Engrailed-1 in eccrine sweat gland development and regeneration: from evolutionary innovation to therapeutic perspectives.

Functional regeneration of eccrine sweat glands (ESGs) remains a major unmet need in burns and trauma, where extensive skin loss leads to permanent anhidrosis, life-threatening hyperthermia, and impaired quality of life. Here we synthesize evidence pointing to the homeodomain transcription factor Engrailed-1 (EN1) as a central regulator that coordinates ESG development across four interconnected biological functions: (1) establishing a permissive ventral limb field by repressing the WNT7A-LMX1B dorsalization pathway; (2) executing a binary fate switch that suppresses the hair follicle program while activating a pro-glandular program; (3) functioning as a quantitative rheostat whose expression levels determine ESG density-a mechanism potentiated by primate-specific enhancers such as ECE18; and (4) being required for the establishment of a transient eccrine niche (EDEN) that provides mesenchymal support during gland morphogenesis in mouse models. Drawing on this framework, we propose a tiered pathogenesis-regeneration model (Tiers 1-4) mapping to distinct EN1 failures: Tier 1 (ventral field loss) points to bioengineered glabrous skin substitutes; Tier 2 (lineage specification failure) guides strategies to redirect progenitor fate; Tier 3 (aborted morphogenesis) highlights the need for EDEN-like niche reconstruction; and Tier 4 (compromised adult integrity) suggests approaches to reinforce the mechanical anchor of existing glands through EN1-mediated regulation of epidermal isoform of Bullous Pemphigoid Antigen 1 (BPAG1e), a proposed function inferred from in vitro evidence but untested in vivo. This multi-tiered model offers a conceptual framework for functional ESG restoration while addressing translational gaps between embryonic programs and adult tissue competency-a framework that remains speculative and requires rigorous validation in human tissues, as many mechanisms are supported primarily by mouse studies.