Hox genes have been implicated in specifying positional values along the anteroposterior axis of the caudal central nervous system, but their nested and overlapping expression has complicated the understanding of how they confer specific neural identity. We have employed a direct gain-of-function approach using retroviral vectors to misexpress Hoxa2 and Hoxb1 outside of the normal Hox expression domains, thereby avoiding complications resulting from possible interactions with endogenous Hox genes. Misexpression of either Hoxa2 or Hoxb1 in the anteriormost hindbrain (rhombomere1, r1) leads to the generation of motor neurons in this territory, even though it is normally devoid of this cell type. These ectopic neurons have the specific identity of branchiomotor neurons and, in the case of Hoxb1-induced cells, their axons leave the hindbrain either by fasciculating with the resident cranial motor axons at isthmic (trochlear) or r2 (trigeminal) levels of the axis or via novel ectopic exit points in r1. Next, we have attempted to identify the precise branchiomotor subtypes that are generated after misexpression and our results suggest that the ectopic motor neurons generated following Hoxa2 misexpression are trigeminal-like, while those generated following Hoxb1 misexpression are facial-like. Our data demonstrate, therefore, that at least to a certain extent and for certain cell types, the singular activities of individual Hox genes (compared to a combinatorial mode of action, for example) are sufficient to impose on neuronal precursor cells the competence to generate distinctly specified cell types. Moreover, as these particular motor neuron subtypes are normally generated in the most anterior domains of Hoxa2 and Hoxb1 expression, respectively, our data support the idea that the main site of individual Hox gene action is in the anteriormost subdomain of their expression, consistent with the phenomenon of posterior dominance.

Alexandre
D.
,
Clarke
J. D. W.
,
Oxtoby
E.
,
Yan
Y.-L.
,
Jowett
T.
,
Holder
N.
(
1996
)
Ectopic expression of Hoxa-1 in the zebrafish alters the fate of the mandibular arch neural crest and phenocopies a retinoic acid-induced phenotype.
Development
122
,
735
746
Carpenter
E. M.
,
Goddard
J. M.
,
Chisaka
O.
,
Manley
N. R.
,
Capecchi
M. R.
(
1993
).
Loss of Hoxa-1 (Hox-1.6) function results in the reorganization of the murine hindbrain.
Development
118
,
1063
1075
Chisaka
O.
,
Musci
T.
,
Capecchi
M.
(
1992
).
Developmental defects of the ear, cranial nerves and hindbrain resulting from targeted disruption of the mouse homeobox gene Hox-1.6.
Nature
355
,
516
520
Clarke
J. D. W.
,
Lumsden
A.
(
1993
)
Segmental repetition of neuronal phenotype sets in the chick embryo hindbrain.
Development
118
,
151
162
Covell
D. A.
,
Noden
D. M.
(
1989
)
Embryonic development of the chick primary trigeminal sensory-motor complex.
J. Comp. Neurol
286
,
488
503
Dolle
P.
,
Lufkin
T.
,
Krumlauf
R.
,
Mark
M.
,
Duboule
D.
,
Chambon
P.
(
1993
).
Local alterations of Krox-20 and Hox gene expression in the hindbrain of Hoxa-1 (Hox-1.6) homozygote null mutant embryos.
Proc. Natl. Acad. Sci. USA
90
,
7666
7670
Duboule
D.
(
1991
)
Patterning the vertebrate limb.
Curr. Opin. Genet. Dev
1
,
211
216
Duboule
D.
,
Morata
G.
(
1994
)
Colinearity and functional hierarchy among genes of the homeotic complexes.
Trends Genet
10
,
358
364
Gavalas
A.
,
Davenne
M.
,
Lumsden
A.
,
Chambon
P.
,
Rijli
F. M.
(
1997
)
Role of Hoxa2 in axon pathfinding and rostral hindbrain patterning.
Development
124
,
3693
3702
Gavalas
A.
,
Studer
M.
,
Lumsden
A.
,
Rijli
F. M.
,
Krumlauf
R.
,
Chambon
R.
(
1998
)
Hoxa1 and Hoxb1 synergize in patterning the hindbrain, cranial nerves and second pharyngeal arch.
Development
125
,
1123
1136
Goddard
J. M.
,
Rossel
M.
,
Manley
N. R.
,
Capecchi
M. R.
(
1996
)
Mice with targeted disruption of Hoxb1 fail to form the motor nucleus of the VIIth nerve.
Development
122
,
3217
3228
Guthrie
S.
,
Lumsden
A.
(
1992
)
Motor neuron pathfinding following rhombomere reversal in the chick embryo hindbrain.
Development
114
,
663
673
Hamburger
V.
,
Hamilton
H. L.
(
1951
)
A series of normal stages in the development of the chick embryo.
J. morph
88
,
49
92
Heaton
M. B.
,
Moody
S. A.
(
1980
)
Early development and migration of the trigeminal motor nucleus in the chick embryo.
J. Comp. Neurol
189
,
61
99
Hughes
S. H.
,
Greenhouse
J. J.
,
Petropoulos
C. J.
,
Sutrave
P.
(
1987
)
Adaptor plasmids simplify the insertion of foreign DNA into helper-independent retroviral vectors.
J. Virol
61
,
3004
3012
Krumlauf
R.
(
1993
)
Hox genes and pattern formation in the branchial region of the vertebrate head.
Trends Genet
9
,
106
111
Lufkin
T.
,
Dierich
A.
,
LeMeur
M.
,
Mark
M.
,
Chambon
P.
(
1991
).
Disruption of the Hox-1.6 homeobox gene results in defects in a region corresponding to its rostral domain of expression.
Cell
66
,
1105
1119
Lumsden
A.
,
Krumlauf
R.
(
1996
)
Patterning the vertebrate neuraxis.
Science
274
,
1109
1115
Maconochie
M. K.
,
Nonchev
S.
,
Studer
M.
,
Chan
S.-K.
,
Popperl
H.
,
Sham
M. H.
,
Mann
R. S.
,
Krumlauf
R.
(
1997
)
Cross-regulation in the mouse HoxB complex: the expression of Hoxb2 in rhombomere 4 is regulated by Hoxb1.
Genes Dev
11
,
1885
1895
Mark
M.
,
Lufkin
T.
,
Vonesc
J.-L.
,
Ruberte
E.
,
Olivo
J.-C.
,
Dolle
P.
,
Gorry
P.
,
Lumsden
A.
,
Chambon
P.
(
1993
)
Two rhombomeres are altered in Hoxa-1 mutant mice.
Development
119
,
319
338
McGinnis
W.
,
Krumlauf
R.
(
1992
)
Homeobox genes and axial patterning.
Cell
68
,
283
302
McKay
I.
,
Lewis
J.
,
Lumsden
A.
(
1997
)
Organization and developmentof facial motor neurons in the kreisler mutant mouse.
Eur. J. Neurosci
9
,
1499
1506
Popperl
H.
,
Blenz
M.
,
Studer
M.
,
Chan
S.-K.
,
Aparicio
S.
,
Brenner
S.
,
Mann
R. S.
,
Krumlauf
R.
(
1995
)
Segmental expression of Hoxb1 is controlled by a highly conserved autoregulatory loop dependent upon exd/pbx.
Cell
81
,
1031
1042
Pourquie
O.
,
Coltey
M.
,
Thomas
J. L.
,
LeDouarin
N.
(
1990
)
A widely distributed antigen developmentally regulated in the nervous system.
Development
109
,
743
752
Prince
V.
,
Lumsden
A.
(
1994
)
Hoxa2 expression in normal and transposed rhombomeres: independent regulation in the neural tube and neural crest.
Development
120
,
911
923
Simon
H.
,
Guthrie
S.
,
Lumsden
A.
(
1994
)
Regulation of SC1/DM-GRASP during the migration of motor neurons in the chick embryo brain stem.
J. Neurobiol
25
,
1129
1143
Simon
H.
,
Hornbruch
A.
,
Lumsden
A.
(
1995
)
Independent assignment of anteroposterior and dorsoventral positional values in the developing chick hindbrain.
Curr. Biol
5
,
205
214
Studer
M.
,
Lumsden
A.
,
Ariza-McNaughton
,
Bradley
A.
,
Krumlauf
R.
(
1996
)
Altered segmental identity and abnormal migration of motor neurons in mice lacking Hoxb1.
Nature
384
,
630
634
Tanabe
Y.
,
Jessell
T. M.
(
1996
)
Diversity and pattern in the developing spinal cord.
Science
274
,
1115
1123
Thor
S.
,
Ericson
J.
,
Brannstrom
T.
,
Edlund
T.
(
1991
)
The homeodomain LIM protein Isl-1 is expressed in subsets of neurons and endocrine cells in the adult rat.
Neuron
7
,
881
889
Tsuchida
T.
,
Ensini
M.
,
Morton
S. B.
,
Baldassare
M.
,
Edlund
T.
,
Jessell
T. M.
,
Pfaff
S. L.
(
1994
)
Topographic organisation of embryonic motor neurons defined by expression of LIM homeobox genes.
Cell
79
,
957
970
Varela-Echavarria
A.
,
Pfaff
S. L.
,
Guthrie
S.
(
1996
)
Differential expression of LIM homeobox genes among motor neuron subpopulations in the developing chick brain stem.
Mol. Cell. Neurosci
8
,
242
257
Zhang
M.
,
Kim
H.-J.
,
Marshall
H.
,
Gendron-Maguire
M.
,
Lucas
D. A.
,
Baron
A.
,
Gudas
L. J.
,
Gridley
T.
,
Krumlauf
R.
,
Grippo
J. F.
(
1994
)
Ectopic Hoxa-1 induces rhombomere transformation in mouse hindbrain.
Development
120
,
2431
24
This content is only available via PDF.