Co-workers:

We have determined the protein complement of nuclear envelopes (NEs) isolated from liver, muscle and two activation states of leukocytes, finding that ~60% of NE transmembrane proteins (NETs) differed among these tissues. This variation together with functions we found for several novel NETs argue that the NE is as complex a signaling node as the plasma membrane. This could underlie in part the many diseases now linked to the NE that range from muscular dystrophies to lipodystrophies, neuropathy, osteopoikilosis, and premature aging syndromes. The high degree of tissue-specificity in the NE proteome might also explain how some NE-linked diseases preferentially yield pathology in just a small subset of tissues despite that the mutated proteins are widely expressed: complexes containing the tissuespecific NETs might be disrupted only in certain tissues. We have cloned roughly 80 uncharacterized tissuespecific NETs and used them in screens to identify those that influence cell cycle progression, differentiation and genome organization.

Of particular note, several tissue-specific NETs affect the positioning of certain chromosomes with respect to the nuclear periphery. Their expression in cell types where they are not normally expressed resulted in these chromosomes repositioning from the nuclear interior to the nuclear periphery while their knockdown in cells where they are normally expressed resulted in the relocation of certain chromosomes away from the nuclear periphery. For example, chromosome 5 is normally in the nuclear interior in fibroblast cells and at the nuclear periphery in liver. Exogenous expression of liver-specific NET47 in a general fibroblast line recruited chromosome 5 to the periphery while its knockdown in  liver cells caused chromosome 5 to move away from the periphery. The change in chromosome position was accompanied by changes in gene expression. NET47 expression in the fibroblasts induced several genes that are normally upregulated in liver differentiation. Other NETs that were preferentially expressed in muscle cells or adipocytes had the same types of effects and could thus contribute to muscular dystrophies and lipodystrophies linked to the NE and potentially also to diabetes and obesity.

The screens also identified several proteins that affect cytoskeletal organization. This is another mechanism by which NETs could contribute to muscular dystrophy disease pathologies and we are currently trying to determine their interaction partners and performing exome sequencing in unlinked muscular dystrophy patients to determine if these or the chromosomerepositioning NETs contribute additional disease variants.

Selected publications:

Korfali, N., Srsen, V., Waterfall, M., Batrakou, D. G., Pekovic, V., Hutchison, C. J., and Schirmer, E. C. (2011) A flow cytometrybased screen of nuclear envelope transmembrane proteins identifies NET4/Tmem53 as involved in stress-dependent cell cycle withdrawal. PLoS ONE 6, e18762.

Zuleger, N., Kelly, D. A., Richardson, A. C., Kerr, A. R. W., Goldberg, M. W., Goryachev, A. B., and Schirmer, E. C. (2011) System analysis shows distinct mechanisms and common principles of nuclear envelope protein dynamics. J. Cell Biol. 193, 109-123.

Wilkie, G. S., Korfali, N., Swanson, S. K., Malik, P., Srsen, V., Batrakou, D. G., de las Heras, J., Zuleger, N., Kerr, A. R. W., Florens, L., and Schirmer, E. C. (2011) Several novel nuclear envelope proteins from muscle have cytoskeletal associations. Mol. Cell. Proteomics 10, M110.003129.