Glymphatic system clears extracellular tau and protects from tau aggregation and neurodegeneration.

Accumulation of tau has been implicated in various neurodegenerative diseases termed tauopathies. Tau is a microtubule-associated protein but is also actively released into the extracellular fluids including brain interstitial fluid and cerebrospinal fluid (CSF). However, it remains elusive whether clearance of extracellular tau impacts tau-associated neurodegeneration. Here, we show that aquaporin-4 (AQP4), a major driver of the glymphatic clearance system, facilitates the elimination of extracellular tau from the brain to CSF and subsequently to deep cervical lymph nodes. Strikingly, deletion of AQP4 not only elevated tau in CSF but also markedly exacerbated phosphorylated tau deposition and the associated neurodegeneration in the brains of transgenic mice expressing P301S mutant tau. The current study identified the clearance pathway of extracellular tau in the central nervous system, suggesting that glymphatic clearance of extracellular tau is a novel regulatory mechanism whose impairment contributes to tau aggregation and neurodegeneration.

In vivo kinetic approach reveals slow SOD1 turnover in the CNS.

Therapeutic strategies that target disease-associated transcripts are being developed for a variety of neurodegenerative syndromes. Protein levels change as a function of their half-life, a property that critically influences the timing and application of therapeutics. In addition, both protein kinetics and concentration may play important roles in neurodegeneration; therefore, it is essential to understand in vivo protein kinetics, including half-life. Here, we applied a stable isotope-labeling technique in combination with mass spectrometric detection and determined the in vivo kinetics of superoxide dismutase 1 (SOD1), mutation of which causes amyotrophic lateral sclerosis. Application of this method to human SOD1-expressing rats demonstrated that SOD1 is a long-lived protein, with a similar half-life in both the cerebral spinal fluid (CSF) and the CNS. Additionally, in these animals, the half-life of SOD1 was longest in the CNS when compared with other tissues. Evaluation of this method in human subjects demonstrated successful incorporation of the isotope label in the CSF and confirmed that SOD1 is a long-lived protein in the CSF of healthy individuals. Together, the results of this study provide important insight into SOD1 kinetics and support application of this technique to the design and implementation of clinical trials that target long-lived CNS proteins.

Mechanistic insights into the roles of three linked single-stranded template binding residues of MMLV reverse transcriptase in misincorporation and mispair extension fidelity of DNA synthesis.

To obtain some insights into the structure-function relationship of Moloney murine leukemia virus (MMLV) reverse transcriptase (RT), we modeled the catalytic state ternary complexes of this protein using the corresponding RT from human immunodeficiency virus type 1 (HIV-1) and available structures of MMLV RT. We observed that three MMLV RT single-stranded template binding residues, Y64, D114, and R116, act as a linked set through mutual interactions, including hydrogen bonding and ion-pairing. The analogous residues of HIV-1 RT have a somewhat different environment and they lack this linked phenomenon. To understand the functional implication of this linked set of MMLV RT, we performed site-directed mutagenesis at these three positions. Then the mutant enzymes were examined for their biochemical properties and nucleotide selectivity. Mutagenesis of these residues (Y64A, D114A, and R116A) resulted in enzymes with slight to modest changes in polymerase activities. The processivity of DNA synthesis correlated positively with the binding affinity of the MMLV RT variants. Lower fidelity in mutants was indicated by measurements of misincorporation and mispair extension fidelity of wild type (WT) and mutant RTs, in contrast to earlier works that indicate that mutations at the analogous positions in HIV-1 RT result in relatively higher fidelity. These data together with structural analysis suggest that this structural set may therefore be a key factor responsible for the different fidelity of these two RTs.