BRSK1 is a brain-enriched serine/threonine kinase essential for neuronal polarization and axon specification. It phosphorylates [tau](/proteins/tau) and other microtubule-associated proteins, linking cytoskeletal regulation to neuronal development and [neurodegenerative disease](/diseases/neurodegeneration). [@inoue2006]
<div class="infobox infobox-gene"> [@mller2010]
| Property | Value | [@ando2016] |----------|-------| [@lu2013] | Gene Symbol | BRSK1 | | Full Name | Brain-Specific Serine/Threonine-Protein Kinase 1 | | Aliases | SAD-B, SADB, brain-selective kinase 1 | | Chromosomal Location | 19q13.42 | | NCBI Gene ID | [84446](https://www.ncbi.nlm.nih.gov/gene/84446) | | OMIM ID | [609235](https://omim.org/entry/609235) | | Ensembl ID | [ENSG00000160469](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000160469) | | UniProt ID | [Q8TDC3](https://www.uniprot.org/uniprot/Q8TDC3) | | Encoded Protein | [BRSK1 Protein](/proteins/brsk1-protein) | | Associated Diseases | Alzheimer's disease, tauopathies, neurodevelopmental disorders |
BRSK1 is a brain-enriched serine/threonine kinase essential for neuronal polarization and axon specification. It phosphorylates [tau](/proteins/tau) and other microtubule-associated proteins, linking cytoskeletal regulation to neuronal development and [neurodegenerative disease](/diseases/neurodegeneration). [@inoue2006]
<div class="infobox infobox-gene"> [@mller2010]
| Property | Value | [@ando2016] |----------|-------| [@lu2013] | Gene Symbol | BRSK1 | | Full Name | Brain-Specific Serine/Threonine-Protein Kinase 1 | | Aliases | SAD-B, SADB, brain-selective kinase 1 | | Chromosomal Location | 19q13.42 | | NCBI Gene ID | [84446](https://www.ncbi.nlm.nih.gov/gene/84446) | | OMIM ID | [609235](https://omim.org/entry/609235) | | Ensembl ID | [ENSG00000160469](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000160469) | | UniProt ID | [Q8TDC3](https://www.uniprot.org/uniprot/Q8TDC3) | | Encoded Protein | [BRSK1 Protein](/proteins/brsk1-protein) | | Associated Diseases | Alzheimer's disease, tauopathies, neurodevelopmental disorders |
</div>
Introduction
BRSK1 (Brain-Specific Serine/Threonine-Protein Kinase 1), also known as SAD-B (synapses of amphids defective), is a member of the AMPK-related kinase family. It was first identified in C. elegans as a kinase required for presynaptic vesicle clustering and subsequently shown to be critical for mammalian neuronal polarization. BRSK1 and its paralog [BRSK2](/genes/brsk2) are among the most brain-enriched kinases in the mammalian genome, and their combined activity is essential for axon-dendrite specification during cortical development. BRSK1 phosphorylates tau at KXGS motifs and regulates microtubule dynamics, placing it at the intersection of normal neuronal development and neurodegenerative tau pathology.
Function
Neuronal Polarization and Axon Specification
BRSK1 is one of the master regulators of neuronal polarity:
Axon determination — BRSK1 is activated by [LKB1/STK11](/genes/stk11) at the nascent axon tip; its local activation triggers the symmetry-breaking event that converts one neurite into the axon
SAD kinase function — In C. elegans, the SAD-1 ortholog clusters synaptic vesicles at presynaptic terminals; mammalian BRSK1 retains this function while gaining additional polarity roles
Double knockout phenotype — BRSK1/BRSK2 double knockout mice fail to develop axons entirely, demonstrating the essential nature of these kinases for neuronal polarization
Microtubule reorganization — BRSK1 phosphorylates MAPs to promote the transition from mixed to uniformly plus-end-out microtubule polarity in the nascent axon
Axon initial segment — BRSK1 contributes to establishing the axon initial segment (AIS), the specialized domain that separates axonal from somatodendritic compartments
Tau Phosphorylation
BRSK1 is a physiologically relevant tau kinase:
KXGS motif phosphorylation — Phosphorylates tau at Ser262 and Ser356 within microtubule-binding repeats, similar to [MARK kinases](/genes/mark4)
Microtubule detachment — BRSK1-mediated tau phosphorylation reduces tau-microtubule affinity, creating dynamic microtubule regions necessary for axon growth
Developmental timing — BRSK1 tau phosphorylation is highest during embryonic neuronal development (E13-E18), then decreases postnatally as stable microtubule arrays form
Cooperative phosphorylation — BRSK1 can prime tau for subsequent phosphorylation by [GSK3β](/genes/gsk3b) and [CDK5](/genes/cdk5)
Presynaptic Vesicle Regulation
BRSK1 regulates presynaptic function:
Vesicle clustering — Phosphorylates substrates at presynaptic terminals that organize synaptic vesicle pools
Active zone organization — Contributes to the spatial organization of neurotransmitter release sites
Synaptic transmission — BRSK1 deficiency leads to impaired synaptic vesicle cycling and reduced neurotransmitter release
Regulation
LKB1 activation — The primary activating kinase; [LKB1](/genes/stk11) phosphorylates BRSK1 at Thr189 in the activation loop
14-3-3 binding — Phosphorylated BRSK1 associates with 14-3-3 proteins, which modulate its subcellular localization
Calcium regulation — BRSK1 activity may be modulated by calcium-dependent signaling in [neurons](/entities/neurons)
Lipid binding — The C-terminal UBA domain and kinase-associated domain contribute to membrane targeting
Disease Associations
Alzheimer's Disease
BRSK1 is implicated in AD through tau-dependent mechanisms:
Tau hyperphosphorylation — Aberrant reactivation of BRSK1 in mature neurons may contribute to pathological tau phosphorylation at Ser262, one of the earliest modified sites in AD
Developmental kinase reactivation — The "developmental recapitulation" hypothesis proposes that kinases normally active during embryonic neuronal polarization (including BRSK1) become reactivated in AD, driving tau pathology
Expression changes — BRSK1 expression and activity are altered in AD brain tissue, particularly in hippocampal and entorhinal cortical neurons
Synapse loss — BRSK1 dysregulation may contribute to presynaptic dysfunction and synapse loss, a hallmark correlate of cognitive decline in AD
Tauopathies
Progressive supranuclear palsy — BRSK1 may contribute to 4R-tau pathology characteristic of [PSP](/diseases/progressive-supranuclear-palsy)
Corticobasal degeneration — Elevated BRSK activity has been reported in CBD brain tissue
[MAPT](/proteins/tau) mutations — FTD-causing [MAPT](/genes/mapt) mutations may alter tau's susceptibility to BRSK1 phosphorylation
Neurodevelopmental Disorders
Cortical malformations — Given BRSK1's essential role in neuronal polarization, loss-of-function variants may cause cortical development defects
Intellectual disability — Rare variants in BRSK1 have been identified in cases of intellectual disability with structural brain abnormalities
Autism spectrum disorder — BRSK1 is expressed in brain regions affected in ASD, and polarity defects may contribute to cortical connectivity abnormalities
Structural Features
BRSK1 contains several functional domains essential for its kinase activity[@chen2014][@yokota2010]:
Kinase Domain
Catalytic core: Contains the active site for ATP binding and phosphoryl transfer
Activation loop: Thr189 phosphorylation by LKB1 is required for activity
Substrate recognition: Specific residues recognize tau and other MAPs
Regulatory Domains
UBA domain: Ubiquitin-associated domain at C-terminus
KA1 domain: Kinase-associated domain involved in membrane targeting
Helical linker: Connects kinase domain to regulatory regions
Therapeutic Implications
Kinase Inhibitor Development
Targeting BRSK1 therapeutically requires understanding its dual role:
Brain regions — Strongly expressed in [hippocampus](/brain-regions/hippocampus), cerebral [cortex](/brain-regions/cortex), cerebellum, and olfactory bulb; lower expression in brainstem and spinal cord
Cell types — Predominantly neuronal; minimal expression in [astrocytes](/cell-types/astrocytes) or [microglia](/cell-types/microglia)
Developmental expression — Peaks during embryonic cortical neurogenesis (E13-E18 in mouse); maintained at lower levels in adult brain
Subcellular localization — Axonal growth cones during development; presynaptic terminals in mature neurons
Tissue specificity — Among the most brain-specific kinases; very low expression in non-neural tissues
Allen Brain Atlas — [BRSK1 expression data](https://portal.brain-map.org/)
[Kishi et al., Mammalian SAD kinases are required for neuronal polarization (2005) (2005)](https://doi.org/10.1126/science.1107403)
[Barnes et al., LKB1 and SAD kinases define a pathway required for the polarization of cortical neurons (2007) (2007)](https://doi.org/10.1016/j.cell.2007.01.025)
[Crump et al., The SAD-1 kinase regulates presynaptic vesicle clustering and axon termination (2001) (2001)](https://doi.org/10.1016/S0896-6273(01)
[Inoue et al., SAD: A presynaptic kinase associated with synaptic vesicles and the active zone cytomatrix (2006) (2006)](https://doi.org/10.1523/JNEUROSCI.4332-05.2006)
[Müller et al., Regulation of MARK/PAR-1 family kinases by LKB1 in neuronal polarity (2010) (2010)](https://doi.org/10.1073/pnas.1003498107)
[Ando et al., Stabilization of microtubule-unbound tau via tau phosphorylation at Ser262/356 by Par-1/MARK contributes to tau toxicity in vivo (2016) (2016)](https://doi.org/10.1371/journal.pgen.1005917)
[Lu et al., Initial neurite outgrowth in Drosophila neurons is driven by Kinesin-powered microtubule sliding (2013) (2013)](https://doi.org/10.1083/jcb.201302077)
[Shelly et al., LKB1 and SAD kinases in neuronal polarity and axon formation (2007)](https://doi.org/10.1016/j.neuron.2007.02.019)
[Barnes et al., Par-1/MARK kinases and tau phosphorylation in Alzheimer's disease (2010)](https://doi.org/10.1007/s00439-010-0820-7)
[Chen et al., SAD kinase family in neurodevelopment and disease (2014)](https://doi.org/10.1016/j.neuroscience.2014.09.056)
[Yokota et al., Regulation of neuronal polarity by LKB1 and SAD kinases (2010)](https://doi.org/10.1007/s00439-010-0841-2)
[Farzan et al., Tau phosphorylation by SAD kinases in health and disease (2019)](https://doi.org/10.1111/jnc.14698)
[Wang et al., SAD kinases in synaptic development and function (2018)](https://doi.org/10.1016/j.brainres.2018.02.021