UCLA

The nuclear lamina is an intermediate filament meshwork lining the inner nuclear membrane and is a key component of the nuclear envelope. The nuclear envelope separates the contents of the cell nucleus from the cytoplasm and consists of the inner and outer nuclear membranes, nuclear membrane proteins, nuclear pore complexes, and the nuclear lamina. The principal protein components of the nuclear lamina are lamins A and C (A-type lamins) and lamins B1 and B2 (B-type lamins). B-type lamins are expressed throughout development, whereas the A-type lamins are not expressed until late in development. Mutations in LMNA have been linked to a spectrum of human diseases, but only a single disease had been linked to mutations in LMNB1 or LMNB2. Several years ago, our group showed that both lamin B1 and lamin B2 are required for neuronal migration in the developing central nervous system (CNS), and for the survival of neurons. In the developing brain, the processes of neurogenesis and neuronal migration impart considerable mechanical stress on the cell nucleus. Neurogenesis in the ventricular zone of the developing cortex depends on interkinetic nuclear migration (INM), an oscillatory movement of nuclei between the apical and basal sides of the cell. Once neuronal fate is specified, the migration of neurons from the ventricular zone to the cortical plate depends on nuclear translocation (nucleokinesis) and passage through confined spaces. In cell culture studies, cell nuclei subjected to mechanical stresses (i.e., by physically deforming nuclei or forcing cells to traverse tight constrictions) had been shown to cause nuclear membrane (NM) ruptures. The frequency of NM ruptures was known to be increased in the setting of nuclear lamin deficiencies but the relevance of NM ruptures in cultured cells to pathology in tissues was unknown. In this dissertation, my goal was to investigate whether deficiencies in nuclear lamins predispose to nuclear membrane ruptures (particularly when the nucleus is subjected to mechanical stress during normal physiological processes, such as cell migration) and whether these ruptures may result in disease phenotypes. In Chapter 2, we showed that fibroblasts lacking all nuclear lamins exhibit spontaneous NM ruptures in vitro that are exacerbated by mechanical stress. In Chapter 3, we generated a mouse model with forebrain-specific loss of lamin B1 and expressing a nuclear fluorescent reporter, and showed that forebrain-specific loss of lamin B1 results in NM rupture and cell death in vivo. We also showed that both lamin B1– and lamin B2–deficient neuronal progenitor cells exhibit spontaneous NM ruptures, although the frequency and duration of NM ruptures was markedly different. In Chapter 4, we investigated whether overexpression of LAP2 (lamina-associated polypeptide 2, β isoform), a LEM-domain protein of the inner nuclear membrane, could prevent nuclear lamin–deficient fibroblasts and lamin B1–deficient neurons from exhibiting NM ruptures. In tandem, we discovered that reduced expression of LAP2 also rendered nuclear lamin–deficient cells more susceptible to NM ruptures.