Phylum defining feature : The apical complex

The Phylum Apicomplexa is a large and uniquely diverse collection of unicellular, eukaryotic organisms that share amongst other common features, one that is capable of conferring the honor of being placed in this phylum, and one from which this blog derives its name . Ladies and gentlemen, let me introduce (cue fanfare ♪♫♪), the Apical Complex.

Ontologically, the apical complex is a constituent cellular component, found at the anterior end of the mature parasite. It is made up of various cytoskeletal components and membrane bound organelles, and among the latter , we find the micronemes, rhoptries, polar rings and conoids. (All that is standard textbook fare).

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(Source: http://tolweb.org/tree/ToLimages/zoite-tolv1.png; under the Creative Commons License)

Zooming in closer on Toxoplasma gondii, an archetypical apicomplexan, as Hu et al. have done in their paper , we find that the conoid is spirally arranged, which naturally allows for its role in invasion. The conoid is associated with the preconoidal rings, the polar rings and intraconoid microtubules. The polar rings serve as an anchor for microtubules that run from the anterior end of the organism to the posterior end, right under the pellicle. These, because of their location, are called subpellicular microtubules. Microtubule numbers vary across the phylum.When viewed under a phase contrast microscope, the entire apical complex appears as a single dark spot on the anterior end of the parasite.

The conoid is quite remarkable in that although it is made up of alpha-beta tublulins that are similar to the ones found in mammals, the tubulins form an unique ribbon-like polymer. When calcium levels in the cell rise, the conoid along with the associated upper polar ring protrudes beyond the base of the lower polar ring. This is shown in these figures on PubMed, in which alpha-tublin was modified with yellow fluorescent protein labelling.

The micronemes are involved in attachment and penetration. The proteins, named MICs, in these organelles are secreted, before the exocytosis of the rhoptry proteins. Before they get to their compartment, these proteins which have a classic hydrophobic amino terminal signal peptide move through the endoplasmic reticulum and exit the network at the golgi complex. They additionally have two conserved motifs at the C terminus - the first being a SYHYY and the second, an acidic series, EIEYE. These are important in folding, sorting and the spanning of the membrane. There are also distinct adhesive domains in the middle.

As soon as the tachyzoite attaches to the host cell, there is a marked increase in calcium levels in the parasitic cell, as calcium from the acidocalcisomes is released. This triggers the activation of at least two calcium dependant protein kinases that cause further signal cascades, and ultimately the release of micronemal proteins. The adhesive domains are involved in the adhesion and further invasion may take place.

Rhoptry proteins, reviewed in an article in Nature Reviews Microbiology,2008 , by Boothroyd, is conserved in its presence across the phylum and yet is varied in its architecture and numbers. There are 12 in the tachyzoite stages of Toxoplasma, each 2 to 3 micrometers in length. They appear to have compartments when studied with IFA. Protein compartmentalization post-transcriptionally occurs by understudied mechanisms. Rhoptries are even visible under a phase contrast microscope and appear to be related to exosomes. Of the 29 proteins identified in the rhoptries, 24 ROP proteins localize in the bulb region and 5 RON proteins localize in the neck region, with 28 newly recognized proteins yet to be allotted designations/names. Some of the ROP proteins are enzymes like kinases, proteases and phosphatases, that do not have orthologs in related genera. The, one gene- one protein, RON proteins however, have orthologs in related extant sps, although their exact functions are yet to be fully elucidated.  The rhoptries also seem to possess lipids rich in cholesterol. These are suspected to play a role during invasion, by forming vesicles around secreted proteins and ultimately fusing with the parasitophorous vacuole (PV).

During cellular invasion, the rhoptry contents are released. Some of the RON proteins are involved in the formation of the moving junction, which forms the physical interface, that is , the point of contact during invasion. The ROP proteins, after their release, localize in the PV or the PV membrane (PVM) or inside the host cell (including the nucleus), with many other targets being yet unknown. ROP1 is released in the PV as a vesicle and later moves to the membrane. ROP2 and its associates are putative integral PVM proteins that recruit the host mitochondria to the cytoplasmic face of the PVM. PP2C-hn and ROP16 contain typical NLS (Nuclear Localization Signal) sequences that cause them to move to the nucleus. All these molecules seem to play a key role in the biology of Toxoplasma sps.

The exact mechanism of conservation of the apical complex across the phylum seems to be a curious thing, as the phylum seems to be inhabited by protozoans that are as different, from each other in their pathogenesis and habitat, as the night is from the day. It certainly is true that the use of the apical complex is not always the same in all species, and certainly not all have the above described mechanism of Toxoplasma gondii. Thus, the Toxoplasma model, despite being the best that we have got right now, gives us only a glimpse of the preeminence of the great phylum-defining feature. It surely is our starting point. Where we will yet reach from here is unknown . Will we ravel the marvels of the pathogenesis of, say Cyclospora, by studying this feature? Will we bring to light the reason for the bizzare behaviour of the other Apicomplexans ? We can only hope so, for now, because, we have miles to sail in our research before we reach the shores of understanding .

References :

1. Boothroyd JC, Dubremetz JF. Kiss and spit: the dual roles of Toxoplasma rhoptries. Nat Rev Microbiol 2008;6:79-88.

2. Hu K, Roos DS, Murray JM. A novel polymer of tubulin forms the conoid of Toxoplasma gondii. J Cell Biol 2002;156:1039-1050.

3. Soldati D, Dubremetz JF, Lebrun M. Microneme proteins: structural and functional requirements to promote adhesion and invasion by the apicomplexan parasite Toxoplasma gondii. Int J Parasitol 2001;31:1293-1302.