C99 is the transmembrane carboxyl-terminal domain of the amyloid precursor protein that is cleaved by γ-secretase to release the amyloid-β polypeptides, which are associated with Alzheimer’s disease. Nuclear magnetic resonance and electron paramagnetic resonance spectroscopy show that the extracellular amino terminus of C99 includes a surface-embedded “N-helix” followed by a short “N-loop” connecting to the transmembrane domain (TMD). The TMD is a flexibly curved α helix, making it well suited for processive cleavage by γ-secretase. Titration of C99 reveals a binding site for cholesterol, providing mechanistic insight into how cholesterol promotes amyloidogenesis. Membrane-buried GXXXG motifs (G, Gly; X, any amino acid), which have an established role in oligomerization, were also shown to play a key role in cholesterol binding. The structure and cholesterol binding properties of C99 may aid in the design of Alzheimer’s therapeutics.

The 99 residue transmembrane C-terminal domain (C99, also known as β-CTF) of the amyloid precursor protein (APP) is the product of β-secretase cleavage of full length APP and the substrate for γ-secretase cleavage. The latter cleavage releases the amyloid-β polypeptides that are closely associated with Alzheimer’s disease. C99 is thought to form homodimers; however, the free energy in favor of dimerization has not previously been quantitated. It was also recently documented that cholesterol forms a 1:1 complex with monomeric C99 in bicelles. Here, the affinities for both homodimerization and cholesterol binding to C99 were measured in bilayered lipid vesicles using both electron paramagnetic resonance (EPR) and Förster resonance energy transfer (FRET) methods. Homodimerization and cholesterol binding were seen to be competitive processes, which center on the transmembrane G700XXXG704XXXG709 glycine zipper motif and the adjacent Gly709. The observed Kd for cholesterol binding (Kd = 2.7 ± 0.3 mol%) is on the low end of the physiological cholesterol concentration range in mammalian cell membranes. On the other hand, the observed Kd for homodimerization (Kd = 0.47 ± 0.15 mol%) likely exceeds the physiological concentration range for C99. These results suggest that the 1:1 cholesterol:C99 complex will be more highly populated than C99 homodimers under most physiological conditions, observations that are of relevance to understanding γ-secretase cleavage of C99.

Charles Sanders is the co-recipient of the Protein Society 2013 Hans Neurath Award.Abstract: As of mid 2013 a Medline search on "cholesterol" yielded over 200,000 hits, reflecting the prominence of this lipid in numerous aspects of animal cell biology and physiology under conditions of health and disease. Aberrations in cholesterol homeostasis underlie both a number of rare genetic disorders and contribute to common sporadic and complex disorders including heart disease, stroke, type II diabetes, and Alzheimer's disease. The corresponding author of this review and his lab stumbled only recently into the sprawling area of cholesterol research when they discovered that the amyloid precursor protein (APP) binds cholesterol, a topic covered by the Hans Neurath Award lecture at the 2013 Protein Society Meeting. Here, we first provide a brief overview of cholesterol-protein interactions and then offer our perspective on how and why binding of cholesterol to APP and its C99 domain (b-CTF) promotes the amyloidogenic pathway, which is closely related to the etiology of Alzheimer's disease.

Real‐time monitoring of bioprocesses by the integration of analytics at critical unit operations is one of the paramount necessities for quality by design manufacturing and real‐time release (RTR) of biopharmaceuticals. A well‐defined process analytical technology (PAT) roadmap enables the monitoring of critical process parameters and quality attributes at appropriate unit operations to develop an analytical paradigm that is capable of providing real‐time data. We believe a comprehensive PAT roadmap should entail not only integration of analytical tools into the bioprocess but also should address automated‐data piping, analysis, aggregation, visualization, and smart utility of data for advanced‐data analytics such as machine and deep learning for holistic process understanding. In this review, we discuss a broad spectrum of PAT technologies spanning from vibrational spectroscopy, multivariate data analysis, multiattribute chromatography, mass spectrometry, sensors, and automated‐sampling technologies. We also provide insights, based on our experience in clinical and commercial manufacturing, into data automation, data visualization, and smart utility of data for advanced‐analytics in PAT. This review is catered for a broad audience, including those new to the field to those well versed in applying these technologies. The article is also intended to give some insight into the strategies we have undertaken to implement PAT tools in biologics process development with the vision of realizing RTR testing in biomanufacturing and to meet regulatory expectations.

C99 (also known as β-CTF) is the 99 residue transmembrane C-terminal domain (residues 672–770) of the amyloid precursor protein and is the immediate precursor of the amyloid-β (Aβ) polypeptides. To test the dependence of the C99 structure on the composition of the host model membranes, NMR studies of C99 were conducted both in anionic lyso-myristoylphosphatidylglycerol (LMPG) micelles and in a series of five zwitterionic bicelle compositions involving phosphatidylcholine and sphingomyelin in which the acyl chain lengths of these lipid components varied from 14 to 24 carbons. Some of these mixtures are reported for the first time in this work and should be of broad utility in membrane protein research. The site-specific backbone 15N and 1H chemical shifts for C99 in LMPG and in all five bicelle mixtures were seen to be remarkably similar, indicating little dependence of the backbone structure of C99 on the composition of the host model membrane. However, the length of the transmembrane span was seen to vary in a manner that alters the positioning of the γ-secretase cleavage sites with respect to the center of the bilayer. This observation may contribute to the known dependency of the Aβ42-to-Aβ40 production ratio on both membrane thickness and the length of the C99 transmembrane domain.