Science - 31 January 2020

have been reported to move within cells at
an average speed of 2mm/s ( 53 ), and poten-
tially greater mobility of translating mono-
somes may allow them to patrol and serve a
larger number of synapses. Given that one
polysome translates a single mRNA result-
ing in multiple copies of a single protein, the
relative scarcity of ribosomes imposes con-
straints on both the timing and diversity of
locally synthesized proteins. Neuropil-localized
transcripts exhibited a greater monosome pref-
erence than somatic transcripts, potentially
allowing for the production of a more diverse
set of proteins from a limited pool of available
ribosomes at synapses.
Monosome-preferring transcripts encoded
proteins that span a broad range of abun-
dancesintheneuropil.Becausemanysynap-

tic proteins are present at very low copy

numbers within the pre- and postsynaptic

compartments [e.g., AMPA receptors; esti-

mated ~15 to 20 per postsynaptic density

(PSD)] ( 54 ), their local translation by single

ribosomes may suffice to maintain or even

alter synaptic activity. We also uncovered a

subset of monosome-preferring transcripts

that encode surprisingly high-abundance pro-

teins, including the scaffolding proteins Bsn

and Dlg3. This subset also exhibited increased

RNA levels and translation rates within the

neuropil. These features might underlie the

ability of these monosome-preferring tran-

scripts to encode abundant proteins. On the

other hand, predominant polysome transla-

tion was observed for key signaling, scaffolding,

or cytoskeletal proteins (e.g., Camk2a, PSD95,

and actin), which are present at very high

copy numbers within synapses ( 54 ). Many

studies investigating translational control in

synaptic plasticity or neurological disorders

have focused their analysis on transcripts that

cosediment with polysomes ( 9 , 55 – 57 ). Given

that monosomes are key contributors to the

neuronal translatome, focusing on polysome-

associated transcripts may provide an incom-

plete picture of translational regulation.

Most transcripts exhibited a similar M/P

preference in both the somata and neuropil,

suggesting that ribosome occupancy is often

an intrinsic feature of the transcript. Consist-

entwiththisfinding,wedetectedaposi-

tive correlation between M/P ratio and ORF

length, in agreement with previous studies

reporting decreased ribosome density and

Bieveret al.,Science 367 , eaay4991 (2020) 31 January 2020 6of14

0

12

0

12

0

12

0

12

neuropil mono

neuropil poly

somata mono

somata poly

Arc 1 kb

B

0

64

0

64

0

64

0

64

neuropil mono

neuropil poly

somata mono

somata poly

Serpini1 1 kb

D

2

0.00

0.25

0.50

0.75

1.00

ecdf

monosome vs polysome (log 2 FC)

-2 -1 0 1

p = 9.215e-15

I (npl mono, soma mono)

IV (npl poly, soma mono)

3

2

1

0

-1

-2

monosome vs polysome (log-3

FC) 2

neuropil

monosome vs polysome (log 2 FC)

somata

3210-1-2-3

A

II (npl mono, soma poly)

III (npl poly, soma poly)

mono:poly higher

in neuropil

mono:poly higher

in somata

E

neuropil

somata

mono:poly higher

in neuropil

F

0.00

0.25

0.50

0.75

1.00

ecdf

monosome vs polysome (log 2 FC)

-2 -1 0 1 2

p = 1.692e-08

C

0.00

0.25

0.50

0.75

1.00

ecdf

monosome vs polysome (log 2 FC)

-2 -1 0 1 2

p = 6.128e-05

mono:poly higher

in somata

neuropil

somata

mono:poly

overall neuronal

neuropil

somata

n = 36

n = 136

Fig. 5. Localization influences the translational status of selective transcripts.
(A)M/Plog 2 fold changes (FC) in the neuropil (yaxis) versus the somata (xaxis).
The majority of transcripts exhibited correlated (R^2 =0.6,P<2.2×10−^16 )M/P
enrichments between both compartments. Colored dots highlight transcripts that
exhibit significantly increased (cyan,n= 136 replicates) or decreased (purple,n=36
replicates) M/P log 2 fold changes in the neuropil compared with the somata. DESeq2
was used for analysis, with a threshold of 0.05 on the adjustedPvalue (see Materials
and methods). Numerals represent thedifferent quadrants. Gray dots denote
transcripts that do not exhibit significant changes in M/P log 2 fold changes between
compartments. (BandC) Example (Arc) (B) and cumulative distribution frequency of

the M/P log 2 fold changes (C) of transcripts exhibiting significantly higher M/P ratios

in the somata (purple) compared with the neuropil (dark purple).P=6.128×10−^5 ,

Kolmogorov-Smirnov test. ecdf, empirical cumulative distribution function.

(DandE) Example (Serpini1) (D) and cumulative distribution frequency of the

M/P log 2 fold changes (E) of transcripts exhibiting significantly higher M/P ratios

in the neuropil (dark cyan) compared to the somata (cyan).P= 9.215 × 10−^15 ,

Kolmogorov-Smirnov test. (F) Cumulative distribution frequency depicting the

M/P log 2 fold changes of all neuronal genes [cyan, purple, and gray dots in (A)]

in the somata (red) and neuropil (blue), indicating an overall tendency toward

higher M/P ratios in the neuropil.P=1.692×10−^8 , Kolmogorov-Smirnov test.

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