A Very Large Organelle in Sympathetic Neurons

Background: Recently a large, non-membranous organelle, named the loukomasome (Greek: “doughnut” body) was discovered within specific subpopulations of rat peripheral autonomic ganglia neurons. This proteinaceous organelle is found throughout the sympathetic chain with the highest proportions in the pelvic and stellate ganglia (innervating the pelvic organs and heart, respectively). This organelle is restricted to neurons which co-express neuropeptide Y (NPY) and calbindin-D28K, indicating that target-derived factors common to these tissues may specify whether a neuron contains this organelle. The loukoumasome appears singly, in one of multiple possible conformations, which is dependent on its position within the cell. The three most common conformations identified are: toroidal when on the nuclear side of the trans-golgi network (TGN), twisted when on the boundary of the TGN, and rod shaped when peripheral to the TGN. This research aims to further characterize the structure, protein composition and cellular interactions of the loukoumasome in order to shed light on the functional significance of this organelle.  Methods: An antibody raised against pericentrin, a protein normally located in the region surrounding the centrosome, was found to bind exclusively to the loukoumasome in immunohistological sections of the stellate and pelvic ganglia. This antibody was used to examine how the proportions of the three conformations differ between the ganglia in the rat sympathetic chain. In addition, we screened the loukoumasome with antibodies specific for different cytoskeletal elements and regulators and used electron microscopy studies to examine the loukoumasome’s ultrastructure. Results: When comparing the loukoumasomes in the pelvic and stellate ganglia we identified a significant difference in the proportions of the conformations of this organelle. Within pelvic ganglia there is a higher proportion of toroidal and lower proportion of linearized loukoumasomes when compared to the loukoumasomes found within neurons of the stellate ganglia. Examination of the protein composition of the loukoumasome revealed that it contains multiple proteins involved in regulating the cytoskeleton including the microtubule nucleating protein g-tubulin and the neuron specific bIII-tubulin. Furthermore, through electron microscopy and immunohistochemistry studies we identified the loukoumasome as an electron lucent toroid that often interacts with the nematosome, another enigmatic, proteinaceous organelle. Conclusions: The protein and structural composition studies performed here are the first steps in elucidating the roles that these enigmatic organelles play within the neurons in which they reside. This research into the newly discovered loukoumasome, as well as further studies on other poorly understood organellar structures such as the nematosome, contributes to the aim of fully understanding the complete make-up of a cell.