A wide variety of cellular processes, including fertilization, cell division, cell migration, and cell polarity, depend on nuclear migration events. Inner nuclear membrane SUN proteins and outer nuclear membrane KASH proteins couple nuclei to the cytoskeleton. Gaps remain in understanding how KASH-SUN bridges are formed and function. Specifically, the molecular mechanisms of how proteins are trafficked to the inner nuclear mem- brane, how microtubules and motors are coordinated to move nuclei, and how KASH and SUN proteins interact to connect cytoplasmic forces to nuclei remain unknown. Our hypothesis is that forces generated by microtubule motors in the cytoplasm are connected to the nucleus by a bridge of conserved KASH and SUN proteins. Understanding how forces are transferred across the nuclear envelope will allow us to elucidate mechanisms of how nuclei are positioned in a cell, how chromosomes are moved inside the nucleus, and how perturbations of these processes disrupt cell and developmental processes. Our model will be tested by three specific aims:
(Aim 1) Elucidate mechanisms of inner nuclear membrane biogenesis. The current paradigm is that membrane proteins diffuse within the ER membrane to the nuclear envelope. Our preliminary data support an alternative active transport model for inner nuclear membrane trafficking, using a combination of the soluble nuclear import machinery, membrane-bound importins, and a Golgi trafficking intermediate. We hypothesize that multiple inner-nuclear-membrane-localization signals function to first actively transport UNC-84 from the peripheral ER to the nuclear envelope and to then mediate movement across the nuclear pore.
(Aim 2) Deter- mine how kinesin, dynein, and microtubules function to move nuclei. Tug-of-war, interdependent regulation, and bi-directional movement are proposed models to explain how motors of opposite polarity function together to move a cargo. Our hypothesis is that kinesin-1 provides the force to move nuclei and that dynein mediates backwards movements and rolling to bypass roadblocks. We will distinguish between two models for how NOCA-1 regulates polarized microtubule arrays-by regulating either plus-end tip dynamics or nucleation of microtubules.
(Aim 3) Determine how forces generated in the cytoplasm are coupled to the nucleus. Two models could explain the role of the KASH-SUN bridge in nuclear migration;they could serve simply as outer nuclear docking sites or, also as transducers of force across the nuclear envelope. We hypothesize that forces generated in the cytoplasm are directly linked to the nuclear lamina by KASH-SUN bridges. Our approach is innovative because it takes advantage of a C. elegans model with unique genetic and molecular strengths with the ability to film and quantify nuclear migration. The proposed research is significant because it is expected to (A) elucidate mechanisms of protein transport to the inner nuclear membrane, (B) elucidate mechanisms of bi- directional nuclear migration along polarized microtubules that will be applicable to other large cargos, and (C) determine how the forces that move nuclei are transferred across the nuclear envelope.
The proposed research is relevant to public health because mutations in KASH and SUN proteins have been shown to cause or to be linked to muscular dystrophies, ataxias, lissencephaly, progeria, and multiple cancers. Moreover, defects in nuclear migration contribute to many of the neuromuscular defects of these diseases. Thus, the proposed research is relevant to the part of the NIH's mission that fosters fundamental discoveries in basic cell and developmental biology with great potential for a positive impact on human health.
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