**6. Role of the ECM in control of adult learning behavior and cognitive flexibility**

Experimental weakening of the ECM by local injection of matrix-digesting enzymes can promote functional neurorehabilitation in the injured brain. This has been related mostly to injuries on the level of the peripheral nervous system and spinal cord [4,30,31]. Experiencedriven plasticity does, however, not only lead sensory development or neuronal rehabilitation, but is also indispensable during learning, memory formation, and re-consolidation throughout life. The question now arises how forms of ECM-dependent plasticity in the adult brain might govern learning-related plasticity, lifelong memory reformation, and the organization of cognitively flexible behavior. In this respect, several studies have investigated the involvement of ECM functions in memory storage in adult animals. This has been characterized the best for long-term plasticity in the hippocampus and fear memory in the amygdala. However, available evidence is controversial about how ECM functions may impact on learning and memory processes.

For instance, it has been reported that tenascin-R knockout mice show normal hippocampusdependent spatial memory acquisition in a Water maze. In subsequent reversal learning though animals showed more vulnerable spatial long-term memory yielding enhanced relearning performance due to less conflicting past and actual learning contingencies [32]. Another study found an already impaired acquisition of hippocampus-dependent contextual memory in same knockout mice [33]. Injection of chABC in the bilateral striatum, however, has been related to an improvement of water maze acquisition learning, while the recall of the learned values was unaffected [34].

In addition to deficits in matrix components, studies also found effects of deficits in exopro‐ teases modeling the ECM. Loss of MMP9 activity has been associated with impaired hippo‐ campal-dependent learning and amygdala-dependent learning. This is in line with findings of wild-type mice trained in an inhibitory avoidance (IA) learning paradigm [35,36]. Hippo‐ campal LTP has been related to increased levels of MMP3 and MMP9. Both proteases were upregulated for at least ~48 h promoting local plastic synaptic environments underlying the learning performance. Intra-hippocampal injections of MMP9 blockers completely abrogated memory for the IA response when tested days later. Comparably, hippocampal MMP3 and MMP9 were found to be increased during water maze acquisition learning in a NMDAdependent manner. Hippocampal injection of the broad-spectrum MMP9 inhibitor FN-439 also prevented elevated MMP9 levels, altered hippocampal LTP, and prevented spatial acquisition learning [35].

We have shown that the ECM in the adult brain is a plastic structural scaffold shaped by network activity. Depending on the current activity level, the ECM can incorporate secreted components or release signaling messengers by proteolytic cleavage. Cleaved products can trigger signaling through diverse ECM receptors and modulate the activities of transmitter receptor, ion channels, or integrin signaling impacting on plastic shaping of individual

150 Composition and Function of the Extracellular Matrix in the Human Body

**6. Role of the ECM in control of adult learning behavior and cognitive**

Experimental weakening of the ECM by local injection of matrix-digesting enzymes can promote functional neurorehabilitation in the injured brain. This has been related mostly to injuries on the level of the peripheral nervous system and spinal cord [4,30,31]. Experiencedriven plasticity does, however, not only lead sensory development or neuronal rehabilitation, but is also indispensable during learning, memory formation, and re-consolidation throughout life. The question now arises how forms of ECM-dependent plasticity in the adult brain might govern learning-related plasticity, lifelong memory reformation, and the organization of cognitively flexible behavior. In this respect, several studies have investigated the involvement of ECM functions in memory storage in adult animals. This has been characterized the best for long-term plasticity in the hippocampus and fear memory in the amygdala. However, available evidence is controversial about how ECM functions may impact on learning and

For instance, it has been reported that tenascin-R knockout mice show normal hippocampusdependent spatial memory acquisition in a Water maze. In subsequent reversal learning though animals showed more vulnerable spatial long-term memory yielding enhanced relearning performance due to less conflicting past and actual learning contingencies [32]. Another study found an already impaired acquisition of hippocampus-dependent contextual memory in same knockout mice [33]. Injection of chABC in the bilateral striatum, however, has been related to an improvement of water maze acquisition learning, while the recall of the

In addition to deficits in matrix components, studies also found effects of deficits in exopro‐ teases modeling the ECM. Loss of MMP9 activity has been associated with impaired hippo‐ campal-dependent learning and amygdala-dependent learning. This is in line with findings of wild-type mice trained in an inhibitory avoidance (IA) learning paradigm [35,36]. Hippo‐ campal LTP has been related to increased levels of MMP3 and MMP9. Both proteases were upregulated for at least ~48 h promoting local plastic synaptic environments underlying the learning performance. Intra-hippocampal injections of MMP9 blockers completely abrogated memory for the IA response when tested days later. Comparably, hippocampal MMP3 and MMP9 were found to be increased during water maze acquisition learning in a NMDAdependent manner. Hippocampal injection of the broad-spectrum MMP9 inhibitor FN-439

synapses.

**flexibility**

memory processes.

learned values was unaffected [34].

Similarly, spatial training in a water maze in wild-type rats has been found to correlate with increased levels of hippocampal brevican and versican in the membrane fraction [37]. These findings indicate that hippocampal-dependent learning induced a period of intrinsic activityinduced focal MMP-mediated proteolysis driving long-lasting synaptic modifications under‐ lying learning and memory consolidation. Effects of changes in the ECM on initial learning are, however, still unclear [25,33].

More recently, insights into the impact of the ECM onto behavior came from studies using experimental, enzymatic, and local weakening of the ECM. The study of Gogolla et al. manipulating adult fear extinction suggested that memory acquisition differs in juvenile and adult brains due to changes of the mature ECM functions. The authors further argued that intra-amygdala injections of chABC in adult rats had no effect on acquisition learning of fear, but only on extinction, reinstatement, and renewal of the fear memory [15]. A further study showed that intra-hippocampal and prefrontal injection of chABC and HYase in mice impair long-term trace contextual fear conditioning [38]. This finding has been related to the impair‐ ment in the L-VDCC-dependent component of hippocampal LTP by cleaved extracellular HA [20].

In addition to spatial memory, another set of studies examined the function of the ECM in memory consolidation of drug seeking. A recent study showed that intracerebral injection of FN-439 impaired the acquisition of a cocaine-induced conditioned place preference (CPP) of rats. FN-439 injection 30 min prior to cocaine memory re-activation further attenuated the reinstatement of CPP in extinguished animals. The study further showed that intra-amygdala injections of chABC during active extinction of cocaine-induced CPP prevented its subsequent priming-induced reinstatement. ChABC injections alone had no effect on the retention, retrieval, or relearning of CPP [39]. Similarly, enzymatic weakening of PNNs in the prelimbic cortex or in the amygdala of adult rats impaired the acquisition and reconsolidation of druginduced memories [40,41].

With respect to cognitively flexible adaptation of behavior, we have recently shown that weakening of the ECM in auditory cortex promotes complex forms of cortex-dependent relearning in the Mongolian gerbil [42]. In our experiments, we trained animals on frequencymodulated tone discrimination based on the rising or falling modulation direction in a go-/ nogo-task. Such auditory learning is known to depend on learning-induced plastic reorgani‐ zation of neuronal circuits in the auditory cortex. After acquiring robust discrimination of the stimulus contingencies, the animals were trained to reverse their choice. We found that ECM weakening by local HYase injection in bilateral auditory cortex accelerated the demanding relearning performance (**Figure 3**). Specifically, animals had to inhibit the obsolete initial behavioral strategy and then establish its successful reversal. Importantly, attenuation of the ECM did neither affect the acquisition learning nor erased already established, learned memory traces (**Figure 3B**). That means attenuation of the ECM in sensory cortex of these animals promoted the flexible adaptation of the effectively appropriate strategy during cortexdependent learning behavior that bases on "reprogramming" previously acquired auditory memories. The ECM reconstitutes after several days to weeks limiting again the promoting effects onto cognitive flexibility (**Figure 3A**). A comparable finding investigated log-term object recognition memory in knockout mice of the link protein Crtl1/Hapln1—a key molecule for stabilization of PNNs. The Ctrl1/Hapln1 knockout mice have attenuated PNNs in the perirhi‐ nal cortex. Long-term object recognition memory, a task depending on perirhinal cortex, was enhanced in these mice. Local injection of chABC in wild-type mice had the same memoryprolonging effect in the object recognition task, but also attenuated over time [43]. In this study, the attenuation of the PNNs was accompanied by enhanced perirhinal LTD, which is thought to be the major synaptic mechanism underlying object recognition memory.

**Figure 3. Local enzymatic weakening of the ECM in auditory cortex of Mongolian gerbils enhanced the cognitive flexibility in a relearning paradigm**. (**A**) *Right*, Quantification of ECM weakening after local injection of HYase in uni‐ lateral auditory cortex of Mongolian gerbils (right) compared to control (left) based on WFA staining. *Left*, HYase injec‐ tion significantly weakened the ECM for about 1 week and reconstituted fully after 2 weeks. (**B**) Mongolian gerbils were trained in a two-compartmental go/no-go Shuttle-box in order to discriminate two frequency-modulated sounds (modulation direction indicated by rising and falling arrows). Gerbils showed successful acquisition depending on the contingency of the stimuli as a go-stimulus (red) or Nogo-stimulus (green). In two groups, the contingency was re‐ versed after seven training days (*left and middle*). Conditioned response rates were strongly reduced in both experimen‐ tal groups indicating the active inhibition of the previously established discrimination strategy. HYase-treated animals were significantly better in correcting the behavioral strategy and successfully relearn the task (middle). Interestingly, HYase treatment did not interfere with the recall of already established cortex-dependent auditory memories (*right*). Modified from Ref. [42].

The both last-mentioned studies therefore promote the view that the perineuronal ECM in the adult brain actively organizes the balance between memory stability and flexibility. Cortical attenuation of the ECM in the mature brain might hence promote the cognitive flexibility that can build on learned behaviors and allows for an enhanced activity-dependent memory reorganization (see also Ref. [44]). And regeneration of the ECM gradually restores normal, restrictive adult plasticity levels. Generally, all studies summarized in this review emphasized that the increased experience-based plasticity by acute, enzymatic PNN diminution is activitydependent and the rather inconspicuous effect of mere ECM attenuation in general. Mecha‐ nistically, enzymatic ECM degradation might facilitate the rearrangement of functional network connectivity by a shift in the balance between excitation and inhibition leading to destabilized existing patterns of neuronal network interactions [45].
