Lamins are nuclear intermediate filament proteins present in all metazoan cells. They are the main constituents of the nuclear lamina, a scaffold structure underlying the inner nuclear membrane that provides mechanical stability for the nucleus. The view of a mostly mechanical role of lamins has been challenged in the past years by the discovery that mutations in lamina proteins are linked to several human diseases ranging from muscular dystrophies to accelerated aging syndromes. There is increasing evidence that lamins and associated proteins are also involved in the control of gene expression and cell signaling.

We are studying the dynamics and functions of lamins in chromatin organization and in the regulation of gene expression, cell signaling and genome stability in tissue progenitor cells. Furthermore we aim at elucidating the molecular disease mechanisms of lamin-linked diseases in order to explore new avenues for therapeutic interventions.

Current Projects:

1. Regulation and functions of nucleoplasmic lamins in health and disease

(Project leader: Thomas Dechat, supported by FWF grant P22043-B12)

As intermediate filament-type proteins lamins have traditionally been viewed as immobile structural components of eukaryotic nuclei that provide mechanical stability and organize chromatin at the nuclear envelope. We and others have recently described a tightly regulated and highly mobile pool of lamins in the nuclear interior, which may have major roles in the regulation of gene expression and may be responsible for several pathologies in lamin-linked diseases. Currently, we analyze the regulation and activities of this poorly characterized lamin pool.

To address these questions we have been studying lamin-binding proteins and their role in lamin assembly and lamin functions. Among the best-characterized lamin-binding proteins are members of the LEM (LAP-Emerin-MAN1) domain protein family, which associate with chromatin via their LEM domain. We found that a unique member of the LEM protein family, termed Lamina-Associated Polypeptide 2α (LAP2α) binds specifically to the nucleoplasmic pool of lamins and regulates its nuclear targeting in a cell cycle and differentiation-dependent manner. Nucleoplasmic lamin-LAP2α complexes bind the cell cycle regulator protein retinoblastoma (pRb) and regulate the pRb/E2F signaling pathway during cell proliferation and differentiation of tissue progenitor cells. LAP2α knockout mice lack lamins in the nuclear interior and show impaired pRb activity, leading to hyperproliferation of early progenitor cells and hyperplasia in regenerating tissues, such as epidermis (Figure 1), intestine, skeletal muscle and the hematopoietic system. We propose that lamin-LAP2α complexes regulate the activity of adult stem cells and early progenitors in tissue homeostasis and regeneration.

In order to find out whether the functions of nucleoplasmic lamin-LAP2α complexes are impaired in lamin-linked diseases and contribute to the tissue pathologies, we investigate the dynamics and regulation of lamins and LAP2α in cells from Hutchinson-Gilford Progeria Syndrome (HGPS) patients and in HGPS model cells and mice. HGPS patients bear a mutation in the lamin A-encoding gene. Patients are born normally but age at a rate ten times faster than their healthy siblings showing many features similar to the normal ageing process. They die early in their teens due to cardiovascular problems. The molecular mechanisms are poorly understood and efficient therapies are missing. Currently, we test the hypothesis that impairment of the lamin-LAP2α complex in HGPS affects cell proliferation and differentiation.

2. Ankle 1, a nuclease involved in genomic reorganization

(Project leader: Andreas Brachner, supported by FWF grant P22569-B09)

We recently identified Ankyrin and LEM domain-containing protein 1 (Ankle1) as a novel LEM domain protein that is conserved from C. elegans to man. Intriguingly, Ankle1 contains a unique C-terminal GIY-YIG motif, which is a hallmark of a subgroup of unrelated homing endonucleases, and possesses nuclease activity in vitro and in vivo. Ankle1 shuttles between cytoplasm and nucleus, and Ankle1 enriched in the nucleus induces DNA double strand breaks and DNA damage signaling (Figure 2). Based on the predominant expression of mammalian Ankle1 in hematopoietic cells, we propose that Ankle1 is a new component involved in DNA repair and/or DNA rearrangements during lymphocyte development. Currently, we test the regulation of Ankle1 localization and activity in various cell lines and analyze its putative in vivo functions using Ankle1-deficient mice.

3. Dynamics of inner nuclear membrane proteins

(Project leader: Mirta Boban, supported by FWF grant P23805-B20)

The nuclear envelope of eukaryotic cells consists of two concentric membrane layers, the inner (INM) and the outer (ONM) nuclear membrane, and of nuclear pore complexes (NPCs), which form transport channels through the double membrane system. Although ONM and INM are connected at the sites of NPCs by the pore membrane, they contain different sets of integral membrane proteins. To reach the INM, these membrane proteins that are synthesized at the ER diffuse laterally within the ER-ONM to the cytoplasmic side of NPCs, where they have to pass the physical barrier of the NPC at the pore membrane. However, their destiny after reaching the INM is one of the intriguing, but barely addressed aspects of INM protein dynamics. Protein quality control, involving protein turnover and degradation, is an important process protecting cells against aggregation of misfolded or damaged proteins. While protein quality control degradation machineries have mostly been characterized in the cytoplasm, the existence and function of such machineries in the nucleus are not well characterized. To unravel the mechanisms of INM protein turnover we test i) whether INM proteins have to be transported back to the cytoplasm to be degraded by cytoplasmic machineries, ii) which molecular degradation pathways are involved, and iii) what is the functional significance and regulation of INM protein turnover. Currently we study the dynamics of several integral INM proteins in yeast Saccharomyces cerevisiae and in mammalian cells. Our project explores an important biological process and may yield new insights into the formation of aberrant protein aggregates in the nucleus leading to disease.