Juxtacrine signaling

Juxtacrine signaling is a form of cell-to-cell communication that requires direct physical contact between the signaling cell and the responding cell. Unlike endocrine (long-range), paracrine (short-range diffusion), or autocrine (self-signaling) modes, juxtacrine signals are transmitted across the narrow intercellular space (typically 15–30 nm) via membrane-anchored proteins. This contact-dependent mechanism ensures highly localized, spatially restricted communication that is essential for developmental patterning, immune responses, tissue polarity, and synaptic maintenance (Singer, 1992).

There are two principal forms of juxtacrine signaling. In the receptor-ligand mode, a membrane-bound ligand on the surface of one cell binds directly to a receptor on an adjacent cell. In the direct membrane contact mode, gap junctions or tunneling nanotubes allow the passive diffusion of small molecules (e.g., ions, cAMP, IP₃) between contiguous cells, a process sometimes termed gap junctional intercellular communication (GJIC) (Goodenough & Paul, 2009). However, the term juxtacrine is most commonly reserved for contact-dependent signaling via surface-bound ligands.

The most extensively characterized juxtacrine pathway is the Notch signaling system. The Notch receptor is a transmembrane protein expressed on the surface of a signal-receiving cell, while its ligands—Delta (Dll1, Dll3, Dll4) and Jagged (Jag1, Jag2) in mammals—are also transmembrane proteins on the signaling cell. Upon direct cell-cell contact, the ligand engages the Notch receptor, triggering a proteolytic cascade (ADAM protease and γ-secretase) that releases the Notch intracellular domain (NICD). The NICD translocates to the nucleus, where it associates with the transcription factor CSL (CBF1/RBPJκ) to activate target genes such as Hes and Hey. This mechanism is fundamental to lateral inhibition and boundary formation during development (Bray, 2016).

Another major juxtacrine system involves the ephrin-Eph receptor family. Eph receptors (receptor tyrosine kinases) and their ephrin ligands are both membrane-anchored. Bidirectional signaling can occur: forward signaling through Eph receptors and reverse signaling through ephrins. This contact-dependent communication guides cell migration, axon pathfinding, topographic mapping in the visual system, and boundary formation during tissue segmentation (Kullander & Klein, 2002).

In the immune system, juxtacrine signaling is essential for the formation and function of the immunological synapse—the structured interface between a T lymphocyte and an antigen-presenting cell (APC). Interactions between T cell receptors (TCR) and peptide-MHC complexes, as well as co-stimulatory pairs such as CD28-CD80/86, are contact-dependent. Additionally, the membrane-bound cytokine mTGF-β (membrane-bound transforming growth factor beta) directly signals to adjacent cells, modulating regulatory T cell function (Dustin & Depoil, 2011).

Cell adhesion molecules (CAMs), including cadherins, integrins, and selectins, can also function as juxtacrine signaling platforms. Classical cadherins (E-cadherin, N-cadherin) mediate homophilic adhesion and, through interaction with catenins, transduce signals that regulate the Wnt/β-catenin pathway and cytoskeletal dynamics, thereby controlling epithelial polarity, contact inhibition of proliferation, and collective cell migration (Takeichi, 2014).

A specialized form of juxtacrine signaling occurs in gap junctions, where connexin proteins assemble into channels that directly connect the cytoplasm of adjacent cells. These channels permit the direct intercellular transfer of ions and small signaling molecules (<1 kDa), including Ca²⁺, cAMP, IP₃, and ATP. This electrical and metabolic coupling coordinates cardiac myocyte contraction, neuronal synchrony, and embryonic development (Goodenough & Paul, 2009).

Dysregulation of juxtacrine signaling is associated with human diseases. Loss-of-function mutations in Notch pathway components cause Alagille syndrome (Jag1 mutations) and cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL, Notch3 mutations). Aberrant ephrin-Eph signaling promotes tumor invasion and metastasis. Connexin mutations underlie peripheral neuropathies (Cx32), sensorineural deafness (Cx26, Cx30), and congenital heart disease (Cx43) (Laird, 2006).

Références

Bray, S. J. (2016). Notch signalling in context. Nature Reviews Molecular Cell Biology, 17(11), 722–735.

Dustin, M. L., & Depoil, D. (2011). New insights into the T cell synapse from single molecule techniques. Nature Reviews Immunology, 11(10), 672–684.

Goodenough, D. A., & Paul, D. L. (2009). Gap junctions. Cold Spring Harbor Perspectives in Biology, 1(1), a002576.

Kullander, K., & Klein, R. (2002). Mechanisms and functions of Eph and ephrin signalling. Nature Reviews Molecular Cell Biology, 3(7), 475–486.

Laird, D. W. (2006). Life cycle of connexins in health and disease. Biochemical Journal, 394(3), 527–543.

Singer, S. J. (1992). Intercellular communication and cell-cell adhesion. Science, 255(5052), 1671–1677.

Takeichi, M. (2014). Dynamic contacts: rearranging adherens junctions to drive epithelial remodelling. Nature Reviews Molecular Cell Biology, 15(6), 397–410.