Calmodulin binds the N-terminus of the functional amyloid Orb2A inhibiting fibril formation

The cytoplasmic polyadenylation element-binding protein Orb2 is a key regulator of long-term memory (LTM) in Drosophila. The N-terminus of the Orb2 isoform A is required for LTM and forms cross-β fibrils on its own. However, this N-terminus is not part of the core found in ex vivo fibrils. We previously showed that besides forming cross-β fibrils, the N-terminus of Orb2A binds anionic lipid membranes as an amphipathic helix. Here, we show that the Orb2A N-terminus can similarly interact with calcium activated calmodulin (CaM) and that this interaction prevents fibril formation. Because CaM is a known regulator of LTM, this interaction could potentially explain the regulatory role of Orb2A in LTM.

Droplet and fibril formation of the functional amyloid Orb2

The functional amyloid Orb2 belongs to the cytoplasmic polyadenylation element binding (CPEB) protein family and plays an important role in long-term memory formation in Drosophila. The Orb2 domain structure combines RNA recognition motifs with low-complexity sequences similar to many RNA-binding proteins shown to form protein droplets via liquid–liquid phase separation (LLPS) in vivo and in vitro. This similarity suggests that Orb2 might also undergo LLPS. However, cellular Orb2 puncta have very little internal protein mobility, and Orb2 forms fibrils in Drosophila brains that are functionally active indicating that LLPS might not play a role for Orb2. In the present work, we reconcile these two views on Orb2 droplet formation. Using fluorescence microscopy, we show that soluble Orb2 can indeed phase separate into protein droplets. However, fluorescence recovery after photobleaching (FRAP) data shows that these droplets have either no or only an extremely short-lived liquid phase and appear maturated right after formation. Orb2 fragments that lack the C-terminal RNA-binding domain (RBD) form fibrils out of these droplets. Solid-state NMR shows that these fibrils have well-ordered static domains in addition to the Gln/His-rich fibril core. Further, we find that full-length Orb2B, which is by far the major component of Orb2 fibrils in vivo, does not transition into fibrils but remains in the droplet phase. Together, our data suggest that phase separation might play a role in initiating the formation of functional Orb2 fibrils.

Participant-Centered Strategies for Overcoming Barriers to Biospecimen Collection among Spanish-Speaking Latina Breast Cancer Survivors

BACKGROUND

Latinos are underrepresented in biomedical research, particularly biomarker research, yet they constitute the nation's largest ethnic/racial minority. Optimal methods for obtaining biospecimens for biomarker research among Latinos need to be identified. To minimize barriers and enhance participation, this study developed and tested tailored strategies for collecting biomarkers of chronic stress and premature aging among Spanish-speaking Latina breast cancer survivors.

METHODS

This study used a community-based participatory approach and selected hair and saliva as noninvasive biospecimens to assess telomere length, the cortisol awakening response (CAR), and hair cortisol concentration. We developed bilingual multimedia instructional materials, and community health workers assisted in collections. Telephone surveys assessed willingness to participate in future studies, barriers to sample collection, and recommendations for improving the strategies.

RESULTS

A total of 103 participants were recruited over 18 months from two rural sites in California, and 88 were retained at 6-month follow-up. At baseline, rates of donating salivary DNA for telomere length measurement, saliva for CAR analysis, and hair for cortisol concentration were 98%, 89%, and 52%, respectively. At follow-up, rates were 83%, 76%, and 55%, respectively. The majority of participants reported being very willing to provide hair (72%) or saliva (74%) for future studies.

CONCLUSIONS

Our results support the feasibility of including minorities in biomedical research. We report excellent rates of saliva collection when community partners are engaged in the process, and when patient-centered and culturally tailored recruitment methods are implemented.

IMPACT

The development of methods to facilitate the inclusion of minorities in biomedical research is critical to eliminate racial/ethnic health disparities.

Dynamics of the Proline-Rich C-Terminus of Huntingtin Exon-1 Fibrils

Intrinsically disordered protein domains not only are found in soluble proteins but also can be part of large protein complexes or protein aggregates. For example, several amyloid fibrils have intrinsically disordered domains framing a rigid β-sheet-rich core. These disordered domains can often be observed using solution NMR methods in combination with modest magic angle spinning and without perdeuteration. But how can these regions be detected using solution NMR methods when they are part of a fibril that is not tumbling isotropically in solution? Here we addressed this question by investigating the dynamic C-terminus of huntingtin exon-1 (HTT_ex1) fibrils that are important in Huntington's disease. We assigned the most dynamic regions of the C-terminus of three HTT_ex1 variants. On the basis of this assignment, we measured site-specific secondary chemical shifts, peak intensities, and R₁, R'₂, and R_1ρ ¹⁵N relaxation rates. In addition, we determined the residual ¹H-¹⁵N dipolar couplings of this region. Our results show that the dipolar couplings are averaged to a very high degree, resulting in an order parameter that is essentially zero. Together, our data show that the C-terminus of HTT_ex1 is intrinsically disordered and undergoes motions in the high picosecond to low nanosecond range.

The Functional Amyloid Orb2A Binds to Lipid Membranes

Lipid membranes interact with and influence the aggregation of many amyloid-forming proteins. Orb2 is a cytoplasmic polyadenylation element-binding protein homolog in Drosophila melanogaster that forms functional amyloids necessary for long-term memory. One isoform, Orb2A, has a unique N-terminus that has been shown to be important for the formation of amyloid-like aggregates and long-term memory in vivo. Orb2A is also found enriched in the synaptic membrane fraction. Our sequence and hydropathy analysis suggests that it can form an amphipathic helix, which is ideal for lipid membrane interaction. We used circular dichroism and site-directed spin labeling coupled with electron paramagnetic resonance to test the first 88 amino acids of Orb2A for lipid interaction. We show that Orb2A1-88 interacts with anionic lipid membranes using an amphipathic helix at its unique N-terminus. This interaction depends on the charge of the lipid membrane and the degree of membrane curvature. We used transmission electron microscopy and electron paramagnetic resonance to show that the presence of anionic small unilamellar vesicles inhibits amyloid fibril formation by Orb2A. This inhibition by anionic membranes could be a potential mechanism regulating Orb2A amyloid formation in vivo.