The filaments can exhibit a length of > 100 m and contain the F-actin isoform actin 2. gametes, likePfs230,Pfs48/45 orPfs25, but not the zygote surface proteinPfs28. We show that these tubular structures represent long-distance cell-to-cell connections between sexual stage parasites and demonstrate that they meet the characteristics of nanotubes. We propose that malaria parasites utilize these adhesive nanotubes in order to facilitate intercellular contact between gametes during reproduction in the mosquito midgut. Keywords:malaria, nanotube, Plasmodium, gamete, fertilization, transmission, mosquito == Introduction == Cell-cell contact is an important mode of intercellular interactions in multicellular organisms as well as in communities of unicellular microbes. While adjacent cells FH1 (BRD-K4477) in a tissue form features like gap junctions or synapses for the exchange of information, long distance communication is primarily mediated by the release of hormones or growth factors, without any physical contact between the communicating cells. Such exchange of soluble signaling molecules is particularly important for cells that are not embedded within a tissue, like bacteria or immune cells of higher eukaryotes. A novel type of cell-to-cell connections was identified in 2004, which represents a transient filamentous structure between two interacting cells [reviewed in1,2]. These filaments were first described in rat pheochromocytoma (PC12) cells3and were later reported in a variety of mammalian immune cells, such as NK cells, dendritic cells, T cells and mast cells4,5,6,7,8, thus cells that are either singular or form loose networks. Due to their morphological appearance they were termed membrane nanotubes or tunneling nanotubes, respectively. Nanotubes can either formde novoby outgrowth of filopodia-like protrusions via actin polymerization3or they represent elongating membrane channels, which form as adhering cells move apart4,9. During recent years, extensive research on nanotubes has revealed that these structures represent a general mechanism for operational connectivity between cells10, but instead of exerting a particular function, they reveal hN-CoR heterogeneity in their properties and have to be divided into subclasses1,2. Nanotubes are typically 50-200 nm in diameter and exhibit a length of up to 100 m. Such filaments consist of F-actin, while both F-actin and microtubules can be found in so-called thick nanotubes, which are > 700 nm in diameter9. Nanotubes were reported to either display a continuous membrane between two connected cells, as described for PC12 and dendritic cells, or are close-ended, like T-cell-specific nanotubes6. Several functions were attributed to nanotubes, depending on the cell type, from which they originate. Assigned functions range from trafficking of vesicles or mitochondria, as described for PC12 cells and macrophages3,9,11to mediating intercellular Ca2+signaling, as was shown for myeloid cells5. We here report similar filamentous structures in the sexual stages of the human malaria parasitePlasmodium falciparum, which form in the mosquito midgut during reproduction. The sexual phase of the malaria parasite is initiated by the differentiation of intraerythrocytic sexual precursor cells FH1 (BRD-K4477) in the human host, the gametocytes. Upon uptake by the blood-feeding female anopheline mosquito, a drop in temperature as well as factors in the mosquito midgut trigger the activation of gametocytes, resulting in their FH1 (BRD-K4477) egress from the enveloping erythrocyte [reviewed in12,13]. The activated female gametocytes round up and transform into macrogametes, while activated male gametocytes undergo three rounds of nuclear division and form eight flagella-like microgametes, which actively detach from the central body in a process called exflagellation and move freely in the medium before fertilizing a receptive macrogamete. Within a day post activation, the resulting zygote transforms into the infective ookinete, which is able to exit the midgut lumen by disrupting the midgut epithelium [reviewed in12]. In this study, we describe that activated gametocytes develop membranous filaments by means of cell-derived surface extensions, which can form membranous bridges between gametes. The filaments display typical features of nanotubes and might represent a novel type of long distance cell-to-cell connections between the sexual stages of malaria parasites within the mosquito midgut. == Results == ==.