of CRISPR-Cas genome-editing technology
has made it possible to successfully inactivate 62 PERVs in immortalized pig cells (12),
and now the same group, in Niu et al., inactivated all PERVs in primary cells and used
these cells to generate live, healthy, genetically modified pigs (1) (see the figure). The
pig strain used for this experiment normally
carries 25 copies of PERV-A and PERV-B. Using CRISPR-Cas to introduce an inactivating
mutation in a highly conserved region in the
pol gene that encodes reverse transcriptase,
all 25 PERVs in the primary cells were inactivated and unable to produce infectious virus
particles. Using nuclei from these cells, embryos were produced by somatic cell nuclear
transfer and transferred into surrogate sows
with no PERV-C and minimal PERV numbers. They produced 37 PERV-inactivated
piglets from 17 sows—15 piglets remain alive,
and the oldest healthy pigs were four months
old at the time of publication. Although it remains unclear whether PERVs can actually
infect humans (even though they can infect
human cells in culture) and induce diseases
that are typical for retroviruses, such as immunodeficiency or cancer, this new achievement will allay fears of PERV infection after
xenotransplantation. This is a step forward
in the clinical application of xenotransplantation; however, the other problems—
immune rejection, physiological compatibility,
and the elimination of other potentially zoonotic viruses—have to be solved.
There is another interesting piece of information gained from Niu et al.: In numerous
species, including humans, the envelope proteins of endogenous retroviruses play an important role in the generation of the placenta
(13). These proteins, known as syncytins,
help generate the syncytiotrophoblast in the
placenta and may have immunosuppressive
properties (14). The fact that the genetically
engineered pigs were born healthy indicates
either that the disruption of the reverse transcriptase does not affect the function of the
envelope proteins or that placentogenesis in
pigs does not, after all, require retroviral envelope proteins. j
1. D. Niu et al., Science 357, 1303 (2017).
2. H.Niemann, B.Petersen, Transgenic Res. 25,361(2016).
3. N. Klymiuk et al., Mol. Reprod. Dev. 77, 209 (2010).
4. D. K. Cooper et al ., Transplantation 84, 1 (2007).
5. J. Denner, Xenotransplantation22, 167 (2015).
6. J.A.Fishman,Am.J. Transplant. 17,856(2017).
7. J. Denner, Xenotransplantation 22, 329 (2015).
8. J. Denner, in Retroviruses: Molecular Biology, Genomics,
and Pathogenesis, R. Kurth, N. Bannert, Eds. (Caister
Academic Press, 2010), pp. 35–69.
9. J.Denner,R.R.Tönjes, Clin. Microbiol. Rev.25,318(2012).
10. J. Denner, Viruses 9, 213 (2017).
11. M. Semaan et al ., PLOS ONE 10, e0122059 (2015).
12. L. Yang et al., Science 350, 1101 (2015).
13. J. Denner, APMIS 124, 31 (2016).
14. M.Mangeney etal., Proc.Natl.Acad.Sci.U.S.A. 104,20534
Angular momentum can
slow down photoemission
By Vladislav S. Yakovlev and
Photoemission spectroscopy, where the absorption of an energetic photon by a material results in the emission of an electron, is an invaluable source of information about electronic struc- ture. Electrons gain their kinetic energies by interacting with both light and
their surroundings. In a solid, for example,
this makes it possible to measure band energies, energies and lifetimes of quasiparticles,
spectral density of states, surface states, and
both elastic and inelastic scattering processes. Since the photoelectric effect was explained by Max Planck and Albert Einstein,
the fundamental processes behind photoemission have been thoroughly studied in
both experiment and theory, but do we fully
understand the dynamics of electron emission? On page 1274 of this issue, Siek et al.
(1) show that the angular momentum of the
electron affects which electrons are emitted
first from an atom in a solid.
When an energetic photon is absorbed by
an atom, it takes a very short time for the
outgoing electron wave packet to form. The
formation time can be roughly estimated
as the ratio of the Bohr radius to the elec-
tron’s final velocity, which gives 2 3 10–17 s
(20 as) for a 30-eV electron. Direct access
to processes that occur on such short time
scales was once unimaginable, but at the
beginning of this century, a series of break-
throughs led to what is currently known
as attosecond physics (2). Its toolbox al-
lows for such measurements, especially at-
tosecond streaking (3). In this technique,
the electron is set free by a pulse of ener-
getic photons lasting <1 fs. Photoemission
takes place in the presence of a laser field
that does not ionize on its own, but instead
shifts the energy of the electron depending
on the moment at which it is emitted. In
these measurements, time can be mapped
onto electron energy or momentum.
Why is this valuable? Whereas conventional photoemission measurements are
only sensitive to real-valued photoemission cross sections, attosecond streaking
gives access to complex-valued probability
amplitudes of photoemission into a certain
direction. In this framework, the electron
wave packet emerging from a given energy
level is a duplicate of the attosecond pulse,
but with its phase shifted by the phase of
the responsible transition matrix element.
The first derivative of this phase with respect to energy is related to the Eisenbud-Wigner-Smith delay, originally studied in
the context of electron scattering (4). In a
single-electron atomic system, calculating
this delay is simple, but for more complex
quantum systems, theorists must make approximations, and experimental verification of their predictions puts these models
through a new line of scrutiny.
Apart from testing various assumptions
and approximations, one of the persisting
challenges is explaining these delays in
clear physical terms. For atoms, the centrifugal barrier experienced by an electron
wave with a nonzero angular momentum
was identified as one of the important factors that determine the group delay of an
electron wave packet. Does the centrifugal
barrier matter in a solid, where electrons
are exposed to a complex environment created by a crystal lattice and other electrons?
This was the question that Siek et al. asked.
Solids are much more complex than individual atoms, so the answer was not trivial.
To investigate this question, they per-
formed attosecond streaking measurements
on tungsten diselenide, WSe2 (see the fig-
ure). This van der Waals material consists
of alternating sheets of W and Se, with the
Electrons with high angular momentum are the last
to emerge from a solid
Max-Planck-Institut für Quantenoptik, Garching, Germany.
“…the local environment
of a bound electron, which
is dominated by atomic
potentials, leaves measurable
signatures in time-resolved