Lamin Networks

Lamin proteins are type-V IF proteins that, unlike other family members, assemble to branched filaments. Besides the cage-like framework at the inner nuclear membrane, lamins are also found throughout the nucleus: fusions of lamins A and B with GFP reveal a homogenous nucleoplasmic "veil" in addition to the intensely fluorescing nuclear lamina. Mobility measurements show that within the veil lam-ins are more mobile and somewhat less resistant to the conventional extraction steps. Jackson (2005) has described a branched intermediate filament network that ramifies throughout the nucleus of human cells. RNAi techniques were applied to displace each of the nuclear lamin proteins from the filaments with the consequence that the structural changes correlated with profound effects on both RNA and DNA synthesis. An almost complete cessation of transcription by RNA polymerase II and an approximately 70% decrease in the number of S-phase cells suggest that lamin networks contribute to the regulation of chromatin function.

Metazoan cells express A- and B-type lamins, which differ in their length and pI value. While B-type lamins are present in every cell, A-type lamins are only expressed following gastrulation. In humans there are three differentially regulated genes: Lamin A and C are splice variants of the LMNA gene found at 1q21, whereas lamins B1 and B2 are expressed from the LMNB1 and LMNB2 genes on 5q23 and 19q13, respectively. Lamins B (and later A) have been determined as major S/MAR-binding partners within the nuclear scaffold. The relevant binding properties could be reproduced with paracrystal-like lamin polymers revealing two activity-dependent modes that appear to be related to different features of S/MARs. One type involves the regions with single-strand potential and the other the minor groove of the DNA double strand. Both modes of association are interdependent as S/MAR binding is almost completely inhibited by the presence of single-stranded competitors.

3.1.1 Laminopathies

The recent discoveries that mutated lamins and lamin-binding nuclear membrane proteins can be linked to numerous rare human diseases (laminopathies) have changed the cell biologist's view of lamins as mere structural nuclear scaffold proteins (review: Zastrow et al. 2004). So far, however, we can only speculate why mutations in lamin A/C or in the associated emerin or the lamin B receptor genes result in such a wide range of tissue-specific phenotypes and how different mutations in the same gene can give rise to such a diverse set of diseases: Emery-Dreifuss/limb girdle muscular dystrophies, dilated cardiomyopathy (DCM), familial partial lipodystrophy (FPLD), autosomal recessive axonal neuropathy (Charcot-S/MARie-Tooth disorder, CMT2), mandibuloacral dysplasia (MAD), Hutchison Gilford Progeria syndrome (HGS), Greenberg skeletal dysplasia and Pelger-Huet anomaly (PHA). All the mentioned matrix constituents are known to interact with DNA and/or chromosomal proteins, including the core histones, and they provide a complex dynamic link between the peripheral lamina and nucle-oskeletal structures. It is anticipated that understanding the cellular dysfunctions that lead to laminopathies will further enhance our insight into the specific roles of the lamina in nuclear organization (review: Burke and Stewart 2002, 2006).

A particular group of LMNA mutations leads to a progeroid disease called "atypical Werner's syndrome" (WS). Fibroblasts from affected patients show a substantially enhanced proportion of nuclei with altered morphology and a disordered lamin structure. So far, there is no molecular explanation for a progeroid disease associated with lamin functions. However, a clue may arise from relating this atypical form to the prototype WS, an inherited disease characterized by sensitivity to DNA-damaging agents, by genomic instability and premature aging: Prototype WS is caused by a missense mutation in the gene of a RecQ family helicase/exonuclease (WRN) for which one of the postulated functions is the participation in a replication complex (Chen et al. 2003). For WS cells the poly(ADP-ribosyl)ation of cellular proteins is severely impaired, suggesting a relation between WRN and PARP-1. Immunoprecipitation studies and protein interaction assays in fact indicate direct association of PARP-1 with WRN and the assembly of a complex together with Ku70/80. In the presence of DNA and NAD+, PARP-1 modifies Ku70/80, but not WRN, and it undergoes the typical automodification reaction. These events reduce the DNA binding capacity of Ku70/80 and its potential to stimulate WRN activity, demonstrating that PARP-1 is definitely involved in its regulation (Li et al. 2004). A report by Vidakovic et al. (2004) proving that the association with lamins modulates the activity of PARP-1 may provide the missing link between the two forms of the syndrome (atypical and prototype; cf. Fig. 2).

Poly(ADP-ribosyl)ation has frequently been linked to longevity, as differences in the catalytic activity of PARP-1 closely correlate with differences in life span. These findings together suggest a functional link between WRN, PARP-1 and Ku70/80, which can consequently be considered as caretakers of genome integrity.

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