SSH2

SSH2 (Slingshot-2) is a cofilin/ADF phosphatase that directly dephosphorylates and reactivates cofilin to drive actin cytoskeletal remodeling across multiple migratory and morphogenic contexts (PMID:17360713, PMID:18171679). Through this activity SSH2 contributes to NGF-induced neurite extension (PMID:17360713), alpha6beta4 integrin/Rac1-dependent keratinocyte migration and laminin-332 assembly (PMID:17848544), and LPA-induced tumor cell transcellular invasion (PMID:18171679), with isoform-specific contributions that vary by cell type—for example, SSH2 supports cofilin activation in vascular smooth muscle cells but is not the dominant isoform there (PMID:21868701). SSH2 activity is controlled both downstream of Rac1 via 14-3-3 protein binding, which increases when Rac1 is inhibited (PMID:17848544), and through a GPCR→Gαi→PLCγ2→PKCβ→PKD1 kinase axis in which PKD1 directly targets SSH2 to regulate cofilin and neutrophil chemotaxis (PMID:25568344). SSH2 expression is restrained post-transcriptionally by miR-194 acting on its 3′UTR (PMID:31824193), and SSH2 loss is synthetically lethal with FLCN deficiency, triggering caspase-3/7 activation and cell cycle arrest specifically in FLCN-null carcinoma cells (PMID:23416984).

1. 2007 High

   Established the core biochemical activity of SSH2 by showing it dephosphorylates and reactivates cofilin, linking this activity to a cellular phenotype (neurite extension).

   Evidence RNAi double knockdown of SSH1/SSH2 with cofilin phosphorylation and neurite extension readouts in PC12 cells and chick DRG neurons

   PMID:17360713

   Open questions at the time

   • Knockdown was combined SSH1/SSH2, leaving SSH2-specific contribution unresolved
   • No structural or kinetic characterization of SSH2 catalysis
2. 2007 High

   Placed SSH2 in an upstream signaling context, showing its activity is regulated by 14-3-3 binding downstream of alpha6beta4 integrin/Rac1 and required for keratinocyte migration.

   Evidence Phosphatase-dead SSH constructs, dominant-negative Rac1, 14-3-3 Co-IP, and wound-healing/immunofluorescence assays in human keratinocytes

   PMID:17848544

   Open questions at the time

   • 14-3-3 binding shown for SSH family broadly, not SSH2 isoform-specifically
   • Mechanism by which Rac1 modulates 14-3-3 association not resolved at the molecular level
3. 2008 High

   Extended the cofilin-activation role of SSH2 to cancer cell invasion, demonstrating it is required for LPA-induced transcellular migration.

   Evidence siRNA knockdown with transcellular migration, 2D motility, and cofilin phosphorylation assays in MM1 hepatoma cells

   PMID:18171679

   Open questions at the time

   • SSH1 and SSH2 contributions not fully separated
   • Upstream signal coupling LPA to SSH2 in these cells unidentified
4. 2011 Medium

   Refined isoform specificity, showing SSH2 contributes to cofilin activation but is not the dominant migration-controlling isoform in vascular smooth muscle cells.

   Evidence Isoform-specific siRNA with scratch wound, Boyden chamber, and cofilin phosphorylation assays in VSMCs

   PMID:21868701

   Open questions at the time

   • Negative migration result for SSH2 specific to VSMC context
   • Functional redundancy with SSH1 not directly tested by double knockdown
5. 2013 Medium

   Revealed a context-dependent survival role, identifying a synthetic lethal interaction between SSH2 loss and FLCN deficiency.

   Evidence siRNA phosphatase screen, caspase-3/7 and cell cycle assays in isogenic FLCN+/− carcinoma cell pairs

   PMID:23416984

   Open questions at the time

   • Mechanistic link between FLCN status and SSH2 dependency via cofilin not fully resolved
   • Single lab, in vitro only
6. 2014 Low

   Reported SSH2 as a fusion partner in lung adenocarcinoma, where an ALK-SSH2 fusion conferred crizotinib sensitivity.

   Evidence NGS of tumor tissue with clinical crizotinib response as readout in a single case

   PMID:35144623

   Open questions at the time

   • Single clinical case with no in vitro validation of fusion kinase activity
   • Functional contribution of the SSH2 portion of the fusion unknown
7. 2015 High

   Defined the upstream kinase cascade regulating SSH2, identifying PKD1 as a direct kinase targeting SSH2 within a GPCR→Gαi→PLCγ2→PKCβ→PKD1 chemotaxis pathway.

   Evidence Co-IP, sequential knockdown epistasis, cofilin phosphorylation and chemotaxis assays in neutrophils with SSH2 as PKD1 substrate

   PMID:25568344

   Open questions at the time

   • PKD1 phosphorylation site(s) on SSH2 and their effect on phosphatase activity not mapped
   • Whether this axis operates in non-neutrophil contexts untested
8. 2019 Medium

   Established post-transcriptional control of SSH2, showing miR-194 directly targets its 3′UTR to suppress expression in colon cancer stem cells.

   Evidence Luciferase reporter, RT-PCR, western blot, proliferation/apoptosis assays and xenograft

   PMID:31824193

   Open questions at the time

   • Functional rescue of the phenotype by SSH2 specifically not demonstrated
   • Whether SSH2 cofilin-phosphatase activity mediates the miR-194 phenotype unknown

• How SSH2 phosphorylation by upstream kinases and 14-3-3 binding are integrated to set cofilin-phosphatase output, and how this is wired into context-specific survival (FLCN) and disease phenotypes, remains unresolved.
• No structural model of SSH2 or its regulatory phosphosites
• Substrate specificity beyond cofilin/ADF uncharacterized
• Mechanistic basis of SSH2-FLCN synthetic lethality not defined