FIGURE 5.

Role of the biosynthetic and endocytic networks in the organization of endomembranes and Golgi positioning in differentiated cell types. In all cells the centrosomal (or centrosome-derived) and non-centrosomal (Golgi-nucleated) MTs with plus-minus polarity are indicated by orange and brown color, respectively. The IC elements (dark green) and REs (light green) are also depicted by different colors. (A) Highly schematic model of a neuron with its cell body, axon and dendritic tree. Golgi stacks (blue) are present in the cell body (Golgi ribbon) and in the proximal branchpoints of the dendritic tree (Golgi outposts), but are lacking from axons. By contrast, in addition to being present in the cell body, IC elements and REs are found throughout the neuronal periphery. The blow-up highlights synapses with local secretory ERES-IC-RE units. Whether axons contain similar structures is presently unclear. (B) Schematic diagram of a small portion of a long multinucleated skeletal muscle cell. In a terminally differentiated myofiber small Golgi outposts are found in the nuclear periphery and – together with ERES – at specific sites within the myofibrillar system. These sites, which function in the nucleation of longitudinal and vertical bundles of non-centrosomal (Golgi-nucleated) MTs most likely contain also IC elements and REs. (C) In a polarized epithelial cell the tight junctions divide the PM into apical and basolateral domains. The apical PM further consists of ciliary and non-ciliary subdomains. In epithelial cells, as in neurons and muscle cells, the centrosome loses its major role as MT-organizing center and (in this case) forms a basal body at the base of the primary cilium. This function is taken over by sub-apical nucleation sites which, however, remain enigmatic. These sites generate a vertical array of MTs typical for the polarized epithelial cell. The apical region may also contain a lateral array of MTs of mixed polarity (not shown). Moreover, a sub-population of non-centrosomal MTs are nucleated by the Golgi apparatus and grow apically. Rab11-containing apical recycling endosomes (AREs) pile-up at the minus ends of the vertical MTs. Similarly as in the pericentrosomal region of a fibroblastic cell (see Figure 4), a pool of IC elements are proposed join the REs at this location. This conclusion is supported by the existence of a circular membrane compartment at the base of the primary cilium, which is known to contain Rab11 and the IC/cis-Golgi protein GM130.