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Commentary
9
The tubby-like proteins, a family with roles in neuronal
development and function
Akihiro Ikeda, Patsy M. Nishina and Jürgen K. Naggert*
The Jackson Laboratory, 600 Main Street, Bar Harbor, ME 04609, USA
*Author for correspondence (e-mail: [email protected])
Journal of Cell Science 115, 9-14 (2002) © The Company of Biologists Ltd
Summary
The identification of a mutation at the tubby (Tub) locus,
which causes obesity and neurosensory degeneration, led to
the discovery of the tubby-like proteins (TULPs). Tub and
the genes that encode three tubby-like proteins (TULP1TULP3) form a novel, small gene family that plays an
important role in maintenance and function of neuronal
cells during development and post-differentiation.
Although exploration of the molecular function of these
genes is still in its infancy, recent biochemical studies have
provided ‘entry points’ into pathways whose elucidation
will further our understanding of TULP action. In
addition, mRNA expression and translocation of the TUB
protein have been shown to be regulated by thyroid
hormone and by G-protein-coupled receptor signaling,
respectively. These latter findings may help to link the
cellular function of TUB to known mechanisms for energy
homeostasis.
Introduction
Currently, four tubby gene family members (Tub, Tulp1, Tulp2
and Tulp3) (Kleyn et al., 1996; Noben-Trauth et al., 1996;
North et al., 1997; Nishina et al., 1998) have been identified in
humans and mice. These four genes probably comprise the
entire vertebrate family because only one gene has been found
in Drosophila (GenBank accession number: AE003453) and C.
elegans (AF143297), which is consistent with the proposed
two genome duplications that occurred during vertebrate
evolution (Gibson and Spring, 2000). Tubby-like genes are also
found in other organisms (i.e. Gallus (AJ396643), Xenopus
(AW638855), Zea Mays (AI737314) and Arabidopsis
(AC005309)). Plants in particular appear to harbor a large
number of tubby-like proteins (TULPs). The strong
conservation of Tub across members (Fig.1) and through
evolution suggests that the TULPs carry out some basic
function in cells in which they are expressed. This suggestion
is strengthened by the occurrence of disease phenotypes that
result from mutations within tub and Tulp1 (Kleyn et al., 1996;
Noben-Trauth et al., 1996; Heckenlively et al., 1995;
Ohlemiller et al., 1997; Hagstrom et al., 1998; Banerjee et al.,
1998; Lewis et al., 1999; Paloma et al., 2000) and by the
embryonic lethality of mice homozygous for a disrupted Tulp3
allele (Ikeda et al., 2001).
Determining the cellular function of this family is, therefore,
important not only for understanding the mechanisms that
cause the observed disease phenotypes but also for
understanding how they maintain the normal function of
TULP-expressing cells. Experimental evidence suggests that
TULPs have cellular roles in vesicular trafficking (Hagstrom
et al., 1999; Hagstrom et al., 2001), in mediation of insulin
signaling (Kapeller et al., 1999) and in gene transcription
(Boggon et al., 1999). Molecular studies also provide us with
clues about how expression of Tub and translocation of the
TUB protein is affected by pathways regulating feeding
behavior and energy balance (Koritschoner et al., 2001;
Santagata et al., 2001). Here, we discuss the recent advances
in genetic and molecular studies and the future directions that
might be explored in order to further our understanding of the
role of tubby gene family, particularly in neuronal cells.
Key words: Tubby, Tubby-like protein, neuron
TULPs are important for normal neuronal function in
animals
The original phenotype associated with a mutation in the Tub
gene, maturity onset obesity associated with insulin resistance
(Table 1) (Coleman and Eicher, 1990), was later expanded
to include sensory neural defects (retinal and cochlear
degeneration) (Heckenlively et al., 1995; Ohlemiller et al.,
1997). As the tubby gene is predominantly expressed in
neuronal cells, including those of the hypothalamus, it was
suggested that the late-onset obesity in tubby mice might reflect
defects in the neurons of the hypothalamic ‘satiety center’
(Kleyn et al., 1996). Pro-opiomelanocortin (POMC) and
neuropeptide Y (NPY) mRNA levels are significantly reduced
in the arcuate nucleus (ARC), whereas NPY levels are
increased 30-fold in the dorsomedial (DMH) and ventromedial
(VMH) hypothalamic nuclei of tubby mice compared with
wild-type controls (Guan et al., 1998). Although a reduction in
αMSH (melanin-stimulating hormone), an anorexigenic
peptide derived from POMC, might be expected to result in
increased body weight, the consequences of increased NPY
levels in the VMH and DMH are not known. In addition, the
αMSH expression changes were observed after the disease
onset and, therefore, may not be causative. The exact role of
TUB in the etiology of the obese phenotype thus remains
unclear. As TUB is expressed at comparable levels in all
neurons in the brain, it is possible that additional, more subtle,
phenotypes remain to be discovered.
Retinal degeneration characterized by apoptotic loss of
10
Journal of Cell Science 115 (1)
N-terminal region
Transcriptional activation
C-terminal region
DNA binding
PtdIns(4,5)P2 binding
Exon1
Fig. 1. Organization of the TUB, TULP3 and
TULP1 proteins. Each box represents amino
TUB
acids translated from each exon, and the
percentage amino-acid sequence similarity
18%
58% 62%
48% 62% 77% 93% 76% 90% 94% 92%
with TUB is shown. Green boxes indicate the
N-terminal regions with relatively low
TULP3
homology between family members. Red
28% 28% 35% 19% 39% 60% 91% 75% 90% 92% 97%
boxes indicate the C-terminal regions with
21%
high homology. The blue boxes represent the
TULP1
common acceptor site for alternative Nterminal exons observed in both human and
Nuclear localization signal
mouse TUB. Yellow and pink boxes represent
exons that are not conserved among family members. The nuclear localization signal is indicated. The N-terminal regions of TUB and TULP1
are able to activate transcription; the C-terminal region of TUB can bind to double-stranded DNA and to phosphatidylinositol-phosphates.
Table 1. Expression patterns and phenotypes arising from
mutations of the tubby gene family
Tub
Tulp1
Tulp2
Tulp3
Tissue expression
Phenotypes of mutants
Brain, eye, testis
Brain, eye
Testis
Ubiquitous
Obesity, retinal and cochlear degeneration
Retinal degeneration
ND*
Embryonic lethal with neural tube defect
*Not determined.
photoreceptor cells is a common phenotype observed in both
C57BL6-tub/tub (B6-tub/tub) and C57BL6.129-Tulp1tm1Jax
(Tulp1–/–) mice (Ikeda et al., 1999b; Hagstrom et al., 1999;
Ikeda et al., 2000). Although there are differences in
progression and severity of the degeneration, morphological
features of photoreceptor cell abnormalities in both the
spontaneous tub mutant and the Tulp1 knockout are similar
(Heckenlively et al., 1995; Hagstrom et al., 1999; Ikeda et al.,
2000).
Unlike tubby and Tulp1–/– mice, B6.129-Tulp3tm1Jax
(Tulp3–/–) mice exhibit embryonic lethality, with failure of
neural tube closure characterized by neuroepithelial apoptosis,
specifically in the hindbrain and the caudal neural tube (Ikeda
et al., 2001). Tulp3 is ubiquitously expressed throughout
embryonic development. The earliest observable phenotype of
the knockout mice is a significant reduction in the number of
βIII-tubulin-positive neurons in the hindbrain at embryonic
days (E) 9.5-10.5. Tulp3 appears, therefore, to be essential for
the maintenance or function of normally differentiating
neuronal cell populations. The neuroepithelial cell population
is known to be spatially subdivided into distinct classes of
neuron (Briscoe and Ericson, 2001). As apoptosis is restricted
to the ventral region of the neuroepithelium in the hindbrain of
Tulp3–/– embryos, there is selective cell death in specific cell
types in this targeted mutant model.
Taken together, the genetic analyses suggest that the primary
site of action of the tubby family genes is in neuronal cells,
including hypothalamic, sensory and differentiating embryonic
neurons. ‘Apoptosis’ of neuronal cells is common to all
mutations in tubby family genes. Do mutations within Tulp
genes directly drive the cell death pathway or does a functional
defect lead to the eventual cell death? In the case of the sensory
neural defects, the latter alternative is likely. Auditory
brainstem response (ABR) and electroretinographic (ERG)
recordings in tubby mice show a functional loss of hearing and
vision, respectively, before cell death (Ikeda et al., 1999a) (A.I.
et al., unpublished).
Gene-gene interactions determine the tubby
phenotype
The severity of the phenotypes observed in tubby mice is
affected by the genetic background (Ikeda et al., 1999a;
Nishina et al., 2000). For example, the profound hearing
loss in B6-tub/tub mice is completely rescued in tub/tub
homozygous progeny derived from intercrossing B6-tub/tub
mice with the mouse strains AKR/J, CAST/Ei and 129/Ola.
Linkage analysis identified a major modifier locus – modifier
of tubby hearing 1 (moth1) (Ikeda et al., 1999). A single moth1
allele from several different inbred strains protects tubby mice
from the hearing loss associated with a B6-derived recessive
moth1 allele. Homozygosity of the B6 moth1 allele itself
cannot induce hearing impairment in wild-type B6 mice. This
suggests that the allelic difference in the moth1 gene of B6
compared to the other inbred strains, therefore, is not critical
for maintaining their sensory neurons but becomes significant
only when the tubby gene is mutated and unable to function
properly. In addition, the moth1 locus maps to a region on
mouse chromosome 2 that also contains a gene that protects
tubby mice from retinal degeneration (Nishina et al., 2000).
This finding suggests that the same gene on chromosome 2 is
involved in a common pathway through which retinal and
cochlear degeneration are induced in tubby mice.
Molecular structure and genetic studies suggest a
common functional domain among TULPs
The C-terminal region is highly conserved among TULPs (Fig.
1), suggesting that this region might contain a functionally
important domain. This idea is supported by genetic studies.
The mutation within the tub gene is at a splice donor site in
the last intron, which results in the replacement of the last 44
amino-acid residues with 24 that are not observed in the normal
protein (Kleyn et al., 1996; Noben-Trauth et al., 1996). The
fact that the targeted tub null allele (Stubdal et al., 2000) has
the same phenotype as the spontaneous mutant confirms that
this splice-site mutation is also the molecular basis for the
The tubby-like proteins
spontaneous loss-of-function mutation. In addition, the same
type of splice-site mutation was observed in the TULP1 gene
among patients affected with retinitis pigmentosa (RP) 14 in a
large Dominican Republic kindred (Banerjee et al., 1998). A
clustering of mutations in the C-terminal region was also found
in spontaneous cases of RP, again emphasizing the functional
importance of this domain in Tubby family members.
To probe the function of this conserved region, Boggon et
al. (Boggon et al., 1999) crystallized the C-terminal domain of
TUB and performed X-ray crystallographic analyses. The
electron-density map obtained was interpreted as a unique
protein structure, a 12-stranded β-barrel conformation filled
with a central hydrophobic core that traverses the entire barrel.
Two prominent features of the folded protein are a large
groove of positively charged residues and a smaller region of
negatively charged residues on the opposite side of the groove.
This positively charged groove is expected to form proteinprotein or protein-DNA binding sites. Additional molecular
biological experiments showed that the TUB C-terminal
domain has the potential to bind double-stranded DNA
oligomers (Boggon et al., 1999). Given that a chimeric protein
composed of the N-terminus of TUB or TULP1 and a GAL4
DNA-binding domain can activate transcription, Boggon et al.
(Boggon et al., 1999) suggested that TUB might be a novel
class of transcription factor (Fig. 2b). So far, there is no
experimental evidence that TUB or TULP1 can recognize
specific DNA sequences, and targets for TUB transcriptional
activation have not yet been identified. Nevertheless, it is
tempting to speculate that the reduced Pomc and Npy mRNA
levels in the arcuate nucleus are the result of reduced TUB
transcriptional activity in the tubby mutant.
Compared with the C-terminus, the N-terminal region of
members of the tubby family is less well conserved; however,
clear similarities between the family members exist. In
particular, the exons containing the acceptor sites for the Nterminal splice forms and the putative nuclear-localization
signals are well conserved (Nishina et al., 1998) (Fig. 1).
Consistent with the presence of potential nuclear localization
signals, the N-terminus defines the nuclear localization of TUB
(He et al., 2000; Santagata et al., 2001).
Protein sequences are highly conserved among tubby gene
family members, which suggests that they perform the same
type of function within the cells in which they are expressed.
As described below, the observed mislocalization of rhodopsin
in both tubby and Tulp1–/– mice implies a common function
(Hagstrom et al., 1999; Hagstrom et al., 2001) (A.I., P.M.N.
and J.K.N., unpublished data). However, although expression
of each gene appears to overlap in several cell types, genetic
analyses suggest that they are not functionally redundant in all
tissues. Although both TUB and TULP1 are expressed in the
same hypothalamic neurons, tubby mice become obese but
Tulp1–/– mice do not, which suggests unique tasks for each
protein (Ikeda et al., 2000). This idea is also supported by the
observation that the distribution of TUB and TULP1 within the
nucleus of hypothalamic neurons is not overlapping (Ikeda et
al., 2000). In Tulp3–/– embryos, only a specific population of
neuronal cells undergoes apoptosis. One explanation for the
restricted cell death is that the ubiquitously expressed Tulp3 is
interacting with specific genes in a subset of developing
neuronal cells. Alternatively, although on an organismal level
there does not appear to be functional redundancy, it remains
11
possible that family members can substitute for each other in
specific cell types. In this context, it is interesting that
abnormalities are observed only in adult tub/tub mice, even
though the Tub gene is highly expressed in the neuroepithelium
at embryonic day E10.5 (Sahly et al., 1998), a time at which
the Tulp3 gene is also expressed (Ikeda et al., 2001). It is
possible, therefore, that Tulp3 compensates for Tub during
neuronal cell development.
Thyroid hormones T3 and T4 regulate Tub
expression
Koritschoner et al. (Koritschoner et al., 2001) identified Tub as
a potential T3-regulated gene in a differential display
experiment. The level of expression of Tub is lower in
hypothyroid rats, most significantly in Purkinje cells of the
cerebellum. Normal expression levels were restored by T3/T4
treatment. Tub expression was also upregulated by T3
treatment of cultured neuronal cells. These results suggest that
the expression of Tub is regulated by thyroid hormone, a
hormone known to be important in the regulation of body
weight (Krotkiewski, 2000) and neurosensory development
(Forrest et al., 1996; Ng et al., 2001). However, the significance
of these findings, although interesting, is not yet readily
apparent, because the major difference in expression levels was
observed in Purkinje cells. Although a role for the cerebellum
in food intake operant behavior has been suggested (Morimoto
et al., 1984; Lucchi et al., 1998; Mahler et al., 1993), whether
acute thyroid regulation in the cerebellum affects feeding
behavior is currently not known. Further studies of the
regulation of Tub by thyroid hormone in other phenotypically
relevant sites, such as the hypothalamic nuclei, could lead to
important contributions to our understanding of the functional
role of TULPs.
TUB as a mediator in the insulin signaling pathway
Kapeller et al. (Kapeller et al., 1999) reported that when the
Tub gene was transfected into chinese hamster ovary cells that
stably express insulin receptor (CHO-IR), the resultant TUB
protein was phosphorylated at tyrosine residues in response to
insulin and insulin-like growth factor treatment (Fig. 2a).
Furthermore, in vitro studies showed that TUB is directly
phosphorylated by the purified insulin receptor kinase, as well
as by ABL and JAK2 kinases, but not by the EGF receptor or
SRC kinases. The phosphorylated form of TUB appears to bind
to the SRC homology 2 (SH2) domains of ABL, LCK and
phospholipase Cγ. Therefore, TUB might function as an
adapter protein, linking the insulin receptor to SH2-containing
proteins. Since the tubby gene is prominently expressed in the
brain, this signaling pathway is likely to be active in the
neuronal cells in vivo. Interestingly, TUB and the insulin
receptors colocalize in neurons of the hypothalamus (Kleyn et
al., 1996; He et al., 2000; Unger et al., 1991; Carvalheira et al.,
2001), a structure known to be important for regulating energy
homeostasis. Recent studies using gene-targeted mice lacking
insulin receptor substrate 2 (IRS-2) and brain insulin receptor
(INSR) revealed that insulin signaling plays an important role
in the energy homeostasis in the brain (Burks et al., 2000;
Brüning et al., 2000). It will be interesting to see whether TUB
phosphorylation and/or translocation to the nucleus is impaired
12
Journal of Cell Science 115 (1)
(a) Signal transduction
(b) Translocation and
transcriptional activation
(c) Rhodopsin transport
Discs
+
Insulin receptor
G protein coupled receptor
microtubules
Gαq
PLC-γ
TUB
OS
PtdIns(4,5)P2
TUB P
PLC-β
CC
_
P
ABL
P
IS
TUB
JAK2
Cell
Body
Gene expression?
+
+
+
Gene expression?
TUB
Spherule
Vesicle
Rhodopsin;
Kinesin-II
Mislocalized rhodopsin
Fig. 2. Proposed molecular functions of TULPs. (a) TUB as a mediator of insulin signaling. TUB is phosphorylated by the insulin receptor
kinase domain, as well as by ABL and JAK2. This phosphorylation increases the binding capacity of TUB for proteins that have SH2 domains,
such as PLC-γ. (b) TUB as a nuclear signaling molecule. TUB is attached to the plasma membrane through PtdIns(4,5)P2. The signals from Gprotein-coupled receptors phosphorylate Gαq and release TUB through PLC-β activity. TUB released from the plasma membrane is
translocated to the nucleus where it may act as a transcription factor. (c) TUB as a facilitator of rhodopsin transport in rod photoreceptors. The
rod photoreceptor structure has five distinct components: the rod outer segment (OS) containing membrane discs stacked within a plasma
membrane envelope; the inner segment (IS) containing the biosynthetic machinery; the connecting cilium (CC) joining the OS and IS; the cell
body containing the nucleus; and the spherule representing the synaptic ending of the cell (Tai et al., 1999). Rhodopsin is manufactured in the
Golgi apparatus of the IS and packaged into vesicles. The C-terminal tail of opsin interacts with the dynein light chain and is transported along
microtubules to the cilium where the rhodopsin-containing vesicles fuse with the plasma membrane. Rhodopsin is then thought to be
transported to the outer segment (blue arrows) by kinesin-II (Tai et al., 1999; Marszalek et al., 2000). In Tulp1–/– mice, mislocalization of
rhodopsin is observed in the plasma membrane (red arrows). Since TULP1 is localized in the IS, it may be involved in the transport the vesicles
containing rhodopsin.
in the brains of knockout mice in which INSR has been
specifically ablated.
Regulation of TULPs by subcellular localization
A distinct functional role of the TULPs may also be conferred
by their subcellular localization. In hypothalamic neurons,
TUB is localized in the cytoplasm and nucleus (He et al.,
2000), whereas in photoreceptor cells it appears to be only in
the cytoplasm. In addition, although TUB was only detected in
the nucleolus in the hypothalamic neurons, TULP1 was present
in structures that are likely to represent the perinucleolar cap
and coiled bodies or gems in the nucleus but not the nucleolus
(Ikeda et al., 2000). Although it is currently not known why
TUB is differentially localized in different cell populations,
it is possible that regulatory factors that determine the
localization of TUB confer multiple functions upon the
TULPs. For example, we found that TUB is localized in the
nucleus of the precursor cells of the retinal photoreceptor cells,
which undergo proliferation, and TUB localization shifts to the
cytoplasm when these cells differentiate (unpublished data).
Nuclear transport of TUB protein induced by Gprotein-coupled receptor signaling
TUB is localized in the nucleus as well as in the plasma
membrane of cultured cells (Kapeller et al., 1999; Koritschoner
et al., 2001; Santagata et al., 2001). Santagata et al. (Santagata
et al., 2001) report that TUB is transported from the plasma
membrane to the nucleus through G protein αq (Gαq) activation
(Fig. 2b). Association of TUB with the plasma membrane
is thought to occur after its binding to PtdIns(4,5)P2
The tubby-like proteins
(phosphatidylinositol-4,5-bisphosphate), a phospholipid that is
highly enriched in the plasma membrane. Release of TUB is
triggered by receptor-mediated activation of Gαq through the
action of phospholipase C-β. These findings suggest potential
candidates for receptors that might regulate TUB in vivo
through Gαq, such as serotonin (Tecott et al., 1995), bombesin
(Ohki-Hamazaki et al., 1997), dopamine D1 (Sidhu, 1998),
melanocortin 4 (Huszar et al., 1997) and melaninconcentrating hormone receptors (Chambers et al., 1999;
Saito et al., 1999). These receptors are expressed in the
hypothalamus and influence feeding behavior and energy
homeostasis. Although transfection of serotonin receptor 2c
(5HT2c) induced nuclear translocation of TUB in vitro
(Santagata et al., 2001), and a mild obesity phenotype is
observed in 5HT2c null mutant mice (Tecott et al., 1995), other
hypothalamic receptors may also play a role in the obesity
observed in tubby mice. There is much that still needs to be
learned about the regulation of TULPs. Generally, nuclear
localization of proteins is regulated by their binding proteins
or conformational changes owing to post-translational
modification such as phosphorylation. Phosphorylation of
TUB may be important for its nuclear localization or,
alternatively, additional proteins may be necessary for TUB
translocation.
Mutations in TUB and TULP1 impair rhodopsin
transport
Extracellular vesicles accumulate near the inner segments of
the photoreceptor cells in both Tulp1–/– and tubby mice
(Hagstrom et al., 1999; Ikeda et al., 2000). Hagstrom et al.
(Hagstrom et al., 1999) hypothesized that the tubby family
normally functions in intracellular vesicular trafficking of
opsin and other transported factors (Fig. 2c). Indeed, ectopic
localization of rhodopsin is observed (Hagstrom et al., 2001)
(A.I., P.M.N. and J.K.N, unpublished) in both Tulp1–/– and
tubby mice. In addition, yeast two-hybrid analysis using TUB
as bait identified proteins that are associated with secretory
vesicles as binding partners (Hagstrom et al., 2000). These
results support a role for TUB and TULP1 in protein trafficking
in the photoreceptor cells of the retina.
Conclusion/Perspectives
Genetic and biochemical studies, which have focused on
identifying a common function for the members of the tubby
family, have provided insights into potential functions of
the TULPs in a wide variety of neuronal cell populations.
Continued functional studies aimed at elucidating tissuespecific functions may clarify how TULPs function in cells and
how the mutations within these genes cause particular disease
pathologies. For example, if TULPs act as transcription factors,
do they recognize specific DNA sequences within specific
tissues? What kinds of genes are regulated by TULPs? These
genes will certainly become key factors in understanding the
pathways through which TULPs act. But how can a role as a
transcription factor be reconciled with the observed rhodopsin
mislocalization and the localization of TUB to the nucleolus?
At this point, our current understanding of the function of the
tubby-like gene family is reminiscent of the three blind men
and the elephant, who after examining different parts of the
13
elephant came to vastly different conclusions about the nature
of the beast. Mutants for each tubby family gene studied thus
far show significant abnormalities and phenotypes that do not
always overlap despite overlapping expression patterns. It is
therefore not clear that all of the observed phenotypic
characteristics of Tulp mutations can be ascribed to a single
pathway with one function. Indeed, it seems possible that
TULPS are multifunctional proteins that may play multiple,
independent roles in normal cellular function and integrity.
We are grateful to Barbara K. Knowles and Edward H. Leiter for
careful review of the manuscript. Research in our laboratory was
supported by grants from Foundation for Fighting Blindness, NIH
DK59641, and AXYS Pharmaceuticals Inc. Institutional shared
services are supported by National Cancer Institute Support grant CA34196.
References
Banerjee, P., Kleyn, P. W., Knowls, J. A., Lewis, C. A., Ross, B. M., Parano,
E., Kovats, S. G., Lee, J. J., Penchaszadeh, G. K., Ott, J. et al. (1998).
TULP1 mutation in two extended Dominican kindreds with autosomal
recessive Retinitis pigmentosa. Nat. Genet. 18, 177-179.
Boggon, T. J., Shan, W. S., Santagata, S., Myers, S. C. and Shapiro, L.
(1999). Implication of tubby proteins as transcription factors by structurebased functional analysis. Science 286, 2119-2125.
Briscoe, J. and Ericson, J. (2001). Specification of neuronal fates in the
ventral neural tube. Curr. Opin. Neurobiol. 11, 43-49.
Brüning, J. C., Gautam, D., Burks, D. J., Gillette, J., Schubert, M., Orban,
P. C., Klein, R., Krone, W., Muller-Wieland, D. and Kahn, C. R. (2000).
Role of brain insulin receptor in control of body weight and reproduction.
Science 289, 2122-2125.
Burks, D. J., de Mora, J. F., Schubert, M., Withers, D. J., Myers, M. G.,
Towery, H. H., Altamuro, S. L, Flint, C. L and White, M. F. (2000). IRS2 pathways integrate female reproduction and energy homeostasis. Nature
407, 377-382.
Carvalheira, J. B., Siloto, R. M., Ignacchitt, I., Brenelli, S. L., Carvalho,
C. R., Leite, A., Velloso, L. A., Gontijo, J. A. and Saad, M. J. (2001).
Insulin modulates leptin-induced STAT3 activation in rat hypothalamus.
FEBS Lett. 500, 119-124.
Chambers, J., Ames, R. S., Bergsma, D., Muir, A., Fitzgerald, L. R.,
Hervieu, G., Dytko, G. M., Foley, J. J., Martin, J., Liu, W. S. et al. (1999).
Melanin-concentrating hormone is the cognate ligand for the orphan Gprotein-coupled receptor SLC-1. Nature 400, 261-265.
Coleman, D. L. and Eicher, E. M. (1990). Fat (fat) and tubby (tub): two
autosomal recessive mutations causing obesity syndromes in the mouse. J.
Hered. 81, 424-427.
Forrest, D., Erway, L. C., Ng, L., Altschuler, R. and Curran, T. (1996).
Thyroid hormone receptor beta is essential for development of auditory
function. Nat. Genet. 13, 354-357.
Gibson, T. J. and Spring, J. (2000). Evolution of sequences, structures, and
genomes. Biochem. Soc. Trans. 28, 259-264.
Guan, X. M., Yu, H. and Van der Ploeg, L. H. (1998). Evidence of altered
hypothalamic pro-opiomelanocortin/neuropeptide Y mRNA expression in
tubby mice. Brain Res. Mol. Brain Res. 59, 273-279.
Hagstrom, S. A., North, M., Nishina, P. M., Berson, E. and Dryja, T.
(1998). Recessive mutations in the gene encoding the tubby-like protein,
TULP1, in patients with retinitis pigmentosa. Nat. Genet. 18, 174-176.
Hagstrom, S. A., Duyao, M., North, M. A. and Li, T. (1999). Retinal
degeneration in tulp1–/– mice: vesicular accumulation in the
interphotoreceptor matrix. Invest. Ophthalmol. Vis. Sci. 40, 2795-2802.
Hagstrom, S. H., Yue, G., Scimeca, M. and Li, T. (2000). Search for TULP1
function by analyzing animal models and by screening for interacting
proteins. Invest. Ophthalmol. Vis. Sci. 41, S532 (Abstract 2833).
Hagstrom, S. A., Adamian, M., Scimeca, M., Pawlyk, B. S., Yue, G. and
Li, T. (2001). A role for the Tubby-like protein 1 in rhodopsin transport.
Invest. Ophthalmol. Vis. Sci. 42, 1955-1962.
He, W., Ikeda, S., Bronson, R. T., Yan, G., Nishina, P. M., North, M. A.
and Naggert, J. K. (2000). GFP-tagged expression and
immunohistochemical studies to determine the subcellular localization of
the tubby gene family members. Mo. Brain Res. 81, 109-117.
14
Journal of Cell Science 115 (1)
Heckenlively, J. R., Chang, B., Erway, L. C., Peng, C., Hawes, N. L.,
Hageman, G. S. and Roderick, T. H. (1995). Mouse model for Usher
syndrome: Linkage mapping suggests homology to Usher type I reported at
human chromosome 11p15. Proc. Natl. Acad. Sci. 92, 11100-11104.
Huszar, D., Lynch, C. A., Fairchild-Huntress, V., Dunmore, J. H., Fang,
Q., Berkemeier, L. R., Gu, W., Kesterson, R. A., Boston, B. A., Cone, R.
D. et al. (1997). Targeted disruption of the melanocortin-4 receptor results
in obesity in mice. Cell 88, 131-141.
Ikeda, A., Zheng, Q. Y., Rosenstiel, P., Maddatu, T., Zuberi, A. R.,
Roopenian, D. C., North, M. A., Naggert, J. K., Johnson, K. R. and
Nishina, P. M. (1999a). Genetic modification of hearing in tubby mice:
evidence for the existence of a major gene (moth1) which protects tubby
mice from hearing loss. Hum. Mol. Genet. 8, 1761-1767.
Ikeda, A., Ikeda, S., Gridley, T., Nishina, P. M. and Naggert, J. K. (2001).
Neural tube defects and neuroepithelial cell death in Tulp3 knockout mice.
Hum. Mol. Genet. 10, 1325-1334.
Ikeda, S., He, W., Ikeda, A., Naggert, J. K., North, M. A. and Nishina, P.
M. (1999b). Cell specific expression of tubby gene family members in the
retina. Invest. Ophthalmol. Vis. Sci. 40, 2706-2712.
Ikeda, S., Shiva, N., Ikeda, A., Smith, R. S., Nusinowitz, S., Yan, G., Lin,
T. R., Chu, S., Heckenlively, J. R., North, M. A. et al. (2000). Retinal
degeneration but not obesity is observed in null mutants of the tubby-like
protein 1 gene. Hum. Mol. Genet. 22, 155-163.
Kapeller, R., Moriarty, A., Strauss, A., Stubdal, H., Theriault, K., Siebert,
E., Chickering, T., Morgenstern, J. P., Tartaglia, L. A. and Lillie, J.
(1999). Tyrosine phosphorylation of tub and its association with Src
homology 2 domain-containing proteins implicate tub in intracellular
signaling by insulin. J. Biol. Chem. 274, 24980-24986.
Kleyn, P. W., Fan, W., Kovats, S. G., Lee, J. J., Pulido, J. C., Wu, Y.,
Berkemeier, L. R., Misumi, D. J., Holmgren, L., Charlat, O. et al. (1996).
Identification and characterization of the mouse obesity gene tubby: a
member of a novel gene family. Cell 85, 281-290.
Koritschoner, N. P., Alvarez-Dolado, M., Kurz, S. M., Heikenwalder, M.
F., Hacker, C., Vogel, F., Munoz, A. and Zenke, M. (2001). Thyroid
hormone regulates the obesity gene tub. EMBO Rep. 2, 499-504.
Krotkiewski, M. (2000). Thyroid hormones and treatment of obesity. Int. J.
Obes. Relat. Metab. Disord. 24, S116-S119.
Lucchi, M. L., Callegari, E., Barazzoni, A. M., Chiocchetti, R., Clavenzani,
P. and Bortolami, R. (1998). Cerebellar and spinal projections of the
coeruleus complex in the duck: a fluorescent retrograde double-labeling
study. Anat. Rec. 251, 392-397.
Lewis, C. A., Batlle, I. R., Batlle, K. G., Banerjee, P., Cideciyan, A. V.,
Huang, J., Aleman, T. S., Huang, Y., Ott, J., Gilliam, T. C. et al. (1999).
Tubby-like protein 1 homozygous splice-site mutation causes early-onset
severe retinal degeneration. Invest. Ophthalmol. Vis. Sci. 40, 2106-2114.
Mahler, P., Guastavino, J. M., Jacquart, G. and Strazielle, C. (1993). An
unexpected role of the cerebellum: involvement in nutritional organization.
Physiol. Behav. 54, 1063-1067.
Marszalek, J. R., Liu, X., Roberts, E. A., Chui, D., Marth, J. D., Williams,
D. S. and Goldstein, L. S. (2000). Genetic evidence for selective transport
of opsin and arrestin by kinesin-II in mammalian photoreceptors. Cell 102,
175-187.
Morimoto, A., Suzumi, M., Sakata, Y. and Murakami, N. (1984).
Activation of brain regions in rats during food-intake operant behavior.
Physiol. Behav. 33, 965-968.
Ng, L., Hurley, J. B., Dierks, B., Srinivas, M., Salto, C., Vennstrom, B.,
Reh, T. A. and Forrest, D. (2001). A thyroid hormone receptor that is
required for the development of green cone photoreceptors. Nat. Genet. 27,
94-98.
Nishina, P. M., North, M. A., Ikeda, A., Yan, Y. and Naggert, J. K. (1998).
Molecular characterization of a novel tubby gene family member, TULP3,
in mouse and human. Genomics 54, 215-220.
Nishina, P. M., Ikeda, A. and Naggert, J. K. (2000). Identification of QTLs
for protection from photoreceptor cell degeneration in tubby mice. Invest.
Ophthalmol. Vis. Sci. 41, S202 (Abstract 1057).
Noben-Trauth, K., Naggert, J. K., North, M. A. and Nishina, P. M. (1996).
A candidate gene for the mouse mutation tubby. Nature 380, 534-538.
North, M. A., Naggert, J. K., Yan, Y., Noben-Trauth, K. and Nishina, P.
M. (1997). Molecular characterization of TUB, TULP1, and TULP2,
members of the novel tubby gene family and their possible relation to ocular
diseases. Proc. Natl. Acad. Sci. 94, 3128-3133.
Ohki-Hamazaki, H., Watase, K., Yamamoto, K., Ogura, H., Yamano, M.,
Yamada, K., Maeno, H., Imaki, J., Kikuyama, S., Wada, E. and Wada,
K. (1997). Mice lacking bombesin receptor subtype-3 develop metabolic
defects and obesity. Nature 390, 165-169.
Ohlemiller, K. K., Hughes, R. M., Lett, J. M., Ogilvie, J. M., Speck, J.
D., Wright, J. S. and Faddis, B. T. (1997). Progression of cochlear and
retinal degeneration in the tubby (rd5) mouse. Audiol. Neuro-Otol. 2, 175185.
Paloma, E., Hjelmqvist, L., Bayes, M., Garcia-Sandoval, B., Ayuso, C.,
Balcells, S. and Gonzalez-Duarte, R. (2000). Novel mutations in the
TULP1 gene causing autosomal recessive retinitis pigmentosa. Invest.
Ophthalmol. Vis. Sci. 41, 656-659.
Saito, Y., Nothacker, H. P., Wang, Z., Lin, S. H., Leslie, F. and Civelli, O.
(1999). Molecular characterization of the melanin-concentrating-hormone
receptor. Nature 400, 265-269.
Sahly, I., Gogat, K., Kobetz, A., Marchant, D., Menasche, M., Castel, M.,
Revah, F., Dufier, J., Guerre-Millo, M. and Abitbol, M. M. (1998).
Prominent neuronal-specific tub gene expression in cellular targets of tubby
mice mutation. Hum. Mol. Genet. 7, 1437-1447.
Santagata, S., Boggon, T. J., Baird, C. L., Gomez, C. A., Zhao, J., Shan,
W. S., Myszka, D. G. and Shapiro, L. (2001). G-protein signaling through
tubby proteins. Science 292, 2041-2050.
Sidhu, A. (1998). Coupling of D1 and D5 dopamine receptors to multiple G
proteins: Implications for understanding the diversity in receptor-G protein
coupling. Mol. Neurobiol. 16, 125-134.
Stubdal, H., Lynch, C. A., Moriarty, A., Fang, Q., Chickering, T., Deeds,
J. D., Fairchild-Huntress, V., Charlat, O., Dunmore, J. H., Kleyn, P. et
al. (2000). Targeted deletion of the tub mouse obesity gene reveals that
tubby is a loss-of-function mutation. Mol. Cell Biol. 20, 878-882.
Tai, A. W., Chuang, J. Z., Bode, C., Wolfrum, U. and Sung, C. H. (1999).
Rhodopsin’s carboxy-terminal cytoplasmic tail acts as a membrane receptor
for cytoplasmic dynein by binding to the dynein light chain Tctex-1. Cell
97, 877-887.
Tecott, L. H., Sun, L. M., Akana, S. F., Strack, A. M., Lowenstein, D. H.,
Dallman, M. F. and Julius, D. (1995). Eating disorder and epilepsy in mice
lacking 5-HT2c serotonin receptors. Nature 374, 542-546.
Troutt, L. L. and Burnside, B. (1988). Microtubule polarity and distribution
in teleost photoreceptors. J. Neurosci. 8, 2371-2380.
Unger, J. W., Livingston, J. N. and Moss, A. M. (1991). Insulin receptors in
the central nervous system: localization, signalling mechanisms and
functional aspects. Prog. Neurobiol. 36, 343-362.