the cornea and bulbar conjunctiva to (1) determine baseline PL profiles of these tissues, (2) compare and contrast these individual PL metabolite profiles as well as groups of metabolites, and (3) describe pathway-specific metabolic interrelations among these tissues. Corneal and conjunctival tissue samples were isolated from rabbit eyes (n = 30) and extracted with chloroform-methanol using a modified Folch procedure. 31P nuclear magnetic resonance spectroscopy was used to qualitatively and quantitatively measure tissue PL profiles. The cornea and conjunctiva, respectively, have the following PL composition (mole % of total detected phospholipid): phosphatidylglycerol (PG) -, 0.4; lysophosphatidylethanolamine 1.2, -; phosphatidic acid -, 0.4; diPG (cardiolipin) 2.1, 3.5; unknown PL at the chemical shift of 0.13 δ 1.5, 0.9; ethanolamine plasmalogen 11.2, 13.0; phosphatidylethanolamine 11.5, 12.8; phosphatidylserine 8.9, 10.1; sphingomyelin 10.2, 10.7; lysophosphatidylcholine 0.9, 1.4; phosphatidylinositol 5.3, 5.3; phosphatidylcholine (PC) plasmalogen or alkylacylPC 2.2, 1.9; PC 45.1, 40.0. In addition, 28 PL metabolic indices were calculated from these data, which permitted pathway-specific lipid analyses. This study (1) establishes PL profiles of the two ocular tissues of the integument that cover the surface of the eye, (2) compares and contrasts indices comprised of ratios and combinations of PL, and (3) describes pathway-specific metabolic interrelations among these tissues to serve as baselines for studies involving the distribution of tissue phospholipids.

A physicochemical perspective on cellular ageing

Cellular ageing described at the molecular level is a multifactorial process that leads to a spectrum of ageing trajectories. There has been recent discussion about whether a decline in physicochemical homeostasis causes aberrant phase transitions, which are a driver of ageing. Indeed, the function of all biological macromolecules, regardless of their participation in biomolecular condensates, depends on parameters such as pH, crowding, and redox state. We expand on the physicochemical homeostasis hypothesis and summarise recent evidence that the intracellular milieu influences molecular processes involved in ageing.

Adenosine Triphosphate (ATP) and Protein Aggregation in Age-Related Vision-Threatening Ocular Diseases

Protein aggregation is the etiopathogenesis of the three most profound vision-threatening eye diseases: age-related cataract, presbyopia, and age-related macular degeneration. This perspective organizes known information on ATP and protein aggregation with a fundamental unrecognized function of ATP. With recognition that maintenance of protein solubility is related to the high intracellular concentration of ATP in cells, tissues, and organs, we hypothesize that (1) ATP serves a critical molecular function for organismal homeostasis of proteins and (2) the hydrotropic feature of ATP prevents pathological protein aggregation while assisting in the maintenance of protein solubility and cellular, tissue, and organismal function. As such, the metabolite ATP plays an extraordinarily important role in the prevention of protein aggregation in the leading causes of vision loss or blindness worldwide.

Toward a molecular mechanism for the interaction of ATP with alpha-synuclein

The ability of Adenosine Triphosphate (ATP) to modulate protein solubility establishes a critical link between ATP homeostasis and proteinopathies, such as Parkinson's (PD). The most significant risk factor for PD is aging, and ATP levels decline dramatically with age. However, the mechanism by which ATP interacts with alpha-synuclein (αS), whose aggregation is characteristic of PD, is currently not fully understood, as is ATP's effect on αS aggregation. Here, we use nuclear magnetic resonance spectroscopy as well as fluorescence, dynamic light scattering and microscopy to show that ATP affects multiple species in the αS self-association cascade. The triphosphate moiety of ATP disrupts long-range electrostatic intramolecular contacts in αS monomers to enhance initial aggregation, while also inhibiting the formation of late-stage β-sheet fibrils by disrupting monomer-fibril interactions. These effects are modulated by magnesium ions and early onset PD-related αS mutations, suggesting that loss of the ATP hydrotropic function on αS fibrillization may play a role in PD etiology.

ATP induces folding of ALS-causing C71G-hPFN1 and nascent hSOD1

ALS-causing C71G-hPFN1 coexists in both folded and unfolded states, while nascent hSOD1 is unfolded. So far, the mechanisms underlying their ALS-triggering potential remain enigmatic. Here we show by NMR that ATP completely converts C71G-hPFN1 into the folded state at a 1:2 ratio, while inducing nascent hSOD1 into two co-existing states at a 1:8 ratio. Surprisingly, the inducing capacity of ATP comes from its triphosphate, but free triphosphate triggers aggregation. The inducing capacity ranks as: ATP = ATPP = PPP > ADP = AMP-PNP = AMP-PCP = PP, while AMP, adenosine, P, and NaCl show no conversion. Mechanistically, ATP and triphosphate appear to enhance the intrinsic folding capacity encoded in the sequences, as unveiled by comparing conformations and dynamics of ATP- and Zn2+-induced hSOD1 folded states. Our study provides a mechanism for the finding that some single-cell organisms employ polyphosphates as primordial chaperones, and sheds light on the enigma of age-related onset of familial ALS and risk increase of neurodegenerative diseases.

Tuning the ATP-ATP and ATP-disordered protein interactions in high ATP concentration by altering water models

The adenosine triphosphate (ATP)-protein interactions have been of great interest since the recent experimental finding of ATP's role as a hydrotrope. The interaction between ATP and disordered proteins is fundamental to the dissolution of protein aggregates and the regulation of liquid-liquid phase separation by ATP. Molecular dynamics simulation is a powerful tool for analyzing these interactions in molecular detail but often suffers from inaccuracies in describing disordered proteins and ATPs in high concentrations. Recently, several water models have been proposed to improve the description of the protein-disordered states, yet how these models work with ATP has not been explored. To this end, here, we study how water models affect ATP and alter the ATP-ATP and ATP-protein interactions for the intrinsically disordered protein, α-Synuclein. Three water models, TIP4P-D, OPC, and TIP3P, are compared, while the protein force field is fixed to ff99SBildn. The results show that ATP over-aggregates into a single cluster in TIP3P water, but monomers and smaller clusters are found in TIP4P-D and OPC waters. ATP-protein interaction is also over-stabilized in TIP3P, whereas repeated binding/unbinding of ATP to α-Synuclein is observed in OPC and TIP4P-D waters, which is in line with the recent nuclear magnetic resonance experiment. The adenine ring-mediated interaction is found to play a major role in ATP-ATP and ATP-protein contacts. Interestingly, changing Mg2+ into Na+ strengthened the electrostatic interaction and promoted ATP oligomerization and ATP-α-Synuclein binding. Overall, this study shows that changing the water model can be an effective approach to improve the properties of ATP in high concentration.

Systemic Anti-Inflammatory Therapy Aided by Curcumin-Laden Double-Headed Nanoparticles Combined with Injectable Long-Acting Insulin in a Rodent Model of Diabetes Eye Disease

Therapeutic interventions that counter emerging targets in diabetes eye diseases are lacking. We hypothesize that a combination therapy targeting inflammation and hyperglycemia can prevent diabetic eye diseases. Here, we report a multipronged approach to prevent diabetic cataracts and retinopathy by combining orally bioavailable curcumin-laden double-headed (two molecules of gambogic acid conjugated to terminal carboxyl groups of poly(d,l-lactide-co-glycolide)) nanoparticles and injectable basal insulin. The combination treatment led to a significant delay in the progression of diabetic cataracts and retinopathy, improving liver function and peripheral glucose homeostasis. We found a concurrent reduction in lens aggregate protein, AGEs, and increased mitochondrial ATP production. Importantly, inhibition of Piezo1 protected against hyperglycemia-induced retinal vascular damage suggesting possible involvement of Piezo1 in the regulation of retinal phototransduction. Histologic evaluation of murine small intestines revealed that chronic administration of curcumin-laden double-headed nanoparticles was well tolerated, circumventing the fear of nanoparticle toxicity. These findings establish the potential of anti-inflammatory and anti-hyperglycemic combination therapy for the prevention of diabetic cataracts and retinopathy.

ATP modulates self-perpetuating conformational conversion generating structurally distinct yeast prion amyloids that limit autocatalytic amplification

Prion-like self-perpetuating conformational conversion of proteins into amyloid aggregates is associated with both transmissible neurodegenerative diseases and non-Mendelian inheritance. The cellular energy currency ATP is known to indirectly regulate the formation, dissolution, or transmission of amyloid-like aggregates by providing energy to the molecular chaperones that maintain protein homeostasis. In this work, we demonstrate that ATP molecules, independent of any chaperones, modulate the formation and dissolution of amyloids from a yeast prion domain (NM domain of Saccharomyces cerevisiae Sup35) and restricts autocatalytic amplification by controlling the amount of fragmentable and seeding-competent aggregates. ATP, at (high) physiological concentrations in the presence of Mg²⁺, kinetically accelerates NM aggregation. Interestingly, ATP also promotes phase-separation-mediated aggregation of a human protein harboring a yeast prion-like domain. We also show that ATP disaggregates preformed NM fibrils in a dose-independent manner. Our results indicate that ATP-mediated disaggregation, unlike the disaggregation by the disaggregase Hsp104, yields no oligomers that are considered one of the critical species for amyloid transmission. Furthermore, high concentrations of ATP delimited the number of seeds by giving rise to compact, ATP-bound NM fibrils that exhibited nominal fragmentation by either free ATP or Hsp104 disaggregase to generate lower molecular weight amyloids. Additionally, (low) pathologically relevant ATP concentrations restricted autocatalytic amplification by forming structurally distinct amyloids which are found seeding-inefficient due to their reduced β-content. Our results provide key mechanistic underpinnings of concentration-dependent chemical chaperoning by ATP against prion-like transmissions of amyloids.

Nonspecific Binding of Adenosine Tripolyphosphate and Tripolyphosphate Modulates the Phase Behavior of Lysozyme

Adenosine tripolyphosphate (ATP) is a small polyvalent anion that has recently been shown to interact with proteins and have a major impact on assembly processes involved in biomolecular condensate formation and protein aggregation. However, the nature of non-specific protein-ATP interactions and their effects on protein solubility are largely unknown. Here, the binding of ATP to the globular model protein is characterized in detail using X-ray crystallography and nuclear magnetic resonance (NMR). Using NMR, we identified six ATP binding sites on the lysozyme surface, with one known high-affinity nucleic acid binding site and five non-specific previously unknown sites with millimolar affinities that also bind tripolyphosphate (TPP). ATP binding occurs primarily through the polyphosphate moiety, which was confirmed by the X-ray structure of the lysozyme-ATP complex. Importantly, ATP binds preferentially to arginine over lysine in non-specific binding sites. ATP and TPP have similar effects on solution-phase protein-protein interactions. At low salt concentrations, ion binding to lysozyme causes precipitation, while at higher salt concentrations, redissolution occurs. The addition of an equimolar concentration of magnesium to ATP does not alter ATP binding affinities but prevents lysozyme precipitation. These findings have important implications for both protein crystallization and cell biology. Crystallization occurs readily in ATP solutions outside the well-established crystallization window. In the context of cell biology, the findings suggest that ATP binds non-specifically to folded proteins in physiological conditions. Based on the nature of the binding sites identified by NMR, we propose several mechanisms for how ATP binding can prevent the aggregation of natively folded proteins.

A macrocyclic molecule with multiple antioxidative activities protects the lens from oxidative damage

Growing evidence links oxidative stress to the development of a cataract and other diseases of the eye. Treatments for lens-derived diseases are still elusive outside of the standard surgical interventions, which still carry risks today. Therefore, a potential drug molecule OHPy2N2 was explored for the ability to target multiple components of oxidative stress in the lens to prevent cataract formation. Several pathways were identified. Here we show that the OHPy2N2 molecule activates innate catalytic mechanisms in primary lens epithelial cells to prevent damage induced by oxidative stress. This protection was linked to the upregulation of Nuclear factor erythroid-2-related factor 2 and downstream antioxidant enzyme for glutathione-dependent glutaredoxins, based on Western Blot methods. The anti-ferroptotic potential was established by showing that OHPy2N2 increases levels of glutathione peroxidase, decreases lipid peroxidation, and readily binds iron (II) and (III). The bioenergetics pathway, which has been shown to be negatively impacted in many diseases involving oxidative stress, was also enhanced as evidence by increased levels of Adenosine triphosphate product when the lens epithelial cells were co-incubated with OHPy2N2. Lastly, OHPy2N2 was also found to prevent oxidative stress-induced lens opacity in an ex vivo organ culture model. Overall, these results show that there are multiple pathways that the OHPy2N2 has the ability to impact to promote natural mechanisms within cells to protect against chronic oxidative stress in the eye.

Defunctionalizing intracellular organelles such as mitochondria and peroxisomes with engineered phospholipase A/acyltransferases

Organelles vitally achieve multifaceted functions to maintain cellular homeostasis. Genetic and pharmacological approaches to manipulate individual organelles are powerful in probing their physiological roles. However, many of them are either slow in action, limited to certain organelles, or rely on toxic agents. Here, we design a generalizable molecular tool utilizing phospholipase A/acyltransferases (PLAATs) for rapid defunctionalization of organelles via remodeling of the membrane phospholipids. In particular, we identify catalytically active PLAAT truncates with minimal unfavorable characteristics. Chemically-induced translocation of the optimized PLAAT to the mitochondria surface results in their rapid deformation in a phospholipase activity dependent manner, followed by loss of luminal proteins as well as dissipated membrane potential, thus invalidating the functionality. To demonstrate wide applicability, we then adapt the molecular tool in peroxisomes, and observe leakage of matrix-resident functional proteins. The technique is compatible with optogenetic control, viral delivery and operation in primary neuronal cultures. Due to such versatility, the PLAAT strategy should prove useful in studying organelle biology of diverse contexts.

Mechanistic Insight on General Protein-Binding Ability of ATP and the Impacts of Arginine Residues

Recent experiments suggested that adenosine triphosphate (ATP) can regulate liquid-liquid phase separation (LLPS) of various proteins and inhibit protein aggregations at its physiological concentration, which is highly correlated with the nonspecific interactions of ATP to a wide variety of proteins. However, the mechanism underlying the general binding capability of ATP largely remains unclear. In this work, we used molecular dynamics simulation to study the binding of ATPs to three proteins with distinct net charges: TDP-43 NTD (-7 e), TAF15-RRM (0 e), HWEL (+8 e). Negatively charged ATP exhibits a strong trend to accumulate around all of these proteins. While only a fraction of the accumulated ATPs directly binds to the limited regions of the protein surface, additional ATPs indirectly bind to proteins by aggregating into ATP clusters. Hence, the proportion of the directly bound ATPs in the clusters as well as their binding regions can be adjusted in response to different proteins, which makes ATP well adapted to a variety of proteins. Moreover, our results suggest that ATP tightly binds to Arg with high affinity, and Arg dominates the direct binding of ATP. Meanwhile, Arg also affects the self-association of accumulated ATPs. The size of the ATP cluster is effectively regulated by the distribution of Arg. Considering the ubiquity of Arg in proteins, our findings are helpful to understand the general binding capability of ATP.

Intracellular ATP Concentration and Implication for Cellular Evolution

Crystalline lens and striated muscle exist at opposite ends of the metabolic spectrum. Lens is a metabolically quiescent tissue, whereas striated muscle is a mechanically dynamic tissue with high-energy requirements, yet both tissues contain millimolar levels of ATP (>2.3 mM), far exceeding their underlying metabolic needs. We explored intracellular concentrations of ATP across multiple cells, tissues, species, and domains to provide context for interpreting lens/striated muscle data. Our database revealed that high intracellular ATP concentrations are ubiquitous across diverse life forms including species existing from the Precambrian Era, suggesting an ancient highly conserved role for ATP, independent of its widely accepted view as primarily "metabolic currency". Our findings reinforce suggestions that the primordial function of ATP was non-metabolic in nature, serving instead to prevent protein aggregation.

ATP and Tri-Polyphosphate (TPP) Suppress Protein Aggregate Growth by a Supercharging Mechanism

A common strategy to increase aggregation resistance is through rational mutagenesis to supercharge proteins, which leads to high colloidal stability, but often has the undesirable effect of lowering conformational stability. We show this trade-off can be overcome by using small multivalent polyphosphate ions, adenosine triphosphate (ATP) and tripolyphosphate (TPP) as excipients. These ions are equally effective at suppressing aggregation of ovalbumin and bovine serum albumin (BSA) upon thermal stress as monitored by dynamic and static light scattering. Monomer loss kinetic studies, combined with measurements of native state protein–protein interactions and ζ-potentials, indicate the ions reduce aggregate growth by increasing the protein colloidal stability through binding and overcharging the protein. Out of three additional proteins studied, ribonuclease A (RNaseA), α-chymotrypsinogen (α-Cgn), and lysozyme, we only observed a reduction in aggregate growth for RNaseA, although overcharging by the poly-phosphate ions still occurs for lysozyme and α-Cgn. Because the salts do not alter protein conformational stability, using them as excipients could be a promising strategy for stabilizing biopharmaceuticals once the protein structural factors that determine whether multivalent ion binding will increase colloidal stability are better elucidated. Our findings also have biological implications. Recently, it has been proposed that ATP also plays an important role in maintaining intracellular biological condensates and preventing protein aggregation in densely packed cellular environments. We expect electrostatic interactions are a significant factor in determining the stabilizing ability of ATP towards maintaining proteins in non-dispersed states in vivo.

Adenosine triphosphate energy-independently controls protein homeostasis with unique structure and diverse mechanisms

Proteins function in the crowded cellular environments with high salt concentrations, thus facing tremendous challenges of misfolding/aggregation which represents a pathological hallmark of aging and an increasing spectrum of human diseases. Recently, intrinsically disordered regions (IDRs) were recognized to drive liquid-liquid phase separation (LLPS), a common principle for organizing cellular membraneless organelles (MLOs). ATP, the universal energy currency for all living cells, mysteriously has concentrations of 2-12 mM, much higher than required for its previously-known functions. Only recently, ATP was decoded to behave as a biological hydrotrope to inhibit protein LLPS and aggregation at mM. We further revealed that ATP also acts as a bivalent binder, which not only biphasically modulates LLPS driven by IDRs of human and viral proteins, but also bind to the conserved nucleic-acid-binding surfaces of the folded proteins. Most unexpectedly, ATP appears to act as a hydration mediator to antagonize the crowding-induced destabilization as well as to enhance folding of proteins without significant binding. Here, this review focuses on summarizing the results of these biophysical studies and discussing their implications in an evolutionary context. By linking triphosphate with unique hydration property to adenosine, ATP appears to couple the ability for establishing hydrophobic, π-π, π-cation and electrostatic interactions to the capacity in mediating hydration of proteins, which is at the heart of folding, dynamics, stability, phase separation and aggregation. Consequently, ATP acquired a category of functions at ~mM to energy-independently control protein homeostasis with diverse mechanisms, thus implying a link between cellular ATP concentrations and protein-aggregation diseases.

Did Cyclic Metaphosphates Have a Role in the Origin of Life?

How life began still eludes science life, the initial progenote in the context presented herein, being a chemical aggregate of primordial inorganic and organic molecules capable of self-replication and evolution into ever increasingly complex forms and functions.Presented is a hypothesis that a mineral scaffold generated by geological processes and containing polymerized phosphate units was present in primordial seas that provided the initiating factor responsible for the sequestration and organization of primordial life's constituents. Unlike previous hypotheses proposing phosphates as the essential initiating factor, the key phosphate described here is not a polynucleotide or just any condensed phosphate but a large (in the range of at least 1 kilo-phosphate subunits), water soluble, cyclic metaphosphate, which is a closed loop chain of polymerized inorganic phosphate residues containing only phosphate middle groups. The chain forms an intrinsic 4-phosphate helix analogous to its structure in Na Kurrol's salt, and as with DNA, very large metaphosphates may fold into hairpin structures. Using a Holliday-junction-like scrambling mechanism, also analogous to DNA, rings may be manipulated (increased, decreased, exchanged) easily with little to no need for additional energy, the reaction being essentially an isomerization.A literature review is presented describing findings that support the above hypothesis. Reviewed is condensed phosphate inorganic chemistry including its geological origins, biological occurrence, enzymes and their genetics through eukaryotes, polyphosphate functions, circular polynucleotides and the role of the Holliday junction, previous biogenesis hypotheses, and an Eoarchean Era timeline.

Tethering-induced destabilization and ATP-binding for tandem RRM domains of ALS-causing TDP-43 and hnRNPA1

TDP-43 and hnRNPA1 contain tandemly-tethered RNA-recognition-motif (RRM) domains, which not only functionally bind an array of nucleic acids, but also participate in aggregation/fibrillation, a pathological hallmark of various human diseases including amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), alzheimer's disease (AD) and Multisystem proteinopathy (MSP). Here, by DSF, NMR and MD simulations we systematically characterized stability, ATP-binding and conformational dynamics of TDP-43 and hnRNPA1 RRM domains in both tethered and isolated forms. The results reveal three key findings: (1) upon tethering TDP-43 RRM domains become dramatically coupled and destabilized with Tm reduced to only 49 °C. (2) ATP specifically binds TDP-43 and hnRNPA1 RRM domains, in which ATP occupies the similar pockets within the conserved nucleic-acid-binding surfaces, with the affinity slightly higher to the tethered than isolated forms. (3) MD simulations indicate that the tethered RRM domains of TDP-43 and hnRNPA1 have higher conformational dynamics than the isolated forms. Two RRM domains become coupled as shown by NMR characterization and analysis of inter-domain correlation motions. The study explains the long-standing puzzle that the tethered TDP-43 RRM1–RRM2 is particularly prone to aggregation/fibrillation, and underscores the general role of ATP in inhibiting aggregation/fibrillation of RRM-containing proteins. The results also rationalize the observation that the risk of aggregation-causing diseases increases with aging.

ATP differentially antagonizes the crowding-induced destabilization of human γS-crystallin and its four cataract-causing mutants

αβγ-crystallins account for ∼90% of ocular proteins in lens with concentrations ≥400 mg/ml, which has to be soluble for the whole life-span and their aggregation results in cataract. So far, four cataract-causing mutants G18V, D26G, S39C and V42 M have been identified for human γS-crystallin. Mysteriously, lens maintains ATP concentrations of 3-7 mM despite being a metabolically-quiescent organ. Here by DSF and NMR, we characterized the binding of ATP to three cataract-causing mutants of human γS-crystallin as well as its effect on the solution conformations and thermal stability. The results together decode several novel findings: 1) ATP shows no detectable binding to WT and mutants, as well as no significant alternation of their conformations even at molar ratio of 1:200.2) Cataract-causing mutants show distinctive patterns of the crowding-induced destabilization. 3) ATP differentially antagonizes their crowding-induced destabilization. Our studies suggest that the crowding-induced destabilization of human γS-crystallin is also critically dependent of the hydration shell which could be differentially altered by four mutations. Most unexpectedly, ATP acts as an effective mediator for the protein hydration shell to antagonize the crowding-induced destabilization.

The Role of Pi, Glutamine and the Essential Amino Acids in Modulating the Metabolism in Diabetes and Cancer

Purpose

Re-examine the current metabolic models.

Methods

Review of literature and gene networks.

Results

Insulin activates Pi uptake, glutamine metabolism to stabilise lipid membranes. Tissue turnover maintains the metabolic health. Current model of intermediary metabolism (IM) suggests glucose is the source of energy, and anaplerotic entry of fatty acids and amino acids into mitochondria increases the oxidative capacity of the TCA cycle to produce the energy (ATP). The reduced cofactors, NADH and FADH2, have different roles in regulating the oxidation of nutrients, membrane potentials and biosynthesis. Trans-hydrogenation of NADH to NADPH activates the biosynthesis. FADH2 sustains the membrane potential during the cell transformations. Glycolytic enzymes assume the non-canonical moonlighting functions, enter the nucleus to remodel the genetic programmes to affect the tissue turnover for efficient use of nutrients. Glycosylation of the CD98 (4F2HC) stabilises the nutrient transporters and regulates the entry of cysteine, glutamine and BCAA into the cells. A reciprocal relationship between the leucine and glutamine entry into cells regulates the cholesterol and fatty acid synthesis and homeostasis in cells. Insulin promotes the Pi transport from the blood to tissues, activates the mitochondrial respiratory activity, and glutamine metabolism, which activates the synthesis of cholesterol and the de novo fatty acids for reorganising and stabilising the lipid membranes for nutrient transport and signal transduction in response to fluctuations in the microenvironmental cues. Fatty acids provide the lipid metabolites, activate the second messengers and protein kinases. Insulin resistance suppresses the lipid raft formation and the mitotic slippage activates the fibrosis and slow death pathways.

Cataract-causing G18V eliminates the antagonization by ATP against the crowding-induced destabilization of human γS-crystallin

In lens, ∼90% of ocular proteins are αβγ-crystallins with concentrations ≥400 mg/ml, which need to remain soluble for the whole life-span and their aggregation leads to cataract. The G18V mutation of human γS-crystallin causes hereditary childhood-onset cortical cataract. Mysteriously, despite being a metabolically-quiescent organ, lens maintains ATP concentrations of 3-7 mM. Very recently, we found that ATP has no significant binding to γS-crystallin as well as no alternation of its conformation. Nevertheless, ATP antagonizes the crowding-induced destabilization of γS-crystallin even at 1:1, most likely by interacting with the hydration shell. Here by DSF and NMR, we characterized the effect of ATP on binding, conformation, stability of G18V γS-crystallin and its interactions with α-crystallin. The results reveal: 1) G18V significantly accelerates the crowding-induced destabilization with Tm of 67 °C reduced to 50.5 °C at 1 mM. 2) Most unexpectedly, G18V almost completely eliminates the antagonizing effect of ATP against the crowding-induced destabilization. 3) ATP shows no significant effect on the interactions of α-crystallin with both WT and G18V γS-crystallin. Results together decode for the first time that G18V causes cataract not only by accelerating the crowding-induced destabilization, but also by eliminating the antagonizing effect of ATP against the crowding-induced destabilization.

ATP antagonizes the crowding-induced destabilization of the human eye-lens protein γS-crystallin

In lens, αβγ-crystallins accounting for ∼90% of ocular proteins with concentrations >400 mg/ml need to remain soluble for the whole life-span and their aggregation can lead to cataract. Mysteriously, despite being a metabolically-quiescent organ, lens maintains ATP concentrations of 3-7 mM. Very recently, ATP was proposed to hydrotropically prevent aggregation of crystallins but the mechanism remains unexplored. Here by NMR, DLS and DSF, we characterized the association, thermal stability and conformation of the 178-residue human γS-crystallin at concentrations from 2 to 100 mg/ml in the absence and in the presence of ATP. Results together reveal for the first time that ATP does antagonize the crowding-induced destabilization, although it has no significant binding to γS-crystallin as well as no alteration of its conformation. Therefore, ATP prevents aggregation in lens by a novel mechanism, thus rationalizing the fact that declining concentrations of ATP upon being aged is related to age-related cataractogenesis. To restore the normal concentrations of ATP in lens may represent a promising therapeutic strategy to treat aggregation-causing eye diseases.