Amyloid-like Assembly Activates a Phosphatase in the Developing Drosophila Embryo

Functional Amyloids: Where Supramolecular Amyloid Assembly Controls Biological Activity or Generates New Functionality

Functional amyloids are a rapidly expanding class of fibrillar protein structures, with a core cross-β scaffold, where novel and advantageous biological function is generated by the assembly of the amyloid. The growing number of amyloid structures determined at high resolution reveal how this supramolecular template both accommodates a wide variety of amino acid sequences and also imposes selectivity on the assembly process. The amyloid fibril can no longer be considered a generic aggregate, even when associated with disease and loss of function. In functional amyloids the polymeric β-sheet rich structure provides multiple different examples of unique control mechanisms and structures that are finely tuned to deliver assembly or disassembly in response to physiological or environmental cues. Here we review the range of mechanisms at play in natural, functional amyloids, where tight control of amyloidogenicity is achieved by environmental triggers of conformational change, proteolytic generation of amyloidogenic fragments, or heteromeric seeding and amyloid fibril stability. In the amyloid fibril form, activity can be regulated by pH, ligand binding and higher order protofilament or fibril architectures that impact the arrangement of associated domains and amyloid stability. The growing understanding of the molecular basis for the control of structure and functionality delivered by natural amyloids in nearly all life forms should inform the development of therapies for amyloid-associated diseases and guide the design of innovative biomaterials.

Herzog is not required for mushroom body development or courtship learning & memory but is required for eye development in Drosophila melanogaster

Herzog (Hzg, CG5830) shares similarity to members of the haloacid dehalogenase subfamily of small CTD phosphatases. In Drosophila it is a maternal gene essential for establishment of embryonic segment polarity, and oligomerization is required for activation of phosphatase activity. While Hzg is expressed in the brain, its role has not been investigated. To that end, we further characterized Hzg expression in the brain and found that it is highly expressed in neurons of the mushroom body where it localises to axons, and is also expressed in cortical glia. We investigated its role in mushroom body development as well as courtship learning and memory, but found that knockdown of Hzg had no impact on these processes. In contrast, knockdown in post-mitotic neurons in the eye resulted in disruption to ommatidial patterning and pigmentation, indicating it plays an important role in eye development.

Essence determines phenomenon: Assaying the material properties of biological condensates

Intracellular spaces are partitioned into separate compartments to ensure that numerous biochemical reactions and cellular functions take place in a spatiotemporally controlled manner. Biomacromolecules including proteins and RNAs undergo liquid–liquid phase separation and subsequent phase transition to form biological condensates with diverse material states. The material/physical properties of biological condensates are crucial for fulfilling their distinct physiological functions, and abnormal material properties can cause deleterious effects under pathological conditions. Here, we review recent studies showing the role of the material properties of biological condensates in their physiological functions. We also summarize several classic methods as well as newly emerging techniques for characterization and/or measurement of the material properties of biological condensates.

Neurodevelopmental Clues to Neurodegeneration

Neurodegenerative disorders are characterized by neuronal loss, usually in late life. But recently, abnormalities of proteins implicated in neurodegenerative disorders have been identified in disorders of childhood, raising the possibility that clues to susceptibility to and prevention of neurodegenerative disorders may be identifiable before symptoms of disease arise. This review leverages these new and evolving findings to test our hypothesis, first proposed in 2010, that proteins implicated in neurodegenerative disorders play important roles in brain development by examining evidence in the peer-reviewed literature published in the past five years for the relevance of these proteins in normal and disease-associated brain development.

DUSP5 promotes osteogenic differentiation through SCP1/2-dependent phosphorylation of SMAD1

Dual‐specificity phosphatases (DUSPs) are defined by their capability to dephosphorylate both phosphoserine/phosphothreonine (pSer/pThr) and phosphotyrosine (pTyr). DUSP5, a member of DUSPs superfamily, is located in the nucleus and plays crucially regulatory roles in the signaling pathway transduction. In our present study, we discover that DUSP5 significantly promotes osteogenic differentiation of mesenchymal stromal cells (MSCs) by activating SMAD1 signaling pathway. Mechanistically, DUSP5 physically interacts with the phosphatase domain of small C‐terminal phosphatase 1/2 (SCP1/2, SMAD1 phosphatases) by the linker region. In addition, we further confirm that DUSP5 activates SMAD1 signaling through a SCP1/2‐dependent manner. Specifically, DUSP5 attenuates the SCP1/2‐SMAD1 interaction by competitively binding to SCP1/2, which is responsible for the SMAD1 dephosphorylation, and thus results in the activation of SMAD1 signaling. Importantly, DUSP5 expression in mouse bone marrow MSCs is significantly reduced in ovariectomized (OVX) mice in which osteogenesis is highly passive, and overexpression of Dusp5 via tail vein injection reverses the bone loss of OVX mice efficiently. Collectively, this work demonstrates that the linker region of DUSP5 maybe a novel chemically modifiable target for controlling MSCs fate choices and for osteoporosis treatment.

Protein self-assembly: A new frontier in cell signaling

Long viewed as paradigm-shifting, but rare, prions have recently been discovered in all domains of life. Protein sequences that can drive this form of self-assembly are strikingly common in eukaryotic proteomes, where they are enriched in proteins involved in information flow and signal transduction. Although prions were thought to be a consequence of random errors in protein folding, recent studies suggest that prion formation can be a controlled process initiated by defined cellular signals. Many are present in normal biological contexts, yet are invisible to most technologies used to interrogate the proteome. Here, we review mechanisms by which protein self-assembly can create a stable record of past stimuli, altering adaptive responses, and how prion behavior is controlled by signaling processes. We touch on the diverse implications that this has for normal biological function and regulation, ranging from drug resistance in fungi to the innate immune response in humans. Finally, we discuss the potential for prion domains in transcription factors and RNA-binding proteins to orchestrate heritable gene expression changes in response to transient signals, such as during development.

FMRP-PKA Activity Negative Feedback Regulates RNA Binding-Dependent Fibrillation in Brain Learning and Memory Circuitry

Fragile X mental retardation protein (FMRP) promotes cyclic AMP (cAMP) signaling. Using an in vivo protein kinase A activity sensor (PKA-SPARK), we find that Drosophila FMRP (dFMRP) and human FMRP (hFMRP) enhance PKA activity in a central brain learning and memory center. Increasing neuronal PKA activity suppresses FMRP in Kenyon cells, demonstrating an FMRP-PKA negative feedback loop. A patient-derived R140Q FMRP point mutation mislocalizes PKA-SPARK activity, whereas deletion of the RNA-binding argi-nine-glycine-glycine (RGG) box (hFMRP-ΔRGG) produces fibrillar PKA-SPARK assemblies colocalizing with ribonucleoprotein (RNP) and aggregation (thioflavin T) markers, demonstrating fibrillar partitioning of cytosolic protein aggregates. hFMRP-ΔRGG reduces dFMRP levels, indicating RGG-independent regulation. Short-term hFMRP-ΔRGG induction produces activated PKA-SPARK puncta, whereas long induction drives fibrillar assembly. Elevated temperature disassociates hFMRP-ΔRGG aggregates and blocks activated PKA-SPARK localization. These results suggest that FMRP regulates compartmentalized signaling via complex assembly, directing PKA activity localization, with FMRP RGG box RNA binding restricting separation via low-complexity interactions.

FMRP is required for brain cAMP induction and cAMP-dependent PKA activation, but the FMRP mechanism is uncharacterized. Sears and Broadie test FXS patient-derived and FMRP domain-deficient mutants to reveal conserved FMRP functions regulating PKA activation, subcellular localization, and reversible partitioning into elongated fibrillar assemblies in brain learning/ memory circuit neurons.

Mechanisms Linking Mitochondrial Dysfunction and Proteostasis Failure

Maintaining cellular protein homeostasis (proteostasis) is an essential task for all eukaryotes. Proteostasis failure worsens with aging and is considered a cause of and a therapeutic target for age-related diseases including neurodegenerative disorders. The cellular networks regulating proteostasis and the pathogenic events driving proteostasis failure in disease remain poorly understood. Model organism studies in yeast and Drosophila reveal an intriguing link between mitochondrial function and proteostasis. In this review we examine recent findings on mitochondrial outer membrane (MOM)-associated mRNA translation, how this process is sensitive to mitochondrial dysfunction and constantly surveyed by ribosome-associated quality control (RQC), and how defects in this process generate aberrant proteins with unusual C-terminal extensions (CTEs) that promote aggregation and drive proteostasis failure. We also discuss the implications for human diseases.