Sarah Waters

Interactions with LC3 and polyubiquitin chains link nbr1 to autophagic protein turnover

MINT‐7034220: NBR1 (uniprotkb:Q14596) physically interacts (MI:0218) with ube2o (uniprotkb:Q6ZPJ3) by two hybrid (MI:0018)

Neighbor of Brca1 gene (Nbr1) functions as a negative regulator of postnatal osteoblastic bone formation and p38 MAPK activity

The neighbor of Brca1 gene (Nbr1) functions as an autophagy receptor involved in targeting ubiquitinated proteins for degradation. It also has a dual role as a scaffold protein to regulate growth-factor receptor and downstream signaling pathways. We show that genetic truncation of murine Nbr1 leads to an age-dependent increase in bone mass and bone mineral density through increased osteoblast differentiation and activity. At 6 mo of age, despite normal body size, homozygous mutant animals (Nbr1tr/tr) have ~50% more bone than littermate controls. Truncated Nbr1 (trNbr1) co-localizes with p62, a structurally similar interacting scaffold protein, and the autophagosome marker LC3 in osteoblasts, but unlike the full-length protein, trNbr1 fails to complex with activated p38 MAPK. Nbr1tr/tr osteoblasts and osteoclasts show increased activation of p38 MAPK, and significantly, pharmacological inhibition of the p38 MAPK pathway in vitro abrogates the increased osteoblast differentiation of Nbr1tr/tr cells. Nbr1 truncation also leads to increased p62 protein expression. We show a role for Nbr1 in bone remodeling, where loss of function leads to perturbation of p62 levels and hyperactivation of p38 MAPK that favors osteoblastogenesis.

MAP1B Interaction with the FW Domain of the Autophagic Receptor Nbr1 Facilitates Its Association to the Microtubule Network.

Selective autophagy is a process whereby specific targeted cargo proteins, aggregates, or organelles are sequestered into double-membrane-bound phagophores before fusion with the lysosome for protein degradation. It has been demonstrated that the microtubule network is important for the formation and movement of autophagosomes. Nbr1 is a selective cargo receptor that through its interaction with LC3 recruits ubiquitinated proteins for autophagic degradation. This study demonstrates an interaction between the evolutionarily conserved FW domain of Nbr1 with the microtubule-associated protein MAP1B. Upon autophagy induction, MAP1B localisation is focused into discrete vesicles with Nbr1. This colocalisation is dependent upon an intact microtubule network as depolymerisation by nocodazole treatment abolishes starvation-induced MAP1B recruitment to these vesicles. MAP1B is not recruited to autophagosomes for protein degradation as blockage of lysosomal acidification does not result in significant increased MAP1B protein levels. However, the protein levels of phosphorylated MAP1B are significantly increased upon blockage of autophagic degradation. This is the first evidence that links the ubiquitin receptor Nbr1, which shuttles ubiquitinated proteins to be degraded by autophagy, to the microtubule network.

Autophagic receptors Nbr1 and p62 coregulate skeletal remodeling.

Skeletal remodelling is an ongoing process requiring the coordinated action of different cell types to maintain homeostatic control of bone synthesis and degradation. Mutations in p62/SQSTM1 are associated with sporadic and 5q35-linked Paget’s Disease of Bone (PDB), characterized by focal increased bone turnover. These mutations cluster in the ubiquitin associated (UBA) domain and are thought to lead to enhancement of NFκB pathway activation involved in osteoclastogenesis and hyper-responsiveness to receptor activator of nuclear factor-κB ligand (RANKL). The structurally similar selective autophagic receptor, Nbr1, binds to LC3 and p62, and is sequestered into autophagosomes, whereas it accumulates in autophagic-deficient tissues. We have shown that truncation of Nbr1 in a murine model, where it can still interact with p62 but not LC3, leads to increased osteoblast differentiation and activity in vivo. This results in an age-dependent increase in bone mass and bone mineral density. This is a molecular consequence of loss of autophagy receptor function via deletion of its C-terminal UBA domain, and/or modulation of the p38 MAPK cellular signalling pathway.

The cardiac physiome: foundations and future prospects for mathematical modelling of the heart.

Advances in techniques for measuring the structure and function of cells, tissues, and organs continue to provide remarkable data on physiological function. Combined with the evolution of quantitative mathematical and computational approaches the integration of this information is now entering an exciting new phase through the development of whole organ computational models. These measurement and mathematical drivers are both enabling and motivating the progression of this work. Specifically the rich sources of data are key to parameterising detailed models that, in turn, are often required for analysis and interpretation of physiological information. Technological developments have further accelerated this interaction by facilitating the addition of multi-physics functionality, as part of initiatives such as the International Union of Physiological Sciences (IUPS) sponsored Physiome project and the European Commission funded Virtual Physiological Human (VPH) project. Across the scope of this work the heart is arguably the most advanced current exemplar of an organ for which an integrated physiological model exists. Fifty years after the publication of the first cardiac cell model, we are now at a point that this approach is increasingly being exploited to provide novel scientific insights and clinical applications. This issue of Progress of Biophysics and Molecular Biology is focused on substantive, if inevitably incomplete, reviews of the state of the art of multi-physics computational modelling of the heart. These articles result from a programme on ‘The Cardiac Physiome: Multi-scale and Multi-physics Mathematical Modelling Applied to the Heart’, which took place at the Newton Institute for Mathematical Sciences in Cambridge during July 2009. Key to the theme of this programme, this issue, and the Physiome concept itself, is the integration of multiple data collected across scales and modalities within biophysically detailed mathematical models that represent different domains of cardiac physiology, that is: cardiac cell behaviour, electrical activation of cardiac tissue, coronary fluid flow, and cardiac tissue mechanics. However, in addition to considering the development and application of models in each domain, these articles and the wider literature they refer provide a framework to consider the general challenges and opportunities ahead for the Physiome approach. Starting at the protein and whole cardiac myocyte scale, Fink et al. reviewed the development of integrated cellular models. Presentation of both the historical context and current on-going work highlights the importance of interaction between model parameters and experimental data sets. Enabled by computational developments these cellular models are also key components