Naegleria's Split Mor­phol­ogy Dis­or­der

by Psi Wave­func­tion

By de­fault, a mem­brane-bound en­tity like a cell should be a spher­i­cal, form­less blob. How­ever, most cells are not such form­less blobs, but rather have adopted one or more forms from a vast reper­tory of stun­ningly com­plex mor­pholo­gies. To wit, see (and ad­mire) the ra­di­o­lar­i­ans, meta­zoan neu­rons, gi­ant parabasalians or the end­lessly weird and so­phis­ti­cated cil­i­ates. Even prokary­otes have a highly com­plex cel­lu­lar struc­ture, and are not, as some bio­chemists are prone to think, mere bags of en­zymes. The deeper you ven­ture into the realm of cel­lu­lar diver­sity, the more awe-in­spir­ing be­comes the cor­nu­copia of cel­lu­lar struc­tural and mor­pho­log­i­cal va­ri­ety. Luck­ily, there is some or­der to it as there are two fun­da­men­tal 'gen­res' of cel­lu­lar mor­phol­ogy, at least in the pro­tists: fla­gel­lates and amoe­bae. Of course, there are also cysts, but since those are mostly rest­ing stages (be­ing a round ball isn't par­tic­u­larly help­ful while feed­ing or flee­ing from preda­tors...) they can be ig­nored for now.

The com­plex struc­ture of fla­gel­lates. EM sec­tion through a ma­ture fla­gel­late Te­tra­mitus. K1 = kine­tosome. Cm = cyto­stomal canal. FV = food vac­uole. Rz = rhi­zo­plast. Source

Cell shape de­pends on the cy­toskele­ton. As you know, its two main com­po­nent sys­tems are actin and tubu­lin, ig­nor­ing the plethora of mis­cel­la­neous pro­teins that are used for var­i­ous struc­tural jobs. Tubu­lin makes mi­cro­tubules, the spin­dle fibers of mi­to­sis, but is also im­por­tant for the fla­gellar ap­pa­ra­tus (we've yet to find one com­posed of actin and prob­a­bly for good rea­sons). You al­so know that actin is a key player in cell motil­ity and mor­phol­ogy in an­i­mal sys­tems. It is also hea­vily in­volved in en­domem­brane traf­fick­ing within a cell, as well as endo- and ex­o­cy­to­sis. If inter­ested, a re­cent is­sue of Sci­ence has a nice overview of actin in mor­pho­gen­e­sis and cell move­ment.

The role of the cy­toskele­ton in mor­pho­gen­e­sis is much less clearly de­fined. It de­pends largely on the species. Plants, for ex­am­ple, rely very heav­ily on tubu­lin for mor­phology, with actin be­ing a mi­nor player. Amoe­boid cells are pri­mar­ily actin-based. In fact, amoe­boid cells re­sort to tubu­lin largely for spin­dle for­ma­tion dur­ing mi­to­sis. They hate tubu­lin about as much as plants hate actin. Actin-based cells don't have to be amor­phous; they are still able to achieve com­plex mor­pholo­gies. But there is a pos­i­tive cor­re­la­tion be­tween amoe­boid-ness and actin-ness ('ac­tin­ity'?).

In con­trast, fla­gel­lates are pri­mar­ily tubu­lin-based. Of course, they still use actin for some intra­cellular work, but the shape de­pends largely on the whims of their mi­cro­tubules. Per­haps not re­ly­ing much on fla­gella al­lows the amoe­boids to dis­pense with the mi­cro­tubule or­ga­ni­za­tion path­ways, thereby switch­ing to actin. Fla­gel­lates, re­ly­ing heav­ily on in­tact tubu­lin sys­tems, may be less prone to los­ing their struc­ture. Also, if you're a fla­gel­late, you need shape for a mod­icum of stream­lin­ing. Try swim­ming around as a form­less or floppy blob of some sort! Keep in mind that life at that scale is very dif­fer­ent. Vis­cos­ity calls the shots when con­sid­er­ing uni­cel­lu­lar motil­ity. Per­haps be­ing hy­dro­dy­namic isn't even as im­por­tant as sim­ply re­tain­ing shape. Oth­er­wise you'd be like a blob of mo­lasses try­ing to swim through a sea of maple syrup. Not gonna get very far.

What­ever the rea­son, amoe­boid cells tend to have a pre­dom­i­nantly actin-based cy­toskele­ton, fla­gellates have a pen­chant for tubu­lin. Of course, not all or­gan­isms are de­ci­sive enough to make this com­mit­ment, so we've got amoe­bofla­gel­lates in the mid­dle.

Plenty of other or­gan­isms fancy tran­si­tion­ing be­tween be­ing more amoe­boid or more fla­gel­late. But few cells ac­tu­ally dis­pense with fla­gella and basal bod­ies al­to­gether, to form them anew when spe­cial con­di­tions arise. It's time to in­tro­duce one that does.

Nae­g­le­ria and Tetrami­tus: Het­erolo­boseans with 'split mor­phol­ogy dis­or­der'

Meet Nae­g­le­ria, a Het­erolo­bosean, fa­mous to med­ical bi­ol­o­gists as a brain-eat­ing op­por­tunist, and to ac­tual bi­ol­o­gists as the mas­ter of de novo fla­gel­lar cre­ation.

Nae­g­le­ria gru­beri.Source

And be­fore some­one ac­cuses me of fo­cus­ing on bio­me­dically rel­e­vant or­gan­isms, Tetrami­tus, an­other Het­erolo­bosean, does it too, and has more fla­gella to oogle at.

Dra­matic trans­for­mation of Tetrami­tus be­tween amoe­boid and fla­gel­late forms. Source

The nice pic­ture of the Tetrami­tus fla­gel­late form at the be­gin­ning of this post re­veals its mar­velous com­plex­ity. Keep in mind this thing was a 'form­less' amoeba a few hours be­fore. Quite a com­plex struc­ture to be formed within a 4 hour pe­riod! Note that while the or­ganelles seem to po­si­tion them­selves freely in amoe­boid cells, they be­come sub­stan­tially more fixed in the fla­gel­late form. This one even has sub­pel­lic­u­lar mi­cro­tubule bun­dles, a cy­tostome, and the oral groove char­ac­ter­is­tic of its 'king­dom,' the Ex­ca­vata.

Of Nae­g­le­ria's culi­nary in­dul­gences

Be­cause of Nae­g­le­ria's pesky habit to oc­ca­sion­ally get into hu­man brains and eat them (oops!), it gets a lot more re­search at­ten­tion (which, in the world of Het­erolo­bosean bi­ol­ogy, doesn't re­ally mean very much). It should be noted that while there are some waves of Nae­gler­a­pho­bia out there, in­ci­dences of hu­man in­fec­tion are very very VERY rare (small hand­ful of cases a year; some years with­out any cases what­so­ever). Un­for­tu­nately, it is al­most uni­ver­sally fa­tal, but so are more ubiq­ui­tous dis­eases. But that's enough ex­cuse for the me­dia to put up a scary ar­ti­cle on Nae­g­le­ria every once in a blue moon, of­ten ac­com­pa­nied by an im­age of Amoeba pro­teus or Chaos spp. Hey, at least they're still eu­kary­otes!

Be­fore we con­tinue on with our hard core cell bi­ol­ogy, let's clar­ify that Nae­g­le­ria is not a par­a­site, but rather an op­por­tunist that seems to not mind the 37 °C heat of the hu­man body. It gets into the hu­man brain through the nose, gen­er­ally from swim­ming in warm wa­ter. Once in­side, it de­vours the brain, in­ad­ver­tently killing its 'host', but it does not re­quire trans­mis­sion to an­other host to live on. So Nae­g­le­ria's gas­tro­nomic in­cli­na­tions may well just be an ac­ci­dent, al­beit a rather costly one to the few hu­mans it in­fects.

Amoe­boid to fla­gel­late trans­for­ma­tion

Sum­mary of Nae­g­le­ria's life cy­cle based on C. Ful­ton in Meth­ods in Cell Phy­siology, Vol. 4. 1970. (Don't pay any at­tention to the por­trayal of the fla­gel­lar root ap­pa­ra­tus.)

Our ex­plo­ration of Nae­g­le­ria's life cy­cle be­gins with its dom­i­nant life stage, the eat­ing and mi­tot­i­cally-able amoe­boid form. In­ter­est­ingly, mi­to­sis hap­pens with­out basal bod­ies (like it does in land plants and di­atoms). The ad­dition of wa­ter in­duces the fla­gel­late form, which is ca­pable of feed­ing via its elab­o­rate oral groove and cyto­stome, but not di­vid­ing. While an amoe­boid shape is op­timised for a ben­thic en­vi­ron­ment (crawl­ing on sur­faces), ex­ces­sively fluid en­vi­ron­ments favour the swim­ming fla­gellate. The third life stage is a rest­ing cyst, which it forms and hi­ber­nates in when times are bad (e.g., drought). There seems to be no men­tion of a sex­ual life cy­cle, most likely due to this group be­ing so un­der­stud­ied. (Each year brings us more and more ev­i­dence of sex in or­gan­isms pre­vi­ously thought to be asex­ual, with ac­cu­mu­lat­ing evi­dence sug­gest­ing sex is a shared eu­kary­otic fea­ture.) Nae­g­le­ria's life cy­cle con­ve­niently in­volves all three fun­damental cell types, which is why it's such a won­der­ful sys­tem for study­ing the trans­for­ma­tions be­tween them (and cel­lu­lar dif­fer­en­ti­a­tion.)

The tran­si­tions be­tween the prin­ci­pal cell mor­pholo­gies in­volve the reg­u­la­tion of actin and tubu­lin monomer lev­els: that is, the amount and con­cen­tra­tion of the 'build­ing blocks' for each cytoskele­tal sys­tem. In fact, the rea­son many or­gan­isms en­cyst for mi­to­sis (or at least re­duce their fla­gel­lar length) is to redi­rect tubu­lin from the cy­toskele­ton and fla­gel­lum to form the mi­totic spin­dle. Oth­erwise, you'd have to syn­the­sise a bunch of new tubu­lin that would soon be­come use­less. If you look at some im­ages of Nae­g­le­ria amoe­bae un­der­go­ing mi­to­sis, you'll see that the cells are much rounder than nor­mal.

There's prob­a­bly also a reg­u­la­tion of the actin- and mi­cro­tubule-as­so­ci­ated pro­teins in­volved in cy­toskele­tal or­ga­ni­za­tion for each sys­tem (and the in­ter­ac­tions be­tween them), but that doesn't seem to have been stud­ied at all in Nae­g­le­ria. Sim­ply syn­the­sis­ing tubu­lin is in­suf­fi­cient for any sem­blance of a well-or­gan­ised cy­toskele­ton; aux­il­iary pro­teins are re­quired to reg­u­late the dis­tinctly non-ran­dom mor­phol­ogy of a fla­gel­late.

Changes in the actin and mi­cro­tubule cy­toskeleton as amoe­bae dif­fer­en­ti­ate into fla­gel­lates. Im­munos­tain­ing of the cyto­skeleton: red = anti-actin; green = anti‑α–tubulin; yel­low = co-lo­cal­i­sa­tion of anti-actin and anti-α-tubu­lin. Time: 0 to 120 min­utes. Bar: 10 µm. Note that these are dif­fer­ent cells, as they had to be fixed (killed) for im­munos­tain­ing. I would love to see a live cell time­lapse of this pro­cess! Note how the fla­gel­late mor­phol­ogy is 'molded' by the lon­gi­tu­di­nal micro­tubule bun­dles. Source

The Nae­g­le­ria sys­tem could be fruit­ful for iso­lat­ing mi­cro­­tubule-as­so­ci­ated pro­teins and other mi­cro­tubule regu­lators (e.g., via mi­croar­ray chips). Con­sid­er­ing the cyto­skeleton is the most ex­cit­ing cell struc­ture (fact), cell bio­logists should stop ne­glect­ing Het­erolo­boseans. We do know that the com­mit­ment to fla­gel­la­tion in­volves a ra­pid in­crease of tubu­lin pro­duc­tion, since the amoe­boid form has barely any of it.

Nae­g­le­ria takes ~120 min at 25 °C to tran­si­tion from an amoeba to a fla­gel­late. The ear­li­est event in this process is the ini­ti­a­tion of tubu­lin syn­the­sis, fol­lowed by basal body as­sem­bly 30 min later. Cells round up as actin fi­laments de­poly­merise, and basal bod­ies be­gin to sprout fla­gella at about the one hour mark. In an­other 20 min, the tubu­lin cy­toskele­ton is molded into the proper fla­gellar body shape, with fla­gella reach­ing full length at ~110–120 min.

Doesn't look like Nae­g­le­ria has had its genome se­quen­ced yet, al­though ESTs and a draft for N. gru­beri are un­derway. Of course, the other het­erolo­boseans are be­ing thor­oughly ne­glected. And yet we may be about to se­quence ten thou­sand ver­te­brate genomes.

Hope­fully I've con­vinced you at least that Nae­g­le­ria would make an in­ter­est­ing model for study­ing cell dif­fer­en­ti­a­tion and tran­si­tions in mor­phol­ogy, as well as cy­toskele­tal dy­nam­ics. While the field is still minis­cule – at least its non-clin­i­cal com­po­nent – some work is be­ing done right now on the reg­u­la­tion of cy­toskele­tal de­vel­op­ment in Nae­g­le­ria (such as this pa­per on the lo­cal­i­sa­tion of mi­cro­tubule-re­lated mR­NAs), but not as much as you'd ex­pect. I'm not aware of any other sys­tem that in­volves such a com­plete tran­si­tion be­tween three fun­da­men­tal cell types. For stud­ies of mor­phology, what more can you ask for?

Note: For a list of re­search pa­pers drawn upon, click here.

Yana, aka Psi Wave­func­tion, is an un­der­grad­u­ate in the De­part­ment of Botany, Uni­ver­sity of British Co­lum­bia, and the host of the blog Skep­tic Won­der: Pro­tists, Memes and Ran­dom Mus­ings (in­ac­tive since 2015). Her orig­i­nal post on Nae­g­le­ria pre­sented on her own blog was adapted, with her per­mis­sion, for pub­li­ca­tion here.