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HEMATOPOIESIS
From the Ontario Cancer Institute and the Department of
Medical Biophysics, University of Toronto, Ontario, Canada.
Although hematopoiesis is known to proceed from stem cells through
a graded series of multipotent, oligopotent, and unipotent precursor
cells, it has been difficult to resolve these cells physically
one from another. There is, therefore, corresponding uncertainty about
the exact distribution and timing of the expression of genes known to
be important in hematopoietic differentiation. In earlier work, the
generation of a set of amplified complementary DNAs (cDNAs) from
single precursor cells was described, whose biologic potential
was determined by the outcome of cultured sibling cells. In this study,
the new acquisition of cDNA from multipotent myeloid precursor cells is
described, as is the mapping of RNA-level expression of 17 distinct
cytokine receptors (c-kit, Flk-1, Flk-2/Flt-3, c-fms,
gp130, erythropoietin receptor, GM-CSFR Maintenance of the various hematopoietic lineages
is achieved by the continuous growth and differentiation of a hierarchy of precursor cells in the blood-forming tissues. Positioned at the top
are rare, multipotent stem cells possessing a unique capacity for
extensive, if not permanent, self-renewal.1 Stem cell
progeny progress through a series of pluripotent and eventually
bipotent intermediates of diminishing self-renewal ability and
differentiative repertoire.2-5 These yield, in
turn, to precursors restricted to development along single
hematopoietic lineages.
The genes and mechanisms that specify lineage divergence are still only
dimly understood. Cell purification has been generally considered a
crucial step to eventual identification of the genes whose differential
expression would account for key differentiation decisions. However,
presently known markers have not cleanly resolved the variety of
differentiation intermediates known to exist on the basis of clonal
analyses. Although multipotent cell lines have frequently been assumed
to offer a solution to the problem of heterogeneity,6 the
evidence clearly indicates heterogeneity comparable to normal
hematopoietic tissue and that only infrequent cells within the lines
actually remain multipotential.7-10
A different approach was recently described4 that allows
isolation of precursor subsets in homogeneous form and supports analysis of global patterns of gene expression. The method is based on
the synchronous nature of differentiation observed in nascent colonies
or "starts" consisting of 4 to 8 cells. Single precursor cells are
drawn from such starts, and their transcripts are amplified by global
reverse transcription-polymerase chain reaction (RT-PCR). The
remaining siblings are cultured individually in conditions supportive
of multilineage development, and their clonal outcomes are determined.
Identical differentiative outcomes of each sibling reporter allow
tentative assignment of the same potential to the cells processed for
complementary DNA (cDNA).
In our earlier study, we described construction in this way of a set of
cDNA samples from tripotent, bipotent, and unipotent hematopoietic precursor cells derived from murine bone
marrow.4 Hybridization analysis of the sample set with a
small number of probes for lineage-specific transcripts provided
confirmation of the lineage assignments, particularly of the more
mature elements of the set. However, there was at that time only a
limited representation of pluripotent elements. Moreover, validation of
pluripotential assignments was less likely to be achievable with probes
characteristic of maturing cells, leaving the challenge of identifying
probes that might have known specificity for individual pluripotent intermediates.
Hematopoiesis is known to be driven by a variety of cytokines, some of
which act with distinct differential specificity on pluripotential
precursors at particular stages or on committed precursors in
particular lineages.11 Such response patterns indicate
that the timed initiation of expression of the cognate factor receptors
is likely to play a central role in the sequence of events leading to
lineage divergence and establishment in the hematopoietic system. An
understanding of the earliest steps of the process will, therefore,
require a description of where and when expression of the key receptors
begins, particularly in the precursor intermediates that are presently
unresolved by physical techniques. Such an analysis is now possible
based on our single-cell cDNA approach. In addition to its importance
in resolving receptor expression, such an analysis, if congruent with
known patterns of differential responsiveness of precursor subsets
to particular cytokines, would add substantially to confidence in the
validity and specificity of our cDNA sample matrix.
In this study, we describe the acquisition of new cDNA samples from
multipotent precursor cells from adult marrow, including pentapotent
and tetrapotent instances, and report on the distribution of expression
of transcripts for 17 distinct cytokine receptors using the expanded
cDNA sample matrix. The resultant data are the first to resolve
expression of these transcripts among hematopoietic precursors at
different commitment stages, and they provide essential validation of
the integrity and fidelity of our hierarchy sample set.
The cDNA samples characterized in this study were generated in 2 sets. Collection and characterization of the initial set was described
in detail.4 It comprised 69 individual cDNA samples from
hematopoietic cells ranging from tripotent precursors to terminally
maturing cells in erythroid, myeloid, and lymphoid lineages. In this
study we describe the generation of 86 additional samples. The new
instances significantly augmented the sample sizes of precursor stages
already included in the original set (tripotent E/Meg/Mac, bipotent
E/Meg, committed Meg and Mac precursors) and of terminally
maturing cells. Of greatest interest, the work extended representation
of the sample archive to more primitive tetrapotent and
pentapotent cells.
Cells and cultures
Terminally maturing hematopoietic cells were sampled from single
lineage colonies growing in methylcellulose cultures containing IL-1,
IL-3, and erythropoietin.4 Mast cells were generated from
marrow cells cultured 4 to 7 weeks in medium containing IL-3 and
concanavalin A.4 Mature T cells were sampled from
ConA-stimulated, 2- to 4-day cultures of CD4+ or
CD8+ T cells isolated from adult C57BL/6J spleens. NIH3T3
and 95/1.7 fibroblast monolayers14 were grown in
serum-containing medium.
For sampling of precursor cell cDNA and for assessment of developmental
potential of sibling cells, colony starts from marrow cell precursors
were initiated in sparsely seeded methyl cellulose cultures4,13 containing IL-1, c-kit ligand,
IL-3, IL-11, 5637 bladder carcinoma cell-conditioned medium, and
erythropoietin. The differentiation potential of sister cells was
tested at 4 days by micromanipulating single siblings into gridded
secondary methylcellulose plates and culturing them in the same
cytokines. Small numbers of cells were drawn at intervals from growing
secondary colonies and were stained on glass slides with
May-Grünwald-Giemsa for morphologic assessment.4
Megakaryocytic lineage cells were identified by histochemical staining
for acetylcholinesterase.15 For detailed assessment of
lineage content of large, actively growing secondary colonies, cells
were dispersed and subcultured at 7 to 9 days into tertiary
methylcellulose plates where erythroid and other myeloid growth could
more readily be identified. They were also subcultured into liquid
medium containing IL-3 and ConA for 4 to 7 weeks, after which cells
were stained with Alcian blue for specific detection of mast
cells.16
Amplified cDNA
The individual cDNA samples generated in this study were hybridized
with cDNA probes for constitutive housekeeping transcripts (L32) and
lineage-specific transcripts For hybridization analysis of cytokine receptor probes, amplified cDNA samples from cells at similar positions in the hematopoietic differentiation hierarchy were pooled (0.5 µL per sample) and re-amplified. Seventeen such pools, each representative of a distinct stage, were prepared, and 20 µL each was slot-blotted onto Hybond N+ nylon membranes (Amersham, Uppsala, Sweden). Each blot also included a "blank" control consisting of a similar quantity of DNA amplified in a global RT-PCR reaction performed in the absence of a cellular template and determined by sequencing to consist of primer concatamers. For secondary target-specific PCR, globally amplified cDNA was diluted 1:10 000 into PCR buffer (10 mM Tris, pH 8.3, 50 mM KCl, 1.5 mM Mg++, 2 mM dNTPs, 1 µM specific upstream and downstream primers) and amplified with Taq polymerase through 35 or 40 cycles (94°C at 15 seconds/60°C at 30 seconds/72°C at 60 seconds). The amplified product was visualized after electrophoresis on 1.8% agarose gels containing ethidium bromide. Gels were then blotted onto Hybond N+ nylon membranes, and the membranes were probed with the corresponding radiolabeled specific amplification product. Probes Specific probes, detailed in Table 1, were designed to target terminal 3' untranslated sequences close to the polyadenylation sites. Probes prepared as PCR fragments were amplified with the indicated primer pairs enclosing terminal 3' sequences, using globally amplified cDNA from appropriate cell lines or primary cell sources as templates. For the design of PCR primers, extra care was taken to ensure proximity to the true 3' termini. From posted GenBank cDNA sequences, the 3' termini were used to probe the public murine EST databases to confirm termination at the same positions in ESTs prepared with oligo(dT) primers. For c-kit, IL-2R , and IL-7R , which lacked
classical polyadenylation signals in the GenBank sequences, EST
sequence clusters were found that extended the posted sequences (for
c-kit, by 19 bases; IL-2R, 660 bases; IL-7R, 250 and
650 bases, to alternative polyadenylation sites) and contained
appropriate polyadenylation signals (Table 1 footnote). These revisions
have been posted as annotations to the NCBI reference sequences listed
in the footnote to Table 1. Once primers directed against extreme 3'
tracts were designed, the enclosed sequences of the intended
probes were screened against the GenBank nonredundant nucleotide
database to ensure unique specificity for the target transcripts. All
amplified fragments were validated by size determination and
sequencing.
After labeling of the probes with 32P by random priming
(oligolabeling kit; Pharmacia, Uppsala, Sweden), blots were hybridized with 10 mL probe containing at least 106 cpm/mL in buffer
containing 1% sodium dodecyl sulfate, 1M NaCl, 10% dextran sulfate,
and 100 µgm/mL salmon sperm DNA, at 70°C (c-kit, 60°C)
for 16 hours. Blots were washed twice for 1 hour in 0.2 × SSC and 0.1% sodium dodecyl sulfate at 70°C. Autoradiographs were
exposed at
Sampling of cDNA from individual multipotent hematopoietic precursor cells Genes with roles in self-renewal and lineage commitment decisions are likely to be expressed in multipotent cells that possess these capacities, and a subset of such genes might be expected to be expressed with greatest intensity in the earliest precursor stages. This study describes a crucial step in establishing a system able to detect genes that display strongly stage-specific patterns of expression.Our earlier work4 established that when precursor cells in
adult marrow initiate colony growth in culture, individual cells replated from a given 4- to 8-cell colony start usually generate colonies of identical composition. When colonies are grown from unseparated murine marrow cells, the great majority contain only a
single differentiating lineage, and, not surprisingly, individual cells
from such colony starts display identical unilineage potential (Figure
1, top panels).
To enhance the proportion of colony starts developing from pluripotential precursors in our earlier study, we eliminated the most rapidly cycling precursors from marrow in vitro by cold thymidine arrest overnight4 before plating in semisolid medium. The middle panels in Figure 1 summarize a small result set obtained with the same technique used in the present work, showing the enhanced proportion of clonal starts displaying potential for generating progeny in 2 and occasionally 3 lineages. Of all starts with bilineage outcomes, 60% yielded a uniform set of bilineage colonies from the subcultured siblings (middle right panel). As in our original study, colony starts displaying potential to generate cells in more than 3 myeloid lineages were rare. To improve further the potential for sampling colony starts from uncommitted precursor subsets, we tested marrow cells that had been enriched by cell sorting for the stem cells measured by long-term in vivo reconstitution. The RholoLy6Ahi subfraction of adult marrow contains 85% of the long-term reconstituting activity of marrow while excluding 99% of the more advanced clonogenic cells.12,13 When colony starts were initiated from this fraction, half the starts with dispersed morphology13 contained cells able to differentiate into 3 to 5 distinct myeloid lineages (Figure 1, bottom panels). Mast cell potential was monitored specifically by testing the ability of sibling subcolonies to spawn 6- to 12-week IL-3-dependent liquid cultures of Alcian blue-positive mast cells. The proportion of multiple (3-4) lineage starts yielding uniformly multilineage subclones was 45%. Even in the case of pentapotent starts, half yielded subclones that uniformly contained at least 4 lineages (erythroid, megakaryocyte, macrophage, and neutrophil), though some subclones failed to spawn long-term mast cell cultures (Figure 1, lower right panel). Thus, approximately half of the multipotent clonogenic cells obtained from the stem cell fraction were able to undergo 2 to 4 serial divisions without restricting their myeloid differentiation potential. In view of these results, we concentrated on dispersed morphology starts generated from the stem cell-enriched fraction. From starts consisting of 4 to 16 cells, one or more cells were individually processed to global cDNA, while their siblings were subcultured to determine their lineage potential. Eight starts were identified with potential to differentiate into at least erythroid, megakaryocyte, monocyte, and neutrophil lineages. From these, 15 single-cell cDNA samples were harvested. Sibling cells from 5 of the 8 starts generated tetralineage colonies in methylcellulose. In 3 of these 5, all siblings generated tetralineage colonies, whereas in 2 others some siblings generated tetralineage colonies and others yielded colonies containing 3 or fewer lineages. In the remaining 3 of 8 multilineage starts, most of the colonies from sibling cells also yielded long-term liquid cultures of mast cells. The 4 associated cDNA samples from these "pentapotent" instances were segregated for separate analysis (Figure 1). Because these numbers are small, it was not anticipated that hybridization differences between the tetrapotent and pentapotent sets could be considered significant. Additional cDNA samples were harvested from tripotent, bipotent, and
unipotent precursor cells
For investigation of stage-specific gene expression, single-cell cDNA
samples from each precursor or mature cell type were pooled, and each
pool was transferred as a single slot onto hierarchy blots. Pooling was
intended not only to simplify the generation of blots but also to
average out the marked variation in transcript expression observed
at the single cell but not population levels.4,20 Two
alternative blot layouts are shown in Figure 2, indicating the numbers
of samples pooled for application to each slot. Also shown is the
result obtained when a blot with layout A was hybridized with a probe
for L32, a constitutively expressed "housekeeping" transcript
encoding a ribosomal subunit protein. The transcript was detected as
expected in all cellular samples. A background control ("blank"),
consisting of an equivalent amount of primer elongation artifact
amplified in the global RT-PCR reaction in the absence of cell-derived
mRNA template, did not bind the probe. In the lower right panel of
Figure 2, the autoradiographic images from each individual slot were
redimensioned and positioned over the corresponding locations in the
lineage tree. This arrangement allows rapid assessment of differential
hybridization intensities Hybridization of the L32 probe to the pools prepared from pentapotent and tetrapotent cells was frequently weaker than for most of the other pools, though the difference was not marked in the exposure shown in Figure 2. When the single-cell cDNA components of each pool were tested individually, each of the 4 pentapotent samples hybridized weakly, whereas 6 of the 11 tetrapotent samples hybridized more strongly. To investigate the basis for the difference, the pentapotent pool and a pool of the strongly hybridizing tetrapotent subset were cloned separately into plasmid libraries. A random sampling of 31 plasmid inserts from the pentapotent pool revealed 84% to consist of primer artifact rather than bona fide cDNA. In contrast, all 39 inserts from the strongly hybridizing tetrapotent subset were cell-derived. Hierarchy blot analysis of cytokine receptor transcript expression To assess expression of cytokine receptor transcripts, hierarchy blots were hybridized with radiolabeled receptor cDNA probes. Because the global RT-PCR procedure begins with oligo(dT) priming, efficient detection of transcripts in the amplified product requires probes that include 3' terminal sequence. Two levels of analysis were performed. An initial screen was carried out with probes prepared simply by isolating nominal 3' termini from supplied plasmid probes. In most instances, the resultant hybridization signals were convincingly above background, and in those instances further experimentation was not performed. However, a number of probes failed to hybridize above background even though cells at particular stages were known to be responsive to the corresponding cytokines. These negative results prompted a second level of analysis. The first step was to ensure that the probes were indeed inclusive of the extreme 3' ends of their target transcripts. In 3 instances (c-kit, IL-2R, IL-7R), the available GenBank
mRNA sequences were incomplete. These were successfully extended by
identification of contiguous EST clusters containing polyadenylation
signals followed by polyA tails (detailed in "Materials and
methods"). Suitable primer pairs enclosing terminal sequence were
devised and used to amplify the corresponding fragments from globally amplified cDNA, as indicated. Once the sequence of the amplified fragments was confirmed, they were radiolabeled and used as probes on
hierarchy blots. Additionally, the primers were used in secondary PCR
reactions applied to each global cDNA pool represented on the
hierarchy blots.
Cytokine receptor transcripts expected to demonstrate lineage-restricted expression. Once a matrix of cDNA samples was assembled that had an expanded representation of uncommitted stages, it became feasible to compare RNA-level expression patterns of receptors for cytokines acting before and after lineage commitment to the corresponding biologic response profiles. For heteromultimeric receptors, we chose to probe only for subunits associated with ligand specificity. Subunits of 17 distinct cytokine receptors were analyzed. Most hematopoietic cytokines have been described as influencing cellular targets at multiple stages and frequently in more than a single lineage. Three cytokines most associated with dominant action in a single lineage provided an opportunity to compare expression both across lineages and through transitions from multiple to single potentiality. Erythropoietin is the archetypal lineage-specific cytokine whose biologic effects are considered confined mainly to the erythroid lineage. Moreover, its stage of action within that lineage is well defined, commencing only after unique commitment has occurred and centering on precursor cells called colony-forming unit (CFU-E) that immediately precede the onset of globin synthesis.21 As shown in Figure 3, hybridization of an EpR probe was decidedly strongest over cDNA from CFU-E, exactly in accord with the biologic response profile.
The receptor for the macrophage growth factor M-CSF is c-fms22; its biologic activity and physical binding are associated with committed cells in the macrophage lineage.23,24 Genetic disruption of c-fms leads mainly to a failure of development of osteoclasts from their macrophage precursors.25 Figure 3 confirms the expression of c-fms transcripts in committed macrophage precursors and in terminally maturing monocytes-macrophages. Distinct hybridization was also observed in maturing populations of other hemopoietic lineage cells. Because these samples of terminally maturing cells were derived from populations that could have contained a small macrophage component, the significance of these signals is not yet clear. Very strong hybridization was also detected in single tetrapotent precursor cells but was absent in downstream precursors that had dual macrophage-neutrophil potential. Ambient granulocyte-colony-stimulating factor (G-CSF) levels limit neutrophil production in vivo,26 and G-CSF stimulates predominantly committed neutrophil precursor growth in cultures of murine marrow.27 Although it mobilizes multipotent precursors from marrow in vivo and effects on earlier precursors have been described in bulk cultures, compelling evidence for direct action on uncommitted cells is sparse. Binding studies localized receptors to neutrophilic and monocytic lineages, with receptor numbers increasing with maturation.28,29 Expression of transcripts for G-CSF receptor was, therefore, anticipated to show a corresponding pattern. Using a 241-bp probe extending to 50 bases upstream of the poly(dA) tail, we observed strong hybridization throughout the precursor hierarchy (not shown). To determine whether this diffuse hybridization could reflect the presence of a repetitive motif, the probe was compared against the GenBank database and revealed the presence of a tract (positions 3114-3245, accession #M58288), 25 b upstream of the polyadenylation signal, that is represented frequently in the mouse genome. Accordingly, we used a new 3' probe that excluded this motif (Table 1 footnote) and observed strongest hybridization to cDNA from maturing cells in most lineages, including neutrophils, and weaker signals in bipotent and committed neutrophilic precursors (Figure 3). Fibroblasts were also distinctly positive. IL-4 is best known as a regulator of growth and immunoglobulin switching during B lymphocyte development, but it also has pleiotropic effects on various kinds of myeloid colony formation in culture.30 However, its best-established primary myeloid targets are mast cells31 and monocytic lineage cells32 that respond by deactivation,33 cytokine down-regulation,34 and dendritic cell conversion.35 In general correspondence with this pattern of biologic activity, strongest hybridization of the IL-4R probe was
detected in maturing macrophage populations and more weakly in their
immediate committed precursors. Hybridization was also detected
distinctly in tripotent precursors and just above background in mast
cell cDNA.
Receptor transcripts expected to be expressed in very primitive
hematopoietic precursors.
C-kit is expressed on reconstituting stem
cells,36-38 and its ligand promotes their survival and
growth in culture.13 It is also required for transit of
early stages of erythroid commitment upstream of CFU-E39
and for mast cell differentiation and growth in vivo.40
Although hybridization of the 3' UTR probe (A probe, Table 1) was
expected not only to cDNA from multipotent stages but also to committed
erythroid precursors and mast cells, we obtained significant
hybridization only to pentapotent cell cDNA (Figure
4). To extend the sensitivity of
detection, we subjected each globally amplified cDNA sample pool to
secondary c-kit-specific amplification using the B primers
indicated in Table 1. A single fragment of the expected size was
amplified from the pentapotent cDNA sample (Figure 4) and confirmed as
c-kit by sequencing. The same fragment was also amplified
from cDNA corresponding to tripotent and bipotent E/Meg and Neut/Mac
precursors and to committed erythroid precursors (BFU-E), but not from
any of the other committed precursor cDNA pools and not from long-term
mast cell cDNA.
Flk-1 encodes the receptor for vascular endothelial growth factor. It is required for emergence of endothelium and blood cells in early embryogenesis41 and for survival and growth of a common endothelial and hematopoietic progenitor differentiating from embryonic stem cells in culture.42 The receptor protein has been reported on primitive human hematopoietic precursors,43 and transcripts have been detected in human hematopoietic precursors and in megakaryocytes.44 On hierarchy blots we detected a robust hybridization signal only in maturing megakaryocyte cDNA. Secondary Flk-1-specific PCR additionally detected transcripts in maturing macrophages but not in any other lineages or precursor stages (Figure 4). These results suggest possible secondary effects of vascular endothelial growth factor, the cognate ligand of Flk-1, on hematopoiesis through interaction with megakaryocytes or monocytes. Flk-2/Flt-3 is required for the generation of B lymphoid precursor cells in vivo,45 and its ligand enhances survival and renewal of primitive myeloid precursor cells in culture.46-49 We were unable to detect significant probe hybridization on hierarchy blots (not shown). PCR primers directed against the 3' terminus successfully amplified their target from cDNA globally amplified from a reconstituting stem cell fraction purified from adult mouse marrow (Figure 4), a result confirmed by sequencing of the amplified fragment. However, specific PCR failed to detect transcripts in any of the globally amplified hierarchy cDNAs. The result suggested that although transcripts may be expressed in resting reconstituting stem cells, they are down-regulated in more advanced, though still multipotent, myeloid progeny growing in culture. Transcripts expected to be expressed with distinctive specificities
in pluripotent precursors.
The receptors in this group recognize ligands known to affect
precursors in more than one lineage and, in most instances, also
precursors that have more than one lineage potential. Despite this
common feature, the associated agonists differ from one another in the
spectrum of targets each affects. The group was considered to provide an informative overall test of the fidelity of the staging
and lineage assignments of our single-cell cDNA samples. Hybridization
results for the group are shown in Figure
5.
IL-3 is known to target committed precursors in the erythroid, megakaryocyte, neutrophil, and mast cell lineages and their pluripotent precursors.50-52 However, in the mouse there is little evidence that direct responsiveness extends further upstream to the level of long-term reconstituting cells. The IL-3-specific receptor
subunit, present in the mouse but lacking in humans, confers high
ligand affinity to the subunit53,54 and is required for maximum affinity of the receptors on hematopoietic cells in vivo.55 As anticipated, moderate to strong hybridization
of the -specific probe was seen in all erythroid-myeloid lineages and in bipotent and tripotent precursor stages, whereas distinctly less
was observed to tetrapotent and pentapotent samples.
Granulocyte macrophage-colony-stimulating factor (GM-CSF) stimulates
almost exclusively neutrophil and macrophage lineages and their
bipotent precursors in murine marrow cultures,56 and it
particularly differs from IL-3 in not stimulating the growth of
erythroid colonies. Receptors have been detected on macrophage, neutrophil, and eosinophil lineage cells.57 The receptor subunit is specific for the GM-CSF ligand.58,59
The pattern of hybridization of a GM-CSFR probe differed distinctly
from that of the IL-3R-specific probe: strong hybridization was
obtained to tetrapotent cDNA, but high-level expression was maintained
as expected into macrophage and neutrophil, but not erythroid or
megakaryocyte, lineages.
IL-7 is a growth factor required by committed B and T lymphocyte
precursors.60,61 It also enhances the growth of macrophage and neutrophil lineage cells in culture provided other cytokines are
present, including c-kit ligand, or the classical
colony-stimulating factors GM-CSF, M-CSF, or IL-3.62,63
The receptor subunit has been demonstrated by FACS analysis on
macrophages and their precursors and on B and T
lymphocytes,64 and monocytes are directly responsive.65 A 3' probe prepared on the basis of the
available GenBank sequence (accession #M29697.1) for the IL-7 receptor sequence yielded little hybridization to a hierarchy blot (not shown). As referenced in the footnote to Table 1, we extended the mRNA
sequence several hundred bases downstream to 2 potential alternative
polyadenylation sites by identifying EST clusters in the NCBI murine
EST database. Primer pairs specific to each of the alternative 3' ends
were used to amplify products from our globally amplified B cell cDNA
pool. Products of the predicted sizes were obtained (Figure 5,
"upstream" and "downstream"), and their identities were
confirmed by sequencing. The upstream receptor probe hybridized
most clearly to cDNA from macrophages and B cells on the hierarchy blot
(Figure 5). To obtain additional sensitivity, both primer pairs were
used to probe our cDNA sample matrix by specific secondary PCR
amplification. As also shown in Figure 5, the upstream primers detected
transcripts in maturing B cell and macrophage cDNA, while the
downstream primers detected targets in the same maturing samples and in
mast cell, T cell, and, at a lower level, tetrapotent cDNAs. Overall,
IL-7R transcripts were most abundant in cDNA from maturing
macrophages and growing B cells, but they were also detected in
multipotent cells and in mast- and T-cell populations. The results
reinforce the evidence that macrophages are a likely target of IL-7.
Because macrophages are potential sources of G-CSF and GM-CSF, the
findings also suggest a possible indirect mechanism for reported
effects of IL-7 on myeloid colony formation.
Tumor necrosis factor (TNF)- and - exert inhibitory action on the
growth of hemopoietic precursor cells in bulk cultures, with macrophage
precursor growth the most affected,66 and on the growth of
neutrophil-macrophage precursors in single-cell cultures.67 The TNF receptor subtype 1 is the main
mediator of these responses, as shown by loss of the inhibitory effect in cultures of marrow from TNFR1 / mice.67
Our TNFR1 probe hybridized predominantly to cDNA from bipotent
neutrophil-macrophage precursor cDNA and from committed and maturing
cells in the macrophage lineage (Figure 5). A strong signal was also
seen on cDNA from NIH3T3 fibroblasts.
Although the effects of IL-6 have been reported in bulk cultures on
various hematopoietic stages and lineages, binding of labeled IL-6 was
more narrowly distributed on macrophage and megakaryocyte lineage cells
and morphologically undifferentiated precursor cells.68 In
good agreement, we observed hybridization of our IL-6R probe predominantly to cDNA from macrophages and their committed precursors and on upstream bipotent neutrophil-macrophage and tetrapotent precursor cells.
IL-11, when combined with c-kit ligand and other cytokines
in cultures of murine marrow, strikingly promotes the growth of in vivo
short-term erythroid reconstituting cells13 and
multilineage and committed erythroid and megakaryocyte
progenitors.69 These effects on pluripotent cells in
culture are more pronounced than those of IL-6. Little information is
available on differential binding of IL-11 to murine marrow cells in
different lineages. As shown in Figure 5, the actions on more primitive
cells are paralleled by distinct hybridization of an IL-11R probe to
cDNA from bipotent and tripotent precursors upstream of the erythroid and megakaryocyte lineages, samples that showed significantly less
signal with the IL-6R probe. Committed neutrophil precursors and ConA-activated T cells also hybridized the IL-11R probe
strongly, but little signal was detected in megakaryocytes or their
committed precursors.
Receptors with little prior indication of myeloid stage- or
lineage- specificity of expression.
IL-6, IL-11, LIF, and Oncostatin M have distinct cognate receptors that
incorporate gp130 as a common signaling subunit. Because the gp130
receptor subunit is ubiquitously expressed in various tissues,70 it was expected to show a wider distribution of
expression than the IL-6 and IL-11 receptors individually. In
agreement, our probe hybridized more widely than either IL-6R
IL-1 shows a variety of enhancing and inhibitory effects in bulk cultures of myeloid and lymphoid cells in the presence of other primary agonists.52,71 Cells in murine marrow fractions enriched for either primitive stem cells or more advanced and committed progenitors bind IL-1 specifically,38 and mature neutrophils and monocytes are also primary responders,72 suggesting widespread receptor expression in the hematopoietic hierarchy. The type 1 receptor is the principal signaling element responsible for biologic responses to IL-1, while the evidence suggests that type 2 receptors play modulating rather than primary roles and have a more restricted pattern of expression.72,73 As seen in Figure 6, type 1 receptor transcripts were detected ubiquitously in the marrow hierarchy, whereas type 2 transcript expression was found predominantly in maturing macrophages. These patterns thus also conform to expectations. Although IL-2 plays its dominant biologic role in regulating lymphocyte growth downstream of T cell activation, granulocyte and macrophage lineage cells are reported to express receptor components for IL-2 and respond to it.74-76 The receptor subunit is essential
for the formation of a high-affinity receptor complex and transduction
of a proliferative signal.77 Human monocytes are variably
reported to express or not express the subunit protein,75,77 while in mice it (CD122) is expressed on
maturing cells in the neutrophil series.76 An IL-2
receptor plasmid probe (Table 1) initially showed little
hybridization on our hierarchy blots (not shown) and lacked an obvious
polyadenylation signal. As described earlier for IL-7, it was possible
to identify by EST cluster analysis a likely 3' polyadenylation site
located 660 bases downstream of the 3' end of the available murine
sequence in the GenBank database (accession #M28052.1). Using a primer pair directed against the identified terminus, we were able to amplify
the correct target sequence from our globally amplified T cell cDNA,
and the resultant probe hybridized robustly to cDNA from T cells but
not from any of our other hierarchy samples (Figure 6). Target-specific
secondary PCR additionally detected transcripts in tetrapotent cDNA but
not in cDNA from more advanced cells, including the neutrophilic or
monocytic lineages.
The single-cell approach described in this study was designed to resolve gene expression patterns to individual stages during lineage divergence from multipotent stem cells. Generation of nearly homogeneous samples of cells at various individual stages is beyond the range of current cell-sorting strategies. However, we showed in an earlier study that homogeneity at specific stages could be achieved by sampling individual cells and described the generation and probing of a set of stage-specific cDNAs that included unipotent, bipotent, and a small number of tripotent precursors of various myeloid lineages.4 The present study describes the acquisition of new cDNA samples from single cells that were tetrapotent and pentapotent for the generation of progeny in erythroid, megakaryocyte, macrophage, neutrophil, and mast cell lineages. Multipotent cells were obtained in 2 stages, first by FACS enrichment of marrow for the fraction containing the long-term reconstituting stem cells, and then by selecting the disperse morphology colony starts generated from this fraction in culture. After 4 days in culture, only 1 in 60 of such starts had the capacity to reconstitute recipient mice.13 However, approximately half of them yielded multilineage differentiation, and approximately half of those yielded subclones that were uniformly multilineage. Thus, the approach allows access to small homogeneous populations of cells that are downstream of the long-term reconstituting cells but are still multipotent. In most instances, precursors were tetrapotent for erythroid, megakaryocyte, macrophage, and neutrophil differentiation. Where sibling cells exhibited the additional potential for spawning long-term mast cell cultures, the associated cDNA samples were segregated from the rest with the designation "pentapotent." The numbers of samples were, however, too small for differences in hybridization behavior between tetrapotent and pentapotent cDNAs to be considered significant. As determined by sibling outcomes, numerous cDNA samples were also obtained from downstream stages representing tripotent, bipotent, and unipotent hematopoietic precursor cells. In all these instances, samples were entered into the archive only if sibling outcomes were uniform. Overall, the new entries enlarged our sample set from 69 in our original study to 168, of which 120 derived from single growth-competent precursor cells and the rest derived from terminally maturing populations. Abundance of regulated, but not of constitutive, transcripts varies widely from cell to cell, and there is evidence that much of the variability originates in the processed cells themselves rather than in the RT-PCR procedure.4,20 We analyzed sample pools rather than the entire array of single-cell cDNA samples, a tactic that allowed us to average the results from multiple samples at a particular stage without compromising the homogeneity assured by their single-cell origin. Many of our receptor probes hybridized robustly to the hierarchy blots,
yielding coherent patterns of differential expression. However, other
probes were more problematic, either failing to hybridize above
background to any of the pools (IL-2R The global RT-PCR procedure was designed to amplify all transcripts
without bias. We and others have provided evidence that the procedure
preserves relative abundance relationships in the original mRNA
population.17,19 The amplification product thus differs
substantially from that obtained by target-specific RT-PCR, by which,
given enough cycles, even a few transcripts can amplify to yield a
strong band. It was anticipated that receptor transcripts in precursors
at very low abundance would be more difficult to detect by
hybridization to our cDNA samples than those in higher abundance. We
indeed found that our second-generation probes for c-kit,
IL-2R In the multipotent sample set comprising pentapotent and tetrapotent
cDNAs, we detected transcripts for c-kit, gp130, IL-6R |
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Abstract
Introduction
70°C with intensifying screens.
again, as determined by sibling outcomes.
Such samples were entered into our archive only if sibling outcomes
were identical. New samples were also obtained from terminally differentiating myeloid and T lymphocytic cells. Relative to the sample
collection described originally,
