Dynamics scaling behavior and control of nuclear wrinkling Jonathan A. Jackson1 2Nicolas Romeo3 4Alexander Mietke3 5Keaton J. Burns3 Jan F. Totz3Adam C. Martin1J orn Dunkel3and Jasmin Imran Alsous6

2025-05-03 0 0 6.42MB 43 页 10玖币
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Dynamics, scaling behavior, and control of nuclear wrinkling
Jonathan A. Jackson,1, 2, Nicolas Romeo,3, 4, Alexander Mietke,3, 5 Keaton J. Burns,3
Jan F. Totz,3Adam C. Martin,1orn Dunkel,3, and Jasmin Imran Alsous6,
1Department of Biology, Massachusetts Institute of Technology
2Graduate Program in Biophysics, Harvard University
3Department of Mathematics, Massachusetts Institute of Technology
4Department of Physics, Massachusetts Institute of Technology
5School of Mathematics, University of Bristol
6Center for Computational Biology, Flatiron Institute, Simons Foundation
(Dated: June 27, 2023)
The cell nucleus is enveloped by a complex membrane, whose wrinkling has been implicated in
disease and cellular aging. The biophysical dynamics and spectral evolution of nuclear wrinkling
during multicellular development remain poorly understood due to a lack of direct quantitative
measurements. Here, we combine live-imaging experiments, theory, and simulations to characterize
the onset and dynamics of nuclear wrinkling during egg development in the fruit fly, Drosophila
melanogaster, when nurse cell nuclei increase in size and display stereotypical wrinkling behavior.
A spectral analysis of three-dimensional high-resolution data from several hundred nuclei reveals
a robust asymptotic power-law scaling of angular fluctuations consistent with renormalization and
scaling predictions from a nonlinear elastic shell model. We further demonstrate that nuclear wrin-
kling can be reversed through osmotic shock and suppressed by microtubule disruption, providing
tunable physical and biological control parameters for probing mechanical properties of the nuclear
envelope. Our findings advance the biophysical understanding of nuclear membrane fluctuations
during early multicellular development.
Wrinkling and flickering of flexible sheet-like structures
essentially determine mechanics and transport in a wide
range of physical and biological systems, from graphene
[1, 2] and DNA origami [3] to nuclear envelopes (NEs)
[4, 5, 12, 14] and cell membranes [8, 9]. Over the last
decade, much progress has been made through experi-
mental and theoretical work in understanding the effects
of environmental fluctuations on the bending behaviors
of carbon-based monolayers [10] and the shape deforma-
tions of lipid bilayer membranes of vesicles [11–13] and
cells [14, 61]. In contrast, the emergence and dynami-
cal evolution of surface deformations in NEs [12, 14, 15]
at different length- and timescales, to which we re-
fer throughout this paper simply as ‘wrinkling’, still
pose fundamental open questions, as performing three-
dimensional (3D) observations at high spatio-temporal
resolution remains challenging under natural physiolog-
ical and developmental growth conditions. Specifically,
it is unclear how NE wrinkle formation proceeds during
cellular development, which biophysical processes govern
wrinkle morphology, and whether there exist character-
istic scaling laws for NE surface fluctuations [5, 12, 17].
Addressing these questions through quantitative mea-
surements promises insights into the physics of complex
membranes and can clarify the biological and biomedical
implications of NE deformations that have been linked
to gene expression [12], cellular aging [18], and diseases
like progeria syndrome [4, 19].
These authors contributed equally
dunkel@mit.edu
jalsous@flatironinstitute.org
Here, we combine 3D confocal microscopy, theoretical
analysis, and simulations to characterize the wrinkling
morphology and dynamics of nuclear surfaces in fruit
fly egg chambers. A spectral analysis of over 300 nu-
clei provides evidence for an asymptotic power-law scal-
ing of the surface fluctuations, consistent with predic-
tions from renormalization calculations [48, 52] and scal-
ing arguments based on a nonlinear elasticity model for
thin shells. Although the scaling is found to be highly
robust against physical and biological perturbations, its
magnitude (prefactor) can be tuned via osmotic pressure
variation and microtubule disruption. These two differ-
ent control mechanisms enable the tuning and probing
of the NE’s spectral and mechanical properties, and pro-
vide biophysical strategies for suppressing and reversing
nuclear wrinkling.
The NE is a double membrane that separates the cell’s
nuclear interior from the surrounding cytoplasm. The
two concentric 4 nm-thick lipid bilayers are 20-50 nm
apart and are supported by the nuclear lamina, a non-
contractile meshwork of intermediate filaments that lie
adjacent to the inner nuclear membrane, conferring me-
chanical stability and affecting essential cellular processes
through regulation of chromatin organization and gene
expression [23]. Among other proteins, the NE contains
nuclear pore complexes, multi-protein channels that pri-
marily regulate passage of macromolecules between the
nucleus and the cytoplasm [24, 25]. Recent in vitro stud-
ies have provided key insights into the role of lamins,
cytoplasmic structures, and the physical environment in
affecting NE morphology, as well as evidence for the crit-
ical importance of nuclear shape for many cellular and
nuclear functions [4, 17], including transcriptional dy-
arXiv:2210.11581v4 [physics.bio-ph] 25 Jun 2023
2
d
e f
(i)
(ii)
(iii)
(iv)
Older egg chambers
Younger egg chambers
aB
g
Time proxy
100 150 200
10-3
10-2
10-1
Roughness
100101
Power spectrum
Angular number
10-6
10-4
10-2
0
1
-1
Time proxy
100 120 140 160 180 200
c
Nurse cell nuclei Oocyte
Follicle cell nuclei
b
Spectral
reconstruction
0 min 10 min
20 min 30 min
Nup107
Nup107
Nup107
Nup107
Data
80 109 140 166 186 217
FIG. 1. Dynamic wrinkling of nurse cell nuclear envelopes during Drosophila egg development. a, Maximum-
intensity projection (MIP) of a 3D image of an egg chamber expressing GFP-labeled Nup107, a component of the nuclear pore
complex. The wrinkled nuclei of the 15 nurse cells are substantially larger than those of the surrounding follicle cells. b, MIP of
four egg chambers showing an increase in nurse cell nuclear size and nuclear surface deformation as egg chambers age. Curved
arrows indicate developmental progression from youngest (i) to oldest (iv). c, MIPs of individual nurse cell nuclei from six egg
chambers spanning all ages included in our dataset, showing an increase in nuclear radius and NE wrinkling with age. Note
that scale bar is the same size for each image; oldest nucleus shown is about 2.3 times the diameter of youngest shown. d,
Spectral reconstruction of NE surfaces shown in cfrom 3D microscopy data using spherical harmonics with an angular number
up to lmax = 25 (Eq. (1), Methods). Time proxy values for each nucleus are included above the reconstructions. e, Power
spectra normalized by average radius for N= 78 nuclei from 39 egg chambers in nurse cells directly connected to the oocyte
(results are qualitatively similar for nuclei farther away from the oocyte; Supp. Fig. S4). Hashed area indicates approximate
noise threshold for young nuclei; color indicates the time proxy (corresponding to the color bar in d) as defined in the text and
detailed in SI Sec. III 1. f, NE roughness R=Pl3(2l+ 1)Plfor the same nuclei as in eincreases exponentially with time
proxy (see also Supp. Fig. S4). g, Snapshots of the same nucleus at four different time points illustrate that NE wrinkling
is a dynamic process (Supplementary Video 2). Blue and orange arrowheads point to wrinkles that disappear and appear,
respectively, between subsequent frames. Scale bars: 50 µm (a, b), 10 µm (c, g).
3
namics [12]. Despite notable progress, a quantitative
understanding of how wrinkling phenomenology and 3D
spectral properties of nuclear surfaces evolve in time and
during cellular development has remained elusive.
To investigate the biophysical dynamics, scaling be-
haviors, and reversibility of nuclear wrinkling, we used
the egg chamber of the fruit fly Drosophila melanogaster,
a powerful system amenable to 3D high-resolution live
imaging and targeted biological and physical perturba-
tions [26]. The egg chamber contains 15 nurse cells and
the oocyte (the immature egg cell), all connected via cy-
toplasmic bridges and enclosed by a thin layer of hun-
dreds of follicle cells (Fig. 1a, with schematics in Supp.
Fig. S1a, [27]). For most of the 3 days of oogenesis, the
nurse cells supply proteins, mRNAs, and organelles to
the oocyte through diffusion and microtubule-mediated
directed transport [31, 33, 34, 60]. To provide the prodi-
gious amount of material and nutrients that the oocyte
needs, each nurse cell replicates its DNA 10 times with-
out undergoing cell division, thereby notably increasing
its nuclear and cell sizes [35]. In the 30-hour window
studied here, the diameter of nurse cell nuclei in the cells
directly connected to the oocyte increases from approxi-
mately 16 to about 40 micrometers [33, 35], accompanied
by the progressive appearance of fold-like deformations in
the NE, providing an ideal test bed for studying the onset
and evolution of NE wrinkling (Fig. 1b,c).
To compare nurse cell nuclei within the same egg cham-
ber and across different egg chambers, we defined a proxy
measurement for developmental time (referred to here as
the ‘time proxy’) based on the geometric average of the
egg chamber’s length and width (Methods, SI Sec. III 1,
Supp. Fig. S1b,c). Since egg chamber geometry cor-
relates closely with developmental progression, adopt-
ing this continuous geometric characterization offers finer
temporal resolution than the traditional approach of dis-
tinguishing 14 discrete morphological stages [33, 34] (for
a comparison between the time proxy and developmen-
tal stage, see Supp. Fig. S1c). By time-ordering nu-
clei according to this metric, we could more accurately
determine the time of emergence of nuclear wrinkling
and reconstruct its evolution (Fig. 1b,c). To track the
NEs of the nurse cells in space and time, we used a
fluorescently-tagged version of the nuclear pore complex
protein Nup107 that delineates the nucleus (Supplemen-
tary Video 1; qualitatively similar wrinkling patterns
were observed using a different labeled protein in the
NE and via label-free imaging, see Supp. Fig. S2); note
that this label allows observation only of deformations
that include both membranes of the nuclear envelope,
but is unlikely to label deformations that include only
the inner membrane, such as Type I nucleoplasmic retic-
ula [34, 35]. Having acquired highly resolved 3D imag-
ing data (Fig. 1c, Supp. Fig. S3), we reconstructed the
nuclear surface radius R(θ, ϕ) relative to the geometric
center of the nucleus, where θand ϕare the spherical
polar angles.
To obtain a compact 3D spectral representation of
the nuclear surface deformations, we computed the real
spherical harmonic coefficients flm, defined by
R(θ, ϕ) =
lmax
X
l=0
l
X
m=l
flmYlm(θ, ϕ),(1)
where Ylm is the spherical harmonic with angular num-
ber land order m(Methods). Equation (1) allows for
a continuous reconstruction of the NEs (Fig 1d, Supp.
Fig. S3), with the mode-cutoff lmax setting the angular
resolution of the spectral representation (Methods). The
coefficient values {flm}depend on the choice of coordi-
nate system, that is, the orientation of the nuclei. To
obtain a rotation-invariant characterization of the sur-
face wrinkles, we consider the power spectrum of radial
out-of-plane deformations
Pl=4π
(2l+ 1)f2
00
l
X
m=l
f2
lm,(2)
normalized by the average radius of the shell R=
f00/4π. The non-negative numbers Plmeasure the
average power in a mode of angular wavenumber l. A
single-valued summary statistic of surface wrinkling can
be given in terms of the ‘roughness’ parameter R=
Pl3(2l+ 1)Pl, the total power contained in angular
numbers l3. By ignoring the long-wavelength modes
l < 3, Rmeasures the contribution of finer-scale wrinkles
to NE deformations. Our analysis of over 300 nurse cell
nuclei shows that the power spectrum of NEs maintains
an approximately constant shape as development pro-
gresses, but with a steadily-increasing amplitude (Fig. 1e;
Supp. Fig. S4), reflecting the fact that wrinkling becomes
more pronounced as nuclei increase in size. Rincreases
exponentially with the time proxy (Fig. 1f), suggesting
that nurse cell nuclei transition smoothly from an un-
wrinkled to a wrinkled state.
Nuclear surface wrinkling is a highly dynamic pro-
cess [12]. By imaging individual nurse cells at 40 s inter-
vals, we too observed that NE surface shapes fluctuate
substantially, with smaller features appearing and dis-
appearing faster than larger ones (Fig. 1g, Supplemen-
tary Video 2). Specifically, power spectra Plof repeat-
edly imaged nuclei changed on timescales of minutes or
faster (Supp. Fig. S4 and Supp. Fig. S5). The rotational
invariance of spectra implies that these fluctuations are
not the result of whole body rotations, but instead reflect
a rapid shape dynamics of NE surfaces. Experimental
limitations prevented quantification of timescales for the
entire 3D surface, but our observations are qualitatively
consistent with findings that smaller wrinkles typically
decay faster [36, 61]. Furthermore, the fact that the de-
formation spectrum is monotonically decreasing (Fig. 1e)
implies that there is no preferred wavelength, suggesting
that the observed NE shapes do not correspond to fluc-
tuations about the steady-states of buckled shells, but
instead reflect dynamic wrinkling across all experimen-
tally resolved angular scales.
4
ec
b
Angular number
Power spectrum
100101
Normalized Lamin C intensity
d
“Young”
“Old”
LamC
Nup107
a
80 100 120 140 160
Developmental coordinate
100101
Angular number
Power spectrum
60 100 140 180
0
0.1
0.2
0.3
0.4
0.5
0.6
Time proxy
Time proxy
60 100 140 180
0-1 1
100102.5 25
10-6
10-5
10-4
10-3
10-2 0.1 0.40.05 0.2
0.05 0.1 0.2 0.4
3.104
10-6
10-5
10-4
10-3
10-2
FIG. 2. Fluctuating elastic shell theory predicts a scaling law with exponent 3for the wrinkle power
spectrum, in agreement with experiments.a, Equilibrium simulation snapshots of nuclei at temperature Teff = 10Teq ,
undeformed radius R= 25 µm and Rc/R = 20, at fixed FvK number γ= 3 ×104for varying elastic moduli controlled by
kTeff. Color indicates the normalized deviation of the surface from the mean shell radius. b, Time-averaged spectra of
simulated NEs of undeformed radius R= 25 µm, Rc/R = 20, Teff = 10Teq for different moduli κ, Y at fixed γ= 3 ×104,
showing the transition from weak nonlinearity to strong nonlinearity as bending rigidity decreases. Color bar matches the dots
from a.c, Binned averages of spectra from nuclei in nurse cells directly connected to the oocyte reveal that shape fluctuations
follow a scaling law with an exponent between 3.2 and 8/3 that is obeyed throughout development. ‘Young’ nuclei have a
time proxy between 80 140, N= 29 nuclei, from 12 egg chambers; ‘Old’ nuclei have a time proxy between 160 220, N= 40
nuclei, from 22 egg chambers. Bars show extremal values. Hashed area indicates approximate noise threshold for young nuclei.
(See Supp. Fig. S4 for comparison between nuclei at different positions in the egg chamber) d, Fixed egg chambers expressing
Nup107::RFP and stained for Lamin C, showing a decrease in Lamin C intensity in nurse cell nuclei as egg chambers increase
in age. In contrast, Nup107::RFP intensity stays relatively constant. The same trend is observed in live imaging of ex vivo egg
chambers expressing LamC::GFP and Nup107::RFP (Supp. Fig. S6a). Wrinkling of nuclei in younger egg chambers (all but
the rightmost) is a result of fixation and is not observed in live imaging until later stages. Arrows indicate increasing age; egg
chamber boundaries are shown in dashed outlines. Scale bar: 50 µm. e, Normalized Lamin C fluorescence intensity decreases
by approximately 5-fold over time. Normalization details are specified in the Methods. N= 337 nuclei from 23 egg chambers;
colored dots show means for each egg chamber.
Both maximum-intensity projections and spectral re-
constructions show that NE wrinkles and creases are
sharp, with narrow bent regions separated by flatter areas
(Fig. 1). This morphology is reminiscent of the nonlinear
stress-focusing characteristic of crumpled elastic sheets
and shells such as ordinary paper sheets, which are much
more easily bent than stretched [1, 37, 38]. In partic-
ular, these geometric nonlinearities lead to anisotropic
responses when point forces are applied to the shell [38].
To rationalize the experimentally observed wrinkle mor-
phology at spatial scales larger than the NE thickness, we
constructed a minimal effective elastic model, describing
the NE as a deformed spherical shell (equilibrium radius
R). In spherical coordinates r= (θ, ϕ), the shell has an
5
isotropic elastic free energy [42, 48]
Fshell =Zd2rκ
2(2f)2+λ
2ϵ2
ii +µϵ2
ij ,(3)
where i, j ∈ {θ, ϕ}and using the Einstein summation
convention. The energy functional (3), accounts for
bending stiffness through a Helfrich-like bending term
that penalizes out-of-plane deformation f(positive when
pointing inwards), and the stretching of the membrane
through the nonlinear strain tensor ϵij . The 2D Lam´e
parameters λ, µ are proportional to the 2D Young’s mod-
ulus Y. The strain combines contributions from fand
from the in-plane deformation u(r) (SI Sec. IV). We also
allow for a preferred radius of curvature Rcof the shell
mismatched with the radius Rof the shell RcR, which
in the large-F¨oppl-von K´arm´an (FvK) regime leads to a
strain tensor ϵij =1
2(iuj+jui+ifjf)δij f/Rc
(SI Sec. IV). Previous work [44, 45, 67] has shown the
NE to be stiffer than most biological membranes and to
be well described as a thin membrane of a 3D isotropic
elastic material with an effective 3D Young’s modulus
E1 kPa and thickness of h10 100 nm (for a
more detailed discussion of limitations of fluid membrane
models, see SI Sec. IV 5), leading to a bending rigidity of
κ= 100300 kTeq 1018 J, where Teq is the room tem-
perature, and a stretching rigidity, captured by the 2D
Young’s modulus, of Y104N/m [43]. By construc-
tion, these moduli are approximately related through the
effective thickness hpκ/Y [48]. Note that Yis a fac-
tor of 103smaller than the stretching rigidity of a lipid
bilayer, potentially explained by the presence of ‘area
reservoirs’ in NEs and by transmembrane protein confor-
mational changes [44]. For a shell of radius R, one can
define the FvK number γ=Y R2which describes the
relative propensity of the material to bend rather than to
stretch. Using the above values, we find that the NE has
a large FvK number γ104106, comparable to that
of a sheet of paper or graphene [1]. Accordingly, the NE
is more amenable to bending than to stretching, and de-
formations are expected to appear as sharp wrinkles and
creases, in agreement with our observations (Fig. 1).
To compare the surface shapes and fluctuation pre-
dicted by Eq. (3) with our experimental data, we sim-
ulated the equilibrium Langevin PDE derived from this
free energy (see Methods and SI Sec. IV 4 for simulation
details). The simulations account for hydrodynamic cou-
pling and both passive and active fluctuations, which are
modeled by an effective temperature kTeff . Despite the
model’s minimal character and theoretical limitations of
Eq. (3) at long wavelengths where l0 (SI Sec. IV), the
numerically obtained shapes (Fig. 2a) are qualitatively
similar to those in the experiments (Fig. 1d). In the
experimentally accessible range of low-to-intermediate
angular wave numbers 3 l11, the angular spec-
tra extracted from the simulations at different ratios of
kTeff [0.05,0.5] (Fig. 2b) and experimental data
(Fig. 1e) also show an approximately similar decay, sug-
gesting that the minimal elastic shell model in Eq. (3)
captures relevant features of the NE, providing a basis
for further analysis and predictions.
A main feature of the experimentally measured spec-
tra is that both younger and older nuclei exhibit a sim-
ilar asymptotic power law decay in the limit of small
angular numbers l10 (Fig. 2c). To rationalize this
observation, we first note that the scaling behavior in
our experiments deviates from the basic linear response
theory predictions, which is expected because, even for
younger nuclei, the radial fluctuations ftypically exceed
the NE thickness h103R(Fig. 1c-f). More precisely,
for small fluctuations (fhR) and small thermo-
dynamic pressure (ppc= 4κY /R2
c, where pcis the
critical buckling pressure of the sphere), linear response
theory predicts that the power spectrum Plexhibits a
plateau for llcand falls of as l4for llcwith
a crossover value lcγ1/4pR/Rc(SI Sec. IV)[45, 48],
which is not seen in our experiments (Figs. 1e and 2c).
Indeed, classical shell theory [2] states that nonlinear ef-
fects become important when the out-of-plane deforma-
tions fbecome comparable to or exceed the shell thick-
ness h, which is generally the case in our data where
hfR(Fig. 1c,d,g). Nonlinear analysis of elastic
plates and shells has a long history [46, 52] and has seen
major advances in the last decade [42, 48], motivated
in part by the discovery of graphene [47]. As demon-
strated above, the FvK number of the NE is comparable
to that of graphene, so we can borrow and apply recent
theoretical results to understand the fluctuation spectra
of the NE. Specifically, a detailed renormalization group
(RG) analysis [48] of Eq. (3) showed that, for sufficiently
small plate fluctuations, elastic nonlinearities lead to a
modified asymptotic decay of Pll3.2, consistent with
our experimental and simulated data (Figs. 1e and 2b,c)
and with previous experiments in red blood cell spec-
trin networks [49]. Notably, earlier studies [42, 48, 52]
also predicted that the interplay of elastic nonlinearities
and fluctuations can cause the spontaneous collapse of
sufficiently large shells, suggesting a physical mechanism
that could contribute to the eventual breakdown of the
nurse cell NE when these cells donate their contents to
the oocyte [21, 60].
The previously mentioned RG methods can give rise
to divergences in large deformation regimes, where non-
linearities dominate the shell’s response (SI Sec. IV,
Fig. S9). To obtain an analytical prediction for the scal-
ing in the larger-deformation regime hfR < Rc,
relevant to older nuclei, we performed an asymptotic di-
mensional analysis that provides additional insight into
how NE wrinkling can be controlled. To that end, we
added to the elastic free energy Fshell an effective pres-
sure term Fp=Rd2rpefff, where peff accounts for a
normal load, which may arise from osmotic pressure dif-
ferences or microtubule-induced local stresses. Denoting
by Lthe characteristic surface variation length scale and
omitting numerical prefactors that depend on details of
the adopted thin-shell modeling approach (SI Sec. IV),
one finds for shells of thickness hpκ/Y that the vari-
摘要:

Dynamics,scalingbehavior,andcontrolofnuclearwrinklingJonathanA.Jackson,1,2,∗NicolasRomeo,3,4,∗AlexanderMietke,3,5KeatonJ.Burns,3JanF.Totz,3AdamC.Martin,1J¨ornDunkel,3,†andJasminImranAlsous6,‡1DepartmentofBiology,MassachusettsInstituteofTechnology2GraduatePrograminBiophysics,HarvardUniversity3Departm...

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Dynamics scaling behavior and control of nuclear wrinkling Jonathan A. Jackson1 2Nicolas Romeo3 4Alexander Mietke3 5Keaton J. Burns3 Jan F. Totz3Adam C. Martin1J orn Dunkel3and Jasmin Imran Alsous6.pdf

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