NUMB (gene)

Protein numb homolog is a protein that in humans is encoded by the NUMB gene. The protein encoded by this gene plays a role in the determination of cell fates during development. The encoded protein, whose degradation is induced in a proteasome-dependent manner by MDM2, is a membrane-bound protein that has been shown to associate with EPS15, LNX1, and NOTCH1. Four transcript variants encoding different isoforms have been found for this gene.

The protein Numb is coded for by the gene, NUMB, whose mechanism appears to be evolutionarily conserved. Numb has been extensively studied in both invertebrates and mammals, though its function is best understood in Drosophila. Numb plays a crucial role in asymmetrical cell division during development, allowing for differential cell fate specification in the central and peripheral nervous systems. During neurogenesis, Numb localizes to one side of the mother cell such that it is distributed selectively to one daughter cell. This asymmetric division allows a daughter cell containing Numb to acquire a different fate than the other daughter cell.

The numb gene protein product controls binary cell fate decisions in the peripheral and central nervous systems of both invertebrates and mammals during neurogenesis. During cell division, Numb is asymmetrically localized to one end of the progenitor cell and subsequently segregates to only one daughter cell where it intrinsically determines cell fate. Numb protein signaling plays a key role in binary cell fate decisions following asymmetric cell divisions. One daughter cell, generally that receiving the Numb, is able to adopt a neuronal fate and innervate the developing nervous system. The other daughter cell becomes a progenitor cell to fill the lost role of the parent cell and maintain proliferation. In addition to its role in proliferation and differentiation, Numb has also been shown to play a role in tumorigenesis and the response of neural progenitors to chemotactic cues during migration.

In mammals, there are four alternatively spliced forms of the Numb protein. In addition, there is a Numb homolog called “Numb-like,” or NUMBL. Numb proteins in mammals are not as well understood as their fly counterparts. The various forms of Numb have differential progenitor-promoting and differentiation-promoting functions. More research is necessary to understand the complex relationships between these forms of Numb and their functions.

In both invertebrates and mammals, Numb is localized using the Pins/GαI complex and the PAR complex of Bazooka (Par3 in mammals), Par6, and aPKC (atypical protein kinase C). In the sensory organ precursor (SOP) cell, the PAR proteins localize to the posterior pole of the cell, and the Pins/GαI complex is localized to the anterior pole of the cell. This results in an anterior/posterior cell division with daughter cells of similar size. In neuroblasts, both complexes are localized to the apical cortex, causing apical/basal cell division and daughter cells exhibiting strong size asymmetry. In the SOP, one mechanism for Numb localization has been proposed based on the PAR complex. It states that a complex phosphorylation cascade enables aPKC to phosphorylate Numb in the pre-mitotic cell, decreasing its affinity for the plasma membrane. This releases Numb from the aPKC pole, increasing its presence in the non-aPKC pole. This establishes the asymmetric distribution of Numb, with the Numb/Pon crescent on one side of the mother cell.

Another proposed component of the localization complex is Partner of Numb (PON), which is asymmetrically localized during mitosis and acts as an adaptor protein by binding and mediating the anchoring of Numb. The localization of PON is controlled by either Insc or the Frizzled-Wnt signaling pathway.

Numb's primary function in cell differentiation is as an inhibitor of Notch signaling which is essential for maintaining self-renewal potential in stem and progenitor cells. Notch is a transmembrane signaling receptor that is activated by DSL family ligands. Notch binds the ligands Delta and Serrate in Drosophila. The human ligands are Delta-like and Jagged, respectively. These ligands are themselves integral membrane proteins. Following ligand binding of the Notch receptor, the intracellular fragment of Notch (NICD, or notch intracellular domain) is released into the cytoplasm and transported to the nucleus, where it can form a complex with binding partners such as EP300 and histone acetyltransferase and act as a transcription factor for Notch target genes. Among the Notch target genes are members of the HES and HEY gene families whose protein products can act as transcriptional repressors for tissue-specific transcription factors, thus maintaining the cell's potential for self-renewal.

Numb exerts its functional role on cell fate decisions by antagonizing Notch signaling activities. The molecular mechanisms underlying this relationship appear to rely on the ubiquitination of the membrane bound Notch1 receptor and the subsequent degradation of its NICD following receptor activation. In support of this, Numb's ability to ubiquinate Notch1 directly correlated with its functional inhibition of Notch1 signaling activities. The ubiquitination pathway directs protein recycling by directly tagging specific proteins for proteasome degradation. Through a multi-step process, free ubiquitin is first attached to an activating enzyme (E1) and then transferred to a conjugating enzyme (E2) which partners with a ligase (E3) which functions as an adaptor to selectively transfer the ubiquitin to specific protein substrates. Numb expression was found to selectively tag the membrane Notch1 receptor for ubiquitination through the interaction of its Phosphotyrosine-binding domain with the E3 ubiquitin ligase Itch. Numb and Itch work in concert to promote the ubiquitination of the full-length membrane-tethered Notch receptor prior to activation. However, Numb only appears to promote the degradation of the NICD cleavage product following receptor activation, targeting it for proteasome degradation and preventing its translocation to the nucleus.