Biology subjects

Julian, R.

Publications and source records attributed to Julian, R..

5 recordsLinked to original sources

bioRxiv2025

The microtubule nexus linking amyloid beta and tau: A simple and unifying theory for the underlying cause of Alzheimer's Disease

Alzheimers disease (AD) is defined by cognitive decline in conjunction with accumulation of aggregated amyloid {beta} (A{beta}) and tau, yet existing models of AD fail to provide a simple connection between A{beta} and tau. However, microtubules provide an intriguing nexus for pathological interactions between the two. Tau binds to microtubules and is critical to maintain their proper function. We demonstrate that A{beta} also binds to microtubules with affinity comparable that of tau itself. We hypothesize that displacement of tau by A{beta} leads to microtubule dysfunction and facilitates tau phosphorylation and aggregation. Importantly, in this model, aggregation of A{beta} is not the primary cause of toxicity, which allows many of the apparent contradictions between A{beta} pathology and cognition to be rationalized. This new model highlights the importance of both tau and A{beta} and enables novel therapeutic and intervention strategies to be considered.

neuroscience↗

bioRxiv2024

Sex differences in the cardiopulmonary and neuromuscular response to high-intensity interval exercise

Sex differences exist in the integrative response to exercise, however, these are typically researched during constant-load exercise. Interval exercise involves high-intensity efforts interspersed with recovery periods to repeatedly stress physiological systems, and it is currently unknown whether the response to this form of exercise differs between sexes. Ten males and ten females (age: 25{+/-}3 years) completed two experimental visits. First, an incremental treadmill exercise test was performed to obtain submaximal (lactate threshold) and maximal ([Formula] O2peak) data. Thereafter, visit two involved 4 x 3-min running intervals at 90% of the final incremental test velocity (v[Formula] O2peak), with 90 secs rest between intervals. Before exercise and after each interval, maximal voluntary contraction (MVC), quadriceps potentiated twitch (Qtw.pot), and voluntary activation (VA) were recorded. The rates of oxygen uptake ([Formula] O2), carbon dioxide production ([Formula] CO2) and ventilation ([Formula] E) were continuously recorded throughout. There was no sex difference in relative [Formula] O2peak (males: 47.2{+/-}6.0 vs. females: 44.4{+/-}5.8 ml.kg- 1.min-1, p=0.292). When expressed relative to peak values, there were no sex differences in the [Formula] O2 or [Formula] CO2 response to the interval task (p[≥]0.781). Females had greater [Formula] E, [Formula] E/[Formula] O2, and [Formula] E/[Formula] CO2 values during the first two intervals (p[≤]0.046). There were no sex differences in the reductions in MVC, Qtw.pot, and VA during the interval task (p[≥]0.150), however females had lesser reductions in Qtw.pot values post-exercise (-24{+/-}9 vs. -15{+/-}8%, p=0.044). Sex differences exist in the physiological response to interval exercise. Compared to males, females experienced greater hyperpnoea during the initial stages, and had lesser decreases in contractile function post-exercise.

physiology↗

bioRxiv2020

A two-trick pony: lysosomal protease cathepsin B possesses surprising ligase activity

Cathepsin B is an important protease within the lysosome, where it helps recycle proteins to maintain proteostasis. It is also known to degrade proteins elsewhere but has no other known functionality. However, by carefully monitoring peptide digestion with liquid chromatography and mass spectrometry, we observed synthesis of novel peptides during cathepsin B incubations. This ligation activity was explored further with a variety of peptide substrates to establish mechanistic details and was found to operate through a two-step mechanism with proteolysis and ligation occurring separately. Further explorations using varied sequences indicated increased affinity for some substrates, though all were found to ligate to some extent. Finally, experiments with a proteolytically inactive form of the enzyme yielded no ligation, indicating that the ligation reaction occurs in the same active site but in the reverse direction of proteolysis. These results clearly establish that cathepsin B can act as both a protease and ligase, although protease action eventually dominates over longer periods of time.

biochemistry↗

bioRxiv2019

Spontaneous isomerization of long-lived proteins provides a molecular mechanism for the lysosomal failure observed in Alzheimer’s disease

Proteinaceous aggregation is a well-known observable in Alzheimers disease (AD), but failure and storage of lysosomal bodies within neurons is equally ubiquitous and actually precedes bulk accumulation of extracellular amyloid plaque. In fact, AD shares many similarities with certain lysosomal storage disorders though establishing a biochemical connection has proven difficult. Herein, we demonstrate that isomerization and epimerization, which are spontaneous chemical modifications that occur in long-lived proteins, prevent digestion by the proteases in the lysosome (namely the cathepsins). For example, isomerization of aspartic acid into L-isoAsp prevents digestion of the N-terminal portion of A{beta} by cathepsin L, one of the most aggressive lysosomal proteases. Similar results were obtained after examination of various target peptides with a full series of cathepsins, including endo-, amino-, and carboxy-peptidases. In all cases peptide fragments too long for transporter recognition or release from the lysosome persisted after treatment, providing a mechanism for eventual lysosomal storage and bridging the gap between AD and lysosomal storage disorders. Additional experiments with microglial cells confirmed that isomerization disrupts proteolysis in active lysosomes. These results are easily rationalized in terms of protease active sites, which are engineered to precisely orient the peptide backbone and cannot accommodate the backbone shift caused by isoaspartic acid or side chain dislocation resulting from epimerization. Although A{beta} is known to be isomerized and epimerized in plaques present in AD brains, we further establish that the rates of modification for aspartic acid in positions 1 and 7 are fast and could accrue prior to plaque formation. Spontaneous chemistry can therefore provide modified substrates capable of inducing gradual lysosomal failure, which may play an important role in the cascade of events leading to the disrupted proteostasis, amyloid formation, and tauopathies associated with AD.

biochemistry↗

bioRxiv2018

Structural and Functional Consequences of Age-Related Isomerization in α Crystallins

Long-lived proteins are subject to spontaneous degradation and may accumulate a range of modifications over time, including subtle alterations such as isomerization. Recently, tandem-mass spectrometry approaches have enabled the identification and detailed characterization of such peptide isomers, including those differing only in chirality. However, the structural and functional consequences of these perturbations remain largely unexplored. Here we examine the site-specific impact of isomerization of aspartic acid and epimerization of serine in human A- and B-crystallin. From a total of 81 sites of modification identified in aged eye lenses, four (BSer59, ASer162, BAsp62, BAsp109) map to crucial oligomeric interfaces. To characterize the effect of isomerization on quaternary assembly, molecular dynamics calculations and native mass spectrometry experiments were performed on recombinant forms of A- and B-crystallin that incorporate, or mimic, isomerized residues. In all cases, oligomerization is significantly affected, with epimerization of a single serine residue (ASer162) sufficing to weaken inter-subunit binding dramatically. Furthermore, phosphorylation of BSer59, known to play an important regulatory role in oligomerization, is severely inhibited by serine epimerization and altered by isomerization of nearby BAsp62. Similarly, isomerization of BAsp109 disrupts a vital salt-bridge with BArg120, a loss previously shown to yield aberrant oligomerization and aggregation in several disease variants. Our results illustrate how isomerization of amino-acid residues, which may seem like a minor structural perturbation, can have profound consequences on protein assembly and activity by disrupting specific hydrogen bonds and salt bridges.\n\nSignificance StatementProteins play numerous critical roles in our bodies but suffer damage with increasing age. For example, isomerization is a spontaneous post-translational modification that alters the three-dimensional connectivity of an amino acid, yet remains invisible to traditional proteomic experiments. Herein, radical-based fragmentation was used for isomer identification while molecular dynamics and native mass spectrometry were utilized to assess structural consequences. The results demonstrate that isomerization disrupts both oligomeric assembly and phosphorylation in the -crystallins, which are long-lived proteins in the lens of the eye. The loss of function associated with these modifications is likely connected to age-related diseases such as cataract and neurodegenerative disorders, while the methodologies we present represent a framework for structure-function studies on other isomerized proteins.

biochemistry↗