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A major limitation to the widespread use of hematopoietic stem cells (HSC) is the relatively crude level of our knowledge of how to maintain these cells in vitro without loss of the long-term multilineage growth and differentiation properties required for their clinical utility. An experimental and theoretical framework for predicting and controlling the outcome of HSC stimulation by exogenous cytokines would thus be useful. An emerging theme from recent HSC expansion studies is that a net gain in HSC numbers requires the maintenance of critical signaling ligand(s) above a threshold level. These ligand-receptor complex thresholds can be maintained, for example, by high concentrations of soluble cytokines or by extracellular matrix- or cell-bound cytokine presentation. According to such a model, when the relevant ligand-receptor interaction falls below a critical level, the probability of a differentiation response is increased; otherwise, self-renewal is favored. Thus, in addition to the identity of a particular receptor-ligand interaction being important to the regulation of stem cell responses, the quantitative nature of this interaction, as well as the dynamics of receptor expression, internalization, and signaling, may have a significant influence on stem cell fate decisions. This review uses examples from hematopoiesis and other tissue systems to examine existing evidence for a role of receptor activation thresholds in regulating hematopoietic stem cell self-renewal versus differentiation events. (Blood. 2000;96:1215-1222)

Intracellular trafficking of the epidermal growth factor receptor (EGF-R) is regulated by receptor occupancy. To investigate this, we developed an assay to study endosomal sorting under steady-state conditions. Using a cell line transfected with EGF-R variants, we found that the fraction of internalized EGF.EGF-R complexes sorted to lysosomes was a function of the number of intracellular complexes and required sequences in the cytoplasmic domain of the receptor. As the number of intracellular occupied wild-type receptors increased from 3 x 10(2) to 2 x 10(5)/cell, the fraction of internalized EGF that was degraded dropped from 70 to 20%. Transforming growth factor-alpha, which dissociates from the EGF-R at endosomal pH, was degraded to a uniform extent of approximately 50% at all intracellular ligand concentrations. EGF internalized by receptors lacking a cytoplasmic domain (c'647) was degraded to an extent of only 5-10% independent of the number of intracellular complexes. Mutant receptors truncated either at residues 1022 or 973 displayed sorting patterns intermediate between wild-type and c'647 receptors. Despite large differences in their internalization rates, the fractional sorting patterns of c'1022 and c'973 receptors were indistinguishable. Receptor tyrosine kinase activity appeared to have a small effect on sorting pattern, but only in the context of full-length receptors. Our results indicate that the default pathway of internalized receptors is rapidly recycling and that lysosomal targeting of occupied EGF-R is due to endosomal retention that is both specific and saturable. In addition, internalization and endosomal retention of EGF-R appear to be mediated by distinct structural elements.

Little is known about the regulation of EGF receptor (EGF-R) trafficking following endocytosis. We investigated this by using a series of EGF-R with altered cytoplasmic tails and comparing their ability to undergo recycling and lysosomal targeting in both the occupied and empty state. We found that 2-3% of empty EGF-R are internalized each minute, but rapidly recycle (t1/2 approximately 5 min). This constitutive internalization and recycling of empty receptors was independent of cytoplasmic receptor sequences. Occupied EGF-R, in contrast, displayed a much slower rate of recycling (t1/2 between 10-23 min) due to retention within recycling endosomes. Endosomal retention of different EGF-R correlated with lysosomal targeting of EGF. Intrinsic receptor tyrosine kinase activity had no discernible effect on postendocytic trafficking of EGF. Although sequences within the cytoplasmic tail of the EGF-R appear to be required for occupancy-dependent endosomal retention, they are distinct from those required for ligand-induced endocytosis. Our studies indicate that intracellular trafficking of the EGF-R is regulated by endosomal components that preferentially recognize occupied receptors. Down-regulation of the EGF-R thus involves two distinct regulatory processes: one at the level of internalization and one at the level of recycling.

Occupancy-induced down-regulation of cell surface epidermal growth factor (EGF) receptors attenuates signal transduction. To define mechanisms through which down-regulation of this class of growth factor receptors occurs, we have investigated the relative roles of ligand-induced internalization and recycling in this process. Occupied, kinase-active EGF receptors were internalized through a high affinity, saturable endocytic system at rates up to 10-fold faster than empty receptors. In contrast, full length EGF receptors lacking tyrosine kinase activity underwent internalization at a rate independent of occupancy. This "kinase-independent" internalization rate appeared to reflect constitutive receptor internalization since it was similar to the internalization rate of both receptors lacking a cytoplasmic domain and of antibodies bound to empty receptors. EGF internalized by either kinase-active or kinase-inactive receptors was efficiently recycled and was found within endosomes containing recycling transferrin receptors. However, targeting of internalized receptors to lysosomes did not require receptor kinase activity. All receptors that displayed ligand-induced internalization also underwent down-regulation, indicating that the proximal cause of down-regulation is occupancy-induced endocytosis. Tyrosine kinase activity greatly enhances this process by stabilizing receptor association with the endocytic apparatus.