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Loss of A-type lamin expression compromises nuclear envelope integrity leading to muscular dystrophy.

Sullivan T, Escalante-Alcalde D, Bhatt H, Anver M, Bhat N, Nagashima K, Stewart CL, Burke B - J. Cell Biol. (1999)

Bottom Line: Whereas B-type lamins are found in all mammalian cells, A-type lamin expression is developmentally regulated.This phenotype is associated with ultrastructural perturbations to the nuclear envelope.In skeletal and cardiac muscles, this is manifest as a dystrophic condition related to EDMD.

View Article: PubMed Central - PubMed

Affiliation: Advanced BioScience Laboratories-Basic Research Program, National Cancer Institute-Frederick Cancer Research and Development Center, Maryland 21702, USA.

ABSTRACT
The nuclear lamina is a protein meshwork lining the nucleoplasmic face of the inner nuclear membrane and represents an important determinant of interphase nuclear architecture. Its major components are the A- and B-type lamins. Whereas B-type lamins are found in all mammalian cells, A-type lamin expression is developmentally regulated. In the mouse, A-type lamins do not appear until midway through embryonic development, suggesting that these proteins may be involved in the regulation of terminal differentiation. Here we show that mice lacking A-type lamins develop to term with no overt abnormalities. However, their postnatal growth is severely retarded and is characterized by the appearance of muscular dystrophy. This phenotype is associated with ultrastructural perturbations to the nuclear envelope. These include the mislocalization of emerin, an inner nuclear membrane protein, defects in which are implicated in Emery-Dreifuss muscular dystrophy (EDMD), one of the three major X-linked dystrophies. Mice lacking the A-type lamins exhibit tissue-specific alterations to their nuclear envelope integrity and emerin distribution. In skeletal and cardiac muscles, this is manifest as a dystrophic condition related to EDMD.

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(a) Western blot analysis of emerin (34 kD) and lamins A and C (74 and 65 kD, respectively) in +/+, +/−, and −/− Lmna MEFs showing no difference in emerin levels between different genotypes. (b) In wild-type cells, emerin is localized exclusively to the nuclear envelope. (c) In the heterozygotes, some emerin localization can be detected in the cytoplasm. (d) In the lamin A/C −/− cells, emerin is largely cytoplasmic although some localization to the nuclear envelope is still apparent. Expression of human lamin A in Lmna  MEFs results in relocalization of emerin to the nuclear envelope. (e) DAPI staining revealing the nuclei of two MEFs. (f) The upper MEF expresses human lamin A localized exclusively to the nuclear envelope. (g) Both cells express emerin but in the lamin A  cell, emerin is localized predominantly in the cytoplasmic compartment, whereas in the lamin A positive MEF, emerin is localized almost entirely to the nuclear envelope. (h) Merged image showing colocalization of emerin and human lamin A in the nuclear envelope. (i and j) Emerin distribution in P19EC cells (i) and (j) their differentiated derivatives P19MES that express A-type lamins. Both cell types exhibit NE-associated emerin.
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Figure 5: (a) Western blot analysis of emerin (34 kD) and lamins A and C (74 and 65 kD, respectively) in +/+, +/−, and −/− Lmna MEFs showing no difference in emerin levels between different genotypes. (b) In wild-type cells, emerin is localized exclusively to the nuclear envelope. (c) In the heterozygotes, some emerin localization can be detected in the cytoplasm. (d) In the lamin A/C −/− cells, emerin is largely cytoplasmic although some localization to the nuclear envelope is still apparent. Expression of human lamin A in Lmna MEFs results in relocalization of emerin to the nuclear envelope. (e) DAPI staining revealing the nuclei of two MEFs. (f) The upper MEF expresses human lamin A localized exclusively to the nuclear envelope. (g) Both cells express emerin but in the lamin A cell, emerin is localized predominantly in the cytoplasmic compartment, whereas in the lamin A positive MEF, emerin is localized almost entirely to the nuclear envelope. (h) Merged image showing colocalization of emerin and human lamin A in the nuclear envelope. (i and j) Emerin distribution in P19EC cells (i) and (j) their differentiated derivatives P19MES that express A-type lamins. Both cell types exhibit NE-associated emerin.

Mentions: EDMD was originally described as an X-linked disorder that mapped to the gene for emerin, a ubiquitous INM protein (Bione et al. 1994). While the function of emerin is unknown, the finding that A-type lamin defects result in an EDMD-like disorder suggests these proteins might functionally interact. Consistent with this, a recent study by Bonne et al. 1999 revealed that in humans an autosomal variant of EDMD maps to the lamin A/C (LMNA) gene. Since the localization of integral proteins to the INM is thought to involve a process of selective retention (Powell and Burke 1990; Ellenberg et al. 1997; Yang et al. 1997), it is possible that A-type lamins serve to immobilize emerin within the INM. Therefore, we examined the distribution of emerin in wild-type and lamin A/C −/− MEFs. While the overall levels of emerin in these cells is identical (Fig. 5 a), immunofluorescence microscopy revealed dramatic differences in emerin subcellular localization. In wild-type cells, emerin is concentrated within the nuclear envelope (Manilal et al. 1996; Nagano et al. 1996) (Fig. 5 b). In the −/− cells, there is partial loss of NE-associated emerin with a more general cytoplasmic distribution identical to that observed for resident ER proteins (Fig. 5 d). Presumably, emerin is no longer retained within the INM, and is free to access the outer nuclear membrane and ER via the membrane continuities at the periphery of NPCs (Ostlund et al. 1999). MEFs from heterozygous embryos exhibited intermediate levels of cytoplasmic emerin (Fig. 5 c). These results indicate that at least in MEFs, correct emerin localization is contingent upon A-type lamin expression. In contrast, LAP2, which shares some sequence homology with emerin but which is known to interact with both chromatin and B-type lamins (Furukawa et al. 1998), still concentrates at the nuclear periphery in −/− cells (Fig. 2 c).


Loss of A-type lamin expression compromises nuclear envelope integrity leading to muscular dystrophy.

Sullivan T, Escalante-Alcalde D, Bhatt H, Anver M, Bhat N, Nagashima K, Stewart CL, Burke B - J. Cell Biol. (1999)

(a) Western blot analysis of emerin (34 kD) and lamins A and C (74 and 65 kD, respectively) in +/+, +/−, and −/− Lmna MEFs showing no difference in emerin levels between different genotypes. (b) In wild-type cells, emerin is localized exclusively to the nuclear envelope. (c) In the heterozygotes, some emerin localization can be detected in the cytoplasm. (d) In the lamin A/C −/− cells, emerin is largely cytoplasmic although some localization to the nuclear envelope is still apparent. Expression of human lamin A in Lmna  MEFs results in relocalization of emerin to the nuclear envelope. (e) DAPI staining revealing the nuclei of two MEFs. (f) The upper MEF expresses human lamin A localized exclusively to the nuclear envelope. (g) Both cells express emerin but in the lamin A  cell, emerin is localized predominantly in the cytoplasmic compartment, whereas in the lamin A positive MEF, emerin is localized almost entirely to the nuclear envelope. (h) Merged image showing colocalization of emerin and human lamin A in the nuclear envelope. (i and j) Emerin distribution in P19EC cells (i) and (j) their differentiated derivatives P19MES that express A-type lamins. Both cell types exhibit NE-associated emerin.
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Related In: Results  -  Collection

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Figure 5: (a) Western blot analysis of emerin (34 kD) and lamins A and C (74 and 65 kD, respectively) in +/+, +/−, and −/− Lmna MEFs showing no difference in emerin levels between different genotypes. (b) In wild-type cells, emerin is localized exclusively to the nuclear envelope. (c) In the heterozygotes, some emerin localization can be detected in the cytoplasm. (d) In the lamin A/C −/− cells, emerin is largely cytoplasmic although some localization to the nuclear envelope is still apparent. Expression of human lamin A in Lmna MEFs results in relocalization of emerin to the nuclear envelope. (e) DAPI staining revealing the nuclei of two MEFs. (f) The upper MEF expresses human lamin A localized exclusively to the nuclear envelope. (g) Both cells express emerin but in the lamin A cell, emerin is localized predominantly in the cytoplasmic compartment, whereas in the lamin A positive MEF, emerin is localized almost entirely to the nuclear envelope. (h) Merged image showing colocalization of emerin and human lamin A in the nuclear envelope. (i and j) Emerin distribution in P19EC cells (i) and (j) their differentiated derivatives P19MES that express A-type lamins. Both cell types exhibit NE-associated emerin.
Mentions: EDMD was originally described as an X-linked disorder that mapped to the gene for emerin, a ubiquitous INM protein (Bione et al. 1994). While the function of emerin is unknown, the finding that A-type lamin defects result in an EDMD-like disorder suggests these proteins might functionally interact. Consistent with this, a recent study by Bonne et al. 1999 revealed that in humans an autosomal variant of EDMD maps to the lamin A/C (LMNA) gene. Since the localization of integral proteins to the INM is thought to involve a process of selective retention (Powell and Burke 1990; Ellenberg et al. 1997; Yang et al. 1997), it is possible that A-type lamins serve to immobilize emerin within the INM. Therefore, we examined the distribution of emerin in wild-type and lamin A/C −/− MEFs. While the overall levels of emerin in these cells is identical (Fig. 5 a), immunofluorescence microscopy revealed dramatic differences in emerin subcellular localization. In wild-type cells, emerin is concentrated within the nuclear envelope (Manilal et al. 1996; Nagano et al. 1996) (Fig. 5 b). In the −/− cells, there is partial loss of NE-associated emerin with a more general cytoplasmic distribution identical to that observed for resident ER proteins (Fig. 5 d). Presumably, emerin is no longer retained within the INM, and is free to access the outer nuclear membrane and ER via the membrane continuities at the periphery of NPCs (Ostlund et al. 1999). MEFs from heterozygous embryos exhibited intermediate levels of cytoplasmic emerin (Fig. 5 c). These results indicate that at least in MEFs, correct emerin localization is contingent upon A-type lamin expression. In contrast, LAP2, which shares some sequence homology with emerin but which is known to interact with both chromatin and B-type lamins (Furukawa et al. 1998), still concentrates at the nuclear periphery in −/− cells (Fig. 2 c).

Bottom Line: Whereas B-type lamins are found in all mammalian cells, A-type lamin expression is developmentally regulated.This phenotype is associated with ultrastructural perturbations to the nuclear envelope.In skeletal and cardiac muscles, this is manifest as a dystrophic condition related to EDMD.

View Article: PubMed Central - PubMed

Affiliation: Advanced BioScience Laboratories-Basic Research Program, National Cancer Institute-Frederick Cancer Research and Development Center, Maryland 21702, USA.

ABSTRACT
The nuclear lamina is a protein meshwork lining the nucleoplasmic face of the inner nuclear membrane and represents an important determinant of interphase nuclear architecture. Its major components are the A- and B-type lamins. Whereas B-type lamins are found in all mammalian cells, A-type lamin expression is developmentally regulated. In the mouse, A-type lamins do not appear until midway through embryonic development, suggesting that these proteins may be involved in the regulation of terminal differentiation. Here we show that mice lacking A-type lamins develop to term with no overt abnormalities. However, their postnatal growth is severely retarded and is characterized by the appearance of muscular dystrophy. This phenotype is associated with ultrastructural perturbations to the nuclear envelope. These include the mislocalization of emerin, an inner nuclear membrane protein, defects in which are implicated in Emery-Dreifuss muscular dystrophy (EDMD), one of the three major X-linked dystrophies. Mice lacking the A-type lamins exhibit tissue-specific alterations to their nuclear envelope integrity and emerin distribution. In skeletal and cardiac muscles, this is manifest as a dystrophic condition related to EDMD.

Show MeSH
Related in: MedlinePlus