Are Signal Peptides Hidden Regulators of Neurodegenerative Disease?

Canonical signal peptides (SPs) are short N-terminal sequences that direct nascent proteins into the secretory pathway, but their role extends far beyond protein targeting. Advances in sequencing and computational tools have enabled their systematic identification across proteomes, highlighting SPs as critical regulators of protein biogenesis, including endoplasmic reticulum (ER) targeting, translocation, folding, and proteostasis. Clinically, mutations affecting SP function underlie a distinct group of human disorders, while SP-derived fragments are emerging as diagnostic biomarkers and therapeutic targets. In biotechnology, SPs are engineered to enhance recombinant protein production and serve as molecular tags for intracellular delivery. Together, these developments position SPs at the intersection of fundamental cell biology, medicine, and biotechnology. While this review primarily focuses on canonical SPs, it also considers selected non-canonical targeting and topogenic sequences whose dysfunction contributes to protein misfolding, impaired ER translocation, disrupted degradation pathways, and altered intracellular trafficking in neurodegenerative diseases. Aberrations involving both conventional SPs and alternative targeting/topogenic elements contribute to pathological protein aggregation, a hallmark of major neurodegenerative disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington Disease (HD), prion diseases, and amyotrophic lateral sclerosis/frontotemporal dementia (ALS/FTD); in multiple sclerosis (MS) is primarily an inflammatory demyelinating disease, where abnormal protein exposure, potentially linked to misprocessed SPs, can activate immune responses. By synthesizing current knowledge, the review explores how alterations in targeting determinants influence key proteostasis pathways, acting as upstream modulators of disease-relevant molecular cascades. It further discusses the emerging concept that SP-derived fragments may participate in intercellular communication, adding an additional layer of regulatory complexity.