If we're seeing this, it's already too
late. This is the naked capsit of the
hepatitis B virus moments after it has
infiltrated a normal liver cell by
hacking a run-of-the-mill gate on its
surface. For hundreds of millions of
people, this represents the point of no
return. Hepes capsid is now being
rapidly transported to the nucleus of
the cell, where it will break apart and
unleash one of the most intense takeover
programs in nature. In a world where
most viruses try to live fast and
transform an entire host cell into a
rapidly dying capsid bomb, hepatitis B
opts to take a slower, clumsier
approach, stumbling its way to gradually
converting this liver cell into a viral
factory, co-opting cellular export
machinery to infect other liver cells
and to deploy chaff that confounds the
immune system. Hep B is a barely
optimized infectious agent, relying
almost entirely on this liver cell's
metabolism to assemble new viral
particles. And yet somehow this sloppy
strategy has resulted in hepatitis B
being one of the deadliest infections of
all time, touching the lives of nearly
two billion of the people on Earth
today, with up to 4% of the entire human
population settled with a chronic near
incurable hepatitis B infestation.
Today, let's explore the life cycle of
one of our deadliest opponents and see
how the stealth approach shatters our
defenses. Let's get into it.
And hey, welcome back to Clockwork, a
show about the molecular foundations of
life. Subscribe so you don't miss new
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show to see our perks like monthly
sticker packs and our great molecular
papercraft series. But we've got a lot
to cover here, so let's not waste any
time. In this video, we're going to
loosely follow the life cycle of the
hepatitis B virus. from entry to HEP B's
incorporation into the nucleus to the
wild and complex assembly and export
process for new virus particles. Doing
this is going to show us the strange
stumbling path hep has taken to world
domination. But let's cover the ultra
basics. Hepatitis B is a virus, a kind
of genetic aberration that exists to
repurpose your cellular machinery to
build more copies of itself. Hep's quote
unquote goal is to smuggle its own very
weird DNA code into cells in your liver.
Despite the fact that hepatitis B only
spreads through direct contact with
bodily fluids, it is remarkably
resilient and infectious. And here's the
headline. According to the World Health
Organization, there are over 200 million
folks suffering from some form of
chronic hepatitis B infection with
estimates that 2 billion people, you
know, over a full quarter of the world's
population have been exposed to the
virus. And those same WHO estimates pin
over 1 million deaths per year on
hepatitis B. But the numbers are fuzzy
because those deaths come from liver
failure and the staggering amount of
liver cancer that hepatitis B can cause.
Only our old nemesis tuberculosis can
really edge out those numbers and just
barely at that. The molecular biology
we're about to explore will help explain
why the burden from hep is so high. So
let's just jump right in. If you're
going to actually wrap your head around
how hepatitis B works, you first need a
quick primer on its point of entry. The
hpatocy or liver cell. Hep B enters via
the bloodstream and is perfectly
structured for infecting your liver
cells specifically. Since all roads lead
to the liver eventually in our
circulatory system, it's a great vector
for infection. More importantly, these
surface apatocytes are just primed to be
the perfect partner for hepatitis. Your
liver cells are very active, constantly
pulling metabolites or harmful chemicals
out of your bloodstream and then
processing those chemicals for whatever
their next step is. Your liver is a
dizzying factory floor and water
treatment facility all bundled into one
organ. Hepatitis B is going to exploit a
lot of these processing pipelines to
quietly turn this apatic into a
long-term virus factory. And here's how
it starts. This is the full hepatitis B
virion. You're looking at the outer
membrane of the virus. This outer
membrane protects a little protein
shell. More on that later. But there are
also these little protein nubs embedded
in the membrane, the hepatitis B surface
antigens. Unlike a lot of other viruses,
only a very small part of the surface
protein actually pokes out of the
membrane. This little nub is like a
lockpick and it fits perfectly into
receptors on the surface of our hpatocy
here. These receptors are meant for
pulling bile salts out of your
bloodstream. But HPV can activate and
hitch a ride on this exact conveyor
belt. A bunch of proteins work together
to bring the hepatitis B viron into this
liver cell once this receptor gets
activated. And I really need to
emphasize how this liver cell is going
to automatically do most of the work
here in terms of establishing and
maintaining a long-term hepatitis B
infection. For real, hepatitis B is one
of the most stop hating yourself
infections I've ever studied. Once
inside, hepatitis B will lose its outer
membrane, exposing the core protein
capsit of the virus. The HEP B capsid is
just along for the ride at this stage.
This is just around 150 to 200 copies of
a single protein assembled into the
spiky sphere. The capsid protein is just
the right shape needed to establish this
infection. Each copy of the protein has
a little tag on the end of it. A small
string of protein called a nuclear
localization signal. Again, essentially
hepatitis B doesn't need to do anything
once it gets inside. The moment it
enters, it will bump into and get picked
up by a group of proteins called
importins, which bind to that nuclear
localization signal and then
automatically pull hepatitis B's capsid
down into the nucleus. These importins
also create a shield that allows this
whole little complex to be pulled across
the nuclear pore. One of the most
well-defended gates in any one of your
cells. Hep B is going to get an all
access pass here essentially for free.
Which means now hepatitis B can get down
to business. Once inside the nucleus,
the hep capsid will break apart, but the
exact details of how that goes down are
poorly understood. Regardless, breaking
down the capsid reveals the actually
infectious core of hepatitis B, an
incredibly weird and broken circular
packet of viral doublestranded DNA. I
cannot emphasize enough how bizarre
hepatitis B's genome is. This is a
roughly scale accurate simulation of
hepatitis B's DNA packet. Hepatitis B is
only packing 3.2 kilobase pairs of DNA
here. 3,200 letters of DNA, which is
basically nothing. That much DNA
information is only enough to encode
like two copies of your hemoglobin
protein. And the average intron of
non-oding DNA in your genes is no joke
double the length of the entire
hepatitis B genome. But quick fun fact,
this incredibly small DNA packet only
makes hepatitis B the second smallest
viral genome that infects humans. What's
the number one smallest virus, you ask?
Hang tight, we'll actually get to that
by the end of this video. So he's genome
is super small, but look carefully.
Turns out Heepb's DNA also arrives
broken and incomplete. You've got this
intact doublestranded section here
attached to a long stretch of
singlestranded DNA that's still attached
to the viral polymerase protein that
built this busted structure in the first
place. And by this part of the infection
cycle, my jaw is already on the floor.
So far, hepatitis B has effectively done
zero actual work, prepared a vanishingly
small DNA payload, and arrived with a
halfbusted genome in order to take on
and defeat one of the most robust and
multi-layered immune systems known to
the natural world. The difference in
scale here is astounding to me. Hep B is
basically packing a spork to take on and
defeat an entire aircraft carrier. But
keep watching. We'll see why the system
is secretly brilliant. Because your
cells simply cannot tolerate the
presence of broken DNA of any kind.
Instead of purging this dumb gambit from
hepatitis B, your own DNA repair
mechanisms are going to take over here
and quote unquote fix this broken double
DNA strand, completing HEP B's genome
for it. And I mean, incredibly, these
repair mechanisms will even go so far as
to package and process the hepatitis B
genome using your own histones, turning
an incomplete viral genome into a mini
chromosome. This convealantly closed
circular DNA packet is now ready to just
pretend it's part of your genome and get
to work quietly building virus parts. So
many other viruses come in guns blazing,
completely hijacking cellular metabolism
and blowing up their victims to spread
as many copies of themselves as possible
as quickly as possible. Hepatitis B is
here to take things slow to slip in
through the cracks and pull off a
gradual takeover which is going to cause
so many downstream problems. And those
problems, according to this recent
paper, are kicked off the moment this
mini chromosome forms. Because in this
section of the mini chromosome right
here, the liver cell's own machinery
will predictably add histones in this
exact same spot, which will guarantee
that DNA transcription proteins will
attach and start assembling viral RNA
from hep's smallest protein coding
section. This is critical because this
tiny 465 letter segment of DNA codes for
hepatitis B's ace in the hole, the
appropriately named X protein. This is
effectively the only tool in hepatitis
B's offensive arsenal. What you're
looking at here is a simulation of the
X- protein, as the shape of this thing
is honestly very loose and hard to pin
down. The X- protein is here, in short,
to cause pandemonium. Most of this
little glob is inherently disordered,
but it can cohhere into a few different
shapes based on the other proteins it is
close to. This allows X to shapeshift
its way into disrupting nearly 200 known
proteins and pathways in human cells.
One weapon, hundreds of targets. You
could almost get away with describing X
as a molecular cluster bomb. Protein X
could have far-reaching effects like
this because it's more of an adapter
protein. It likely redirects cellular
metabolism by bringing different
complexes together. And let me
illustrate that with one of X's most
important jobs, which needs to happen
right now if HEP has any hope of gaining
a real foothold in the hepatite. X needs
to be the first hepatitis protein on the
scene because the whole smuggling viral
DNA into a human nucleus thing really
shouldn't work out. like not even a
little because you've got a whole army
of proteins patrolling the nucleus to
make sure that no foreign DNA has
infiltrated this inner sanctum. We've
evolved tons of nucleus-based
countermeasures to ensure our cells are
only doing what they're supposed to do.
Managing your genome is one of your most
important jobs. And if you fail at that
job, you either get sick or get cancer.
So like it's a huge deal. And protein X
is going to immediately start blowing
that up by first binding to this DNA
repair protein called DDB1. Proteins
like DDB1 helps with DNA repair, but it
also helps detect faulty proteins inside
the nucleus, working with cellular
garbage disposal systems like this E3
ubiquitin liase. Anyway, one side of X
will bind to DDB1, and then the other
side will attach to a central component
in this beefy DNA maintenance complex
called SMC 56. And now that these two
systems are in contact, the ubiquitin
complex will just automatically tag SMC
56 for destruction, despite the fact
there's nothing wrong with this complex
at all. So, as more and more X-rotein
makes its way into the nucleus, more and
more SMC56 will get destroyed. But,
okay, like why is that bad? What does
this SMC John do that's so important?
SMC 56 is a chromatin maintenance
system, one of many that helps clean up
and protect your DNA. SMC 56's specific
job is to recognize and bind to any
suspicious or viral DNA it finds,
blocking it from getting transcribed. So
sure enough, blowing up this antiviral
defense protein is going to open the
floodgates and allow hepatitis B's mini
chromosome to start producing viral
proteins unchecked. This hepatitis X
protein messes with DNA maintenance in
so many more ways. But the upshot is is
that X interference will disable viral
countermeasures while also increasing
the number of transcription factors that
are recruited to the hepatitis B mini
chromosome, which ensures that this
liver cell is constantly producing more
hepatitis B parts no matter what. But I
guess that begs the question, what
exactly can hepatitis build with such a
limited genome anyway? And you know
what? I've gone too far with the limited
language here. It's better to think of
hepatitis B as one of the most efficient
DNA systems that can infect a person.
Most of hepatitis B's genes actually
overlap. So the virus is getting a ton
of mileage out of a compact and easy to
propagate DNA sequence. In total, there
are four places where DNA transcription
machinery can bind to the HEP mini
chromosome. We've already seen what the
shortest one does with the X- protein.
In total, these four binding sites can
build five unique mRNA strands, which
can, through the magic of molecular
biology, yield seven different proteins.
This is the full hepatitis B toolkit
right here. And let's break down
everyone's roles real quick and see how
this all plays out. Hepatitis B's
biggest need is a viral polymerase
protein. This will get packaged inside
the virus and help complete the assembly
process by transcribing a long RNA back
into hepatitis B DNA. Then we have the
capsid protein which will help form the
main structure of the final viron. On
top of that, there's these surface
antigen proteins which can come in three
distinct flavors. And now the longest
mRNA transcript is technically just here
to act as a template to rebuild
hepatitis B's DNA sequence in new
virons. But if there's a small mistake
and the transcription machinery binds
around 30 letters upstream, then that
long mRNA will eventually get assembled
into this goofy variant of the capsid
protein, the HEP antigen that comes with
a little bonus tail. So, all in all,
we've got a copy machine, two structural
scaffolds with some variations, one tiny
agent of pure biochemical chaos, and one
mistake. That's all you need if you want
to impact the lives of over 2 billion
people on Earth today. So, hepatitis B
wins when these structural proteins self
assemble around a full copy of that
pre-genomic mRNA attached to the viral
polymerase. But hepatitis B doesn't
encode for anything that helps push
liver cell metabolism towards assembling
hep virons. Right now, at the start of
things, protein X is just blowing up
enough DNA maintenance systems to
guarantee that the hepatitis B mini
chromosome will pump out as many copies
of these structural proteins as
possible. Beyond that, hepatitis B just
goes with the flow, hoping that enough
capsid proteins will bump into each
other to allow the viron to self
assemble. And I've said it before, but
this borderline Zen playbook is going to
have incredible downstream consequences.
But right now, let's just assemble one
virion and see how this is supposed to
play out. Remember this long pre-genomic
mRNA strand is all we need to make a new
copy of the hepatitis B genome. This
mRNA will meet up with the viral
polymerase and capsid proteins will
gradually self assemble around this
genetic payload. After the capsid is
complete, the polymerase will activate
and use that pregenomic mRNA to build
the half-complete hepatitis B DNA strand
that's going to infect the next cell
down the line. Meanwhile, these surface
antigens really hate being out in the
open cytoplasm and will either clump
together or embed themselves in whatever
membrane they can find. Most likely,
they'll end up concentrating in this
liver cell's endopplasmic reticulum.
Now, all we got to do is put these two
main components together. Ideally, this
freshly complete viral capsid will bump
into a section of endopplasmic reticulum
that's chalk full of HPV surface
proteins, which will just surround that
capsid and butt off the membrane,
completing our freshly new hepatitis B
viral particle, which is called a DANE
particle. By the way, I never mentioned
that, but just want to have that out
there. But this is rarely how things
work out at first. I can't emphasize
this enough. Hep B relies almost
completely on normal liver business to
assemble itself. The main thing this
capsid is built to do is infiltrating
the nucleus. Remember at the beginning
when I showed you that the capsid
proteins have a nuclear localization
signal baked into their structure. That
hasn't changed and it can't change. So
right here when the capsid is fully
assembled and getting ready to envelop
itself, this moment where hepatitis B
can claim a quick victory and get out
scot-free, nuclear importance will
probably just pick it back up and pull
it back into the nucleus. For me, this
is the crux of the video. Hepatitis B
doesn't really have a plan. and beyond
getting into the nucleus. So, a lot of
the initial virons that get assembled
are just going to end up right back at
the starting point. When folks talk
about viral infections, you hear a lot
about plans and intention and about
hijacking cellular machinery to build
new viruses. Hepatitis B is a great
window into how haphazard all this is.
Hepatitis B has bet everything on an
efficient genome, and that sometimes
makes the virus appear downright clutsy,
even though this little fella has
stumbled its way to touching the lives
of a full quarter of the world's
population. But back to this infection
cycle, hepatitis B can still turn this
quote unquote clumsiness into a huge
advantage. Because look, once this
additional capsid makes it back into the
nucleus, it will also break apart,
allowing this copy of the hepatitis B
genome to get repaired and turned into
an additional mini chromosome. So just
in case the initial infecting virion
can't synthesize enough X-rotein in time
to completely shut down the DNA defense
squad, there's now double the number of
mini chromosomes in this nucleus, which
will produce double the amount of
hepatitis mRNA and proteins. And this
process is just going to keep repeating
over and over until a kind of
equilibrium is reached. Eventually,
there are enough copies of the hepatitis
B genome stuffed into the cell's nucleus
and enough capsins are being produced
that the nuclear import system gets
completely overwhelmed. On average,
there's usually about 50 copies of the
mini chromosome in an infected nucleus,
but sometimes that number can go up to
100 to 150. So, hepatitis B has multiple
backup copies of its genome in case one
or more get damaged, and more and more
hepatitis B DNA in its nucleus will
increase viral, mRNA, and protein
production. This is going to become the
central pillars of hepatitis B's
success. It doesn't matter that this
infection cycle is inefficient at
assembling and releasing new virons. If
we flood this liver cell with more and
more hepatitis proteins, we'll have a
better chance of enough of them randomly
bumping into each other to assemble
complete and infectionready virons. But
of course, this comes with a lot of
complications. First of all, there's
going to be downstream effects that work
in hepatitis B's favor. As these
hepatitis B proteins flood into the
cytoplasm, they're going to start
messing with some cellular mechanics.
Notably, a lot of these hepatitis
proteins are going to automatically
limit the frontline immune response that
could blow up this burgeoning viral
factory before things get out of hand.
First up, the X-rotein sub centrifuge is
going to extend into the cytoplasm,
obviously hampering all sorts of
internal immune pathways. Notably, X
disrupts the mitochondrial antiviral
signaling system or MAVs, which
recognizes viral infections and
activates the production of type 1
interferons. Interferons are one of our
best frontline defenses against viruses.
At a high level, these proteins activate
RNA destroying machines that completely
shut down all protein production in an
infected cell temporarily. They also
signal to neighboring cells. So there's
an infection happening which will help
call in immune helpers like these
natural killer cells which will activate
the self-destruct sequence in the
infected hpatocy. While there's lots of
ways the cell can activate type 1
interferon production. Disrupting MAVs
is going to turn off a critical early
warning circuit for this cell.
Meanwhile, it's not just protein X doing
all the damage out here. Some of
hepatitis B's structural proteins also
just happen to inhibit internal immune
responses as well. And while there's
dozens of examples, I don't want us to
be here all day. So, I'm just going to
show you the one that blew my mind the
most because this is about the wildest
thing I've seen in any infection cycle.
Hepatitis B's service antigens mess with
the major vault protein. If you've been
plugged into the life sciences space at
all, you've heard about these massive,
mysterious vault complex present in all
of our cells. We aren't exactly sure
what these things do, but there are lots
of solid theories, and we're even using
them to help study internal cellular
mechanics. Based on studies focused on
hepatitis B, we know that cells produce
more of these vaults when a viral
infection kicks off. And we also know
that the major vault protein or MVP
interacts with another immune signaling
system, MYD88. But somehow hepatitis B
surface antigen proteins crowd around
these vaults and prevent my88 from
joining the party. Okay, cool. But who
cares? What does the interaction between
MVP and MYD88 actually do? And you know,
it helps kick off type 1 interferon
production. Nice. So, in short,
hepatitis B's flood of the zone strategy
actually works out really well as it
kneecaps the frontline immune response.
However, this chaos does kind of cut
both ways and things don't work out
perfectly for hepatitis B here because
the assembly process for these hepatitis
B virons is actually incredibly
inefficient. Sure, this cell is
gradually pumping out complete
infectious hepatitis B virus particles,
but because there is zero control of
this process, the cell is actually
pumping out a lot more useless stuff.
First up, the endopplasmic reticulum of
infected cells is going to get so
completely stuffed with hepatitis B
service antigens that empty little viral
bubbles are going to spontaneously bud
off and leave without any payload. The
same thing happens with these capsid
proteins. They'll self assemble even
without that pregenomic mRNA and
polymerase inside them. And those empty
capsids and empty surface shells will
still get exported out of this liver
cell. And the same actually goes for
this misprint of the core protein, the
hepatitis B E antigen. So while this
hpatocy is building and releasing
infectious hep virants ready to
infiltrate nearby cells, it's also
spewing a bunch of nonfunctional hep
detritus that seemingly seemingly does
nothing. But of course, that can't be
the end. How do you think hepatitis B is
going to use this mess in its favor?
Hilariously, all these quote unquote
wasted particles are going to act like a
kind of chaff that overwhelms and almost
literally exhausts the host immune
system. Immune coordinators like te-c
cells will get so overburdened by viral
stimulus that they'll just stop
responding to the presence of hepatitis
B proteins, which can slow the immune
response long enough to make a hepatitis
B infection chronic. And I could really
go on all day here, but these basic
details illustrate the foundations of
what makes hepatitis B so infectious and
so pervasive. Sure, for most infections,
hepatitis B will eventually get
eradicated once the immune system gets
its act together and mounts a
coordinated enough response to overcome
all the viral chaff and interference
silencing. But depending on the age
person gets infected, hepatitis B can
infiltrate enough liver cells and
exhaust the immune system enough that
the infection becomes chronic and
borderline incurable. Without a strong
immune response, a huge percentage of
the 250 million people affected by
chronic hepatitis B don't have very
noticeable symptoms until things start
going completely off the rails. For one
thing, the X-roin's reign of terror will
gradually lead to more and more genomic
instability in these liver cells,
leading to the development of severe
liver cancers. Even without the cancer,
a hepatitis B infection lives at the DNA
level. So the immune system only has
scorched earth options like apoptosis
and severe inflammation to try and
eradicate a hep infestation. After
decades of chronic infection, this can
cause profound liver damage that leads
to liver failure. So for a lot of folks
in the endstages of a chronic hepatitis
B infection, a liver transplant is their
only option. To me, hepatitis B is just
a deeply humbling disease to learn
about. Last year, I took y'all on a deep
dive into tuberculosis and how that
bacteria had developed so many
incredible, seemingly specific hacks
that hamper the immune system enough for
it to establish an infection. Hepatitis
B is almost the exact opposite kind of
problem where our enemies can sloppily
stumble through the cracks of our
metabolism and gradually build up enough
of a defensive front that we irrevocably
become hepatitis B viral factories. Hep
B is a critical reminder that biology is
just the whims of chemistry and physics
at the end of the day. And sometimes all
you need is a few interlin feedback
loops to become incredibly dominant. But
this haphazard and explosively
inefficient infection cycle has one more
astounding complication. The most
mind-blowing one of all. Hepatitis B is
so uncontrolled and so messy that this
infection cycle actually leaves the
virus open to other viral parasites or
satellite viruses that can only exist in
the presence of a hepatitis B infection.
In which I mean I cannot believe that
something like that exists. Remember
like a million years ago when I
mentioned that hepatitis B has only the
second shortest genetic code of any
human infecting virus? Well, hepatitis B
directly enables the number one player,
hepatitis D. This unbelievably short
bundle of viral RNA and scaffold protein
can only survive if it can envelop
itself in hepatitis B's surface
antigens. It doesn't code for them. This
RNA just codes for its scaffold protein.
And the way that this satellite virus
works is just astounding. But here's the
thing. Satellite viruses and co-
infections are honestly miles above my
ability to explain. But luckily, my good
friend and collaborator Fi the Neutrfil
has already made an incredible video
that can take you through the topic. And
here he is now with just a little
teaser.
>> Woah, woah, woah, woah, woah. Where the
heck am I? Am I in the Clockwork Blender
space? I'm not used to feeling so thick.
Give me a second.
>> Ah, much better. That's right, folks.
Heep B and HEP D, also known as the
Delta virus, form what is possibly the
weirdest dynamic duo in all of verology.
Come on over to my channel to learn more
where we're going to talk about all the
bizarre interactions that happen when
you get infected with two viruses at
once.
Okay, seriously, go watch that video
now. I actually do make a cameo in there
and explain hepatitis D in just a tiny
bit more detail. But here, there's
nothing more in this video. Quick, let's
just abruptly cut to the outro music.
And hey, thanks for watching all the way
to the end of this video. Sorry for the
abrupt finish here, but I really want to
guarantee that you go check out my
friend Fi the Neutrifll right now. Like
literally right now. Why are you still
here? If you like this video though and
you're still here for whatever reason,
make sure you like and subscribe. Also,
please comment on Fi's video and tell
him that Clockwork sent you. It actually
provides us a lot of invaluable data
that we can use to plan future videos.
Otherwise, I would also really
appreciate it if you checked out
patreon.com/clockworkshow
to join our monthly sticker club or
molecular paperclav club to support my
channel specifically. But I'll catch you
all in the next video. For now, I'd like
to leave you with peace, love, and
nuclear localization signals. Everyone,
be well. Thank you so much.